{"gene":"GFPT1","run_date":"2026-04-28T18:06:52","timeline":{"discoveries":[{"year":2001,"finding":"GFPT1 encodes a rate-limiting enzyme (glutamine:fructose-6-phosphate amidotransferase) that catalyzes the first step of the hexosamine biosynthesis pathway, converting fructose-6-phosphate and glutamine to glucosamine-6-phosphate; a muscle-selective splice variant (GFAT1Alt/GFAT1-L) containing an 18-amino-acid insert at position 229 displays altered kinetics: ~2-fold higher apparent Km for fructose-6-phosphate and ~5-fold lower Ki for UDP-GlcNAc compared to ubiquitous GFAT1.","method":"Recombinant adenovirus expression in COS-7 cells, enzyme activity and kinetic assays, RT-PCR, cDNA cloning and sequencing","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 1 — reconstituted enzyme activity with kinetic characterization, replicated independently in two labs (PMID 11679416 and PMID 11587069)","pmids":["11679416","11587069"],"is_preprint":false},{"year":2001,"finding":"GFAT1 activity in Drosophila (ortholog Dmel/GFAT1) is subject to feedback inhibition by UDP-N-acetylglucosamine (UDP-GlcNAc) and is phosphorylated and regulated by protein kinase A (PKA); one of the two putative PKA sites is conserved, and PKA-dependent regulation was demonstrated in a yeast total protein extract system.","method":"Recombinant expression in yeast, enzyme activity assays in cell extracts, site conservation analysis","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro enzyme assay; single lab, Drosophila ortholog","pmids":["11716769"],"is_preprint":false},{"year":2016,"finding":"mTORC2 promotes GFPT1 expression by sustaining sufficient levels of glutaminolysis catabolites (particularly α-ketoglutarate) and by enabling nuclear accumulation of the GFPT1 transcriptional regulator Xbp1s; loss of mTORC2 reduces GFPT1 expression and HBP flux.","method":"siRNA knockdown of mTORC2 components, metabolite measurements, nuclear fractionation, reporter assays, metabolomics","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (knockdown, metabolomics, fractionation) in a single rigorous study","pmids":["27570073"],"is_preprint":false},{"year":2017,"finding":"AMPK directly phosphorylates GFPT1 at serine 243 in response to VEGF stimulation or AICAR treatment; this phosphorylation inhibits GFAT1 enzymatic activity, reduces cellular O-GlcNAc levels, and promotes VEGF-induced endothelial sprouting/angiogenesis. A non-phosphorylatable S243A-GFAT1 mutant abolishes these effects.","method":"Label-free phosphoproteomics, siRNA knockdown, site-directed mutagenesis (S243A), GFAT activity assays, O-GlcNAc immunoblotting, in vitro angiogenesis (sprouting) assays","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — phosphoproteomics identification + mutagenesis + enzymatic assay + functional readout in same study","pmids":["28008135"],"is_preprint":false},{"year":2018,"finding":"mTORC2 modulates both the amplitude and duration of GFPT1 Ser-243 phosphorylation during nutrient starvation; Ser-243 phosphorylation promotes GFPT1 protein expression and maintains UDP-GlcNAc production when nutrients are limiting, independently of AMPK in some conditions.","method":"Immunoblotting for pSer243-GFAT1, pharmacological inhibition of glycolysis (2-DG), glutamine deprivation, mTORC2-deficient cells, O-GlcNAcylation measurement","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal perturbations; replicates and extends PMID 28008135 findings","pmids":["30201609"],"is_preprint":false},{"year":2020,"finding":"Full-length crystal structure of human GFAT-1 was solved in complex with various ligands; UDP-GlcNAc directly binds GFAT-1 and inhibits its catalytic activity via feedback inhibition. The longevity-associated G451E variant (corresponding to C. elegans gain-of-function) shows drastically reduced sensitivity to UDP-GlcNAc inhibition; structural and biochemical data indicate the interdomain linker is critical for UDP-GlcNAc-mediated inhibition.","method":"X-ray crystallography (full-length human GFAT-1), enzyme activity assays with UDP-GlcNAc titration, site-directed mutagenesis (G451E), UDP-GlcNAc level measurement in mammalian cells","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structure + in vitro enzyme assay + mutagenesis + cellular validation in single study","pmids":["32019926"],"is_preprint":false},{"year":2021,"finding":"Protein kinase A (PKA) phosphorylates human GFAT-1 at Ser205 with two distinct effects: it lowers baseline GFAT-1 catalytic activity AND abolishes UDP-GlcNAc feedback inhibition, thereby uncoupling the metabolic feedback loop. The R203H variant interferes with both UDP-GlcNAc inhibition and PKA-mediated Ser205 phosphorylation.","method":"Enzyme activity assays, site-directed mutagenesis (R203H, phospho-site variants), UDP-GlcNAc measurements in C. elegans and mammalian cells, structural analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with enzymatic assays and cellular UDP-GlcNAc measurements","pmids":["33846315"],"is_preprint":false},{"year":2013,"finding":"Mutations in GFPT1 cause reduced cell-surface expression of acetylcholine receptor (AChR) at the neuromuscular junction; this is due to reduced steady-state levels of AChR α, δ, and ε subunits (but not β), not altered transcription, mediated through defective N-linked glycosylation (UDP-GlcNAc substrate reduction). siRNA silencing or pharmacological inhibition of GFPT1 enzymatic activity both recapitulate the reduction in surface AChR.","method":"Patient-derived myotube culture, cell-surface AChR quantification, siRNA knockdown, pharmacological GFPT1 inhibition, Western blot for AChR subunits, gene expression analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal loss-of-function methods (patient cells, siRNA, pharmacological) with defined mechanistic readout","pmids":["23569079"],"is_preprint":false},{"year":2018,"finding":"Muscle-specific knockout of Gfpt1 in mice (Gfpt1tm1d/tm1d) causes myasthenia and myopathy with postsynaptic NMJ morphological changes (loss of junctional folds) accompanied by presynaptic alterations and tubular aggregates in muscle, demonstrating that muscle-derived GFPT1 is sufficient and necessary for NMJ integrity and neurotransmission.","method":"Cre/LoxP muscle-specific conditional knockout mouse, electrophysiology, NMJ morphology (electron microscopy, immunofluorescence), grip strength testing, proteomics","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with defined cellular and morphological phenotypes at NMJ","pmids":["29905857"],"is_preprint":false},{"year":2022,"finding":"Under glucose deprivation, GFAT1 interacts with TAB1 in a TAB1-S438 phosphorylation-dependent manner; GFAT1 binding to TAB1 facilitates TTLL5-GFAT1-TAB1 complex formation, and GFAT1's metabolic activity producing glutamate enables TTLL5-mediated TAB1 glutamylation, which recruits p38α MAPK and drives p38 activation to promote autophagy and tumor cell survival.","method":"Co-immunoprecipitation, proximity ligation assay, phospho-mutant constructs (TAB1-S438A), TTLL5 knockdown, autophagy and cell viability assays, mass spectrometry identification of glutamylation","journal":"Cell