{"gene":"SLC6A19","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":2004,"finding":"SLC6A19 encodes the neutral amino acid transporter B0AT1, which functions as a sodium-dependent, chloride-independent neutral amino acid transporter (system B0) expressed primarily in intestinal mucosa and renal proximal tubules; loss-of-function mutations cause Hartnup disorder with defective neutral amino acid transport in epithelia.","method":"Homozygosity mapping, gene cloning, mutation identification in affected families, in vitro transport assays showing reduced neutral amino acid transport by disease-causing mutations","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 1-2 — two independent concurrent papers using cloning + functional transport assays + human genetics, strongly replicated","pmids":["15286787","15286788"],"is_preprint":false},{"year":2005,"finding":"Mouse B0AT1 (Slc6a19) mediates Na+-amino acid co-transport with a 1:1 stoichiometry; all neutral amino acids are substrates but large neutral non-aromatic amino acids are preferred; transport is electrogenic and voltage-dependent; a random binding order model with a positive charge on the ternary [Na+-substrate-transporter] complex is consistent with the kinetic data.","method":"Two-electrode voltage-clamp in Xenopus oocytes, tracer uptake studies, kinetic modeling","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — rigorous electrophysiology with mechanistic modeling, independently confirmed by a second Xenopus study","pmids":["15804236"],"is_preprint":false},{"year":2005,"finding":"B0AT1 transports one Na+ per neutral amino acid via an ordered simultaneous mechanism in which the amino acid binds prior to Na+, followed by simultaneous translocation; Li+ can partially substitute for Na+; Cl- and H+ concentrations influence current magnitude.","method":"Simultaneous electrophysiology and radiolabeled leucine uptake in Xenopus oocytes, steady-state kinetic analysis","journal":"Pflugers Archiv : European journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — simultaneous charge and substrate flux measurement in same cell with rigorous kinetic analysis","pmids":["16133263"],"is_preprint":false},{"year":2009,"finding":"SLC6A19 requires either collectrin (TMEM27) for surface expression in the kidney or angiotensin-converting enzyme 2 (ACE2) for surface expression in the intestine.","method":"Review citing experimental data on ancillary protein requirements for plasma membrane expression","journal":"IUBMB life","confidence":"High","confidence_rationale":"Tier 2 — supported by multiple independent experimental papers cited therein, widely replicated","pmids":["19472175"],"is_preprint":false},{"year":2010,"finding":"SGK1, SGK2, and SGK3 stimulate SLC6A19 transport activity by increasing transporter surface expression (Vmax) without altering substrate affinity; the effect involves stabilization of the transporter in the plasma membrane rather than inhibiting retrieval, and is additive with ACE2 co-expression.","method":"Two-electrode voltage-clamp in Xenopus oocytes, quantitative immunoassay of surface protein, brefeldin A chase experiments, co-expression with active vs. inactive kinase constructs","journal":"Cellular physiology and biochemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (electrophysiology, surface immunoassay, brefeldin A chase) in a single study with kinase-dead controls","pmids":["20511718"],"is_preprint":false},{"year":2011,"finding":"JAK2 (and gain-of-function V617F JAK2) stimulates SLC6A19 activity by increasing transporter maximal transport rate (Vmax) via enhanced carrier insertion into the plasma membrane, without altering substrate affinity.","method":"Two-electrode voltage-clamp in Xenopus oocytes, JAK2 inhibitor AG490, brefeldin A chase, chemiluminescence surface protein quantification, kinase-dead (K882E) JAK2 control","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods in single study with kinase-dead control, single lab","pmids":["21964291"],"is_preprint":false},{"year":2011,"finding":"Mice lacking B0AT1 (Slc6a19) show abolished Na+-dependent neutral amino acid uptake in intestinal and renal brush-border membrane vesicles, reduced body weight, blunted postprandial insulin secretion, reduced mTOR signaling and activated GCN2/ATF4 stress response in intestinal epithelial cells, demonstrating a role in systemic amino acid signaling and body weight regulation.","method":"Slc6a19 knockout mouse analysis, brush-border membrane vesicle transport assays, Western blot of mTOR and GCN2/ATF4 pathway components, insulin measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple defined cellular and physiological phenotypes and mechanistic pathway placement","pmids":["21636576"],"is_preprint":false},{"year":2012,"finding":"B0AT1 forms functional complexes in the intestinal brush-border membrane with the peptidases aminopeptidase N (APN/CD13) and ACE2; APN increases B0AT1 apparent substrate affinity up to 2.5-fold and increases surface expression, with the catalytic site of APN involved in the affinity change, likely by increasing local substrate concentration.","method":"Immunoprecipitation of brush-border membrane proteins, Blue native electrophoresis, Xenopus oocyte functional co-expression, site-directed mutagenesis of APN, peptide competition, in silico modeling","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1-2 — Co-IP plus functional reconstitution in oocytes plus mutagenesis of binding partner in single study","pmids":["22677001"],"is_preprint":false},{"year":2012,"finding":"PKB/Akt stimulates SLC6A19 transport activity by increasing transporter maximal rate via enhanced carrier insertion into the plasma membrane; PIKfyve (phosphatidylinositol-3-phosphate-5-kinase), when activated by PKB/Akt phosphorylation at S318, further augments this effect.","method":"Two-electrode voltage-clamp in Xenopus oocytes, co-expression with active vs. kinase-dead PKB/Akt (T308A/S473A) and PKB/Akt-resistant PIKfyve (S318A), brefeldin A chase","journal":"Cellular physiology and biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — multiple mutant controls and brefeldin A chase, single lab","pmids":["23234856"],"is_preprint":false},{"year":2013,"finding":"Slc6a19 gene expression in the intestine is regulated at three levels: (1) CpG promoter methylation (hypermethylated in crypts, demethylated in villi), (2) histone H3K27Ac modification (active in villi), and (3) transcription factor control where HNF1a and HNF4a activate transcription in villus enterocytes while SOX9 represses it in crypts.","method":"Enterocyte/crypt fractionation, gene expression analysis, chromatin immunoprecipitation, promoter methylation analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal epigenetic and transcription factor methods with cell-type-specific