Removal of C-Terminal Src Kinase from the Immune Synapse by a New Binding Protein

Abstract

The Csk tyrosine kinase negatively regulates the Src family kinases Lck and Fyn in T cells. Engagement of the T-cell antigen receptor results in a removal of Csk from the lipid raft-associated transmembrane protein PAG/Cbp. Instead, Csk becomes associated with an ∼72-kDa tyrosine-phosphorylated protein, which we identify here as G3BP, a phosphoprotein reported to bind the SH3 domain of Ras GTPase-activating protein. G3BP reduced the ability of Csk to phosphorylate Lck at Y505 by decreasing the amount of Csk in lipid rafts. As a consequence, G3BP augmented T-cell activation as measured by interleukin-2 gene activation. Conversely, elimination of endogenous G3BP by RNA interference increased Lck Y505 phosphorylation and reduced TCR signaling. In antigen-specific T cells, endogenous G3BP moved into a intracellular location adjacent to the immune synapse, but deeper inside the cell, upon antigen recognition. Csk colocalization with G3BP occurred in this “parasynaptic” location. We conclude that G3BP is a new player in T-cell-antigen receptor signaling and acts to reduce the amount of Csk in the immune synapse.

ACKNOWLEDGMENTS

This study was supported by fellowships from Rotary International, the Van Beirs Foundation, the Centre Anticancereux pres L'Universite de Liege, and the Spanish Ministry of Education and Culture and by grants CA81261 (S.P.S.) and AI53585, AI35603, AI55741, CA96949, and AI48032 (T.M.) from the National Institutes of Health. T.V. is supported by a postdoctoral fellowship from the Norwegian Cancer Society.