discovery","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, complex reconstitution elements, mutagenesis, and functional readout in same study","pmids":["35945223"],"is_preprint":false},{"year":2021,"finding":"GFAT1 inhibition reduces N-glycosylation of PD-L1, accelerating its proteasomal degradation; GFAT1 is required for glycosylation-dependent PD-L1 protein stability in lung cancer cells. Loss of GFAT1 activity after IFNγ stimulation enhances T cell activation and NK cell cancer-killing.","method":"Pharmacological GFAT1 inhibition, siRNA knockdown, PD-L1 glycosylation assessment (lectin blot/glycoprotein assay), proteasome inhibitor rescue, T cell and NK cell co-culture functional assays","journal":"Carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal loss-of-function methods with mechanistic glycosylation readout; single lab","pmids":["34270713"],"is_preprint":false},{"year":2016,"finding":"In human keratinocytes, GFAT1 (not GFAT2) is the primary driver of UDP-GlcNAc pool maintenance and hyaluronan synthesis; GFAT1 siRNA silencing reduces UDP-GlcNAc and hyaluronan, while simultaneously inducing compensatory upregulation of glucosamine-6-phosphate deaminases (GNPDAs) and hyaluronan synthase 2 (HAS2), and inhibiting cell migration.","method":"siRNA silencing of GFAT1, GFAT2, GNPDA1, GNPDA2; UDP-GlcNAc quantification; hyaluronan ELISA; gene expression analysis; cell migration assay","journal":"Glycobiology","confidence":"Medium","confidence_rationale":"Tier 2 — systematic siRNA knockdown with multiple pathway readouts; single lab","pmids":["26887390"],"is_preprint":false},{"year":2015,"finding":"A 3'-UTR c.*22C>A mutation in GFPT1 creates a de novo binding site for miR-206*, a muscle-abundant microRNA; this illegitimate miRNA binding reduces GFPT1 protein levels in patient myoblasts, and inhibition of miR-206* with a specific anti-miR rescues GFPT1 expression, establishing a post-transcriptional pathomechanism for CMS.","method":"Luciferase reporter assays (wild-type vs mutant 3'-UTR), anti-miR-206* inhibitor treatment, GFPT1 protein quantification in patient myoblasts, miRNA expression profiling","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — reporter assay + patient cell rescue experiment; multiple orthogonal methods in single study","pmids":["25765662"],"is_preprint":false},{"year":2021,"finding":"GFPT1 is the primary GFAT isoform in cardiomyocytes (not GFAT2); GFPT1 but not GFPT2 knockdown reduces stress-induced protein O-GlcNAcylation in neonatal cardiac preparations. GFAT2 expression is restricted to cardiac fibroblasts, while GFAT1 is expressed in both myocytes and fibroblasts.","method":"Targeted siRNA knockdown of GFPT1 and GFPT2 separately, O-GlcNAcylation Western blot, immunostaining of rodent cardiac preparations, single-cell RNA-seq data analysis, Western blot in human iPSC-derived cardiomyocytes","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — isoform-specific knockdown with O-GlcNAc readout plus cell-type localization; single lab","pmids":["34735873"],"is_preprint":false},{"year":2007,"finding":"In zebrafish, a nonsense mutation in gfpt1 (eartha mutant) specifically disrupts late-stage melanocyte differentiation (melanin production) during regeneration but not development; genetic analysis shows gfpt1 acts cell-autonomously within melanocytes to promote ontogenetic melanocyte darkening, placing gfpt1 in a regeneration-specific differentiation pathway.","method":"Forward genetic screen, positional cloning, melanocyte ablation (MoTP), mosaic analysis (cell autonomy), in situ hybridization","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis/genetic screen with cell-autonomy test; zebrafish ortholog","pmids":["17542649"],"is_preprint":false},{"year":2000,"finding":"Overexpression of GFAT in primary mesangial cells activates the PAI-1 promoter 2-3 fold in a manner dependent on GFAT enzymatic activity (abrogated by GFAT inhibitors azaserine and DON); downstream glucosamine also activates the PAI-1 promoter, and GFAT overexpression increases TGF-β1 and TGF-β receptor mRNAs, establishing GFAT activity as a regulator of fibrogenic gene expression via hexosamine flux.","method":"GFAT overexpression in primary mesangial cells, luciferase reporter assay, pharmacological enzyme inhibition, glucosamine supplementation, RT-PCR for TGF-β mRNAs","journal":"American journal of physiology. Renal physiology","confidence":"Medium","confidence_rationale":"Tier 2 — overexpression plus pharmacological inhibition with reporter assay readout; single lab","pmids":["10997922"],"is_preprint":false},{"year":1997,"finding":"The mouse GFAT promoter lacks a canonical TATA box and is controlled by three Sp1-binding elements in a proximal GC-rich region (~-120 bp upstream of TSS); Sp1 binding was confirmed by DNase I footprinting, EMSA, and site-directed mutagenesis. This proximal region also confers EGF-responsiveness.","method":"Promoter cloning, primer extension, luciferase reporter assays, DNase I footprinting, EMSA, site-directed mutagenesis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 — multiple complementary in vitro methods (footprinting, EMSA, mutagenesis) with functional reporter assay","pmids":["9060444"],"is_preprint":false},{"year":2017,"finding":"Mutations in GFPT1-related LG-CMS are associated with loss of postsynaptic junctional folds, denervation-reinnervation at NMJs, reduced O-glycosylation, and reduced sialylation of transmembrane proteins in extra-junctional areas of patient muscle biopsies, demonstrating that GFPT1 deficiency causes glycosylation defects that structurally compromise all three NMJ components.","method":"Neuromuscular biopsy analysis (EM ultrastructure), immunohistochemistry, glycan analysis (lectin histochemistry), NMJ morphometry from three unrelated patients","journal":"Journal of neurology","confidence":"Medium","confidence_rationale":"Tier 2 — direct patient tissue analysis with multiple glycan and morphological readouts; no in vitro reconstitution","pmids":["28712002"],"is_preprint":false},{"year":2022,"finding":"GFAT1 interacts with PTEN in cervical cancer cells (co-localization by immunofluorescence, confirmed by co-immunoprecipitation) and promotes PTEN ubiquitination and proteasomal degradation; GFPT1 knockdown stabilizes PTEN and inhibits proliferation, which is rescued by PTEN silencing.","method":"Co-immunoprecipitation, immunofluorescence co-localization, GFPT1 overexpression and knockdown, ubiquitination assay, colony formation/Edu/MTT proliferation assays, in vivo xenograft","journal":"Carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP plus functional rescue; single lab","pmids":["36040914"],"is_preprint":false},{"year":2018,"finding":"2-Deoxy-D-glucose treatment causes AMPK-mediated phosphorylation of GFAT1, leading to reduction of total N-glycoproteins and ER stress-associated apoptosis in pancreatic cancer cells; metformin (AMPK activator) synergistically enhances this effect, linking GFAT1 phosphorylation to disruption of protein N-glycosylation.","method":"Proteomics (2DG-treated cells), GFAT1 phosphorylation