Abstract

Protooncogenic protein c-Cbl undergoes tyrosine phosphorylation in response to stimulation through the receptors for antigens, immunoglobulins, cytokines, and growth factors as well as through the integrins. Tyrosine phosphorylation of c-Cbl may play a functional role in signal transduction, since c-Cbl interacts with many crucial signaling molecules including protein-tyrosine kinases, adaptor proteins, and phosphatidylinositol 3'-kinase. Therefore, it is essential for our understanding of the functions of c-Cbl in signal transduction to identify its tyrosine phosphorylation sites, to determine the protein-tyrosine kinases that phosphorylate these sites, and to elucidate the role of these sites in the interactions of c-Cbl with other signaling proteins. In this report, we demonstrate that tyrosines 700, 731, and 774 are the major tyrosine phosphorylation sites of c-Cbl in T cells in response to pervanadate treatment, as well as in response to TcR/CD3 ligation. Coexpression experiments in COS cells demonstrate that among T cell-expressed Src- and Syk-related protein-tyrosine kinases, Fyn, Yes, and Syk appear to play a major role in phosphorylation of c-Cbl, whereas Lck and Zap phosphorylate c-Cbl ineffectively. Fyn, Yes, and Syk phosphorylate the same sites of c-Cbl that become phosphorylated in stimulated T cells. Among these kinases, Fyn and Yes demonstrate strong binding to c-Cbl, which involves both phosphotyrosine-dependent and phosphotyrosine-independent mechanisms.