Activation of the COOH-terminal Src kinase (Csk) by cAMP-dependent proteinkinase inhibits signaling through the T cell receptor

In T cells, cAMP-dependent protein kinase (PKA) type I colocalizes with the T cell receptor-CD3 complex (TCR/CD3) and inhibits T cell function via a p reviously unknown proximal target. Here we examine the mechanism for this P KA-mediated immunomodulation. cAMP treatment of Jurkat and normal T cells r educes Lck-mediated tyrosine phosphorylation of the TCR/CD3 zeta chain afte r T cell activation, and decreases Lck activity. Phosphorylation of residue Y505 in Lck by COOH-terminal Src kinase (Csk), which negatively regulates Lck, is essential for the inhibitory effect of cAMP on zeta chain phosphory lation. PKA phosphorylates Csk at S364 in vitro and in vivo leading to a tw o- to fourfold increase in Csk activity that is necessary for cAMP-mediated inhibition of TCR-induced interleukin 2 secretion. Both PKA type I and Csk are targeted to lipid rafts where proximal T cell activation occurs, and p hosphorylation of raft-associated Lck by Csk is increased in cells treated with forskolin. We propose a mechanism whereby PKA through activation of Cs k intersects signaling by Src kinases and inhibits T cell activation.