{"gene":"PTPN4","run_date":"2026-04-28T19:45:45","timeline":{"discoveries":[{"year":2000,"finding":"PTPN4 (PTPMEG) interacts with glutamate receptor delta2 (GluRδ2) and NMDA receptor GluRε1 (GluN2B) via its PDZ domain binding to the C-terminal PDZ target sequences of these receptors; additionally, PTPN4 enhances Fyn-mediated tyrosine phosphorylation of GluRε1 in a PTPase activity-dependent manner","method":"Yeast two-hybrid screening, co-immunoprecipitation from cultured cells and brain tissue, PDZ domain binding assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP in cultured cells and brain, domain mapping, functional enzymatic readout; replicated in multiple contexts","pmids":["10748123"],"is_preprint":false},{"year":1996,"finding":"PTPN4 (PTPMEG) is primarily localized to the membrane and cytoskeletal fractions of cells; it is phosphorylated on serine and threonine residues within an intermediate domain (aa 386–503) containing PEST sequences and proline-rich motifs; proteolytic cleavage by calpain in this region activates the phosphatase 4–8-fold; in platelets, thrombin and calcium ionophore stimulation triggers calpain-mediated proteolysis and activation of PTPN4","method":"Subcellular fractionation, recombinant protein expression (Sf9 and COS-7 cells), trypsin/calpain cleavage assays, immunoprecipitation from human platelets, calpain inhibitor (calpeptin) treatment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution of proteolytic activation, subcellular fractionation, and biochemical characterization with inhibitor validation","pmids":["8910369"],"is_preprint":false},{"year":1996,"finding":"Overexpression of PTPN4 in COS-7 cells reduces cell growth rate, lowers saturation density, and inhibits anchorage-independent colony formation; the catalytically inactive C→S mutant also inhibits proliferation and colony formation, though less potently, suggesting both enzymatic and non-enzymatic mechanisms; endogenous PTPN4 is localized to membrane and cytoskeletal fractions","method":"Stable COS-7 cell lines overexpressing wild-type and active-site C→S mutant PTPN4, soft-agar colony formation assay, subcellular fractionation","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — loss/gain-of-function with defined phenotype and mutagenesis, single lab","pmids":["8917530"],"is_preprint":false},{"year":2007,"finding":"PTPN4 knockout mice show impaired motor learning (accelerated rotarod), impaired cerebellar delay eyeblink conditioning, and significantly attenuated long-term depression (LTD) at parallel fiber–Purkinje cell synapses, establishing that PTPN4 tyrosine dephosphorylation events are required for cerebellar synaptic plasticity and motor learning; developmental climbing fiber elimination and basal synaptic properties are unaffected","method":"PTPN4 knockout mouse generation, behavioral testing (rotarod, eyeblink conditioning), electrophysiology (LTD at PF-PC synapses)","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple orthogonal behavioral and electrophysiological phenotypes; replicated in vivo","pmids":["17953619"],"is_preprint":false},{"year":2011,"finding":"The PTPN4 PDZ domain binds peptides mimicking C-terminal sequences of known ligands (e.g., GluN2A C-terminus, rabies virus G protein); crystal structures of PTPN4-PDZ complexed with two different peptides define the structural determinants of binding; intracellular delivery of high-affinity PDZ ligand peptides induces glioblastoma cell death, and killing efficiency correlates with PDZ binding affinity","method":"Crystal structure determination of PTPN4-PDZ/peptide complexes, binding affinity measurements, intracellular peptide delivery assays in glioblastoma cells","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 — crystal structures with functional validation (cell death assay correlated with affinity), multiple peptide ligands tested","pmids":["22000519"],"is_preprint":false},{"year":2015,"finding":"PTPN4 directly dephosphorylates TRAM (TICAM2) on tyrosine residues upon TLR4 activation, thereby inhibiting cytoplasmic translocation of TRAM, disrupting TRAM–TRIF interaction, and specifically suppressing TRIF-dependent IRF3 activation and IFN-β production downstream of TLR4","method":"Co-immunoprecipitation, tyrosine phosphorylation assays, overexpression/knockdown of PTPN4 in macrophages, IRF3 activation and IFN-β reporter assays","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"High","confidence_rationale":"Tier 2 — substrate identification with functional epistasis (TRAM phosphorylation → IRF3 → IFN-β), multiple orthogonal methods, single lab","pmids":["25825441"],"is_preprint":false},{"year":2008,"finding":"PTPN4 substrate-trapping mutants complex with TCR ζ-chain ITAMs and can dephosphorylate ITAM phosphotyrosines; substrate-trapping derivative augments basal and TCR-induced NF-κB activation; however, PTPN4-deficient mice show no defect in T cell development, TCR signaling, ITAM phosphorylation, or immune responses, indicating functional redundancy with other phosphatases in T cells","method":"Substrate-trapping mutant co-immunoprecipitation, PTPN4-deficient mouse generation, T cell signaling assays, NF-κB reporter assay","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — substrate-trapping identifies ITAM as substrate, KO mouse provides epistasis; functional redundancy limits phenotypic interpretation","pmids":["18614237"],"is_preprint":false},{"year":2008,"finding":"PTPN4 and PTPN3 are both dispensable for TCR signal transduction; PTPN4/PTPN3 double-KO and PTPN4/PTPN3/PTPN13 triple-KO mice show normal T cell development, cytokine production, and Th1/Th2/Th17 differentiation, establishing genetic epistasis that these three FERM-PDZ PTPs do not redundantly control TCR signaling","method":"PTPN4-deficient, double-KO, and triple-KO mouse generation; T cell development, proliferation, cytokine, and differentiation assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis via multiple KO combinations; clean null phenotype with multiple assays","pmids":["19107198"],"is_preprint":false},{"year":2013,"finding":"PTPN4 interacts with CrkI via the SH3 domain of CrkI and the proline-rich region (aa 462–468) of PTPN4; overexpression of PTPN4 reduces CrkI tyrosine phosphorylation and inhibits CrkI-mediated cell proliferation and migration; PTPN4 knockdown enhances CrkI-mediated cell growth and motility","method":"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, co-localization, overexpression/siRNA knockdown with proliferation and wound-healing assays","journal":"Cellular & molecular biology