immunoblotting, lectin-based N-glycoprotein detection, ER stress marker quantification, AMPK inhibitor/activator co-treatment, cell growth assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — proteomic identification plus pharmacological mechanistic follow-up; single lab","pmids":["29753740"],"is_preprint":false}],"current_model":"GFPT1 (GFAT1) is the rate-limiting enzyme of the hexosamine biosynthesis pathway that converts fructose-6-phosphate and glutamine to glucosamine-6-phosphate, ultimately producing UDP-GlcNAc for N- and O-linked glycosylation; its catalytic activity is allosterically inhibited by feedback from UDP-GlcNAc, and this feedback is further tuned by PKA-mediated phosphorylation at Ser205 (which both lowers basal activity and abolishes UDP-GlcNAc inhibition) and by AMPK-mediated phosphorylation at Ser243 (which inhibits activity and promotes angiogenesis); mTORC2 controls GFPT1 expression and the amplitude/duration of Ser243 phosphorylation in response to nutrient status; in skeletal muscle, GFPT1 is essential for glycosylation of neuromuscular junction proteins (including AChR) and its loss causes limb-girdle congenital myasthenic syndrome with tubular aggregates, and it additionally participates in non-canonical signaling by forming a complex with TAB1 and TTLL5 under glucose starvation to drive p38 MAPK activation via TAB1 glutamylation."},"narrative":{"teleology":[{"year":1997,"claim":"Defining the transcriptional control of GFPT1 revealed a TATA-less, Sp1-driven promoter architecture with EGF-responsiveness, establishing how GFPT1 expression is basally maintained and growth-factor regulated.","evidence":"Promoter cloning, DNase I footprinting, EMSA, mutagenesis and reporter assays in mouse cells","pmids":["9060444"],"confidence":"High","gaps":["Human promoter regulation not directly tested","Upstream signaling cascades linking EGF to Sp1 at the GFPT1 locus not delineated"]},{"year":2001,"claim":"Biochemical characterization of GFAT1 and its muscle-selective splice variant (GFAT1-L) established the kinetic basis for tissue-specific HBP regulation, showing that the 18-amino-acid insert alters substrate affinity and tightens UDP-GlcNAc feedback inhibition.","evidence":"Recombinant expression in COS-7 cells, enzymatic kinetics, RT-PCR and cDNA cloning across two independent labs","pmids":["11679416","11587069"],"confidence":"High","gaps":["Physiological significance of the muscle-selective variant in vivo not demonstrated","Structural basis of kinetic differences was unknown until 2020"]},{"year":2001,"claim":"Identification of PKA-dependent regulation and UDP-GlcNAc feedback inhibition of the Drosophila GFAT1 ortholog indicated that cAMP/PKA signaling modulates HBP flux, though the precise phosphorylation site in the human enzyme was not yet mapped.","evidence":"Recombinant expression in yeast, enzyme activity assays with UDP-GlcNAc and PKA, site conservation analysis","pmids":["11716769"],"confidence":"Medium","gaps":["Drosophila ortholog study; direct PKA phosphorylation site on human GFAT1 unresolved at this time","In vivo PKA regulation not demonstrated"]},{"year":2013,"claim":"Demonstrating that GFPT1 loss reduces steady-state levels of AChR subunits via defective N-glycosylation — not transcriptional changes — resolved how HBP deficiency leads to NMJ dysfunction in congenital myasthenic syndrome patients.","evidence":"Patient-derived myotube culture, surface AChR quantification, siRNA and pharmacological GFPT1 inhibition, Western blot","pmids":["23569079"],"confidence":"High","gaps":["Whether all CMS-associated GFPT1 mutations act through the same glycosylation deficiency mechanism","Presynaptic contributions not addressed"]},{"year":2015,"claim":"Discovery that a 3′-UTR mutation creates a de novo miR-206* binding site that suppresses GFPT1 protein revealed a post-transcriptional pathomechanism for CMS, rescuable by anti-miR treatment.","evidence":"Luciferase reporter assay comparing WT vs mutant 3′-UTR, anti-miR-206* rescue in patient myoblasts","pmids":["25765662"],"confidence":"High","gaps":["In vivo rescue not tested","Whether other CMS patients carry similar regulatory mutations"]},{"year":2016,"claim":"Placing GFPT1 expression under mTORC2 control — via Xbp1s nuclear accumulation and glutaminolysis-derived α-ketoglutarate — linked growth-factor/nutrient sensing to HBP transcriptional output.","evidence":"siRNA knockdown of mTORC2 components, metabolomics, nuclear fractionation, reporter assays","pmids":["27570073"],"confidence":"High","gaps":["Whether mTORC2 regulation of GFPT1 is tissue-specific","Direct transcription factor binding to GFPT1 promoter not shown"]},{"year":2017,"claim":"Identification of AMPK-mediated Ser243 phosphorylation as an inhibitory modification of GFAT1 that reduces O-GlcNAcylation and promotes angiogenesis connected metabolic stress sensing to HBP flux control and vascular biology.","evidence":"Label-free phosphoproteomics, S243A mutagenesis, GFAT activity assays, O-GlcNAc blotting, endothelial sprouting assays","pmids":["28008135"],"confidence":"High","gaps":["In vivo angiogenesis phenotype not tested","Whether Ser243 phosphorylation affects protein stability vs. catalysis differently in different tissues"]},{"year":2017,"claim":"Patient biopsy analysis showed that GFPT1-CMS causes glycosylation defects (reduced O-glycosylation and sialylation) extending beyond the NMJ to extra-junctional muscle membrane proteins, broadening the pathological scope beyond AChR.","evidence":"EM ultrastructure, lectin histochemistry, immunohistochemistry on biopsies from three unrelated patients","pmids":["28712002"],"confidence":"Medium","gaps":["Specific glycoprotein substrates affected not individually identified","No reconstitution of glycosylation rescue"]},{"year":2018,"claim":"Muscle-specific Gfpt1 knockout in mice recapitulated CMS with NMJ structural disintegration and tubular aggregates, proving that muscle-intrinsic GFPT1 is necessary and sufficient for NMJ maintenance.","evidence":"Cre/LoxP conditional knockout, electrophysiology, EM, immunofluorescence, grip strength, proteomics","pmids":["29905857"],"confidence":"High","gaps":["Whether presynaptic defects are secondary to postsynaptic glycosylation loss","Rescue by UDP-GlcNAc supplementation not attempted"]},{"year":2018,"claim":"mTORC2 was shown to modulate the amplitude and duration of GFPT1 Ser243 phosphorylation under nutrient stress, integrating growth-factor and energy-sensing pathways at GFPT1 to sustain UDP-GlcNAc production.","evidence":"pSer243-GFAT1 immunoblotting, 2-DG and glutamine deprivation, mTORC2-deficient cells, O-GlcNAcylation measurements","pmids":["30201609"],"confidence":"High","gaps":["Whether mTORC2 acts directly on GFAT1 or indirectly through AMPK modulation","Structural basis for Ser243 effect on activity not resolved"]},{"year":2020,"claim":"The full-length crystal structure of human GFAT1 provided the first atomic-resolution model of UDP-GlcNAc feedback inhibition and revealed that the interdomain linker is the critical structural element mediating allosteric regulation, explaining how the