{"gene":"CSK","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1991,"finding":"CSK specifically phosphorylates the negative regulatory C-terminal tyrosine of Src family kinases (p60c-src, p56lyn, p59fyn), and this phosphorylation represses their kinase activities. Tyrosine 508 of p56lyn was identified as essential for this phosphorylation.","method":"In vitro kinase assay, site-directed mutagenesis, Western blot","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro kinase assay with mutagenesis, replicated across multiple SFK substrates, foundational study widely confirmed","pmids":["1722201"],"is_preprint":false},{"year":1999,"finding":"Crystal structure of the CSK kinase domain co-complexed with staurosporine resolved at 2.4 Å. The structure revealed the ATP-binding cleft between N- and C-terminal lobes, conserved Lys328 and Glu267 forming a molecular pivot for lobe movement, and unique orientations of Asp332 and Phe333 suggesting inhibitor binding via induced fit.","method":"X-ray crystallography (molecular replacement)","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure at 2.4 Å resolution with full refinement statistics reported","pmids":["9878439"],"is_preprint":false},{"year":2002,"finding":"Full-length crystal structure of Csk revealed that SH2 and SH3 domains are oriented outward (distinct from inactive SFKs) with binding pockets accessible for intermolecular interaction. In active conformations, the SH2-kinase linker stabilizes the active state through direct SH2-kinase domain linkage; SH2 domain rotation destroys this linkage in inactive conformations, suggesting Csk is regulated through SH2-kinase domain coupling.","method":"X-ray crystallography (full-length protein)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — full-length crystal structure with multiple conformational states analyzed","pmids":["11884384"],"is_preprint":false},{"year":1994,"finding":"Csk suppression of Src requires functional SH2, SH3, and catalytic domains. An SH3 deletion mutant retained in vitro kinase activity but failed to suppress Src in cells. When Src is active, Csk relocalizes from cytosol to podosomes via its SH3 and SH2 domains; SH3 or SH2 domain mutants do not relocalize, explaining their failure to regulate Src.","method":"Expression of Csk mutants in csk-deficient cells, immunofluorescence localization, in vitro kinase assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — functional mutant analysis in defined cell system combined with localization, replicated across multiple mutants","pmids":["7518562"],"is_preprint":false},{"year":1995,"finding":"Overexpressed Csk localizes to focal adhesions via its SH2 and SH3 domains (not catalytic activity). Csk forms a complex with paxillin in cells and its SH2 domain interacts with FAK and paxillin in vitro. Catalytically active Csk overexpression causes cell rounding, loss of adhesion, and redistribution of αvβ5 integrin; catalytically inactive mutant does not produce these effects.","method":"Inducible overexpression, immunofluorescence, co-immunoprecipitation, in vitro binding (SH2 domain GST pulldown), point and deletion mutagenesis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (localization, co-IP, in vitro binding, mutagenesis) in a single study","pmids":["7529872"],"is_preprint":false},{"year":1998,"finding":"Csk phosphorylates and inactivates Src only when Src is not previously autophosphorylated at its activation loop tyrosine. Autophosphorylated Src can still be phosphorylated by Csk at the C-terminal tail but is not inactivated, because the SH2 binding site for phospho-Tyr is inaccessible. PTP1B dephosphorylation of the Src autophosphorylation site restores Csk-mediated inactivation.","method":"In vitro kinase assay, pre-incubation with ATP-Mg, site-directed mutagenesis (Yes Y-to-F mutant), PTP1B dephosphorylation rescue","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and phosphatase rescue, single lab","pmids":["9794236"],"is_preprint":false},{"year":1998,"finding":"CSK catalyzes phosphoryl transfer via a ternary complex random-substrate-binding mechanism. ADP release is rate-limiting with Mn as cofactor; the chemical (phosphoryl transfer) step is fully rate-determining with Mg. Asp314 enhances gamma-phosphate reactivity toward electrophilic attack (thio-effect analysis with D314E mutant).","method":"Steady-state kinetics, dead-end inhibitor analysis, viscosity experiments, thio effects with wild-type and D314E mutant Csk","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — detailed kinetic mechanism with multiple substrates, mutant analysis, and metal substitution studies","pmids":["9425036","9048573"],"is_preprint":false},{"year":1995,"finding":"The catalytic base Asp314 of Csk is not required for early tyrosine deprotonation (trifluorotyrosine substrate is phosphorylated with similar efficiency). D314E mutation reduces kcat ~10,000-fold with minimal effect on substrate Km, and thio-effect analysis suggests Asp314 enhances gamma-phosphate electrophilicity.","method":"In vitro kinase assay with unnatural amino acid substrate, site-directed mutagenesis (D314E), thio-effect measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — active-site mutagenesis combined with unnatural substrate chemistry","pmids":["7673185"],"is_preprint":false},{"year":1999,"finding":"The Csk SH3 domain intramolecularly interacts with the catalytic domain to stimulate kinase activity (~100-fold reduction when catalytic domain alone is expressed). The interaction surface on SH3 involves the SH3-SH2 linker region, distinct from the classical type-II polyproline helix-binding surface.","method":"Expression of isolated Csk domains in E. coli, in vitro kinase assay, trans-stimulation, surface plasmon resonance, site-directed mutagenesis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution of domain interactions in vitro with kinetic and SPR measurements","pmids":["10460171"],"is_preprint":false},{"year":2001,"finding":"Pre-steady-state kinetic analysis shows CSK phosphoryl transfer to peptide substrate is fast (k3 ≥ 140 s⁻¹) and favorable, with rate-limiting ADP release (koff ~0.6 s⁻¹ ≈ kcat). Viscosometric studies indicate slow conformational changes are coupled to ADP dissociation.","method":"Rapid quench flow, stopped-flow fluorescence, viscosometric experiments","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — pre-steady-state kinetics with multiple transient-state methods establishing catalytic mechanism","pmids":["11551213"],"is_preprint":false},{"year":2001,"finding":"The SH3 and SH2 domains of Csk's substrate Src are dispensable for efficient Csk-catalyzed phosphorylation; the catalytic domain and tail of Src are sufficient. Key Src tail residues Glu (Y-3) and Gln (Y-1) are critical determinants of substrate efficiency for Csk. Src dimerization/oligomerization appears important for high-efficiency Csk phosphorylation.","method":"In vitro kinase assay with purified recombinant proteins and alanine-scanning mutants of Src tail","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis of substrate combined with in vitro reconstitution","pmids":["11329267"],"is_preprint":false},{"year":2001,"finding":"Csk physically interacts with PTP-HSCF (third member of PEP phosphatase family) via the Csk SH2 domain binding to tyrosine-phosphorylated sites in PTP-HSCF's non-catalytic region. Csk and PTP-HSCF synergize to inhibit Src family kinases; this cooperativity depends on the domains mediating their interaction. PTP-HSCF selectively dephosphorylates the positive regulatory tyrosine of Src-related kinases.","method":"Yeast two-hybrid, co-immunoprecipitation in mammalian cells and hematopoietic cells, transfection assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — yeast two-hybrid confirmed by co-IP in multiple cell systems, functional synergy demonstrated","pmids":["11158295"],"is_preprint":false},{"year":2000,"finding":"PAG (phosphoprotein associated with GEMs/Cbp) is a transmembrane adaptor that constitutively binds Csk via its phosphorylated cytoplasmic tyrosines in resting T cells. After T cell activation, PAG is dephosphorylated and dissociates from Csk. PAG expression in COS cells recruits endogenous Csk and alters Src kinase activity. Overexpression in Jurkat cells downregulates TCR-mediated NFAT activation.","method":"Co-immunoprecipitation, mass spectrometry identification, COS cell transfection, Jurkat overexpression with reporter assay","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, multiple cell systems, functional consequence demonstrated; independently replicated","pmids":["10790433"],"is_preprint":false},{"year":2001,"finding":"PKA phosphorylates Csk at Ser364 in vitro and in vivo, leading to a 2-4-fold increase in Csk kinase activity. This PKA-mediated Csk activation is necessary for cAMP-mediated inhibition of TCR signaling and IL-2 secretion. Both PKA type I and Csk are targeted to lipid rafts where Lck phosphorylation by activated Csk is enhanced.","method":"In vitro PKA phosphorylation of Csk, kinase activity assay, lipid raft fractionation, IL-2 secretion assay, phosphorylation site mutants","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro phosphorylation + in vivo confirmation + functional readout, independently replicated","pmids":["11181701"],"is_preprint":false},{"year":2001,"finding":"Csk is constitutively present in lipid rafts through interaction with PAG/Cbp. Upon TCR triggering, PAG/Cbp is dephosphorylated leading to Csk dissociation from rafts. A dominant-negative Csk that displaces endogenous Csk from lipid rafts results in elevated basal TCR-zeta phosphorylation and enhanced TCR responses, establishing that raft-localized Csk tonically inhibits T cell activation.","method":"Lipid raft fractionation, dominant-negative Csk expression, TCR-zeta phosphorylation assay, NFAT-AP1 reporter assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization-function link using dominant-negative approach with multiple functional readouts","pmids":["11390365"],"is_preprint":false},{"year":2003,"finding":"PKA regulates Csk by two mechanisms: (1) increasing Cbp/PAG phosphorylation to augment Csk recruitment to lipid rafts, reducing TCR-induced Csk dissociation; and (2) directly increasing Csk phosphotransferase activity. cAMP-mediated inhibition of TCR signaling requires raft-associated Csk, as displacing Csk from rafts abolishes PGE2-mediated inhibition.","method":"Raft fractionation, co-immunoprecipitation, FRET-based kinase activity assay, NFAT-AP1 reporter","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — two orthogonal mechanisms established, spatial and enzymatic regulation combined in one study","pmids":["12665526"],"is_preprint":false},{"year":2002,"finding":"Csk is required for G protein (Gβγ, Gα12, Gα13, Gαq)-induced actin stress fiber formation. In Csk-deficient MEFs, GPCR-induced actin reorganization is completely blocked and reintroduction of Csk restores it. Csk catalytic activity is required; Gβγ both translocates Csk to the plasma membrane and directly increases Csk kinase activity.","method":"Csk-deficient MEFs, Csk reintroduction, catalytic mutant rescue, biochemical Gβγ-Csk interaction and kinase activation assay","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic (KO rescue) plus biochemical interaction and kinase activation, multiple G proteins tested","pmids":["12062086"],"is_preprint":false},{"year":2002,"finding":"DXMS analysis of CSK shows that ATP-analog AMPPNP and product ADP protect overlapping but distinct regions including the active site, activation loop, helix G, SH2 domain, and SH2-kinase linker, demonstrating that phosphoryl transfer induces unique conformational changes propagated from active site to regulatory domains.","method":"Hydrogen-deuterium exchange mass spectrometry (DXMS) with nucleotide analogs","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative HX-MS method covering 63% of protein, multiple