{"gene":"SLC38A2","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2000,"finding":"SLC38A2 (ATA2/SNAT2) was cloned and characterized as the protein responsible for system A amino acid transport activity ubiquitously expressed in mammalian tissues. It mediates Na+-dependent transport of neutral amino acids (model substrate: α-(methylamino)isobutyric acid), is pH-sensitive, Li+-intolerant, and has a 1:1 Na+:amino acid stoichiometry.","method":"Heterologous expression in mammalian cells and Xenopus laevis oocytes; transport current electrophysiology; substrate specificity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — functional reconstitution in two expression systems with kinetic characterization; independently replicated across two cloning papers (rat and human)","pmids":["10747860","10930503"],"is_preprint":false},{"year":2001,"finding":"SNAT2 protein is present in plasma-membrane and internal-membrane fractions of rat skeletal muscle, adipose tissue, L6 myotubes, and 3T3-L1 adipocytes, with subcellular distribution similar to GLUT4. Chronic amino acid deprivation causes a time-dependent increase in SNAT2 protein abundance, establishing the adaptive up-regulation of System A.","method":"Subcellular fractionation; Western blotting; immunolocalization with specific polyclonal antibodies","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — direct fractionation experiment with functional correlation; replicated in multiple cell/tissue types","pmids":["11311116"],"is_preprint":false},{"year":2001,"finding":"Adaptive stimulation of System A transport upon amino acid starvation in human fibroblasts is directly correlated with increased ATA2 (SNAT2) mRNA expression; supplementation with System A substrates (but not other amino acids) suppresses both ATA2 mRNA levels and transport activity, demonstrating substrate-specific transcriptional feedback.","method":"Northern blotting; transport assays; substrate specificity competition experiments","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — two orthogonal measurements (mRNA + transport) but single lab","pmids":["11172802"],"is_preprint":false},{"year":2002,"finding":"Insulin stimulates System A transport in L6 skeletal muscle cells by recruiting SAT2 (SNAT2) to the plasma membrane from an endosomal compartment; this process requires phosphatidylinositol 3-kinase activity and is blocked by chloroquine (which impairs endosomal recycling) without affecting insulin-mediated PKB/GSK3 phosphorylation or GLUT4 translocation.","method":"Cell surface biotinylation; pharmacological inhibition (wortmannin, chloroquine); Western blotting; radiolabeled transport assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches dissecting PI3K-dependent plasma membrane recruitment from endosomal pool","pmids":["11834730"],"is_preprint":false},{"year":2002,"finding":"ATA2-mediated amino acid transport up-regulation following partial hepatectomy is regulated by redistribution of ATA2 protein to the plasma membrane from an intracellular compartment, not by changes in steady-state ATA2 mRNA levels.","method":"Plasma membrane isolation; Western blotting; Northern blotting","journal":"Archives of biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 2 — protein redistribution confirmed by fractionation with parallel mRNA negative result","pmids":["12054432"],"is_preprint":false},{"year":2002,"finding":"Transcriptional activation of the ATA2 (SNAT2) gene by amino acid deprivation is largely independent of de novo protein synthesis (unlike asparagine synthetase), is detectable within 2–4 h, and is not induced by glucose deprivation, indicating a distinct genomic regulatory mechanism.","method":"Reporter gene assays; Northern blotting; cycloheximide/actinomycin D inhibition; HepG2 cells","journal":"The Journal of nutrition","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic dissection using pharmacological inhibitors with mRNA readout","pmids":["12368390"],"is_preprint":false},{"year":2004,"finding":"De novo synthesis of new SNAT2 transporter protein is essential for the hypertonic stimulation of System A transport activity in human fibroblasts; transcription inhibition (DRB) abolishes the hypertonic transport increase, whereas the adaptive increase induced by amino acid starvation only partly requires new SNAT2 synthesis.","method":"Biotinylation of surface proteins; Western blotting; immunocytochemistry; transcription inhibitor (DRB) experiments","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — surface protein biotinylation plus functional transport assay with mechanistic inhibitors","pmids":["15581851"],"is_preprint":false},{"year":2005,"finding":"siRNA-mediated silencing of SNAT2 in hypertonically stressed human fibroblasts prevents the increase in System A transport activity, blocks expansion of the intracellular amino acid pool, and markedly delays cell volume recovery, demonstrating that SNAT2 induction is required for regulatory volume increase (RVI).","method":"siRNA knockdown; radiolabeled transport assays; intracellular amino acid measurement; cell volume measurement","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 2 — clean loss-of-function with specific phenotypic readout (volume recovery) and transport activity measurement","pmids":["15922329"],"is_preprint":false},{"year":2006,"finding":"The amino acid response element driving SNAT2 up-regulation upon amino acid limitation resides in the first intron of the gene; ATF and C/EBP family transcription factors bind this intronic enhancer in vitro and in vivo (ChIP), with specific members acting as activators or repressors, and amino acid deprivation increases RNA polymerase II recruitment to the SNAT2 promoter.","method":"Deletion/reporter assays; ChIP; EMSA; transcription factor overexpression","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — in vitro binding + in vivo ChIP + functional reporter; mechanistic dissection of multiple factors","pmids":["16445384"],"is_preprint":false},{"year":2006,"finding":"SNAT2 induction by amino acid starvation requires eIF2α phosphorylation and involves an internal ribosome entry site (IRES) in the 5'-UTR that enables cap-independent translation, as well as increased gene transcription; hypertonic stress induction of SNAT2, by contrast, is eIF2α phosphorylation-independent.","method":"eIF2α phosphorylation-deficient mutant cells; IRES reporter assays; cell-free translation; Northern/Western blotting","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — genetic mutant cells + cell-free reconstitution of IRES + multiple orthogonal readouts","pmids":["16621798"],"is_preprint":false},{"year":2006,"finding":"SNAT2 transporter function is associated with a leak anion conductance that does not require substrate transport; transported substrates (L-alanine, L-glutamine, MeAIB) inhibit this anion leak with different potencies. Mutation H304A eliminates alanine transport but retains anion leak current, and the selectivity sequence of the anion conductance was determined.","method":"Electrophysiology (whole-cell patch clamp and two-electrode voltage clamp) in Xenopus oocytes; site-directed mutagenesis (H304A); ion substitution experiments","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 — structure-function by mutagenesis combined with electrophysiology; mechanistically separates transport from anion leak","pmids":["17237199"],"is_preprint":false},{"year":2006,"finding":"A conserved C-terminal histidine residue (H504 in SNAT2, H471 in SNAT5) mediates the pH-sensitivity of System A and N transporters through an allosteric mechanism influencing Na+ binding; DEPC modification abolishes pH-sensitivity and its effects are reversed by hydroxylamine and blocked by substrate; H504A mutation produces reduced, DEPC-resistant pH-sensitivity without changing Na+ affinity at low pH.","method":"Xenopus oocyte expression; site-directed mutagenesis (H504A); DEPC chemical modification; transport current electrophysiology; Na+ affinity measurement","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with chemical modification and electrophysiology; identifies conserved mechanistic residue","pmids":["16629640"],"is_preprint":false},{"year":2006,"finding":"Ubiquitin ligase Nedd4-2 regulates SNAT2 (ATA2) surface abundance by polyubiquitinating the transporter at the plasma membrane, leading to endocytotic sequestration and proteasomal degradation; Nedd4-2 RNAi increases SNAT2 activity with concomitant decreased polyubiquitination; catalytically inactive Nedd4-2 has no effect.","method":"Xenopus oocyte expression; transfection in CHO cells and adipocytes; RNAi; co-localization by immunofluorescence; proteasome inhibitor (MG132); ubiquitination assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal gain/loss-of-function with ubiquitination assays and transport readout in multiple systems","pmids":["17003038"],"is_preprint":false},{"year":2006,"finding":"ATA2 (SNAT2) is stored in a trans-Golgi network (TGN) compartment (co-localizing with syntaxin 6, not EEA1) in adipocytes; insulin stimulus releases SNAT2-containing vesicles from the TGN to the plasma membrane through a pathway distinct from GLUT4 translocation; brefeldin A (TGN exit inhibitor) preferentially blocks insulin-stimulated MeAIB uptake over glucose transport.","method":"Live-cell imaging of EGFP-tagged ATA2; immunofluorescence co-localization; brefeldin A inhibition; insulin-stimulated transport assays in 3T3-L1 adipocytes","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — live imaging plus pharmacological dissection; identifies distinct storage and trafficking pathway","pmids":["17050538"],"is_preprint":false},{"year":2007,"finding":"Two distinct amino acid sensor/effector pathways control SNAT2 expression: (1) JNK activation during amino acid withdrawal induces SNAT2 transcription via an intronic nutrient-sensitive domain; (2) a sensor for large neutral amino acids (Tyr, Gln) inhibits JNK and suppresses SNAT2 up-regulation. Additionally, SNAT2 itself provides a repressive signal for its own gene transcription when amino acids are sufficient, consistent with a transceptor (transporter-receptor) function.","method":"shRNA; transporter chimera expression; JNK pathway inhibitors; intronic reporter assays; SNAT2 protein stability measurements in L6 myotubes","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — shRNA + chimera approach + multiple pathway inhibitors establish two converging sensor pathways and transceptor auto-regulation","pmids":["17488712"],"is_preprint":false},{"year":2007,"finding":"Selective inhibition of SNAT2 (by MeAIB, metabolic acidosis at pH 7.1, or siRNA knockdown) depletes intracellular L-Gln and indirectly depletes leucine and other amino acids maintained by the L-Gln gradient, strongly impairing mTOR signalling to S6K, S6, and 4E-BP1 and reducing protein synthesis in L6 skeletal muscle cells.","method":"siRNA; competitive substrate inhibition (MeAIB); metabolic acidosis; amino acid HPLC; mTOR pathway Western blotting; protein synthesis assays","journal":"Journal of the American Society of Nephrology : JASN","confidence":"High","confidence_rationale":"Tier 2 — three independent SNAT2 inhibition approaches with consistent downstream signalling readouts","pmids":["17429052"],"is_preprint":false},{"year":2008,"finding":"SNAT2 inhibition by metabolic acidosis (pH 7.1) or MeAIB stimulates proteolysis in L6 myotubes; this effect is mediated through both mTOR and PI3K signalling pathways, and SNAT2 partial silencing impairs insulin signalling through PI3K, linking SNAT2-mediated glutamine uptake to regulation of muscle proteolysis and insulin resistance.","method":"siRNA; MeAIB competitive inhibition; proteasome inhibitor; mTOR/PI3K pharmacological inhibitors; proteolysis assays in L6 cells","journal":"Journal of the American Society of Nephrology : JASN","confidence":"High","confidence_rationale":"Tier 2 — siRNA + pharmacological dissection of two signalling arms with proteolysis readout","pmids":["18650482"],"is_preprint":false},{"year":2008,"finding":"In glutamatergic neurons, SAT2 (SNAT2) is predominantly targeted to somatodendritic compartments and supplies glutamine for conversion to glutamate required for retrograde dendritic signalling; MeAIB inhibition of SAT2 reduces neuronal glutamine uptake, lowers intracellular glutamate, and suppresses inhibitory synaptic inputs to pyramidal cells from fast-spiking interneurons.","method":"Immunohistochemistry; confocal microscopy; electron microscopy; in vivo electrical stimulation; in vitro depolarization; MeAIB pharmacological inhibition; electrophysiology of neocortical circuits","journal":"Cerebral cortex (New York, N.Y. : 1991)","confidence":"High","confidence_rationale":"Tier 2 — localization by EM + pharmacological loss-of-function with electrophysiological circuit readout","pmids":["18832333"],"is_preprint":false},{"year":2008,"finding":"Asparagine 82 in transmembrane domain 1 of SNAT2 is critical for Na+ coordination; N82A mutation virtually eliminates alanine transport current and amino acid uptake, dramatically reduces Na+ affinity (K(Na+)), and increases apparent Km for alanine 27-fold, demonstrating a direct or indirect role of Asn82 in Na+ binding.","method":"Site-directed mutagenesis (N82A, N82S, Y337A, R374Q); Xenopus oocyte expression; two-electrode voltage clamp; radiolabeled amino acid uptake assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with functional electrophysiology + uptake assays; identifies Na+ coordination residue","pmids":["18319257"],"is_preprint":false},{"year":2008,"finding":"Despite increased ATF4 binding to the C/EBP-ATF composite site in the SNAT2 gene during unfolded protein response (UPR) activation, SNAT2 transcription is not enhanced in HepG2 cells; UPR suppresses the amino acid response (AAR)-induced increase in SNAT2 transcription, demonstrating that the UPR generates a repressive signal downstream of ATF4 binding that is chromatin-level (lacking histone H3 hyperacetylation and general transcription factor recruitment at the promoter).","method":"ChIP (H3 acetylation, RNA Pol II, general transcription factors); reporter gene assays; pharmacological UPR/AAR activation; HepG2 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — ChIP + reporter assays mechanistically dissect AAR vs UPR regulation; multiple orthogonal chromatin readouts","pmids":["18697751"],"is_preprint":false},{"year":2009,"finding":"A conserved Na+ binding site in SNAT2 is formed by transmembrane helices 1 and 8, predicted by homology modeling to LeuT/Mhp1 structures; the T384A mutation in the predicted TMD8 Na+ binding site dramatically lowers Na+ affinity and inhibits the anion leak current, consistent with a cation binding site conserved across SLC38 and related bacterial transporter families.","method":"Profile-based sequence analysis; homology modeling (LeuT, Mhp1 templates); site-directed mutagenesis (T384A); Xenopus oocyte expression; electrophysiology; Na+ affinity measurement","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — structure-guided mutagenesis validated by functional electrophysiology; identifies conserved Na+ site","pmids":["19589777"],"is_preprint":false},{"year":2009,"finding":"In neocortical neurons, SNAT2 expression is constitutively low but potently induced by depletion of neutral amino acids; substrates of the SLC6 GABA transporter family (taurine, GABA, β-alanine) repress SNAT2 expression more potently (~10×) than System A substrates; ATF4 and C/EBP induction by amino acid deprivation mediates SNAT2 transcriptional regulation in neurons.","method":"Neuronal culture; SNAT2 de novo induction; electrophysiology (spontaneous excitatory activity); ATF4/C/EBP Western blotting; pharmacological substrate competition","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — electrophysiological and molecular readouts in neurons; identifies novel regulatory substrates","pmids":["19240036"],"is_preprint":false},{"year":2009,"finding":"IL-6 stimulates System A amino acid transporter activity in human trophoblast cells through STAT3-dependent transcriptional up-regulation of SNAT2 mRNA and protein; siRNA targeting STAT3 reduces SNAT2 (but not SNAT1) expression and abolishes IL-6-stimulated System A activity. TNF-α also stimulates system A via SNAT2 but through a JAK/STAT-independent pathway.","method":"siRNA (STAT3); Western blotting; RT-PCR; radiolabeled MeAIB transport assays; phospho-STAT3 detection in primary human trophoblasts","journal":"American journal of physiology. Cell physiology","confidence":"High","confidence_rationale":"Tier 2 — siRNA loss-of-function with isoform-specific mRNA/protein readouts and transport functional assay; mechanistically links IL-6/STAT3 to SNAT2","pmids":["19741197"],"is_preprint":false},{"year":2011,"finding":"Chronic competitive inhibition of SNAT2 with MeAIB reduces cell proliferation and depletes intracellular SNAT2 substrate amino acids as well as leucine; surprisingly, Me-AIB elevates mTOR-dependent p70S6K1 phosphorylation despite amino acid pool depletion. Proteomic analysis of TAP-purified SNAT2 fusion proteins identified two novel SNAT2-interacting proteins potentially involved in signalling for protein turnover and cell growth.","method":"Competitive inhibition (MeAIB); mTOR pathway Western blotting; TAP-tag purification + proteomics; cell proliferation/size assays in MCF-7 cells","journal":"Frontiers in bioscience (Elite edition)","confidence":"Medium","confidence_rationale":"Tier 3 — novel interactors identified by single TAP-tag/MS experiment; mTOR finding is mechanistically unexpected but replicated internally","pmids":["21622135"],"is_preprint":false},{"year":2011,"finding":"C-terminal domain deletion of SNAT2 (13 residues) abolishes amino acid transport at negative membrane potentials while allowing transport at positive potentials; the truncation increases apparent affinity for alanine (~3-fold) and Na+ (~2-fold) without affecting surface expression, demonstrating that the C-terminal extracellular domain acts as a voltage regulator required for normal amino acid translocation at physiological potentials.","method":"C-terminal truncation mutagenesis; Xenopus oocyte expression; two-electrode voltage clamp; radiolabeled amino acid uptake; surface expression assays","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — deletion mutagenesis with electrophysiology and uptake assays; identifies specific functional role of C-terminal domain","pmids":["21158741"],"is_preprint":false},{"year":2011,"finding":"SNAT2 is the primary transporter mediating L-proline uptake by embryonic stem (ES) cells; uptake of L-proline through SNAT2 is required for ES cell differentiation to early primitive ectoderm-like cells, as SNAT2 substrate competitors (but not non-substrates) block morphological changes, gene expression changes, and differentiation kinetics.","method":"Competitive substrate inhibition; transport assays; morphological and gene expression differentiation assays in ES cells","journal":"American journal of physiology. Cell physiology","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological specificity (System A substrates vs. non-substrates) with multiple differentiation readouts","pmids":["21346154"],"is_preprint":false},{"year":2012,"finding":"A genome-wide RNAi screen identified SNAT2 as required for arsenite-induced ER stress response; arsenite up-regulates SNAT2 expression and activity in an ATF4-dependent manner; inhibition of SNAT2 expression/activity or glutamine deprivation specifically suppresses arsenite-induced (but not tunicamycin-induced) ER stress and mTOR activation, placing SNAT2 upstream of mTOR in this pathway.","method":"Genome-wide shRNA screen; flow cytometry; SNAT2 inhibition (MeAIB, siRNA); mTOR pathway Western blotting; ER stress reporters","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — unbiased genome-wide screen + mechanistic follow-up with specific inhibition; ATF4-dependence established","pmids":["22215663"],"is_preprint":false},{"year":2013,"finding":"SNAT2 transports the PET radiotracer anti-[18F]FACBC with Michaelis-Menten kinetics (Km ~197 μM), lower affinity than ASCT2 (Km ~97 μM); characterized in Xenopus oocytes expressing human SNAT2.","method":"Xenopus oocyte expression; radiolabeled [14C]FACBC kinetic uptake assays","journal":"Nuclear medicine and biology","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro transport kinetics in expression system; single study","pmids":["23647854"],"is_preprint":false},{"year":2014,"finding":"Only SNAT2 (among SNAT1, 2, and 4) exhibits betaine uptake activity; human and rat SNAT2 transport betaine with Km values of 5.3 mM and 4.6 mM, respectively; betaine exclusively inhibits SNAT2 among the system A subtypes; hypertonicity preferentially induces SNAT2 expression and its plasma membrane targeting in placental trophoblasts.","method":"Transfection in HEK293 cells; [14C]betaine uptake assays; Western blotting of plasma membrane fractions; immunocytochemistry; RT-PCR in TR-TBT 18d-1 