{"gene":"PTPN4","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":2000,"finding":"PTPN4 (PTPMEG) interacts with glutamate receptor delta2 (GluRdelta2) and NMDA receptor GluRepsilon1 (GluN2A) via its PDZ domain binding to their C-terminal PDZ target sequences, as shown by yeast two-hybrid screening, co-immunoprecipitation in cultured cells and brain tissue.","method":"Yeast two-hybrid screening, co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP in both cultured cells and native brain tissue, replicated across two receptor substrates with defined domain mapping","pmids":["10748123"],"is_preprint":false},{"year":2000,"finding":"PTPN4 enhances Fyn-mediated tyrosine phosphorylation of GluRepsilon1 (GluN2A) in a PTPase activity-dependent manner, suggesting it promotes rather than simply opposes phosphorylation at this substrate.","method":"Cell-based phosphorylation assay with wild-type and catalytic mutant PTPN4","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based assay with PTPase-inactive mutant control, single lab","pmids":["10748123"],"is_preprint":false},{"year":1996,"finding":"PTPN4 is primarily localized to the membrane and cytoskeletal fractions of A172 and COS-7 cells, as determined by subcellular fractionation.","method":"Subcellular fractionation, immunoblotting with polyclonal antibodies","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — fractionation in two cell lines with antibody validation, single lab","pmids":["8910369"],"is_preprint":false},{"year":1996,"finding":"PTPN4 is phosphorylated on serine and threonine residues within the intermediate domain (amino acids 386–503) containing PEST sequences and proline-rich motifs. Proteolytic cleavage by trypsin or calpain in this region activates the phosphatase 4–8 fold.","method":"Recombinant protein phosphorylation analysis, in vitro protease treatment, phosphatase activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical reconstitution with defined cleavage sites and activity measurement, multiple orthogonal methods","pmids":["8910369"],"is_preprint":false},{"year":1996,"finding":"In platelets stimulated with calcium ionophore or thrombin, PTPN4 undergoes calpain-mediated proteolytic activation; pre-treatment with calpeptin (calpain inhibitor) blocks this agonist-induced proteolysis.","method":"Immunoprecipitation of platelet extracts, calpain inhibitor experiment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — physiological activation in native platelets with pharmacological inhibitor control, two different agonists tested","pmids":["8910369"],"is_preprint":false},{"year":1996,"finding":"Overexpression of wild-type PTPN4 in COS-7 cells inhibits cell proliferation, reduces saturation density, and blocks anchorage-independent growth (colony formation in soft agar ~30-fold reduction). The catalytically inactive C→S mutant also inhibits these processes but less potently (~10-fold), indicating both phosphatase-dependent and -independent mechanisms.","method":"Stable COS-7 cell lines overexpressing PTPN4 or C→S mutant; growth curves, soft-agar colony assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — defined cellular phenotype with active-site mutagenesis, single lab","pmids":["8917530"],"is_preprint":false},{"year":2007,"finding":"PTPN4-knockout mice show severe impairment in accelerated rotarod performance, rapid acquisition of delay eyeblink conditioning, and significantly attenuated long-term depression at parallel fiber–Purkinje cell synapses, establishing that PTPN4 tyrosine dephosphorylation is required for motor learning and cerebellar synaptic plasticity.","method":"Knockout mouse behavioral testing (rotarod, eyeblink conditioning), electrophysiology (LTD recordings at PF-PC synapses)","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with multiple orthogonal phenotypic readouts (behavioral + electrophysiological), single lab but rigorous","pmids":["17953619"],"is_preprint":false},{"year":2006,"finding":"Drosophila Ptpmeg (ortholog of vertebrate PTPN3/PTPN4) is required in neurons for proper establishment and maintenance of mushroom body axon projections. Phosphatase activity is essential for both alpha and beta lobe formation; the FERM domain is specifically required for preventing alpha lobe retraction but not beta lobe overextension, indicating domain-specific roles in distinct axon-projection processes.","method":"Drosophila loss-of-function genetics, neuroanatomical analysis, domain-specific rescue experiments","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple mutant alleles, domain-specific rescue, dual phenotypic readouts in well-defined neuron subtypes","pmids":["17138662"],"is_preprint":false},{"year":2011,"finding":"The PDZ domain of PTPN4 binds the C-terminal PDZ-binding motifs of glutamate receptor subunit GluN2A and rabies virus G protein; peptides targeting this domain trigger glioblastoma cell death. Crystal structures of two PTPN4-PDZ/peptide complexes identified the structural determinants of binding, and killing efficiency correlated with peptide affinity for PTPN4-PDZ.","method":"Crystal structure determination, cell death assays with peptide delivery, binding affinity measurements","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus functional cell-death assays with affinity-activity correlation, multiple peptide ligands tested","pmids":["22000519"],"is_preprint":false},{"year":2014,"finding":"The PDZ domain of PTPN4 inhibits its own phosphatase catalytic activity in cis; binding of a PDZ ligand to the PDZ domain is sufficient to release this autoinhibition. The active PTPN4 two-domain construct adopts a compact conformation in solution, and PDZ ligand binding disrupts transient interdomain communication to restore catalytic competence.","method":"Analytical ultracentrifugation, small-angle X-ray scattering, NMR, kinetic phosphatase assays","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal structural and biochemical methods (AUC, SAXS, NMR, kinetics) in one study demonstrating mechanism","pmids":["25158884"],"is_preprint":false},{"year":2015,"finding":"PTPN4 inhibits TRIF-dependent TLR4 signaling by dephosphorylating TRAM (TICAM2) upon TLR4 activation, preventing cytoplasmic translocation of TRAM and its interaction with TRIF, thereby specifically suppressing IRF3 activation and IFN-β production.","method":"Co-immunoprecipitation, phosphorylation assays, IRF3 activation and IFN-β production measurements, TRAM localization assays","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional readouts with defined substrate (TRAM) and pathway placement, single lab","pmids":["25425441"],"is_preprint":false},{"year":2016,"finding":"PTPN4 PDZ domain forms a high-affinity complex with the C-terminus of p38γ MAP kinase. The crystal structure of this complex was solved. Binding of the p38γ C-terminus to the PDZ domain abolishes PTPN4 catalytic autoinhibition and enables efficient dephosphorylation of the p38γ activation loop by PTPN4.","method":"Crystal structure determination, kinetic phosphatase assays, binding affinity measurements, cell death assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus in vitro kinetic assays demonstrating allosteric activation and substrate dephosphorylation, multiple orthogonal methods","pmids":["27246854"],"is_preprint":false},{"year":2017,"finding":"The