{"gene":"SKAP1","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":1997,"finding":"SKAP55 (SKAP1) was identified as a substrate that associates with p59fyn in human T-lymphocytes via its SH2 domain, and selectively binds to isolated SH2 domains of Lck, Lyn, Src, and Fyn but not ZAP70, Syk, Shc, SLP-76, Grb2, PI3K, or c-abl in vitro. The protein contains a pleckstrin homology domain, a C-terminal SH3 domain, and several potential tyrosine phosphorylation sites.","method":"GST-SH2 domain pulldown, molecular cloning, in vitro binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro binding assays with multiple SH2 domains, replicated across labs","pmids":["9195899"],"is_preprint":false},{"year":1998,"finding":"SKAP55 (SKAP1) binds directly to SLAP-130 (ADAP/FYB) via the SH3 domain of SKAP55 and the proline-rich sequence of SLAP-130, as demonstrated by co-immunoprecipitation, co-expression in COS cells, and two-hybrid analysis.","method":"Two-hybrid screen, co-expression/co-IP in COS cells, truncation mutants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — direct binding established by two-hybrid and co-expression with truncation mutants, replicated across labs","pmids":["9748251","9671755"],"is_preprint":false},{"year":1998,"finding":"FYB (ADAP/SLAP-130) serves as a binding partner for SKAP55 (SKAP1) in T cells; both proteins colocalize in the perinuclear region and SKAP55 acts as a substrate for FYN kinase. The interaction is mediated through the SH3 domain of SKAP55.","method":"Two-hybrid screen with FYB as bait, immunofluorescence confocal microscopy, kinase assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — two-hybrid plus localization plus kinase substrate assay, replicated","pmids":["9671755"],"is_preprint":false},{"year":2000,"finding":"The SH3 domain of FYN binds to a novel proline-independent RKxxYxxY motif in SKAP55 (SKAP1). This non-canonical motif binding overlaps with the proline-based binding site on the charged surface of the SH3 domain. Expression of the RKGDYASY peptide inhibited TcRzeta/CD3-mediated NF-AT transcription in T cells.","method":"Peptide precipitation, alanine scanning, 2D NMR of FYN-SH3 with bound peptide, in vivo co-expression","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — NMR structure plus mutagenesis plus functional in vivo assay in single rigorous study","pmids":["10856234"],"is_preprint":false},{"year":2002,"finding":"SKAP55 (SKAP1) is phosphorylated at Tyr-232 and associates with CD45 in vivo; anti-CD3 stimulation promotes SKAP55 tyrosine phosphorylation and translocation from cytoplasm to membrane. SKAP55 couples CD45 with Src family kinases, positively regulating TCR-mediated IL-2 promoter transcriptional activation.","method":"Yeast two-hybrid, mutational analysis (Y232F), overexpression in Jurkat, IL-2 promoter reporter assay","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — two-hybrid plus mutagenesis plus functional transcription assay, single lab","pmids":["11909961"],"is_preprint":false},{"year":2002,"finding":"SKAP55 (SKAP1) forms homodimers through its SH3 domain and SK region, translocates to lipid rafts upon TCR activation, and interacts with Fyn kinase and Grb-2 at Tyr-271. Overexpression of SKAP55 in Jurkat cells activates MAPK following TCR engagement.","method":"Co-immunoprecipitation, mutational analysis, stable overexpression, lipid raft fractionation","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal co-IP with mutagenesis and fractionation, single lab","pmids":["12171928"],"is_preprint":false},{"year":2003,"finding":"SKAP55 (SKAP1) regulates LFA-1-mediated adhesion and T cell-APC conjugate formation. SKAP55 colocalizes with actin at the T cell-APC synapse and promotes LFA-1 clustering. Conjugate formation requires the SKAP-55 SH3 domain and is accompanied by translocation of SKAP-55 to membrane rafts regulated by both LFA-1 and TCR ligation.","method":"Overexpression, adhesion assays (fibronectin/ICAM-1), confocal microscopy, lipid raft fractionation, SH3 deletion mutants","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (adhesion assay, confocal, fractionation, domain deletion) in defined cellular context","pmids":["12652296"],"is_preprint":false},{"year":2003,"finding":"In mast cells, SKAP55 (SKAP1) forms a complex with SLAP-130 and MIST; collaboration of SLAP-130 with SKAP55 recruits MIST to Fyn rather than Lyn, regulated by higher affinity binding of SLAP-130/SKAP55 to the Fyn-SH2 domain compared to the Lyn-SH2 domain.","method":"Co-immunoprecipitation, direct binding assays, affinity comparison","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 3 — co-IP and affinity comparison, single lab","pmids":["12681493"],"is_preprint":false},{"year":2005,"finding":"siRNA knockdown of SKAP-55 (SKAP1) identified an essential role for this adaptor in TCR-mediated inside-out signaling for LFA-1 clustering and T cell-APC conjugation, distinct from TCR-CD3 clustering. SKAP-55R cannot compensate for loss of SKAP-55 in LFA-1 clustering.","method":"siRNA knockdown, LFA-1 clustering assay, conjugation assay","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — specific KD with defined phenotypic readouts and specificity controls (SKAP-55R non-rescue)","pmids":["15939789"],"is_preprint":false},{"year":2005,"finding":"SKAP55 (SKAP1) protein is rapidly degraded (half-life ~15-20 min) in the absence of ADAP, and ADAP stabilizes SKAP55 by causing a 5-fold decrease in its proteolysis rate. This stabilization requires the SH3 domain-mediated SKAP55-ADAP interaction.","method":"ADAP-deficient Jurkat cell line, protein stability/pulse-chase analysis, SH3 domain inactivation, restoration of ADAP expression","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — biochemical stability assays with domain mutant controls and rescue, single lab with multiple orthogonal methods","pmids":["15849195"],"is_preprint":false},{"year":2006,"finding":"The ADAP/SKAP55 signaling module is required for TCR-mediated inside-out signaling; disruption of ADAP/SKAP55 interaction displaces activated Rap1 from the plasma membrane without affecting its GTPase activity. Membrane targeting of the ADAP/SKAP55 module induces T-cell adhesion in the absence of TCR stimulation.","method":"Module disruption experiments, Rap1 membrane localization assay, constitutive membrane targeting constructs","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches including constitutive membrane targeting with functional readout","pmids":["16980616"],"is_preprint":false},{"year":2006,"finding":"Fyn kinase phosphorylates Tyr-294 in the RKXXY294XXY297 motif of SKAP-55 (SKAP1), blocking the ADAP C-terminal SH3 domain binding to this motif (confirmed by plasmon resonance). Phosphorylation of Tyr-294 (but not Tyr-297) disrupts ADAP-SH3c binding, and Y294F mutation blocks TCR-induced LFA-1-mediated adhesion to ICAM-1 and IL-2 promoter activity.","method":"In vivo phosphorylation by Fyn, plasmon resonance interaction analysis, Y294F and Y297F mutants, LFA-1 adhesion assay, IL-2 promoter reporter","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — plasmon resonance with mutagenesis plus functional in vivo