longevity-associated G451E variant escapes feedback.","evidence":"X-ray crystallography with multiple ligand complexes, enzyme activity titrations, G451E mutagenesis, cellular UDP-GlcNAc measurements","pmids":["32019926"],"confidence":"High","gaps":["Structure of the muscle-specific GFAT1-L splice variant not determined","Dynamics of the conformational change during catalytic cycle not captured"]},{"year":2021,"claim":"Mapping PKA phosphorylation to Ser205 of human GFAT1 and showing it simultaneously lowers basal activity and abolishes UDP-GlcNAc feedback inhibition resolved how cAMP signaling uncouples HBP flux from its metabolic feedback loop.","evidence":"Enzyme activity assays, site-directed mutagenesis (S205 and R203H), UDP-GlcNAc measurements in C. elegans and mammalian cells, structural analysis","pmids":["33846315"],"confidence":"High","gaps":["In vivo physiological contexts where PKA-Ser205 regulation predominates not defined","Interplay between Ser205 and Ser243 phosphorylation not addressed"]},{"year":2022,"claim":"Discovery of a non-canonical signaling role for GFAT1 — forming a ternary complex with TAB1 and TTLL5 under glucose starvation to drive TAB1 glutamylation and p38 MAPK activation — established that GFAT1 functions beyond metabolic catalysis as a signaling scaffold.","evidence":"Reciprocal Co-IP, proximity ligation assay, TAB1-S438A mutagenesis, TTLL5 knockdown, mass spectrometry, autophagy and viability assays","pmids":["35945223"],"confidence":"High","gaps":["Whether the GFAT1-TAB1-TTLL5 complex forms in non-cancer cell types","Structural basis of the ternary complex unknown","Relative contribution of catalytic vs. scaffolding functions under starvation not separated"]},{"year":null,"claim":"Key unresolved questions include the structural basis for the GFAT1-L splice variant's altered kinetics, the interplay between Ser205 and Ser243 phosphorylation in vivo, whether the GFAT1-TAB1-TTLL5 signaling axis operates in muscle or other non-cancer tissues, and the potential for therapeutic UDP-GlcNAc supplementation to rescue CMS phenotypes.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of the GFAT1-L muscle variant","Dual phosphorylation crosstalk unexplored","In vivo metabolic rescue of CMS not attempted"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,5,6]},{"term_id":"GO:0016853","term_label":"isomerase activity","supporting_discovery_ids":[0,5]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[7,9]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,3,5,6,11]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[7,10,17]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,4,9]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[7,8,12,17]}],"complexes":["GFAT1-TAB1-TTLL5 complex"],"partners":["TAB1","TTLL5","PTEN","MAPK14"],"other_free_text":[]},"mechanistic_narrative":"GFPT1 encodes glutamine:fructose-6-phosphate amidotransferase (GFAT1), the rate-limiting enzyme of the hexosamine biosynthesis pathway (HBP), catalyzing the conversion of fructose-6-phosphate and glutamine to glucosamine-6-phosphate for UDP-GlcNAc production [PMID:11679416, PMID:32019926]. GFAT1 activity is allosterically inhibited by its end-product UDP-GlcNAc, and this feedback is modulated by PKA phosphorylation at Ser205 (which lowers basal activity while abolishing UDP-GlcNAc inhibition) and by AMPK phosphorylation at Ser243 (which inhibits activity and reduces cellular O-GlcNAcylation), with mTORC2 controlling both GFPT1 expression and the dynamics of Ser243 phosphorylation under nutrient stress [PMID:33846315, PMID:28008135, PMID:27570073, PMID:30201609]. In skeletal muscle, GFPT1 is essential for N-glycosylation of neuromuscular junction (NMJ) proteins including acetylcholine receptor subunits, and loss-of-function mutations cause limb-girdle congenital myasthenic syndrome with tubular aggregates, as demonstrated by patient myotubes and muscle-specific knockout mice [PMID:23569079, PMID:29905857]. Under glucose starvation, GFAT1 engages in a non-canonical signaling role by forming a complex with TAB1 and TTLL5, in which its glutamate-producing activity enables TTLL5-mediated TAB1 glutamylation to activate p38 MAPK and promote autophagy [PMID:35945223]."},"prefetch_data":{"uniprot":{"accession":"Q06210","full_name":"Glutamine--fructose-6-phosphate aminotransferase [isomerizing] 1","aliases":["D-fructose-6-phosphate amidotransferase 1","Glutamine:fructose-6-phosphate amidotransferase 1","GFAT 1","GFAT1","Hexosephosphate aminotransferase 1"],"length_aa":699,"mass_kda":78.8,"function":"Rate-limiting enzyme of the hexosamine biosynthetic pathway (HBP) that catalyzes the formation of glucosamine-6-phosphate from fructose-6-phosphate and glutamine, thereby controlling the flux of glucose into this pathway (PubMed:32019926, PubMed:35229715). Inhibited by UDP-N-acetylglucosamine (UDP-GlcNAc) through a feedback loop (PubMed:32019926, PubMed:35229715). Fine-tunes the metabolic fluctuations of UDP-GlcNAc and its impacts on hyaluronan synthesis during tissue remodeling (PubMed:26887390). 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behavior","url":"https://pubmed.ncbi.nlm.nih.gov/34978387","citation_count":13,"is_preprint":false},{"pmid":"24874596","id":"PMC_24874596","title":"Delayed development of GFA immunoreactivity in the parietal cortex during thyroid hormone deficiency.","date":"1985","source":"International journal of developmental neuroscience : the official journal of the International Society for Developmental Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/24874596","citation_count":13,"is_preprint":false},{"pmid":"99269","id":"PMC_99269","title":"[Disappearance of the gliofibrillar protein (GFA) during the cultivation of glioblastoma cells].","date":"1978","source":"Comptes rendus hebdomadaires des seances de l'Academie des sciences. 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Recherches chirurgicales europeennes","url":"https://pubmed.ncbi.nlm.nih.gov/34986481","citation_count":10,"is_preprint":false},{"pmid":"33438142","id":"PMC_33438142","title":"Novel compound heterozygous variants in the GFPT1 gene leading to rare limb-girdle congenital myasthenic syndrome with rimmed vacuoles.","date":"2021","source":"Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/33438142","citation_count":10,"is_preprint":false},{"pmid":"1992564","id":"PMC_1992564","title":"Combined lead acetate and disulfiram treatment-induced alterations of glial fibrillary acidic protein (GFA) immunoreactive astrocytes in brain smears.","date":"1991","source":"Toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/1992564","citation_count":10,"is_preprint":false},{"pmid":"14988277","id":"PMC_14988277","title":"Scrutiny of the glutamine-fructose-6-phosphate transaminase 1 (GFPT1) locus reveals conserved haplotype block structure not associated with diabetic nephropathy.","date":"2004","source":"Diabetes","url":"https://pubmed.ncbi.nlm.nih.gov/14988277","citation_count":10,"is_preprint":false},{"pmid":"35722419","id":"PMC_35722419","title":"GFAT1 is highly expressed in cancer stem cells of pancreatic cancer.","date":"2022","source":"Annals