InteractionofTcellreceptorwithspecificantigeninthecontextofMHCIIactivatesasignaltransductionpathwaythatleadstoTcellactivation.IntheTcellreceptorsignalingpathway,thesrcfamilykinasesLckandFynareactivatedtophosphorylateproteinsintheTcellreceptorcomplexwhichrecruitandactivatetheZAP70kinase.TheactivationofZAP70phosphorylatesdownstreamtargetsthatactivateMAPkinasepathwaysandcauseTcellactivation.TheCD45phosphorylasealsoplaysaroleinTcellreceptorsignaling,dephosphorylatedLckandFyntoactivatethem.OtherfactorsmodulateTcellreceptoractivation.Csk(COOH-terminalSrkkinase)phosphorylatesLckanddeactivatesit,opposingtheactionofCD45.ThephosphorylationofLckbyCskinhibitsTcellreceptorsignalingandinhibitsTcellactivation.CskactivityisregulatedinTcellsbyPKA,thecAMP-dependentproteinkinaseactivatedbythesecondmessengercAMP.TheactivityofCskalsoappearstodependonotherfactorssuchasCBP,whichrecruitsCsktotheplasmamembraneinlipidraftswhereothersignalingfactorssuchtheTcellreceptorcomplexarelocalized.CBPalsodirectlyactivatesCsk.

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