assays","pmids":["16461356"],"is_preprint":false},{"year":2007,"finding":"SKAP55 (SKAP1) co-immunoprecipitates with RasGRP1, the Ras guanine nucleotide exchange factor. Binding requires the C-terminus of SKAP55 and is enhanced by tyrosine phosphorylation. Overexpression of SKAP55 disrupts TCR-to-Ras-Erk-AP1 signaling and IL-2 gene transcription, while knockdown decreases AP-1 reporter activity and ERK phosphorylation.","method":"Co-immunoprecipitation, RNAi knockdown, overexpression, reporter gene assay, ERK phosphorylation assay","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP with domain mapping plus gain- and loss-of-function with multiple readouts, single lab","pmids":["17658605"],"is_preprint":false},{"year":2008,"finding":"SKAP-55 (SKAP1) binds to RasGRP1 via its C-terminus and negatively regulates the p21ras-ERK pathway. SKAP-55 deficient primary T-cells show hyper-activation of ERK and increased RasGRP1 in the trans-Golgi network (where p21ras is activated), indicating SKAP-55 restricts RasGRP1 availability at the TGN.","method":"Skap1-/- primary T-cells, ERK phosphorylation assay, RasGRP1 localization by microscopy, C-terminal binding mutant, RNAi knockdown in T-cell lines","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — KO primary T-cells plus localization plus binding domain mutants, single lab","pmids":["18320039"],"is_preprint":false},{"year":2010,"finding":"SKAP1 (SKAP-55) N-terminal domain binds the C-terminal SARAH domain of RapL to form a SKAP1-RapL-Rap1 complex that binds LFA-1. In Skap1-/- primary T-cells, TCR-induced Rap1-RapL complex formation and LFA-1 binding are absent. A RapL mutation (L224A) that abrogates SKAP1 binding disrupts vesicular colocalization and T cell-DC conjugation, and reduces T cell dwell times with DCs in lymph nodes.","method":"Skap1-/- primary T-cells, co-IP, RapL point mutation (L224A), two-photon intravital imaging in lymph nodes, conjugation assays","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1/2 — KO primary T-cells, mutagenesis, and in vivo imaging with multiple functional readouts","pmids":["20346707"],"is_preprint":false},{"year":2011,"finding":"The PH domain of SKAP1 (specifically residue R131) is required for RapL translocation to plasma membranes and subsequent Rap1 and LFA-1 binding. PI3K pathway-dependent membrane targeting of SKAP1 via its PH domain enables RapL membrane recruitment. N-terminal myristoylated SKAP1 constitutively recruits RapL to membranes and activates LFA-1 without TCR ligation.","method":"PH domain R131M mutation, membrane fractionation, LFA-1/ICAM-1 binding assay, myr-tagged SKAP1 rescue, PI3K inhibitor","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1/2 — mutagenesis plus constitutive membrane targeting rescue plus PI3K pathway dissection","pmids":["21669874"],"is_preprint":false},{"year":2011,"finding":"Physical association of SKAP55 (SKAP1) with ADAP, through the PH domain of SKAP55 (residue R131), recruits ADAP to LFA-1 integrin complexes after TCR stimulation and is necessary and sufficient for integrin function rescue in ADAP-deficient T-cells. This SKAP55-ADAP association restricts ADAP's ability to interact with the NF-κB signalosome (CARMA1/TAK1), defining distinct functional pools.","method":"SKAP-ADAP chimeric fusion protein, R131M PH domain mutant, integrin activation assays, NF-κB reporter, LFA-1 co-IP in ADAP-/- T cells","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — chimeric protein, point mutant, KO rescue, multiple functional readouts in single study","pmids":["21525391"],"is_preprint":false},{"year":2011,"finding":"Two independent pools of the ADAP/SKAP55 module exist in T cells: one interacting with a RAPL/Mst1 complex and another linked to a RIAM/Mst1/Kindlin-3 complex; both complexes require ADAP/SKAP55 for binding to LFA-1 upon CCR7 stimulation, regulating both affinity and avidity of LFA-1.","method":"Co-immunoprecipitation, CCR7-stimulated adhesion assays, ADAP/SKAP55 module loss-of-function, in vivo homing assays","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP plus functional adhesion and in vivo homing, single lab","pmids":["22117043"],"is_preprint":false},{"year":2013,"finding":"SKAP55 (SKAP1) dimerizes via its N-terminal region and the dimerization enables coimmunoprecipitation of RIAM, talin recruitment into TCR-induced adhesive junctions, and inside-out signaling to β1 integrins. SKAP55 dimerization and ADAP binding together are required for SLP-76 microcluster persistence and movement.","method":"Tandem dimer constructs, co-IP, timelapse/TIRF microscopy of SLP-76 microclusters, integrin signaling assays","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — engineered dimer constructs plus live cell imaging plus co-IP plus functional signaling assays","pmids":["24368808"],"is_preprint":false},{"year":2015,"finding":"The ADAP-SKAP55 signaling module promotes PD-1 expression on CD8+ CTLs in a Fyn-, Ca2+-, and NFATc1-dependent manner. SKAP55 or ADAP knockout reduces PD-1 expression and enhances anti-tumor CTL activity. Adoptive transfer of SKAP55-deficient CD8+ CTLs blocks tumor growth.","method":"Skap55-/- and Adap-/- mice, DC vaccine tumor models, adoptive transfer, NFATc1 inhibitor (CsA), flow cytometry for PD-1","journal":"EMBO molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — KO mice plus pharmacological inhibition plus adoptive transfer, single lab","pmids":["25851535"],"is_preprint":false},{"year":2016,"finding":"Skap1-/- T-cells show reduced translocation of talin and RIAM to the T cell-DC contact interface, and an altered pattern of talin cleavage. Expression of a calpain cleavage-resistant talin (L432G) restored impaired adhesion of Skap1-/- T-cells with DCs, demonstrating SKAP1 affects talin function at the LFA-1 activation interface.","method":"Skap1-/- T cells, confocal imaging of talin/RIAM translocation, calpain-resistant talin rescue, T cell-DC conjugation assay","journal":"Immunology letters","confidence":"Medium","confidence_rationale":"Tier 2 — KO T-cells, localization, and rescue with talin mutant, single lab","pmids":["26905930"],"is_preprint":false},{"year":2017,"finding":"LFA-1 cross-linking activates FAK1/PYK2, which phosphorylates LAT at Y171 to enable binding of the GRB2-SKAP1 adaptor complex. These LAT-GRB2-SKAP1 complexes are distinct from canonical LAT-GADs-SLP-76 complexes and mediate LFA-1-induced de-adhesion/decreased T cell-DC dwell times. LAT-Y171 mutation reduced T cell-DC binding and proliferation.","method":"Co-IP, kinase assay, LAT-Y171 point mutation, T cell-DC dwell time imaging, DO11.10 T cell proliferation assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1/2 — kinase assay with mutagenesis plus live imaging plus functional proliferation readout","pmids":["28699640"],"is_preprint":false},{"year":2017,"finding":"Within the PH domain of SKAP55 (SKAP1), residue D120 retains SKAP55 in the cytoplasm of resting T cells, while K152 promotes membrane recruitment via actin binding upon TCR triggering. K152-dependent actin interaction promotes talin binding to LFA-1, facilitating LFA-1 affinity modulation.","method":"Point mutations D120 and K152, subcellular localization assays, actin binding