of translational medicine","url":"https://pubmed.ncbi.nlm.nih.gov/35722419","citation_count":10,"is_preprint":false},{"pmid":"31255525","id":"PMC_31255525","title":"Novel compound heterozygous GFPT1 mutations in a family with limb-girdle myasthenia with tubular aggregates.","date":"2019","source":"Neuromuscular disorders : NMD","url":"https://pubmed.ncbi.nlm.nih.gov/31255525","citation_count":9,"is_preprint":false},{"pmid":"7007903","id":"PMC_7007903","title":"Immunoperoxidase staining of glial fibrillary acidic (GFA) protein polymerized in vitro: an ultramicroscopic study.","date":"1980","source":"Neurochemical research","url":"https://pubmed.ncbi.nlm.nih.gov/7007903","citation_count":9,"is_preprint":false},{"pmid":"6479239","id":"PMC_6479239","title":"Cell-specific domains of glial- and muscle-type intermediate filament proteins. Immunoaffinity chromatography and immunoblotting study of GFA protein and desmin.","date":"1984","source":"Experimental cell research","url":"https://pubmed.ncbi.nlm.nih.gov/6479239","citation_count":9,"is_preprint":false},{"pmid":"36464356","id":"PMC_36464356","title":"Knockdown of GFAT disrupts chitin synthesis in Hyphantria cunea larvae.","date":"2022","source":"Pesticide biochemistry and physiology","url":"https://pubmed.ncbi.nlm.nih.gov/36464356","citation_count":8,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47940,"output_tokens":5500,"usd":0.11316},"stage2":{"model":"claude-opus-4-6","input_tokens":9031,"output_tokens":3477,"usd":0.19812},"total_usd":0.31128,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"GFPT1 encodes a rate-limiting enzyme (glutamine:fructose-6-phosphate amidotransferase) that catalyzes the first step of the hexosamine biosynthesis pathway, converting fructose-6-phosphate and glutamine to glucosamine-6-phosphate; a muscle-selective splice variant (GFAT1Alt/GFAT1-L) containing an 18-amino-acid insert at position 229 displays altered kinetics: ~2-fold higher apparent Km for fructose-6-phosphate and ~5-fold lower Ki for UDP-GlcNAc compared to ubiquitous GFAT1.\",\n      \"method\": \"Recombinant adenovirus expression in COS-7 cells, enzyme activity and kinetic assays, RT-PCR, cDNA cloning and sequencing\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted enzyme activity with kinetic characterization, replicated independently in two labs (PMID 11679416 and PMID 11587069)\",\n      \"pmids\": [\"11679416\", \"11587069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"GFAT1 activity in Drosophila (ortholog Dmel/GFAT1) is subject to feedback inhibition by UDP-N-acetylglucosamine (UDP-GlcNAc) and is phosphorylated and regulated by protein kinase A (PKA); one of the two putative PKA sites is conserved, and PKA-dependent regulation was demonstrated in a yeast total protein extract system.\",\n      \"method\": \"Recombinant expression in yeast, enzyme activity assays in cell extracts, site conservation analysis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzyme assay; single lab, Drosophila ortholog\",\n      \"pmids\": [\"11716769\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"mTORC2 promotes GFPT1 expression by sustaining sufficient levels of glutaminolysis catabolites (particularly α-ketoglutarate) and by enabling nuclear accumulation of the GFPT1 transcriptional regulator Xbp1s; loss of mTORC2 reduces GFPT1 expression and HBP flux.\",\n      \"method\": \"siRNA knockdown of mTORC2 components, metabolite measurements, nuclear fractionation, reporter assays, metabolomics\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (knockdown, metabolomics, fractionation) in a single rigorous study\",\n      \"pmids\": [\"27570073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"AMPK directly phosphorylates GFPT1 at serine 243 in response to VEGF stimulation or AICAR treatment; this phosphorylation inhibits GFAT1 enzymatic activity, reduces cellular O-GlcNAc levels, and promotes VEGF-induced endothelial sprouting/angiogenesis. A non-phosphorylatable S243A-GFAT1 mutant abolishes these effects.\",\n      \"method\": \"Label-free phosphoproteomics, siRNA knockdown, site-directed mutagenesis (S243A), GFAT activity assays, O-GlcNAc immunoblotting, in vitro angiogenesis (sprouting) assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — phosphoproteomics identification + mutagenesis + enzymatic assay + functional readout in same study\",\n      \"pmids\": [\"28008135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"mTORC2 modulates both the amplitude and duration of GFPT1 Ser-243 phosphorylation during nutrient starvation; Ser-243 phosphorylation promotes GFPT1 protein expression and maintains UDP-GlcNAc production when nutrients are limiting, independently of AMPK in some conditions.\",\n      \"method\": \"Immunoblotting for pSer243-GFAT1, pharmacological inhibition of glycolysis (2-DG), glutamine deprivation, mTORC2-deficient cells, O-GlcNAcylation measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal perturbations; replicates and extends PMID 28008135 findings\",\n      \"pmids\": [\"30201609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Full-length crystal structure of human GFAT-1 was solved in complex with various ligands; UDP-GlcNAc directly binds GFAT-1 and inhibits its catalytic activity via feedback inhibition. The longevity-associated G451E variant (corresponding to C. elegans gain-of-function) shows drastically reduced sensitivity to UDP-GlcNAc inhibition; structural and biochemical data indicate the interdomain linker is critical for UDP-GlcNAc-mediated inhibition.\",\n      \"method\": \"X-ray crystallography (full-length human GFAT-1), enzyme activity assays with UDP-GlcNAc titration, site-directed mutagenesis (G451E), UDP-GlcNAc level measurement in mammalian cells\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure + in vitro enzyme assay + mutagenesis + cellular validation in single study\",\n      \"pmids\": [\"32019926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Protein kinase A (PKA) phosphorylates human GFAT-1 at Ser205 with two distinct effects: it lowers baseline GFAT-1 catalytic activity AND abolishes UDP-GlcNAc feedback inhibition, thereby uncoupling the metabolic feedback loop. The R203H variant interferes with both UDP-GlcNAc inhibition and PKA-mediated Ser205 phosphorylation.\",\n      \"method\": \"Enzyme activity assays, site-directed mutagenesis (R203H, phospho-site variants), UDP-GlcNAc measurements in C. elegans and mammalian cells, structural analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with enzymatic assays and cellular UDP-GlcNAc measurements\",\n      \"pmids\": [\"33846315\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Mutations in GFPT1 cause reduced cell-surface expression of acetylcholine receptor (AChR) at the neuromuscular junction; this is due to reduced steady-state levels of AChR α, δ, and ε subunits (but not β), not altered transcription, mediated through defective N-linked glycosylation (UDP-GlcNAc substrate reduction). siRNA silencing or pharmacological inhibition of GFPT1 enzymatic activity both recapitulate the reduction in surface AChR.