inter-domain linker connecting the PDZ and phosphatase domains of PTPN4 is required for PDZ-mediated autoinhibition and PDZ-ligand-mediated activation; a conserved hydrophobic patch in the linker mediates interdomain communication, as shown by site-directed mutagenesis affecting phosphatase regulation without altering PDZ ligand binding.","method":"Site-directed mutagenesis of linker residues, kinetic phosphatase assays, comparative sequence analysis","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — mutagenesis plus kinetic assays in single lab with bioinformatic support","pmids":["28801650"],"is_preprint":false},{"year":2008,"finding":"PTPN4 substrate-trapping mutant complexes with and dephosphorylates the ITAMs of the TCR zeta subunit in vitro; substrate-trapping derivative augments basal and TCR-induced NF-κB activation in T cells. However, PTPN4-deficient mice show no alteration in ITAM phosphorylation, TCR signaling, T cell development, or immune responses, indicating functional redundancy in vivo.","method":"Substrate-trapping mutant co-immunoprecipitation, NF-κB reporter assays, PTPN4-KO mouse analysis","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — substrate trapping identifies ITAM as substrate in vitro with functional NF-κB readout, but KO shows no in vivo phenotype indicating redundancy","pmids":["18614237"],"is_preprint":false},{"year":2008,"finding":"PTPN4 single-KO, PTPN4/PTPN3 double-KO, and PTPN4/PTPN3/PTPN13 triple-KO mice all show normal T cell development, TCR signaling, cytokine production, and T cell differentiation, establishing that PTPN4 (and its closest paralogs) are dispensable for TCR signal transduction.","method":"Knockout mouse analysis, TCR signaling assays, cytokine measurements, T cell differentiation assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — rigorous negative result confirmed across three KO genotypes with multiple readouts, single lab","pmids":["19107198"],"is_preprint":false},{"year":2013,"finding":"PTPN4 interacts with CrkI via the SH3 domain of CrkI and a proline-rich region (amino acids 462–468) of PTPN4. Overexpression of PTPN4 reduces CrkI phosphorylation and inhibits CrkI-mediated proliferation and migration; PTPN4 knockdown enhances CrkI-mediated cell growth and motility.","method":"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, co-localization, siRNA knockdown, wound healing assay","journal":"Cellular & molecular biology letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal binding assays plus functional loss-of-function and gain-of-function, single lab","pmids":["23666597"],"is_preprint":false},{"year":2019,"finding":"PTPN4 directly interacts with and dephosphorylates pSTAT3 at Tyr705, suppressing STAT3 transcriptional activity; loss of PTPN4 in colorectal cancer accelerates cell growth and tumor formation in vivo.","method":"Co-immunoprecipitation, phosphorylation assay, STAT3 luciferase reporter, PTPN4 overexpression/knockdown, xenograft tumor model","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus phosphorylation assay plus in vivo xenograft, single lab","pmids":["31025789"],"is_preprint":false},{"year":2018,"finding":"A de novo missense variant in PTPN4 (p.Leu72Ser) located in the FERM domain does not abolish protein expression in neurons but abolishes localization of PTPN4 to dendritic spines in mouse hippocampal neurons, establishing that the FERM domain controls subcellular distribution to synaptic compartments.","method":"Transfection of mutant vs. wild-type PTPN4 in mouse hippocampal neurons, fluorescence imaging of dendritic spines","journal":"Clinical genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct imaging of localization with isogenic mutant comparison, single lab, single variant","pmids":["30238967"],"is_preprint":false},{"year":2020,"finding":"NSPA (neuronal surface P antigen), an RBR-type E3 ubiquitin ligase, ubiquitinates PTPMEG (PTPN4), leading to its proteasomal degradation. Absence of NSPA causes increased PTPMEG levels, which correlates with reduced tyrosine phosphorylation (including GluN2B Tyr1472) and removal of GluN2A and GluN2B from postsynaptic densities.","method":"Cell-based ubiquitination assay, biochemical fractionation of synaptic regions, KO mouse immunoblotting, phosphorylation analysis","journal":"BMC biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based ubiquitination assay plus KO mouse biochemistry, single lab, substrate inferred by correlation","pmids":["33158444"],"is_preprint":false},{"year":2022,"finding":"The PDZ domain of PTPN4 directly binds the C-terminal PDZ-binding motif of HPV16 E6 oncoprotein; crystal structure determination revealed that hydrophobic interactions (key role of Leu158 of E6) and intermolecular hydrogen bonds sustain the complex. Corresponding motifs from HPV18, 31, 33, and 45 bind PTPN4-PDZ with comparable affinities.","method":"X-ray crystallography, isothermal titration calorimetry","journal":"Journal of microbiology (Seoul, Korea)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus ITC thermodynamic binding measurements, multiple HPV genotypes tested","pmids":["35089587"],"is_preprint":false},{"year":2000,"finding":"The mouse testis-enriched phosphatase (TEP), a putative murine homolog of human PTPN4 (94% amino acid identity), is specifically expressed in spermatocytes and spermatids within seminiferous tubules and its recombinant phosphatase domain possesses PTP catalytic activity.","method":"Northern blot, immunohistochemistry, recombinant protein activity assay in E. coli","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro activity assay on recombinant domain, single lab, limited mechanistic follow-up","pmids":["11054567"],"is_preprint":false},{"year":2023,"finding":"MARCH8 E3 ubiquitin ligase promotes PTPN4 protein degradation via ubiquitination; loss of PTPN4 activates STAT3 phosphorylation at Tyr705 and its downstream transcriptional activity, promoting pancreatic cancer growth and invasion.","method":"Ubiquitination assay, Western blotting, Co-IP, overexpression/knockdown functional assays, xenograft in vivo","journal":"Pancreas","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ubiquitination assay identifies MARCH8 as E3 ligase for PTPN4 degradation, consistent with STAT3 phosphorylation readout, single lab","pmids":["37747937"],"is_preprint":false},{"year":2025,"finding":"KPNA5 recognizes nuclear localization signals (NLS) in PTPN4 and mediates its nuclear transport; nuclear PTPN4 inhibits STAT3 phosphorylation and downstream signaling, suppressing ovarian cancer cell proliferation and invasion.","method":"Co-immunoprecipitation, Western blotting, nuclear fractionation, overexpression functional assays, xenograft model","journal":"Cancer medicine","confidence":"Low","confidence_rationale":"Tier 3 / Weak — nuclear localization of PTPN4 implied by functional assays and Co-IP, NLS mapping stated but mechanistic detail limited to single paper","pmids":["40145330"],"is_preprint":false}],"current_model":"PTPN4 (PTPMEG) is a cytoplasmic, FERM-PDZ-phosphatase domain-containing protein tyrosine phosphatase whose catalytic activity is autoinhibited by its own PDZ domain in a linker-dependent compact conformation; PDZ ligand binding (from GluRdelta2, GluN2A/GluN2B, p38γ, or viral proteins) disrupts this interdomain autoinhibition to activate the phosphatase, which then dephosphorylates substrates including the NMDA receptor-associated TRAM adaptor, STAT3-Tyr705, CrkI, p38γ activation loop, and TCR ITAM tyrosines, placing PTPN4 at the intersection of synaptic plasticity (cerebellar LTD and motor learning via Purkinje cell GluR signaling), innate immune regulation (TLR4-TRIF/TRAM pathway), and cell survival; its stability is regulated by calpain-mediated proteolytic activation in platelets and by MARCH8-driven ubiquitin–proteasomal degradation, and its FERM domain controls localization to dendritic spines."