assay, talin co-IP, LFA-1 affinity assay","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — structure-function mutagenesis plus localization plus downstream functional assays, single lab","pmids":["28052935"],"is_preprint":false},{"year":2018,"finding":"SKAP1 forms homodimers mediated by residues A17 to L21 in its N-terminal region. SKAP1 homodimer formation is not required for RapL binding.","method":"Co-IP of truncation/deletion mutants, N-terminal region mapping","journal":"BMC research notes","confidence":"Medium","confidence_rationale":"Tier 3 — co-IP with mutants, single lab, moderate mechanistic follow-up","pmids":["30522503"],"is_preprint":false},{"year":2019,"finding":"SKAP1 is phosphorylated by PLK1 and binds PLK1 at its N-terminal serine 31 (S31); this interaction is required for optimal PLK1 kinase activity. siRNA knockdown of SKAP1 reduces the rate of T-cell division and delays expression of PLK1, Cyclin A, and pH3. Reconstitution with WT SKAP1 but not SKAP1-S31 mutant restores normal cell division.","method":"Co-IP, PLK1 kinase assay, siRNA knockdown, S31 point mutant rescue, cell cycle marker analysis","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — kinase assay with mutagenesis plus KD plus rescue, single lab","pmids":["31320682"],"is_preprint":false},{"year":2024,"finding":"Two SKAP1 modules control interaction with SRC kinases: one composed of motifs in the second interdomain interacting with the SH2 domain of SRC kinases, and another composed of the DIM domain modulated by the SH3 domain and SRC kinase activation status.","method":"Modular dissection with deletion constructs, comparison with SKAP2, binding assays","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 3 — single lab, binding domain mapping without full reconstitution or structural validation","pmids":["38483858"],"is_preprint":false}],"current_model":"SKAP1 (SKAP55) is a T cell-specific adaptor protein that functions as a central node in TCR inside-out signaling: it constitutively binds ADAP (via SH3-proline interaction) for protein stability and plasma membrane recruitment, uses its PH domain (R131, D120, K152) to sense PI3K-generated lipids and recruit RapL to the membrane, forms an SKAP1-RapL-Rap1 complex that binds and activates LFA-1 for integrin clustering and T cell-APC conjugation, negatively regulates the Ras-ERK pathway by sequestering RasGRP1, interacts with FAK1/PYK2-phosphorylated LAT-Y171 via GRB2 to mediate LFA-1-driven de-adhesion, promotes PD-1 expression through a Fyn-Ca²⁺-NFATc1 pathway, and acts as a scaffold for PLK1 at N-terminal S31 to optimize T-cell division."},"narrative":{"teleology":[{"year":1997,"claim":"Identification of SKAP55 as a Fyn-SH2-binding substrate in T cells established it as a novel PH- and SH3-domain-containing signaling adaptor downstream of Src-family kinases.","evidence":"GST-SH2 pulldowns and molecular cloning from human T lymphocytes","pmids":["9195899"],"confidence":"High","gaps":["No known function or downstream partners beyond Src-family kinase binding","Physiological role of PH and SH3 domains unknown"]},{"year":1998,"claim":"Discovery of the constitutive SKAP55–ADAP (SLAP-130/FYB) interaction via the SH3 domain defined SKAP1 as part of a stable adaptor module, raising the question of what this module does in TCR signaling.","evidence":"Yeast two-hybrid, co-IP in COS cells, truncation mutants, confocal colocalization","pmids":["9748251","9671755"],"confidence":"High","gaps":["Functional consequence of the SKAP55–ADAP module unknown","How the module is regulated by TCR stimulation unclear"]},{"year":2000,"claim":"Structural characterization of a non-canonical RKxxYxxY motif in SKAP55 bound by the Fyn SH3 domain, with functional inhibition of NF-AT transcription, showed that Fyn engagement of SKAP55 uses an atypical mechanism with signaling consequences.","evidence":"2D NMR of Fyn-SH3 with RKGDYASY peptide, alanine scanning, NF-AT reporter in T cells","pmids":["10856234"],"confidence":"High","gaps":["Relationship between Fyn-SH3 binding and integrin signaling not yet explored","Whether phosphorylation of the RKxxYxxY motif regulates binding in vivo unknown"]},{"year":2002,"claim":"Demonstration that SKAP55 translocates to lipid rafts upon TCR ligation and interacts with CD45 and Grb2 positioned it as a signal-responsive scaffold linking proximal TCR machinery to downstream effectors.","evidence":"Co-IP, lipid raft fractionation, Y232F and Y271 mutagenesis, IL-2 and MAPK reporter assays in Jurkat cells","pmids":["11909961","12171928"],"confidence":"Medium","gaps":["Overexpression-based system; endogenous stoichiometry not confirmed","Direct connection to integrin activation not yet established"]},{"year":2003,"claim":"SKAP55 was shown to be essential for TCR-induced LFA-1 clustering and T cell–APC conjugate formation, establishing integrin inside-out signaling as its primary biological function.","evidence":"Overexpression/SH3-deletion mutants, adhesion assays to ICAM-1/fibronectin, confocal at immune synapse","pmids":["12652296"],"confidence":"High","gaps":["Mechanism connecting SKAP55 to LFA-1 activation unknown","Loss-of-function (KD/KO) confirmation pending"]},{"year":2005,"claim":"siRNA knockdown confirmed a non-redundant requirement for SKAP55 (not compensated by SKAP55R) in inside-out LFA-1 signaling, and ADAP was shown to stabilize SKAP55 protein, explaining why ADAP deficiency phenocopies SKAP55 loss.","evidence":"siRNA in T cells with LFA-1 clustering readouts; pulse-chase in ADAP-deficient Jurkat cells","pmids":["15939789","15849195"],"confidence":"High","gaps":["Downstream effectors linking SKAP55 to LFA-1 still unidentified","Whether SKAP55 degradation is proteasome-dependent not resolved"]},{"year":2006,"claim":"The ADAP/SKAP55 module was shown to be required for membrane retention of activated Rap1, and Fyn phosphorylation of Tyr-294 in SKAP55 was found to regulate ADAP-SH3c binding, linking kinase input to integrin signaling output.","evidence":"Rap1 membrane localization in module-disrupted cells; SPR for pY294 disruption of ADAP binding; Y294F mutant blocks LFA-1 adhesion","pmids":["16980616","16461356"],"confidence":"High","gaps":["Identity of the Rap1 effector bridging SKAP55 to LFA-1 unknown","Role of dimerization in Rap1 membrane retention unclear"]},{"year":2008,"claim":"SKAP55 was found to negatively regulate Ras–ERK signaling by binding and sequestering RasGRP1 away from the trans-Golgi network, revealing a second major signaling axis beyond integrin regulation.","evidence":"Skap1−/− primary T cells showing ERK hyperactivation and increased TGN-localized RasGRP1; co-IP with C-terminal mapping","pmids":["17658605","18320039"],"confidence":"Medium","gaps":["Whether RasGRP1 sequestration operates simultaneously with integrin signaling unclear","Structural basis of SKAP55–RasGRP1 interaction unresolved"]},{"year":2010,"claim":"Identification of the SKAP1–RapL–Rap1 ternary complex that directly binds LFA-1, with in vivo validation by two-photon imaging showing reduced T cell dwell times in Skap1−/− lymph nodes, provided the molecular bridge between SKAP1 and integrin activation.","evidence":"Skap1−/− primary