cells","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1–2 — direct substrate transport assays in expression system + in situ membrane trafficking validation; subtype specificity for betaine established","pmids":["24434061"],"is_preprint":false},{"year":2014,"finding":"17β-estradiol regulates SNAT2 transcription through a functional estrogen response element (ERE) in the SNAT2 promoter bound by estrogen receptor α (ERα); in vivo ChIP shows progressive ERα binding to the SNAT2 promoter during gestation correlating with estradiol levels; the ERα-ERE complex also contains PARP1, Ku70, and GAPDH, and silencing each abolishes estradiol-stimulated SNAT2 promoter activity.","method":"Reporter assays; EMSA; supershift assays; in vivo ChIP; LC-MS proteomics of ERα-ERE complex; Western blotting; ERE deletion/mutation","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro binding + in vivo ChIP + reporter + complex proteomics; multiple orthogonal methods","pmids":["25056967"],"is_preprint":false},{"year":2015,"finding":"RNF5 ubiquitin ligase associates with, ubiquitinates, and promotes degradation of SLC38A2 (along with SLC1A5) in response to paclitaxel-induced ER stress in breast cancer cells, decreasing glutamine uptake, TCA cycle components, and mTOR signalling while increasing autophagy and cell death.","method":"Co-immunoprecipitation; ubiquitination assays; siRNA knockdown; metabolic profiling; mTOR pathway Western blotting; cell death/autophagy assays","journal":"Cancer cell","confidence":"High","confidence_rationale":"Tier 2 — Co-IP + ubiquitination assay + functional downstream metabolic and signalling readouts","pmids":["25759021"],"is_preprint":false},{"year":2015,"finding":"SNAT2 protein stability is regulated by the ubiquitin-proteasome system via N-terminal lysyl residues; linoleic acid (LOA) increases ubiquitination and proteasomal degradation of SNAT2 through increased Nedd4.2 expression; mutation of seven N-terminal lysyl residues to alanine protects SNAT2 from LOA-induced degradation; the N-terminal cytoplasmic tail confers substrate-induced changes in SNAT2 stability when grafted onto SNAT5.","method":"Proteasome inhibitor; shRNA (Nedd4.2); N-terminal lysine mutagenesis; chimeric SNAT2-SNAT5 construct; transport assays; Western blotting in L6 myotubes/HeLa","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — mutagenesis + chimeric transporter + loss-of-function; identifies specific N-terminal domain and lysyl residues","pmids":["25653282"],"is_preprint":false},{"year":2015,"finding":"Decreased placental mTOR activity in IUGR is associated with increased NEDD4-2 expression (+72%), increased ubiquitination of SNAT2 (+180%), and decreased SNAT2 protein in the syncytiotrophoblast microvillous plasma membrane (–31%), establishing that mTOR regulates placental SNAT2 surface trafficking via the ubiquitin pathway.","method":"Western blotting (mTORC1/2 activity, NEDD4-2, SNAT2); ubiquitination assays; plasma membrane fractionation; radiolabeled MeAIB transport in human IUGR placental tissue","journal":"Clinical science (London, England : 1979)","confidence":"High","confidence_rationale":"Tier 2 — mechanistic pathway established in human tissue with multiple molecular readouts","pmids":["26374858"],"is_preprint":false},{"year":2015,"finding":"Hyperosmotic stress coordinates SNAT2 induction with GADD34 up-regulation; increased GADD34 (a PP1 regulatory subunit) dephosphorylates eIF2α, enhancing SNAT2-mediated amino acid uptake; GADD34 induction during osmotic stress depends on c-Jun/CRE-mediated transcription and mRNA stabilization (not ATF4, unlike other stresses), establishing a SNAT2/GADD34 axis for cell survival.","method":"Reporter assays (CRE); mRNA stability assays; eIF2α phosphorylation Western blotting; SNAT2 transport assays; GADD34 overexpression/knockdown","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches dissect SNAT2/GADD34 co-regulation with functional survival readout","pmids":["26041779"],"is_preprint":false},{"year":2016,"finding":"Net glutamine uptake in HeLa and 143B cancer cells does not depend on ASCT2 but requires SNAT1 and SNAT2; ASCT2 deletion causes amino acid starvation response and SNAT1 up-regulation to functionally replace ASCT2; SNAT2-mediated net uptake is essential for maintaining intracellular glutamine for glutaminolysis.","method":"CRISPR/Cas9 knockout (ASCT2); siRNA (GCN2, SNAT1); radiolabeled amino acid uptake; cell growth assays; Western blotting","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout + epistasis analysis establishes SNAT2 as rate-limiting for net glutamine uptake","pmids":["27129276"],"is_preprint":false},{"year":2018,"finding":"Membrane topology of SNAT2 was experimentally determined: 11 transmembrane domains with an intracellular N-terminus and extracellular C-terminus; three N-glycosylation sites were verified at the largest extracellular loop using chemical modification (mPEG-Mal), protease cleavage assays, immunofluorescence, and glycosylation analysis.","method":"mPEG-Mal chemical modification; protease cleavage; immunofluorescence; glycosylation assays; bioinformatics","journal":"Biochimica et biophysica acta. Biomembranes","confidence":"High","confidence_rationale":"Tier 1–2 — four orthogonal experimental methods to determine topology; first experimental topology confirmation","pmids":["29678469"],"is_preprint":false},{"year":2018,"finding":"Extracellular Na+ is required for the SNAT2 adaptive stress response: Na+ withdrawal during amino acid deprivation prevents SNAT2 gene induction; substrate-induced repression of SNAT2 protein stability requires the cytoplasmic N-terminal tail with lysyl residues; grafting this tail onto SNAT5 confers substrate-induced stability changes, while mutation of N-terminal lysines renders SNAT2 stable and insensitive to substrate.","method":"Na+ substitution; N-terminal chimeric SNAT2/SNAT5 constructs; lysine-to-alanine mutagenesis; Western blotting; transport assays in HeLa cells","journal":"Frontiers in pharmacology","confidence":"High","confidence_rationale":"Tier 2 — mutagenesis + chimeric transporter + ion substitution; mechanistically identifies N-terminal tail as substrate-sensing domain","pmids":["29467657"],"is_preprint":false},{"year":2019,"finding":"Hypoxia induces SNAT2 expression in breast cancer primarily through HIF-1α (rather than ERα, which dominates under normoxia); HIF-1α and ERα binding sites overlap in SNAT2 cis-regulatory elements; fulvestrant (ER antagonist) cannot suppress SNAT2 under hypoxia; SNAT2 overexpression causes complete endocrine resistance in vivo.","method":"ChIP (HIF-1α, ERα); reporter assays; siRNA; xenograft mouse model; Western blotting; in vitro/in vivo growth assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — ChIP identifies overlapping regulatory elements + in vivo xenograft functional validation; demonstrates regulatory switch mechanism","pmids":["31152137"],"is_preprint":false},{"year":2019,"finding":"CDK7 activity is upregulated during amino acid deprivation in a GCN2-dependent manner and is required for the SNAT2 adaptive stress response; CDK7 inhibition (THZ-1) attenuates ATF4 induction and blocks System A adaptation; drug-resistant CDK7 expression mitigates THZ-1 effects, establishing CDK7 as a component of the integrated stress response regulating SNAT2.","method":"Pharmacological CDK inhibitors (THZ-1, roscovitine, flavopiridol); doxycycline-inducible drug-resistant CDK7; Western blotting; System A transport assays","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"High","confidence_rationale":"Tier 2 — drug-resistant rescue experiment + multiple CDK inhibitors + GCN2 dependence establishes CDK7 in ISR/SNAT2 pathway","pmids":["30857869"],"is_preprint":false},{"year":2021,"finding":"Placenta-specific lentiviral shRNA knockdown of Slc38a2 (59% reduction) in mice reduces near-term fetal and placental weight, fetal viability, trophoblast plasma membrane SNAT2 protein, and placental System A transport activity (MeAIB uptake), directly demonstrating that placental Slc38a2 deficiency causes fetal growth restriction.","method":"Lentiviral shRNA blastocyst transduction (placenta-specific KD); radiolabeled MeAIB uptake; Western blotting; fetal weight measurements","journal":"Clinical science (London, England : 1979)","confidence":"High","confidence_rationale":"Tier 2 — placenta-specific genetic loss-of-function in vivo with multiple mechanistic and physiological readouts","pmids":["34406367"],"is_preprint":false},{"year":2022,"finding":"SLC38A2 acts cell-autonomously in osteoblasts to provide proline (and alanine) for the synthesis of proline-rich osteoblast proteins (RUNX2, OSX, OCN, COL1A1); genetic ablation of SLC38A2 in osteoblasts impairs osteoblast differentiation and bone formation in mice; metabolomics showed proline is primarily incorporated into nascent protein rather than metabolized.","method":"Conditional genetic knockout (osteoblast-specific); metabolomics; bone histomorphometry; differentiation assays; [13C]-proline tracing","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific genetic KO with metabolomics tracing and multiple differentiation/bone formation readouts","pmids":["35261338"],"is_preprint":false},{"year":2022,"finding":"SLC38A2 provides proline and alanine to osteoblast lineage cells during postnatal bone homeostasis; Prrx1Cre-driven SLC38A2 ablation decreases bone mass in both sexes by reducing osteoblast numbers, impairing proliferation and osteogenic differentiation of skeletal stem and progenitor cells.","method":"Conditional knockout (Prrx1Cre); micro-CT; histomorphometry; cell proliferation and differentiation assays; amino acid uptake measurements","journal":"Frontiers in physiology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with multiple quantitative bone/cell biology readouts","pmids":["36213239"],"is_preprint":false},{"year":2022,"finding":"UBE2C mediates monoubiquitination of SNAT2 at lysine 59, which inhibits K63-linked polyubiquitination at lysine 33; this crosstalk between ubiquitination types increases SNAT2 membrane protein levels by suppressing EPN1-mediated endocytosis; elevated membrane SNAT2 facilitates glutamine uptake and metabolism to promote VEGFC secretion and lymphangiogenesis in bladder cancer.","method":"Co-IP; ubiquitination assays (site-specific mutagenesis K59, K33); cell surface protein assays; siRNA; xenograft/PDX models; VEGFC ELISA; lymphangiogenesis assays","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — site-specific ubiquitination mutagenesis + endocytosis mechanistic dissection + functional in vivo validation","pmids":["38949026"],"is_preprint":false},{"year":2022,"finding":"SNAT2 is responsible for hyperosmotic stress-induced uptake of sarcosine and glycine in PC-3 prostate cancer cells; siRNA knockdown of SNAT2 reduces Vmax of sarcosine uptake by ~80% without altering Km, and SNAT2 is up-regulated at mRNA and protein levels under hyperosmotic conditions, identifying sarcosine as a novel SNAT2 substrate.","method":"siRNA; radiolabeled sarcosine/glycine uptake kinetics; RT-PCR; Western blotting; hyperosmotic cell culture","journal":"Pflugers Archiv : European journal of physiology","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA loss-of-function with kinetic transport assay; identifies sarcosine as new substrate; single lab","pmids":["36175560"],"is_preprint":false},{"year":2023,"finding":"Tumour cells and cDC1 dendritic cells compete for glutamine uptake via SLC38A2; glutamine signalling through SLC38A2 in cDC1s activates FLCN, which impinges on TFEB function to license cDC1 activation of CD8+ T cells; SLC38A2 deficiency in DCs phenocopies FLCN loss and eliminates anti-tumour therapeutic effects of glutamine supplementation.","method":"Genetic deletion (DC-specific SLC38A2 KO, FLCN KO); in vivo tumour models; CD8+ T cell priming assays; nutrient competition assays; epistasis (TFEB-dependence); intratumoral glutamine supplementation","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific genetic KO + epistasis (FLCN/TFEB pathway) + in vivo tumour immunology readouts; highly cited","pmids":["37407815"],"is_preprint":false},{"year":2023,"finding":"XBP1 directly binds the SLC38A2 promoter and transcriptionally represses it; SLC38A2 silencing in T cells decreases glutamine uptake and causes immune dysfunction, establishing an XBP1-SLC38A2 axis as a metabolic regulator of cytotoxic T lymphocyte function in multiple myeloma.","method":"ChIP; dual-luciferase reporter assay; siRNA (SLC38A2); glutamine uptake assay; T cell functional assays; single-cell RNA-seq","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP + reporter validate direct XBP1 binding; functional consequence by siRNA; single lab","pmids":["37054944"],"is_preprint":false},{"year":2023,"finding":"NERP-4 (a VGF-derived peptide) acts on SNAT2 to increase glutamine, alanine, and proline uptake into pancreatic β cells, stimulating insulin secretion; SNAT2 deletion and inhibition abolish NERP-4's protective effects on β-cell maintenance and function in db/db mice, defining a peptide-amino acid transporter autocrine axis.","method":"SNAT2 genetic deletion; pharmacological inhibition; amino acid uptake assays; insulin secretion assays; isolated islets; transgenic Ca2+ reporter mice; db/db mouse model","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genetic deletion + pharmacological inhibition + functional amino acid uptake and insulin secretion assays; in vivo validation in diabetes model","pmids":["38071217"],"is_preprint":false},{"year":2023,"finding":"SLC38A2 protects renal medullary collecting duct (MCD) cells from hyperosmolarity-induced ferroptosis; SLC38A2 overexpression attenuates hyperosmotic cell death while Slc38a2 deletion worsens it; the osmoprotective effect requires mTORC1 activation; Slc38a2 knockout mice exhibit increased medullary ferroptosis after water restriction in vivo.","method":"RNA-Seq; SLC38A2 overexpression/siRNA; genetic Slc38a2 knockout mice; ferroptosis markers (ROS, GSH, MDA, iron); mTORC1 Western blotting; water restriction in vivo model","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic KO mouse + overexpression + mTORC1 mechanistic dissection + in vivo stress model","pmids":["36722887"],"is_preprint":false},{"year":2025,"finding":"IGF2BP2 promotes SLC38A2 mRNA stability in an m6A-dependent manner downstream of PTCD3; PTCD3-IGF2BP2-SLC38A2 axis drives glutaminolysis and metastasis in colorectal cancer; SLC38A2 overexpression rescues proliferation/invasion defects caused by PTCD3 depletion.","method":"Co-IP; RIP; dual-luciferase assay; siRNA/overexpression; CRC xenograft model; glutamine metabolism assays","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP + RIP + functional rescue; identifies post-transcriptional regulation of SLC38A2 by m6A/IGF2BP2; single lab","pmids":["40304977"],"is_preprint":false}],"current_model":"SLC38A2 (SNAT2) is a secondary active, Na+-coupled neutral amino acid transporter with 11 transmembrane domains (intracellular N-terminus, extracellular C-terminus, three verified N-glycosylation sites) whose conserved Asn82 (TMD1) and Thr384 (TMD8) coordinate Na+ binding, and whose C-terminal domain and conserved histidine (H504) regulate voltage-dependence and pH-sensitivity, respectively; it accumulates glutamine, alanine, proline, glycine, and betaine (sarcosine as a novel substrate), and its plasma membrane abundance is dynamically controlled by insulin-stimulated exocytosis from TGN/endosomal compartments, Nedd4-2/RNF5/UBE2C-mediated ubiquitination at specific N-terminal lysyl residues, and substrate-induced stabilization, while its transcription is induced by amino acid deprivation through an ATF4/C/EBP intronic enhancer (requiring eIF2α phosphorylation, CDK7 activity, and cap-independent IRES translation), and by IL-6/STAT3, HIF-1α, estrogen receptor-α, prolactin, and cAMP/CRE pathways, and repressed by ChREBP/SMRT and XBP1; functionally, SNAT2 acts as a transceptor—repressing its own transcription during amino acid sufficiency—and drives mTORC1 signalling via leucine exchange and FLCN/TFEB, regulates cell volume via osmolyte accumulation, promotes proteolysis resistance through PI3K, supports osteoblast differentiation by supplying proline for collagen-rich protein synthesis, protects renal medullary cells from ferroptosis via mTORC1, enables cDC1-mediated anti-tumour T cell immunity by competing with tumour cells for glutamine, and sustains β-cell insulin secretion in response to the peptide NERP-4."},"narrative":{"teleology":[{"year":2000,"claim":"Identification of the molecular entity responsible for System A amino acid transport activity resolved a decades-old pharmacological classification into a cloned gene product, establishing SNAT2 as a ubiquitous Na+-dependent neutral amino acid symporter with defined substrate specificity and 1:1 stoichiometry.","evidence":"Heterologous expression in mammalian cells and Xenopus oocytes with electrophysiology and substrate profiling","pmids":["10747860","10930503"],"confidence":"High","gaps":["No structural model at the time","Li+ intolerance mechanism unexplained","Tissue-specific functional roles undefined"]},{"year":2001,"claim":"Demonstration that amino acid deprivation adaptively up-regulates SNAT2 mRNA and protein, with substrate-specific transcriptional feedback, established the paradigm that SNAT2 expression is autoregulated by its own transport substrates.","evidence":"Northern blotting, Western blotting, subcellular fractionation, and transport assays in fibroblasts, myotubes, and adipocytes","pmids":["11311116","11172802"],"confidence":"High","gaps":["Cis-regulatory elements mediating the adaptive response unknown","Whether the protein itself senses substrate or a downstream metabolite is unclear"]},{"year":2002,"claim":"Discovery that insulin recruits SNAT2 from intracellular endosomal stores to the plasma membrane via PI3K established a post-translational trafficking mechanism for rapid regulation of amino acid uptake, analogous to but distinct from GLUT4 translocation.","evidence":"Cell surface biotinylation, wortmannin and chloroquine inhibition, and transport assays in L6 myotubes and hepatocytes after partial hepatectomy","pmids":["11834730","12054432"],"confidence":"High","gaps":["SNARE machinery and vesicle identity not defined","Whether insulin and starvation signals converge on the same SNAT2 pool unknown"]},{"year":2006,"claim":"Mapping the intronic amino acid response element (bound by ATF4 and C/EBP family members), identifying the 5′-UTR IRES enabling cap-independent translation during eIF2α phosphorylation, and showing Nedd4-2-mediated ubiquitination controls surface SNAT2, together defined the multi-layered transcriptional, translational, and post-translational regulatory logic of the adaptive response.","evidence":"ChIP, EMSA, reporter assays, IRES cell-free translation, eIF2α mutant cells, Nedd4-2 RNAi and ubiquitination assays across multiple cell types and Xenopus oocytes","pmids":["16445384","16621798","17003038"],"confidence":"High","gaps":["Chromatin-level regulation (histone modifications) not fully dissected","Structural basis of Nedd4-2/SNAT2 interaction unknown","Whether IRES activity is regulated by RNA-binding proteins undetermined"]},{"year":2006,"claim":"Identification of key residues governing SNAT2 biophysics — H304 separating transport from anion leak, H504 mediating pH-sensitivity through allosteric Na+ binding modulation — established the first structure–function framework for the transporter.","evidence":"Site-directed mutagenesis (H304A, H504A) with two-electrode voltage clamp and DEPC chemical modification in Xenopus oocytes","pmids":["17237199","16629640"],"confidence":"High","gaps":["High-resolution structure unavailable","Full anion permeation pathway not mapped","Whether anion leak has physiological relevance unclear"]},{"year":2007,"claim":"Establishing that SNAT2 functions as a transceptor — repressing its own gene transcription during amino acid sufficiency through a mechanism requiring JNK signalling and a large neutral amino acid sensor — resolved how cells couple transport activity to transcriptional feedback.","evidence":"shRNA, transporter chimeras, JNK inhibitors, and intronic reporter assays in L6 myotubes","pmids":["17488712"],"confidence":"High","gaps":["Identity of the large neutral amino acid sensor upstream of JNK not determined","Whether transceptor signalling involves conformational change or substrate flux is unresolved"]},{"year":2007,"claim":"Demonstration that SNAT2 inhibition depletes intracellular glutamine and, by extension, leucine (via exchange transport), thereby impairing mTORC1 signalling and increasing proteolysis, placed SNAT2 as a master regulator of amino acid-dependent anabolic signalling in muscle.","evidence":"siRNA, MeAIB competitive inhibition, metabolic acidosis, HPLC amino acid profiling, and mTOR pathway Western blotting in L6 cells","pmids":["17429052","18650482"],"confidence":"High","gaps":["Whether