nucleotide states compared","pmids":["12417200"],"is_preprint":false},{"year":2004,"finding":"Shp2 promotes Src family kinase activation by dephosphorylating PAG/Cbp, thereby reducing Csk recruitment to SFKs. In Shp2-deficient cells, SFK inhibitory C-terminal tyrosines are hyperphosphorylated. This places Shp2 upstream of Csk in controlling SFK activity and downstream signaling (Plcγ1, Ras/Erk).","method":"Shp2-deficient cells, phosphorylation analysis of SFK regulatory tyrosines, Plcγ1 phosphorylation assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in defined KO cells with mechanistic biochemical readouts, widely cited","pmids":["14967142"],"is_preprint":false},{"year":2002,"finding":"Csk is constitutively associated with integrin αIIbβ3 in platelets. Upon fibrinogen binding, Csk dissociates from αIIbβ3 concomitant with Src Tyr-529 dephosphorylation and Src Tyr-418 phosphorylation (activation), placing Csk as a direct negative regulator of integrin-proximal Src in platelet spreading.","method":"Co-immunoprecipitation from platelets, phospho-specific antibodies, integrin engagement assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP with functional context, well-controlled physiological setting","pmids":["11940607"],"is_preprint":false},{"year":2005,"finding":"Csk binds via its SH2 domain to phosphorylated Tyr685 of VE-cadherin. VE-cadherin recruits Csk to cell contacts; this interaction increases with cell density. VE-cadherin-Y685F mutant abolishes Csk binding and confers higher growth density. Csk overexpression inhibits endothelial cell proliferation in a VE-cadherin phospho-Y685-dependent manner; Csk RNAi enhances proliferation.","method":"Co-immunoprecipitation, GST-SH2 pulldown, inducible Csk overexpression, RNAi knockdown, proliferation assay with Y685F mutant","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP + in vitro pulldown + functional rescue with phosphosite mutant + RNAi","pmids":["15861137"],"is_preprint":false},{"year":2005,"finding":"Binding of a Cbp-derived phosphopeptide to the Csk SH2 domain enhances Src recognition (lowers Km) by increasing phosphoryl transfer rate, detected by rapid quench flow. DXMS shows the Cbp peptide induces conformational changes propagating from the SH2 domain through the SH2-kinase linker to the glycine-rich loop of the active site, suggesting cantilever-like SH2 motion orders the active site.","method":"Rapid quench flow kinetics, DXMS, computational normal mode analysis","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple biophysical methods (HX-MS, pre-steady-state kinetics, normal mode analysis) establishing allosteric mechanism","pmids":["16002086"],"is_preprint":false},{"year":2001,"finding":"NMR chemical shift mapping shows the Csk SH3 domain binds the catalytic domain at the SH3-SH2 linker region and N-terminal tail (not the classical polyproline-binding RT-loop surface). Site-directed mutagenesis of the SH3-SH2 linker reduces Csk kinase activity up to 5-fold, confirming this intradomain interaction activates the catalytic domain.","method":"NMR chemical shift perturbation mapping, site-directed mutagenesis, in vitro kinase assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR mapping with mutagenesis validation, two orthogonal methods","pmids":["11724538"],"is_preprint":false},{"year":2003,"finding":"PKA-mediated activation of Csk by Ser364 phosphorylation depends on the intact SH3 domain. Isolated Csk kinase domain is phosphorylated by PKA at S364 but is not significantly activated unless the SH3 domain is present. PKA phosphorylation facilitates the SH3-kinase domain interaction, measured by surface plasmon resonance.","method":"In vitro PKA phosphorylation of Csk domains, kinase activity assay, surface plasmon resonance","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with domain fragments and SPR measurement","pmids":["12600271"],"is_preprint":false},{"year":2006,"finding":"Csk recognizes Src family kinase substrates through a docking site on its catalytic domain that is distinct from the active site. Docking determinants on Src recognized by this site were identified; similar docking-based recognition was confirmed for Yes. This establishes a docking-based substrate recognition mechanism for Csk.","method":"In vitro binding assays, mutagenesis of Csk docking site and Src docking determinants, kinase assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis of both enzyme and substrate docking elements with kinetic validation","pmids":["16439366"],"is_preprint":false},{"year":2003,"finding":"Csk activation loop lacks tyrosine residues, and specific residues in this loop have little function in Src phosphorylation. Thrombin-cleavage of the activation loop restores full activity in loop-insertion mutants that are nearly inactive toward artificial substrates, indicating the activation loop acts as an inhibitory regulatory structure and is dispensable for catalysis when removed.","method":"Extensive site-directed mutagenesis, loop replacement, engineered thrombin cleavage site, kinase assay with physiological (Src) and artificial substrates","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis plus proteolytic rescue experiment, multiple substrate types compared","pmids":["12686554"],"is_preprint":false},{"year":1994,"finding":"Csk-negative chicken B cells have constitutively active Lyn (hyperphosphorylated at autophosphorylation site) and constitutively activated Syk, demonstrating Csk maintains Lyn and downstream Syk in inactive states. BCR crosslinking was still required for full downstream signaling (Ca2+ mobilization, IP3 generation) despite constitutive kinase activation.","method":"Csk-deficient B cell clones (gene disruption), kinase activity assay, phosphorylation analysis, calcium flux, IP3 measurement","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function in defined KO cells with multiple biochemical and functional readouts","pmids":["7935444"],"is_preprint":false},{"year":1998,"finding":"T cell development is controlled by Csk: conditional inactivation of Csk in immature thymocytes abrogates the requirement for preTCR, αβTCR, and MHC class II for development of DP and CD4+ SP thymocytes, placing Csk as the gatekeeper that enforces receptor-dependent developmental checkpoints by restraining Src-family kinases.","method":"Conditional Csk gene knockout in thymocytes (Cre-lox), flow cytometric analysis of thymic development","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional genetic KO with defined developmental phenotype, epistasis placing Csk upstream of preTCR/αβTCR checkpoints","pmids":["9732874"],"is_preprint":false},{"year":2013,"finding":"Inhibition of an analog-sensitive Csk variant (CskAS) in thymocytes induces potent SFK activation and proximal TCR signaling up to PLCγ1 phosphorylation, but downstream signals (IP3, Ca2+, Erk phosphorylation) are impaired. Actin cytoskeleton remodeling (pharmacological or via CD28 costimulation) rescues these downstream defects, revealing that Csk controls a checkpoint requiring actin remodeling for full TCR signaling.","method":"Analog-sensitive Csk knockin mice, chemical-genetic inhibition, phosphorylation analysis, IP3/Ca2+ measurement, cytoskeletal perturbation","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — chemical-genetic approach with analog-sensitive allele, multiple orthogonal signaling readouts","pmids":["24317039"],"is_preprint":false},{"year":1995,"finding":"CSK directly associates with the GTPase-activating protein-associated p62 (GAP-A.p62) protein via the CSK SH2 domain (reconstituted in vitro with GST-CSK and GST-SH2 domain fusions). This complex localizes to membrane/cytoskeletal fractions in v-Src NIH 3T3 cells, suggesting GAP-A.p62 as a docking protein mediating CSK translocation to sites of Src activity.","method":"Co-immunoprecipitation, GST pulldown in vitro reconstitution, subcellular fractionation","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro GST pulldown plus co-IP from cells, single lab","pmids":["7544435"],"is_preprint":false},{"year":1996,"finding":"Csk interacts with phosphorylated Lck via the Csk SH2 domain; this interaction requires Lck autophosphorylation at Tyr394 (activation loop). Autophosphorylated Lck is preferentially phosphorylated by Csk at its negative regulatory Tyr505, suggesting activated Src-family kinases are preferential Csk substrates.","method":"BIAcore (SPR) real-time binding assay, yeast two-hybrid, in vitro kinase assay with site-directed Lck mutants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — SPR binding measurement + yeast two-hybrid + in vitro kinase assay, two orthogonal interaction methods","pmids":["8631775"],"is_preprint":false},{"year":2004,"finding":"Csk overexpression in colon cancer cells decreases SFK activation, increases E-cadherin-mediated cell-cell contacts, decreases focal contacts, and reduces cell adhesion/migration/invasiveness. Dominant-negative Csk produces opposite effects: elevated SFK/FAK/paxillin phosphorylation, cell scattering, increased focal contacts and invasiveness. All these events are integrin-dependent.","method":"Stable overexpression and dominant-negative expression, kinase assay, cell adhesion/migration/invasion assay, phospho-FAK/paxillin immunoblot","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — paired wild-type/dominant-negative approach with multiple cellular phenotype readouts, single lab","pmids":["14712234"],"is_preprint":false},{"year":2007,"finding":"p140Cap adaptor protein binds Src and activates Csk, leading to Src inhibition. p140Cap silencing increases cell spreading, migration, and Src activity; increased p140Cap expression activates Csk and inhibits Src and downstream signaling, cell motility, and invasion.","method":"RNAi knockdown, co-immunoprecipitation, kinase activity assay, cell migration/invasion assay","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus functional RNAi/overexpression with defined readouts, single lab","pmids":["17525734"],"is_preprint":false},{"year":2009,"finding":"Csk directly phosphorylates Tyr393 of the P2X3 receptor in vitro (in vitro kinase assay in HEK cells), strongly inhibiting receptor currents. Csk knockdown in HEK and trigeminal neurons potentiates P2X3 responses, confirming constitutive Csk-mediated inhibition of P2X3. NGF levels regulate Csk activity in sensory neurons to control P2X3 sensitization.","method":"In vitro kinase assay, site-directed mutagenesis (structural modeling-guided), RNAi knockdown, electrophysiology","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct in vitro phosphorylation of identified site plus RNAi functional validation, two orthogonal methods","pmids":["19509283"],"is_preprint":false},{"year":2012,"finding":"LYP (lymphoid tyrosine phosphatase) inhibits T cell activation when dissociated from CSK. Dissociation of the LYP-CSK complex is necessary for LYP recruitment to the plasma membrane where it downmodulates TCR signaling. The autoimmunity-associated LYP variant that cannot bind CSK shows reduced TCR-mediated signaling.","method":"Spatiotemporal dynamics imaging, selective LYP chemical probe, co-immunoprecipitation, functional T cell activation assays","journal":"Nature chemical biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — chemical probe plus co-IP plus live-cell imaging plus functional assays, multiple orthogonal methods","pmids":["22426112"],"is_preprint":false},{"year":2016,"finding":"PAG (Cbp) genetically cooperates with PTPN22 and Dok adaptors to inhibit effector T cell activation. In PAG-deficient mice, Csk associates more with PTPN22 and Dok adaptors as alternative partners. Combining PAG deficiency with PTPN22 or Dok deficiency further enhances effector T cell responses, establishing a cooperative inhibitory circuit.","method":"PAG-knockout mice, double-knockout genetic epistasis, co-immunoprecipitation, T cell activation assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in