B-CELL ANTIGEN RECEPTOR CROSS-LINKING INDUCES PHOSPHORYLATION OF THE P21(RAS) 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 p21(ras) (Ras). We have investiga ted the effects of AgR ligation on three proteins that have been impli cated as regulators of Ras: SHC, GRB-2, and mSOS1. We show that AgR cr oss-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 pr otein. 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 prote in that GRB-2 bound to in anti-lg-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 c ould be blocked by an inhibitor of protein kinase C. Thus, signaling b y the B cell AgR stimulates phosphorylation of SHC and mSOS1 and induc es the formation of membrane-associated complexes containing SHC, GRB- 2, mSOS1, and a 145-kDa protein. These events may be important for act ivation of Ras by the AgR.