\",\n      \"method\": \"Patient-derived myotube culture, cell-surface AChR quantification, siRNA knockdown, pharmacological GFPT1 inhibition, Western blot for AChR subunits, gene expression analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal loss-of-function methods (patient cells, siRNA, pharmacological) with defined mechanistic readout\",\n      \"pmids\": [\"23569079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Muscle-specific knockout of Gfpt1 in mice (Gfpt1tm1d/tm1d) causes myasthenia and myopathy with postsynaptic NMJ morphological changes (loss of junctional folds) accompanied by presynaptic alterations and tubular aggregates in muscle, demonstrating that muscle-derived GFPT1 is sufficient and necessary for NMJ integrity and neurotransmission.\",\n      \"method\": \"Cre/LoxP muscle-specific conditional knockout mouse, electrophysiology, NMJ morphology (electron microscopy, immunofluorescence), grip strength testing, proteomics\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with defined cellular and morphological phenotypes at NMJ\",\n      \"pmids\": [\"29905857\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Under glucose deprivation, GFAT1 interacts with TAB1 in a TAB1-S438 phosphorylation-dependent manner; GFAT1 binding to TAB1 facilitates TTLL5-GFAT1-TAB1 complex formation, and GFAT1's metabolic activity producing glutamate enables TTLL5-mediated TAB1 glutamylation, which recruits p38α MAPK and drives p38 activation to promote autophagy and tumor cell survival.\",\n      \"method\": \"Co-immunoprecipitation, proximity ligation assay, phospho-mutant constructs (TAB1-S438A), TTLL5 knockdown, autophagy and cell viability assays, mass spectrometry identification of glutamylation\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, complex reconstitution elements, mutagenesis, and functional readout in same study\",\n      \"pmids\": [\"35945223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GFAT1 inhibition reduces N-glycosylation of PD-L1, accelerating its proteasomal degradation; GFAT1 is required for glycosylation-dependent PD-L1 protein stability in lung cancer cells. Loss of GFAT1 activity after IFNγ stimulation enhances T cell activation and NK cell cancer-killing.\",\n      \"method\": \"Pharmacological GFAT1 inhibition, siRNA knockdown, PD-L1 glycosylation assessment (lectin blot/glycoprotein assay), proteasome inhibitor rescue, T cell and NK cell co-culture functional assays\",\n      \"journal\": \"Carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal loss-of-function methods with mechanistic glycosylation readout; single lab\",\n      \"pmids\": [\"34270713\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In human keratinocytes, GFAT1 (not GFAT2) is the primary driver of UDP-GlcNAc pool maintenance and hyaluronan synthesis; GFAT1 siRNA silencing reduces UDP-GlcNAc and hyaluronan, while simultaneously inducing compensatory upregulation of glucosamine-6-phosphate deaminases (GNPDAs) and hyaluronan synthase 2 (HAS2), and inhibiting cell migration.\",\n      \"method\": \"siRNA silencing of GFAT1, GFAT2, GNPDA1, GNPDA2; UDP-GlcNAc quantification; hyaluronan ELISA; gene expression analysis; cell migration assay\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic siRNA knockdown with multiple pathway readouts; single lab\",\n      \"pmids\": [\"26887390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"A 3'-UTR c.*22C>A mutation in GFPT1 creates a de novo binding site for miR-206*, a muscle-abundant microRNA; this illegitimate miRNA binding reduces GFPT1 protein levels in patient myoblasts, and inhibition of miR-206* with a specific anti-miR rescues GFPT1 expression, establishing a post-transcriptional pathomechanism for CMS.\",\n      \"method\": \"Luciferase reporter assays (wild-type vs mutant 3'-UTR), anti-miR-206* inhibitor treatment, GFPT1 protein quantification in patient myoblasts, miRNA expression profiling\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay + patient cell rescue experiment; multiple orthogonal methods in single study\",\n      \"pmids\": [\"25765662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GFPT1 is the primary GFAT isoform in cardiomyocytes (not GFAT2); GFPT1 but not GFPT2 knockdown reduces stress-induced protein O-GlcNAcylation in neonatal cardiac preparations. GFAT2 expression is restricted to cardiac fibroblasts, while GFAT1 is expressed in both myocytes and fibroblasts.\",\n      \"method\": \"Targeted siRNA knockdown of GFPT1 and GFPT2 separately, O-GlcNAcylation Western blot, immunostaining of rodent cardiac preparations, single-cell RNA-seq data analysis, Western blot in human iPSC-derived cardiomyocytes\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — isoform-specific knockdown with O-GlcNAc readout plus cell-type localization; single lab\",\n      \"pmids\": [\"34735873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"In zebrafish, a nonsense mutation in gfpt1 (eartha mutant) specifically disrupts late-stage melanocyte differentiation (melanin production) during regeneration but not development; genetic analysis shows gfpt1 acts cell-autonomously within melanocytes to promote ontogenetic melanocyte darkening, placing gfpt1 in a regeneration-specific differentiation pathway.\",\n      \"method\": \"Forward genetic screen, positional cloning, melanocyte ablation (MoTP), mosaic analysis (cell autonomy), in situ hybridization\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis/genetic screen with cell-autonomy test; zebrafish ortholog\",\n      \"pmids\": [\"17542649\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Overexpression of GFAT in primary mesangial cells activates the PAI-1 promoter 2-3 fold in a manner dependent on GFAT enzymatic activity (abrogated by GFAT inhibitors azaserine and DON); downstream glucosamine also activates the PAI-1 promoter, and GFAT overexpression increases TGF-β1 and TGF-β receptor mRNAs, establishing GFAT activity as a regulator of fibrogenic gene expression via hexosamine flux.\",\n      \"method\": \"GFAT overexpression in primary mesangial cells, luciferase reporter assay, pharmacological enzyme inhibition, glucosamine supplementation, RT-PCR for TGF-β mRNAs\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — overexpression plus pharmacological inhibition with reporter assay readout; single lab\",\n      \"pmids\": [\"10997922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"The mouse GFAT promoter lacks a canonical TATA box and is controlled by three Sp1-binding elements in a proximal GC-rich region (~-120 bp upstream of TSS); Sp1 binding was confirmed by DNase I footprinting, EMSA, and site-directed mutagenesis. This proximal region also confers EGF-responsiveness.\",\n      \"method\": \"Promoter cloning, primer extension, luciferase reporter assays, DNase I footprinting, EMSA, site-directed mutagenesis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple complementary in vitro methods (footprinting, EMSA, mutagenesis) with functional reporter assay\",\n      \"pmids\": [\"9060444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Mutations in GFPT1-related LG-CMS are associated with loss of postsynaptic junctional folds, denervation-reinnervation at NMJs, reduced O-glycosylation, and reduced sialylation of transmembrane proteins in extra-junctional areas of patient muscle biopsies, demonstrating that GFPT1 deficiency causes glycosylation defects that structurally compromise all three NMJ components.