SH3 DOMAINS OF THE ADAPTER MOLECULE GRB2 COMPLEX WITH 2 PROTEINS IN T-CELLS - THE GUANINE-NUCLEOTIDE EXCHANGE PROTEIN SOS AND A 75-KDA PROTEIN THAT IS A SUBSTRATE FOR T-CELL ANTIGEN RECEPTOR-ACTIVATED TYROSINEKINASES

In T lymphocytes activated via the T cell antigen receptor (TCR), the SH2- and SH3-containing adapter molecule Grb2 forms a complex with the Ras guanine nucleotide exchange protein Sos and tyrosine phosphoprote ins. The interaction of Sos with Grb2 is mediated via the Grb2 SH3 dom ains. In this study, it is shown that a 75-kDa protein is also complex ed with the Grb2 SH3 domains in T cells, but not in Rat-1 fibroblasts. The identity of the p75 protein is not known, but immunoblot analysis with phosphotyrosine antibodies indicated that it is rapidly tyrosine -phosphorylated in TCR-activated T cells. This characteristic clearly distinguishes p75 from Sos since Sos is not a phosphotyrosine protein. In vitro binding studies indicated that the p75 phosphotyrosine prote in binds to a glutathione S-transferase fusion protein of intact Grb2, but not to a Grb2 fusion protein mutated in its SH3 domains. p75 can also bind to the single COOH-terminal Grb2 SH3 domain, whereas Sos has an in vitro binding preference for the NH2-terminal Grb2 SH3 domain. Collectively, these data indicate that in T cells, two proteins can co mplex with the Grb2 SH3 domains: Sos and a p75 molecule that is tyrosi ne-phosphorylated in TCR-activated cells. The significance of p75 asso ciation with Grb2 is not clear, but by analogy with Sos, p75 is a pote ntial candidate for a Grb2 effector protein. Data are presented showin g that the interaction of the Grb2 SH2 domains with tyrosine phosphopr oteins may be regulated by conformational restraints imposed by differ ent molecules complexing with the Grb2 SH3 domains. It is thus possibl e to speculate that the interaction of either p75 or Sos with the Grb2 SH3 domain may influence the interaction of the Grb2 SH2 domain with tyrosine phosphoproteins.