fractionation","pmids":["24121511"],"is_preprint":false},{"year":2014,"finding":"Nimesulide (anti-inflammatory drug) potently inhibits B0AT1 transport activity (IC50 ~23 µM) via a noncompetitive mechanism with respect to glutamine but competitive with respect to Na+; molecular docking suggests it binds an external site causing steric hindrance of the translocation path.","method":"Proteoliposome transport assay with rat kidney B0AT1, inhibition kinetic analysis, molecular docking","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 1-2 — reconstituted proteoliposome assay plus kinetic analysis plus computational docking, single lab","pmids":["24704252"],"is_preprint":false},{"year":2015,"finding":"Collectrin is necessary not only for plasma membrane expression of B0AT1 and B0AT3, but also for their catalytic function; syntaxin 1A and syntaxin 3 inhibit B0AT1 membrane expression by competing with collectrin; mutagenesis of transmembrane domains 1α, 5, and 7 of B0AT3 identified residues critical for collectrin interaction and collectrin-dependent surface expression or catalytic activation.","method":"Monocarboxylate-B0AT1/3 fusion constructs, co-expression in Xenopus oocytes, mutagenesis screening, electrophysiological transport assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — systematic mutagenesis plus functional assay in multiple constructs identifying specific interaction residues","pmids":["26240152"],"is_preprint":false},{"year":2017,"finding":"B0AT1 intestinal surface expression requires ACE2 co-expression and is further enhanced by aminopeptidase N (CD13); B0AT1, ACE2, and CD13 co-localize on the luminal membrane of small intestinal villi; B0AT1 protein expression increases in distal direction along the rat small intestine.","method":"Immunofluorescence co-localization, Western blotting along intestinal segments, ring uptake transport assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2-3 — co-localization confirmed by immunofluorescence with functional transport measurements, single lab","pmids":["28915252"],"is_preprint":false},{"year":2017,"finding":"Benztropine is a competitive inhibitor of B0AT1 (IC50 ~44 µM) and blocks neutral amino acid uptake in inverted intestinal sections; identified through high-throughput screening using a CHO cell line stably expressing collectrin and B0AT1 combined with homology model-based computational docking.","method":"Stable CHO cell line with collectrin + B0AT1, fluorescence membrane depolarization assay, computational docking on B0AT1 homology model, ex vivo intestinal uptake assay","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — cell-based assay plus ex vivo validation plus computational docking, single lab","pmids":["28176326"],"is_preprint":false},{"year":2021,"finding":"The ACE2:B0AT1 heterodimer functional unit (molecular weight ~183.7 kDa) constitutes the physiological transport-active unit within the larger [ACE2:B0AT1]2 dimer-of-heterodimers (~345 kDa) quaternary complex in situ in native enterocyte apical membranes.","method":"Radiation inactivation analysis of purified enterocyte brush-border membrane vesicles using high-energy electron radiation, target size theory","journal":"Function (Oxford, England)","confidence":"Medium","confidence_rationale":"Tier 2 — direct biophysical measurement of functional unit size in native membranes, single lab, novel approach","pmids":["34847569"],"is_preprint":false},{"year":2022,"finding":"Nine Hartnup disease-causing B0AT1 variants (R57C, G93R, R95P, R178Q, L242P, G284R, S303L, D517G, P579L) are retained in the endoplasmic reticulum and fail to traffic to the plasma membrane; variants R178Q and S303L also disrupt ACE2 intracellular trafficking and plasma membrane localization.","method":"Subcellular localization assays, biochemical fractionation, immunofluorescence of B0AT1 variants in cell lines, in silico structural analysis","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2-3 — systematic study of 18 variants with subcellular localization readout and ACE2 co-trafficking effect, single lab","pmids":["40852587"],"is_preprint":false},{"year":2022,"finding":"B0AT1 (Slc6a19) is expressed in mouse preimplantation embryos together with ACE2; B0AT1 knockout reduces litter size and impairs preimplantation embryo development in vitro; B0AT1 is the main contributor to L-proline uptake at the 4-8 cell stage.","method":"Slc6a19 knockout mice, radiolabeled proline uptake competition assays in oocytes and embryos, in vitro embryo culture with developmental staging","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 — KO with specific transport readout and competitive inhibition in embryos, single lab","pmids":["36611813"],"is_preprint":false},{"year":2025,"finding":"SLC6A19 actively transports tryptophan into renal cell carcinoma cells, facilitating de novo NAD+ biosynthesis, which activates SIRT1 deacetylase; SIRT1 then deacetylates H3K27, repressing NF-κB p65 transcription and suppressing epithelial-mesenchymal transition.","method":"SLC6A19 overexpression and knockdown in RCC cell lines and in vivo models, NAD+ measurement, SIRT1 activity assays, H3K27 acetylation ChIP, EMT marker analysis, KLF4 transcription factor analysis","journal":"Oncogenesis","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal mechanistic assays in single lab establishing a novel pathway","pmids":["41203631"],"is_preprint":false}],"current_model":"SLC6A19 encodes B0AT1, a sodium-coupled (1:1 stoichiometry), chloride-independent electrogenic transporter of neutral amino acids at the apical membrane of intestinal enterocytes and renal proximal tubule cells; its trafficking to the plasma membrane is obligatorily dependent on the ancillary proteins ACE2 (intestine) or collectrin/TMEM27 (kidney), which also modulate its catalytic activity, while its surface abundance is further regulated by SGK1-3, PKB/Akt-PIKfyve, and JAK2 kinases that promote transporter insertion into the membrane; in the intestine it forms functional digestive complexes with ACE2 and aminopeptidase N (APN), the latter increasing substrate affinity by concentrating substrate locally; disease-causing missense mutations frequently cause ER retention of B0AT1 with secondary mislocalization of ACE2; beyond epithelial amino acid absorption, B0AT1-mediated tryptophan uptake feeds NAD+ biosynthesis to activate SIRT1-dependent chromatin regulation, and whole-body B0AT1 deficiency impairs mTOR signaling and insulin secretion, linking epithelial amino acid transport to systemic metabolic homeostasis."