\",\n      \"method\": \"Neuromuscular biopsy analysis (EM ultrastructure), immunohistochemistry, glycan analysis (lectin histochemistry), NMJ morphometry from three unrelated patients\",\n      \"journal\": \"Journal of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct patient tissue analysis with multiple glycan and morphological readouts; no in vitro reconstitution\",\n      \"pmids\": [\"28712002\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GFAT1 interacts with PTEN in cervical cancer cells (co-localization by immunofluorescence, confirmed by co-immunoprecipitation) and promotes PTEN ubiquitination and proteasomal degradation; GFPT1 knockdown stabilizes PTEN and inhibits proliferation, which is rescued by PTEN silencing.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence co-localization, GFPT1 overexpression and knockdown, ubiquitination assay, colony formation/Edu/MTT proliferation assays, in vivo xenograft\",\n      \"journal\": \"Carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP plus functional rescue; single lab\",\n      \"pmids\": [\"36040914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"2-Deoxy-D-glucose treatment causes AMPK-mediated phosphorylation of GFAT1, leading to reduction of total N-glycoproteins and ER stress-associated apoptosis in pancreatic cancer cells; metformin (AMPK activator) synergistically enhances this effect, linking GFAT1 phosphorylation to disruption of protein N-glycosylation.\",\n      \"method\": \"Proteomics (2DG-treated cells), GFAT1 phosphorylation immunoblotting, lectin-based N-glycoprotein detection, ER stress marker quantification, AMPK inhibitor/activator co-treatment, cell growth assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — proteomic identification plus pharmacological mechanistic follow-up; single lab\",\n      \"pmids\": [\"29753740\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GFPT1 (GFAT1) is the rate-limiting enzyme of the hexosamine biosynthesis pathway that converts fructose-6-phosphate and glutamine to glucosamine-6-phosphate, ultimately producing UDP-GlcNAc for N- and O-linked glycosylation; its catalytic activity is allosterically inhibited by feedback from UDP-GlcNAc, and this feedback is further tuned by PKA-mediated phosphorylation at Ser205 (which both lowers basal activity and abolishes UDP-GlcNAc inhibition) and by AMPK-mediated phosphorylation at Ser243 (which inhibits activity and promotes angiogenesis); mTORC2 controls GFPT1 expression and the amplitude/duration of Ser243 phosphorylation in response to nutrient status; in skeletal muscle, GFPT1 is essential for glycosylation of neuromuscular junction proteins (including AChR) and its loss causes limb-girdle congenital myasthenic syndrome with tubular aggregates, and it additionally participates in non-canonical signaling by forming a complex with TAB1 and TTLL5 under glucose starvation to drive p38 MAPK activation via TAB1 glutamylation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GFPT1 encodes glutamine:fructose-6-phosphate amidotransferase (GFAT1), the rate-limiting enzyme of the hexosamine biosynthesis pathway (HBP), catalyzing the conversion of fructose-6-phosphate and glutamine to glucosamine-6-phosphate for UDP-GlcNAc production [PMID:11679416, PMID:32019926]. GFAT1 activity is allosterically inhibited by its end-product UDP-GlcNAc, and this feedback is modulated by PKA phosphorylation at Ser205 (which lowers basal activity while abolishing UDP-GlcNAc inhibition) and by AMPK phosphorylation at Ser243 (which inhibits activity and reduces cellular O-GlcNAcylation), with mTORC2 controlling both GFPT1 expression and the dynamics of Ser243 phosphorylation under nutrient stress [PMID:33846315, PMID:28008135, PMID:27570073, PMID:30201609]. In skeletal muscle, GFPT1 is essential for N-glycosylation of neuromuscular junction (NMJ) proteins including acetylcholine receptor subunits, and loss-of-function mutations cause limb-girdle congenital myasthenic syndrome with tubular aggregates, as demonstrated by patient myotubes and muscle-specific knockout mice [PMID:23569079, PMID:29905857]. Under glucose starvation, GFAT1 engages in a non-canonical signaling role by forming a complex with TAB1 and TTLL5, in which its glutamate-producing activity enables TTLL5-mediated TAB1 glutamylation to activate p38 MAPK and promote autophagy [PMID:35945223].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Defining the transcriptional control of GFPT1 revealed a TATA-less, Sp1-driven promoter architecture with EGF-responsiveness, establishing how GFPT1 expression is basally maintained and growth-factor regulated.\",\n      \"evidence\": \"Promoter cloning, DNase I footprinting, EMSA, mutagenesis and reporter assays in mouse cells\",\n      \"pmids\": [\"9060444\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human promoter regulation not directly tested\", \"Upstream signaling cascades linking EGF to Sp1 at the GFPT1 locus not delineated\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Biochemical characterization of GFAT1 and its muscle-selective splice variant (GFAT1-L) established the kinetic basis for tissue-specific HBP regulation, showing that the 18-amino-acid insert alters substrate affinity and tightens UDP-GlcNAc feedback inhibition.\",\n      \"evidence\": \"Recombinant expression in COS-7 cells, enzymatic kinetics, RT-PCR and cDNA cloning across two independent labs\",\n      \"pmids\": [\"11679416\", \"11587069\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological significance of the muscle-selective variant in vivo not demonstrated\", \"Structural basis of kinetic differences was unknown until 2020\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identification of PKA-dependent regulation and UDP-GlcNAc feedback inhibition of the Drosophila GFAT1 ortholog indicated that cAMP/PKA signaling modulates HBP flux, though the precise phosphorylation site in the human enzyme was not yet mapped.\",\n      \"evidence\": \"Recombinant expression in yeast, enzyme activity assays with UDP-GlcNAc and PKA, site conservation analysis\",\n      \"pmids\": [\"11716769\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Drosophila ortholog study; direct PKA phosphorylation site on human GFAT1 unresolved at this time\", \"In vivo PKA regulation not demonstrated\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Demonstrating that GFPT1 loss reduces steady-state levels of AChR subunits via defective N-glycosylation — not transcriptional changes — resolved how HBP deficiency leads to NMJ dysfunction in congenital myasthenic syndrome patients.