letters","confidence":"Medium","confidence_rationale":"Tier 2–3 — multiple binding assays plus functional gain/loss-of-function with defined phenotype, single lab","pmids":["23666597"],"is_preprint":false},{"year":2014,"finding":"The PDZ domain of PTPN4 inhibits its own phosphatase catalytic activity through an intramolecular interaction; binding of a PDZ ligand releases this autoinhibition and restores catalytic activity; the two-domain PDZ–phosphatase construct adopts a predominant compact conformation in solution as shown by AUC, SAXS, and NMR","method":"Analytical ultracentrifugation, small-angle X-ray scattering, NMR spectroscopy, in vitro phosphatase activity assays with and without PDZ ligands","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 — multiple biophysical techniques (SAXS, AUC, NMR) plus enzymatic assays reconstituting the regulatory mechanism","pmids":["25158884"],"is_preprint":false},{"year":2016,"finding":"PTPN4 PDZ domain forms a tight complex with the C-terminal sequence of p38γ MAPK; crystal structure of PTPN4-PDZ bound to the p38γ C-terminus reveals molecular basis of recognition; p38γ C-terminus has the highest affinity among known endogenous PTPN4-PDZ ligands; binding of the p38γ C-terminal peptide to the PDZ domain abolishes the catalytic autoinhibition of PTPN4, enabling efficient dephosphorylation of the p38γ activation loop","method":"Crystal structure determination, isothermal titration calorimetry, in vitro phosphatase activity assays, intracellular peptide delivery (cell death assay)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus ITC plus in vitro enzymatic assay demonstrating activation-loop dephosphorylation; multiple orthogonal methods","pmids":["27246854"],"is_preprint":false},{"year":2017,"finding":"The inter-domain linker connecting the PTPN4 PDZ domain and phosphatase domain contains a conserved patch of hydrophobic residues that mediates the PDZ-related autoinhibition and PDZ-ligand-dependent activation of phosphatase activity; mutations in this linker patch disrupt PTPN4 bidomain regulation without affecting PDZ ligand binding affinity","method":"Comparative sequence analysis, site-directed mutagenesis of linker residues, kinetic phosphatase activity assays","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 1–2 — mutagenesis with enzymatic assays demonstrating linker-mediated regulation; single lab","pmids":["28801650"],"is_preprint":false},{"year":2019,"finding":"PTPN4 directly interacts with STAT3 and dephosphorylates pSTAT3 at Tyr705; loss of PTPN4 in colorectal/rectal cancer cells promotes STAT3 transcriptional activity and tumor growth, both in vitro and in vivo","method":"Co-immunoprecipitation, phosphatase assay (pSTAT3 Tyr705), PTPN4 overexpression and knockdown with cell growth and xenograft assays","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct interaction and dephosphorylation shown by Co-IP and phosphorylation assay, supported by in vivo xenograft; single lab","pmids":["31025789"],"is_preprint":false},{"year":2018,"finding":"A de novo missense variant in the FERM domain of PTPN4 (p.Leu72Ser) causes loss of PTPN4 localization to dendritic spines in mouse hippocampal neurons without affecting overall neuronal expression, linking PTPN4 spine localization to neurodevelopmental phenotype","method":"Transfection of wild-type and mutant human PTPN4 in mouse hippocampal neurons, confocal imaging of dendritic spine localization","journal":"Clinical genetics","confidence":"Medium","confidence_rationale":"Tier 2 — direct live-cell imaging with functional domain mutation establishing localization mechanism; single lab","pmids":["30238967"],"is_preprint":false},{"year":2006,"finding":"Drosophila Ptpmeg (ortholog of PTPN4/PTPN3) regulates axonal projection establishment and stabilization in the central brain; phosphatase activity is essential for both mushroom body α and β lobe formation; the FERM domain is selectively required for preventing α lobe retraction but not β lobe overextension; Ptpmeg acts non-cell-autonomously in MB or EB neurons, implicating it in cell-cell signaling","method":"Drosophila ptpmeg loss-of-function mutants, neuronal rescue experiments, domain-specific transgenes (phosphatase-dead, FERM mutants), mosaic analysis","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with domain-specific mutants in an ortholog; multiple neuronal circuit phenotypes characterized","pmids":["17138662"],"is_preprint":false},{"year":2020,"finding":"NSPA (neuronal surface P antigen) ubiquitinates PTPMEG (mouse PTPN4 ortholog), targeting it for proteasomal degradation; in NSPA-KO mice, PTPMEG accumulates at postsynaptic densities, leading to reduced tyrosine phosphorylation of GluN2B (Tyr1472 endocytic signal) and selective removal of GluN2A and GluN2B NMDAR subunits from PSDs, impairing hippocampal LTP and memory","method":"Cell-based ubiquitination assay, immunoblot of synaptic fractions, NSPA-KO mouse electrophysiology (LTP), behavioral memory assays, GluN2B pTyr1472 immunoblot","journal":"BMC biology","confidence":"Medium","confidence_rationale":"Tier 2 — ubiquitination assay plus KO epistasis plus electrophysiology; single lab but orthogonal methods","pmids":["33158444"],"is_preprint":false},{"year":2022,"finding":"HPV16 E6 oncoprotein C-terminal PDZ-binding motif directly binds the PDZ domain of PTPN4; crystal structure and ITC measurements show that hydrophobic interactions (Leu158 of E6) and hydrogen bond networks sustain the complex; high-risk HPV genotypes 16, 18, 31, 33, and 45 all bind PTPN4 PDZ with comparable affinities","method":"X-ray crystallography of PTPN4-PDZ/HPV E6 peptide complex, isothermal titration calorimetry","journal":"Journal of microbiology (Seoul, Korea)","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus ITC quantitative binding; rigorous structural and biochemical validation","pmids":["35089587"],"is_preprint":false},{"year":2023,"finding":"E3 ubiquitin ligase MARCH8 promotes PTPN4 protein degradation via ubiquitination; loss of PTPN4 activates STAT3 phosphorylation (pSTAT3 Tyr705) and its transcriptional activity, promoting pancreatic cancer growth and metastasis","method":"Ubiquitination assay, co-immunoprecipitation, PTPN4 overexpression/knockdown in pancreatic cancer cells, xenograft in vivo model, western blot for pSTAT3","journal":"Pancreas","confidence":"Medium","confidence_rationale":"Tier 2–3 — ubiquitination assay identifies MARCH8 as E3 ligase for PTPN4; STAT3 dephosphorylation pathway confirmed; 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 in ovarian cancer cells","method":"Co-immunoprecipitation, overexpression/knockdown