},"narrative":{"teleology":[{"year":2004,"claim":"Identification of SLC6A19 as the gene encoding the long-sought epithelial system B0 neutral amino acid transporter resolved the molecular basis of Hartnup disorder and established the gene's primary physiological role in intestinal and renal amino acid absorption.","evidence":"Homozygosity mapping and gene cloning in Hartnup families combined with in vitro transport assays of disease-causing mutations","pmids":["15286787","15286788"],"confidence":"High","gaps":["Three-dimensional structure of B0AT1 not determined","Mechanism of tissue-specific expression not addressed","Ancillary protein requirements not yet identified"]},{"year":2005,"claim":"Detailed electrophysiological characterization established the fundamental transport mechanism — 1:1 Na+:amino acid stoichiometry, electrogenic translocation, voltage dependence, and substrate preference for large neutral amino acids — providing the biophysical framework for all subsequent functional studies.","evidence":"Two-electrode voltage-clamp and simultaneous radiolabeled substrate uptake in Xenopus oocytes with kinetic modeling","pmids":["15804236","16133263"],"confidence":"High","gaps":["Structural basis of substrate selectivity unknown","Ion coupling mechanism at the atomic level unresolved"]},{"year":2009,"claim":"Recognition that B0AT1 requires tissue-specific ancillary proteins — collectrin in the kidney and ACE2 in the intestine — for plasma membrane trafficking explained how a single transporter achieves regulated surface expression in two distinct epithelia.","evidence":"Multiple experimental studies showing loss of surface expression in the absence of collectrin or ACE2, consolidated in a review","pmids":["19472175"],"confidence":"High","gaps":["Molecular interface between B0AT1 and ancillary proteins not mapped","Whether ancillary proteins modulate catalytic activity beyond trafficking not resolved"]},{"year":2010,"claim":"Identification of SGK1-3, PKB/Akt–PIKfyve, and JAK2 as kinases that stimulate B0AT1 surface abundance via enhanced membrane insertion revealed a signaling-regulated layer of transporter control beyond the obligatory ancillary protein requirement.","evidence":"Voltage-clamp, surface protein quantification, and brefeldin A chase in Xenopus oocytes co-expressing kinase-active versus kinase-dead constructs","pmids":["20511718","21964291","23234856"],"confidence":"High","gaps":["Direct phosphorylation site(s) on B0AT1 not identified","In vivo relevance of kinase regulation not tested in animal models","Whether kinase effects operate through collectrin/ACE2 or independently unknown"]},{"year":2011,"claim":"The Slc6a19-knockout mouse demonstrated that B0AT1 is essential for intestinal and renal neutral amino acid absorption in vivo and linked transporter loss to blunted postprandial insulin secretion, reduced mTOR signaling, and activated amino acid starvation responses, connecting epithelial transport to systemic metabolic signaling.","evidence":"Slc6a19 knockout mice with brush-border membrane vesicle transport assays, mTOR/GCN2/ATF4 pathway Western blots, and insulin measurements","pmids":["21636576"],"confidence":"High","gaps":["Whether metabolic phenotypes are secondary to intestinal versus renal transport loss not dissected","Contribution of individual amino acids (e.g., tryptophan vs. leucine) to signaling phenotypes not resolved"]},{"year":2012,"claim":"Demonstration that B0AT1 forms a ternary brush-border complex with ACE2 and aminopeptidase N, with APN catalytic activity increasing B0AT1 substrate affinity by local substrate concentration, established a digestive-absorptive supramolecular unit on the enterocyte surface.","evidence":"Co-immunoprecipitation from native brush-border membranes, blue native electrophoresis, Xenopus oocyte co-expression with APN mutagenesis and peptide competition","pmids":["22677001"],"confidence":"High","gaps":["Stoichiometry of the ternary complex not determined","How the complex is assembled during biosynthetic trafficking unknown"]},{"year":2013,"claim":"Multi-level transcriptional control of Slc6a19 by promoter CpG methylation, histone H3K27 acetylation, and opposing transcription factors HNF1α/HNF4α (activating) versus SOX9 (repressing) explained the crypt-to-villus gradient of B0AT1 expression and established the epigenetic logic of its tissue-restricted transcription.","evidence":"Enterocyte/crypt fractionation with ChIP, promoter methylation, and gene expression analysis","pmids":["24121511"],"confidence":"High","gaps":["Whether these regulatory mechanisms are conserved in human intestine not shown","Upstream signals triggering demethylation during enterocyte differentiation unknown"]},{"year":2015,"claim":"Systematic mutagenesis revealed that collectrin is required not only for B0AT1 surface trafficking but also for catalytic activation, and identified transmembrane domains 1α, 5, and 7 as critical interaction interfaces, while syntaxin 1A/3 compete with collectrin to restrict surface expression.","evidence":"Fusion constructs and TM-domain mutagenesis of B0AT1/B0AT3 with electrophysiological assay in Xenopus oocytes","pmids":["26240152"],"confidence":"High","gaps":["High-resolution structural basis of collectrin–B0AT1 interface not determined","Whether syntaxin competition is physiologically regulated unknown"]},{"year":2021,"claim":"Radiation inactivation analysis of native enterocyte membranes demonstrated that the ACE2:B0AT1 heterodimer (~184 kDa) is the minimal functional transport unit within a larger dimer-of-heterodimers quaternary assembly, reconciling structural and functional data.","evidence":"Radiation inactivation of purified brush-border membrane vesicles with target size theory analysis","pmids":["34847569"],"confidence":"Medium","gaps":["Result from a single biophysical approach; independent validation by cryo-EM functional reconstitution lacking","Whether the dimer-of-heterodimers displays cooperativity not tested"]},{"year":2022,"claim":"Characterization of Hartnup-causing missense variants showed that most are retained in the ER, and specific variants (R178Q, S303L) also trap ACE2 intracellularly, revealing that mutant B0AT1 can dominantly impair its trafficking partner.","evidence":"Subcellular localization by immunofluorescence and biochemical fractionation of 18 B0AT1 variants in cell lines","pmids":["40852587"],"confidence":"Medium","gaps":["Whether ACE2 mislocalization contributes to disease phenotype in patients not tested","Rescue by chemical chaperones not explored"]},{"year":2022,"claim":"Detection of B0AT1 and ACE2 in mouse preimplantation embryos and reduced litter size in Slc6a19-knockout mice extended the transporter's known physiological roles beyond adult epithelia to early embryonic development and proline uptake at the 4–8 cell stage.","evidence":"Slc6a19 knockout mice, radiolabeled proline uptake in oocytes and embryos, in vitro embryo culture","pmids":["36611813"],"confidence":"Medium","gaps":["Mechanism by which proline uptake supports preimplantation development not established","Whether human embryos depend on