defined KO mice with biochemical partner-switching and functional readouts","pmids":["27926878"],"is_preprint":false},{"year":2016,"finding":"Connexin43 (Cx43) recruits both PTEN and Csk to the region between residues 266-283 of its C-terminus, forming a ternary complex with c-Src. Pulldown assays showed this Cx43 region is sufficient to recruit c-Src, PTEN, and Csk and to inhibit oncogenic c-Src activity. Csk and PTEN silencing reduces the antiproliferative effect of Cx43.","method":"Confocal microscopy, co-immunoprecipitation, GST pulldown (Cx43 domain fragments), RNAi knockdown, proliferation assay","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus domain pulldown plus functional RNAi, single lab","pmids":["27391443"],"is_preprint":false},{"year":2016,"finding":"Upon αvβ3 integrin engagement of Thy-1, Csk is present in the Thy-1/CBP/Csk complex at the plasma membrane; integrin binding causes Src inactivation and exclusion from this complex. CBP silencing abolishes Csk recruitment and the downstream RhoA/ROCK neurite retraction signaling, establishing CBP as the key transducer linking Thy-1 to Csk-mediated Src inactivation.","method":"Single-molecule tracking, nanoscopy, co-immunoprecipitation, RNAi knockdown, RhoA activation assay","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple imaging methods plus biochemical and genetic approaches, single lab","pmids":["27842221"],"is_preprint":false},{"year":2018,"finding":"Simultaneous ablation of Csk and CD148 in platelets causes dramatic SFK hyperactivation, demonstrating these proteins are essential regulators of platelet SFK activity. Paradoxically, this leads to reduced thrombosis due to downregulation of GPVI-FcRγ and CLEC-2 ITAM receptors and upregulation of inhibitory G6b-B/Shp1/Shp2 signaling as a negative feedback.","method":"Conditional Csk/CD148 double-knockout mice, analog-sensitive Csk model, platelet SFK activity assay, receptor expression analysis, bleeding/thrombosis models","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional double-KO plus analog-sensitive mouse model, multiple in vivo and ex vivo readouts","pmids":["29301754"],"is_preprint":false},{"year":2006,"finding":"Cbp/PAG phosphorylation by Lyn recruits Csk (and Ctk) to suppress Lyn kinase activity within minutes of Epo stimulation. A single phosphotyrosine (Tyr314) on Cbp serves dual function: recruiting Csk/Ctk for rapid Lyn suppression and later recruiting SOCS1 for Lyn ubiquitination and degradation.","method":"Co-immunoprecipitation, in vitro kinase assay, mutagenesis of Cbp Tyr314, ubiquitination assay in Epo-responsive cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic dissection of phosphosite with multiple functional readouts at different timepoints","pmids":["16920712"],"is_preprint":false},{"year":2013,"finding":"CSK interacts with FLT3 and c-Kit receptor tyrosine kinases in a phosphorylation-dependent manner via its SH2 domain; CSK is recruited from cytoplasm to the inner cell membrane upon ligand stimulation. CSK knockdown or inhibition increases FLT3- and c-Kit-mediated Akt and Erk phosphorylation and downstream signaling, demonstrating CSK negatively regulates type III RTK signaling.","method":"Co-immunoprecipitation, SH2-domain binding assays, siRNA knockdown, phosphorylation analysis, subcellular fractionation","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus siRNA with phosphorylation readouts, single lab","pmids":["23707526"],"is_preprint":false},{"year":2007,"finding":"ZO-1 and ZO-2 (tight junction proteins) serve as Csk-binding proteins (identified by tandem affinity purification/LC-MS/MS). ZO-2 is phosphorylated upon Src transformation and binds Csk in a Csk SH2-dependent manner, establishing ZO proteins as novel Src/Csk scaffolds at cell junctions.","method":"Tandem affinity purification, LC-MS/MS proteomics, co-immunoprecipitation, Csk SH2 domain binding assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomic identification confirmed by co-IP and domain-specific binding, single lab","pmids":["18086565"],"is_preprint":false},{"year":2004,"finding":"Csk overexpression in HeLa cells reduces acid-induced c-Src activation and blocks acid-induced NHE-3 mRNA upregulation and Na+/H+ antiporter activity, placing Src-family kinases downstream of acid sensing as required for NHE-3 regulation.","method":"Stable Csk overexpression in OKP cells, Src kinase activity assay, NHE-3 mRNA and transporter activity measurement","journal":"Proceedings of the National Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — defined overexpression with parallel kinase and functional readouts, single lab","pmids":["7541536"],"is_preprint":false},{"year":2015,"finding":"Inhibition of CskAS during TCR stimulation leads to stronger, more prolonged TCR signaling and increased T cell proliferation. Even a small increase in SFK activity (via partial Csk inhibition) is sufficient to potentiate T cell responses to weak agonists, establishing Csk as the key determinant of TCR signaling threshold and affinity discrimination.","method":"Analog-sensitive Csk knockin mice, dose-titrated chemical inhibitor, T cell activation and proliferation assays, signaling phosphorylation analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — chemical-genetic approach with graded inhibition titration, multiple functional and biochemical readouts","pmids":["26302204"],"is_preprint":false},{"year":2023,"finding":"Adipsin acts upstream of Csk in cardiac microvascular endothelial cells. LC-MS/MS and co-IP identified Csk as a direct downstream regulator of Adipsin. Csk knockdown increases Src phosphorylation (Tyr416) and VE-cadherin phosphorylation (Tyr685, Tyr731) and abolishes Adipsin-induced inhibition of VE-cadherin internalization and protection of endothelial barrier integrity.","method":"LC-MS/MS proteomics, co-immunoprecipitation, siRNA knockdown, phospho-specific immunoblot, permeability assay","journal":"BMC medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomic identification + co-IP + siRNA with multiple downstream readouts, single lab","pmids":["37237266"],"is_preprint":false},{"year":1999,"finding":"Csk requires two divalent metal cations for activity: one to form the ATP-Mg complex and a second free Mg2+ for activation. The second site can be occupied by Mn2+, Co2+, Ni2+, or Zn2+ with much higher affinity than Mg2+. Zn2+ acts as a dead-end inhibitor at this site competitive against free Mg2+. Metal identity at this site influences substrate affinity.","method":"Steady-state kinetics with divalent metal substitution, inhibition analysis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic metal substitution kinetics establishing mechanistic role of second metal site","pmids":["10220355"],"is_preprint":false}],"current_model":"CSK is a non-receptor protein tyrosine kinase that serves as the master negative regulator of Src-family kinases (SFKs) by specifically phosphorylating a conserved C-terminal regulatory tyrosine (e.g., Tyr527 of Src), inducing an inactive closed conformation; CSK is itself regulated by (i) recruitment to the plasma membrane via SH2-domain binding to phosphorylated transmembrane adaptors (primarily PAG/Cbp) at lipid rafts, which both localizes it near SFK substrates and allosterically activates its kinase domain through coupled SH2-kinase domain motions, (ii) direct enzymatic activation by PKA-mediated phosphorylation at Ser364 in a SH3-domain-dependent manner, and (iii) G-protein (Gβγ)-induced translocation and kinase activation; substrate recognition involves both an active-site mechanism requiring specific tail residues (Glu at Y-3, Gln at Y-1) and a distal docking site on the catalytic domain; catalysis proceeds via a random ternary complex with rate-limiting ADP release (Mn) or chemical step (Mg), and the activation loop functions as an inhibitory regulatory element rather than requiring autophosphorylation."},"narrative":{"mechanistic_narrative":"CSK is a cytosolic non-receptor tyrosine kinase that functions as the master negative regulator of Src-family kinases (SFKs), phosphorylating their conserved C-terminal regulatory tyrosine (e.g., Lyn Tyr508, Src Tyr529) to repress kinase activity and lock SFKs in an inactive conformation [PMID:1722201, PMID:11940607]. CSK preferentially acts on SFKs that are already activation-loop autophosphorylated and accessible: autophosphorylated, dimerized Src/Lck are favored substrates, while CSK fails to inactivate SFKs whose phospho-tail SH2 docking site is occluded, a state reversed by PTP1B [PMID:9794236, PMID:11329267, PMID:8631775]. Substrate recognition combines an active-site mechanism reading the Src tail residues Glu(Y-3)/Gln(Y-1) with a distinct docking site on the catalytic domain [PMID:11329267, PMID:16439366]. Catalysis proceeds through a random-order ternary complex requiring two divalent metals, with ADP release rate-limiting under Mn and the chemical step rate-limiting under Mg; the catalytic Asp314 enhances gamma-phosphate electrophilicity and the activation loop acts as an inhibitory element that requires no autophosphorylation [PMID:9425036, PMID:9048573, PMID:7673185, PMID:11551213, PMID:12686554, PMID:10220355]. CSK activity is governed by interdomain coupling: its outward-oriented SH2 and SH3 domains brace the active state, the SH3 domain stimulates the catalytic domain via the SH3-SH2 linker, and SH2 engagement of a Cbp/PAG phosphopeptide allosterically orders the active site through cantilever-like motion propagated to the glycine-rich loop [PMID:11884384, PMID:10460171, PMID:11724538, PMID:16002086]. Membrane recruitment is the dominant spatial control: CSK is delivered to lipid rafts and cell contacts through SH2-domain binding to the transmembrane adaptor PAG/Cbp and to scaffolds including FAK/paxillin, VE-cadherin pTyr685, integrin alphaIIbbeta3, and FLT3/c-Kit, positioning it adjacent to its SFK substrates [PMID:10790433, PMID:11390365, PMID:7529872, PMID:15861137, PMID:11940607, PMID:23707526]. Enzymatic activation is layered onto this localization by PKA phosphorylation at Ser364 (in an SH3-dependent manner) and by Gbetagamma, which both translocate CSK and increase its catalytic activity [PMID:11181701, PMID:12600271, PMID:12062086]. Through this regulatory logic CSK sets signaling thresholds in physiological circuits: it enforces receptor-dependent thymocyte developmental checkpoints and tunes the TCR activation threshold and affinity discrimination, restrains BCR/Lyn-Syk signaling, controls platelet integrin-proximal Src, and limits SFK-driven adhesion, migration and proliferation in epithelial and endothelial contexts [PMID:9732874, PMID:26302204, PMID:24317039, PMID:7935444, PMID:29301754, PMID:14712234, PMID:15861137].","teleology":[{"year":1991,"claim":"Established CSK's core biochemical function — what enzyme controls SFK activity and how — by showing it phosphorylates the SFK C-terminal regulatory tyrosine to repress their activity.","evidence":"in vitro kinase assay with multiple SFK substrates and Lyn Tyr508 mutagenesis","pmids":["1722201"],"confidence":"High","gaps":["Did not resolve how CSK is targeted to SFKs in cells","Structural basis of substrate selection unaddressed"]},{"year":1994,"claim":"Demonstrated in cells that SFK suppression requires CSK's SH2 and SH3 domains for relocalization to sites of active Src, separating catalytic competence from in-cell regulatory function.","evidence":"Csk mutant rescue in csk-deficient cells with immunofluorescence and loss-of-function in chicken B cells","pmids":["7518562","7935444"],"confidence":"High","gaps":["Identity of the membrane/scaffold receptors mediating localization not yet defined","Did not address downstream signaling consequences quantitatively"]},{"year":1995,"claim":"Identified scaffold/adaptor partners (paxillin, FAK, GAP-A.p62) that recruit CSK to focal adhesions and cytoskeletal sites, linking CSK localization to adhesion