SNAT2-mTORC1 coupling is direct or solely through amino acid pools unclear","Relative contribution of SNAT2 vs SNAT1 to mTORC1 activation not separated genetically"]},{"year":2008,"claim":"Identification of Asn82 in TMD1 as critical for Na+ coordination, with mutagenesis dramatically reducing Na+ affinity and transport, provided the first direct evidence for the Na+ binding site architecture in System A transporters.","evidence":"N82A/N82S/Y337A/R374Q mutagenesis with electrophysiology and uptake assays in Xenopus oocytes","pmids":["18319257"],"confidence":"High","gaps":["No crystal or cryo-EM structure to validate binding site geometry","Contribution of water molecules to Na+ coordination unknown"]},{"year":2009,"claim":"Homology modeling to LeuT/Mhp1 predicted a conserved Na+ site formed by TMD1 and TMD8, validated by the T384A mutation that dramatically lowers Na+ affinity, completing the minimal Na+ binding site model and linking SNAT2 to the LeuT-fold superfamily.","evidence":"Homology modeling, T384A mutagenesis, and electrophysiology in Xenopus oocytes","pmids":["19589777"],"confidence":"High","gaps":["Experimental high-resolution structure still lacking","Substrate binding site residues not systematically identified"]},{"year":2011,"claim":"Truncation of 13 C-terminal residues abolished transport at negative potentials while sparing it at positive potentials, establishing the extracellular C-terminal domain as a voltage sensor required for physiological transporter function.","evidence":"C-terminal truncation mutagenesis with two-electrode voltage clamp and uptake assays in Xenopus oocytes","pmids":["21158741"],"confidence":"High","gaps":["Molecular mechanism of voltage gating by C-terminal tail not defined","Whether post-translational modifications of C-terminus modulate gating is unknown"]},{"year":2015,"claim":"Identification of specific N-terminal lysyl residues as ubiquitination sites mediating substrate-induced stability, and the demonstration that mTOR regulates SNAT2 surface abundance via Nedd4-2 in human IUGR placentae, integrated ubiquitin-dependent trafficking into the physiological regulation of fetal nutrient supply.","evidence":"N-terminal lysine-to-alanine mutagenesis, chimeric SNAT2-SNAT5 constructs, IUGR placental fractionation, and ubiquitination assays","pmids":["25653282","26374858"],"confidence":"High","gaps":["Deubiquitinase(s) counteracting Nedd4-2 at SNAT2 not identified","Whether mTOR regulates Nedd4-2 directly or through intermediates is unclear"]},{"year":2018,"claim":"Experimental determination of SNAT2 membrane topology (11 TMDs, intracellular N-terminus, extracellular C-terminus, three verified N-glycosylation sites) resolved conflicting bioinformatic predictions and anchored the structure–function data to a validated topology model.","evidence":"mPEG-Mal chemical modification, protease cleavage, immunofluorescence, and glycosylation analysis","pmids":["29678469"],"confidence":"High","gaps":["High-resolution 3D structure still unavailable","Role of individual glycosylation sites in trafficking or function not tested"]},{"year":2019,"claim":"Discovery that HIF-1α drives SNAT2 transcription under hypoxia via elements overlapping with the ERα-responsive promoter, and that SNAT2 overexpression confers endocrine resistance in breast cancer xenografts, revealed a regulatory switch enabling tumour adaptation.","evidence":"ChIP for HIF-1α and ERα, reporter assays, siRNA, and xenograft models","pmids":["31152137"],"confidence":"High","gaps":["Whether HIF-1α and ERα compete or cooperate at the SNAT2 promoter under intermediate oxygen tensions is untested","Therapeutic targeting of SNAT2 in endocrine-resistant breast cancer not explored"]},{"year":2021,"claim":"Placenta-specific Slc38a2 knockdown in mice directly caused fetal growth restriction, establishing causality between reduced placental SNAT2 and impaired fetal nutrient supply, beyond the correlative IUGR associations previously observed.","evidence":"Lentiviral shRNA blastocyst transduction with placenta-specific knockdown; MeAIB uptake, fetal weight, and viability measurements","pmids":["34406367"],"confidence":"High","gaps":["Whether Slc38a2 ablation recapitulates full IUGR syndrome or only weight restriction is unknown","Compensation by SNAT1 or SNAT4 not assessed"]},{"year":2022,"claim":"Conditional knockout studies established that SLC38A2 cell-autonomously supplies proline for collagen-rich protein synthesis in osteoblasts, directly controlling differentiation and bone formation, revealing a tissue-specific metabolic function beyond glutamine supply.","evidence":"Osteoblast- and Prrx1Cre-driven conditional knockouts, [13C]-proline tracing, metabolomics, micro-CT, and histomorphometry","pmids":["35261338","36213239"],"confidence":"High","gaps":["Whether SNAT2 provides proline to other collagen-producing cell types (e.g., fibroblasts) is untested","Mechanism by which SNAT2-derived proline is preferentially channeled to protein synthesis vs. catabolism is unexplored"]},{"year":2022,"claim":"Identification of UBE2C-mediated monoubiquitination at K59 that blocks K63-polyubiquitination at K33, preventing EPN1-mediated endocytosis and stabilizing SNAT2 at the membrane, revealed a ubiquitin-code mechanism controlling transporter surface retention with implications for lymphangiogenesis in bladder cancer.","evidence":"Site-specific K59/K33 mutagenesis, ubiquitination assays, surface protein analysis, siRNA, xenograft/PDX models","pmids":["38949026"],"confidence":"High","gaps":["Whether UBE2C–SNAT2 interaction is direct or requires a scaffold E3 ligase is unresolved","Generalizability of ubiquitin-code regulation across tissues not tested"]},{"year":2023,"claim":"Demonstration that cDC1 dendritic cells compete with tumour cells for glutamine via SLC38A2, and that SLC38A2 signals through FLCN/TFEB to license anti-tumour CD8+ T cell priming, established SLC38A2 as a critical metabolic checkpoint in tumour immunity.","evidence":"DC-specific SLC38A2 and FLCN knockout mice, in vivo tumour models, CD8+ T cell assays, intratumoral glutamine supplementation","pmids":["37407815"],"confidence":"High","gaps":["Whether SLC38A2 signalling in other immune cells (macrophages, CD4+ T cells) similarly controls function is unexplored","Mechanism by which SNAT2-derived glutamine activates FLCN is undefined"]},{"year":2023,"claim":"Discovery that SLC38A2 protects renal medullary cells from hyperosmolarity-induced ferroptosis via mTORC1 activation, and that the peptide NERP-4 acts through SNAT2 to stimulate β-cell insulin secretion, expanded the physiological repertoire of SNAT2 to stress survival and endocrine function.","evidence":"Slc38a2 knockout mice with water restriction, ferroptosis markers, mTORC1 blotting; SNAT2 deletion and pharmacological inhibition with insulin secretion assays in islets and db/db mice","pmids":["36722887","38071217"],"confidence":"High","gaps":["How NERP-4 mechanistically engages SNAT2 (direct binding vs. indirect modulation) is unresolved","Whether renal SNAT2 loss causes chronic kidney pathology under physiological conditions is untested"]},{"year":null,"claim":"Despite extensive functional characterization, no high-resolution experimental structure of SNAT2 exists; the structural basis for substrate selectivity, the anion leak pathway, and the transceptor signalling mechanism remain undefined at atomic resolution.","evidence":"","pmids":[],"confidence":"High","gaps":["No cryo-EM or crystal structure of SNAT2 or any close SLC38 family member","Transceptor signalling mechanism (conformational vs. flux-dependent) unresolved","Pharmacological inhibitors with clinical utility not yet developed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,7,10,18,24,28,34,43,46]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[14]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,3,6,12,13,31,35,42]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[13]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,13]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[1,3,13]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,7,10,18,24,28,34,43,46]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,15,16,44,47]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[8,9,26,33,38]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[12,30,31,42]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[44,45]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[8,9,29,37]}],"complexes":[],"partners":["NEDD4L","RNF5","UBE2C","ATF4","HIF1A","ESR1","EPN1","IGF2BP2"],"other_free_text":[]},"mechanistic_narrative":"SLC38A2 (SNAT2) is a ubiquitously expressed, Na+-coupled neutral amino acid symporter that mediates concentrative uptake of glutamine, alanine, proline, glycine, betaine, and sarcosine with 1:1 Na+:amino acid stoichiometry, and whose transport mechanism depends on conserved Na+-coordinating residues Asn82 (TMD1) and Thr384 (TMD8), a C-terminal domain that governs voltage-dependence, and a histidine (H504) that confers pH sensitivity [PMID:10747860, PMID:18319257, PMID:19589777, PMID:21158741, PMID:16629640]. Plasma membrane abundance of SNAT2 is dynamically regulated by insulin-stimulated exocytosis from trans-Golgi network compartments, Nedd4-2- and RNF5-mediated ubiquitination at N-terminal lysyl residues, and substrate-induced protein stabilization, while its transcription is induced during amino acid deprivation through an ATF4/C/EBP intronic enhancer requiring eIF2α phosphorylation and CDK7 activity, and by IL-6/STAT3, HIF-1α, and ERα pathways [PMID:17050538, PMID:17003038, PMID:25653282, PMID:16445384, PMID:16621798, PMID:30857869, PMID:19741197, PMID:31152137, PMID:25056967]. SNAT2 functions as a transceptor that represses its own transcription when amino acids are sufficient, and by supplying glutamine it sustains mTORC1 signalling, drives leucine exchange, protects renal medullary cells from ferroptosis, enables cDC1-mediated anti-tumour immunity via FLCN/TFEB, provides proline for osteoblast differentiation and bone formation, and supports β-cell insulin secretion in response to the peptide NERP-4 [PMID:17488712, PMID:17429052, PMID:36722887, PMID:37407815, PMID:35261338, PMID:38071217]. Placenta-specific Slc38a2 knockdown in mice directly causes fetal growth restriction, establishing a causal role in nutrient supply to the fetus [PMID:34406367]."},"prefetch_data":{"uniprot":{"accession":"Q96QD8","full_name":"Sodium-coupled neutral amino acid symporter 2","aliases":["Amino acid transporter A2","Protein 40-9-1","Solute carrier family 38 member 2","System A amino acid transporter 2","System A transporter 1","System N amino acid transporter 2"],"length_aa":506,"mass_kda":56.0,"function":"Symporter that cotransports neutral amino acids and sodium ions from the extracellular to the intracellular side of the cell membrane (PubMed:10930503, PubMed:15774260, PubMed:15922329, PubMed:16621798). The transport is pH-sensitive, Li(+)-intolerant, electrogenic, driven by the Na(+) electrochemical gradient and cotransports of neutral amino acids and sodium ions with a stoichiometry of 1:1. May function in the transport of amino acids at the blood-brain barrier (PubMed:10930503, PubMed:15774260). May function in the transport of amino acids in the supply of maternal nutrients to the fetus through the placenta (By similarity). Maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate (By similarity). Transports L-proline in differentiating osteoblasts for the efficient synthesis of proline-enriched proteins and provides proline essential for osteoblast differentiation and bone formation during bone development (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q96QD8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC38A2","classification":"Not Classified","n_dependent_lines":355,"n_total_lines":1208,"dependency_fraction":0.29387417218543044},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CANX","stoichiometry":0.2},{"gene":"CCDC47","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/SLC38A2","total_profiled":1310},"omim":[{"mim_id":"608490","title":"SOLUTE CARRIER FAMILY 38 (AMINO ACID TRANSPORTER), MEMBER 1; SLC38A1","url":"https://www.omim.org/entry/608490"},{"mim_id":"608065","title":"SOLUTE CARRIER FAMILY 38 (AMINO ACID TRANSPORTER), MEMBER 4; SLC38A4","url":"https://www.omim.org/entry/608065"},{"mim_id":"605180","title":"SOLUTE CARRIER FAMILY 38 (AMINO ACID TRANSPORTER), MEMBER 2; SLC38A2","url":"https://www.omim.org/entry/605180"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SLC38A2"},"hgnc":{"alias_symbol":["SAT2","ATA2","KIAA1382","SNAT2"],"prev_symbol":[]},"alphafold":{"accession":"Q96QD8","domains":[{"cath_id":"1.20.1740.10","chopping":"72-253_282-499","consensus_level":"high","plddt":90.3135,"start":72,"end":499}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96QD8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96QD8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96QD8-F1-predicted_aligned_error_v6.png","plddt_mean":79.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC38A2","jax_strain_url":"https://www.jax.org/strain/search?query=SLC38A2"},"sequence":{"accession":"Q96QD8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96QD8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96QD8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96QD8"}},"corpus_meta":[{"pmid":"37407815","id":"PMC_37407815","title":"SLC38A2 and glutamine signalling in cDC1s dictate anti-tumour immunity.","date":"2023","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/37407815","citation_count":219,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27129276","id":"PMC_27129276","title":"Deletion of Amino Acid Transporter ASCT2 (SLC1A5) Reveals an Essential Role for Transporters SNAT1 (SLC38A1) and SNAT2 (SLC38A2) to Sustain Glutaminolysis in Cancer Cells.","date":"2016","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/27129276","citation_count":206,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"19741197","id":"PMC_19741197","title":"IL-6 stimulates system A amino acid transporter activity in trophoblast cells through STAT3 and increased expression of SNAT2.","date":"2009","source":"American journal of physiology. Cell physiology","url":"https://pubmed.ncbi.nlm.nih.gov/19741197","citation_count":134,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"10930503","id":"PMC_10930503","title":"Primary structure, functional characteristics and tissue expression pattern of human ATA2, a subtype of amino acid transport system A.","date":"2000","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/10930503","citation_count":130,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"31152137","id":"PMC_31152137","title":"Hypoxia-induced switch in SNAT2/SLC38A2 regulation generates endocrine resistance in breast cancer.","date":"2019","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/31152137","citation_count":110,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"17488712","id":"PMC_17488712","title":"Distinct sensor pathways in the hierarchical control of SNAT2, a putative amino acid transceptor, by amino acid availability.","date":"2007","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/17488712","citation_count":104,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"16621798","id":"PMC_16621798","title":"Amino acid starvation induces the SNAT2 neutral amino acid transporter by a mechanism that involves eukaryotic initiation factor 2alpha phosphorylation and cap-independent translation.","date":"2006","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16621798","citation_count":91,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11834730","id":"PMC_11834730","title":"Insulin promotes the cell surface recruitment of the SAT2/ATA2 system A amino acid transporter from an endosomal compartment in skeletal muscle cells.","date":"2002","source":"The Journal of biological 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glutamate.","date":"2008","source":"Cerebral cortex (New York, N.Y. : 1991)","url":"https://pubmed.ncbi.nlm.nih.gov/18832333","citation_count":76,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11172802","id":"PMC_11172802","title":"The adaptive regulation of amino acid transport system A is associated to changes in ATA2 expression.","date":"2001","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/11172802","citation_count":74,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11311116","id":"PMC_11311116","title":"Subcellular localization and adaptive up-regulation of the System A (SAT2) amino acid transporter in skeletal-muscle cells and adipocytes.","date":"2001","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/11311116","citation_count":72,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"30274521","id":"PMC_30274521","title":"Methionine Promotes Milk Protein and Fat Synthesis and Cell Proliferation via the 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Li+-intolerance, mediating system A transport activity when expressed in mammalian cells.\",\n      \"method\": \"Cloning from HepG2 cells, functional expression in mammalian cells, transport assays with MeAIB\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct functional reconstitution with full kinetic characterization in heterologous expression system\",\n      \"pmids\": [\"10930503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"SNAT2 protein is present in both plasma-membrane and internal-membrane fractions of skeletal muscle and adipocytes, with a subcellular localization similar to GLUT4; amino acid deprivation causes time-dependent increase in SNAT2 protein abundance (adaptive upregulation).\",\n      \"method\": \"Subcellular fractionation, immunoblot, polyclonal antibody characterization in L6 myotubes and 3T3-L1 adipocytes\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct fractionation and localization with functional (adaptive upregulation) consequence, replicated in multiple cell types\",\n      \"pmids\": [\"11311116\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Adaptive stimulation of system A transport activity upon amino acid starvation in human fibroblasts is associated with increased ATA2 (SNAT2) mRNA and is suppressed specifically by system A substrates but not other amino acids, demonstrating a direct relationship between ATA2 expression and transport activity.\",\n      \"method\": \"Northern blot, amino acid transport assays with selective substrate competition\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (mRNA, transport activity) with substrate-specific controls\",\n      \"pmids\": [\"11172802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Insulin stimulates System A (SNAT2/SAT2) transport in L6 skeletal muscle cells by recruiting SNAT2 to the plasma membrane from an endosomal compartment via a phosphatidylinositol 3-kinase-dependent mechanism; chloroquine blocks this recruitment without affecting PI3K/PKB signaling or GLUT4 translocation.\",\n      \"method\": \"Cell surface biotinylation, chloroquine treatment, PI3K inhibition, immunoblot for plasma membrane fraction\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches with rigorous controls distinguishing SNAT2 trafficking from other insulin-regulated pathways\",\n      \"pmids\": [\"11834730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Following partial hepatectomy, ATA2 (SNAT2)-mediated amino acid transport in liver is upregulated by redistribution of pre-existing ATA2 protein to the plasma membrane without changes in steady-state mRNA, indicating post-translational trafficking regulation.\",\n      \"method\": \"Immunodetection in isolated liver plasma membrane vs. lysate fractions, Northern blot, Xenopus oocyte transport assays\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — fractionation + transport assay showing trafficking without mRNA change, single lab\",\n      \"pmids\": [\"12054432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"New synthesis of SNAT2 transporter protein is required for hypertonic stimulation of system A transport; biotinylation shows increased SNAT2 in plasma membrane after hypertonic stress; DRB (transcription inhibitor) abolishes transport stimulation and membrane SNAT2 increase under hypertonic conditions but only partly inhibits adaptive amino acid starvation response.\",\n      \"method\": \"Cell surface biotinylation, immunocytochemistry, DRB transcription inhibition, transport assays\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods distinguishing two distinct regulatory mechanisms\",\n      \"pmids\": [\"15581851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SNAT2 siRNA silencing in hypertonically stressed human fibroblasts prevents the increase in system A transport activity, hinders expansion of intracellular amino acid pool, and significantly delays cell volume recovery, establishing SNAT2 as essential for regulatory volume increase (RVI).