T cells, RapL-L224A point mutant, co-IP, intravital two-photon imaging in lymph nodes","pmids":["20346707"],"confidence":"High","gaps":["How PH domain lipid sensing drives RapL recruitment mechanistically not yet resolved","Contribution of RIAM-containing complexes versus RapL complexes not delineated"]},{"year":2011,"claim":"The PH domain of SKAP1 (R131) was shown to mediate PI3K-dependent membrane targeting that recruits RapL, and two distinct ADAP/SKAP55 pools (RapL/Mst1 and RIAM/Kindlin-3/Mst1) were identified as parallel arms controlling LFA-1 affinity and avidity.","evidence":"R131M mutant, PI3K inhibitor, myristoylated SKAP1 rescue; co-IP of two pools with CCR7 stimulation and in vivo homing","pmids":["21669874","21525391","22117043"],"confidence":"High","gaps":["Whether both pools operate simultaneously at the same synapse unknown","Lipid species specificity of PH domain not defined"]},{"year":2013,"claim":"SKAP55 dimerization via the N-terminal region was shown to enable RIAM co-immunoprecipitation, talin recruitment, and SLP-76 microcluster persistence, integrating SKAP55 oligomerization with the spatiotemporal dynamics of TCR signaling complexes.","evidence":"Engineered tandem dimer constructs, TIRF/timelapse imaging of SLP-76 microclusters, integrin assays","pmids":["24368808"],"confidence":"High","gaps":["Stoichiometry of dimers versus monomers in resting versus activated T cells unknown","Crystal/cryo-EM structure of SKAP55 dimer absent"]},{"year":2015,"claim":"SKAP55 was discovered to promote PD-1 expression via a Fyn–Ca²⁺–NFATc1 pathway, and Skap55−/− CD8+ CTLs showed enhanced anti-tumor activity, revealing a role beyond adhesion in immune checkpoint regulation.","evidence":"Skap55−/− and Adap−/− mice, DC-vaccine tumor models, adoptive transfer, CsA inhibition, PD-1 flow cytometry","pmids":["25851535"],"confidence":"Medium","gaps":["Whether SKAP55 directly regulates NFATc1 nuclear translocation or acts indirectly unresolved","Mechanism independent of ADAP not excluded","Generalizability to human tumors not tested"]},{"year":2017,"claim":"A distinct LFA-1-triggered de-adhesion pathway was mapped: LFA-1 cross-linking activates FAK1/PYK2, which phosphorylates LAT-Y171 to recruit GRB2–SKAP1 complexes that shorten T cell–DC dwell times, separating adhesion-promoting and de-adhesion functions of SKAP1.","evidence":"Co-IP, kinase assay, LAT-Y171 mutant, T cell–DC dwell time imaging, proliferation assay","pmids":["28699640"],"confidence":"High","gaps":["Downstream effector of LAT-GRB2-SKAP1 in de-adhesion not identified","Whether this pathway intersects with RapL-dependent adhesion signaling unclear"]},{"year":2019,"claim":"SKAP1 was identified as a PLK1 scaffold phosphorylated at S31, required for optimal PLK1 kinase activity and timely T cell division, extending SKAP1 function beyond adhesion into cell cycle regulation.","evidence":"PLK1 kinase assay, S31 mutant rescue of siRNA knockdown, cell cycle marker analysis","pmids":["31320682"],"confidence":"Medium","gaps":["Whether PLK1 scaffolding is T cell-specific or generalizable unknown","How TCR signaling connects to S31 phosphorylation timing unclear"]},{"year":null,"claim":"A structural model of full-length SKAP1 (including the dimer interface, PH domain lipid contacts, and SH3-mediated interactions) is lacking, and the mechanism by which SKAP1 coordinates its multiple signaling outputs — integrin activation, Ras–ERK suppression, PD-1 regulation, and cell cycle control — in space and time remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure of SKAP1 alone or in complex","Spatiotemporal coordination of SKAP1 signaling pools at the immune synapse undefined","In vivo phenotype of SKAP1 point mutants (e.g., R131M, S31A) in mice not reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[1,6,10,14,18]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[15,22]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[4,22]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,5,6,10,15]}],"pathway":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[6,8,14,15,18]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[6,8,14,19,21]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[10,12,13,15]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[24]}],"complexes":["ADAP/SKAP55 module","SKAP1-RapL-Rap1 complex","ADAP/SKAP55-RIAM-Kindlin-3-Mst1 complex"],"partners":["ADAP","FYN","RAPL","RAP1","RIAM","GRB2","RASGPR1","PLK1"],"other_free_text":[]},"mechanistic_narrative":"SKAP1 (SKAP55) is a T cell-specific adaptor protein that orchestrates TCR-mediated inside-out signaling to integrins, coupling antigen receptor engagement to cell adhesion, de-adhesion, and immune synapse formation. SKAP1 constitutively binds ADAP via its SH3 domain, which stabilizes SKAP1 protein levels and enables plasma membrane recruitment; its PH domain senses PI3K-generated lipids (through residues R131, D120, K152) and recruits the RapL–Rap1 complex to the membrane, driving LFA-1 clustering and T cell–APC conjugation [PMID:15939789, PMID:21669874, PMID:20346707, PMID:15849195]. Beyond integrin activation, SKAP1 negatively regulates the Ras–ERK pathway by sequestering RasGRP1 away from the trans-Golgi network, promotes PD-1 expression through a Fyn–Ca²⁺–NFATc1 axis relevant to anti-tumor immunity, and scaffolds PLK1 at N-terminal S31 to optimize T cell division [PMID:18320039, PMID:25851535, PMID:31320682]. A distinct LFA-1-triggered signaling arm uses FAK1/PYK2-phosphorylated LAT-Y171 and GRB2 to recruit SKAP1 into complexes that mediate de-adhesion and modulate T cell–DC dwell times [PMID:28699640]."},"prefetch_data":{"uniprot":{"accession":"Q86WV1","full_name":"Src kinase-associated phosphoprotein 1","aliases":["Src family-associated phosphoprotein 1","Src kinase-associated phosphoprotein of 55 kDa","SKAP-55","pp55"],"length_aa":359,"mass_kda":41.4,"function":"Positively regulates T-cell receptor signaling by enhancing the MAP kinase pathway. Required for optimal conjugation between T-cells and antigen-presenting cells by promoting the clustering of integrin ITGAL on the surface of T-cells. May be involved in high affinity immunoglobulin epsilon receptor signaling in mast cells","subcellular_location":"Cytoplasm; Nucleus; Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q86WV1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SKAP1","classification":"Not Classified","n_dependent_lines":14,"n_total_lines":1208,"dependency_fraction":0.011589403973509934},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SKAP1","total_profiled":1310},"omim":[{"mim_id":"618478","title":"FYN-BINDING PROTEIN 2; FYB2","url":"https://www.omim.org/entry/618478"},{"mim_id":"604969","title":"SRC KINASE-ASSOCIATED PHOSPHOPROTEIN 1; SKAP1","url":"https://www.omim.org/entry/604969"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Approved"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"lymphoid 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that associates with p59fyn in human T-lymphocytes via its SH2 domain, and selectively binds to isolated SH2 domains of Lck, Lyn, Src, and Fyn but not ZAP70, Syk, Shc, SLP-76, Grb2, PI3K, or c-abl in vitro. The protein contains a pleckstrin homology domain, a C-terminal SH3 domain, and several potential tyrosine phosphorylation sites.