control.","evidence":"co-IP, GST-SH2 pulldown, inducible overexpression and subcellular fractionation","pmids":["7529872","7544435"],"confidence":"Medium","gaps":["GAP-A.p62 interaction is single-lab in vitro reconstitution","Direct contribution of each scaffold to physiological SFK regulation not dissected"]},{"year":1998,"claim":"Resolved the catalytic mechanism and a substrate-state requirement — CSK inactivates only non-autophosphorylated SFKs, and phosphoryl transfer proceeds via a random ternary complex with rate-limiting ADP release (Mn) or chemical step (Mg).","evidence":"in vitro reconstitution with PTP1B rescue, steady-state kinetics, dead-end inhibition and metal substitution","pmids":["9794236","9425036","9048573"],"confidence":"High","gaps":["Conformational coupling between active site and regulatory domains not yet visualized","Physiological relevance of the autophosphorylation gate in vivo untested"]},{"year":1999,"claim":"Provided structural and energetic models of catalysis — kinase-domain crystal structure, the intramolecular SH3-to-catalytic-domain activation, and a two-metal requirement.","evidence":"X-ray crystallography, isolated-domain reconstitution with SPR, metal-substitution kinetics","pmids":["9878439","10460171","10220355"],"confidence":"High","gaps":["Full-length conformational regulation not yet captured","How extracellular signals modulate domain coupling unknown"]},{"year":2001,"claim":"Defined the allosteric regulatory architecture — full-length structure showing SH2-kinase coupling, NMR mapping of the SH3/linker activation surface, and substrate determinants in the Src tail.","evidence":"full-length crystallography, NMR chemical-shift mapping with mutagenesis, alanine scanning of Src tail","pmids":["11884384","11724538","11329267"],"confidence":"High","gaps":["Dynamic coupling of SH2 engagement to active-site ordering not yet measured","Role of Src oligomerization in cells unverified"]},{"year":2001,"claim":"Established the membrane-recruitment and PKA-activation paradigm placing CSK in lipid rafts as a tonic inhibitor of TCR signaling, controlled by PAG/Cbp binding and Ser364 phosphorylation.","evidence":"co-IP, raft fractionation, dominant-negative displacement, in vitro PKA phosphorylation with functional IL-2/NFAT readouts; pre-steady-state kinetics","pmids":["10790433","11181701","11390365","11551213","11158295"],"confidence":"High","gaps":["Quantitative contribution of localization vs. enzymatic activation not separated until later","Whether PAG is the sole raft anchor unresolved"]},{"year":2002,"claim":"Showed CSK activation is multi-input — Gbetagamma both translocates and activates CSK to drive actin stress fibers, and nucleotide-induced conformational changes propagate from active site to regulatory domains.","evidence":"Csk-deficient MEF rescue with catalytic mutant, Gbetagamma interaction/activation assay, DXMS with nucleotide analogs, platelet co-IP","pmids":["12062086","12417200","11940607"],"confidence":"High","gaps":["Structural detail of the Gbetagamma-CSK interface absent","Mechanism linking CSK to actin remodeling downstream of SFKs incompletely defined"]},{"year":2003,"claim":"Integrated spatial and enzymatic PKA control and defined the activation loop as an inhibitory element, refining how CSK activity is tuned.","evidence":"raft fractionation with FRET kinase assay, SPR of SH3-dependent PKA activation, activation-loop mutagenesis with thrombin-cleavage rescue","pmids":["12665526","12600271","12686554"],"confidence":"High","gaps":["In vivo stoichiometry of PKA-Ser364 phosphorylation unknown","Physiological signals that relieve activation-loop inhibition unclear"]},{"year":2004,"claim":"Placed CSK within phosphatase-coordinated SFK control circuits and SFK-dependent epithelial phenotypes, defining upstream regulators (Shp2, PTP-HSCF) and adhesion/transport outputs.","evidence":"Shp2-deficient cell epistasis, colon-cancer overexpression/dominant-negative phenotypes, Csk overexpression in OKP cells","pmids":["14967142","14712234","7541536"],"confidence":"High","gaps":["Colon cancer and NHE-3 phenotypes are single-lab","Direct vs. indirect contribution of CSK to invasion not fully separated"]},{"year":2005,"claim":"Demonstrated allosteric activation by SH2-ligand binding and identified VE-cadherin pTyr685 as a junctional CSK anchor coupling cell density to growth control.","evidence":"rapid quench flow with Cbp peptide, DXMS, normal mode analysis; reciprocal co-IP, GST-SH2 pulldown, RNAi and Y685F rescue","pmids":["16002086","15861137"],"confidence":"High","gaps":["In-cell allosteric activation by endogenous PAG not directly measured","How density signals control VE-cadherin Tyr685 phosphorylation unclear"]},{"year":2006,"claim":"Defined a docking-based substrate-recognition mechanism on the CSK catalytic domain and dissected Cbp-mediated dual control of Lyn in cytokine signaling.","evidence":"mutagenesis of CSK docking site and Src/Yes docking determinants with kinetics; Cbp Tyr314 mutagenesis with ubiquitination assays","pmids":["16439366","16920712"],"confidence":"High","gaps":["Generality of docking-based recognition across all SFKs untested","Temporal handoff from CSK to SOCS1 mechanism incompletely defined"]},{"year":2013,"claim":"Using analog-sensitive CSK in vivo, established CSK as the gatekeeper of receptor-dependent developmental checkpoints and revealed an actin-remodeling step downstream of SFK activation.","evidence":"conditional Csk knockout in thymocytes; analog-sensitive Csk knockin with cytoskeletal perturbation; FLT3/c-Kit co-IP and knockdown","pmids":["9732874","24317039","23707526"],"confidence":"High","gaps":["Mechanism by which actin remodeling licenses downstream TCR signaling unresolved","FLT3/c-Kit regulation is single-lab Medium-confidence"]},{"year":2015,"claim":"Quantified CSK as the principal determinant of TCR signaling threshold and ligand affinity discrimination by titrating chemical-genetic inhibition.","evidence":"analog-sensitive Csk knockin mice with graded inhibitor dosing and functional/biochemical readouts","pmids":["26302204"],"confidence":"High","gaps":["Molecular basis of threshold setting not reduced to specific phosphorylation kinetics","Whether the same logic operates in other receptor systems untested here"]},{"year":2018,"claim":"Mapped cooperative inhibitory circuits and negative feedback by combining CSK loss with other regulators (PAG/PTPN22/Dok adaptors; CD148 in platelets; LYP dissociation).","evidence":"PAG-knockout and double-knockout epistasis, conditional Csk/CD148 double-KO platelets, LYP chemical probe with imaging and co-IP","pmids":["27926878","29301754","22426112"],"confidence":"High","gaps":["Quantitative hierarchy among redundant CSK adaptors unresolved","Feedback receptor-downregulation mechanism in platelets only partially defined"]},{"year":2023,"claim":"Extended CSK's negative-regulatory role to additional scaffolds and physiological barriers — connexin43/PTEN ternary complexes, Thy-1/CBP integrin signaling, ZO tight-junction scaffolds, P2X3 receptor inhibition, and Adipsin-driven endothelial barrier protection.","evidence":"co-IP, domain pulldowns, RNAi and phospho-readouts across multiple cell systems; in vitro kinase assay and electrophysiology for P2X3","pmids":["27391443","27842221","18086565","19509283","37237266"],"confidence":"Medium","gaps":["Most of these scaffold/substrate links are single-lab","Direct vs. SFK-mediated contributions not always separated"]},{"year":null,"claim":"How the distinct activation inputs (PAG/Cbp SH2 engagement, PKA-Ser364, Gbetagamma, scaffold recruitment) are integrated quantitatively in a living cell to set net CSK activity, and how CSK partner choice is selected among redundant adaptors, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model integrating spatial and enzymatic activation in vivo","Determinants of adaptor selection among PAG/PTPN22/Dok/VE-cadherin/integrin scaffolds unknown","Structure of full-length CSK engaged with a membrane adaptor on substrate unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,5,24,30,33]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,6,7,45]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[6,9,45]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,14,18]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,40]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[12,14,19,37,40]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[3,4,16]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[12,14,27,43,26]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,16,18,40]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[19,38]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[20,41,31]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[27]}],"complexes":["Thy-1/CBP/Csk complex","Cx43/PTEN/Csk/c-Src complex"],"partners":["PAG/CBP","LCK","PTPN22/LYP","VE-CADHERIN","PAXILLIN","FAK","ITGA2B/INTEGRIN ALPHAIIBBETA3","PTP-HSCF"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P41240","full_name":"Tyrosine-protein kinase CSK","aliases":["C-Src kinase","Protein-tyrosine kinase CYL"],"length_aa":450,"mass_kda":50.7,"function":"Non-receptor tyrosine-protein kinase that plays an important role in the regulation of cell growth, differentiation, migration and immune response. Phosphorylates tyrosine residues located in the C-terminal tails of Src-family kinases (SFKs) including LCK, SRC, HCK, FYN, LYN, CSK or YES1. Upon tail phosphorylation, Src-family members engage in intramolecular interactions between the phosphotyrosine tail and the SH2 domain that result in an inactive conformation. To inhibit SFKs, CSK is recruited to the plasma membrane via binding to transmembrane proteins or adapter proteins located near the plasma membrane. Suppresses signaling by various surface receptors, including T-cell receptor (TCR) and B-cell receptor (BCR) by phosphorylating and maintaining inactive several positive effectors such as FYN or LCK. May act as a negative regulator of EGFR and STAT3 signaling pathways (PubMed:26918609)","subcellular_location":"Cytoplasm; Cell membrane","url":"https://www.uniprot.org/uniprotkb/P41240/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CSK","classification":"Not Classified","n_dependent_lines":76,"n_total_lines":1208,"dependency_fraction":0.06291390728476821},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000103653","cell_line_id":"CID001021","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"focal_adhesions","grade":2}],"interactors":[{"gene":"TNS3","stoichiometry":0.2},{"gene":"RAD50","stoichiometry":0.2},{"gene":"PEAK1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001021","total_profiled":1310},"omim":[{"mim_id":"618323","title":"MYASTHENIC SYNDROME, CONGENITAL, 25, PRESYNAPTIC; CMS25","url":"https://www.omim.org/entry/618323"},{"mim_id":"617344","title":"PEAK1-RELATED KINASE-ACTIVATING PSEUDOKINASE 1; PRAG1","url":"https://www.omim.org/entry/617344"},{"mim_id":"616912","title":"ENAH/VASP-LIKE PROTEIN; EVL","url":"https://www.omim.org/entry/616912"},{"mim_id":"614406","title":"SLP ADAPTOR- AND CSK-INTERACTING MEMBRANE PROTEIN; SCIMP","url":"https://www.omim.org/entry/614406"},{"mim_id":"611435","title":"DOCKING PROTEIN 3; DOK3","url":"https://www.omim.org/entry/611435"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Vesicles","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"lymphoid tissue","ntpm":158.2}],"url":"https://www.proteinatlas.org/search/CSK"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P41240","domains":[{"cath_id":"2.30.30.40","chopping":"11-66","consensus_level":"high","plddt":92.7455,"start":11,"end":66},{"cath_id":"3.30.505.10","chopping":"79-169","consensus_level":"high","plddt":92.7493,"start":79,"end":169},{"cath_id":"3.30.200.20","chopping":"197-266","consensus_level":"high","plddt":87.779,"start":197,"end":266},{"cath_id":"1.10.510.10","chopping":"274-446","consensus_level":"high","plddt":94.2101,"start":274,"end":446}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P41240","model_url":"https://alphafold.ebi.ac.uk/files/AF-P41240-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P41240-F1-predicted_aligned_error_v6.png","plddt_mean":91.