studies in ovarian cancer cells, xenograft model, western blot for pSTAT3, NLS mapping","journal":"Cancer medicine","confidence":"Low","confidence_rationale":"Tier 3 — single Co-IP and functional assay; nuclear transport mechanism partially characterized; single lab","pmids":["40145330"],"is_preprint":false}],"current_model":"PTPN4 is a multi-domain non-receptor protein tyrosine phosphatase containing FERM, PDZ, and catalytic domains, whose phosphatase activity is autoinhibited by an intramolecular PDZ–phosphatase interaction mediated through the inter-domain linker; binding of PDZ ligands (including the C-termini of GluN2A, GluRδ2, p38γ, or viral proteins) relieves this autoinhibition and activates catalysis, enabling PTPN4 to dephosphorylate substrates including TRAM (suppressing TRIF-dependent TLR4/IRF3 signaling), pSTAT3-Tyr705 (suppressing STAT3-driven proliferation), CrkI, TCR ζ-chain ITAMs, and the p38γ activation loop; PTPN4 is regulated post-translationally by calpain cleavage (activating) and MARCH8-mediated ubiquitination/degradation, is localized to membranes, cytoskeleton, and dendritic spines, and is required in Purkinje cells for cerebellar LTD and motor learning."},"narrative":{"teleology":[{"year":1996,"claim":"Establishing that PTPN4 is membrane/cytoskeleton-associated and subject to proteolytic activation answered how the phosphatase is regulated post-translationally: calpain cleavage within the PEST-containing intermediate domain activates catalytic activity 4–8-fold, linking calcium signaling to PTPN4 function in platelets.","evidence":"Subcellular fractionation, recombinant protein calpain/trypsin cleavage assays, and calpain inhibitor treatment in human platelets","pmids":["8910369"],"confidence":"High","gaps":["Whether calpain activation of PTPN4 occurs in neurons or immune cells","Identity of platelet substrates dephosphorylated by activated PTPN4"]},{"year":1996,"claim":"Demonstrating that PTPN4 overexpression suppresses cell growth and anchorage-independent colony formation—partially independent of catalytic activity—established that PTPN4 functions as a growth suppressor with both enzymatic and scaffolding contributions.","evidence":"Stable COS-7 lines expressing wild-type or catalytically inactive C→S mutant PTPN4, soft-agar assays","pmids":["8917530"],"confidence":"Medium","gaps":["The non-catalytic mechanism of growth inhibition was not identified","Relevance to endogenous expression levels unknown"]},{"year":2000,"claim":"Identification of glutamate receptors GluRδ2 and GluN2B as PDZ-domain-dependent interactors of PTPN4 placed the phosphatase at postsynaptic sites and suggested a role in modulating excitatory neurotransmission.","evidence":"Yeast two-hybrid, co-immunoprecipitation from brain tissue and cultured cells, PDZ domain binding assays","pmids":["10748123"],"confidence":"High","gaps":["Whether PTPN4 directly dephosphorylates GluRδ2 or GluN2B was not demonstrated","Functional consequence of the interaction on synaptic transmission was not tested"]},{"year":2006,"claim":"Drosophila Ptpmeg mutant analysis revealed that PTPN4-family phosphatase activity is essential for axon projection and that the FERM domain has a distinct, selective role in axon stabilization, providing the first in vivo genetic evidence for domain-specific functions.","evidence":"Drosophila loss-of-function mutants, domain-specific transgenic rescue, mosaic analysis of mushroom body neurons","pmids":["17138662"],"confidence":"Medium","gaps":["Mammalian axon guidance roles not tested","Cell-autonomous versus non-cell-autonomous substrates not identified"]},{"year":2007,"claim":"PTPN4 knockout mice demonstrated that the phosphatase is required for cerebellar long-term depression and motor learning, converting the receptor-interaction data into a defined physiological function at identified synapses.","evidence":"PTPN4 KO mice, rotarod and eyeblink conditioning behavior, electrophysiology at parallel fiber–Purkinje cell synapses","pmids":["17953619"],"confidence":"High","gaps":["The specific synaptic substrate(s) mediating LTD impairment were not identified","Whether hippocampal synaptic plasticity is also affected was not examined"]},{"year":2008,"claim":"Substrate-trapping identified TCR ζ-chain ITAMs as PTPN4 substrates, but genetic ablation of PTPN4 alone or together with PTPN3 and PTPN13 revealed complete dispensability for T cell signaling, establishing functional redundancy in the immune compartment.","evidence":"Substrate-trapping co-IP, single/double/triple KO mice, T cell development and cytokine assays","pmids":["18614237","19107198"],"confidence":"Medium","gaps":["Identity of the compensating phosphatase(s) in T cells","Whether PTPN4 contributes to T cell signaling under stress or infection conditions"]},{"year":2014,"claim":"Reconstitution of the PDZ–phosphatase bidomain revealed that catalytic activity is autoinhibited by the PDZ domain in a compact conformation, and that PDZ ligand binding relieves this inhibition—establishing the core allosteric mechanism governing PTPN4 activation.","evidence":"AUC, SAXS, NMR of the bidomain construct, in vitro phosphatase assays ± PDZ ligands","pmids":["25158884"],"confidence":"High","gaps":["Full-length structural context including the FERM domain was not resolved","Quantitative coupling between ligand affinity and activation extent was not fully defined"]},{"year":2015,"claim":"Identification of TRAM as a PTPN4 substrate in macrophages demonstrated that the phosphatase negatively regulates innate immune signaling by specifically suppressing TRIF-dependent IRF3 activation and IFN-β production downstream of TLR4.","evidence":"Co-IP, tyrosine phosphorylation assays, PTPN4 overexpression/knockdown in macrophages, IRF3 and IFN-β reporter assays","pmids":["25825441"],"confidence":"High","gaps":["Whether PDZ-ligand-dependent activation is required for TRAM dephosphorylation in cells","Role in other TLR pathways not tested"]},{"year":2016,"claim":"Crystal structures of the PTPN4-PDZ/p38γ complex and enzymatic assays showed that p38γ is both the highest-affinity endogenous PDZ ligand and a direct substrate (activation-loop dephosphorylation), unifying the allosteric activation and substrate-recognition functions of the PDZ domain.","evidence":"X-ray crystallography, ITC, in vitro phosphatase assays on p38γ activation-loop phosphopeptides","pmids":["27246854"],"confidence":"High","gaps":["In vivo validation of p38γ as a PTPN4 substrate in a physiological setting","Whether other MAPKs are similarly regulated"]},{"year":2017,"claim":"Mutagenesis of the conserved