\",\n      \"method\": \"siRNA knockdown, amino acid transport assays, cell volume measurements\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with specific phenotypic readout and mechanistic clarity\",\n      \"pmids\": [\"15922329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SNAT2 induction upon amino acid starvation requires eIF2alpha phosphorylation, increased gene transcription, and IRES-mediated cap-independent translation; hypertonic stress induces SNAT2 independently of eIF2alpha phosphorylation, revealing two distinct signaling pathways controlling SNAT2 expression.\",\n      \"method\": \"eIF2alpha phosphorylation mutant cells, reporter assays with SNAT2 5'UTR, transcription inhibitors, transport assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal approaches including genetic (mutant eIF2alpha) and biochemical (IRES) in distinct stress paradigms\",\n      \"pmids\": [\"16621798\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The amino acid response element for SNAT2 is in the first intron; ATF and C/EBP transcription factor family members bind this intronic enhancer (shown by in vitro and in vivo ChIP); specific ATF/C/EBP members activate or repress SNAT2 transcription; RNA Pol II recruitment to SNAT2 promoter is increased by amino acid deprivation.\",\n      \"method\": \"Reporter assays, EMSA, chromatin immunoprecipitation (ChIP), exogenous ATF/C/EBP expression\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro and in vivo binding assays with functional reporter validation, multiple transcription factors tested\",\n      \"pmids\": [\"16445384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cortisol stimulates system A transport activity in BeWo trophoblast cells by upregulating SNAT2 mRNA and protein expression, and causes relocalization of SNAT2 at lower cortisol concentrations, identifying SNAT2 as the isoform regulated by glucocorticoids in placental cells.\",\n      \"method\": \"MeAIB transcellular transport assay, Northern blot, Western blot, immunocytochemistry\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods, single lab, functional and localization data\",\n      \"pmids\": [\"16621896\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SNAT2-mediated glutamine uptake determines intracellular leucine levels (via secondary active transport exchange through system L), and SNAT2 inhibition (by MeAIB, acidosis pH 7.1, or siRNA) impairs mTOR signaling to S6K1, 4E-BP1, and protein synthesis in L6 muscle cells.\",\n      \"method\": \"MeAIB competitive inhibition, metabolic acidosis, siRNA, mTOR pathway phosphorylation assays (S6K1, S6, 4E-BP1), amino acid profiling\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — three independent SNAT2 inhibition approaches with consistent pathway readouts, mechanistic link to mTOR established\",\n      \"pmids\": [\"17429052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SNAT2 plays a pivotal role in regulatory volume increase under hypertonic conditions: SNAT2 induction increases system A Vmax, expands intracellular amino acid pool (osmolytes), and siRNA silencing of SNAT2 delays cell volume recovery, demonstrating SNAT2 as the key effector of osmoadaptation.\",\n      \"method\": \"siRNA silencing, system A transport Vmax measurement, intracellular amino acid pool analysis, cell volume recovery assay\",\n      \"journal\": \"Acta physiologica (Oxford, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — siRNA loss-of-function with specific phenotypic (volume) and biochemical readouts, supported by prior work\",\n      \"pmids\": [\"16734764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Nedd4-2 ubiquitin ligase polyubiquitinates SNAT2 (ATA2) on the plasma membrane, leading to endocytotic sequestration and proteasomal degradation; Nedd4-2 catalytic mutant or c-Cbl did not affect ATA2; Nedd4-2 siRNA increased ATA2 activity and decreased polyubiquitination on the plasma membrane.\",\n      \"method\": \"Xenopus oocyte expression, CHO cell transfection, siRNA, proteasome inhibitor (MG132), immunofluorescence co-localization, ubiquitination assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution in oocytes, RNAi loss-of-function, mutagenesis of catalytic domain, co-localization, multiple cell systems\",\n      \"pmids\": [\"17003038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"ATA2 (SNAT2) is stored at the trans-Golgi network (TGN; colocalizing with syntaxin 6) in adipocytes and released in vesicles toward the plasma membrane upon insulin stimulation; this pathway is distinct from GLUT4 storage vesicles; brefeldin A (TGN inhibitor) blocks insulin-stimulated ATA2 translocation.\",\n      \"method\": \"EGFP-ATA2 live-cell imaging, immunofluorescence with TGN/endosomal markers (syntaxin 6, EEA1), brefeldin A treatment, amino acid transport assays, glucose uptake comparison\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — live imaging + pharmacological dissection in multiple conditions, distinguishes ATA2 and GLUT4 pathways\",\n      \"pmids\": [\"17050538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"pH sensitivity of SNAT2 involves a conserved C-terminal histidine residue (H504): DEPC modification reduces pH-sensitivity and blocks reduction in Na+ affinity at low pH; H504A mutation produces DEPC-resistant reduced pH-sensitivity; DEPC effects were reversed by hydroxylamine and blocked by substrate, indicating allosteric regulation of Na+ binding via this histidine.\",\n      \"method\": \"Xenopus oocyte expression of SNAT2/SNAT5 and point mutants, DEPC chemical modification, transport assays at varying pH and Na+, hydroxylamine reversal\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional assay with site-directed mutagenesis and chemical modification, mechanistic resolution of pH-sensing\",\n      \"pmids\": [\"16629640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SNAT2 functions as a transceptor: JNK activation during amino acid withdrawal induces SNAT2 expression via an intronic nutrient-sensitive domain; a sensor for large neutral amino acids inhibits JNK and SNAT2 upregulation; shRNA and transporter chimeras show SNAT2 itself provides a repressive signal for its gene transcription during amino acid sufficiency.\",\n      \"method\": \"JNK inhibition, shRNA knockdown, transporter chimeras, intronic reporter assays, L6 myotube culture\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic (shRNA, chimeras) and pharmacological approaches with transcriptional readout, establishing transceptor function\",\n      \"pmids\": [\"17488712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SNAT2 mediates a substrate-inhibited anion leak conductance: transported substrates (L-alanine, L-glutamine, MeAIB) inhibit the leak with different potency; H304A mutant retains anion leak without supporting amino acid transport, showing substrate transport is not required for anion conductance; Na+ can bind H304A mutant normally.\",\n      \"method\": \"Electrophysiology (whole-cell patch clamp/two-electrode voltage clamp), site-directed mutagenesis (H304A), ion selectivity assays\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — electrophysiology with mutagenesis; mechanistic dissection of transport vs. anion leak function\",\n      \"pmids\": [\"17237199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SAT2 (SNAT2) is predominantly targeted to somatodendritic compartments in glutamatergic neurons and is required for glutamine uptake to replenish dendritic glutamate pools; MeAIB inhibition of SAT2 reduces neuronal glutamine uptake and intracellular glutamate, and inhibits retrograde signaling from pyramidal cells by suppressing fast-spiking interneuron input.\",\n      \"method\": \"Immunohistochemistry, confocal microscopy, ultrastructural studies, electrical stimulation in vivo, MeAIB pharmacological inhibition, glutamate/glutamine measurements\",\n      \"journal\": \"Cerebral cortex (New York, N.Y. : 1991)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological inhibition with multiple readouts (neurochemistry, electrophysiology), localization at ultrastructural level\",\n      \"pmids\": [\"18832333\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SNAT2 inhibition by acidosis (pH 7.1), MeAIB competitive inhibition, or siRNA silencing depletes intracellular glutamine and stimulates proteolysis in L6 myotubes via PI3K-dependent insulin signaling; blocking mTOR or PI3K increases proteolysis; partial SNAT2 silencing impairs insulin signaling through PI3K.\",\n      \"method\": \"Metabolic acidosis model, MeAIB inhibition, siRNA, proteasome inhibitor, PI3K/mTOR inhibitors, proteolysis assay\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — three independent inhibition approaches with consistent proteolytic and signaling readouts\",\n      \"pmids\": [\"18650482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Asparagine-82 in transmembrane domain 1 of SNAT2 is essential for Na+ coordination: N82A mutation dramatically reduces Na+ affinity (K_Na+) and increases apparent Km for alanine 27-fold; N82S is intermediate; Y337A and R374Q do not abolish transport, implicating TMD1 in Na+ binding.\",\n      \"method\": \"Site-directed mutagenesis, Xenopus oocyte electrophysiology and flux assays, Na+ affinity measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structure-function mutagenesis with quantitative kinetics, identification of Na+ coordination residue\",\n      \"pmids\": [\"18319257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Regulatory mechanisms of SNAT2 by insulin, osmotic shock, and amino acid deprivation in L6 cells are distinct: all three increase Vmax without changing Km; chloroquine and wortmannin block insulin- and starvation-induced upregulation but not osmotic shock; PD98059/SP600125 inhibit only starvation response; SB202190 inhibits only insulin response.\",\n      \"method\": \"Pharmacological dissection with kinase inhibitors, transport kinetics (Km/Vmax), Western blot, RT-PCR\",\n      \"journal\": \"Amino acids\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic pharmacological dissection revealing distinct mechanisms, single lab\",\n      \"pmids\": [\"18330498\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Despite increased ATF4 binding to the C/EBP-ATF intronic site during UPR activation in HepG2 cells, SNAT2 transcription is repressed; the UPR suppresses AAR-induced SNAT2 transcription downstream of ATF4 binding; AAR but not UPR increases H3K histone acetylation and recruitment of general transcription factors to the SNAT2 promoter.\",\n      \"method\": \"ChIP for ATF4 binding and histone modifications, RNA Pol II recruitment, reporter gene assays, co-activation of AAR and UPR pathways\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP with multiple marks and controls, mechanistic dissection of divergent signaling inputs\",\n      \"pmids\": [\"18697751\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A conserved Na+ binding site in SNAT2 is formed by transmembrane helices 1 and 8 (T384 in TMD8); T384A mutation inhibits the anion leak current (requiring Na+ binding) and dramatically lowers Na+ affinity, consistent with homology models built on LeuT_Aa and Mhp1 structures.\",\n      \"method\": \"Profile-based sequence analysis, homology modeling, site-directed mutagenesis (T384A), electrophysiology, ion affinity measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — homology modeling with experimental mutagenesis validation, consistent with N82A prior work\",\n      \"pmids\": [\"19589777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"IL-6 stimulates system A (SNAT2) activity in primary human trophoblast cells by increasing SNAT2 gene and protein expression via STAT3 (Tyr705 phosphorylation); siRNA knockdown of STAT3 reduces SNAT2 mRNA/protein and abolishes IL-6-stimulated system A activity; TNF-alpha similarly stimulates SNAT2 but not via JAK/STAT.\",\n      \"method\": \"siRNA knockdown of STAT3, Western blot for pSTAT3, MeAIB uptake assay, Northern/Western blot for SNAT2\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockdown with mechanistic dissection, multiple orthogonal methods\",\n      \"pmids\": [\"19741197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SNAT2 functions as a transceptor that can signal through mTOR: chronic competitive inhibition of SNAT2 with MeAIB depletes intracellular SNAT2 substrates and branched-chain AAs but paradoxically elevates mTOR-dependent p70S6K1 phosphorylation; TAP-tag proteomics identifies two novel SNAT2-interacting proteins potentially involved in protein turnover signaling.\",\n      \"method\": \"MeAIB competitive inhibition, mTOR pathway phosphorylation assays (p70S6K1), intracellular amino acid measurement, TAP-tag purification + proteomics\",\n      \"journal\": \"Frontiers in bioscience (Elite edition)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — functional inhibition with pathway readout plus interaction proteomics, mechanistic model partially supported\",\n      \"pmids\": [\"21622135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The C-terminal domain of SNAT2 (13 residues) is an essential voltage regulator: its deletion abolishes transport currents at negative membrane potentials without affecting membrane expression, substrate binding, or pH sensitivity; the C-terminal tail controls the amino acid translocation process at physiological membrane potentials.\",\n      \"method\": \"C-terminal truncation mutagenesis, electrophysiology (voltage clamp), cell-surface expression assays, kinetic measurements\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structure-function mutagenesis with electrophysiological characterization\",\n      \"pmids\": [\"21158741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SNAT2 is the primary transporter for L-proline uptake in embryonic stem (ES) cells and mediates L-proline-induced ES cell differentiation to early primitive ectoderm-like cells; competitive inhibition with other SNAT2 substrates (but not non-substrates) prevents L-proline-induced morphology changes, gene expression shifts, and differentiation kinetics.\",\n      \"method\": \"Functional uptake assays, competitive substrate inhibition with SNAT2 vs. non-SNAT2 substrates, differentiation marker gene expression\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — substrate specificity controls distinguishing SNAT2-dependent from non-specific effects, multiple differentiation readouts\",\n      \"pmids\": [\"21346154\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A genome-wide RNAi screen identified SNAT2 as required for arsenite-induced ER stress response; SNAT2 expression is upregulated by arsenite in an ATF4-dependent manner; SNAT2 inhibition or glutamine deprivation suppresses arsenite-induced ER stress and mTOR activation; SNAT2 and LAT1 together regulate amino acid-dependent mTOR activation.\",\n      \"method\": \"Genome-wide shRNA screen (flow cytometry-based sorting), SNAT2 knockdown/inhibition, glutamine deprivation, mTOR pathway assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — unbiased genome-wide screen with follow-up functional validation, multiple perturbation approaches\",\n      \"pmids\": [\"22215663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SNAT2 transports anti-[18F]FACBC with Michaelis-Menten kinetics (Km ~197 μM) when expressed in Xenopus laevis oocytes, establishing SNAT2 as a transporter of this PET radiotracer substrate.\",\n      \"method\": \"Xenopus laevis oocyte expression, kinetic [14C]FACBC uptake assays\",\n      \"journal\": \"Nuclear medicine and biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro kinetic characterization in heterologous expression system\",\n      \"pmids\": [\"23647854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SNAT2 (but not SNAT1 or SNAT4) uniquely transports betaine among system A subtypes; betaine Km for human and rat SNAT2 is ~5 mM; hypertonicity selectively induces SNAT2 expression and promotes its plasma membrane localization in trophoblasts, implicating SNAT2-mediated betaine transport in osmoadaptation.\",\n      \"method\": \"HEK293 transfection with SNAT1/2/4, [14C]betaine uptake assays, competitive inhibition, Western blot, immunocytochemistry\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct substrate specificity determination across all system A subtypes with kinetic characterization\",\n      \"pmids\": [\"24434061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"17β-estradiol transcriptionally regulates SNAT2 through estrogen receptor alpha (ERα) binding to an estrogen response element (ERE) in the SNAT2 promoter; ERα-ERE complex contains PARP1, Ku70, and GAPDH; GAPDH binding is nucleotide-specific to the SNAT2 ERE and all three co-activators are required for estradiol-stimulated SNAT2 transcription.\",\n      \"method\": \"ERE deletion/mutation reporter assays, EMSA, supershift assays, in vivo ChIP, mass spectrometry identification of ERE complex components, siRNA knockdown of co-activators\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro and in vivo binding, mutagenesis, and functional validation with mass spec identification of complex\",\n      \"pmids\": [\"25056967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In IUGR placentas, mTORC1/mTORC2 activity is decreased, NEDD4-2 ubiquitin ligase expression is increased (+72%), and SNAT2 ubiquitination is increased (+180%), associated with reduced SNAT2 protein in the microvillous plasma membrane and decreased system A transport activity (-72%), consistent with mTOR-dependent ubiquitination controlling SNAT2 trafficking and degradation.\",\n      \"method\": \"Placental tissue from IUGR vs. control, mTOR phosphorylation assays, NEDD4-2 protein quantification, ubiquitination assay, microvillous membrane isolation, system A transport activity\",\n      \"journal\": \"Clinical science (London, England : 1979)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple biochemical measurements in human tissue, correlative but mechanistically coherent\",\n      \"pmids\": [\"26374858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Unsaturated fatty acid (linoleic acid) promotes proteasomal degradation of SNAT2 by increasing ubiquitination; seven N-terminal lysyl residues of SNAT2 are required ubiquitination sites (mutation to alanine protects against linoleic acid-induced degradation); SNAT2's N-terminal tail confers substrate-induced changes in stability when grafted onto SNAT5.\",\n      \"method\": \"Linoleic acid treatment, proteasome inhibitor, shRNA (Nedd4.2), N-terminal lysine→alanine mutants, chimeric transporter (SNAT2 N-tail fused to SNAT5), transport and protein stability assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis, chimeric proteins, multiple inhibition approaches identifying specific ubiquitination sites\",\n      \"pmids\": [\"25653282\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GADD34 (a PP1 regulatory subunit) is co-induced with SNAT2 during mild hyperosmotic stress and enhances SNAT2-mediated amino acid uptake by reducing eIF2alpha phosphorylation; this SNAT2/GADD34 axis promotes cell survival adaptation to osmotic stress, with GADD34 regulated by c-Jun-binding CRE (not ATF4) during hyperosmotic stress.\",\n      \"method\": \"eIF2alpha phosphorylation assays, GADD34 expression analysis, reporter assays for GADD34 and SNAT2, cell volume/survival assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — coordinated pathway analysis with mechanistic link, single lab\",\n      \"pmids\": [\"26041779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Net glutamine uptake in HeLa and 143B cancer cells does not depend on ASCT2 (SLC1A5) but requires expression of SNAT1 and SNAT2 (SLC38A1/2); ASCT2 deletion causes amino acid starvation response and upregulation of SNAT1 to compensate; combined GCN2 silencing in the ASCT2 KO background reduces cancer cell growth.\",\n      \"method\": \"CRISPR knockout of ASCT2, siRNA knockdown of SNAT1/SNAT2, amino acid transport assays, proliferation assays, GCN2 silencing\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout and knockdown with multiple cell types and compensatory mechanism identification\",\n      \"pmids\": [\"27129276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SNAT2 membrane stability requires extracellular Na+: removal of Na+ dramatically reduces SNAT2 stability independent of substrate; substrate (MeAIB and glutamine, but not non-substrates) represses SNAT2 protein stability; this repression is mediated through the cytoplasmic N-terminal tail containing lysyl residues (mutation of N-terminal lysines to alanine renders SNAT2 stable and insensitive to substrate-induced changes).\",\n      \"method\": \"HeLa cells, extracellular Na+ removal, SNAT2 N-terminal lysine mutants, chimeric SNAT2-5 transporter, protein stability assays\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis and chimeric transporter experiments revealing substrate- and ion-sensing mechanism for stability\",\n      \"pmids\": [\"29467657\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SNAT2 membrane topology was experimentally determined: 11 transmembrane domains with intracellular N-terminus and extracellular C-terminus; three N-glycosylation sites confirmed at the largest extracellular loop.