\",\n      \"method\": \"GST-SH2 domain pulldown, molecular cloning, in vitro binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro binding assays with multiple SH2 domains, replicated across labs\",\n      \"pmids\": [\"9195899\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"SKAP55 (SKAP1) binds directly to SLAP-130 (ADAP/FYB) via the SH3 domain of SKAP55 and the proline-rich sequence of SLAP-130, as demonstrated by co-immunoprecipitation, co-expression in COS cells, and two-hybrid analysis.\",\n      \"method\": \"Two-hybrid screen, co-expression/co-IP in COS cells, truncation mutants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — direct binding established by two-hybrid and co-expression with truncation mutants, replicated across labs\",\n      \"pmids\": [\"9748251\", \"9671755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"FYB (ADAP/SLAP-130) serves as a binding partner for SKAP55 (SKAP1) in T cells; both proteins colocalize in the perinuclear region and SKAP55 acts as a substrate for FYN kinase. The interaction is mediated through the SH3 domain of SKAP55.\",\n      \"method\": \"Two-hybrid screen with FYB as bait, immunofluorescence confocal microscopy, kinase assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two-hybrid plus localization plus kinase substrate assay, replicated\",\n      \"pmids\": [\"9671755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The SH3 domain of FYN binds to a novel proline-independent RKxxYxxY motif in SKAP55 (SKAP1). This non-canonical motif binding overlaps with the proline-based binding site on the charged surface of the SH3 domain. Expression of the RKGDYASY peptide inhibited TcRzeta/CD3-mediated NF-AT transcription in T cells.\",\n      \"method\": \"Peptide precipitation, alanine scanning, 2D NMR of FYN-SH3 with bound peptide, in vivo co-expression\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure plus mutagenesis plus functional in vivo assay in single rigorous study\",\n      \"pmids\": [\"10856234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"SKAP55 (SKAP1) is phosphorylated at Tyr-232 and associates with CD45 in vivo; anti-CD3 stimulation promotes SKAP55 tyrosine phosphorylation and translocation from cytoplasm to membrane. SKAP55 couples CD45 with Src family kinases, positively regulating TCR-mediated IL-2 promoter transcriptional activation.\",\n      \"method\": \"Yeast two-hybrid, mutational analysis (Y232F), overexpression in Jurkat, IL-2 promoter reporter assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — two-hybrid plus mutagenesis plus functional transcription assay, single lab\",\n      \"pmids\": [\"11909961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"SKAP55 (SKAP1) forms homodimers through its SH3 domain and SK region, translocates to lipid rafts upon TCR activation, and interacts with Fyn kinase and Grb-2 at Tyr-271. Overexpression of SKAP55 in Jurkat cells activates MAPK following TCR engagement.\",\n      \"method\": \"Co-immunoprecipitation, mutational analysis, stable overexpression, lipid raft fractionation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP with mutagenesis and fractionation, single lab\",\n      \"pmids\": [\"12171928\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"SKAP55 (SKAP1) regulates LFA-1-mediated adhesion and T cell-APC conjugate formation. SKAP55 colocalizes with actin at the T cell-APC synapse and promotes LFA-1 clustering. Conjugate formation requires the SKAP-55 SH3 domain and is accompanied by translocation of SKAP-55 to membrane rafts regulated by both LFA-1 and TCR ligation.\",\n      \"method\": \"Overexpression, adhesion assays (fibronectin/ICAM-1), confocal microscopy, lipid raft fractionation, SH3 deletion mutants\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (adhesion assay, confocal, fractionation, domain deletion) in defined cellular context\",\n      \"pmids\": [\"12652296\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In mast cells, SKAP55 (SKAP1) forms a complex with SLAP-130 and MIST; collaboration of SLAP-130 with SKAP55 recruits MIST to Fyn rather than Lyn, regulated by higher affinity binding of SLAP-130/SKAP55 to the Fyn-SH2 domain compared to the Lyn-SH2 domain.\",\n      \"method\": \"Co-immunoprecipitation, direct binding assays, affinity comparison\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — co-IP and affinity comparison, single lab\",\n      \"pmids\": [\"12681493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"siRNA knockdown of SKAP-55 (SKAP1) identified an essential role for this adaptor in TCR-mediated inside-out signaling for LFA-1 clustering and T cell-APC conjugation, distinct from TCR-CD3 clustering. SKAP-55R cannot compensate for loss of SKAP-55 in LFA-1 clustering.\",\n      \"method\": \"siRNA knockdown, LFA-1 clustering assay, conjugation assay\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — specific KD with defined phenotypic readouts and specificity controls (SKAP-55R non-rescue)\",\n      \"pmids\": [\"15939789\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SKAP55 (SKAP1) protein is rapidly degraded (half-life ~15-20 min) in the absence of ADAP, and ADAP stabilizes SKAP55 by causing a 5-fold decrease in its proteolysis rate. This stabilization requires the SH3 domain-mediated SKAP55-ADAP interaction.\",\n      \"method\": \"ADAP-deficient Jurkat cell line, protein stability/pulse-chase analysis, SH3 domain inactivation, restoration of ADAP expression\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical stability assays with domain mutant controls and rescue, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"15849195\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The ADAP/SKAP55 signaling module is required for TCR-mediated inside-out signaling; disruption of ADAP/SKAP55 interaction displaces activated Rap1 from the plasma membrane without affecting its GTPase activity. Membrane targeting of the ADAP/SKAP55 module induces T-cell adhesion in the absence of TCR stimulation.\",\n      \"method\": \"Module disruption experiments, Rap1 membrane localization assay, constitutive membrane targeting constructs\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches including constitutive membrane targeting with functional readout\",\n      \"pmids\": [\"16980616\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Fyn kinase phosphorylates Tyr-294 in the RKXXY294XXY297 motif of SKAP-55 (SKAP1), blocking the ADAP C-terminal SH3 domain binding to this motif (confirmed by plasmon resonance). Phosphorylation of Tyr-294 (but not Tyr-297) disrupts ADAP-SH3c binding, and Y294F mutation blocks TCR-induced LFA-1-mediated adhesion to ICAM-1 and IL-2 promoter activity.