Co-ligation of the antigen and Fc receptors gives rise to the selective modulation of intracellular signaling in B cells. Regulation of the association of phosphatidylinositol 3-kinase and inositol 5'-phosphatase with the antigen receptor complex

Cross-linking of the Fc receptor (FcR) to surface immunoglobulin (sIg) on B cells inhibits the influx of extracellular calcium and abrogates the proliferative signal. The mechanism by which this occurs is not well understood. In this report we show that co-cross-linking the FcR to the antigen receptor gives rise to very selective modulation of signal transduction in B cells. Co-cross-linking sIg and the FcR enhanced the phosphorylation of the FcR, the adapter protein, Shc, and the inositol 5'-phosphatase Ship. Furthermore, phosphorylation of the FcR induced its association with Ship. Cross-linking of the FcR and sIg decreased the tyrosine phosphorylation of CD19, which led to a reduction in the association of phosphatidylinositol 3-kinase. In addition, the phosphorylation of several other proteins of 73, 39, and 34 kDa was reduced. Activation of the cells with either F(ab')2 or intact anti-IgG induced very similar changes in levels of tyrosine phosphorylation of most other proteins, and no differences in the activation of several protein kinases were observed. These results indicate that the inhibitory signal that is transmitted through the FcR is not mediated by a global shutdown of tyrosine phosphorylation but is, rather, a selective mechanism involving localized changes in the interactions of adapter proteins and the enzymes Ship and phosphatidylinositol 3-kinase with the antigen receptor complex.