PTPRCAP

PTPRCAP (CD45-AP/LPAP) is a leukocyte-restricted type I transmembrane adapter protein that regulates the receptor tyrosine phosphatase CD45 and proximal antigen-receptor signaling in lymphocytes (PMID:8300558, PMID:9607926). It was originally purified by its specific physical association with CD45, and reconstituted in vitro binding confirmed the interaction is direct (PMID:8300558); domain mapping established that the transmembrane segment of PTPRCAP docks onto the transmembrane portion of CD45, with the bulk of the adapter oriented in the cytoplasm (PMID:7673147). By opposing CD45 dimer formation, PTPRCAP stabilizes the active monomeric phosphatase and elevates CD45 catalytic activity, since CD45-AP-null cells accumulate inactive CD45 dimers and show reduced phosphatase activity (PMID:14715639). Through an acidic segment in its cytoplasmic region, PTPRCAP binds directly and selectively to the kinase domains of Lck and ZAP-70 in a TCR-stimulation-dependent manner, the Lck interaction scaling with Lck catalytic activation, thereby assembling a multimolecular complex containing CD45, TCR/CD3, CD4/CD8 coreceptors, and p56(lck) (PMID:10318863, PMID:9880514). Functionally, loss of PTPRCAP in mice reduces antigen-receptor-driven T and B cell proliferation and cytotoxic responses, weakens CD45 association with the TCR/CD3 complex and Lck, and lowers the threshold for T-cell activation under low-potency ligand stimulation (PMID:9607926, PMID:17428310); it also restrains transitional B-cell maturation into the marginal-zone and follicular pools (PMID:25717326). Beyond the immune system, PTPRCAP behaves as a tumor suppressor in lung adenocarcinoma, where its overexpression promotes apoptosis via increased Bax and cleaved caspase-3 with reduced Bcl-2 and inhibits Wnt/β-catenin signaling, and where it is suppressed by miR-582-3p binding to its 3'-UTR (PMID:41411247, PMID:41064095).

1. 1994 High

   Established that a leukocyte-specific ~30 kDa protein physically associates with CD45, defining PTPRCAP as a candidate CD45 adapter.

   Evidence Protein purification by CD45 association, in vitro translation/binding, and cDNA cloning

   PMID:8300558

   Open questions at the time

   • Did not localize the binding interface
   • No functional consequence of the interaction established
2. 1995 High

   Mapped the CD45 docking interface to the transmembrane segments and defined PTPRCAP's membrane topology, explaining how it engages CD45 within the membrane.

   Evidence Deletion/chimeric mutant binding analysis, cell fractionation, protease protection in lymphocytes

   PMID:7673147

   Open questions at the time

   • Did not address effect on CD45 enzymatic activity
   • Cytoplasmic functional partners unidentified
3. 1997 Medium

   Showed PTPRCAP exists in two N-terminal variant forms, that its stability depends on CD45, and that expression is confined to T, B, and pre-B cells, refining its cellular distribution.

   Evidence Northern blot, Western blot, pulse-chase half-life analysis, cell-type expression survey

   PMID:9045908

   Open questions at the time

   • Functional difference between the two isoforms unresolved
   • Mechanism of CD45-dependent stabilization unknown
4. 1996 Medium

   Defined the human and mouse gene structure and chromosomal location, providing the genomic foundation for the locus.

   Evidence FISH, multilocus cross Southern hybridization, genomic sequencing in two independent groups

   PMID:8954783 PMID:8975722

   Open questions at the time

   • No regulatory element function characterized
   • Does not address protein function
5. 1998 High

   Demonstrated in vivo that PTPRCAP supports antigen-receptor responses and is required for efficient CD45–Lck association, linking the adapter to functional immune signaling.

   Evidence CD45-AP knockout mice with co-IP, proliferation, MLR, and CTL assays

   PMID:9607926

   Open questions at the time

   • Whether the CD45–Lck effect is direct or indirect not resolved
   • Molecular basis of impaired proliferation undefined
6. 1999 High

   Identified the cytoplasmic acidic segment as a direct, activity-dependent binding site for the Lck and ZAP-70 kinase domains and placed PTPRCAP within a CD45/TCR/coreceptor/Lck complex.

   Evidence Co-IP in multiple cell contexts, structure-function deletion analysis, recombinant protein binding, TCR stimulation

   PMID:10318863 PMID:9880514

   Open questions at the time

   • Selectivity over Fyn/Csk mechanistically unexplained
   • Stoichiometry of the multimolecular complex unknown
7. 1999 Medium

   Provided a contrasting knockout phenotype in which CD45 surface expression was reduced but CD45–Lck assembly and polyclonal responses were intact, complicating the model of PTPRCAP as a pure positive regulator.

   Evidence Exon 2 disruption KO, immunofluorescence, co-IP, T-cell proliferation assay

   PMID:10602004

   Open questions at the time

   • Conflicts with the CD45–Lck disruption reported in other KO lines
   • Mechanism of reduced CD45 surface expression not defined
8. 2004 High

   Resolved how PTPRCAP regulates CD45 activity by showing it suppresses inactive CD45 dimer formation and thereby raises phosphatase activity.

   Evidence KO T cells and a CD45-AP-null T-cell line (ALST-1) with reconstitution, CD45 dimer assay, microsomal PTP activity assay

   PMID:14715639

   Open questions at the time

   • Structural basis for monomer stabilization unknown
   • Link between dimer suppression and downstream signaling outputs not directly traced
9. 2007 High

   Showed PTPRCAP stabilizes CD45 association with the TCR/CD3 complex and Lck and lowers the activation threshold for weak-affinity ligands, sharpening its role in signal sensitivity.

   Evidence KO mice with co-IP, proliferation, IL-2 ELISA, calcium flux, altered peptide ligand stimulation

   PMID:17428310

   Open questions at the time

   • Why only low-potency ligand responses are affected unexplained
   • Quantitative contribution to substrate recruitment unknown
10. 2015 Medium

    Extended PTPRCAP function to B-cell development, showing it controls transitional-to-mature B-cell subset transitions.

    Evidence RNA-seq of sorted B-cell subsets with gene-deleted and overexpressing transgenic mice

    PMID:25717326

    Open questions at the time

    • Molecular mechanism of subset control not resolved
    • Whether effect depends on CD45 phosphatase regulation untested
11. 2025 Medium

    Identified a non-immune tumor-suppressor role in lung adenocarcinoma via apoptosis induction and Wnt/β-catenin inhibition, with miR-582-3p as an upstream repressor of PTPRCAP.

    Evidence Overexpression in LUAD lines, CCK-8/migration/apoptosis assays, Western blot for Bax/Bcl-2/caspase-3 and Wnt components, xenografts, dual-luciferase reporter, rescue experiments

    PMID:41064095 PMID:41411247

    Open questions at the time

    • Whether tumor suppression involves CD45/Lck or is phosphatase-independent unknown
    • Single-lab cancer cell line data without independent replication
    • Direct molecular link between PTPRCAP and Wnt/β-catenin not established

• How PTPRCAP mechanistically connects its membrane CD45-regulatory function to the cytoplasmic Wnt/apoptosis effects observed in epithelial tumors remains unresolved.
• No structural model of the CD45–PTPRCAP transmembrane complex
• Conflicting KO phenotypes on CD45–Lck not reconciled
• Mechanism bridging immune and tumor-suppressor roles unknown