hydrophobic patch in the PDZ–phosphatase inter-domain linker pinpointed the molecular element mediating autoinhibition and ligand-dependent activation, refining the allosteric model to a specific structural element.","evidence":"Site-directed mutagenesis of linker residues, kinetic phosphatase assays","pmids":["28801650"],"confidence":"Medium","gaps":["No full-length structure showing linker conformation in autoinhibited state","Contribution of FERM domain to regulation not addressed"]},{"year":2018,"claim":"A de novo FERM-domain missense variant (p.Leu72Ser) was shown to abolish PTPN4 localization to dendritic spines, linking the FERM domain to subcellular targeting and implicating PTPN4 dysfunction in neurodevelopmental disease.","evidence":"Transfection of WT/mutant PTPN4 in mouse hippocampal neurons, confocal imaging","pmids":["30238967"],"confidence":"Medium","gaps":["Functional synaptic consequence of mislocalization not measured","Number of patients carrying this variant is very small"]},{"year":2019,"claim":"Identification of STAT3-Tyr705 as a direct PTPN4 substrate established a tumor-suppressive axis: loss of PTPN4 in colorectal cancer derepresses STAT3 transcriptional activity and accelerates tumor growth.","evidence":"Co-IP, pSTAT3 dephosphorylation assay, PTPN4 overexpression/knockdown, xenograft model","pmids":["31025789"],"confidence":"Medium","gaps":["Whether PDZ-dependent activation is required for STAT3 dephosphorylation","Contribution of other PTPN4 substrates to growth suppression"]},{"year":2020,"claim":"Discovery that NSPA/MARCH8 ubiquitinates PTPN4 targeting it for degradation, and that PTPN4 accumulation in NSPA-KO mice causes NMDAR removal from PSDs and impaired hippocampal LTP, revealed ubiquitin-dependent turnover as a key regulatory layer and extended PTPN4's synaptic role beyond the cerebellum.","evidence":"Cell-based ubiquitination assay, NSPA-KO mouse synaptic fractionation, LTP electrophysiology, behavioral memory assays","pmids":["33158444"],"confidence":"Medium","gaps":["Whether NSPA-mediated and MARCH8-mediated ubiquitination are redundant or context-specific","Direct substrate identity at hippocampal synapses not fully defined"]},{"year":2022,"claim":"Crystal structures of the PTPN4-PDZ domain bound to high-risk HPV E6 C-terminal peptides revealed the structural basis of viral hijacking of PTPN4 and showed that multiple oncogenic HPV genotypes converge on PTPN4-PDZ binding, suggesting a conserved viral immune/growth evasion strategy.","evidence":"X-ray crystallography, ITC with HPV16/18/31/33/45 E6 peptides","pmids":["35089587"],"confidence":"High","gaps":["Functional consequence of HPV E6–PTPN4 interaction on phosphatase activity or cell signaling not tested in cells","Whether E6 binding activates or sequesters PTPN4 in vivo unknown"]},{"year":2023,"claim":"MARCH8 was identified as an E3 ligase promoting PTPN4 ubiquitination and degradation, and loss of PTPN4 activated STAT3 signaling to drive pancreatic cancer growth and metastasis, extending the PTPN4–STAT3 tumor-suppressive axis to a second cancer type and identifying an upstream regulatory E3.","evidence":"Ubiquitination assay, co-IP, PTPN4 overexpression/knockdown in pancreatic cancer cells, xenograft model","pmids":["37747937"],"confidence":"Medium","gaps":["Whether MARCH8 ubiquitinates PTPN4 at the same sites as NSPA","Structural basis of MARCH8–PTPN4 interaction unknown"]},{"year":null,"claim":"A full-length structure of PTPN4 integrating FERM, PDZ, and phosphatase domains has not been determined, leaving unknown how the FERM domain coordinates with PDZ-dependent allosteric regulation and subcellular targeting to control substrate access in vivo.","evidence":"","pmids":[],"confidence":"High","gaps":["No full-length atomic structure","Mechanism by which FERM domain directs dendritic spine localization is structurally uncharacterized","In vivo substrates responsible for cerebellar LTD remain unidentified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,5,6,9,10,12]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,12]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[1,2]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[18]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[5,6,7]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,10,12]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,3,15]}],"complexes":[],"partners":["GRIN2A","GRID2","MAPK12","TICAM2","STAT3","CRK","MARCH8","GRIN2B"],"other_free_text":[]},"mechanistic_narrative":"PTPN4 is a non-receptor protein tyrosine phosphatase that functions as a signal-modulating enzyme in synaptic plasticity, innate immunity, and growth control, with its catalytic activity gated by an intramolecular PDZ–phosphatase autoinhibitory mechanism. The PDZ domain directly inhibits phosphatase activity through an inter-domain linker containing a conserved hydrophobic patch; binding of C-terminal PDZ ligands—including glutamate receptor subunits GluN2A and GluRδ2, p38γ MAPK, and high-risk HPV E6 oncoproteins—relieves this autoinhibition and activates catalysis [PMID:25158884, PMID:28801650, PMID:27246854, PMID:35089587]. Identified substrates include TRAM, whose dephosphorylation suppresses TRIF-dependent TLR4/IRF3 signaling and IFN-β production [PMID:25825441]; pSTAT3-Tyr705, whose dephosphorylation restrains STAT3-driven tumor proliferation [PMID:31025789, PMID:37747937]; and CrkI, whose dephosphorylation inhibits cell migration [PMID:23666597]. In Purkinje neurons, PTPN4 is required for cerebellar long-term depression and motor learning, and its FERM domain directs localization to dendritic spines where it regulates NMDA receptor phosphorylation [PMID:17953619, PMID:30238967, PMID:33158444]."