\",\n      \"method\": \"In vivo phosphorylation by Fyn, plasmon resonance interaction analysis, Y294F and Y297F mutants, LFA-1 adhesion assay, IL-2 promoter reporter\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — plasmon resonance with mutagenesis plus functional in vivo assays\",\n      \"pmids\": [\"16461356\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SKAP55 (SKAP1) co-immunoprecipitates with RasGRP1, the Ras guanine nucleotide exchange factor. Binding requires the C-terminus of SKAP55 and is enhanced by tyrosine phosphorylation. Overexpression of SKAP55 disrupts TCR-to-Ras-Erk-AP1 signaling and IL-2 gene transcription, while knockdown decreases AP-1 reporter activity and ERK phosphorylation.\",\n      \"method\": \"Co-immunoprecipitation, RNAi knockdown, overexpression, reporter gene assay, ERK phosphorylation assay\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP with domain mapping plus gain- and loss-of-function with multiple readouts, single lab\",\n      \"pmids\": [\"17658605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SKAP-55 (SKAP1) binds to RasGRP1 via its C-terminus and negatively regulates the p21ras-ERK pathway. SKAP-55 deficient primary T-cells show hyper-activation of ERK and increased RasGRP1 in the trans-Golgi network (where p21ras is activated), indicating SKAP-55 restricts RasGRP1 availability at the TGN.\",\n      \"method\": \"Skap1-/- primary T-cells, ERK phosphorylation assay, RasGRP1 localization by microscopy, C-terminal binding mutant, RNAi knockdown in T-cell lines\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO primary T-cells plus localization plus binding domain mutants, single lab\",\n      \"pmids\": [\"18320039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SKAP1 (SKAP-55) N-terminal domain binds the C-terminal SARAH domain of RapL to form a SKAP1-RapL-Rap1 complex that binds LFA-1. In Skap1-/- primary T-cells, TCR-induced Rap1-RapL complex formation and LFA-1 binding are absent. A RapL mutation (L224A) that abrogates SKAP1 binding disrupts vesicular colocalization and T cell-DC conjugation, and reduces T cell dwell times with DCs in lymph nodes.\",\n      \"method\": \"Skap1-/- primary T-cells, co-IP, RapL point mutation (L224A), two-photon intravital imaging in lymph nodes, conjugation assays\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — KO primary T-cells, mutagenesis, and in vivo imaging with multiple functional readouts\",\n      \"pmids\": [\"20346707\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The PH domain of SKAP1 (specifically residue R131) is required for RapL translocation to plasma membranes and subsequent Rap1 and LFA-1 binding. PI3K pathway-dependent membrane targeting of SKAP1 via its PH domain enables RapL membrane recruitment. N-terminal myristoylated SKAP1 constitutively recruits RapL to membranes and activates LFA-1 without TCR ligation.\",\n      \"method\": \"PH domain R131M mutation, membrane fractionation, LFA-1/ICAM-1 binding assay, myr-tagged SKAP1 rescue, PI3K inhibitor\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — mutagenesis plus constitutive membrane targeting rescue plus PI3K pathway dissection\",\n      \"pmids\": [\"21669874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Physical association of SKAP55 (SKAP1) with ADAP, through the PH domain of SKAP55 (residue R131), recruits ADAP to LFA-1 integrin complexes after TCR stimulation and is necessary and sufficient for integrin function rescue in ADAP-deficient T-cells. This SKAP55-ADAP association restricts ADAP's ability to interact with the NF-κB signalosome (CARMA1/TAK1), defining distinct functional pools.\",\n      \"method\": \"SKAP-ADAP chimeric fusion protein, R131M PH domain mutant, integrin activation assays, NF-κB reporter, LFA-1 co-IP in ADAP-/- T cells\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — chimeric protein, point mutant, KO rescue, multiple functional readouts in single study\",\n      \"pmids\": [\"21525391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Two independent pools of the ADAP/SKAP55 module exist in T cells: one interacting with a RAPL/Mst1 complex and another linked to a RIAM/Mst1/Kindlin-3 complex; both complexes require ADAP/SKAP55 for binding to LFA-1 upon CCR7 stimulation, regulating both affinity and avidity of LFA-1.\",\n      \"method\": \"Co-immunoprecipitation, CCR7-stimulated adhesion assays, ADAP/SKAP55 module loss-of-function, in vivo homing assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP plus functional adhesion and in vivo homing, single lab\",\n      \"pmids\": [\"22117043\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SKAP55 (SKAP1) dimerizes via its N-terminal region and the dimerization enables coimmunoprecipitation of RIAM, talin recruitment into TCR-induced adhesive junctions, and inside-out signaling to β1 integrins. SKAP55 dimerization and ADAP binding together are required for SLP-76 microcluster persistence and movement.\",\n      \"method\": \"Tandem dimer constructs, co-IP, timelapse/TIRF microscopy of SLP-76 microclusters, integrin signaling assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — engineered dimer constructs plus live cell imaging plus co-IP plus functional signaling assays\",\n      \"pmids\": [\"24368808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The ADAP-SKAP55 signaling module promotes PD-1 expression on CD8+ CTLs in a Fyn-, Ca2+-, and NFATc1-dependent manner. SKAP55 or ADAP knockout reduces PD-1 expression and enhances anti-tumor CTL activity. Adoptive transfer of SKAP55-deficient CD8+ CTLs blocks tumor growth.\",\n      \"method\": \"Skap55-/- and Adap-/- mice, DC vaccine tumor models, adoptive transfer, NFATc1 inhibitor (CsA), flow cytometry for PD-1\",\n      \"journal\": \"EMBO molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice plus pharmacological inhibition plus adoptive transfer, single lab\",\n      \"pmids\": [\"25851535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Skap1-/- T-cells show reduced translocation of talin and RIAM to the T cell-DC contact interface, and an altered pattern of talin cleavage. Expression of a calpain cleavage-resistant talin (L432G) restored impaired adhesion of Skap1-/- T-cells with DCs, demonstrating SKAP1 affects talin function at the LFA-1 activation interface.\",\n      \"method\": \"Skap1-/- T cells, confocal imaging of talin/RIAM translocation, calpain-resistant talin rescue, T cell-DC conjugation assay\",\n      \"journal\": \"Immunology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO T-cells, localization, and rescue with talin mutant, single lab\",\n      \"pmids\": [\"26905930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"LFA-1 cross-linking activates FAK1/PYK2, which phosphorylates LAT at Y171 to enable binding of the GRB2-SKAP1 adaptor complex. These LAT-GRB2-SKAP1 complexes are distinct from canonical LAT-GADs-SLP-76 complexes and mediate LFA-1-induced de-adhesion/decreased T cell-DC dwell times. LAT-Y171 mutation reduced T cell-DC binding and proliferation.\",\n      \"method\": \"Co-IP, kinase assay, LAT-Y171 point mutation, T cell-DC dwell time imaging, DO11.10 T cell proliferation assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — kinase assay with mutagenesis plus live imaging plus functional proliferation readout\",\n      \"pmids\": [\"28699640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Within the PH domain of SKAP55 (SKAP1), residue D120 retains SKAP55 in the cytoplasm of resting T cells, while K152 promotes membrane recruitment via actin binding upon TCR triggering. K152-dependent actin interaction promotes talin binding to LFA-1, facilitating LFA-1 affinity modulation.