Interactions of Cbl with two adapter proteins, Grb2 and Crk, upon T cell activation.

Several recent studies have demonstrated that Grb2, composed entirely of SH2 and SH3 domains, serves as an adaptor protein in tyrosine kinase signaling pathways. Cb1, the protein product of c-cbl proto-oncogene, has been reported to be phosphorylated on tyrosine residues upon T cell receptor (TCR) engagement. Here we show that in unstimulated Jurkat cells Cbl is co-immunoprecipitated with monoclonal antibody against Grb2. However, in lymphocytes activated through the TCR, Cbl loses its ability to bind to Grb2 precipitated either with anti-Grb2 antibody or with an immobilized tyrosine phosphopeptide, Y1068-P, derived from the epidermal growth factor receptor. In vitro studies confirm that the ability of Cb1 to bind to both SH3 domains of Grb2 is strongly reduced in activated T lymphocytes. Investigation of the time course of Cbl dissociation from Grb2 reveals that it is transient and correlates with the kinetics of tyrosine phosphorylation of Cbl. Moreover, Cb1 is co-immunoprecipitated with Crk, another SH2/SH3 domain-containing protein, upon TCR stimulation. Tyrosine-phosphorylated Cbl binds exclusively to the SH2 domain of Crk. These results suggest that different adaptor proteins may have different roles in the regulation of c-cbl proto-oncogene product.