},"narrative":{"mechanistic_narrative":"PTPN4 (PTPMEG) is a cytoplasmic FERM–PDZ–phosphatase protein tyrosine phosphatase that operates at the intersection of synaptic plasticity, immune signaling, and cell survival [PMID:10748123, PMID:17953619, PMID:25425441]. Its catalytic activity is held in an autoinhibited state by an intramolecular interaction in which the PDZ domain suppresses the phosphatase domain in cis; the active two-domain protein adopts a compact solution conformation, and binding of a PDZ ligand releases this autoinhibition to restore catalytic competence [PMID:25158884]. This regulation depends on the inter-domain linker, where a conserved hydrophobic patch mediates the interdomain communication required for both autoinhibition and ligand-triggered activation [PMID:28801650]. A range of C-terminal PDZ-binding motifs engage the PTPN4 PDZ domain and can disrupt autoinhibition, including glutamate receptor subunits GluRdelta2 and GluN2A, the p38γ MAP kinase C-terminus, and viral proteins such as rabies virus G protein and HPV16 E6, with structures defining the binding determinants [PMID:10748123, PMID:22000519, PMID:27246854, PMID:35089587]. Once activated, PTPN4 dephosphorylates substrates spanning distinct pathways: it dephosphorylates the p38γ activation loop [PMID:27246854], the TLR4 adaptor TRAM (TICAM2) to suppress TRIF-dependent IRF3 activation and IFN-β production [PMID:25425441], and STAT3 at Tyr705 to restrain STAT3 transcriptional activity, a function lost in colorectal cancer to accelerate tumor growth [PMID:31025789]. In the nervous system, PTPN4 is required for cerebellar long-term depression at parallel fiber–Purkinje cell synapses and for motor learning, and its FERM domain directs localization to dendritic spines [PMID:17953619, PMID:30238967]. PTPN4 activity is also controlled at the protein level: calpain-mediated proteolytic cleavage in the PEST-containing intermediate domain activates the phosphatase several-fold and occurs during platelet activation [PMID:8910369], while the E3 ubiquitin ligases MARCH8 and NSPA drive its ubiquitin–proteasomal degradation [PMID:33158444, PMID:37747937].","teleology":[{"year":1996,"claim":"Established that PTPN4 catalytic activity is post-translationally controlled by proteolysis, defining a regulatory mode beyond constitutive phosphatase function and linking it to physiological agonist signaling.","evidence":"In vitro protease treatment and phosphatase assays on recombinant protein, plus calpain-inhibitor experiments in agonist-stimulated platelets","pmids":["8910369"],"confidence":"High","gaps":["The physiological substrates dephosphorylated by the calpain-activated form in platelets were not identified","Whether proteolytic activation operates in non-platelet contexts was not tested"]},{"year":1996,"claim":"Showed PTPN4 overexpression suppresses proliferation and anchorage-independent growth, providing early evidence for a growth-suppressive role with both catalytic and non-catalytic components.","evidence":"Stable COS-7 lines expressing wild-type or catalytically dead PTPN4; growth curves and soft-agar colony assays","pmids":["8917530"],"confidence":"Medium","gaps":["The substrate(s) mediating growth suppression were not defined","Overexpression in a single cell line does not establish endogenous role"]},{"year":2000,"claim":"Identified the PDZ domain as the device that targets PTPN4 to glutamate receptor subunits, placing the phosphatase physically at synaptic receptor complexes.","evidence":"Yeast two-hybrid, reciprocal Co-IP in cultured cells and brain tissue, with domain mapping; cell-based phosphorylation assay with catalytic mutant","pmids":["10748123"],"confidence":"High","gaps":["The functional consequence of enhanced GluN2A phosphorylation was not mechanistically resolved","Direct receptor dephosphorylation in vivo was not demonstrated"]},{"year":2006,"claim":"Demonstrated through the Drosophila ortholog that distinct PTPN4 domains have separable roles in neuronal axon projection, with catalytic activity and the FERM domain controlling different processes.","evidence":"Drosophila loss-of-function genetics with domain-specific rescue and neuroanatomical analysis of mushroom body lobes","pmids":["17138662"],"confidence":"High","gaps":["Substrates underlying axon-projection phenotypes were not identified","Direct extrapolation to vertebrate PTPN4 not established"]},{"year":2007,"claim":"Established a definitive in vivo neural function: PTPN4 is required for cerebellar synaptic plasticity and motor learning.","evidence":"PTPN4-knockout mouse behavioral testing (rotarod, eyeblink conditioning) and electrophysiology of PF–PC LTD","pmids":["17953619"],"confidence":"High","gaps":["The synaptic substrate dephosphorylated to enable LTD was not identified","Molecular link between phosphatase activity and LTD machinery left open"]},{"year":2008,"claim":"Tested a candidate immune function (TCR ζ ITAM dephosphorylation) and found in vitro substrate engagement but no in vivo requirement, revealing functional redundancy among PTPN4 and its paralogs in T cells.","evidence":"Substrate-trapping Co-IP and NF-κB reporter assays contrasted with PTPN4 single, double and triple KO mouse TCR/cytokine analyses","pmids":["18614237","19107198"],"confidence":"Medium","gaps":["Redundant phosphatase(s) compensating in vivo not identified","In vitro ITAM trapping does not establish physiological substrate"]},{"year":2011,"claim":"Provided the structural basis of PDZ-ligand recognition and showed that peptide ligands targeting the PDZ domain trigger glioblastoma cell death, linking ligand binding to pro-apoptotic outcomes.","evidence":"Crystal structures of PTPN4-PDZ/peptide complexes (GluN2A, rabies G) with affinity measurements and cell-death assays correlating killing with affinity","pmids":["22000519"],"confidence":"High","gaps":["The downstream signaling between PDZ occupancy and cell death was not mapped","Endogenous physiological ligand competition not addressed"]},{"year":2014,"claim":"Resolved the core regulatory mechanism: the PDZ domain autoinhibits the phosphatase in cis and PDZ-ligand binding relieves this inhibition.","evidence":"AUC, SAXS, NMR and kinetic phosphatase assays on the two-domain construct","pmids":["25158884"],"confidence":"High","gaps":["Structural snapshot of the autoinhibited interdomain interface not directly visualized","Whether all PDZ ligands activate to equivalent extents not quantified here"]},{"year":2015,"claim":"Connected the activated phosphatase to innate immune control by identifying TRAM as a substrate whose dephosphorylation restrains TLR4/TRIF-driven IFN-β.","evidence":"Co-IP, phosphorylation assays, TRAM localization, and