},"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 protein-tyrosine phosphatase PTPMEG interacts with glutamate receptor delta 2 and epsilon subunits.","date":"2000","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10748123","citation_count":76,"is_preprint":false},{"pmid":"31178952","id":"PMC_31178952","title":"miR-181c-5p Exacerbates Hypoxia/Reoxygenation-Induced Cardiomyocyte Apoptosis via Targeting PTPN4.","date":"2019","source":"Oxidative medicine and cellular longevity","url":"https://pubmed.ncbi.nlm.nih.gov/31178952","citation_count":51,"is_preprint":false},{"pmid":"8910369","id":"PMC_8910369","title":"The properties of the protein tyrosine phosphatase PTPMEG.","date":"1996","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/8910369","citation_count":49,"is_preprint":false},{"pmid":"36042375","id":"PMC_36042375","title":"MicroRNA-375 is a therapeutic target for castration-resistant prostate cancer through the PTPN4/STAT3 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assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP in cultured cells and brain, domain mapping, functional enzymatic readout; replicated in multiple contexts\",\n      \"pmids\": [\"10748123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"PTPN4 (PTPMEG) is primarily localized to the membrane and cytoskeletal fractions of cells; it is phosphorylated on serine and threonine residues within an intermediate domain (aa 386–503) containing PEST sequences and proline-rich motifs; proteolytic cleavage by calpain in this region activates the phosphatase 4–8-fold; in platelets, thrombin and calcium ionophore stimulation triggers calpain-mediated proteolysis and activation of PTPN4\",\n      \"method\": \"Subcellular fractionation, recombinant protein expression (Sf9 and COS-7 cells), trypsin/calpain cleavage assays, immunoprecipitation from human platelets, calpain inhibitor (calpeptin) treatment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution of proteolytic activation, subcellular fractionation, and biochemical characterization with inhibitor validation\",\n      \"pmids\": [\"8910369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Overexpression of PTPN4 in COS-7 cells reduces cell growth rate, lowers saturation density, and inhibits anchorage-independent colony formation; the catalytically inactive C→S mutant also inhibits proliferation and colony formation, though less potently, suggesting both enzymatic and non-enzymatic mechanisms; endogenous PTPN4 is localized to membrane and cytoskeletal fractions\",\n      \"method\": \"Stable COS-7 cell lines overexpressing wild-type and active-site C→S mutant PTPN4, soft-agar colony formation assay, subcellular fractionation\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss/gain-of-function with defined phenotype and mutagenesis, single lab\",\n      \"pmids\": [\"8917530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"PTPN4 knockout mice show impaired motor learning (accelerated rotarod), impaired cerebellar delay eyeblink conditioning, and significantly attenuated long-term depression (LTD) at parallel fiber–Purkinje cell synapses, establishing that PTPN4 tyrosine dephosphorylation events are required for cerebellar synaptic plasticity and motor learning; developmental climbing fiber elimination and basal synaptic properties are unaffected\",\n      \"method\": \"PTPN4 knockout mouse generation, behavioral testing (rotarod, eyeblink conditioning), electrophysiology (LTD at PF-PC synapses)\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple orthogonal behavioral and electrophysiological phenotypes; replicated in vivo\",\n      \"pmids\": [\"17953619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The PTPN4 PDZ domain binds peptides mimicking C-terminal sequences of known ligands (e.g., GluN2A C-terminus, rabies virus G protein); crystal structures of PTPN4-PDZ complexed with two different peptides define the structural determinants of binding; intracellular delivery of high-affinity PDZ ligand peptides induces glioblastoma cell death, and killing efficiency correlates with PDZ binding affinity\",\n      \"method\": \"Crystal structure determination of PTPN4-PDZ/peptide complexes, binding affinity measurements, intracellular peptide delivery assays in glioblastoma cells\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures with functional validation (cell death assay correlated with affinity), multiple peptide ligands tested\",\n      \"pmids\": [\"22000519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PTPN4 directly dephosphorylates TRAM (TICAM2) on tyrosine residues upon TLR4 activation, thereby inhibiting cytoplasmic translocation of TRAM, disrupting TRAM–TRIF interaction, and specifically suppressing TRIF-dependent IRF3 activation and IFN-β production downstream of TLR4\",\n      \"method\": \"Co-immunoprecipitation, tyrosine phosphorylation assays, overexpression/knockdown of PTPN4 in macrophages, IRF3 activation and IFN-β reporter assays\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — substrate identification with functional epistasis (TRAM phosphorylation → IRF3 → IFN-β), multiple orthogonal methods, single lab\",\n      \"pmids\": [\"25825441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PTPN4 substrate-trapping mutants complex with TCR ζ-chain ITAMs and can dephosphorylate ITAM phosphotyrosines; substrate-trapping derivative augments basal and TCR-induced NF-κB activation; however, PTPN4-deficient mice show no defect in T cell development, TCR signaling, ITAM phosphorylation, or immune responses, indicating functional redundancy with other phosphatases in T cells\",\n      \"method\": \"Substrate-trapping mutant co-immunoprecipitation, PTPN4-deficient mouse generation, T cell signaling assays, NF-κB reporter assay\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — substrate-trapping identifies ITAM as substrate, KO mouse provides epistasis; functional redundancy limits phenotypic interpretation\",\n      \"pmids\": [\"18614237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PTPN4 and PTPN3 are both dispensable for TCR signal transduction; PTPN4/PTPN3 double-KO and PTPN4/PTPN3/PTPN13 triple-KO mice show normal T cell development, cytokine production, and Th1/Th2/Th17 differentiation, establishing genetic epistasis that these three FERM-PDZ PTPs do not redundantly control TCR signaling\",\n      \"method\": \"PTPN4-deficient, double-KO, and triple-KO mouse generation; T cell development, proliferation, cytokine, and differentiation assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via multiple KO combinations; clean null phenotype with multiple assays\",\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 the proline-rich region (aa 462–468) of PTPN4; overexpression of PTPN4 reduces CrkI tyrosine phosphorylation and inhibits CrkI-mediated cell proliferation and migration; PTPN4 knockdown enhances CrkI-mediated cell growth and motility\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, co-localization, overexpression/siRNA knockdown with proliferation and wound-healing assays\",\n      \"journal\": \"Cellular & molecular biology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple binding assays plus functional gain/loss-of-function with defined phenotype, single lab\",\n      \"pmids\": [\"23666597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The PDZ domain of PTPN4 inhibits its own phosphatase catalytic activity through an intramolecular interaction; binding