B0AT1 at equivalent stages unknown"]},{"year":2025,"claim":"A tryptophan→NAD+→SIRT1→H3K27 deacetylation axis downstream of B0AT1 transport was delineated in renal cell carcinoma, linking the transporter to chromatin regulation and epithelial-mesenchymal transition suppression, and broadening its function beyond nutrient absorption to metabolic-epigenetic signaling.","evidence":"SLC6A19 overexpression/knockdown in RCC cell lines and xenograft models with NAD+ quantification, SIRT1 activity assays, H3K27Ac ChIP, and EMT marker analysis","pmids":["41203631"],"confidence":"Medium","gaps":["Whether this NAD+–SIRT1 axis operates in normal renal tubular cells not tested","Specificity of B0AT1 versus other tryptophan transporters in this context not established","In vivo validation in non-cancer physiological settings lacking"]},{"year":null,"claim":"Major unresolved questions include the high-resolution structural basis of B0AT1 interaction with collectrin and ACE2 in transport-active states, the identity of direct phosphorylation sites mediating kinase regulation, tissue-specific dissection of intestinal versus renal contributions to systemic metabolic phenotypes, and whether the NAD+–SIRT1 chromatin pathway operates in normal epithelial physiology.","evidence":"","pmids":[],"confidence":"Low","gaps":["No experimental B0AT1 structure in transport cycle intermediates","Direct kinase phosphorylation sites on B0AT1 not mapped","Conditional tissue-specific knockouts not reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,2,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,3,4,7,11,12,14]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[15]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,1,2,6]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[6,17]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[6,17]}],"complexes":["ACE2:B0AT1 heterodimer","ACE2:B0AT1:APN ternary complex","collectrin:B0AT1 complex"],"partners":["ACE2","TMEM27","ANPEP","SGK1","SGK3","PIKFYVE","STX3"],"other_free_text":[]},"mechanistic_narrative":"SLC6A19 encodes B0AT1, the principal apical sodium-dependent neutral amino acid transporter in intestinal enterocytes and renal proximal tubule epithelium, whose loss-of-function mutations cause Hartnup disorder [PMID:15286787, PMID:15286788]. B0AT1 co-transports one Na+ per neutral amino acid in an electrogenic, chloride-independent cycle with a random or ordered binding mechanism, preferring large non-aromatic neutral amino acids [PMID:15804236, PMID:16133263]. Surface expression of B0AT1 is obligatorily dependent on the ancillary proteins ACE2 in the intestine and collectrin (TMEM27) in the kidney, which also modulate catalytic activity; the transporter forms a functional heterodimeric unit with ACE2 that further associates with aminopeptidase N to increase apparent substrate affinity, while kinases SGK1-3 and PKB/Akt–PIKfyve promote membrane insertion [PMID:19472175, PMID:26240152, PMID:22677001, PMID:20511718, PMID:23234856]. Beyond epithelial amino acid absorption, B0AT1-mediated tryptophan uptake feeds de novo NAD+ biosynthesis to activate SIRT1-dependent chromatin regulation, and whole-body B0AT1 deficiency impairs mTOR signaling and insulin secretion, linking apical amino acid transport to systemic metabolic homeostasis [PMID:21636576, PMID:41203631]."},"prefetch_data":{"uniprot":{"accession":"Q695T7","full_name":"Sodium-dependent neutral amino acid transporter B(0)AT1","aliases":["Solute carrier family 6 member 19","System B(0) neutral amino acid transporter AT1"],"length_aa":634,"mass_kda":71.1,"function":"Transporter that mediates resorption of neutral amino acids across the apical membrane of renal and intestinal epithelial cells (PubMed:15286787, PubMed:15286788, PubMed:18424768, PubMed:18484095, PubMed:19185582, PubMed:26240152). This uptake is sodium-dependent and chloride-independent (PubMed:15286787, PubMed:15286788, PubMed:19185582). Requires CLTRN in kidney or ACE2 in intestine for cell surface expression and amino acid transporter activity (PubMed:18424768, PubMed:19185582)","subcellular_location":"Cell membrane; Apical cell membrane","url":"https://www.uniprot.org/uniprotkb/Q695T7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC6A19","classification":"Not Classified","n_dependent_lines":6,"n_total_lines":1208,"dependency_fraction":0.004966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SLC6A19","total_profiled":1310},"omim":[{"mim_id":"610300","title":"SOLUTE CARRIER FAMILY 6 (NEUROTRANSMITTER TRANSPORTER), MEMBER 18; SLC6A18","url":"https://www.omim.org/entry/610300"},{"mim_id":"608893","title":"SOLUTE CARRIER FAMILY 6 (NEUROTRANSMITTER TRANSPORTER), MEMBER 19; SLC6A19","url":"https://www.omim.org/entry/608893"},{"mim_id":"608331","title":"SOLUTE CARRIER FAMILY 36 (PROTON/AMINO ACID SYMPORTER), MEMBER 2; SLC36A2","url":"https://www.omim.org/entry/608331"},{"mim_id":"605616","title":"SOLUTE CARRIER FAMILY 6 (PROLINE IMINO TRANSPORTER), MEMBER 20; SLC6A20","url":"https://www.omim.org/entry/605616"},{"mim_id":"300335","title":"ANGIOTENSIN I-CONVERTING ENZYME 2; ACE2","url":"https://www.omim.org/entry/300335"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":134.2},{"tissue":"kidney","ntpm":82.9}],"url":"https://www.proteinatlas.org/search/SLC6A19"},"hgnc":{"alias_symbol":["B0AT1"],"prev_symbol":[]},"alphafold":{"accession":"Q695T7","domains":[{"cath_id":"-","chopping":"12-611","consensus_level":"medium","plddt":92.1522,"start":12,"end":611}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q695T7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q695T7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q695T7-F1-predicted_aligned_error_v6.png","plddt_mean":90.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC6A19","jax_strain_url":"https://www.jax.org/strain/search?query=SLC6A19"},"sequence":{"accession":"Q695T7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q695T7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q695T7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q695T7"}},"corpus_meta":[{"pmid":"15286787","id":"PMC_15286787","title":"Mutations in SLC6A19, encoding B0AT1, cause Hartnup disorder.","date":"2004","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/15286787","citation_count":210,"is_preprint":false},{"pmid":"15286788","id":"PMC_15286788","title":"Hartnup disorder is caused by mutations in the gene encoding the neutral amino acid transporter SLC6A19.","date":"2004","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/15286788","citation_count":183,"is_preprint":false},{"pmid":"15804236","id":"PMC_15804236","title":"Characterization of mouse amino acid transporter B0AT1 (slc6a19).","date":"2005","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/15804236","citation_count":105,"is_preprint":false},{"pmid":"19472175","id":"PMC_19472175","title":"The role of the neutral amino acid transporter B0AT1 (SLC6A19) in Hartnup disorder and protein nutrition.","date":"2009","source":"IUBMB life","url":"https://pubmed.ncbi.nlm.nih.gov/19472175","citation_count":71,"is_preprint":false},{"pmid":"21636576","id":"PMC_21636576","title":"Impaired nutrient signaling and body weight control in a Na+ neutral amino acid cotransporter (Slc6a19)-deficient mouse.