\",\n      \"evidence\": \"Patient-derived myotube culture, surface AChR quantification, siRNA and pharmacological GFPT1 inhibition, Western blot\",\n      \"pmids\": [\"23569079\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether all CMS-associated GFPT1 mutations act through the same glycosylation deficiency mechanism\", \"Presynaptic contributions not addressed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Discovery that a 3′-UTR mutation creates a de novo miR-206* binding site that suppresses GFPT1 protein revealed a post-transcriptional pathomechanism for CMS, rescuable by anti-miR treatment.\",\n      \"evidence\": \"Luciferase reporter assay comparing WT vs mutant 3′-UTR, anti-miR-206* rescue in patient myoblasts\",\n      \"pmids\": [\"25765662\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo rescue not tested\", \"Whether other CMS patients carry similar regulatory mutations\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Placing GFPT1 expression under mTORC2 control — via Xbp1s nuclear accumulation and glutaminolysis-derived α-ketoglutarate — linked growth-factor/nutrient sensing to HBP transcriptional output.\",\n      \"evidence\": \"siRNA knockdown of mTORC2 components, metabolomics, nuclear fractionation, reporter assays\",\n      \"pmids\": [\"27570073\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mTORC2 regulation of GFPT1 is tissue-specific\", \"Direct transcription factor binding to GFPT1 promoter not shown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identification of AMPK-mediated Ser243 phosphorylation as an inhibitory modification of GFAT1 that reduces O-GlcNAcylation and promotes angiogenesis connected metabolic stress sensing to HBP flux control and vascular biology.\",\n      \"evidence\": \"Label-free phosphoproteomics, S243A mutagenesis, GFAT activity assays, O-GlcNAc blotting, endothelial sprouting assays\",\n      \"pmids\": [\"28008135\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo angiogenesis phenotype not tested\", \"Whether Ser243 phosphorylation affects protein stability vs. catalysis differently in different tissues\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Patient biopsy analysis showed that GFPT1-CMS causes glycosylation defects (reduced O-glycosylation and sialylation) extending beyond the NMJ to extra-junctional muscle membrane proteins, broadening the pathological scope beyond AChR.\",\n      \"evidence\": \"EM ultrastructure, lectin histochemistry, immunohistochemistry on biopsies from three unrelated patients\",\n      \"pmids\": [\"28712002\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific glycoprotein substrates affected not individually identified\", \"No reconstitution of glycosylation rescue\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Muscle-specific Gfpt1 knockout in mice recapitulated CMS with NMJ structural disintegration and tubular aggregates, proving that muscle-intrinsic GFPT1 is necessary and sufficient for NMJ maintenance.\",\n      \"evidence\": \"Cre/LoxP conditional knockout, electrophysiology, EM, immunofluorescence, grip strength, proteomics\",\n      \"pmids\": [\"29905857\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether presynaptic defects are secondary to postsynaptic glycosylation loss\", \"Rescue by UDP-GlcNAc supplementation not attempted\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"mTORC2 was shown to modulate the amplitude and duration of GFPT1 Ser243 phosphorylation under nutrient stress, integrating growth-factor and energy-sensing pathways at GFPT1 to sustain UDP-GlcNAc production.\",\n      \"evidence\": \"pSer243-GFAT1 immunoblotting, 2-DG and glutamine deprivation, mTORC2-deficient cells, O-GlcNAcylation measurements\",\n      \"pmids\": [\"30201609\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mTORC2 acts directly on GFAT1 or indirectly through AMPK modulation\", \"Structural basis for Ser243 effect on activity not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The full-length crystal structure of human GFAT1 provided the first atomic-resolution model of UDP-GlcNAc feedback inhibition and revealed that the interdomain linker is the critical structural element mediating allosteric regulation, explaining how the longevity-associated G451E variant escapes feedback.\",\n      \"evidence\": \"X-ray crystallography with multiple ligand complexes, enzyme activity titrations, G451E mutagenesis, cellular UDP-GlcNAc measurements\",\n      \"pmids\": [\"32019926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the muscle-specific GFAT1-L splice variant not determined\", \"Dynamics of the conformational change during catalytic cycle not captured\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Mapping PKA phosphorylation to Ser205 of human GFAT1 and showing it simultaneously lowers basal activity and abolishes UDP-GlcNAc feedback inhibition resolved how cAMP signaling uncouples HBP flux from its metabolic feedback loop.\",\n      \"evidence\": \"Enzyme activity assays, site-directed mutagenesis (S205 and R203H), UDP-GlcNAc measurements in C. elegans and mammalian cells, structural analysis\",\n      \"pmids\": [\"33846315\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo physiological contexts where PKA-Ser205 regulation predominates not defined\", \"Interplay between Ser205 and Ser243 phosphorylation not addressed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery of a non-canonical signaling role for GFAT1 — forming a ternary complex with TAB1 and TTLL5 under glucose starvation to drive TAB1 glutamylation and p38 MAPK activation — established that GFAT1 functions beyond metabolic catalysis as a signaling scaffold.\",\n      \"evidence\": \"Reciprocal Co-IP, proximity ligation assay, TAB1-S438A mutagenesis, TTLL5 knockdown, mass spectrometry, autophagy and viability assays\",\n      \"pmids\": [\"35945223\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the GFAT1-TAB1-TTLL5 complex forms in non-cancer cell types\", \"Structural basis of the ternary complex unknown\", \"Relative contribution of catalytic vs. scaffolding functions under starvation not separated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis for the GFAT1-L splice variant's altered kinetics, the interplay between Ser205 and Ser243 phosphorylation in vivo, whether the GFAT1-TAB1-TTLL5 signaling axis operates in muscle or other non-cancer tissues, and the potential for therapeutic UDP-GlcNAc supplementation to rescue CMS phenotypes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of the GFAT1-L muscle variant\", \"Dual phosphorylation crosstalk unexplored\", \"In vivo metabolic rescue of CMS not attempted\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 5, 6]},\n      {\"term_id\": \"GO:0016853\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [7, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 3, 5, 6, 11]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [7, 10, 17]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 4, 9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [7, 8, 12, 17]}\n    ],\n    \"complexes\": [\n      \"GFAT1-TAB1-TTLL5 complex\"\n    ],\n    \"partners\": [\n      \"TAB1\",\n      \"TTLL5\",\n      \"PTEN\",\n      \"MAPK14\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}