of a PDZ ligand releases this autoinhibition and restores catalytic activity; the two-domain PDZ–phosphatase construct adopts a predominant compact conformation in solution as shown by AUC, SAXS, and NMR\",\n      \"method\": \"Analytical ultracentrifugation, small-angle X-ray scattering, NMR spectroscopy, in vitro phosphatase activity assays with and without PDZ ligands\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple biophysical techniques (SAXS, AUC, NMR) plus enzymatic assays reconstituting the regulatory mechanism\",\n      \"pmids\": [\"25158884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PTPN4 PDZ domain forms a tight complex with the C-terminal sequence of p38γ MAPK; crystal structure of PTPN4-PDZ bound to the p38γ C-terminus reveals molecular basis of recognition; p38γ C-terminus has the highest affinity among known endogenous PTPN4-PDZ ligands; binding of the p38γ C-terminal peptide to the PDZ domain abolishes the catalytic autoinhibition of PTPN4, enabling efficient dephosphorylation of the p38γ activation loop\",\n      \"method\": \"Crystal structure determination, isothermal titration calorimetry, in vitro phosphatase activity assays, intracellular peptide delivery (cell death assay)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus ITC plus in vitro enzymatic assay demonstrating activation-loop dephosphorylation; multiple orthogonal methods\",\n      \"pmids\": [\"27246854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The inter-domain linker connecting the PTPN4 PDZ domain and phosphatase domain contains a conserved patch of hydrophobic residues that mediates the PDZ-related autoinhibition and PDZ-ligand-dependent activation of phosphatase activity; mutations in this linker patch disrupt PTPN4 bidomain regulation without affecting PDZ ligand binding affinity\",\n      \"method\": \"Comparative sequence analysis, site-directed mutagenesis of linker residues, kinetic phosphatase activity assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis with enzymatic assays demonstrating linker-mediated regulation; single lab\",\n      \"pmids\": [\"28801650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PTPN4 directly interacts with STAT3 and dephosphorylates pSTAT3 at Tyr705; loss of PTPN4 in colorectal/rectal cancer cells promotes STAT3 transcriptional activity and tumor growth, both in vitro and in vivo\",\n      \"method\": \"Co-immunoprecipitation, phosphatase assay (pSTAT3 Tyr705), PTPN4 overexpression and knockdown with cell growth and xenograft assays\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct interaction and dephosphorylation shown by Co-IP and phosphorylation assay, supported by 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 the FERM domain of PTPN4 (p.Leu72Ser) causes loss of PTPN4 localization to dendritic spines in mouse hippocampal neurons without affecting overall neuronal expression, linking PTPN4 spine localization to neurodevelopmental phenotype\",\n      \"method\": \"Transfection of wild-type and mutant human PTPN4 in mouse hippocampal neurons, confocal imaging of dendritic spine localization\",\n      \"journal\": \"Clinical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct live-cell imaging with functional domain mutation establishing localization mechanism; single lab\",\n      \"pmids\": [\"30238967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Drosophila Ptpmeg (ortholog of PTPN4/PTPN3) regulates axonal projection establishment and stabilization in the central brain; phosphatase activity is essential for both mushroom body α and β lobe formation; the FERM domain is selectively required for preventing α lobe retraction but not β lobe overextension; Ptpmeg acts non-cell-autonomously in MB or EB neurons, implicating it in cell-cell signaling\",\n      \"method\": \"Drosophila ptpmeg loss-of-function mutants, neuronal rescue experiments, domain-specific transgenes (phosphatase-dead, FERM mutants), mosaic analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with domain-specific mutants in an ortholog; multiple neuronal circuit phenotypes characterized\",\n      \"pmids\": [\"17138662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NSPA (neuronal surface P antigen) ubiquitinates PTPMEG (mouse PTPN4 ortholog), targeting it for proteasomal degradation; in NSPA-KO mice, PTPMEG accumulates at postsynaptic densities, leading to reduced tyrosine phosphorylation of GluN2B (Tyr1472 endocytic signal) and selective removal of GluN2A and GluN2B NMDAR subunits from PSDs, impairing hippocampal LTP and memory\",\n      \"method\": \"Cell-based ubiquitination assay, immunoblot of synaptic fractions, NSPA-KO mouse electrophysiology (LTP), behavioral memory assays, GluN2B pTyr1472 immunoblot\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ubiquitination assay plus KO epistasis plus electrophysiology; single lab but orthogonal methods\",\n      \"pmids\": [\"33158444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HPV16 E6 oncoprotein C-terminal PDZ-binding motif directly binds the PDZ domain of PTPN4; crystal structure and ITC measurements show that hydrophobic interactions (Leu158 of E6) and hydrogen bond networks sustain the complex; high-risk HPV genotypes 16, 18, 31, 33, and 45 all bind PTPN4 PDZ with comparable affinities\",\n      \"method\": \"X-ray crystallography of PTPN4-PDZ/HPV E6 peptide complex, isothermal titration calorimetry\",\n      \"journal\": \"Journal of microbiology (Seoul, Korea)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus ITC quantitative binding; rigorous structural and biochemical validation\",\n      \"pmids\": [\"35089587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"E3 ubiquitin ligase MARCH8 promotes PTPN4 protein degradation via ubiquitination; loss of PTPN4 activates STAT3 phosphorylation (pSTAT3 Tyr705) and its transcriptional activity, promoting pancreatic cancer growth and metastasis\",\n      \"method\": \"Ubiquitination assay, co-immunoprecipitation, PTPN4 overexpression/knockdown in pancreatic cancer cells, xenograft in vivo model, western blot for pSTAT3\",\n      \"journal\": \"Pancreas\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — ubiquitination assay identifies MARCH8 as E3 ligase for PTPN4; STAT3 dephosphorylation pathway confirmed; 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 in ovarian cancer cells\",\n      \"method\": \"Co-immunoprecipitation, overexpression/knockdown studies in ovarian cancer cells, xenograft model, western blot for pSTAT3, NLS mapping\",\n      \"journal\": \"Cancer medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP and functional assay; nuclear transport mechanism partially characterized; single lab\",\n      \"pmids\": [\"40145330\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTPN4 is a multi-domain non-receptor protein tyrosine phosphatase containing FERM, PDZ, and catalytic domains, whose phosphatase activity is autoinhibited by an intramolecular PDZ–phosphatase interaction mediated through the inter-domain linker; binding of PDZ ligands (including the C-termini of GluN2A, GluRδ2, p38γ, or viral proteins) relieves this autoinhibition and activates catalysis, enabling PTPN4 to dephosphorylate substrates including TRAM (suppressing TRIF-dependent TLR4/IRF3 signaling), pSTAT3-Tyr705 (suppressing STAT3-driven proliferation), CrkI, TCR ζ-chain ITAMs, and the p38γ activation loop; PTPN4 is regulated post-translationally by calpain cleavage (activating) and MARCH8-mediated ubiquitination/degradation, is localized to membranes, cytoskeleton, and dendritic spines, and is required in Purkinje cells for cerebellar LTD and motor learning.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PTPN4 is a non-receptor protein tyrosine phosphatase that functions as a signal-modulating enzyme in synaptic plasticity, innate immunity, and growth control, with its catalytic activity gated by an intramolecular PDZ–phosphatase autoinhibitory mechanism. The PDZ domain directly inhibits phosphatase activity through an inter-domain linker containing a conserved hydrophobic patch; binding of C-terminal PDZ ligands—including glutamate receptor subunits GluN2A and GluRδ2, p38γ MAPK, and high-risk HPV E6 oncoproteins—relieves this autoinhibition and activates catalysis [PMID:25158884, PMID:28801650, PMID:27246854, PMID:35089587]. Identified substrates include TRAM, whose dephosphorylation suppresses TRIF-dependent TLR4/IRF3 signaling and IFN-β production [PMID:25825441]; pSTAT3-Tyr705, whose dephosphorylation restrains STAT3-driven tumor proliferation [PMID:31025789, PMID:37747937]; and CrkI, whose dephosphorylation inhibits cell migration [PMID:23666597]. In Purkinje neurons, PTPN4 is required for cerebellar long-term depression and motor learning, and its FERM domain directs localization to dendritic spines where it regulates NMDA receptor phosphorylation [PMID:17953619, PMID:30238967, PMID:33158444].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that PTPN4 is membrane/cytoskeleton-associated and subject to proteolytic activation answered how the phosphatase is regulated post-translationally: calpain cleavage within the PEST-containing intermediate domain activates catalytic activity 4–8-fold, linking calcium signaling to PTPN4 function in platelets.\",\n      \"evidence\": \"Subcellular fractionation, recombinant protein calpain/trypsin cleavage assays, and calpain inhibitor treatment in human platelets\",\n      \"pmids\": [\"8910369\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether calpain activation of PTPN4 occurs in neurons or immune cells\", \"Identity of platelet substrates dephosphorylated by activated PTPN4\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Demonstrating that PTPN4 overexpression suppresses cell growth and anchorage-independent colony formation—partially independent of catalytic activity—established that PTPN4 functions as a growth suppressor with both enzymatic and scaffolding contributions.\",\n      \"evidence\": \"Stable COS-7 lines expressing wild-type or catalytically inactive C→S mutant PTPN4, soft-agar assays\",\n      \"pmids\": [\"8917530\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The non-catalytic mechanism of growth inhibition was not identified\", \"Relevance to endogenous expression levels unknown\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identification of glutamate receptors GluRδ2 and GluN2B as PDZ-domain-dependent interactors of PTPN4 placed the phosphatase at postsynaptic sites and suggested a role in modulating excitatory neurotransmission.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation from brain tissue and cultured cells, PDZ domain binding assays\",\n      \"pmids\": [\"10748123\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PTPN4 directly dephosphorylates GluRδ2 or GluN2B was not demonstrated\", \"Functional consequence of the interaction on synaptic transmission was not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Drosophila Ptpmeg mutant analysis revealed that PTPN4-family phosphatase activity is essential for axon projection and that the FERM domain has a distinct, selective role in axon stabilization, providing the first in vivo genetic evidence for domain-specific functions.\",\n      \"evidence\": \"Drosophila loss-of-function mutants, domain-specific transgenic rescue, mosaic analysis of mushroom body neurons\",\n      \"pmids\": [\"17138662\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mammalian axon guidance roles not tested\", \"Cell-autonomous versus non-cell-autonomous substrates not identified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"PTPN4 knockout mice demonstrated that the phosphatase is required for cerebellar long-term depression and motor learning, converting the receptor-interaction data into a defined physiological function at identified synapses.\",\n      \"evidence\": \"PTPN4 KO mice, rotarod and eyeblink conditioning behavior, electrophysiology at parallel fiber–Purkinje cell synapses\",\n      \"pmids\": [\"17953619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The specific synaptic substrate(s) mediating LTD impairment were not identified\", \"Whether hippocampal synaptic plasticity is also affected was not examined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Substrate-trapping identified TCR ζ-chain ITAMs as PTPN4 substrates, but genetic ablation of PTPN4 alone or together with PTPN3 and PTPN13 revealed complete dispensability for T cell signaling, establishing functional redundancy in the immune compartment.\",\n      \"evidence\": \"Substrate-trapping co-IP, single/double/triple KO mice, T cell development and cytokine assays\",\n      \"pmids\": [\"18614237\", \"19107198\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the compensating phosphatase(s) in T cells\", \"Whether PTPN4 contributes to T cell signaling under stress or infection conditions\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Reconstitution of the PDZ–phosphatase bidomain revealed that catalytic activity is autoinhibited by the PDZ domain in a compact conformation, and that PDZ ligand binding relieves this inhibition—establishing the core allosteric mechanism governing PTPN4 activation.