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21636576","citation_count":67,"is_preprint":false},{"pmid":"16133263","id":"PMC_16133263","title":"Steady-state kinetic characterization of the mouse B(0)AT1 sodium-dependent neutral amino acid transporter.","date":"2005","source":"Pflugers Archiv : European journal of physiology","url":"https://pubmed.ncbi.nlm.nih.gov/16133263","citation_count":54,"is_preprint":false},{"pmid":"20511718","id":"PMC_20511718","title":"The serum and glucocorticoid inducible kinases SGK1-3 stimulate the neutral amino acid transporter SLC6A19.","date":"2010","source":"Cellular physiology and biochemistry : international journal of experimental cellular physiology, biochemistry, and pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/20511718","citation_count":52,"is_preprint":false},{"pmid":"22677001","id":"PMC_22677001","title":"Intestinal peptidases form functional complexes with the neutral amino acid transporter B(0)AT1.","date":"2012","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/22677001","citation_count":50,"is_preprint":false},{"pmid":"28915252","id":"PMC_28915252","title":"Expression and regulation of the neutral amino acid transporter B0AT1 in rat small intestine.","date":"2017","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/28915252","citation_count":47,"is_preprint":false},{"pmid":"28176326","id":"PMC_28176326","title":"Identification of novel inhibitors of the amino acid transporter B0 AT1 (SLC6A19), a potential target to induce protein restriction and to treat type 2 diabetes.","date":"2017","source":"British journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/28176326","citation_count":45,"is_preprint":false},{"pmid":"26240152","id":"PMC_26240152","title":"Molecular basis for the interaction of the mammalian amino acid transporters B0AT1 and B0AT3 with their ancillary protein collectrin.","date":"2015","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/26240152","citation_count":40,"is_preprint":false},{"pmid":"20448142","id":"PMC_20448142","title":"Luminal leptin inhibits L-glutamine transport in rat small intestine: involvement of ASCT2 and B0AT1.","date":"2010","source":"American journal of physiology. 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cloning, mutation identification in affected families, in vitro transport assays showing reduced neutral amino acid transport by disease-causing mutations\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — two independent concurrent papers using cloning + functional transport assays + human genetics, strongly replicated\",\n      \"pmids\": [\"15286787\", \"15286788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mouse B0AT1 (Slc6a19) mediates Na+-amino acid co-transport with a 1:1 stoichiometry; all neutral amino acids are substrates but large neutral non-aromatic amino acids are preferred; transport is electrogenic and voltage-dependent; a random binding order model with a positive charge on the ternary [Na+-substrate-transporter] complex is consistent with the kinetic data.\",\n      \"method\": \"Two-electrode voltage-clamp in Xenopus oocytes, tracer uptake studies, kinetic modeling\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — rigorous electrophysiology with mechanistic modeling, independently confirmed by a second Xenopus study\",\n      \"pmids\": [\"15804236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"B0AT1 transports one Na+ per neutral amino acid via an ordered simultaneous mechanism in which the amino acid binds prior to Na+, followed by simultaneous translocation; Li+ can partially substitute for Na+; Cl- and H+ concentrations influence current magnitude.\",\n      \"method\": \"Simultaneous electrophysiology and radiolabeled leucine uptake in Xenopus oocytes, steady-state kinetic analysis\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — simultaneous charge and substrate flux measurement in same cell with rigorous kinetic analysis\",\n      \"pmids\": [\"16133263\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"SLC6A19 requires either collectrin (TMEM27) for surface expression in the kidney or angiotensin-converting enzyme 2 (ACE2) for surface expression in the intestine.\",\n      \"method\": \"Review citing experimental data on ancillary protein requirements for plasma membrane expression\",\n      \"journal\": \"IUBMB life\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — supported by multiple independent experimental papers cited therein, widely replicated\",\n      \"pmids\": [\"19472175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SGK1, SGK2, and SGK3 stimulate SLC6A19 transport activity by increasing transporter surface expression (Vmax) without altering substrate affinity; the effect involves stabilization of the transporter in the plasma membrane rather than inhibiting retrieval, and is additive with ACE2 co-expression.\",\n      \"method\": \"Two-electrode voltage-clamp in Xenopus oocytes, quantitative immunoassay of surface protein, brefeldin A chase experiments, co-expression with active vs. inactive kinase constructs\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (electrophysiology, surface immunoassay, brefeldin A chase) in a single study with kinase-dead controls\",\n      \"pmids\": [\"20511718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"JAK2 (and gain-of-function V617F JAK2) stimulates SLC6A19 activity by increasing transporter maximal transport rate (Vmax) via enhanced carrier insertion into the plasma membrane, without altering substrate affinity.\",\n      \"method\": \"Two-electrode voltage-clamp in Xenopus oocytes, JAK2 inhibitor AG490, brefeldin A chase, chemiluminescence surface protein quantification, kinase-dead (K882E) JAK2 control\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods in single study with kinase-dead control, single lab\",\n      \"pmids\": [\"21964291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Mice lacking B0AT1 (Slc6a19) show abolished Na+-dependent neutral amino acid uptake in intestinal and renal brush-border membrane vesicles, reduced body weight, blunted postprandial insulin secretion, reduced mTOR signaling and activated GCN2/ATF4 stress response in intestinal epithelial cells, demonstrating a role in systemic amino acid signaling and body weight regulation.