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CSK","jax_strain_url":"https://www.jax.org/strain/search?query=CSK"},"sequence":{"accession":"P41240","fasta_url":"https://rest.uniprot.org/uniprotkb/P41240.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P41240/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P41240"}},"corpus_meta":[{"pmid":"1722201","id":"PMC_1722201","title":"CSK: 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C-terminal tyrosine of Src family kinases (p60c-src, p56lyn, p59fyn), and this phosphorylation represses their kinase activities. Tyrosine 508 of p56lyn was identified as essential for this phosphorylation.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis, Western blot\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro kinase assay with mutagenesis, replicated across multiple SFK substrates, foundational study widely confirmed\",\n      \"pmids\": [\"1722201\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Crystal structure of the CSK kinase domain co-complexed with staurosporine resolved at 2.4 Å. The structure revealed the ATP-binding cleft between N- and C-terminal lobes, conserved Lys328 and Glu267 forming a molecular pivot for lobe movement, and unique orientations of Asp332 and Phe333 suggesting inhibitor binding via induced fit.\",\n      \"method\": \"X-ray crystallography (molecular replacement)\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure at 2.4 Å resolution with full refinement statistics reported\",\n      \"pmids\": [\"9878439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Full-length crystal structure of Csk revealed that SH2 and SH3 domains are oriented outward (distinct from inactive SFKs) with binding pockets accessible for intermolecular interaction. In active conformations, the SH2-kinase linker stabilizes the active state through direct SH2-kinase domain linkage; SH2 domain rotation destroys this linkage in inactive conformations, suggesting Csk is regulated through SH2-kinase domain coupling.\",\n      \"method\": \"X-ray crystallography (full-length protein)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — full-length crystal structure with multiple conformational states analyzed\",\n      \"pmids\": [\"11884384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Csk suppression of Src requires functional SH2, SH3, and catalytic domains. An SH3 deletion mutant retained in vitro kinase activity but failed to suppress Src in cells. When Src is active, Csk relocalizes from cytosol to podosomes via its SH3 and SH2 domains; SH3 or SH2 domain mutants do not relocalize, explaining their failure to regulate Src.\",\n      \"method\": \"Expression of Csk mutants in csk-deficient cells, immunofluorescence localization, in vitro kinase assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — functional mutant analysis in defined cell system combined with localization, replicated across multiple mutants\",\n      \"pmids\": [\"7518562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Overexpressed Csk localizes to focal adhesions via its SH2 and SH3 domains (not catalytic activity). Csk forms a complex with paxillin in cells and its SH2 domain interacts with FAK and paxillin in vitro. Catalytically active Csk overexpression causes cell rounding, loss of adhesion, and redistribution of αvβ5 integrin; catalytically inactive mutant does not produce these effects.\",\n      \"method\": \"Inducible overexpression, immunofluorescence, co-immunoprecipitation, in vitro binding (SH2 domain GST pulldown), point and deletion mutagenesis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (localization, co-IP, in vitro binding, mutagenesis) in a single study\",\n      \"pmids\": [\"7529872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Csk phosphorylates and inactivates Src only when Src is not previously autophosphorylated at its activation loop tyrosine. Autophosphorylated Src can still be phosphorylated by Csk at the C-terminal tail but is not inactivated, because the SH2 binding site for phospho-Tyr is inaccessible. PTP1B dephosphorylation of the Src autophosphorylation site restores Csk-mediated inactivation.\",\n      \"method\": \"In vitro kinase assay, pre-incubation with ATP-Mg, site-directed mutagenesis (Yes Y-to-F mutant), PTP1B dephosphorylation rescue\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and phosphatase rescue, single lab\",\n      \"pmids\": [\"9794236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CSK catalyzes phosphoryl transfer via a ternary complex random-substrate-binding mechanism. ADP release is rate-limiting with Mn as cofactor; the chemical (phosphoryl transfer) step is fully rate-determining with Mg. Asp314 enhances gamma-phosphate reactivity toward electrophilic attack (thio-effect analysis with D314E mutant).\",\n      \"method\": \"Steady-state kinetics, dead-end inhibitor analysis, viscosity experiments, thio effects with wild-type and D314E mutant Csk\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — detailed kinetic mechanism with multiple substrates, mutant analysis, and metal substitution studies\",\n      \"pmids\": [\"9425036\", \"9048573\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The catalytic base Asp314 of Csk is not required for early tyrosine deprotonation (trifluorotyrosine substrate is phosphorylated with similar efficiency). D314E mutation reduces kcat ~10,000-fold with minimal effect on substrate Km, and thio-effect analysis suggests Asp314 enhances gamma-phosphate electrophilicity.\",\n      \"method\": \"In vitro kinase assay with unnatural amino acid substrate, site-directed mutagenesis (D314E), thio-effect measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — active-site mutagenesis combined with unnatural substrate chemistry\",\n      \"pmids\": [\"7673185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The Csk SH3 domain intramolecularly interacts with the catalytic domain to stimulate kinase activity (~100-fold reduction when catalytic domain alone is expressed). The interaction surface on SH3 involves the SH3-SH2 linker region, distinct from the classical type-II polyproline helix-binding surface.\",\n      \"method\": \"Expression of isolated Csk domains in E. coli, in vitro kinase assay, trans-stimulation, surface plasmon resonance, site-directed mutagenesis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution of domain interactions in vitro with kinetic and SPR measurements\",\n      \"pmids\": [\"10460171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Pre-steady-state kinetic analysis shows CSK phosphoryl transfer to peptide substrate is fast (k3 ≥ 140 s⁻¹) and favorable, with rate-limiting ADP release (koff ~0.6 s⁻¹ ≈ kcat). Viscosometric studies indicate slow conformational changes are coupled to ADP dissociation.\",\n      \"method\": \"Rapid quench flow, stopped-flow fluorescence, viscosometric experiments\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — pre-steady-state kinetics with multiple transient-state methods establishing catalytic mechanism\",\n      \"pmids\": [\"11551213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The SH3 and SH2 domains of Csk's substrate Src are dispensable for efficient Csk-catalyzed phosphorylation; the catalytic domain and tail of Src are sufficient. Key Src tail residues Glu (Y-3) and Gln (Y-1) are critical determinants of substrate efficiency for Csk. Src dimerization/oligomerization appears important for high-efficiency Csk phosphorylation.\",\n      \"method\": \"In vitro kinase assay with purified recombinant proteins and alanine-scanning mutants of Src tail\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis of substrate combined with in vitro reconstitution\",\n      \"pmids\": [\"11329267\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Csk physically interacts with PTP-HSCF (third member of PEP phosphatase family) via the Csk SH2 domain binding to tyrosine-phosphorylated sites in PTP-HSCF's non-catalytic region. Csk and PTP-HSCF synergize to inhibit Src family kinases; this cooperativity depends on the domains mediating their interaction. PTP-HSCF selectively dephosphorylates the positive regulatory tyrosine of Src-related kinases.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation in mammalian cells and hematopoietic cells, transfection assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — yeast two-hybrid confirmed by co-IP in multiple cell systems, functional synergy demonstrated\",\n      \"pmids\": [\"11158295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PAG (phosphoprotein associated with GEMs/Cbp) is a transmembrane adaptor that constitutively binds Csk via its phosphorylated cytoplasmic tyrosines in resting T cells. After T cell activation, PAG is dephosphorylated and dissociates from Csk. PAG expression in COS cells recruits endogenous Csk and alters Src kinase activity. Overexpression in Jurkat cells downregulates TCR-mediated NFAT activation.\",\n      \"method\": \"Co-immunoprecipitation, mass spectrometry identification, COS cell transfection, Jurkat overexpression with reporter assay\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, multiple cell systems, functional consequence demonstrated; independently replicated\",\n      \"pmids\": [\"10790433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PKA phosphorylates Csk at Ser364 in vitro and in vivo, leading to a 2-4-fold increase in Csk kinase activity. This PKA-mediated Csk activation is necessary for cAMP-mediated inhibition of TCR signaling and IL-2 secretion. Both PKA type I and Csk are targeted to lipid rafts where Lck phosphorylation by activated Csk is enhanced.\",\n      \"method\": \"In vitro PKA phosphorylation of Csk, kinase activity assay, lipid raft fractionation, IL-2 secretion assay, phosphorylation site mutants\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro phosphorylation + in vivo confirmation + functional readout, independently replicated\",\n      \"pmids\": [\"11181701\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Csk is constitutively present in lipid rafts through interaction with PAG/Cbp. Upon TCR triggering, PAG/Cbp is dephosphorylated leading to Csk dissociation from rafts. A dominant-negative Csk that displaces endogenous Csk from lipid rafts results in elevated basal TCR-zeta phosphorylation and enhanced TCR responses, establishing that raft-localized Csk tonically inhibits T cell activation.\",\n      \"method\": \"Lipid raft fractionation, dominant-negative Csk expression, TCR-zeta phosphorylation assay, NFAT-AP1 reporter assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct localization-function link using dominant-negative approach with multiple functional readouts\",\n      \"pmids\": [\"11390365\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PKA regulates Csk by two mechanisms: (1) increasing Cbp/PAG phosphorylation to augment Csk recruitment to lipid rafts, reducing TCR-induced Csk dissociation; and (2) directly increasing Csk phosphotransferase activity. cAMP-mediated inhibition of TCR signaling requires raft-associated Csk, as displacing Csk from rafts abolishes PGE2-mediated inhibition.\",\n      \"method\": \"Raft fractionation, co-immunoprecipitation, FRET-based kinase activity assay, NFAT-AP1 reporter\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal mechanisms established, spatial and enzymatic regulation combined in one study\",\n      \"pmids\": [\"12665526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Csk is required for G protein (Gβγ, Gα12, Gα13, Gαq)-induced actin stress fiber formation. In Csk-deficient MEFs, GPCR-induced actin reorganization is completely blocked and reintroduction of Csk restores it. Csk catalytic activity is required; Gβγ both translocates Csk to the plasma membrane and directly increases Csk kinase activity.