\",\n      \"method\": \"Bioinformatics, mPEG-Mal chemical modification, protease cleavage assay, immunofluorescence, glycosylation analysis\",\n      \"journal\": \"Biochimica et biophysica acta. Biomembranes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal biochemical topology methods with experimental validation\",\n      \"pmids\": [\"29678469\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SNAT2 is the amino acid transporter most frequently induced by hypoxia in breast cancer; under hypoxia, SNAT2 regulation switches from ERα-dependent to HIF-1α-dependent (overlapping binding sites in SNAT2 cis-regulatory elements); HIF-1α-driven SNAT2 overexpression causes resistance to antiestrogen therapy in vivo.\",\n      \"method\": \"Reporter assays, ChIP for HIF-1α and ERα, SNAT2 overexpression in xenografts, fulvestrant treatment under normoxia/hypoxia\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro mechanistic dissection of competing TF binding plus in vivo xenograft validation\",\n      \"pmids\": [\"31152137\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CDK7 activity is upregulated in amino acid-deprived cells in a GCN2-dependent manner and is required for ATF4 expression and SNAT2 adaptive transcriptional induction; selective CDK7 inhibitor THZ-1 attenuates ATF4 expression and blocks system A adaptation; drug-resistant CDK7 expression rescues the adaptation.\",\n      \"method\": \"CDK7 inhibitors (THZ-1, roscovitine, flavopiridol), doxycycline-inducible drug-resistant CDK7, ATF4 immunoblot, system A transport assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological and genetic rescue experiment establishing CDK7 as component of ISR-SNAT2 axis\",\n      \"pmids\": [\"30857869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Placenta-specific Slc38a2/SNAT2 knockdown (59% reduction via lentiviral shRNA in blastocysts) reduces fetal and placental weight, fetal viability, trophoblast plasma membrane SNAT2 protein, and placental [14C]-MeAIB uptake, demonstrating that placental SNAT2 deficiency directly causes fetal growth restriction.\",\n      \"method\": \"Lentiviral shRNA blastocyst transduction (placenta-specific KD), fetal weight, [14C]-MeAIB transport assay, trophoblast plasma membrane fractionation\",\n      \"journal\": \"Clinical science (London, England : 1979)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — placenta-specific in vivo knockdown with mechanistic transport readout establishing causal role in FGR\",\n      \"pmids\": [\"34406367\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SLC38A2 acts cell-autonomously to provide proline to osteoblasts for synthesis of proline-rich osteoblast proteins (RUNX2, OSX, OCN, COL1A1); genetic ablation of SLC38A2 in osteoblasts impairs osteoblast differentiation and bone formation in mice; proline is primarily incorporated into nascent protein with little metabolism.\",\n      \"method\": \"Osteoblast-specific genetic ablation in mice, metabolomic tracing, osteoblast differentiation assays, bone formation measurement\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic KO with metabolomic validation and multiple osteoblast function readouts\",\n      \"pmids\": [\"35261338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SLC38A2 provides proline and alanine to osteoblast lineage cells; genetic ablation using Prrx1Cre reduces postnatal bone mass in both sexes due to impaired osteoblast number, proliferation, and osteogenic differentiation of skeletal stem and progenitor cells.\",\n      \"method\": \"Prrx1Cre conditional knockout, bone mass measurement (micro-CT), osteoblast lineage cell analysis, proliferation and differentiation assays\",\n      \"journal\": \"Frontiers in physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo conditional KO with cellular mechanistic readouts\",\n      \"pmids\": [\"36213239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SNAT2 inhibitor (thiophene-2-carboxamide derivative, IC50 ~0.8-3 μM) selectively inhibits SNAT2 over SNAT1; in combination with glucose transport inhibitor Bay-876, it halts proliferative growth of MDA-MB-231 breast and HPAFII pancreatic cancer cells, revealing synergy between glutaminolysis and glycolysis inhibition.\",\n      \"method\": \"High-throughput FMP assay screening of 33,934 compounds, SNAT2 electrogenic mechanism assay, cell proliferation assays\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — functional assay-based screen with kinetic IC50 determination and selectivity profiling plus cell biology validation\",\n      \"pmids\": [\"36210829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SNAT2 is responsible for hyperosmotic-induced sarcosine and glycine uptake in PC-3 prostate cancer cells; SNAT2 siRNA knockdown reduces sarcosine uptake Vmax from ~2653 to ~513 nmol/mg protein/min without altering Km, establishing SNAT2 as responsible for >80% of hyperosmotic sarcosine uptake; sarcosine is identified as a SNAT2 substrate.\",\n      \"method\": \"siRNA knockdown of SNAT2, [14C]sarcosine and [3H]glycine uptake kinetics, RT-PCR and Western blot for SNAT2\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockdown with quantitative kinetic transport assay identifying sarcosine as novel substrate\",\n      \"pmids\": [\"36175560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SLC38A2 in cDC1 dendritic cells mediates glutamine uptake that is required for cDC1 function in activating CD8+ T cells; tumor cells and cDC1s compete for glutamine via SLC38A2; glutamine signaling via FLCN-TFEB pathway controls cDC1 activation; SLC38A2 deficiency in DCs phenocopies FLCN loss by eliminating anti-tumor therapeutic effect of glutamine supplementation.\",\n      \"method\": \"Nutrient screening, SLC38A2 KO in DCs, FLCN KO in DCs (conditional), TFEB-dependent rescue, in vivo tumor models, CD8+ T cell priming assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with in vivo epistasis (FLCN-TFEB pathway), multiple loss-of-function approaches and tumor models\",\n      \"pmids\": [\"37407815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SLC38A2 protects renal medullary collecting duct cells from hyperosmolarity-induced ferroptosis via mTORC1 activation; SLC38A2 overexpression attenuates ferroptosis, while Slc38a2 deletion/silencing worsens it; Slc38a2-knockout mice show increased medullary ferroptosis following water restriction.\",\n      \"method\": \"RNA-Seq identification, SLC38A2 overexpression, Slc38a2 KO mice, water restriction model, mTORC1 pathway assays, ferroptosis markers (ROS, GSH, MDA)\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain and loss of function in vitro and in vivo with mechanistic pathway identification\",\n      \"pmids\": [\"36722887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XBP1 directly binds the SLC38A2 promoter and inhibits its transcription in cytotoxic T cells in multiple myeloma; XBP1-mediated SLC38A2 repression reduces glutamine uptake and impairs T cell immune function.\",\n      \"method\": \"Single-cell RNA sequencing, XBP1 promoter binding (in vitro binding assay), SLC38A2 silencing, glutamine uptake measurement, T cell functional assays\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — direct promoter binding and functional knockdown, single lab\",\n      \"pmids\": [\"37054944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"NERP-4 (VGF-derived peptide) acts on SNAT2 to increase glutamine, alanine, and proline uptake into pancreatic β cells, stimulating glucose-stimulated insulin secretion; SNAT2 deletion or inhibition abolishes NERP-4's protective effects on β-cell maintenance.\",\n      \"method\": \"SNAT2 deletion/inhibition, NERP-4 treatment of isolated islets and MIN6-K8 cells, amino acid uptake assays, insulin secretion assays, Ca2+ imaging in transgenic mice\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic deletion and inhibition with mechanistic transport and secretion readouts establishing peptide-transporter axis\",\n      \"pmids\": [\"38071217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UBE2C mediates SNAT2 monoubiquitination at lysine-59, which inhibits K63-linked polyubiquitination at lysine-33; this crosstalk increases SNAT2 plasma membrane levels by suppressing epsin 1 (EPN1)-mediated endocytosis; increased membrane SNAT2 enhances glutamine uptake, promotes VEGFC secretion, and drives lymphangiogenesis and lymph node metastasis in bladder cancer.\",\n      \"method\": \"Co-IP, ubiquitination assays (site-specific K59 and K33 mutants), endocytosis assays, SNAT2 membrane protein quantification, glutamine uptake, VEGFC measurement, patient-derived xenograft model\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — site-specific mutagenesis identifying ubiquitination crosstalk mechanism, multiple in vitro and in vivo readouts\",\n      \"pmids\": [\"38949026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ChREBP represses SNAT2 expression in response to high-carbohydrate diet by binding to a carbohydrate response element (ChoRE) at -160 bp in the SNAT2 promoter; ChREBP recruits SMRT corepressor to this site; ChIP confirmed in vivo binding; this controls hepatic glucogenic amino acid uptake.\",\n      \"method\": \"ChREBP binding site deletion/mutation, reporter assays, co-immunoprecipitation of ChREBP-SMRT, in vivo ChIP, high-sucrose diet in rats\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo binding with functional reporter and co-IP validation\",\n      \"pmids\": [\"33225719\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC38A2 (SNAT2) is a secondary active Na+-coupled neutral amino acid transporter with 11 transmembrane domains that accumulates glutamine, alanine, proline, glycine, and other small neutral amino acids (including betaine and sarcosine) into cells via a 1:1 Na+:amino acid stoichiometry; its transport activity is allosterically regulated by a conserved C-terminal histidine (pH sensing) and Na+ binding at TMD1/8 (Asn82, Thr384), with voltage regulation by its extracellular C-terminal tail; at the cellular level, SNAT2 is stored in the trans-Golgi network and trafficked to the plasma membrane in response to insulin (PI3K-dependent), amino acid starvation, or hyperosmotic stress, and is subject to Nedd4-2-mediated polyubiquitination and proteasomal degradation, with substrate occupancy and Na+ sensing controlling its stability via N-terminal lysyl residues; SNAT2 functions as a transceptor by sensing amino acid sufficiency and relaying signals to mTORC1 (via FLCN-TFEB), GCN2/ATF4, and PI3K/proteolysis pathways, while its transcription is regulated by ATF4/C/EBP (amino acid response element in intron 1), STAT3 (IL-6), HIF-1α (hypoxia, overriding ERα), ChREBP-SMRT (high carbohydrate), estradiol-ERα-GAPDH, and CDK7, making it a central hub integrating nutrient, hormonal, osmotic, and immune signals to control cellular amino acid homeostasis, mTOR-dependent anabolism, cell volume, osteoblast differentiation, DC-mediated anti-tumor immunity, and placental fetal nutrient supply.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"SLC38A2 (ATA2/SNAT2) was cloned and characterized as the protein responsible for system A amino acid transport activity ubiquitously expressed in mammalian tissues. It mediates Na+-dependent transport of neutral amino acids (model substrate: α-(methylamino)isobutyric acid), is pH-sensitive, Li+-intolerant, and has a 1:1 Na+:amino acid stoichiometry.\",\n      \"method\": \"Heterologous expression in mammalian cells and Xenopus laevis oocytes; transport current electrophysiology; substrate specificity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution in two expression systems with kinetic characterization; independently replicated across two cloning papers (rat and human)\",\n      \"pmids\": [\"10747860\", \"10930503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"SNAT2 protein is present in plasma-membrane and internal-membrane fractions of rat skeletal muscle, adipose tissue, L6 myotubes, and 3T3-L1 adipocytes, with subcellular distribution similar to GLUT4. Chronic amino acid deprivation causes a time-dependent increase in SNAT2 protein abundance, establishing the adaptive up-regulation of System A.\",\n      \"method\": \"Subcellular fractionation; Western blotting; immunolocalization with specific polyclonal antibodies\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct fractionation experiment with functional correlation; replicated in multiple cell/tissue types\",\n      \"pmids\": [\"11311116\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Adaptive stimulation of System A transport upon amino acid starvation in human fibroblasts is directly correlated with increased ATA2 (SNAT2) mRNA expression; supplementation with System A substrates (but not other amino acids) suppresses both ATA2 mRNA levels and transport activity, demonstrating substrate-specific transcriptional feedback.\",\n      \"method\": \"Northern blotting; transport assays; substrate specificity competition experiments\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — two orthogonal measurements (mRNA + transport) but single lab\",\n      \"pmids\": [\"11172802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Insulin stimulates System A transport in L6 skeletal muscle cells by recruiting SAT2 (SNAT2) to the plasma membrane from an endosomal compartment; this process requires phosphatidylinositol 3-kinase activity and is blocked by chloroquine (which impairs endosomal recycling) without affecting insulin-mediated PKB/GSK3 phosphorylation or GLUT4 translocation.\",\n      \"method\": \"Cell surface biotinylation; pharmacological inhibition (wortmannin, chloroquine); Western blotting; radiolabeled transport assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches dissecting PI3K-dependent plasma membrane recruitment from endosomal pool\",\n      \"pmids\": [\"11834730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"ATA2-mediated amino acid transport up-regulation following partial hepatectomy is regulated by redistribution of ATA2 protein to the plasma membrane from an intracellular compartment, not by changes in steady-state ATA2 mRNA levels.\",\n      \"method\": \"Plasma membrane isolation; Western blotting; Northern blotting\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — protein redistribution confirmed by fractionation with parallel mRNA negative result\",\n      \"pmids\": [\"12054432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Transcriptional activation of the ATA2 (SNAT2) gene by amino acid deprivation is largely independent of de novo protein synthesis (unlike asparagine synthetase), is detectable within 2–4 h, and is not induced by glucose deprivation, indicating a distinct genomic regulatory mechanism.\",\n      \"method\": \"Reporter gene assays; Northern blotting; cycloheximide/actinomycin D inhibition; HepG2 cells\",\n      \"journal\": \"The Journal of nutrition\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection using pharmacological inhibitors with mRNA readout\",\n      \"pmids\": [\"12368390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"De novo synthesis of new SNAT2 transporter protein is essential for the hypertonic stimulation of System A transport activity in human fibroblasts; transcription inhibition (DRB) abolishes the hypertonic transport increase, whereas the adaptive increase induced by amino acid starvation only partly requires new SNAT2 synthesis.\",\n      \"method\": \"Biotinylation of surface proteins; Western blotting; immunocytochemistry; transcription inhibitor (DRB) experiments\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — surface protein biotinylation plus functional transport assay with mechanistic inhibitors\",\n      \"pmids\": [\"15581851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"siRNA-mediated silencing of SNAT2 in hypertonically stressed human fibroblasts prevents the increase in System A transport activity, blocks expansion of the intracellular amino acid pool, and markedly delays cell volume recovery, demonstrating that SNAT2 induction is required for regulatory volume increase (RVI).\",\n      \"method\": \"siRNA knockdown; radiolabeled transport assays; intracellular amino acid measurement; cell volume measurement\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean loss-of-function with specific phenotypic readout (volume recovery) and transport activity measurement\",\n      \"pmids\": [\"15922329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The amino acid response element driving SNAT2 up-regulation upon amino acid limitation resides in the first intron of the gene; ATF and C/EBP family transcription factors bind this intronic enhancer in vitro and in vivo (ChIP), with specific members acting as activators or repressors, and amino acid deprivation increases RNA polymerase II recruitment to the SNAT2 promoter.\",\n      \"method\": \"Deletion/reporter assays; ChIP; EMSA; transcription factor overexpression\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro binding + in vivo ChIP + functional reporter; mechanistic dissection of multiple factors\",\n      \"pmids\": [\"16445384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SNAT2 induction by amino acid starvation requires eIF2α phosphorylation and involves an internal ribosome entry site (IRES) in the 5'-UTR that enables cap-independent translation, as well as increased gene transcription; hypertonic stress induction of SNAT2, by contrast, is eIF2α phosphorylation-independent.\",\n      \"method\": \"eIF2α phosphorylation-deficient mutant cells; IRES reporter assays; cell-free translation; Northern/Western blotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic mutant cells + cell-free reconstitution of IRES + multiple orthogonal readouts\",\n      \"pmids\": [\"16621798\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SNAT2 transporter function is associated with a leak anion conductance that does not require substrate transport; transported substrates (L-alanine, L-glutamine, MeAIB) inhibit this anion leak with different potencies. Mutation H304A eliminates alanine transport but retains anion leak current, and the selectivity sequence of the anion conductance was determined.\",\n      \"method\": \"Electrophysiology (whole-cell patch clamp and two-electrode voltage clamp) in Xenopus oocytes; site-directed mutagenesis (H304A); ion substitution experiments\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structure-function by mutagenesis combined with electrophysiology; mechanistically separates transport from anion leak\",\n      \"pmids\": [\"17237199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A conserved C-terminal histidine residue (H504 in SNAT2, H471 in SNAT5) mediates the pH-sensitivity of System A and N transporters through an allosteric mechanism influencing Na+ binding; DEPC modification abolishes pH-sensitivity and its effects are reversed by hydroxylamine and blocked by substrate; H504A mutation produces reduced, DEPC-resistant pH-sensitivity without changing Na+ affinity at low pH.\",\n      \"method\": \"Xenopus oocyte expression; site-directed mutagenesis (H504A); DEPC chemical modification; transport current electrophysiology; Na+ affinity measurement\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with chemical modification and electrophysiology; identifies conserved mechanistic residue\",\n      \"pmids\": [\"16629640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Ubiquitin ligase Nedd4-2 regulates SNAT2 (ATA2) surface abundance by polyubiquitinating the transporter at the plasma membrane, leading to endocytotic sequestration and proteasomal degradation; Nedd4-2 RNAi increases SNAT2 activity with concomitant decreased polyubiquitination; catalytically inactive Nedd4-2 has no effect.\",\n      \"method\": \"Xenopus oocyte expression; transfection in CHO cells and adipocytes; RNAi; co-localization by immunofluorescence; proteasome inhibitor (MG132); ubiquitination assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal gain/loss-of-function with ubiquitination assays and transport readout in multiple systems\",\n      \"pmids\": [\"17003038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"ATA2 (SNAT2) is stored in a trans-Golgi network (TGN) compartment (co-localizing with syntaxin 6, not EEA1) in adipocytes; insulin stimulus releases SNAT2-containing vesicles from the TGN to the plasma membrane through a pathway distinct from GLUT4 translocation; brefeldin A (TGN exit inhibitor) preferentially blocks insulin-stimulated MeAIB uptake over glucose transport.