\",\n      \"method\": \"Slc6a19 knockout mouse analysis, brush-border membrane vesicle transport assays, Western blot of mTOR and GCN2/ATF4 pathway components, insulin measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple defined cellular and physiological phenotypes and mechanistic pathway placement\",\n      \"pmids\": [\"21636576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"B0AT1 forms functional complexes in the intestinal brush-border membrane with the peptidases aminopeptidase N (APN/CD13) and ACE2; APN increases B0AT1 apparent substrate affinity up to 2.5-fold and increases surface expression, with the catalytic site of APN involved in the affinity change, likely by increasing local substrate concentration.\",\n      \"method\": \"Immunoprecipitation of brush-border membrane proteins, Blue native electrophoresis, Xenopus oocyte functional co-expression, site-directed mutagenesis of APN, peptide competition, in silico modeling\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — Co-IP plus functional reconstitution in oocytes plus mutagenesis of binding partner in single study\",\n      \"pmids\": [\"22677001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"PKB/Akt stimulates SLC6A19 transport activity by increasing transporter maximal rate via enhanced carrier insertion into the plasma membrane; PIKfyve (phosphatidylinositol-3-phosphate-5-kinase), when activated by PKB/Akt phosphorylation at S318, further augments this effect.\",\n      \"method\": \"Two-electrode voltage-clamp in Xenopus oocytes, co-expression with active vs. kinase-dead PKB/Akt (T308A/S473A) and PKB/Akt-resistant PIKfyve (S318A), brefeldin A chase\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple mutant controls and brefeldin A chase, single lab\",\n      \"pmids\": [\"23234856\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Slc6a19 gene expression in the intestine is regulated at three levels: (1) CpG promoter methylation (hypermethylated in crypts, demethylated in villi), (2) histone H3K27Ac modification (active in villi), and (3) transcription factor control where HNF1a and HNF4a activate transcription in villus enterocytes while SOX9 represses it in crypts.\",\n      \"method\": \"Enterocyte/crypt fractionation, gene expression analysis, chromatin immunoprecipitation, promoter methylation analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal epigenetic and transcription factor methods with cell-type-specific fractionation\",\n      \"pmids\": [\"24121511\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Nimesulide (anti-inflammatory drug) potently inhibits B0AT1 transport activity (IC50 ~23 µM) via a noncompetitive mechanism with respect to glutamine but competitive with respect to Na+; molecular docking suggests it binds an external site causing steric hindrance of the translocation path.\",\n      \"method\": \"Proteoliposome transport assay with rat kidney B0AT1, inhibition kinetic analysis, molecular docking\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted proteoliposome assay plus kinetic analysis plus computational docking, single lab\",\n      \"pmids\": [\"24704252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Collectrin is necessary not only for plasma membrane expression of B0AT1 and B0AT3, but also for their catalytic function; syntaxin 1A and syntaxin 3 inhibit B0AT1 membrane expression by competing with collectrin; mutagenesis of transmembrane domains 1α, 5, and 7 of B0AT3 identified residues critical for collectrin interaction and collectrin-dependent surface expression or catalytic activation.\",\n      \"method\": \"Monocarboxylate-B0AT1/3 fusion constructs, co-expression in Xenopus oocytes, mutagenesis screening, electrophysiological transport assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — systematic mutagenesis plus functional assay in multiple constructs identifying specific interaction residues\",\n      \"pmids\": [\"26240152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"B0AT1 intestinal surface expression requires ACE2 co-expression and is further enhanced by aminopeptidase N (CD13); B0AT1, ACE2, and CD13 co-localize on the luminal membrane of small intestinal villi; B0AT1 protein expression increases in distal direction along the rat small intestine.\",\n      \"method\": \"Immunofluorescence co-localization, Western blotting along intestinal segments, ring uptake transport assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — co-localization confirmed by immunofluorescence with functional transport measurements, single lab\",\n      \"pmids\": [\"28915252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Benztropine is a competitive inhibitor of B0AT1 (IC50 ~44 µM) and blocks neutral amino acid uptake in inverted intestinal sections; identified through high-throughput screening using a CHO cell line stably expressing collectrin and B0AT1 combined with homology model-based computational docking.\",\n      \"method\": \"Stable CHO cell line with collectrin + B0AT1, fluorescence membrane depolarization assay, computational docking on B0AT1 homology model, ex vivo intestinal uptake assay\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-based assay plus ex vivo validation plus computational docking, single lab\",\n      \"pmids\": [\"28176326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The ACE2:B0AT1 heterodimer functional unit (molecular weight ~183.7 kDa) constitutes the physiological transport-active unit within the larger [ACE2:B0AT1]2 dimer-of-heterodimers (~345 kDa) quaternary complex in situ in native enterocyte apical membranes.\",\n      \"method\": \"Radiation inactivation analysis of purified enterocyte brush-border membrane vesicles using high-energy electron radiation, target size theory\",\n      \"journal\": \"Function (Oxford, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biophysical measurement of functional unit size in native membranes, single lab, novel approach\",\n      \"pmids\": [\"34847569\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Nine Hartnup disease-causing B0AT1 variants (R57C, G93R, R95P, R178Q, L242P, G284R, S303L, D517G, P579L) are retained in the endoplasmic reticulum and fail to traffic to the plasma membrane; variants R178Q and S303L also disrupt ACE2 intracellular trafficking and plasma membrane localization.\",\n      \"method\": \"Subcellular localization assays, biochemical fractionation, immunofluorescence of B0AT1 variants in cell lines, in silico structural analysis\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — systematic study of 18 variants with subcellular localization readout and ACE2 co-trafficking effect, single lab\",\n      \"pmids\": [\"40852587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"B0AT1 (Slc6a19) is expressed in mouse preimplantation embryos together with ACE2; B0AT1 knockout reduces litter size and impairs preimplantation embryo development in vitro; B0AT1 is the main contributor to L-proline uptake at the 4-8 cell stage.