STIMULATION THROUGH THE T-CELL RECEPTOR INDUCES CBL ASSOCIATION WITH CRK PROTEINS AND THE GUANINE-NUCLEOTIDE EXCHANGE PROTEIN C3G

We and others have recently identified Cbl, the protein product of the c-cbl protooncogene, as an early tyrosine kinase substrate upon T cel l activation and have shown that Cbl forms in vivo complexes with Src family tyrosine kinases, Grb2 adaptor protein, and the p85 subunit of PI-3 kinase. Here we show that Cbl associates with all three forms of the human Crk protein, predominantly CrkL, following T cell receptor a ctivation of Jurkat T cells. Association between Cbl and Crk proteins was confirmed in normal human peripheral blood-derived T cells. In vit ro, Cbl was able to interact with the Crk SH2 domain but not the SH3 d omain. A phosphopeptide corresponding to a potential Crk SH2 domain-bi nding motif in Cbl (pYDVP) specifically inhibited binding between Cbl and Crk SH2 domain. Anti-Cbl antibody completely immunodepleted the Cr kL-associated 120-kDa phosphotyrosyl polypeptide, suggesting that the recently described p130(cas)-related Crk-associated p116 of T cells ma y be Cbl. Consistent with this possibility, the 4F4 antibody used to c haracterize the p116 polypeptide cross-reacted with Cbl protein when i t was resolved on one- or two-dimensional gels. CrkL was constitutivel y associated with a substantial amount of the guanine nucleotide excha nge protein C3G, and a fraction of the C3G protein was coimmunoprecipi tated with Cbl in activated Jurkat T cells. These results suggest the possibility that Cbl may participate in a signaling pathway that regul ates guanine nucleotide exchange on small G-proteins in T cells.