Ligation of the T cell antigen receptor induces tyrosine phosphorylation of p105CasL, a member of the p130Cas-related docking protein family, and its subsequent binding to the Src homology 2 domain of c-Crk.

p105CasL (CasL) is a recently identified signaling molecule closely related to the p130Cas (Crk-associated substrate) docking protein. CasL has a single Src homology (SH) 3 domain in its N-terminal portion followed by multiple consensus motifs for binding to SH2 domains. Like original p130Cas, CasL undergoes tyrosine phosphorylation upon integrin-mediated cell adhesion. In the present report, we provide direct evidence that CasL is also involved in T cell antigen receptor (TcR)-mediated signal transduction. In binding studies in vitro using glutathione-S-transferase fusion proteins, we have identified 105- and 120-kDa phosphotyrosyl proteins (pp105 and pp120, respectively) tightly bound to the SH2 domain of the Crk adapter protein in the H9 human T cell line after stimulation through the CD3/TcR complex. pp120, but not pp105, also bound to the SH3 of another adapter protein, Ash/Grb2. Immunoblotting with specific antibodies revealed that pp120 and pp105 were identical to the c-cbl proto-oncogene product (p120cbl) and CasL, respectively. Association between Crk and tyrosine-phosphorylated CasL after TcR stimulation was also confirmed in vivo. CasL phosphorylation induced by TcR ligation reached maximal levels within 2 min and rapidly declined thereafter, whereas the integrin-dependent response occurred slowly and was more prolonged. Finally, we demonstrated that Crk/CasL association occurred in peripheral blood T lymphocytes in response to TcR engagement. Our findings suggest that CasL is involved in T cell activation signals and resides at a point where two distinct receptor-mediated signaling pathways converge. This provides one mechanism by which integrins may mediate T cell co-stimulation.