\",\n      \"method\": \"Slc6a19 knockout mice, radiolabeled proline uptake competition assays in oocytes and embryos, in vitro embryo culture with developmental staging\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with specific transport readout and competitive inhibition in embryos, single lab\",\n      \"pmids\": [\"36611813\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SLC6A19 actively transports tryptophan into renal cell carcinoma cells, facilitating de novo NAD+ biosynthesis, which activates SIRT1 deacetylase; SIRT1 then deacetylates H3K27, repressing NF-κB p65 transcription and suppressing epithelial-mesenchymal transition.\",\n      \"method\": \"SLC6A19 overexpression and knockdown in RCC cell lines and in vivo models, NAD+ measurement, SIRT1 activity assays, H3K27 acetylation ChIP, EMT marker analysis, KLF4 transcription factor analysis\",\n      \"journal\": \"Oncogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal mechanistic assays in single lab establishing a novel pathway\",\n      \"pmids\": [\"41203631\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC6A19 encodes B0AT1, a sodium-coupled (1:1 stoichiometry), chloride-independent electrogenic transporter of neutral amino acids at the apical membrane of intestinal enterocytes and renal proximal tubule cells; its trafficking to the plasma membrane is obligatorily dependent on the ancillary proteins ACE2 (intestine) or collectrin/TMEM27 (kidney), which also modulate its catalytic activity, while its surface abundance is further regulated by SGK1-3, PKB/Akt-PIKfyve, and JAK2 kinases that promote transporter insertion into the membrane; in the intestine it forms functional digestive complexes with ACE2 and aminopeptidase N (APN), the latter increasing substrate affinity by concentrating substrate locally; disease-causing missense mutations frequently cause ER retention of B0AT1 with secondary mislocalization of ACE2; beyond epithelial amino acid absorption, B0AT1-mediated tryptophan uptake feeds NAD+ biosynthesis to activate SIRT1-dependent chromatin regulation, and whole-body B0AT1 deficiency impairs mTOR signaling and insulin secretion, linking epithelial amino acid transport to systemic metabolic homeostasis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SLC6A19 encodes B0AT1, the principal apical sodium-dependent neutral amino acid transporter in intestinal enterocytes and renal proximal tubule epithelium, whose loss-of-function mutations cause Hartnup disorder [PMID:15286787, PMID:15286788]. B0AT1 co-transports one Na+ per neutral amino acid in an electrogenic, chloride-independent cycle with a random or ordered binding mechanism, preferring large non-aromatic neutral amino acids [PMID:15804236, PMID:16133263]. Surface expression of B0AT1 is obligatorily dependent on the ancillary proteins ACE2 in the intestine and collectrin (TMEM27) in the kidney, which also modulate catalytic activity; the transporter forms a functional heterodimeric unit with ACE2 that further associates with aminopeptidase N to increase apparent substrate affinity, while kinases SGK1-3 and PKB/Akt–PIKfyve promote membrane insertion [PMID:19472175, PMID:26240152, PMID:22677001, PMID:20511718, PMID:23234856]. Beyond epithelial amino acid absorption, B0AT1-mediated tryptophan uptake feeds de novo NAD+ biosynthesis to activate SIRT1-dependent chromatin regulation, and whole-body B0AT1 deficiency impairs mTOR signaling and insulin secretion, linking apical amino acid transport to systemic metabolic homeostasis [PMID:21636576, PMID:41203631].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of SLC6A19 as the gene encoding the long-sought epithelial system B0 neutral amino acid transporter resolved the molecular basis of Hartnup disorder and established the gene's primary physiological role in intestinal and renal amino acid absorption.\",\n      \"evidence\": \"Homozygosity mapping and gene cloning in Hartnup families combined with in vitro transport assays of disease-causing mutations\",\n      \"pmids\": [\"15286787\", \"15286788\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure of B0AT1 not determined\", \"Mechanism of tissue-specific expression not addressed\", \"Ancillary protein requirements not yet identified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Detailed electrophysiological characterization established the fundamental transport mechanism — 1:1 Na+:amino acid stoichiometry, electrogenic translocation, voltage dependence, and substrate preference for large neutral amino acids — providing the biophysical framework for all subsequent functional studies.\",\n      \"evidence\": \"Two-electrode voltage-clamp and simultaneous radiolabeled substrate uptake in Xenopus oocytes with kinetic modeling\",\n      \"pmids\": [\"15804236\", \"16133263\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of substrate selectivity unknown\", \"Ion coupling mechanism at the atomic level unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Recognition that B0AT1 requires tissue-specific ancillary proteins — collectrin in the kidney and ACE2 in the intestine — for plasma membrane trafficking explained how a single transporter achieves regulated surface expression in two distinct epithelia.\",\n      \"evidence\": \"Multiple experimental studies showing loss of surface expression in the absence of collectrin or ACE2, consolidated in a review\",\n      \"pmids\": [\"19472175\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular interface between B0AT1 and ancillary proteins not mapped\", \"Whether ancillary proteins modulate catalytic activity beyond trafficking not resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identification of SGK1-3, PKB/Akt–PIKfyve, and JAK2 as kinases that stimulate B0AT1 surface abundance via enhanced membrane insertion revealed a signaling-regulated layer of transporter control beyond the obligatory ancillary protein requirement.\",\n      \"evidence\": \"Voltage-clamp, surface protein quantification, and brefeldin A chase in Xenopus oocytes co-expressing kinase-active versus kinase-dead constructs\",\n      \"pmids\": [\"20511718\", \"21964291\", \"23234856\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct phosphorylation site(s) on B0AT1 not identified\", \"In vivo relevance of kinase regulation not tested in animal models\", \"Whether kinase effects operate through collectrin/ACE2 or independently unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The Slc6a19-knockout mouse demonstrated that B0AT1 is essential for intestinal and renal neutral amino acid absorption in vivo and linked transporter loss to blunted postprandial insulin secretion, reduced mTOR signaling, and activated amino acid starvation responses, connecting epithelial transport to systemic metabolic signaling.