IRF3/IFN-β readouts","pmids":["25425441"],"confidence":"Medium","gaps":["The TRAM phosphosite(s) targeted were not pinpointed","Single-lab pathway placement without in vivo confirmation"]},{"year":2016,"claim":"Linked PDZ-ligand activation to substrate turnover by showing p38γ both binds the PDZ domain to relieve autoinhibition and is itself dephosphorylated on its activation loop.","evidence":"Crystal structure of the PTPN4-PDZ/p38γ complex with kinetic phosphatase assays and cell-death readouts","pmids":["27246854"],"confidence":"High","gaps":["Cellular consequences of p38γ dephosphorylation by PTPN4 not fully traced","Whether p38γ acts as activator, substrate, or both in vivo not resolved"]},{"year":2017,"claim":"Pinpointed the inter-domain linker, and a conserved hydrophobic patch within it, as the structural element required for both autoinhibition and ligand-mediated activation.","evidence":"Site-directed mutagenesis of linker residues with kinetic assays and sequence analysis","pmids":["28801650"],"confidence":"Medium","gaps":["Atomic-resolution view of how the linker couples PDZ occupancy to the active site not obtained","Single-lab mutagenesis data"]},{"year":2018,"claim":"Assigned the FERM domain a localization function in mammalian neurons, showing a disease-associated variant abolishes dendritic spine targeting without affecting expression.","evidence":"Transfection of wild-type vs p.Leu72Ser PTPN4 in mouse hippocampal neurons with spine imaging","pmids":["30238967"],"confidence":"Medium","gaps":["The FERM-domain binding partner mediating spine targeting was not identified","Single variant in a single study"]},{"year":2019,"claim":"Identified STAT3-Tyr705 as a PTPN4 substrate and established a tumor-suppressive role in colorectal cancer through STAT3 restraint.","evidence":"Co-IP, phosphorylation assay, STAT3 reporter, overexpression/knockdown, and xenograft model","pmids":["31025789"],"confidence":"Medium","gaps":["Whether STAT3 dephosphorylation requires PDZ-ligand activation was not addressed","Single-lab tumor model"]},{"year":2020,"claim":"Showed PTPN4 protein levels are controlled by E3-ligase-driven degradation (NSPA), coupling PTPN4 abundance to synaptic receptor phosphorylation and postsynaptic density composition.","evidence":"Cell-based ubiquitination assay, synaptic fractionation, and KO mouse phosphorylation analysis","pmids":["33158444"],"confidence":"Medium","gaps":["The GluN2B substrate link was inferred by correlation, not direct dephosphorylation","Direct NSPA–PTPN4 ubiquitination site not mapped"]},{"year":2021,"claim":"Defined the structural basis for viral hijacking of PTPN4 by showing HPV E6 oncoproteins from multiple genotypes engage the PDZ domain.","evidence":"X-ray crystallography of the PTPN4-PDZ/HPV16 E6 complex and ITC across HPV genotypes","pmids":["35089587"],"confidence":"High","gaps":["The functional consequence of E6 binding on PTPN4 activity or stability was not determined","Cellular impact in HPV-infected cells not tested"]},{"year":2023,"claim":"Extended degradative control to MARCH8 and reinforced the PTPN4–STAT3 tumor-suppressor axis in pancreatic cancer.","evidence":"Ubiquitination assays, Co-IP, overexpression/knockdown and xenograft functional assays","pmids":["37747937"],"confidence":"Medium","gaps":["MARCH8 ubiquitination site on PTPN4 not mapped","Relationship between MARCH8- and NSPA-mediated degradation not reconciled"]},{"year":2025,"claim":"Proposed a nuclear pool of PTPN4 imported by KPNA5 that suppresses STAT3 signaling in ovarian cancer.","evidence":"Co-IP, nuclear fractionation, overexpression functional assays, and xenograft model","pmids":["40145330"],"confidence":"Low","gaps":["Nuclear localization inferred from functional assays with limited NLS mechanistic detail in a single paper","Whether nuclear vs cytoplasmic STAT3 dephosphorylation predominates not resolved"]},{"year":null,"claim":"How the diverse activating PDZ ligands, proteolytic activation, and E3-ligase-driven degradation are integrated to set PTPN4 activity toward specific substrates in a given cell type remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No unified model linking conformational activation to substrate selection across tissues","Atomic structure of the full-length autoinhibited enzyme lacking","Physiological hierarchy among synaptic, immune, and STAT3 substrates undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[3,10,11,16]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[3,20]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[9,12]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[2]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[6,17]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[10]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[11,16]}],"complexes":[],"partners":["GRID2","GRIN2A","MAPK12","TICAM2","STAT3","CRK","MARCHF8","RNF31"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P29074","full_name":"Tyrosine-protein phosphatase non-receptor type 4","aliases":["Protein-tyrosine phosphatase MEG1","MEG","PTPase-MEG1"],"length_aa":926,"mass_kda":105.9,"function":"Phosphatase that plays a role in immunity, learning, synaptic plasticity or cell homeostasis (PubMed:25825441, PubMed:27246854). Regulates neuronal cell homeostasis by protecting neurons against apoptosis (PubMed:20086240). Negatively regulates TLR4-induced interferon beta production by dephosphorylating adapter TICAM2 and inhibiting subsequent TRAM-TRIF interaction (PubMed:25825441). Also dephosphorylates the immunoreceptor tyrosine-based activation motifs/ITAMs of the TCR zeta subunit and thereby negatively regulates TCR-mediated signaling pathway (By similarity). May act at junctions between the membrane and the cytoskeleton","subcellular_location":"Cell membrane; Cytoplasm, cytoskeleton; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/P29074/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PTPN4","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PTPN4","total_profiled":1310},"omim":[{"mim_id":"611730","title":"ERYTHROCYTE MEMBRANE PROTEIN BAND 4.1-LIKE 5; EPB41L5","url":"https://www.omim.org/entry/611730"},{"mim_id":"610340","title":"ERYTHROCYTE MEMBRANE PROTEIN BAND 4.1-LIKE 4B; EPB41L4B","url":"https://www.omim.org/entry/610340"},{"mim_id":"603271","title":"PROTEIN-TYROSINE PHOSPHATASE, NONRECEPTOR-TYPE, 21; PTPN21","url":"https://www.omim.org/entry/603271"},{"mim_id":"603155","title":"PROTEIN-TYROSINE PHOSPHATASE, NONRECEPTOR-TYPE, 14; PTPN14","url":"https://www.omim.org/entry/603155"},{"mim_id":"176878","title":"PROTEIN-TYROSINE PHOSPHATASE, NONRECEPTOR-TYPE, 4; PTPN4","url":"https://www.omim.org/entry/176878"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PTPN4"},"hgnc":{"alias_symbol":["PTPMEG"],"prev_symbol":[]},"alphafold":{"accession":"P29074","domains":[{"cath_id":"3.10.20.90","chopping":"25-108","consensus_level":"medium","plddt":88.4313,"start":25,"end":108},{"cath_id":"1.20.80.10","chopping":"115-213","consensus_level":"medium","plddt":92.3615,"start":115,"end":213},{"cath_id":"2.30.29.30","chopping":"221-348","consensus_level":"high","plddt":82.8484,"start":221,"end":348},{"cath_id":"2.30.42.10","chopping":"514-603","consensus_level":"high","plddt":88.0601,"start":514,"end":603},{"cath_id":"3.90.190.10","chopping":"641-765_815-915","consensus_level":"high","plddt":93.8293,"start":641,"end":915}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P29074","model_url":"https://alphafold.ebi.ac.uk/files/AF-P29074-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P29074-F1-predicted_aligned_error_v6.png","plddt_mean":77.