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Interaction of T cell receptor with specific antigen in the context of MHC II activates a signal transduction pathway that leads to T cell activation. In the T cell receptor signaling pathway, the src family kinases Lck and Fyn are activated to phosphorylate proteins in the T cell receptor complex which recruit and activate the ZAP70 kinase. The activation of ZAP70 phosphorylates downstream targets that activate MAP kinase pathways and cause T cell activation. The CD45 phosphorylase also plays a role in T cell receptor signaling, dephosphorylated Lck and Fyn to activate them. Other factors modulate T cell receptor activation. Csk (COOH-terminal Src kinase) phosphorylates Lck and deactivates it, opposing the action of CD45. The phosphorylation of Lck by Csk inhibits T cell receptor signaling and inhibits T cell activation. Csk activity is regulated in T cells by PKA, the cAMP-dependent protein kinase activated by the second messenger cAMP. The activity of Csk also appears to depend on other factors such as CBP, which recruits Csk to the plasma membrane in lipid rafts where other signaling factors such the T cell receptor complex are localized. CBP also directly activates Csk. (This definition may be outdated - see the DesignNote.)
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