\",\n      \"method\": \"Point mutations D120 and K152, subcellular localization assays, actin binding assay, talin co-IP, LFA-1 affinity assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — structure-function mutagenesis plus localization plus downstream functional assays, single lab\",\n      \"pmids\": [\"28052935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SKAP1 forms homodimers mediated by residues A17 to L21 in its N-terminal region. SKAP1 homodimer formation is not required for RapL binding.\",\n      \"method\": \"Co-IP of truncation/deletion mutants, N-terminal region mapping\",\n      \"journal\": \"BMC research notes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — co-IP with mutants, single lab, moderate mechanistic follow-up\",\n      \"pmids\": [\"30522503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"SKAP1 is phosphorylated by PLK1 and binds PLK1 at its N-terminal serine 31 (S31); this interaction is required for optimal PLK1 kinase activity. siRNA knockdown of SKAP1 reduces the rate of T-cell division and delays expression of PLK1, Cyclin A, and pH3. Reconstitution with WT SKAP1 but not SKAP1-S31 mutant restores normal cell division.\",\n      \"method\": \"Co-IP, PLK1 kinase assay, siRNA knockdown, S31 point mutant rescue, cell cycle marker analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — kinase assay with mutagenesis plus KD plus rescue, single lab\",\n      \"pmids\": [\"31320682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Two SKAP1 modules control interaction with SRC kinases: one composed of motifs in the second interdomain interacting with the SH2 domain of SRC kinases, and another composed of the DIM domain modulated by the SH3 domain and SRC kinase activation status.\",\n      \"method\": \"Modular dissection with deletion constructs, comparison with SKAP2, binding assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, binding domain mapping without full reconstitution or structural validation\",\n      \"pmids\": [\"38483858\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SKAP1 (SKAP55) is a T cell-specific adaptor protein that functions as a central node in TCR inside-out signaling: it constitutively binds ADAP (via SH3-proline interaction) for protein stability and plasma membrane recruitment, uses its PH domain (R131, D120, K152) to sense PI3K-generated lipids and recruit RapL to the membrane, forms an SKAP1-RapL-Rap1 complex that binds and activates LFA-1 for integrin clustering and T cell-APC conjugation, negatively regulates the Ras-ERK pathway by sequestering RasGRP1, interacts with FAK1/PYK2-phosphorylated LAT-Y171 via GRB2 to mediate LFA-1-driven de-adhesion, promotes PD-1 expression through a Fyn-Ca²⁺-NFATc1 pathway, and acts as a scaffold for PLK1 at N-terminal S31 to optimize T-cell division.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SKAP1 (SKAP55) is a T cell-specific adaptor protein that orchestrates TCR-mediated inside-out signaling to integrins, coupling antigen receptor engagement to cell adhesion, de-adhesion, and immune synapse formation. SKAP1 constitutively binds ADAP via its SH3 domain, which stabilizes SKAP1 protein levels and enables plasma membrane recruitment; its PH domain senses PI3K-generated lipids (through residues R131, D120, K152) and recruits the RapL–Rap1 complex to the membrane, driving LFA-1 clustering and T cell–APC conjugation [PMID:15939789, PMID:21669874, PMID:20346707, PMID:15849195]. Beyond integrin activation, SKAP1 negatively regulates the Ras–ERK pathway by sequestering RasGRP1 away from the trans-Golgi network, promotes PD-1 expression through a Fyn–Ca²⁺–NFATc1 axis relevant to anti-tumor immunity, and scaffolds PLK1 at N-terminal S31 to optimize T cell division [PMID:18320039, PMID:25851535, PMID:31320682]. A distinct LFA-1-triggered signaling arm uses FAK1/PYK2-phosphorylated LAT-Y171 and GRB2 to recruit SKAP1 into complexes that mediate de-adhesion and modulate T cell–DC dwell times [PMID:28699640].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Identification of SKAP55 as a Fyn-SH2-binding substrate in T cells established it as a novel PH- and SH3-domain-containing signaling adaptor downstream of Src-family kinases.\",\n      \"evidence\": \"GST-SH2 pulldowns and molecular cloning from human T lymphocytes\",\n      \"pmids\": [\"9195899\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No known function or downstream partners beyond Src-family kinase binding\", \"Physiological role of PH and SH3 domains unknown\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Discovery of the constitutive SKAP55–ADAP (SLAP-130/FYB) interaction via the SH3 domain defined SKAP1 as part of a stable adaptor module, raising the question of what this module does in TCR signaling.\",\n      \"evidence\": \"Yeast two-hybrid, co-IP in COS cells, truncation mutants, confocal colocalization\",\n      \"pmids\": [\"9748251\", \"9671755\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the SKAP55–ADAP module unknown\", \"How the module is regulated by TCR stimulation unclear\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Structural characterization of a non-canonical RKxxYxxY motif in SKAP55 bound by the Fyn SH3 domain, with functional inhibition of NF-AT transcription, showed that Fyn engagement of SKAP55 uses an atypical mechanism with signaling consequences.\",\n      \"evidence\": \"2D NMR of Fyn-SH3 with RKGDYASY peptide, alanine scanning, NF-AT reporter in T cells\",\n      \"pmids\": [\"10856234\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relationship between Fyn-SH3 binding and integrin signaling not yet explored\", \"Whether phosphorylation of the RKxxYxxY motif regulates binding in vivo unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstration that SKAP55 translocates to lipid rafts upon TCR ligation and interacts with CD45 and Grb2 positioned it as a signal-responsive scaffold linking proximal TCR machinery to downstream effectors.\",\n      \"evidence\": \"Co-IP, lipid raft fractionation, Y232F and Y271 mutagenesis, IL-2 and MAPK reporter assays in Jurkat cells\",\n      \"pmids\": [\"11909961\", \"12171928\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Overexpression-based system; endogenous stoichiometry not confirmed\", \"Direct connection to integrin activation not yet established\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"SKAP55 was shown to be essential for TCR-induced LFA-1 clustering and T cell–APC conjugate formation, establishing integrin inside-out signaling as its primary biological function.\",\n      \"evidence\": \"Overexpression/SH3-deletion mutants, adhesion assays to ICAM-1/fibronectin, confocal at immune synapse\",\n      \"pmids\": [\"12652296\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism connecting SKAP55 to LFA-1 activation unknown\", \"Loss-of-function (KD/KO) confirmation pending\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"siRNA knockdown confirmed a non-redundant requirement for SKAP55 (not compensated by SKAP55R) in inside-out LFA-1 signaling, and ADAP was shown to stabilize SKAP55 protein, explaining why ADAP deficiency phenocopies SKAP55 loss.