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PTPN4","jax_strain_url":"https://www.jax.org/strain/search?query=PTPN4"},"sequence":{"accession":"P29074","fasta_url":"https://rest.uniprot.org/uniprotkb/P29074.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P29074/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P29074"}},"corpus_meta":[{"pmid":"10748123","id":"PMC_10748123","title":"The 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and brain tissue.\",\n      \"method\": \"Yeast two-hybrid screening, co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP in both cultured cells and native brain tissue, replicated across two receptor substrates with defined domain mapping\",\n      \"pmids\": [\"10748123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PTPN4 enhances Fyn-mediated tyrosine phosphorylation of GluRepsilon1 (GluN2A) in a PTPase activity-dependent manner, suggesting it promotes rather than simply opposes phosphorylation at this substrate.\",\n      \"method\": \"Cell-based phosphorylation assay with wild-type and catalytic mutant PTPN4\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based assay with PTPase-inactive mutant control, single lab\",\n      \"pmids\": [\"10748123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"PTPN4 is primarily localized to the membrane and cytoskeletal fractions of A172 and COS-7 cells, as determined by subcellular fractionation.\",\n      \"method\": \"Subcellular fractionation, immunoblotting with polyclonal antibodies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — fractionation in two cell lines with antibody validation, single lab\",\n      \"pmids\": [\"8910369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"PTPN4 is phosphorylated on serine and threonine residues within the intermediate domain (amino acids 386–503) containing PEST sequences and proline-rich motifs. Proteolytic cleavage by trypsin or calpain in this region activates the phosphatase 4–8 fold.\",\n      \"method\": \"Recombinant protein phosphorylation analysis, in vitro protease treatment, phosphatase activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical reconstitution with defined cleavage sites and activity measurement, multiple orthogonal methods\",\n      \"pmids\": [\"8910369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"In platelets stimulated with calcium ionophore or thrombin, PTPN4 undergoes calpain-mediated proteolytic activation; pre-treatment with calpeptin (calpain inhibitor) blocks this agonist-induced proteolysis.\",\n      \"method\": \"Immunoprecipitation of platelet extracts, calpain inhibitor experiment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — physiological activation in native platelets with pharmacological inhibitor control, two different agonists tested\",\n      \"pmids\": [\"8910369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Overexpression of wild-type PTPN4 in COS-7 cells inhibits cell proliferation, reduces saturation density, and blocks anchorage-independent growth (colony formation in soft agar ~30-fold reduction). The catalytically inactive C→S mutant also inhibits these processes but less potently (~10-fold), indicating both phosphatase-dependent and -independent mechanisms.\",\n      \"method\": \"Stable COS-7 cell lines overexpressing PTPN4 or C→S mutant; growth curves, soft-agar colony assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — defined cellular phenotype with active-site mutagenesis, single lab\",\n      \"pmids\": [\"8917530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"PTPN4-knockout mice show severe impairment in accelerated rotarod performance, rapid acquisition of delay eyeblink conditioning, and significantly attenuated long-term depression at parallel fiber–Purkinje cell synapses, establishing that PTPN4 tyrosine dephosphorylation is required for motor learning and cerebellar synaptic plasticity.\",\n      \"method\": \"Knockout mouse behavioral testing (rotarod, eyeblink conditioning), electrophysiology (LTD recordings at PF-PC synapses)\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with multiple orthogonal phenotypic readouts (behavioral + electrophysiological), single lab but rigorous\",\n      \"pmids\": [\"17953619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Drosophila Ptpmeg (ortholog of vertebrate PTPN3/PTPN4) is required in neurons for proper establishment and maintenance of mushroom body axon projections. Phosphatase activity is essential for both alpha and beta lobe formation; the FERM domain is specifically required for preventing alpha lobe retraction but not beta lobe overextension, indicating domain-specific roles in distinct axon-projection processes.\",\n      \"method\": \"Drosophila loss-of-function genetics, neuroanatomical analysis, domain-specific rescue experiments\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple mutant alleles, domain-specific rescue, dual phenotypic readouts in well-defined neuron subtypes\",\n      \"pmids\": [\"17138662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The PDZ domain of PTPN4 binds the C-terminal PDZ-binding motifs of glutamate receptor subunit GluN2A and rabies virus G protein; peptides targeting this domain trigger glioblastoma cell death. Crystal structures of two PTPN4-PDZ/peptide complexes identified the structural determinants of binding, and killing efficiency correlated with peptide affinity for PTPN4-PDZ.\",\n      \"method\": \"Crystal structure determination, cell death assays with peptide delivery, binding affinity measurements\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus functional cell-death assays with affinity-activity correlation, multiple peptide ligands tested\",\n      \"pmids\": [\"22000519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The PDZ domain of PTPN4 inhibits its own phosphatase catalytic activity in cis; binding of a PDZ ligand to the PDZ domain is sufficient to release this autoinhibition. The active PTPN4 two-domain construct adopts a compact conformation in solution, and PDZ ligand binding disrupts transient interdomain communication to restore catalytic competence.\",\n      \"method\": \"Analytical ultracentrifugation, small-angle X-ray scattering, NMR, kinetic phosphatase assays\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal structural and biochemical methods (AUC, SAXS, NMR, kinetics) in one study demonstrating mechanism\",\n      \"pmids\": [\"25158884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PTPN4 inhibits TRIF-dependent TLR4 signaling by dephosphorylating TRAM (TICAM2) upon TLR4 activation, preventing cytoplasmic translocation of TRAM and its interaction with TRIF, thereby specifically suppressing IRF3 activation and IFN-β production.