\",\n      \"method\": \"Csk-deficient MEFs, Csk reintroduction, catalytic mutant rescue, biochemical Gβγ-Csk interaction and kinase activation assay\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic (KO rescue) plus biochemical interaction and kinase activation, multiple G proteins tested\",\n      \"pmids\": [\"12062086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"DXMS analysis of CSK shows that ATP-analog AMPPNP and product ADP protect overlapping but distinct regions including the active site, activation loop, helix G, SH2 domain, and SH2-kinase linker, demonstrating that phosphoryl transfer induces unique conformational changes propagated from active site to regulatory domains.\",\n      \"method\": \"Hydrogen-deuterium exchange mass spectrometry (DXMS) with nucleotide analogs\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative HX-MS method covering 63% of protein, multiple nucleotide states compared\",\n      \"pmids\": [\"12417200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Shp2 promotes Src family kinase activation by dephosphorylating PAG/Cbp, thereby reducing Csk recruitment to SFKs. In Shp2-deficient cells, SFK inhibitory C-terminal tyrosines are hyperphosphorylated. This places Shp2 upstream of Csk in controlling SFK activity and downstream signaling (Plcγ1, Ras/Erk).\",\n      \"method\": \"Shp2-deficient cells, phosphorylation analysis of SFK regulatory tyrosines, Plcγ1 phosphorylation assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in defined KO cells with mechanistic biochemical readouts, widely cited\",\n      \"pmids\": [\"14967142\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Csk is constitutively associated with integrin αIIbβ3 in platelets. Upon fibrinogen binding, Csk dissociates from αIIbβ3 concomitant with Src Tyr-529 dephosphorylation and Src Tyr-418 phosphorylation (activation), placing Csk as a direct negative regulator of integrin-proximal Src in platelet spreading.\",\n      \"method\": \"Co-immunoprecipitation from platelets, phospho-specific antibodies, integrin engagement assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP with functional context, well-controlled physiological setting\",\n      \"pmids\": [\"11940607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Csk binds via its SH2 domain to phosphorylated Tyr685 of VE-cadherin. VE-cadherin recruits Csk to cell contacts; this interaction increases with cell density. VE-cadherin-Y685F mutant abolishes Csk binding and confers higher growth density. Csk overexpression inhibits endothelial cell proliferation in a VE-cadherin phospho-Y685-dependent manner; Csk RNAi enhances proliferation.\",\n      \"method\": \"Co-immunoprecipitation, GST-SH2 pulldown, inducible Csk overexpression, RNAi knockdown, proliferation assay with Y685F mutant\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP + in vitro pulldown + functional rescue with phosphosite mutant + RNAi\",\n      \"pmids\": [\"15861137\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Binding of a Cbp-derived phosphopeptide to the Csk SH2 domain enhances Src recognition (lowers Km) by increasing phosphoryl transfer rate, detected by rapid quench flow. DXMS shows the Cbp peptide induces conformational changes propagating from the SH2 domain through the SH2-kinase linker to the glycine-rich loop of the active site, suggesting cantilever-like SH2 motion orders the active site.\",\n      \"method\": \"Rapid quench flow kinetics, DXMS, computational normal mode analysis\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple biophysical methods (HX-MS, pre-steady-state kinetics, normal mode analysis) establishing allosteric mechanism\",\n      \"pmids\": [\"16002086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"NMR chemical shift mapping shows the Csk SH3 domain binds the catalytic domain at the SH3-SH2 linker region and N-terminal tail (not the classical polyproline-binding RT-loop surface). Site-directed mutagenesis of the SH3-SH2 linker reduces Csk kinase activity up to 5-fold, confirming this intradomain interaction activates the catalytic domain.\",\n      \"method\": \"NMR chemical shift perturbation mapping, site-directed mutagenesis, in vitro kinase assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR mapping with mutagenesis validation, two orthogonal methods\",\n      \"pmids\": [\"11724538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PKA-mediated activation of Csk by Ser364 phosphorylation depends on the intact SH3 domain. Isolated Csk kinase domain is phosphorylated by PKA at S364 but is not significantly activated unless the SH3 domain is present. PKA phosphorylation facilitates the SH3-kinase domain interaction, measured by surface plasmon resonance.\",\n      \"method\": \"In vitro PKA phosphorylation of Csk domains, kinase activity assay, surface plasmon resonance\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with domain fragments and SPR measurement\",\n      \"pmids\": [\"12600271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Csk recognizes Src family kinase substrates through a docking site on its catalytic domain that is distinct from the active site. Docking determinants on Src recognized by this site were identified; similar docking-based recognition was confirmed for Yes. This establishes a docking-based substrate recognition mechanism for Csk.\",\n      \"method\": \"In vitro binding assays, mutagenesis of Csk docking site and Src docking determinants, kinase assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis of both enzyme and substrate docking elements with kinetic validation\",\n      \"pmids\": [\"16439366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Csk activation loop lacks tyrosine residues, and specific residues in this loop have little function in Src phosphorylation. Thrombin-cleavage of the activation loop restores full activity in loop-insertion mutants that are nearly inactive toward artificial substrates, indicating the activation loop acts as an inhibitory regulatory structure and is dispensable for catalysis when removed.\",\n      \"method\": \"Extensive site-directed mutagenesis, loop replacement, engineered thrombin cleavage site, kinase assay with physiological (Src) and artificial substrates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis plus proteolytic rescue experiment, multiple substrate types compared\",\n      \"pmids\": [\"12686554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Csk-negative chicken B cells have constitutively active Lyn (hyperphosphorylated at autophosphorylation site) and constitutively activated Syk, demonstrating Csk maintains Lyn and downstream Syk in inactive states. BCR crosslinking was still required for full downstream signaling (Ca2+ mobilization, IP3 generation) despite constitutive kinase activation.\",\n      \"method\": \"Csk-deficient B cell clones (gene disruption), kinase activity assay, phosphorylation analysis, calcium flux, IP3 measurement\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function in defined KO cells with multiple biochemical and functional readouts\",\n      \"pmids\": [\"7935444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"T cell development is controlled by Csk: conditional inactivation of Csk in immature thymocytes abrogates the requirement for preTCR, αβTCR, and MHC class II for development of DP and CD4+ SP thymocytes, placing Csk as the gatekeeper that enforces receptor-dependent developmental checkpoints by restraining Src-family kinases.\",\n      \"method\": \"Conditional Csk gene knockout in thymocytes (Cre-lox), flow cytometric analysis of thymic development\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional genetic KO with defined developmental phenotype, epistasis placing Csk upstream of preTCR/αβTCR checkpoints\",\n      \"pmids\": [\"9732874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Inhibition of an analog-sensitive Csk variant (CskAS) in thymocytes induces potent SFK activation and proximal TCR signaling up to PLCγ1 phosphorylation, but downstream signals (IP3, Ca2+, Erk phosphorylation) are impaired. Actin cytoskeleton remodeling (pharmacological or via CD28 costimulation) rescues these downstream defects, revealing that Csk controls a checkpoint requiring actin remodeling for full TCR signaling.\",\n      \"method\": \"Analog-sensitive Csk knockin mice, chemical-genetic inhibition, phosphorylation analysis, IP3/Ca2+ measurement, cytoskeletal perturbation\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — chemical-genetic approach with analog-sensitive allele, multiple orthogonal signaling readouts\",\n      \"pmids\": [\"24317039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"CSK directly associates with the GTPase-activating protein-associated p62 (GAP-A.p62) protein via the CSK SH2 domain (reconstituted in vitro with GST-CSK and GST-SH2 domain fusions). This complex localizes to membrane/cytoskeletal fractions in v-Src NIH 3T3 cells, suggesting GAP-A.p62 as a docking protein mediating CSK translocation to sites of Src activity.\",\n      \"method\": \"Co-immunoprecipitation, GST pulldown in vitro reconstitution, subcellular fractionation\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro GST pulldown plus co-IP from cells, single lab\",\n      \"pmids\": [\"7544435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Csk interacts with phosphorylated Lck via the Csk SH2 domain; this interaction requires Lck autophosphorylation at Tyr394 (activation loop). Autophosphorylated Lck is preferentially phosphorylated by Csk at its negative regulatory Tyr505, suggesting activated Src-family kinases are preferential Csk substrates.\",\n      \"method\": \"BIAcore (SPR) real-time binding assay, yeast two-hybrid, in vitro kinase assay with site-directed Lck mutants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — SPR binding measurement + yeast two-hybrid + in vitro kinase assay, two orthogonal interaction methods\",\n      \"pmids\": [\"8631775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Csk overexpression in colon cancer cells decreases SFK activation, increases E-cadherin-mediated cell-cell contacts, decreases focal contacts, and reduces cell adhesion/migration/invasiveness. Dominant-negative Csk produces opposite effects: elevated SFK/FAK/paxillin phosphorylation, cell scattering, increased focal contacts and invasiveness. All these events are integrin-dependent.\",\n      \"method\": \"Stable overexpression and dominant-negative expression, kinase assay, cell adhesion/migration/invasion assay, phospho-FAK/paxillin immunoblot\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — paired wild-type/dominant-negative approach with multiple cellular phenotype readouts, single lab\",\n      \"pmids\": [\"14712234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"p140Cap adaptor protein binds Src and activates Csk, leading to Src inhibition. p140Cap silencing increases cell spreading, migration, and Src activity; increased p140Cap expression activates Csk and inhibits Src and downstream signaling, cell motility, and invasion.\",\n      \"method\": \"RNAi knockdown, co-immunoprecipitation, kinase activity assay, cell migration/invasion assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus functional RNAi/overexpression with defined readouts, single lab\",\n      \"pmids\": [\"17525734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Csk directly phosphorylates Tyr393 of the P2X3 receptor in vitro (in vitro kinase assay in HEK cells), strongly inhibiting receptor currents. Csk knockdown in HEK and trigeminal neurons potentiates P2X3 responses, confirming constitutive Csk-mediated inhibition of P2X3. NGF levels regulate Csk activity in sensory neurons to control P2X3 sensitization.