\",\n      \"method\": \"Live-cell imaging of EGFP-tagged ATA2; immunofluorescence co-localization; brefeldin A inhibition; insulin-stimulated transport assays in 3T3-L1 adipocytes\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — live imaging plus pharmacological dissection; identifies distinct storage and trafficking pathway\",\n      \"pmids\": [\"17050538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Two distinct amino acid sensor/effector pathways control SNAT2 expression: (1) JNK activation during amino acid withdrawal induces SNAT2 transcription via an intronic nutrient-sensitive domain; (2) a sensor for large neutral amino acids (Tyr, Gln) inhibits JNK and suppresses SNAT2 up-regulation. Additionally, SNAT2 itself provides a repressive signal for its own gene transcription when amino acids are sufficient, consistent with a transceptor (transporter-receptor) function.\",\n      \"method\": \"shRNA; transporter chimera expression; JNK pathway inhibitors; intronic reporter assays; SNAT2 protein stability measurements in L6 myotubes\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — shRNA + chimera approach + multiple pathway inhibitors establish two converging sensor pathways and transceptor auto-regulation\",\n      \"pmids\": [\"17488712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Selective inhibition of SNAT2 (by MeAIB, metabolic acidosis at pH 7.1, or siRNA knockdown) depletes intracellular L-Gln and indirectly depletes leucine and other amino acids maintained by the L-Gln gradient, strongly impairing mTOR signalling to S6K, S6, and 4E-BP1 and reducing protein synthesis in L6 skeletal muscle cells.\",\n      \"method\": \"siRNA; competitive substrate inhibition (MeAIB); metabolic acidosis; amino acid HPLC; mTOR pathway Western blotting; protein synthesis assays\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — three independent SNAT2 inhibition approaches with consistent downstream signalling readouts\",\n      \"pmids\": [\"17429052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SNAT2 inhibition by metabolic acidosis (pH 7.1) or MeAIB stimulates proteolysis in L6 myotubes; this effect is mediated through both mTOR and PI3K signalling pathways, and SNAT2 partial silencing impairs insulin signalling through PI3K, linking SNAT2-mediated glutamine uptake to regulation of muscle proteolysis and insulin resistance.\",\n      \"method\": \"siRNA; MeAIB competitive inhibition; proteasome inhibitor; mTOR/PI3K pharmacological inhibitors; proteolysis assays in L6 cells\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — siRNA + pharmacological dissection of two signalling arms with proteolysis readout\",\n      \"pmids\": [\"18650482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In glutamatergic neurons, SAT2 (SNAT2) is predominantly targeted to somatodendritic compartments and supplies glutamine for conversion to glutamate required for retrograde dendritic signalling; MeAIB inhibition of SAT2 reduces neuronal glutamine uptake, lowers intracellular glutamate, and suppresses inhibitory synaptic inputs to pyramidal cells from fast-spiking interneurons.\",\n      \"method\": \"Immunohistochemistry; confocal microscopy; electron microscopy; in vivo electrical stimulation; in vitro depolarization; MeAIB pharmacological inhibition; electrophysiology of neocortical circuits\",\n      \"journal\": \"Cerebral cortex (New York, N.Y. : 1991)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — localization by EM + pharmacological loss-of-function with electrophysiological circuit readout\",\n      \"pmids\": [\"18832333\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Asparagine 82 in transmembrane domain 1 of SNAT2 is critical for Na+ coordination; N82A mutation virtually eliminates alanine transport current and amino acid uptake, dramatically reduces Na+ affinity (K(Na+)), and increases apparent Km for alanine 27-fold, demonstrating a direct or indirect role of Asn82 in Na+ binding.\",\n      \"method\": \"Site-directed mutagenesis (N82A, N82S, Y337A, R374Q); Xenopus oocyte expression; two-electrode voltage clamp; radiolabeled amino acid uptake assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with functional electrophysiology + uptake assays; identifies Na+ coordination residue\",\n      \"pmids\": [\"18319257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Despite increased ATF4 binding to the C/EBP-ATF composite site in the SNAT2 gene during unfolded protein response (UPR) activation, SNAT2 transcription is not enhanced in HepG2 cells; UPR suppresses the amino acid response (AAR)-induced increase in SNAT2 transcription, demonstrating that the UPR generates a repressive signal downstream of ATF4 binding that is chromatin-level (lacking histone H3 hyperacetylation and general transcription factor recruitment at the promoter).\",\n      \"method\": \"ChIP (H3 acetylation, RNA Pol II, general transcription factors); reporter gene assays; pharmacological UPR/AAR activation; HepG2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP + reporter assays mechanistically dissect AAR vs UPR regulation; multiple orthogonal chromatin readouts\",\n      \"pmids\": [\"18697751\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A conserved Na+ binding site in SNAT2 is formed by transmembrane helices 1 and 8, predicted by homology modeling to LeuT/Mhp1 structures; the T384A mutation in the predicted TMD8 Na+ binding site dramatically lowers Na+ affinity and inhibits the anion leak current, consistent with a cation binding site conserved across SLC38 and related bacterial transporter families.\",\n      \"method\": \"Profile-based sequence analysis; homology modeling (LeuT, Mhp1 templates); site-directed mutagenesis (T384A); Xenopus oocyte expression; electrophysiology; Na+ affinity measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structure-guided mutagenesis validated by functional electrophysiology; identifies conserved Na+ site\",\n      \"pmids\": [\"19589777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In neocortical neurons, SNAT2 expression is constitutively low but potently induced by depletion of neutral amino acids; substrates of the SLC6 GABA transporter family (taurine, GABA, β-alanine) repress SNAT2 expression more potently (~10×) than System A substrates; ATF4 and C/EBP induction by amino acid deprivation mediates SNAT2 transcriptional regulation in neurons.\",\n      \"method\": \"Neuronal culture; SNAT2 de novo induction; electrophysiology (spontaneous excitatory activity); ATF4/C/EBP Western blotting; pharmacological substrate competition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — electrophysiological and molecular readouts in neurons; identifies novel regulatory substrates\",\n      \"pmids\": [\"19240036\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"IL-6 stimulates System A amino acid transporter activity in human trophoblast cells through STAT3-dependent transcriptional up-regulation of SNAT2 mRNA and protein; siRNA targeting STAT3 reduces SNAT2 (but not SNAT1) expression and abolishes IL-6-stimulated System A activity. TNF-α also stimulates system A via SNAT2 but through a JAK/STAT-independent pathway.\",\n      \"method\": \"siRNA (STAT3); Western blotting; RT-PCR; radiolabeled MeAIB transport assays; phospho-STAT3 detection in primary human trophoblasts\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — siRNA loss-of-function with isoform-specific mRNA/protein readouts and transport functional assay; mechanistically links IL-6/STAT3 to SNAT2\",\n      \"pmids\": [\"19741197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Chronic competitive inhibition of SNAT2 with MeAIB reduces cell proliferation and depletes intracellular SNAT2 substrate amino acids as well as leucine; surprisingly, Me-AIB elevates mTOR-dependent p70S6K1 phosphorylation despite amino acid pool depletion. Proteomic analysis of TAP-purified SNAT2 fusion proteins identified two novel SNAT2-interacting proteins potentially involved in signalling for protein turnover and cell growth.\",\n      \"method\": \"Competitive inhibition (MeAIB); mTOR pathway Western blotting; TAP-tag purification + proteomics; cell proliferation/size assays in MCF-7 cells\",\n      \"journal\": \"Frontiers in bioscience (Elite edition)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — novel interactors identified by single TAP-tag/MS experiment; mTOR finding is mechanistically unexpected but replicated internally\",\n      \"pmids\": [\"21622135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"C-terminal domain deletion of SNAT2 (13 residues) abolishes amino acid transport at negative membrane potentials while allowing transport at positive potentials; the truncation increases apparent affinity for alanine (~3-fold) and Na+ (~2-fold) without affecting surface expression, demonstrating that the C-terminal extracellular domain acts as a voltage regulator required for normal amino acid translocation at physiological potentials.\",\n      \"method\": \"C-terminal truncation mutagenesis; Xenopus oocyte expression; two-electrode voltage clamp; radiolabeled amino acid uptake; surface expression assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — deletion mutagenesis with electrophysiology and uptake assays; identifies specific functional role of C-terminal domain\",\n      \"pmids\": [\"21158741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SNAT2 is the primary transporter mediating L-proline uptake by embryonic stem (ES) cells; uptake of L-proline through SNAT2 is required for ES cell differentiation to early primitive ectoderm-like cells, as SNAT2 substrate competitors (but not non-substrates) block morphological changes, gene expression changes, and differentiation kinetics.\",\n      \"method\": \"Competitive substrate inhibition; transport assays; morphological and gene expression differentiation assays in ES cells\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological specificity (System A substrates vs. non-substrates) with multiple differentiation readouts\",\n      \"pmids\": [\"21346154\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A genome-wide RNAi screen identified SNAT2 as required for arsenite-induced ER stress response; arsenite up-regulates SNAT2 expression and activity in an ATF4-dependent manner; inhibition of SNAT2 expression/activity or glutamine deprivation specifically suppresses arsenite-induced (but not tunicamycin-induced) ER stress and mTOR activation, placing SNAT2 upstream of mTOR in this pathway.\",\n      \"method\": \"Genome-wide shRNA screen; flow cytometry; SNAT2 inhibition (MeAIB, siRNA); mTOR pathway Western blotting; ER stress reporters\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — unbiased genome-wide screen + mechanistic follow-up with specific inhibition; ATF4-dependence established\",\n      \"pmids\": [\"22215663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SNAT2 transports the PET radiotracer anti-[18F]FACBC with Michaelis-Menten kinetics (Km ~197 μM), lower affinity than ASCT2 (Km ~97 μM); characterized in Xenopus oocytes expressing human SNAT2.\",\n      \"method\": \"Xenopus oocyte expression; radiolabeled [14C]FACBC kinetic uptake assays\",\n      \"journal\": \"Nuclear medicine and biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro transport kinetics in expression system; single study\",\n      \"pmids\": [\"23647854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Only SNAT2 (among SNAT1, 2, and 4) exhibits betaine uptake activity; human and rat SNAT2 transport betaine with Km values of 5.3 mM and 4.6 mM, respectively; betaine exclusively inhibits SNAT2 among the system A subtypes; hypertonicity preferentially induces SNAT2 expression and its plasma membrane targeting in placental trophoblasts.\",\n      \"method\": \"Transfection in HEK293 cells; [14C]betaine uptake assays; Western blotting of plasma membrane fractions; immunocytochemistry; RT-PCR in TR-TBT 18d-1 cells\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct substrate transport assays in expression system + in situ membrane trafficking validation; subtype specificity for betaine established\",\n      \"pmids\": [\"24434061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"17β-estradiol regulates SNAT2 transcription through a functional estrogen response element (ERE) in the SNAT2 promoter bound by estrogen receptor α (ERα); in vivo ChIP shows progressive ERα binding to the SNAT2 promoter during gestation correlating with estradiol levels; the ERα-ERE complex also contains PARP1, Ku70, and GAPDH, and silencing each abolishes estradiol-stimulated SNAT2 promoter activity.\",\n      \"method\": \"Reporter assays; EMSA; supershift assays; in vivo ChIP; LC-MS proteomics of ERα-ERE complex; Western blotting; ERE deletion/mutation\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro binding + in vivo ChIP + reporter + complex proteomics; multiple orthogonal methods\",\n      \"pmids\": [\"25056967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"RNF5 ubiquitin ligase associates with, ubiquitinates, and promotes degradation of SLC38A2 (along with SLC1A5) in response to paclitaxel-induced ER stress in breast cancer cells, decreasing glutamine uptake, TCA cycle components, and mTOR signalling while increasing autophagy and cell death.\",\n      \"method\": \"Co-immunoprecipitation; ubiquitination assays; siRNA knockdown; metabolic profiling; mTOR pathway Western blotting; cell death/autophagy assays\",\n      \"journal\": \"Cancer cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP + ubiquitination assay + functional downstream metabolic and signalling readouts\",\n      \"pmids\": [\"25759021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SNAT2 protein stability is regulated by the ubiquitin-proteasome system via N-terminal lysyl residues; linoleic acid (LOA) increases ubiquitination and proteasomal degradation of SNAT2 through increased Nedd4.2 expression; mutation of seven N-terminal lysyl residues to alanine protects SNAT2 from LOA-induced degradation; the N-terminal cytoplasmic tail confers substrate-induced changes in SNAT2 stability when grafted onto SNAT5.\",\n      \"method\": \"Proteasome inhibitor; shRNA (Nedd4.2); N-terminal lysine mutagenesis; chimeric SNAT2-SNAT5 construct; transport assays; Western blotting in L6 myotubes/HeLa\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis + chimeric transporter + loss-of-function; identifies specific N-terminal domain and lysyl residues\",\n      \"pmids\": [\"25653282\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Decreased placental mTOR activity in IUGR is associated with increased NEDD4-2 expression (+72%), increased ubiquitination of SNAT2 (+180%), and decreased SNAT2 protein in the syncytiotrophoblast microvillous plasma membrane (–31%), establishing that mTOR regulates placental SNAT2 surface trafficking via the ubiquitin pathway.\",\n      \"method\": \"Western blotting (mTORC1/2 activity, NEDD4-2, SNAT2); ubiquitination assays; plasma membrane fractionation; radiolabeled MeAIB transport in human IUGR placental tissue\",\n      \"journal\": \"Clinical science (London, England : 1979)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway established in human tissue with multiple molecular readouts\",\n      \"pmids\": [\"26374858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Hyperosmotic stress coordinates SNAT2 induction with GADD34 up-regulation; increased GADD34 (a PP1 regulatory subunit) dephosphorylates eIF2α, enhancing SNAT2-mediated amino acid uptake; GADD34 induction during osmotic stress depends on c-Jun/CRE-mediated transcription and mRNA stabilization (not ATF4, unlike other stresses), establishing a SNAT2/GADD34 axis for cell survival.\",\n      \"method\": \"Reporter assays (CRE); mRNA stability assays; eIF2α phosphorylation Western blotting; SNAT2 transport assays; GADD34 overexpression/knockdown\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches dissect SNAT2/GADD34 co-regulation with functional survival readout\",\n      \"pmids\": [\"26041779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Net glutamine uptake in HeLa and 143B cancer cells does not depend on ASCT2 but requires SNAT1 and SNAT2; ASCT2 deletion causes amino acid starvation response and SNAT1 up-regulation to functionally replace ASCT2; SNAT2-mediated net uptake is essential for maintaining intracellular glutamine for glutaminolysis.\",\n      \"method\": \"CRISPR/Cas9 knockout (ASCT2); siRNA (GCN2, SNAT1); radiolabeled amino acid uptake; cell growth assays; Western blotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout + epistasis analysis establishes SNAT2 as rate-limiting for net glutamine uptake\",\n      \"pmids\": [\"27129276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Membrane topology of SNAT2 was experimentally determined: 11 transmembrane domains with an intracellular N-terminus and extracellular C-terminus; three N-glycosylation sites were verified at the largest extracellular loop using chemical modification (mPEG-Mal), protease cleavage assays, immunofluorescence, and glycosylation analysis.\",\n      \"method\": \"mPEG-Mal chemical modification; protease cleavage; immunofluorescence; glycosylation assays; bioinformatics\",\n      \"journal\": \"Biochimica et biophysica acta. Biomembranes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — four orthogonal experimental methods to determine topology; first experimental topology confirmation\",\n      \"pmids\": [\"29678469\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Extracellular Na+ is required for the SNAT2 adaptive stress response: Na+ withdrawal during amino acid deprivation prevents SNAT2 gene induction; substrate-induced repression of SNAT2 protein stability requires the cytoplasmic N-terminal tail with lysyl residues; grafting this tail onto SNAT5 confers substrate-induced stability changes, while mutation of N-terminal lysines renders SNAT2 stable and insensitive to substrate.\",\n      \"method\": \"Na+ substitution; N-terminal chimeric SNAT2/SNAT5 constructs; lysine-to-alanine mutagenesis; Western blotting; transport assays in HeLa cells\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis + chimeric transporter + ion substitution; mechanistically identifies N-terminal tail as substrate-sensing domain\",\n      \"pmids\": [\"29467657\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Hypoxia induces SNAT2 expression in breast cancer primarily through HIF-1α (rather than ERα, which dominates under normoxia); HIF-1α and ERα binding sites overlap in SNAT2 cis-regulatory elements; fulvestrant (ER antagonist) cannot suppress SNAT2 under hypoxia; SNAT2 overexpression causes complete endocrine resistance in vivo.\",\n      \"method\": \"ChIP (HIF-1α, ERα); reporter assays; siRNA; xenograft mouse model; Western blotting; in vitro/in vivo growth assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP identifies overlapping regulatory elements + in vivo xenograft functional validation; demonstrates regulatory switch mechanism\",\n      \"pmids\": [\"31152137\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CDK7 activity is upregulated during amino acid deprivation in a GCN2-dependent manner and is required for the SNAT2 adaptive stress response; CDK7 inhibition (THZ-1) attenuates ATF4 induction and blocks System A adaptation; drug-resistant CDK7 expression mitigates THZ-1 effects, establishing CDK7 as a component of the integrated stress response regulating SNAT2.\",\n      \"method\": \"Pharmacological CDK inhibitors (THZ-1, roscovitine, flavopiridol); doxycycline-inducible drug-resistant CDK7; Western blotting; System A transport assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — drug-resistant rescue experiment + multiple CDK inhibitors + GCN2 dependence establishes CDK7 in ISR/SNAT2 pathway\",\n      \"pmids\": [\"30857869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Placenta-specific lentiviral shRNA knockdown of Slc38a2 (59% reduction) in mice reduces near-term fetal and placental weight, fetal viability, trophoblast plasma membrane SNAT2 protein, and placental System A transport activity (MeAIB uptake), directly demonstrating that placental Slc38a2 deficiency causes fetal growth restriction.\",\n      \"method\": \"Lentiviral shRNA blastocyst transduction (placenta-specific KD); radiolabeled MeAIB uptake; Western blotting; fetal weight measurements\",\n      \"journal\": \"Clinical science (London, England : 1979)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — placenta-specific genetic loss-of-function in vivo with multiple mechanistic and physiological readouts\",\n      \"pmids\": [\"34406367\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SLC38A2 acts cell-autonomously in osteoblasts to provide proline (and alanine) for the synthesis of proline-rich osteoblast proteins (RUNX2, OSX, OCN, COL1A1); genetic ablation of SLC38A2 in osteoblasts impairs osteoblast differentiation and bone formation in mice; metabolomics showed proline is primarily incorporated into nascent protein rather than metabolized.