B cell antigen receptor cross-linking induces phosphorylation of the p21ras oncoprotein activators SHC and mSOS1 as well as assembly of complexes containing SHC, GRB-2, mSOS1, and a 145-kDa tyrosine-phosphorylated protein.

Ligation of the B cell AgR activates p21ras (Ras). We have investigated the effects of AgR ligation on three proteins that have been implicated as regulators of Ras: SHC, GRB-2, and mSOS1. We show that AgR cross-linking in B cells stimulated tyrosine and serine phosphorylation of SHC. This correlated with the formation of complexes containing SHC, GRB-2, mSOS1, and an unidentified 145-kDa tyrosine-phosphorylated protein. These complexes were present in the cytosol, as well as in the membrane fraction of the cells, where Ras is located. By using a GRB-2 fusion protein to probe blots, we showed that SHC was the major protein that GRB-2 bound to in anti-Ig-stimulated B cells. This argues that SHC couples GRB-2/mSOS1 to the 145-kDa protein and that SHC is likely to be essential for mSOS1 function in B cells. Finally, we found that AgR cross-linking stimulated phosphorylation of mSOS1 and that this could be blocked by an inhibitor of protein kinase C. Thus, signaling by the B cell AgR stimulates phosphorylation of SHC and mSOS1 and induces the formation of membrane-associated complexes containing SHC, GRB-2, mSOS1, and a 145-kDa protein. These events may be important for activation of Ras by the AgR.