\",\n      \"evidence\": \"AUC, SAXS, NMR of the bidomain construct, in vitro phosphatase assays ± PDZ ligands\",\n      \"pmids\": [\"25158884\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length structural context including the FERM domain was not resolved\", \"Quantitative coupling between ligand affinity and activation extent was not fully defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of TRAM as a PTPN4 substrate in macrophages demonstrated that the phosphatase negatively regulates innate immune signaling by specifically suppressing TRIF-dependent IRF3 activation and IFN-β production downstream of TLR4.\",\n      \"evidence\": \"Co-IP, tyrosine phosphorylation assays, PTPN4 overexpression/knockdown in macrophages, IRF3 and IFN-β reporter assays\",\n      \"pmids\": [\"25825441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PDZ-ligand-dependent activation is required for TRAM dephosphorylation in cells\", \"Role in other TLR pathways not tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Crystal structures of the PTPN4-PDZ/p38γ complex and enzymatic assays showed that p38γ is both the highest-affinity endogenous PDZ ligand and a direct substrate (activation-loop dephosphorylation), unifying the allosteric activation and substrate-recognition functions of the PDZ domain.\",\n      \"evidence\": \"X-ray crystallography, ITC, in vitro phosphatase assays on p38γ activation-loop phosphopeptides\",\n      \"pmids\": [\"27246854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo validation of p38γ as a PTPN4 substrate in a physiological setting\", \"Whether other MAPKs are similarly regulated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Mutagenesis of the conserved hydrophobic patch in the PDZ–phosphatase inter-domain linker pinpointed the molecular element mediating autoinhibition and ligand-dependent activation, refining the allosteric model to a specific structural element.\",\n      \"evidence\": \"Site-directed mutagenesis of linker residues, kinetic phosphatase assays\",\n      \"pmids\": [\"28801650\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No full-length structure showing linker conformation in autoinhibited state\", \"Contribution of FERM domain to regulation not addressed\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"A de novo FERM-domain missense variant (p.Leu72Ser) was shown to abolish PTPN4 localization to dendritic spines, linking the FERM domain to subcellular targeting and implicating PTPN4 dysfunction in neurodevelopmental disease.\",\n      \"evidence\": \"Transfection of WT/mutant PTPN4 in mouse hippocampal neurons, confocal imaging\",\n      \"pmids\": [\"30238967\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional synaptic consequence of mislocalization not measured\", \"Number of patients carrying this variant is very small\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identification of STAT3-Tyr705 as a direct PTPN4 substrate established a tumor-suppressive axis: loss of PTPN4 in colorectal cancer derepresses STAT3 transcriptional activity and accelerates tumor growth.\",\n      \"evidence\": \"Co-IP, pSTAT3 dephosphorylation assay, PTPN4 overexpression/knockdown, xenograft model\",\n      \"pmids\": [\"31025789\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PDZ-dependent activation is required for STAT3 dephosphorylation\", \"Contribution of other PTPN4 substrates to growth suppression\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery that NSPA/MARCH8 ubiquitinates PTPN4 targeting it for degradation, and that PTPN4 accumulation in NSPA-KO mice causes NMDAR removal from PSDs and impaired hippocampal LTP, revealed ubiquitin-dependent turnover as a key regulatory layer and extended PTPN4's synaptic role beyond the cerebellum.\",\n      \"evidence\": \"Cell-based ubiquitination assay, NSPA-KO mouse synaptic fractionation, LTP electrophysiology, behavioral memory assays\",\n      \"pmids\": [\"33158444\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether NSPA-mediated and MARCH8-mediated ubiquitination are redundant or context-specific\", \"Direct substrate identity at hippocampal synapses not fully defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Crystal structures of the PTPN4-PDZ domain bound to high-risk HPV E6 C-terminal peptides revealed the structural basis of viral hijacking of PTPN4 and showed that multiple oncogenic HPV genotypes converge on PTPN4-PDZ binding, suggesting a conserved viral immune/growth evasion strategy.\",\n      \"evidence\": \"X-ray crystallography, ITC with HPV16/18/31/33/45 E6 peptides\",\n      \"pmids\": [\"35089587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of HPV E6–PTPN4 interaction on phosphatase activity or cell signaling not tested in cells\", \"Whether E6 binding activates or sequesters PTPN4 in vivo unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"MARCH8 was identified as an E3 ligase promoting PTPN4 ubiquitination and degradation, and loss of PTPN4 activated STAT3 signaling to drive pancreatic cancer growth and metastasis, extending the PTPN4–STAT3 tumor-suppressive axis to a second cancer type and identifying an upstream regulatory E3.\",\n      \"evidence\": \"Ubiquitination assay, co-IP, PTPN4 overexpression/knockdown in pancreatic cancer cells, xenograft model\",\n      \"pmids\": [\"37747937\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether MARCH8 ubiquitinates PTPN4 at the same sites as NSPA\", \"Structural basis of MARCH8–PTPN4 interaction unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A full-length structure of PTPN4 integrating FERM, PDZ, and phosphatase domains has not been determined, leaving unknown how the FERM domain coordinates with PDZ-dependent allosteric regulation and subcellular targeting to control substrate access in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No full-length atomic structure\", \"Mechanism by which FERM domain directs dendritic spine localization is structurally uncharacterized\", \"In vivo substrates responsible for cerebellar LTD remain unidentified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 5, 6, 9, 10, 12]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [18]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [5, 6, 7]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 10, 12]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 3, 15]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GRIN2A\",\n      \"GRID2\",\n      \"MAPK12\",\n      \"TICAM2\",\n      \"STAT3\",\n      \"CRK\",\n      \"MARCH8\",\n      \"GRIN2B\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}