PTPRK

PTPRK is a transmembrane receptor-type protein tyrosine phosphatase that functions as a key regulator of cell-cell adhesion, contact-dependent growth inhibition, and Wnt/EGFR signaling through both catalytic dephosphorylation and homophilic intercellular adhesion. Its extracellular MAM-Ig-fibronectin III domain mediates calcium-independent trans-homophilic binding via a head-to-tail dimer interface dependent on residue W351, while its intracellular D1 catalytic domain dephosphorylates junctional substrates (Afadin, PARD3, delta-catenin family members, ITGB4), the Wnt-receptor-degrading E3 ligase ZNRF3 at a '4Y' endocytic motif to promote Wnt receptor turnover, EGFR, beta-catenin, CD133, and the metabolic enzyme FBP1 (PMID:30924770, PMID:31934854, PMID:18276111, PMID:39496584, PMID:41820225). Substrate recruitment is achieved in part through the D2 pseudophosphatase domain, which binds the Afadin coiled-coil region independently of phosphorylation, conferring specificity distinct from the paralog PTPRM (PMID:36264065). PTPRK suppresses proliferative signaling by inhibiting E2F transcriptional activity through induction of CDK inhibitors p21 and p27, by limiting Wnt/beta-catenin nuclear accumulation and target gene expression—opposed by MET-mediated phosphorylation of ZNRF3—and by restraining EGFR/ERK signaling in intestinal epithelium (PMID:36551956, PMID:34590584, PMID:36611909).

1. 1993 High

   Establishing that PTPRK is processed by furin into two associated subunits resolved how the mature receptor is generated at the cell surface and set the stage for understanding ectodomain regulation.

   Evidence Site-directed mutagenesis of the furin consensus site in the fourth FNIII repeat, antibody-based detection of cleavage products

   PMID:8474452

   Open questions at the time

   • Whether furin cleavage is required for adhesion or phosphatase activity was not addressed
   • Additional ectodomain shedding events (e.g., by ADAM10) were not examined
2. 1994 High

   Demonstrating that PTPRK mediates calcium-independent homophilic adhesion via its extracellular domain—independent of phosphatase activity—established a dual functionality as both an adhesion molecule and a phosphatase.

   Evidence Heterologous expression, purified extracellular domain bead aggregation assay

   PMID:8264577

   Open questions at the time

   • The specific extracellular domain regions sufficient for adhesion were not mapped
   • In vivo contribution of homophilic adhesion versus phosphatase activity to cell-cell contact was unknown
3. 2007 Medium

   Identifying PTPRK as a TGF-beta/Smad2 target gene linked its transcriptional regulation to a major growth-inhibitory pathway and revealed how EBV-encoded EBNA1 suppresses PTPRK to promote lymphoma cell survival.

   Evidence PTPRK gain- and loss-of-function in Hodgkin lymphoma cell lines, Smad2 protein stability assays, EBNA1 expression constructs

   PMID:17720884

   Open questions at the time

   • Whether Smad2 binds the PTPRK promoter directly was not shown
   • Generalizability beyond Hodgkin lymphoma was untested
4. 2007 High

   Genetic deletion and reconstitution in the LEC rat demonstrated that Ptprk is required for CD4 single-positive T cell maturation in the thymus, establishing an in vivo immune function.

   Evidence Linkage mapping of ~380 kb Ptprk deletion in LEC rats, bone marrow reconstitution with lentiviral Ptprk expression

   PMID:17909891

   Open questions at the time

   • The substrate(s) mediating T cell maturation are unknown
   • Whether this reflects a T-cell-intrinsic or stromal function was not resolved
5. 2008 Medium

   Showing that PTPRK dephosphorylates beta-catenin and suppresses its nuclear accumulation, thereby reducing cyclin D1 and c-myc, provided the first mechanistic link between PTPRK and Wnt/beta-catenin target gene suppression.

   Evidence Overexpression and siRNA knockdown in HEK293 and melanoma cells, subcellular fractionation, Western blot

   PMID:18276111

   Open questions at the time

   • Direct in vitro dephosphorylation of beta-catenin was not demonstrated
   • The specific tyrosine residue(s) targeted on beta-catenin were not identified
6. 2019 High

   Unbiased phosphoproteomics and in vitro dephosphorylation assays identified Afadin, PARD3, and delta-catenin family members as direct PTPRK substrates at cell-cell contacts, redefining PTPRK as a junction-focused phosphatase whose loss disrupts epithelial integrity.

   Evidence Quantitative tyrosine phosphoproteomics, BioID proximity labeling, co-immunoprecipitation, in vitro dephosphorylation, phosphatase-dead mutant cell lines

   PMID:30924770

   Open questions at the time

   • Structural basis for substrate recognition at the active site was not determined
   • Relative contribution of each substrate to junction maintenance was not dissected
7. 2020 High

   Identifying the ZNRF3 '4Y' endocytic motif as a direct PTPRK substrate revealed how PTPRK controls Wnt receptor turnover: dephosphorylation of this motif promotes ZNRF3 internalization and Wnt receptor degradation, with PTPRK loss causing Wnt-dependent axial defects in Xenopus.

   Evidence Phosphomutant analysis of ZNRF3 4Y motif, ZNRF3 internalization assays, Wnt reporter assays, Xenopus loss-of-function embryos

   PMID:31934854

   Open questions at the time

   • Whether the PTPRK-ZNRF3 axis operates in mammalian intestinal stem cells in vivo was not shown
   • How PTPRK is recruited to ZNRF3 was not defined
8. 2021 High

   The discovery that MET phosphorylates the same ZNRF3 4Y motif that PTPRK dephosphorylates established a kinase-phosphatase rheostat controlling Wnt receptor degradation downstream of HGF signaling.