\",\n      \"evidence\": \"siRNA in T cells with LFA-1 clustering readouts; pulse-chase in ADAP-deficient Jurkat cells\",\n      \"pmids\": [\"15939789\", \"15849195\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors linking SKAP55 to LFA-1 still unidentified\", \"Whether SKAP55 degradation is proteasome-dependent not resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The ADAP/SKAP55 module was shown to be required for membrane retention of activated Rap1, and Fyn phosphorylation of Tyr-294 in SKAP55 was found to regulate ADAP-SH3c binding, linking kinase input to integrin signaling output.\",\n      \"evidence\": \"Rap1 membrane localization in module-disrupted cells; SPR for pY294 disruption of ADAP binding; Y294F mutant blocks LFA-1 adhesion\",\n      \"pmids\": [\"16980616\", \"16461356\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the Rap1 effector bridging SKAP55 to LFA-1 unknown\", \"Role of dimerization in Rap1 membrane retention unclear\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"SKAP55 was found to negatively regulate Ras–ERK signaling by binding and sequestering RasGRP1 away from the trans-Golgi network, revealing a second major signaling axis beyond integrin regulation.\",\n      \"evidence\": \"Skap1−/− primary T cells showing ERK hyperactivation and increased TGN-localized RasGRP1; co-IP with C-terminal mapping\",\n      \"pmids\": [\"17658605\", \"18320039\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether RasGRP1 sequestration operates simultaneously with integrin signaling unclear\", \"Structural basis of SKAP55–RasGRP1 interaction unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identification of the SKAP1–RapL–Rap1 ternary complex that directly binds LFA-1, with in vivo validation by two-photon imaging showing reduced T cell dwell times in Skap1−/− lymph nodes, provided the molecular bridge between SKAP1 and integrin activation.\",\n      \"evidence\": \"Skap1−/− primary T cells, RapL-L224A point mutant, co-IP, intravital two-photon imaging in lymph nodes\",\n      \"pmids\": [\"20346707\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How PH domain lipid sensing drives RapL recruitment mechanistically not yet resolved\", \"Contribution of RIAM-containing complexes versus RapL complexes not delineated\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The PH domain of SKAP1 (R131) was shown to mediate PI3K-dependent membrane targeting that recruits RapL, and two distinct ADAP/SKAP55 pools (RapL/Mst1 and RIAM/Kindlin-3/Mst1) were identified as parallel arms controlling LFA-1 affinity and avidity.\",\n      \"evidence\": \"R131M mutant, PI3K inhibitor, myristoylated SKAP1 rescue; co-IP of two pools with CCR7 stimulation and in vivo homing\",\n      \"pmids\": [\"21669874\", \"21525391\", \"22117043\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether both pools operate simultaneously at the same synapse unknown\", \"Lipid species specificity of PH domain not defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"SKAP55 dimerization via the N-terminal region was shown to enable RIAM co-immunoprecipitation, talin recruitment, and SLP-76 microcluster persistence, integrating SKAP55 oligomerization with the spatiotemporal dynamics of TCR signaling complexes.\",\n      \"evidence\": \"Engineered tandem dimer constructs, TIRF/timelapse imaging of SLP-76 microclusters, integrin assays\",\n      \"pmids\": [\"24368808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of dimers versus monomers in resting versus activated T cells unknown\", \"Crystal/cryo-EM structure of SKAP55 dimer absent\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"SKAP55 was discovered to promote PD-1 expression via a Fyn–Ca²⁺–NFATc1 pathway, and Skap55−/− CD8+ CTLs showed enhanced anti-tumor activity, revealing a role beyond adhesion in immune checkpoint regulation.\",\n      \"evidence\": \"Skap55−/− and Adap−/− mice, DC-vaccine tumor models, adoptive transfer, CsA inhibition, PD-1 flow cytometry\",\n      \"pmids\": [\"25851535\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether SKAP55 directly regulates NFATc1 nuclear translocation or acts indirectly unresolved\", \"Mechanism independent of ADAP not excluded\", \"Generalizability to human tumors not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A distinct LFA-1-triggered de-adhesion pathway was mapped: LFA-1 cross-linking activates FAK1/PYK2, which phosphorylates LAT-Y171 to recruit GRB2–SKAP1 complexes that shorten T cell–DC dwell times, separating adhesion-promoting and de-adhesion functions of SKAP1.\",\n      \"evidence\": \"Co-IP, kinase assay, LAT-Y171 mutant, T cell–DC dwell time imaging, proliferation assay\",\n      \"pmids\": [\"28699640\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effector of LAT-GRB2-SKAP1 in de-adhesion not identified\", \"Whether this pathway intersects with RapL-dependent adhesion signaling unclear\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"SKAP1 was identified as a PLK1 scaffold phosphorylated at S31, required for optimal PLK1 kinase activity and timely T cell division, extending SKAP1 function beyond adhesion into cell cycle regulation.\",\n      \"evidence\": \"PLK1 kinase assay, S31 mutant rescue of siRNA knockdown, cell cycle marker analysis\",\n      \"pmids\": [\"31320682\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PLK1 scaffolding is T cell-specific or generalizable unknown\", \"How TCR signaling connects to S31 phosphorylation timing unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A structural model of full-length SKAP1 (including the dimer interface, PH domain lipid contacts, and SH3-mediated interactions) is lacking, and the mechanism by which SKAP1 coordinates its multiple signaling outputs — integrin activation, Ras–ERK suppression, PD-1 regulation, and cell cycle control — in space and time remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure of SKAP1 alone or in complex\", \"Spatiotemporal coordination of SKAP1 signaling pools at the immune synapse undefined\", \"In vivo phenotype of SKAP1 point mutants (e.g., R131M, S31A) in mice not reported\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1, 6, 10, 14, 18]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [15, 22]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4, 22]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 5, 6, 10, 15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [6, 8, 14, 15, 18]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [6, 8, 14, 19, 21]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 12, 13, 15]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [24]}\n    ],\n    \"complexes\": [\n      \"ADAP/SKAP55 module\",\n      \"SKAP1-RapL-Rap1 complex\",\n      \"ADAP/SKAP55-RIAM-Kindlin-3-Mst1 complex\"\n    ],\n    \"partners\": [\n      \"ADAP\",\n      \"FYN\",\n      \"RAPL\",\n      \"RAP1\",\n      \"RIAM\",\n      \"GRB2\",\n      \"RASGPR1\",\n      \"PLK1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}