| Instead , Csk becomes associated with an approximately 72 kDa tyrosine phosphorylated protein , which we identify here as G3BP , a phosphoprotein reported to bind the SH 3 domain of Ras GTPase activating protein . ^^^ G3BP reduced the ability of Csk to phosphorylate Lck at Y 505 by decreasing the amount of Csk in lipid rafts . ^^^ 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 . . ^^^ |