   Evidence Reciprocal co-immunoprecipitation of MET-ZNRF3, pharmacological MET inhibition, siRNA, ZNRF3 internalization and Wnt reporter assays

   PMID:34590584

   Open questions at the time

   • In vivo validation of the MET-PTPRK rheostat in tumor models was lacking
   • Other kinases that may phosphorylate the 4Y motif were not surveyed
9. 2022 High

   The crystal structure of the PTPRK N-terminal domains revealed a head-to-tail homodimer, and mutagenesis of W351 abolished dimerization, defining the molecular determinant of homophilic specificity distinguishing PTPRK from PTPRM.

   Evidence X-ray crystallography, SAXS, site-directed mutagenesis of W351, in vitro dimerization assay

   PMID:36436563

   Open questions at the time

   • Full-length ectodomain structure in the context of a membrane is unavailable
   • Whether W351-dependent dimerization regulates phosphatase activity in trans was not tested
10. 2022 High

    Mapping the Afadin-PTPRK interaction to the D2 pseudophosphatase domain (binding Afadin's coiled-coil distal from the substrate pTyr) resolved how PTPRK achieves substrate specificity through a two-site recognition mechanism distinct from PTPRM.

    Evidence Pulldown assays, mutagenesis of D2 domain and Afadin coiled-coil, interaction mapping

    PMID:36264065

    Open questions at the time

    • Whether D2-mediated recruitment applies to substrates beyond Afadin was not determined
    • No co-crystal structure of the D2-Afadin interface exists
11. 2022 Medium

    Demonstrating that PTPRK suppresses E2F transcriptional activity by inducing p21/p27 and inhibiting CDK2 established a cell-cycle arrest mechanism underlying contact-dependent growth inhibition.

    Evidence siRNA knockdown, overexpression, E2F luciferase reporter, CDK2 activity assay, soft agar and xenograft assays

    PMID:36551956

    Open questions at the time

    • The direct PTPRK substrate linking dephosphorylation to CDK inhibitor induction is unidentified
    • Whether this pathway is independent of beta-catenin dephosphorylation was not addressed
12. 2022 Medium

    Showing that PTPRK dephosphorylates EGFR in intestinal organoids and that its loss increases pEGFR/pERK and proliferation connected PTPRK to epithelial homeostasis and suggested relevance to celiac disease pathology.

    Evidence siRNA silencing and overexpression in intestinal organoids, pEGFR/pERK Western blot, BrdU proliferation assay

    PMID:36611909

    Open questions at the time

    • Direct in vitro dephosphorylation of EGFR by PTPRK was not shown
    • EGFR regulation was later shown to be partly catalytic-activity-independent (PMID:38904097)
13. 2024 Medium

    Separation-of-function analysis using phosphatase-dead mutants revealed that PTPRK regulation of EGFR is independent of catalytic activity, establishing additional scaffold/adaptor functions beyond its enzymatic role.

    Evidence Phosphatase-dead PTPRK mutants in CRC cell lines, invasion assays, mouse colitis model, xenograft assays

    PMID:38904097

    Open questions at the time

    • The non-catalytic mechanism by which PTPRK regulates EGFR is undefined
    • Whether other reported substrates are similarly regulated by scaffolding remains untested
14. 2024 High

    Identifying FBP1 as a PTPRK substrate in hepatocytes expanded its functional repertoire to metabolic regulation: PTPRK dephosphorylation of FBP1 promotes glycolysis, PPARγ-driven lipogenesis, and hepatic fat accumulation.

    Evidence Phosphoproteomics in primary hepatocytes, PTPRK knockout mouse on high-fat diet, hepatic metabolomics, xenograft assays

    PMID:39496584

    Open questions at the time

    • The specific FBP1 phosphosite(s) targeted by PTPRK were not structurally characterized
    • How PTPRK accesses the cytosolic enzyme FBP1 given its membrane localization was not explained
15. 2025 Medium

    PTPRK was implicated in postherpetic neuralgia through activation of DUSP1/p38 MAPK signaling in dorsal root ganglia, extending its roles to neuroinflammation and pain.

    Evidence Rat RTX-induced PHN model, PTPRK overexpression/knockdown in DRG, Western blot for DUSP1/phospho-p38, behavioral pain assays

    PMID:41253902

    Open questions at the time

    • Direct dephosphorylation of DUSP1 by PTPRK was not demonstrated
    • The mechanism by which a phosphatase 'activates' a MAPK pathway is counterintuitive and unresolved
16. 2026 Medium

    Identification of ITGB4 as a PTPRK-binding partner and substrate in CRC, with tumor-derived D1 domain mutations impairing ITGB4 dephosphorylation, provided genetic evidence linking PTPRK catalytic loss to integrin-driven tumor progression.

    Evidence Whole exome sequencing of CRC tumors, co-immunoprecipitation, ITGB4 phosphorylation in cells expressing mutant PTPRK, xenograft assays

    PMID:41820225

    Open questions at the time

    • In vitro dephosphorylation of ITGB4 by wild-type vs. mutant PTPRK was not directly shown
    • Which ITGB4 tyrosine site(s) are targeted was not mapped

• Key open questions include: (1) a structural model of the full intracellular tandem phosphatase domain with bound substrate, (2) the identity of the direct substrate(s) mediating CD4 T cell maturation, (3) the non-catalytic mechanism by which PTPRK regulates EGFR, and (4) how PTPRK substrate selection is regulated by cell-contact-dependent trans-dimerization.
• No full-length intracellular domain structure exists
• In vivo substrate identification in T cells has not been performed
• The catalytic-independent EGFR regulatory mechanism is molecularly undefined