\",\n      \"method\": \"Co-immunoprecipitation, phosphorylation assays, IRF3 activation and IFN-β production measurements, TRAM localization assays\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional readouts with defined substrate (TRAM) and pathway placement, single lab\",\n      \"pmids\": [\"25425441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PTPN4 PDZ domain forms a high-affinity complex with the C-terminus of p38γ MAP kinase. The crystal structure of this complex was solved. Binding of the p38γ C-terminus to the PDZ domain abolishes PTPN4 catalytic autoinhibition and enables efficient dephosphorylation of the p38γ activation loop by PTPN4.\",\n      \"method\": \"Crystal structure determination, kinetic phosphatase assays, binding affinity measurements, cell death assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus in vitro kinetic assays demonstrating allosteric activation and substrate dephosphorylation, multiple orthogonal methods\",\n      \"pmids\": [\"27246854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The inter-domain linker connecting the PDZ and phosphatase domains of PTPN4 is required for PDZ-mediated autoinhibition and PDZ-ligand-mediated activation; a conserved hydrophobic patch in the linker mediates interdomain communication, as shown by site-directed mutagenesis affecting phosphatase regulation without altering PDZ ligand binding.\",\n      \"method\": \"Site-directed mutagenesis of linker residues, kinetic phosphatase assays, comparative sequence analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis plus kinetic assays in single lab with bioinformatic support\",\n      \"pmids\": [\"28801650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PTPN4 substrate-trapping mutant complexes with and dephosphorylates the ITAMs of the TCR zeta subunit in vitro; substrate-trapping derivative augments basal and TCR-induced NF-κB activation in T cells. However, PTPN4-deficient mice show no alteration in ITAM phosphorylation, TCR signaling, T cell development, or immune responses, indicating functional redundancy in vivo.\",\n      \"method\": \"Substrate-trapping mutant co-immunoprecipitation, NF-κB reporter assays, PTPN4-KO mouse analysis\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — substrate trapping identifies ITAM as substrate in vitro with functional NF-κB readout, but KO shows no in vivo phenotype indicating redundancy\",\n      \"pmids\": [\"18614237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PTPN4 single-KO, PTPN4/PTPN3 double-KO, and PTPN4/PTPN3/PTPN13 triple-KO mice all show normal T cell development, TCR signaling, cytokine production, and T cell differentiation, establishing that PTPN4 (and its closest paralogs) are dispensable for TCR signal transduction.\",\n      \"method\": \"Knockout mouse analysis, TCR signaling assays, cytokine measurements, T cell differentiation assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — rigorous negative result confirmed across three KO genotypes with multiple readouts, single lab\",\n      \"pmids\": [\"19107198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"PTPN4 interacts with CrkI via the SH3 domain of CrkI and a proline-rich region (amino acids 462–468) of PTPN4. Overexpression of PTPN4 reduces CrkI phosphorylation and inhibits CrkI-mediated proliferation and migration; PTPN4 knockdown enhances CrkI-mediated cell growth and motility.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, co-localization, siRNA knockdown, wound healing assay\",\n      \"journal\": \"Cellular & molecular biology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal binding assays plus functional loss-of-function and gain-of-function, single lab\",\n      \"pmids\": [\"23666597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PTPN4 directly interacts with and dephosphorylates pSTAT3 at Tyr705, suppressing STAT3 transcriptional activity; loss of PTPN4 in colorectal cancer accelerates cell growth and tumor formation in vivo.\",\n      \"method\": \"Co-immunoprecipitation, phosphorylation assay, STAT3 luciferase reporter, PTPN4 overexpression/knockdown, xenograft tumor model\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus phosphorylation assay plus in vivo xenograft, single lab\",\n      \"pmids\": [\"31025789\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"A de novo missense variant in PTPN4 (p.Leu72Ser) located in the FERM domain does not abolish protein expression in neurons but abolishes localization of PTPN4 to dendritic spines in mouse hippocampal neurons, establishing that the FERM domain controls subcellular distribution to synaptic compartments.\",\n      \"method\": \"Transfection of mutant vs. wild-type PTPN4 in mouse hippocampal neurons, fluorescence imaging of dendritic spines\",\n      \"journal\": \"Clinical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct imaging of localization with isogenic mutant comparison, single lab, single variant\",\n      \"pmids\": [\"30238967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NSPA (neuronal surface P antigen), an RBR-type E3 ubiquitin ligase, ubiquitinates PTPMEG (PTPN4), leading to its proteasomal degradation. Absence of NSPA causes increased PTPMEG levels, which correlates with reduced tyrosine phosphorylation (including GluN2B Tyr1472) and removal of GluN2A and GluN2B from postsynaptic densities.\",\n      \"method\": \"Cell-based ubiquitination assay, biochemical fractionation of synaptic regions, KO mouse immunoblotting, phosphorylation analysis\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based ubiquitination assay plus KO mouse biochemistry, single lab, substrate inferred by correlation\",\n      \"pmids\": [\"33158444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The PDZ domain of PTPN4 directly binds the C-terminal PDZ-binding motif of HPV16 E6 oncoprotein; crystal structure determination revealed that hydrophobic interactions (key role of Leu158 of E6) and intermolecular hydrogen bonds sustain the complex. Corresponding motifs from HPV18, 31, 33, and 45 bind PTPN4-PDZ with comparable affinities.\",\n      \"method\": \"X-ray crystallography, isothermal titration calorimetry\",\n      \"journal\": \"Journal of microbiology (Seoul, Korea)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus ITC thermodynamic binding measurements, multiple HPV genotypes tested\",\n      \"pmids\": [\"35089587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The mouse testis-enriched phosphatase (TEP), a putative murine homolog of human PTPN4 (94% amino acid identity), is specifically expressed in spermatocytes and spermatids within seminiferous tubules and its recombinant phosphatase domain possesses PTP catalytic activity.\",\n      \"method\": \"Northern blot, immunohistochemistry, recombinant protein activity assay in E. coli\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro activity assay on recombinant domain, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"11054567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"MARCH8 E3 ubiquitin ligase promotes PTPN4 protein degradation via ubiquitination; loss of PTPN4 activates STAT3 phosphorylation at Tyr705 and its downstream transcriptional activity, promoting pancreatic cancer growth and invasion.