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis (structural modeling-guided), RNAi knockdown, electrophysiology\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct in vitro phosphorylation of identified site plus RNAi functional validation, two orthogonal methods\",\n      \"pmids\": [\"19509283\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"LYP (lymphoid tyrosine phosphatase) inhibits T cell activation when dissociated from CSK. Dissociation of the LYP-CSK complex is necessary for LYP recruitment to the plasma membrane where it downmodulates TCR signaling. The autoimmunity-associated LYP variant that cannot bind CSK shows reduced TCR-mediated signaling.\",\n      \"method\": \"Spatiotemporal dynamics imaging, selective LYP chemical probe, co-immunoprecipitation, functional T cell activation assays\",\n      \"journal\": \"Nature chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — chemical probe plus co-IP plus live-cell imaging plus functional assays, multiple orthogonal methods\",\n      \"pmids\": [\"22426112\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PAG (Cbp) genetically cooperates with PTPN22 and Dok adaptors to inhibit effector T cell activation. In PAG-deficient mice, Csk associates more with PTPN22 and Dok adaptors as alternative partners. Combining PAG deficiency with PTPN22 or Dok deficiency further enhances effector T cell responses, establishing a cooperative inhibitory circuit.\",\n      \"method\": \"PAG-knockout mice, double-knockout genetic epistasis, co-immunoprecipitation, T cell activation assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in defined KO mice with biochemical partner-switching and functional readouts\",\n      \"pmids\": [\"27926878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Connexin43 (Cx43) recruits both PTEN and Csk to the region between residues 266-283 of its C-terminus, forming a ternary complex with c-Src. Pulldown assays showed this Cx43 region is sufficient to recruit c-Src, PTEN, and Csk and to inhibit oncogenic c-Src activity. Csk and PTEN silencing reduces the antiproliferative effect of Cx43.\",\n      \"method\": \"Confocal microscopy, co-immunoprecipitation, GST pulldown (Cx43 domain fragments), RNAi knockdown, proliferation assay\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus domain pulldown plus functional RNAi, single lab\",\n      \"pmids\": [\"27391443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Upon αvβ3 integrin engagement of Thy-1, Csk is present in the Thy-1/CBP/Csk complex at the plasma membrane; integrin binding causes Src inactivation and exclusion from this complex. CBP silencing abolishes Csk recruitment and the downstream RhoA/ROCK neurite retraction signaling, establishing CBP as the key transducer linking Thy-1 to Csk-mediated Src inactivation.\",\n      \"method\": \"Single-molecule tracking, nanoscopy, co-immunoprecipitation, RNAi knockdown, RhoA activation assay\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple imaging methods plus biochemical and genetic approaches, single lab\",\n      \"pmids\": [\"27842221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Simultaneous ablation of Csk and CD148 in platelets causes dramatic SFK hyperactivation, demonstrating these proteins are essential regulators of platelet SFK activity. Paradoxically, this leads to reduced thrombosis due to downregulation of GPVI-FcRγ and CLEC-2 ITAM receptors and upregulation of inhibitory G6b-B/Shp1/Shp2 signaling as a negative feedback.\",\n      \"method\": \"Conditional Csk/CD148 double-knockout mice, analog-sensitive Csk model, platelet SFK activity assay, receptor expression analysis, bleeding/thrombosis models\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional double-KO plus analog-sensitive mouse model, multiple in vivo and ex vivo readouts\",\n      \"pmids\": [\"29301754\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cbp/PAG phosphorylation by Lyn recruits Csk (and Ctk) to suppress Lyn kinase activity within minutes of Epo stimulation. A single phosphotyrosine (Tyr314) on Cbp serves dual function: recruiting Csk/Ctk for rapid Lyn suppression and later recruiting SOCS1 for Lyn ubiquitination and degradation.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, mutagenesis of Cbp Tyr314, ubiquitination assay in Epo-responsive cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic dissection of phosphosite with multiple functional readouts at different timepoints\",\n      \"pmids\": [\"16920712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"CSK interacts with FLT3 and c-Kit receptor tyrosine kinases in a phosphorylation-dependent manner via its SH2 domain; CSK is recruited from cytoplasm to the inner cell membrane upon ligand stimulation. CSK knockdown or inhibition increases FLT3- and c-Kit-mediated Akt and Erk phosphorylation and downstream signaling, demonstrating CSK negatively regulates type III RTK signaling.\",\n      \"method\": \"Co-immunoprecipitation, SH2-domain binding assays, siRNA knockdown, phosphorylation analysis, subcellular fractionation\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus siRNA with phosphorylation readouts, single lab\",\n      \"pmids\": [\"23707526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"ZO-1 and ZO-2 (tight junction proteins) serve as Csk-binding proteins (identified by tandem affinity purification/LC-MS/MS). ZO-2 is phosphorylated upon Src transformation and binds Csk in a Csk SH2-dependent manner, establishing ZO proteins as novel Src/Csk scaffolds at cell junctions.\",\n      \"method\": \"Tandem affinity purification, LC-MS/MS proteomics, co-immunoprecipitation, Csk SH2 domain binding assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomic identification confirmed by co-IP and domain-specific binding, single lab\",\n      \"pmids\": [\"18086565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Csk overexpression in HeLa cells reduces acid-induced c-Src activation and blocks acid-induced NHE-3 mRNA upregulation and Na+/H+ antiporter activity, placing Src-family kinases downstream of acid sensing as required for NHE-3 regulation.\",\n      \"method\": \"Stable Csk overexpression in OKP cells, Src kinase activity assay, NHE-3 mRNA and transporter activity measurement\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — defined overexpression with parallel kinase and functional readouts, single lab\",\n      \"pmids\": [\"7541536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Inhibition of CskAS during TCR stimulation leads to stronger, more prolonged TCR signaling and increased T cell proliferation. Even a small increase in SFK activity (via partial Csk inhibition) is sufficient to potentiate T cell responses to weak agonists, establishing Csk as the key determinant of TCR signaling threshold and affinity discrimination.\",\n      \"method\": \"Analog-sensitive Csk knockin mice, dose-titrated chemical inhibitor, T cell activation and proliferation assays, signaling phosphorylation analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — chemical-genetic approach with graded inhibition titration, multiple functional and biochemical readouts\",\n      \"pmids\": [\"26302204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Adipsin acts upstream of Csk in cardiac microvascular endothelial cells. LC-MS/MS and co-IP identified Csk as a direct downstream regulator of Adipsin. Csk knockdown increases Src phosphorylation (Tyr416) and VE-cadherin phosphorylation (Tyr685, Tyr731) and abolishes Adipsin-induced inhibition of VE-cadherin internalization and protection of endothelial barrier integrity.\",\n      \"method\": \"LC-MS/MS proteomics, co-immunoprecipitation, siRNA knockdown, phospho-specific immunoblot, permeability assay\",\n      \"journal\": \"BMC medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomic identification + co-IP + siRNA with multiple downstream readouts, single lab\",\n      \"pmids\": [\"37237266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Csk requires two divalent metal cations for activity: one to form the ATP-Mg complex and a second free Mg2+ for activation. The second site can be occupied by Mn2+, Co2+, Ni2+, or Zn2+ with much higher affinity than Mg2+. Zn2+ acts as a dead-end inhibitor at this site competitive against free Mg2+. Metal identity at this site influences substrate affinity.\",\n      \"method\": \"Steady-state kinetics with divalent metal substitution, inhibition analysis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic metal substitution kinetics establishing mechanistic role of second metal site\",\n      \"pmids\": [\"10220355\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CSK is a non-receptor protein tyrosine kinase that serves as the master negative regulator of Src-family kinases (SFKs) by specifically phosphorylating a conserved C-terminal regulatory tyrosine (e.g., Tyr527 of Src), inducing an inactive closed conformation; CSK is itself regulated by (i) recruitment to the plasma membrane via SH2-domain binding to phosphorylated transmembrane adaptors (primarily PAG/Cbp) at lipid rafts, which both localizes it near SFK substrates and allosterically activates its kinase domain through coupled SH2-kinase domain motions, (ii) direct enzymatic activation by PKA-mediated phosphorylation at Ser364 in a SH3-domain-dependent manner, and (iii) G-protein (Gβγ)-induced translocation and kinase activation; substrate recognition involves both an active-site mechanism requiring specific tail residues (Glu at Y-3, Gln at Y-1) and a distal docking site on the catalytic domain; catalysis proceeds via a random ternary complex with rate-limiting ADP release (Mn) or chemical step (Mg), and the activation loop functions as an inhibitory regulatory element rather than requiring autophosphorylation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CSK is a cytosolic non-receptor tyrosine kinase that functions as the master negative regulator of Src-family kinases (SFKs), phosphorylating their conserved C-terminal regulatory tyrosine (e.g., Lyn Tyr508, Src Tyr529) to repress kinase activity and lock SFKs in an inactive conformation [#0, #19]. CSK preferentially acts on SFKs that are already activation-loop autophosphorylated and accessible: autophosphorylated, dimerized Src/Lck are favored substrates, while CSK fails to inactivate SFKs whose phospho-tail SH2 docking site is occluded, a state reversed by PTP1B [#5, #10, #30]. Substrate recognition combines an active-site mechanism reading the Src tail residues Glu(Y-3)/Gln(Y-1) with a distinct docking site on the catalytic domain [#10, #24]. Catalysis proceeds through a random-order ternary complex requiring two divalent metals, with ADP release rate-limiting under Mn and the chemical step rate-limiting under Mg; the catalytic Asp314 enhances gamma-phosphate electrophilicity and the activation loop acts as an inhibitory element that requires no autophosphorylation [#6, #7, #9, #25, #45]. CSK activity is governed by interdomain coupling: its outward-oriented SH2 and SH3 domains brace the active state, the SH3 domain stimulates the catalytic domain via the SH3-SH2 linker, and SH2 engagement of a Cbp/PAG phosphopeptide allosterically orders the active site through cantilever-like motion propagated to the glycine-rich loop [#2, #8, #22, #21]. Membrane recruitment is the dominant spatial control: CSK is delivered to lipid rafts and cell contacts through SH2-domain binding to the transmembrane adaptor PAG/Cbp and to scaffolds including FAK/paxillin, VE-cadherin pTyr685, integrin alphaIIbbeta3, and FLT3/c-Kit, positioning it adjacent to its SFK substrates [#12, #14, #4, #20, #19, #40]. Enzymatic activation is layered onto this localization by PKA phosphorylation at Ser364 (in an SH3-dependent manner) and by Gbetagamma, which both translocate CSK and increase its catalytic activity [#13, #23, #16]. Through this regulatory logic CSK sets signaling thresholds in physiological circuits: it enforces receptor-dependent thymocyte developmental checkpoints and tunes the TCR activation threshold and affinity discrimination, restrains BCR/Lyn-Syk signaling, controls platelet integrin-proximal Src, and limits SFK-driven adhesion, migration and proliferation in epithelial and endothelial contexts [#27, #43, #28, #26, #38, #31, #20].