\",\n      \"method\": \"Conditional genetic knockout (osteoblast-specific); metabolomics; bone histomorphometry; differentiation assays; [13C]-proline tracing\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific genetic KO with metabolomics tracing and multiple differentiation/bone formation readouts\",\n      \"pmids\": [\"35261338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SLC38A2 provides proline and alanine to osteoblast lineage cells during postnatal bone homeostasis; Prrx1Cre-driven SLC38A2 ablation decreases bone mass in both sexes by reducing osteoblast numbers, impairing proliferation and osteogenic differentiation of skeletal stem and progenitor cells.\",\n      \"method\": \"Conditional knockout (Prrx1Cre); micro-CT; histomorphometry; cell proliferation and differentiation assays; amino acid uptake measurements\",\n      \"journal\": \"Frontiers in physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with multiple quantitative bone/cell biology readouts\",\n      \"pmids\": [\"36213239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"UBE2C mediates monoubiquitination of SNAT2 at lysine 59, which inhibits K63-linked polyubiquitination at lysine 33; this crosstalk between ubiquitination types increases SNAT2 membrane protein levels by suppressing EPN1-mediated endocytosis; elevated membrane SNAT2 facilitates glutamine uptake and metabolism to promote VEGFC secretion and lymphangiogenesis in bladder cancer.\",\n      \"method\": \"Co-IP; ubiquitination assays (site-specific mutagenesis K59, K33); cell surface protein assays; siRNA; xenograft/PDX models; VEGFC ELISA; lymphangiogenesis assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — site-specific ubiquitination mutagenesis + endocytosis mechanistic dissection + functional in vivo validation\",\n      \"pmids\": [\"38949026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SNAT2 is responsible for hyperosmotic stress-induced uptake of sarcosine and glycine in PC-3 prostate cancer cells; siRNA knockdown of SNAT2 reduces Vmax of sarcosine uptake by ~80% without altering Km, and SNAT2 is up-regulated at mRNA and protein levels under hyperosmotic conditions, identifying sarcosine as a novel SNAT2 substrate.\",\n      \"method\": \"siRNA; radiolabeled sarcosine/glycine uptake kinetics; RT-PCR; Western blotting; hyperosmotic cell culture\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA loss-of-function with kinetic transport assay; identifies sarcosine as new substrate; single lab\",\n      \"pmids\": [\"36175560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Tumour cells and cDC1 dendritic cells compete for glutamine uptake via SLC38A2; glutamine signalling through SLC38A2 in cDC1s activates FLCN, which impinges on TFEB function to license cDC1 activation of CD8+ T cells; SLC38A2 deficiency in DCs phenocopies FLCN loss and eliminates anti-tumour therapeutic effects of glutamine supplementation.\",\n      \"method\": \"Genetic deletion (DC-specific SLC38A2 KO, FLCN KO); in vivo tumour models; CD8+ T cell priming assays; nutrient competition assays; epistasis (TFEB-dependence); intratumoral glutamine supplementation\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific genetic KO + epistasis (FLCN/TFEB pathway) + in vivo tumour immunology readouts; highly cited\",\n      \"pmids\": [\"37407815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XBP1 directly binds the SLC38A2 promoter and transcriptionally represses it; SLC38A2 silencing in T cells decreases glutamine uptake and causes immune dysfunction, establishing an XBP1-SLC38A2 axis as a metabolic regulator of cytotoxic T lymphocyte function in multiple myeloma.\",\n      \"method\": \"ChIP; dual-luciferase reporter assay; siRNA (SLC38A2); glutamine uptake assay; T cell functional assays; single-cell RNA-seq\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP + reporter validate direct XBP1 binding; functional consequence by siRNA; single lab\",\n      \"pmids\": [\"37054944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"NERP-4 (a VGF-derived peptide) acts on SNAT2 to increase glutamine, alanine, and proline uptake into pancreatic β cells, stimulating insulin secretion; SNAT2 deletion and inhibition abolish NERP-4's protective effects on β-cell maintenance and function in db/db mice, defining a peptide-amino acid transporter autocrine axis.\",\n      \"method\": \"SNAT2 genetic deletion; pharmacological inhibition; amino acid uptake assays; insulin secretion assays; isolated islets; transgenic Ca2+ reporter mice; db/db mouse model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic deletion + pharmacological inhibition + functional amino acid uptake and insulin secretion assays; in vivo validation in diabetes model\",\n      \"pmids\": [\"38071217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SLC38A2 protects renal medullary collecting duct (MCD) cells from hyperosmolarity-induced ferroptosis; SLC38A2 overexpression attenuates hyperosmotic cell death while Slc38a2 deletion worsens it; the osmoprotective effect requires mTORC1 activation; Slc38a2 knockout mice exhibit increased medullary ferroptosis after water restriction in vivo.\",\n      \"method\": \"RNA-Seq; SLC38A2 overexpression/siRNA; genetic Slc38a2 knockout mice; ferroptosis markers (ROS, GSH, MDA, iron); mTORC1 Western blotting; water restriction in vivo model\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO mouse + overexpression + mTORC1 mechanistic dissection + in vivo stress model\",\n      \"pmids\": [\"36722887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"IGF2BP2 promotes SLC38A2 mRNA stability in an m6A-dependent manner downstream of PTCD3; PTCD3-IGF2BP2-SLC38A2 axis drives glutaminolysis and metastasis in colorectal cancer; SLC38A2 overexpression rescues proliferation/invasion defects caused by PTCD3 depletion.\",\n      \"method\": \"Co-IP; RIP; dual-luciferase assay; siRNA/overexpression; CRC xenograft model; glutamine metabolism assays\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP + RIP + functional rescue; identifies post-transcriptional regulation of SLC38A2 by m6A/IGF2BP2; single lab\",\n      \"pmids\": [\"40304977\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC38A2 (SNAT2) is a secondary active, Na+-coupled neutral amino acid transporter with 11 transmembrane domains (intracellular N-terminus, extracellular C-terminus, three verified N-glycosylation sites) whose conserved Asn82 (TMD1) and Thr384 (TMD8) coordinate Na+ binding, and whose C-terminal domain and conserved histidine (H504) regulate voltage-dependence and pH-sensitivity, respectively; it accumulates glutamine, alanine, proline, glycine, and betaine (sarcosine as a novel substrate), and its plasma membrane abundance is dynamically controlled by insulin-stimulated exocytosis from TGN/endosomal compartments, Nedd4-2/RNF5/UBE2C-mediated ubiquitination at specific N-terminal lysyl residues, and substrate-induced stabilization, while its transcription is induced by amino acid deprivation through an ATF4/C/EBP intronic enhancer (requiring eIF2α phosphorylation, CDK7 activity, and cap-independent IRES translation), and by IL-6/STAT3, HIF-1α, estrogen receptor-α, prolactin, and cAMP/CRE pathways, and repressed by ChREBP/SMRT and XBP1; functionally, SNAT2 acts as a transceptor—repressing its own transcription during amino acid sufficiency—and drives mTORC1 signalling via leucine exchange and FLCN/TFEB, regulates cell volume via osmolyte accumulation, promotes proteolysis resistance through PI3K, supports osteoblast differentiation by supplying proline for collagen-rich protein synthesis, protects renal medullary cells from ferroptosis via mTORC1, enables cDC1-mediated anti-tumour T cell immunity by competing with tumour cells for glutamine, and sustains β-cell insulin secretion in response to the peptide NERP-4.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SLC38A2 (SNAT2) is a sodium-coupled neutral amino acid transporter that serves as the principal mediator of system A transport activity, importing glutamine, alanine, proline, glycine, betaine, and sarcosine with 1:1 Na⁺:amino acid stoichiometry, and functioning as a nutrient sensor (transceptor) that integrates amino acid availability with mTORC1 signaling, cell volume regulation, and protein turnover [PMID:10930503, PMID:17429052, PMID:17488712, PMID:37407815]. Na⁺ coordination at TMD1/TMD8 (Asn82, Thr384) and pH sensing through a conserved C-terminal histidine (His504) govern transport kinetics, while a 13-residue extracellular C-terminal tail is required for voltage-dependent translocation [PMID:18319257, PMID:19589777, PMID:16629640, PMID:21158741]. SNAT2 resides in the trans-Golgi network and is trafficked to the plasma membrane by insulin (PI3K-dependent), amino acid starvation, or hyperosmotic stress through distinct signaling pathways, with surface abundance controlled by Nedd4-2–mediated polyubiquitination at N-terminal lysyl residues and a UBE2C-dependent monoubiquitination/K63-polyubiquitination crosstalk that regulates endocytosis [PMID:17050538, PMID:11834730, PMID:17003038, PMID:25653282, PMID:38949026]. Transcription is governed by an intronic ATF4/C/EBP amino acid response element, ERα, HIF-1α (overriding ERα under hypoxia), STAT3 (IL-6), ChREBP-SMRT, and CDK7-dependent integrated stress response signaling, and SNAT2 is physiologically required for placental nutrient supply, osteoblast differentiation, dendritic cell–mediated anti-tumor immunity, renal osmoadaptation, and pancreatic β-cell insulin secretion [PMID:16445384, PMID:31152137, PMID:19741197, PMID:33225719, PMID:30857869, PMID:34406367, PMID:35261338, PMID:37407815, PMID:36722887, PMID:38071217].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Cloning of SLC38A2 established it as the molecular identity of system A transport activity — a long-sought Na⁺-coupled neutral amino acid transporter with pH sensitivity and 1:1 Na⁺:substrate stoichiometry.\",\n      \"evidence\": \"Cloned from HepG2 cells and functionally expressed in mammalian cells with MeAIB transport assays\",\n      \"pmids\": [\"10930503\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of Na⁺ coupling unknown\", \"Membrane topology not experimentally determined\", \"Regulation of transporter expression uncharacterized\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Discovery that SNAT2 protein resides in both plasma membrane and intracellular compartments, with amino acid deprivation increasing its abundance, revealed that regulated trafficking and adaptive gene expression are core regulatory mechanisms.\",\n      \"evidence\": \"Subcellular fractionation and immunoblot in L6 myotubes, 3T3-L1 adipocytes, and human fibroblasts; Northern blot showing mRNA upregulation upon amino acid starvation\",\n      \"pmids\": [\"11311116\", \"11172802\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of intracellular storage compartment unknown\", \"Mechanism of adaptive transcriptional upregulation unresolved\", \"Post-translational vs. transcriptional contribution not separated\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Insulin was shown to recruit SNAT2 from an endosomal compartment to the plasma membrane via PI3K, establishing hormonal control of SNAT2 trafficking as a distinct regulatory mode from adaptive upregulation.\",\n      \"evidence\": \"Cell surface biotinylation, PI3K inhibitor (wortmannin), chloroquine treatment in L6 cells and hepatocyte membrane fractions after partial hepatectomy\",\n      \"pmids\": [\"11834730\", \"12054432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of exact endosomal/Golgi compartment unclear\", \"Vesicular machinery mediating translocation unknown\", \"Whether insulin and starvation pathways converge not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Hyperosmotic stress was identified as a third distinct stimulus inducing SNAT2, requiring new protein synthesis rather than redistribution alone, and separating the osmotic and amino acid starvation pathways mechanistically.\",\n      \"evidence\": \"DRB transcription inhibition, cell surface biotinylation, immunocytochemistry comparing hypertonic vs. amino acid deprivation responses\",\n      \"pmids\": [\"15581851\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Osmotic stress signaling pathway to SNAT2 transcription unidentified\", \"Role of SNAT2 in cell volume recovery not yet functionally tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"A convergence of studies in 2006 resolved four major mechanistic questions: (1) SNAT2 is essential for regulatory volume increase (RVI) under hypertonic stress; (2) SNAT2 is stored at the trans-Golgi network (colocalizing with syntaxin 6) and trafficked in vesicles distinct from GLUT4; (3) Nedd4-2 polyubiquitinates SNAT2 for proteasomal degradation; (4) a conserved C-terminal His504 mediates allosteric pH regulation of Na⁺ binding; and (5) the intronic ATF4/C/EBP amino acid response element was identified.\",\n      \"evidence\": \"siRNA-mediated SNAT2 silencing with cell volume assays; EGFP-ATA2 live imaging with TGN markers and brefeldin A; Nedd4-2 mutagenesis, ubiquitination assays, and siRNA in oocytes and CHO cells; H504A mutagenesis with DEPC modification and electrophysiology; ChIP, EMSA, and reporter assays for intronic enhancer\",\n      \"pmids\": [\"15922329\", \"16734764\", \"17050538\", \"17003038\", \"16629640\", \"16445384\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of TGN retention unclear\", \"Specific lysine residues targeted by Nedd4-2 not identified\", \"How ATF4 binding alone is insufficient during UPR not yet explained\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"SNAT2-mediated glutamine uptake was shown to control intracellular leucine supply (via system L exchange) and mTOR signaling to S6K1 and 4E-BP1, establishing SNAT2 as a rate-limiting upstream input to mTORC1.\",\n      \"evidence\": \"Three independent SNAT2 inhibition approaches (MeAIB, acidosis, siRNA) with mTOR pathway phosphorylation assays and amino acid profiling in L6 muscle cells\",\n      \"pmids\": [\"17429052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SNAT2 signals to mTOR independently of amino acid supply not resolved\", \"Direct physical interaction with mTOR pathway components not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"SNAT2 was established as a transceptor — its own transport activity provides a repressive signal for its gene transcription during amino acid sufficiency via JNK-dependent intronic regulation, and it conducts a substrate-inhibited anion leak current separable from transport via H304.\",\n      \"evidence\": \"shRNA knockdown, transporter chimeras, JNK inhibition, and intronic reporter assays; electrophysiology with H304A mutagenesis\",\n      \"pmids\": [\"17488712\", \"17237199\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream targets of the receptor-like signal unclear\", \"Physiological relevance of anion leak conductance unknown\", \"How substrate occupancy is translated into transcriptional repression not fully resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of Asn82 (TMD1) as a Na⁺ coordination residue, demonstration that SNAT2 inhibition stimulates muscle proteolysis via PI3K, and revelation that the UPR represses AAR-induced SNAT2 transcription despite ATF4 binding (via chromatin-level control) defined the transport mechanism, catabolic consequences, and transcriptional integration of SNAT2.\",\n      \"evidence\": \"N82A/N82S mutagenesis with electrophysiology; MeAIB/acidosis/siRNA with proteolysis and PI3K assays; ChIP for ATF4, histone acetylation, and Pol II at SNAT2 promoter under AAR vs. UPR\",\n      \"pmids\": [\"18319257\", \"18650482\", \"18697751\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full Na⁺ binding site architecture incomplete (TMD8 residue not yet identified)\", \"How UPR represses SNAT2 chromatin remodeling mechanistically unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Thr384 in TMD8 was identified as the second Na⁺ coordination residue, completing the structural model of the Na⁺ binding site at the TMD1-TMD8 interface, and STAT3 was shown to mediate IL-6–stimulated SNAT2 transcription in trophoblasts.\",\n      \"evidence\": \"T384A mutagenesis with electrophysiology and homology modeling based on LeuT/Mhp1; STAT3 siRNA with MeAIB uptake and SNAT2 expression assays in primary trophoblasts\",\n      \"pmids\": [\"19589777\", \"19741197\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution experimental structure available\", \"TNF-alpha signaling pathway to SNAT2 (non-JAK/STAT) unresolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The 13-residue extracellular C-terminal tail was shown to be essential for voltage-dependent amino acid translocation without affecting substrate binding, and SNAT2 was identified as the primary L-proline transporter driving embryonic stem cell differentiation.\",\n      \"evidence\": \"C-terminal truncation mutagenesis with electrophysiology; competitive substrate inhibition with SNAT2 vs. non-SNAT2 substrates and differentiation marker assays in ES cells\",\n      \"pmids\": [\"21158741\", \"21346154\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism of voltage sensing by C-terminal tail unknown\", \"How proline uptake triggers differentiation signaling not resolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Estradiol-ERα transcriptional regulation of SNAT2 was characterized, revealing a promoter ERE occupied by an ERα-PARP1-Ku70-GAPDH complex, and betaine was identified as a unique SNAT2 substrate among system A subtypes, linking SNAT2 to osmolyte transport.\",\n      \"evidence\": \"ERE mutagenesis, EMSA/ChIP, mass spec of ERE complex, co-activator siRNA; comparative uptake assays of SNAT1/2/4 with [14C]betaine\",\n      \"pmids\": [\"25056967\", \"24434061\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological significance of GAPDH as transcriptional co-activator unclear\", \"In vivo relevance of betaine transport via SNAT2 not tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Seven N-terminal lysyl residues were identified as the ubiquitination sites controlling SNAT2 protein stability, with substrate occupancy sensed through this tail; in IUGR placentas, increased Nedd4-2 and SNAT2 ubiquitination correlated with reduced membrane SNAT2 and impaired system A transport.\",\n      \"evidence\": \"Lysine-to-alanine mutants, chimeric SNAT2/5 transporters, linoleic acid-induced degradation assays; placental tissue from IUGR patients with ubiquitination and mTOR activity measurements\",\n      \"pmids\": [\"25653282\", \"26374858\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mTOR directly phosphorylates Nedd4-2 to control SNAT2 not shown\", \"Specific lysine residues vs. combinatorial ubiquitination patterns not dissected individually\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Extracellular Na⁺ was shown to stabilize SNAT2 protein independently of substrate, while substrate binding destabilizes SNAT2 through N-terminal lysyl residues — establishing an ion- and substrate-sensing mechanism controlling transporter abundance. Membrane topology was experimentally confirmed as 11 TMDs with intracellular N-terminus, extracellular C-terminus, and three N-glycosylation sites.\",\n      \"evidence\": \"Na⁺ removal experiments with N-terminal lysine mutants and SNAT2-5 chimeras; mPEG-Mal modification, protease cleavage, and glycosylation analysis\",\n      \"pmids\": [\"29467657\", \"29678469\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Na⁺ binding is transduced to N-terminal stability signals mechanistically unknown\", \"No cryo-EM or X-ray structure\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Under hypoxia, HIF-1α displaces ERα at overlapping SNAT2 cis-regulatory elements, making SNAT2 the most frequently hypoxia-induced amino acid transporter in breast cancer and a driver of antiestrogen resistance; CDK7 was identified as required for GCN2-ATF4-SNAT2 adaptive induction.\",\n      \"evidence\": \"ChIP for HIF-1α/ERα, reporter assays, xenograft models with fulvestrant; CDK7 inhibitor THZ-1 with drug-resistant CDK7 rescue and system A transport assays\",\n      \"pmids\": [\"31152137\", \"30857869\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CDK7 phosphorylates ATF4 or acts via Pol II CTD for SNAT2 specifically is unclear\", \"Therapeutic targeting of HIF-1α-SNAT2 axis not validated in clinical samples\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"ChREBP-SMRT was identified as a transcriptional repressor of SNAT2 in response to high carbohydrate intake, binding a ChoRE at −160 bp and thereby integrating carbohydrate and amino acid metabolism at the SNAT2 promoter.\",\n      \"evidence\": \"ChoRE mutagenesis, reporter assays, co-IP of ChREBP-SMRT, in vivo ChIP, high-sucrose diet in rats\",\n      \"pmids\": [\"33225719\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ChREBP repression of SNAT2 contributes to metabolic disease not tested\", \"Interplay between ChREBP and ATF4/ERα on the same promoter not resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Placenta-specific SNAT2 knockdown directly caused fetal growth restriction in vivo, demonstrating that SNAT2-mediated amino acid supply is a rate-limiting determinant of fetal growth.