SHC CONTAINS 2 GRB2 BINDING-SITES NEEDED FOR EFFICIENT FORMATION OF COMPLEXES WITH SOS IN B-LYMPHOCYTES

Cross-linking of the B-cell antigen receptor (BCR) induces tyrosine ph osphorylation of She, which is believed to lead to the activation of P as, Previous work has shown that tyrosine-phosphorylated She forms com plexes with another adapter protein, Grb2, and the Pas guanine nucleot ide exchange factor SOS. Here, we demonstrate that phosphorylation of She by the hematopoietic cell-specific tyrosine kinase Syk induces bin ding of Grb2 to She, suggesting that Syk phosphorylates She in stimula ted B cells, Surprisingly, Syk-phosphorylated She possesses two Grb2 b inding sites rather than the one site that has been previously reporte d, Both of these sites are required for efficient formation of Shc-Grb 2-SOS complexes in vitro and in vivo. We suggest that two Grb2 protein s anchored by a single She protein bind simultaneously to one SOS mole cule, resulting in a complex that is more stable than a complex contai ning only a single Grb2 protein bound to one SOS molecule. This model is consistent with our observation that BCR stimulation greatly increa ses the amount of SOS associated with Grb2.