\",\n      \"evidence\": \"Slc6a19 knockout mice with brush-border membrane vesicle transport assays, mTOR/GCN2/ATF4 pathway Western blots, and insulin measurements\",\n      \"pmids\": [\"21636576\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether metabolic phenotypes are secondary to intestinal versus renal transport loss not dissected\", \"Contribution of individual amino acids (e.g., tryptophan vs. leucine) to signaling phenotypes not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstration that B0AT1 forms a ternary brush-border complex with ACE2 and aminopeptidase N, with APN catalytic activity increasing B0AT1 substrate affinity by local substrate concentration, established a digestive-absorptive supramolecular unit on the enterocyte surface.\",\n      \"evidence\": \"Co-immunoprecipitation from native brush-border membranes, blue native electrophoresis, Xenopus oocyte co-expression with APN mutagenesis and peptide competition\",\n      \"pmids\": [\"22677001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of the ternary complex not determined\", \"How the complex is assembled during biosynthetic trafficking unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Multi-level transcriptional control of Slc6a19 by promoter CpG methylation, histone H3K27 acetylation, and opposing transcription factors HNF1α/HNF4α (activating) versus SOX9 (repressing) explained the crypt-to-villus gradient of B0AT1 expression and established the epigenetic logic of its tissue-restricted transcription.\",\n      \"evidence\": \"Enterocyte/crypt fractionation with ChIP, promoter methylation, and gene expression analysis\",\n      \"pmids\": [\"24121511\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether these regulatory mechanisms are conserved in human intestine not shown\", \"Upstream signals triggering demethylation during enterocyte differentiation unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Systematic mutagenesis revealed that collectrin is required not only for B0AT1 surface trafficking but also for catalytic activation, and identified transmembrane domains 1α, 5, and 7 as critical interaction interfaces, while syntaxin 1A/3 compete with collectrin to restrict surface expression.\",\n      \"evidence\": \"Fusion constructs and TM-domain mutagenesis of B0AT1/B0AT3 with electrophysiological assay in Xenopus oocytes\",\n      \"pmids\": [\"26240152\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structural basis of collectrin–B0AT1 interface not determined\", \"Whether syntaxin competition is physiologically regulated unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Radiation inactivation analysis of native enterocyte membranes demonstrated that the ACE2:B0AT1 heterodimer (~184 kDa) is the minimal functional transport unit within a larger dimer-of-heterodimers quaternary assembly, reconciling structural and functional data.\",\n      \"evidence\": \"Radiation inactivation of purified brush-border membrane vesicles with target size theory analysis\",\n      \"pmids\": [\"34847569\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Result from a single biophysical approach; independent validation by cryo-EM functional reconstitution lacking\", \"Whether the dimer-of-heterodimers displays cooperativity not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Characterization of Hartnup-causing missense variants showed that most are retained in the ER, and specific variants (R178Q, S303L) also trap ACE2 intracellularly, revealing that mutant B0AT1 can dominantly impair its trafficking partner.\",\n      \"evidence\": \"Subcellular localization by immunofluorescence and biochemical fractionation of 18 B0AT1 variants in cell lines\",\n      \"pmids\": [\"40852587\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether ACE2 mislocalization contributes to disease phenotype in patients not tested\", \"Rescue by chemical chaperones not explored\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Detection of B0AT1 and ACE2 in mouse preimplantation embryos and reduced litter size in Slc6a19-knockout mice extended the transporter's known physiological roles beyond adult epithelia to early embryonic development and proline uptake at the 4–8 cell stage.\",\n      \"evidence\": \"Slc6a19 knockout mice, radiolabeled proline uptake in oocytes and embryos, in vitro embryo culture\",\n      \"pmids\": [\"36611813\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which proline uptake supports preimplantation development not established\", \"Whether human embryos depend on B0AT1 at equivalent stages unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A tryptophan→NAD+→SIRT1→H3K27 deacetylation axis downstream of B0AT1 transport was delineated in renal cell carcinoma, linking the transporter to chromatin regulation and epithelial-mesenchymal transition suppression, and broadening its function beyond nutrient absorption to metabolic-epigenetic signaling.\",\n      \"evidence\": \"SLC6A19 overexpression/knockdown in RCC cell lines and xenograft models with NAD+ quantification, SIRT1 activity assays, H3K27Ac ChIP, and EMT marker analysis\",\n      \"pmids\": [\"41203631\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether this NAD+–SIRT1 axis operates in normal renal tubular cells not tested\", \"Specificity of B0AT1 versus other tryptophan transporters in this context not established\", \"In vivo validation in non-cancer physiological settings lacking\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Major unresolved questions include the high-resolution structural basis of B0AT1 interaction with collectrin and ACE2 in transport-active states, the identity of direct phosphorylation sites mediating kinase regulation, tissue-specific dissection of intestinal versus renal contributions to systemic metabolic phenotypes, and whether the NAD+–SIRT1 chromatin pathway operates in normal epithelial physiology.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No experimental B0AT1 structure in transport cycle intermediates\", \"Direct kinase phosphorylation sites on B0AT1 not mapped\", \"Conditional tissue-specific knockouts not reported\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 2, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 3, 4, 7, 11, 12, 14]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 2, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [6, 17]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [6, 17]}\n    ],\n    \"complexes\": [\n      \"ACE2:B0AT1 heterodimer\",\n      \"ACE2:B0AT1:APN ternary complex\",\n      \"collectrin:B0AT1 complex\"\n    ],\n    \"partners\": [\n      \"ACE2\",\n      \"TMEM27\",\n      \"ANPEP\",\n      \"SGK1\",\n      \"SGK3\",\n      \"PIKfyve\",\n      \"STX3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}