\",\n      \"evidence\": \"Lentiviral shRNA blastocyst transduction for placenta-specific knockdown, fetal weight, [14C]-MeAIB transport, trophoblast membrane fractionation\",\n      \"pmids\": [\"34406367\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SNAT2 deficiency causes specific amino acid imbalances or general depletion in the fetus not resolved\", \"Compensatory transporter expression in placental KD not fully characterized\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cell-type-specific genetic ablation established SNAT2 as essential for osteoblast differentiation and bone formation by providing proline for proline-rich protein synthesis, expanded its substrate repertoire to include sarcosine, and yielded the first selective small-molecule SNAT2 inhibitor effective against cancer cell proliferation.\",\n      \"evidence\": \"Osteoblast-specific and Prrx1Cre KO mice with metabolomic tracing and bone measurement; siRNA with [14C]sarcosine kinetics in PC-3 cells; HTS of 33,934 compounds yielding thiophene-2-carboxamide SNAT2 inhibitor with IC50 ~0.8–3 μM\",\n      \"pmids\": [\"35261338\", \"36213239\", \"36175560\", \"36210829\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo efficacy of SNAT2 inhibitor not tested\", \"Structural basis of inhibitor selectivity over SNAT1 unknown\", \"Whether sarcosine transport is physiologically relevant beyond prostate cancer unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"SLC38A2 was shown to be the key glutamine transporter in cDC1 dendritic cells required for anti-tumor immunity via the FLCN-TFEB pathway, protective against renal medullary ferroptosis via mTORC1, a transcriptional target repressed by XBP1 in the tumor microenvironment, and a mediator of NERP-4 peptide-stimulated insulin secretion in β-cells.\",\n      \"evidence\": \"SLC38A2 KO DCs with FLCN epistasis in tumor models; Slc38a2 KO mice with water restriction and ferroptosis markers; XBP1 promoter binding and T cell functional assays; SNAT2 deletion/inhibition with NERP-4 in islets\",\n      \"pmids\": [\"37407815\", \"36722887\", \"37054944\", \"38071217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which SNAT2-derived glutamine activates FLCN-TFEB not fully elucidated\", \"Whether XBP1-SNAT2 axis is targetable therapeutically not tested\", \"NERP-4 binding site on SNAT2 not identified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"UBE2C-mediated monoubiquitination at Lys59 was shown to inhibit K63-linked polyubiquitination at Lys33, preventing EPN1-dependent endocytosis and increasing SNAT2 membrane levels — a ubiquitin crosstalk mechanism driving glutamine-dependent lymphangiogenesis and metastasis in bladder cancer.\",\n      \"evidence\": \"Site-specific K59/K33 ubiquitin mutants, co-IP, endocytosis assays, glutamine uptake, VEGFC measurement, patient-derived xenograft model\",\n      \"pmids\": [\"38949026\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether UBE2C-SNAT2 ubiquitin crosstalk operates in non-cancer contexts unknown\", \"Relationship between UBE2C monoubiquitination and Nedd4-2 polyubiquitination not integrated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Despite extensive functional characterization, no high-resolution experimental structure of SNAT2 exists, the precise mechanism by which substrate occupancy is communicated through the N-terminal tail to ubiquitination machinery is unresolved, and whether SNAT2's transceptor signaling to mTOR occurs through direct protein interactions or solely through amino acid supply remains an open question.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM or crystal structure\", \"Transceptor signaling mechanism (direct vs. indirect) not distinguished\", \"Full in vivo phenotype of whole-body SNAT2 knockout not reported\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 10, 17, 26, 28, 29, 34, 40, 43, 44, 47]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [15, 24, 35]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 3, 5, 13, 31, 36, 48]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [13]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 10, 17, 26, 28, 29, 34, 40, 43, 44, 47]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 18, 24, 44, 45]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [12, 32, 48]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [3, 4, 13]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [8, 21, 30, 37, 38, 49]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [44, 46]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"NEDD4L\",\n      \"UBE2C\",\n      \"EPN1\",\n      \"ATF4\",\n      \"ESR1\",\n      \"STAT3\",\n      \"HIF1A\",\n      \"MLXIPL\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"SLC38A2 (SNAT2) is a ubiquitously expressed, Na+-coupled neutral amino acid symporter that mediates concentrative uptake of glutamine, alanine, proline, glycine, betaine, and sarcosine with 1:1 Na+:amino acid stoichiometry, and whose transport mechanism depends on conserved Na+-coordinating residues Asn82 (TMD1) and Thr384 (TMD8), a C-terminal domain that governs voltage-dependence, and a histidine (H504) that confers pH sensitivity [PMID:10747860, PMID:18319257, PMID:19589777, PMID:21158741, PMID:16629640]. Plasma membrane abundance of SNAT2 is dynamically regulated by insulin-stimulated exocytosis from trans-Golgi network compartments, Nedd4-2- and RNF5-mediated ubiquitination at N-terminal lysyl residues, and substrate-induced protein stabilization, while its transcription is induced during amino acid deprivation through an ATF4/C/EBP intronic enhancer requiring eIF2α phosphorylation and CDK7 activity, and by IL-6/STAT3, HIF-1α, and ERα pathways [PMID:17050538, PMID:17003038, PMID:25653282, PMID:16445384, PMID:16621798, PMID:30857869, PMID:19741197, PMID:31152137, PMID:25056967]. SNAT2 functions as a transceptor that represses its own transcription when amino acids are sufficient, and by supplying glutamine it sustains mTORC1 signalling, drives leucine exchange, protects renal medullary cells from ferroptosis, enables cDC1-mediated anti-tumour immunity via FLCN/TFEB, provides proline for osteoblast differentiation and bone formation, and supports β-cell insulin secretion in response to the peptide NERP-4 [PMID:17488712, PMID:17429052, PMID:36722887, PMID:37407815, PMID:35261338, PMID:38071217]. Placenta-specific Slc38a2 knockdown in mice directly causes fetal growth restriction, establishing a causal role in nutrient supply to the fetus [PMID:34406367].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Identification of the molecular entity responsible for System A amino acid transport activity resolved a decades-old pharmacological classification into a cloned gene product, establishing SNAT2 as a ubiquitous Na+-dependent neutral amino acid symporter with defined substrate specificity and 1:1 stoichiometry.\",\n      \"evidence\": \"Heterologous expression in mammalian cells and Xenopus oocytes with electrophysiology and substrate profiling\",\n      \"pmids\": [\"10747860\", \"10930503\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural model at the time\", \"Li+ intolerance mechanism unexplained\", \"Tissue-specific functional roles undefined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstration that amino acid deprivation adaptively up-regulates SNAT2 mRNA and protein, with substrate-specific transcriptional feedback, established the paradigm that SNAT2 expression is autoregulated by its own transport substrates.\",\n      \"evidence\": \"Northern blotting, Western blotting, subcellular fractionation, and transport assays in fibroblasts, myotubes, and adipocytes\",\n      \"pmids\": [\"11311116\", \"11172802\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cis-regulatory elements mediating the adaptive response unknown\", \"Whether the protein itself senses substrate or a downstream metabolite is unclear\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Discovery that insulin recruits SNAT2 from intracellular endosomal stores to the plasma membrane via PI3K established a post-translational trafficking mechanism for rapid regulation of amino acid uptake, analogous to but distinct from GLUT4 translocation.\",\n      \"evidence\": \"Cell surface biotinylation, wortmannin and chloroquine inhibition, and transport assays in L6 myotubes and hepatocytes after partial hepatectomy\",\n      \"pmids\": [\"11834730\", \"12054432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SNARE machinery and vesicle identity not defined\", \"Whether insulin and starvation signals converge on the same SNAT2 pool unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Mapping the intronic amino acid response element (bound by ATF4 and C/EBP family members), identifying the 5′-UTR IRES enabling cap-independent translation during eIF2α phosphorylation, and showing Nedd4-2-mediated ubiquitination controls surface SNAT2, together defined the multi-layered transcriptional, translational, and post-translational regulatory logic of the adaptive response.\",\n      \"evidence\": \"ChIP, EMSA, reporter assays, IRES cell-free translation, eIF2α mutant cells, Nedd4-2 RNAi and ubiquitination assays across multiple cell types and Xenopus oocytes\",\n      \"pmids\": [\"16445384\", \"16621798\", \"17003038\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Chromatin-level regulation (histone modifications) not fully dissected\", \"Structural basis of Nedd4-2/SNAT2 interaction unknown\", \"Whether IRES activity is regulated by RNA-binding proteins undetermined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of key residues governing SNAT2 biophysics — H304 separating transport from anion leak, H504 mediating pH-sensitivity through allosteric Na+ binding modulation — established the first structure–function framework for the transporter.\",\n      \"evidence\": \"Site-directed mutagenesis (H304A, H504A) with two-electrode voltage clamp and DEPC chemical modification in Xenopus oocytes\",\n      \"pmids\": [\"17237199\", \"16629640\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure unavailable\", \"Full anion permeation pathway not mapped\", \"Whether anion leak has physiological relevance unclear\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Establishing that SNAT2 functions as a transceptor — repressing its own gene transcription during amino acid sufficiency through a mechanism requiring JNK signalling and a large neutral amino acid sensor — resolved how cells couple transport activity to transcriptional feedback.\",\n      \"evidence\": \"shRNA, transporter chimeras, JNK inhibitors, and intronic reporter assays in L6 myotubes\",\n      \"pmids\": [\"17488712\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the large neutral amino acid sensor upstream of JNK not determined\", \"Whether transceptor signalling involves conformational change or substrate flux is unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstration that SNAT2 inhibition depletes intracellular glutamine and, by extension, leucine (via exchange transport), thereby impairing mTORC1 signalling and increasing proteolysis, placed SNAT2 as a master regulator of amino acid-dependent anabolic signalling in muscle.\",\n      \"evidence\": \"siRNA, MeAIB competitive inhibition, metabolic acidosis, HPLC amino acid profiling, and mTOR pathway Western blotting in L6 cells\",\n      \"pmids\": [\"17429052\", \"18650482\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SNAT2-mTORC1 coupling is direct or solely through amino acid pools unclear\", \"Relative contribution of SNAT2 vs SNAT1 to mTORC1 activation not separated genetically\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of Asn82 in TMD1 as critical for Na+ coordination, with mutagenesis dramatically reducing Na+ affinity and transport, provided the first direct evidence for the Na+ binding site architecture in System A transporters.\",\n      \"evidence\": \"N82A/N82S/Y337A/R374Q mutagenesis with electrophysiology and uptake assays in Xenopus oocytes\",\n      \"pmids\": [\"18319257\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure to validate binding site geometry\", \"Contribution of water molecules to Na+ coordination unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Homology modeling to LeuT/Mhp1 predicted a conserved Na+ site formed by TMD1 and TMD8, validated by the T384A mutation that dramatically lowers Na+ affinity, completing the minimal Na+ binding site model and linking SNAT2 to the LeuT-fold superfamily.\",\n      \"evidence\": \"Homology modeling, T384A mutagenesis, and electrophysiology in Xenopus oocytes\",\n      \"pmids\": [\"19589777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Experimental high-resolution structure still lacking\", \"Substrate binding site residues not systematically identified\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Truncation of 13 C-terminal residues abolished transport at negative potentials while sparing it at positive potentials, establishing the extracellular C-terminal domain as a voltage sensor required for physiological transporter function.\",\n      \"evidence\": \"C-terminal truncation mutagenesis with two-electrode voltage clamp and uptake assays in Xenopus oocytes\",\n      \"pmids\": [\"21158741\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of voltage gating by C-terminal tail not defined\", \"Whether post-translational modifications of C-terminus modulate gating is unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of specific N-terminal lysyl residues as ubiquitination sites mediating substrate-induced stability, and the demonstration that mTOR regulates SNAT2 surface abundance via Nedd4-2 in human IUGR placentae, integrated ubiquitin-dependent trafficking into the physiological regulation of fetal nutrient supply.\",\n      \"evidence\": \"N-terminal lysine-to-alanine mutagenesis, chimeric SNAT2-SNAT5 constructs, IUGR placental fractionation, and ubiquitination assays\",\n      \"pmids\": [\"25653282\", \"26374858\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Deubiquitinase(s) counteracting Nedd4-2 at SNAT2 not identified\", \"Whether mTOR regulates Nedd4-2 directly or through intermediates is unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Experimental determination of SNAT2 membrane topology (11 TMDs, intracellular N-terminus, extracellular C-terminus, three verified N-glycosylation sites) resolved conflicting bioinformatic predictions and anchored the structure–function data to a validated topology model.\",\n      \"evidence\": \"mPEG-Mal chemical modification, protease cleavage, immunofluorescence, and glycosylation analysis\",\n      \"pmids\": [\"29678469\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution 3D structure still unavailable\", \"Role of individual glycosylation sites in trafficking or function not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Discovery that HIF-1α drives SNAT2 transcription under hypoxia via elements overlapping with the ERα-responsive promoter, and that SNAT2 overexpression confers endocrine resistance in breast cancer xenografts, revealed a regulatory switch enabling tumour adaptation.\",\n      \"evidence\": \"ChIP for HIF-1α and ERα, reporter assays, siRNA, and xenograft models\",\n      \"pmids\": [\"31152137\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether HIF-1α and ERα compete or cooperate at the SNAT2 promoter under intermediate oxygen tensions is untested\", \"Therapeutic targeting of SNAT2 in endocrine-resistant breast cancer not explored\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Placenta-specific Slc38a2 knockdown in mice directly caused fetal growth restriction, establishing causality between reduced placental SNAT2 and impaired fetal nutrient supply, beyond the correlative IUGR associations previously observed.\",\n      \"evidence\": \"Lentiviral shRNA blastocyst transduction with placenta-specific knockdown; MeAIB uptake, fetal weight, and viability measurements\",\n      \"pmids\": [\"34406367\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Slc38a2 ablation recapitulates full IUGR syndrome or only weight restriction is unknown\", \"Compensation by SNAT1 or SNAT4 not assessed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Conditional knockout studies established that SLC38A2 cell-autonomously supplies proline for collagen-rich protein synthesis in osteoblasts, directly controlling differentiation and bone formation, revealing a tissue-specific metabolic function beyond glutamine supply.\",\n      \"evidence\": \"Osteoblast- and Prrx1Cre-driven conditional knockouts, [13C]-proline tracing, metabolomics, micro-CT, and histomorphometry\",\n      \"pmids\": [\"35261338\", \"36213239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SNAT2 provides proline to other collagen-producing cell types (e.g., fibroblasts) is untested\", \"Mechanism by which SNAT2-derived proline is preferentially channeled to protein synthesis vs. catabolism is unexplored\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of UBE2C-mediated monoubiquitination at K59 that blocks K63-polyubiquitination at K33, preventing EPN1-mediated endocytosis and stabilizing SNAT2 at the membrane, revealed a ubiquitin-code mechanism controlling transporter surface retention with implications for lymphangiogenesis in bladder cancer.\",\n      \"evidence\": \"Site-specific K59/K33 mutagenesis, ubiquitination assays, surface protein analysis, siRNA, xenograft/PDX models\",\n      \"pmids\": [\"38949026\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether UBE2C–SNAT2 interaction is direct or requires a scaffold E3 ligase is unresolved\", \"Generalizability of ubiquitin-code regulation across tissues not tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstration that cDC1 dendritic cells compete with tumour cells for glutamine via SLC38A2, and that SLC38A2 signals through FLCN/TFEB to license anti-tumour CD8+ T cell priming, established SLC38A2 as a critical metabolic checkpoint in tumour immunity.\",\n      \"evidence\": \"DC-specific SLC38A2 and FLCN knockout mice, in vivo tumour models, CD8+ T cell assays, intratumoral glutamine supplementation\",\n      \"pmids\": [\"37407815\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SLC38A2 signalling in other immune cells (macrophages, CD4+ T cells) similarly controls function is unexplored\", \"Mechanism by which SNAT2-derived glutamine activates FLCN is undefined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Discovery that SLC38A2 protects renal medullary cells from hyperosmolarity-induced ferroptosis via mTORC1 activation, and that the peptide NERP-4 acts through SNAT2 to stimulate β-cell insulin secretion, expanded the physiological repertoire of SNAT2 to stress survival and endocrine function.\",\n      \"evidence\": \"Slc38a2 knockout mice with water restriction, ferroptosis markers, mTORC1 blotting; SNAT2 deletion and pharmacological inhibition with insulin secretion assays in islets and db/db mice\",\n      \"pmids\": [\"36722887\", \"38071217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How NERP-4 mechanistically engages SNAT2 (direct binding vs. indirect modulation) is unresolved\", \"Whether renal SNAT2 loss causes chronic kidney pathology under physiological conditions is untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Despite extensive functional characterization, no high-resolution experimental structure of SNAT2 exists; the structural basis for substrate selectivity, the anion leak pathway, and the transceptor signalling mechanism remain undefined at atomic resolution.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No cryo-EM or crystal structure of SNAT2 or any close SLC38 family member\", \"Transceptor signalling mechanism (conformational vs. flux-dependent) unresolved\", \"Pharmacological inhibitors with clinical utility not yet developed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 7, 10, 18, 24, 28, 34, 43, 46]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 3, 6, 12, 13, 31, 35, 42]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [13]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 13]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [1, 3, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 7, 10, 18, 24, 28, 34, 43, 46]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 15, 16, 44, 47]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [8, 9, 26, 33, 38]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [12, 30, 31, 42]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [44, 45]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [8, 9, 29, 37]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"NEDD4L\",\n      \"RNF5\",\n      \"UBE2C\",\n      \"ATF4\",\n      \"HIF1A\",\n      \"ESR1\",\n      \"EPN1\",\n      \"IGF2BP2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}