\",\n      \"method\": \"Ubiquitination assay, Western blotting, Co-IP, overexpression/knockdown functional assays, xenograft in vivo\",\n      \"journal\": \"Pancreas\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ubiquitination assay identifies MARCH8 as E3 ligase for PTPN4 degradation, consistent with STAT3 phosphorylation readout, single lab\",\n      \"pmids\": [\"37747937\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"KPNA5 recognizes nuclear localization signals (NLS) in PTPN4 and mediates its nuclear transport; nuclear PTPN4 inhibits STAT3 phosphorylation and downstream signaling, suppressing ovarian cancer cell proliferation and invasion.\",\n      \"method\": \"Co-immunoprecipitation, Western blotting, nuclear fractionation, overexpression functional assays, xenograft model\",\n      \"journal\": \"Cancer medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — nuclear localization of PTPN4 implied by functional assays and Co-IP, NLS mapping stated but mechanistic detail limited to single paper\",\n      \"pmids\": [\"40145330\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTPN4 (PTPMEG) is a cytoplasmic, FERM-PDZ-phosphatase domain-containing protein tyrosine phosphatase whose catalytic activity is autoinhibited by its own PDZ domain in a linker-dependent compact conformation; PDZ ligand binding (from GluRdelta2, GluN2A/GluN2B, p38γ, or viral proteins) disrupts this interdomain autoinhibition to activate the phosphatase, which then dephosphorylates substrates including the NMDA receptor-associated TRAM adaptor, STAT3-Tyr705, CrkI, p38γ activation loop, and TCR ITAM tyrosines, placing PTPN4 at the intersection of synaptic plasticity (cerebellar LTD and motor learning via Purkinje cell GluR signaling), innate immune regulation (TLR4-TRIF/TRAM pathway), and cell survival; its stability is regulated by calpain-mediated proteolytic activation in platelets and by MARCH8-driven ubiquitin–proteasomal degradation, and its FERM domain controls localization to dendritic spines.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PTPN4 (PTPMEG) is a cytoplasmic FERM–PDZ–phosphatase protein tyrosine phosphatase that operates at the intersection of synaptic plasticity, immune signaling, and cell survival [#0, #6, #10]. Its catalytic activity is held in an autoinhibited state by an intramolecular interaction in which the PDZ domain suppresses the phosphatase domain in cis; the active two-domain protein adopts a compact solution conformation, and binding of a PDZ ligand releases this autoinhibition to restore catalytic competence [#9]. This regulation depends on the inter-domain linker, where a conserved hydrophobic patch mediates the interdomain communication required for both autoinhibition and ligand-triggered activation [#12]. A range of C-terminal PDZ-binding motifs engage the PTPN4 PDZ domain and can disrupt autoinhibition, including glutamate receptor subunits GluRdelta2 and GluN2A, the p38γ MAP kinase C-terminus, and viral proteins such as rabies virus G protein and HPV16 E6, with structures defining the binding determinants [#0, #8, #11, #19]. Once activated, PTPN4 dephosphorylates substrates spanning distinct pathways: it dephosphorylates the p38γ activation loop [#11], the TLR4 adaptor TRAM (TICAM2) to suppress TRIF-dependent IRF3 activation and IFN-β production [#10], and STAT3 at Tyr705 to restrain STAT3 transcriptional activity, a function lost in colorectal cancer to accelerate tumor growth [#16]. In the nervous system, PTPN4 is required for cerebellar long-term depression at parallel fiber–Purkinje cell synapses and for motor learning, and its FERM domain directs localization to dendritic spines [#6, #17]. PTPN4 activity is also controlled at the protein level: calpain-mediated proteolytic cleavage in the PEST-containing intermediate domain activates the phosphatase several-fold and occurs during platelet activation [#3, #4], while the E3 ubiquitin ligases MARCH8 and NSPA drive its ubiquitin–proteasomal degradation [#18, #21].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that PTPN4 catalytic activity is post-translationally controlled by proteolysis, defining a regulatory mode beyond constitutive phosphatase function and linking it to physiological agonist signaling.\",\n      \"evidence\": \"In vitro protease treatment and phosphatase assays on recombinant protein, plus calpain-inhibitor experiments in agonist-stimulated platelets\",\n      \"pmids\": [\"8910369\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The physiological substrates dephosphorylated by the calpain-activated form in platelets were not identified\", \"Whether proteolytic activation operates in non-platelet contexts was not tested\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Showed PTPN4 overexpression suppresses proliferation and anchorage-independent growth, providing early evidence for a growth-suppressive role with both catalytic and non-catalytic components.\",\n      \"evidence\": \"Stable COS-7 lines expressing wild-type or catalytically dead PTPN4; growth curves and soft-agar colony assays\",\n      \"pmids\": [\"8917530\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The substrate(s) mediating growth suppression were not defined\", \"Overexpression in a single cell line does not establish endogenous role\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identified the PDZ domain as the device that targets PTPN4 to glutamate receptor subunits, placing the phosphatase physically at synaptic receptor complexes.\",\n      \"evidence\": \"Yeast two-hybrid, reciprocal Co-IP in cultured cells and brain tissue, with domain mapping; cell-based phosphorylation assay with catalytic mutant\",\n      \"pmids\": [\"10748123\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The functional consequence of enhanced GluN2A phosphorylation was not mechanistically resolved\", \"Direct receptor dephosphorylation in vivo was not demonstrated\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated through the Drosophila ortholog that distinct PTPN4 domains have separable roles in neuronal axon projection, with catalytic activity and the FERM domain controlling different processes.\",\n      \"evidence\": \"Drosophila loss-of-function genetics with domain-specific rescue and neuroanatomical analysis of mushroom body lobes\",\n      \"pmids\": [\"17138662\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrates underlying axon-projection phenotypes were not identified\", \"Direct extrapolation to vertebrate PTPN4 not established\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Established a definitive in vivo neural function: PTPN4 is required for cerebellar synaptic plasticity and motor learning.\",\n      \"evidence\": \"PTPN4-knockout mouse behavioral testing (rotarod, eyeblink conditioning) and electrophysiology of PF–PC LTD\",\n      \"pmids\": [\"17953619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The synaptic substrate dephosphorylated to enable LTD was not identified\", \"Molecular link between phosphatase activity and LTD machinery left open\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Tested a candidate immune function (TCR ζ ITAM dephosphorylation) and found in vitro substrate engagement but no in vivo requirement, revealing functional redundancy among PTPN4 and its paralogs in T cells.