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Established CSK's core biochemical function — what enzyme controls SFK activity and how — by showing it phosphorylates the SFK C-terminal regulatory tyrosine to repress their activity.\",\n      \"evidence\": \"in vitro kinase assay with multiple SFK substrates and Lyn Tyr508 mutagenesis\",\n      \"pmids\": [\"1722201\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve how CSK is targeted to SFKs in cells\", \"Structural basis of substrate selection unaddressed\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Demonstrated in cells that SFK suppression requires CSK's SH2 and SH3 domains for relocalization to sites of active Src, separating catalytic competence from in-cell regulatory function.\",\n      \"evidence\": \"Csk mutant rescue in csk-deficient cells with immunofluorescence and loss-of-function in chicken B cells\",\n      \"pmids\": [\"7518562\", \"7935444\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the membrane/scaffold receptors mediating localization not yet defined\", \"Did not address downstream signaling consequences quantitatively\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identified scaffold/adaptor partners (paxillin, FAK, GAP-A.p62) that recruit CSK to focal adhesions and cytoskeletal sites, linking CSK localization to adhesion control.\",\n      \"evidence\": \"co-IP, GST-SH2 pulldown, inducible overexpression and subcellular fractionation\",\n      \"pmids\": [\"7529872\", \"7544435\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"GAP-A.p62 interaction is single-lab in vitro reconstitution\", \"Direct contribution of each scaffold to physiological SFK regulation not dissected\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Resolved the catalytic mechanism and a substrate-state requirement — CSK inactivates only non-autophosphorylated SFKs, and phosphoryl transfer proceeds via a random ternary complex with rate-limiting ADP release (Mn) or chemical step (Mg).\",\n      \"evidence\": \"in vitro reconstitution with PTP1B rescue, steady-state kinetics, dead-end inhibition and metal substitution\",\n      \"pmids\": [\"9794236\", \"9425036\", \"9048573\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational coupling between active site and regulatory domains not yet visualized\", \"Physiological relevance of the autophosphorylation gate in vivo untested\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Provided structural and energetic models of catalysis — kinase-domain crystal structure, the intramolecular SH3-to-catalytic-domain activation, and a two-metal requirement.\",\n      \"evidence\": \"X-ray crystallography, isolated-domain reconstitution with SPR, metal-substitution kinetics\",\n      \"pmids\": [\"9878439\", \"10460171\", \"10220355\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length conformational regulation not yet captured\", \"How extracellular signals modulate domain coupling unknown\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the allosteric regulatory architecture — full-length structure showing SH2-kinase coupling, NMR mapping of the SH3/linker activation surface, and substrate determinants in the Src tail.\",\n      \"evidence\": \"full-length crystallography, NMR chemical-shift mapping with mutagenesis, alanine scanning of Src tail\",\n      \"pmids\": [\"11884384\", \"11724538\", \"11329267\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamic coupling of SH2 engagement to active-site ordering not yet measured\", \"Role of Src oligomerization in cells unverified\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Established the membrane-recruitment and PKA-activation paradigm placing CSK in lipid rafts as a tonic inhibitor of TCR signaling, controlled by PAG/Cbp binding and Ser364 phosphorylation.\",\n      \"evidence\": \"co-IP, raft fractionation, dominant-negative displacement, in vitro PKA phosphorylation with functional IL-2/NFAT readouts; pre-steady-state kinetics\",\n      \"pmids\": [\"10790433\", \"11181701\", \"11390365\", \"11551213\", \"11158295\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of localization vs. enzymatic activation not separated until later\", \"Whether PAG is the sole raft anchor unresolved\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Showed CSK activation is multi-input — Gbetagamma both translocates and activates CSK to drive actin stress fibers, and nucleotide-induced conformational changes propagate from active site to regulatory domains.\",\n      \"evidence\": \"Csk-deficient MEF rescue with catalytic mutant, Gbetagamma interaction/activation assay, DXMS with nucleotide analogs, platelet co-IP\",\n      \"pmids\": [\"12062086\", \"12417200\", \"11940607\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural detail of the Gbetagamma-CSK interface absent\", \"Mechanism linking CSK to actin remodeling downstream of SFKs incompletely defined\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Integrated spatial and enzymatic PKA control and defined the activation loop as an inhibitory element, refining how CSK activity is tuned.\",\n      \"evidence\": \"raft fractionation with FRET kinase assay, SPR of SH3-dependent PKA activation, activation-loop mutagenesis with thrombin-cleavage rescue\",\n      \"pmids\": [\"12665526\", \"12600271\", \"12686554\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo stoichiometry of PKA-Ser364 phosphorylation unknown\", \"Physiological signals that relieve activation-loop inhibition unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Placed CSK within phosphatase-coordinated SFK control circuits and SFK-dependent epithelial phenotypes, defining upstream regulators (Shp2, PTP-HSCF) and adhesion/transport outputs.\",\n      \"evidence\": \"Shp2-deficient cell epistasis, colon-cancer overexpression/dominant-negative phenotypes, Csk overexpression in OKP cells\",\n      \"pmids\": [\"14967142\", \"14712234\", \"7541536\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Colon cancer and NHE-3 phenotypes are single-lab\", \"Direct vs. indirect contribution of CSK to invasion not fully separated\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstrated allosteric activation by SH2-ligand binding and identified VE-cadherin pTyr685 as a junctional CSK anchor coupling cell density to growth control.\",\n      \"evidence\": \"rapid quench flow with Cbp peptide, DXMS, normal mode analysis; reciprocal co-IP, GST-SH2 pulldown, RNAi and Y685F rescue\",\n      \"pmids\": [\"16002086\", \"15861137\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In-cell allosteric activation by endogenous PAG not directly measured\", \"How density signals control VE-cadherin Tyr685 phosphorylation unclear\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defined a docking-based substrate-recognition mechanism on the CSK catalytic domain and dissected Cbp-mediated dual control of Lyn in cytokine signaling.\",\n      \"evidence\": \"mutagenesis of CSK docking site and Src/Yes docking determinants with kinetics; Cbp Tyr314 mutagenesis with ubiquitination assays\",\n      \"pmids\": [\"16439366\", \"16920712\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Generality of docking-based recognition across all SFKs untested\", \"Temporal handoff from CSK to SOCS1 mechanism incompletely defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Using analog-sensitive CSK in vivo, established CSK as the gatekeeper of receptor-dependent developmental checkpoints and revealed an actin-remodeling step downstream of SFK activation.\",\n      \"evidence\": \"conditional Csk knockout in thymocytes; analog-sensitive Csk knockin with cytoskeletal perturbation; FLT3/c-Kit co-IP and knockdown\",\n      \"pmids\": [\"9732874\", \"24317039\", \"23707526\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which actin remodeling licenses downstream TCR signaling unresolved\", \"FLT3/c-Kit regulation is single-lab Medium-confidence\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Quantified CSK as the principal determinant of TCR signaling threshold and ligand affinity discrimination by titrating chemical-genetic inhibition.\",\n      \"evidence\": \"analog-sensitive Csk knockin mice with graded inhibitor dosing and functional/biochemical readouts\",\n      \"pmids\": [\"26302204\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of threshold setting not reduced to specific phosphorylation kinetics\", \"Whether the same logic operates in other receptor systems untested here\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Mapped cooperative inhibitory circuits and negative feedback by combining CSK loss with other regulators (PAG/PTPN22/Dok adaptors; CD148 in platelets; LYP dissociation).\",\n      \"evidence\": \"PAG-knockout and double-knockout epistasis, conditional Csk/CD148 double-KO platelets, LYP chemical probe with imaging and co-IP\",\n      \"pmids\": [\"27926878\", \"29301754\", \"22426112\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative hierarchy among redundant CSK adaptors unresolved\", \"Feedback receptor-downregulation mechanism in platelets only partially defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended CSK's negative-regulatory role to additional scaffolds and physiological barriers — connexin43/PTEN ternary complexes, Thy-1/CBP integrin signaling, ZO tight-junction scaffolds, P2X3 receptor inhibition, and Adipsin-driven endothelial barrier protection.\",\n      \"evidence\": \"co-IP, domain pulldowns, RNAi and phospho-readouts across multiple cell systems; in vitro kinase assay and electrophysiology for P2X3\",\n      \"pmids\": [\"27391443\", \"27842221\", \"18086565\", \"19509283\", \"37237266\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Most of these scaffold/substrate links are single-lab\", \"Direct vs. SFK-mediated contributions not always separated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the distinct activation inputs (PAG/Cbp SH2 engagement, PKA-Ser364, Gbetagamma, scaffold recruitment) are integrated quantitatively in a living cell to set net CSK activity, and how CSK partner choice is selected among redundant adaptors, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model integrating spatial and enzymatic activation in vivo\", \"Determinants of adaptor selection among PAG/PTPN22/Dok/VE-cadherin/integrin scaffolds unknown\", \"Structure of full-length CSK engaged with a membrane adaptor on substrate unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 5, 24, 30, 33]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 6, 7, 45]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [6, 9, 45]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 14, 18]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 40]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [12, 14, 19, 37, 40]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [3, 4, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [12, 14, 27, 43, 26]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 16, 18, 40]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [19, 38]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [20, 41, 31]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [27]}\n    ],\n    \"complexes\": [\n      \"Thy-1/CBP/Csk complex\",\n      \"Cx43/PTEN/Csk/c-Src complex\"\n    ],\n    \"partners\": [\n      \"PAG/Cbp\",\n      \"LCK\",\n      \"PTPN22/LYP\",\n      \"VE-cadherin\",\n      \"paxillin\",\n      \"FAK\",\n      \"ITGA2B/integrin alphaIIbbeta3\",\n      \"PTP-HSCF\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}