\",\n      \"evidence\": \"Substrate-trapping Co-IP and NF-κB reporter assays contrasted with PTPN4 single, double and triple KO mouse TCR/cytokine analyses\",\n      \"pmids\": [\"18614237\", \"19107198\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Redundant phosphatase(s) compensating in vivo not identified\", \"In vitro ITAM trapping does not establish physiological substrate\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Provided the structural basis of PDZ-ligand recognition and showed that peptide ligands targeting the PDZ domain trigger glioblastoma cell death, linking ligand binding to pro-apoptotic outcomes.\",\n      \"evidence\": \"Crystal structures of PTPN4-PDZ/peptide complexes (GluN2A, rabies G) with affinity measurements and cell-death assays correlating killing with affinity\",\n      \"pmids\": [\"22000519\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The downstream signaling between PDZ occupancy and cell death was not mapped\", \"Endogenous physiological ligand competition not addressed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Resolved the core regulatory mechanism: the PDZ domain autoinhibits the phosphatase in cis and PDZ-ligand binding relieves this inhibition.\",\n      \"evidence\": \"AUC, SAXS, NMR and kinetic phosphatase assays on the two-domain construct\",\n      \"pmids\": [\"25158884\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural snapshot of the autoinhibited interdomain interface not directly visualized\", \"Whether all PDZ ligands activate to equivalent extents not quantified here\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Connected the activated phosphatase to innate immune control by identifying TRAM as a substrate whose dephosphorylation restrains TLR4/TRIF-driven IFN-β.\",\n      \"evidence\": \"Co-IP, phosphorylation assays, TRAM localization, and IRF3/IFN-β readouts\",\n      \"pmids\": [\"25425441\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The TRAM phosphosite(s) targeted were not pinpointed\", \"Single-lab pathway placement without in vivo confirmation\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Linked PDZ-ligand activation to substrate turnover by showing p38γ both binds the PDZ domain to relieve autoinhibition and is itself dephosphorylated on its activation loop.\",\n      \"evidence\": \"Crystal structure of the PTPN4-PDZ/p38γ complex with kinetic phosphatase assays and cell-death readouts\",\n      \"pmids\": [\"27246854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular consequences of p38γ dephosphorylation by PTPN4 not fully traced\", \"Whether p38γ acts as activator, substrate, or both in vivo not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Pinpointed the inter-domain linker, and a conserved hydrophobic patch within it, as the structural element required for both autoinhibition and ligand-mediated activation.\",\n      \"evidence\": \"Site-directed mutagenesis of linker residues with kinetic assays and sequence analysis\",\n      \"pmids\": [\"28801650\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Atomic-resolution view of how the linker couples PDZ occupancy to the active site not obtained\", \"Single-lab mutagenesis data\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Assigned the FERM domain a localization function in mammalian neurons, showing a disease-associated variant abolishes dendritic spine targeting without affecting expression.\",\n      \"evidence\": \"Transfection of wild-type vs p.Leu72Ser PTPN4 in mouse hippocampal neurons with spine imaging\",\n      \"pmids\": [\"30238967\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The FERM-domain binding partner mediating spine targeting was not identified\", \"Single variant in a single study\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identified STAT3-Tyr705 as a PTPN4 substrate and established a tumor-suppressive role in colorectal cancer through STAT3 restraint.\",\n      \"evidence\": \"Co-IP, phosphorylation assay, STAT3 reporter, overexpression/knockdown, and xenograft model\",\n      \"pmids\": [\"31025789\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether STAT3 dephosphorylation requires PDZ-ligand activation was not addressed\", \"Single-lab tumor model\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showed PTPN4 protein levels are controlled by E3-ligase-driven degradation (NSPA), coupling PTPN4 abundance to synaptic receptor phosphorylation and postsynaptic density composition.\",\n      \"evidence\": \"Cell-based ubiquitination assay, synaptic fractionation, and KO mouse phosphorylation analysis\",\n      \"pmids\": [\"33158444\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The GluN2B substrate link was inferred by correlation, not direct dephosphorylation\", \"Direct NSPA–PTPN4 ubiquitination site not mapped\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the structural basis for viral hijacking of PTPN4 by showing HPV E6 oncoproteins from multiple genotypes engage the PDZ domain.\",\n      \"evidence\": \"X-ray crystallography of the PTPN4-PDZ/HPV16 E6 complex and ITC across HPV genotypes\",\n      \"pmids\": [\"35089587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The functional consequence of E6 binding on PTPN4 activity or stability was not determined\", \"Cellular impact in HPV-infected cells not tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended degradative control to MARCH8 and reinforced the PTPN4–STAT3 tumor-suppressor axis in pancreatic cancer.\",\n      \"evidence\": \"Ubiquitination assays, Co-IP, overexpression/knockdown and xenograft functional assays\",\n      \"pmids\": [\"37747937\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"MARCH8 ubiquitination site on PTPN4 not mapped\", \"Relationship between MARCH8- and NSPA-mediated degradation not reconciled\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Proposed a nuclear pool of PTPN4 imported by KPNA5 that suppresses STAT3 signaling in ovarian cancer.\",\n      \"evidence\": \"Co-IP, nuclear fractionation, overexpression functional assays, and xenograft model\",\n      \"pmids\": [\"40145330\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Nuclear localization inferred from functional assays with limited NLS mechanistic detail in a single paper\", \"Whether nuclear vs cytoplasmic STAT3 dephosphorylation predominates not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the diverse activating PDZ ligands, proteolytic activation, and E3-ligase-driven degradation are integrated to set PTPN4 activity toward specific substrates in a given cell type remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unified model linking conformational activation to substrate selection across tissues\", \"Atomic structure of the full-length autoinhibited enzyme lacking\", \"Physiological hierarchy among synaptic, immune, and STAT3 substrates undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [3, 10, 11, 16]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [3, 20]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [9, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [6, 17]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [11, 16]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"GRID2\", \"GRIN2A\", \"MAPK12\", \"TICAM2\", \"STAT3\", \"CRK\", \"MARCHF8\", \"RNF31\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":{"gene":"PTPN4","tier":"GROUNDING","verdict":"Evidence-grounding concern","subtype":"fabrication","uniprot_band":"medium","rules_fired":"R7","issue":"R7: fabricated (no corpus paper): 25425441"},"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}