Membrane Ig (mIg) functions in binding and internalization of Ag for subsequent processing and presentation to T cells, as well as in transmembrane transduction of signals that lead to cell activation, proliferation, and differentiation. Tyrosine kinase activation and subsequent phosphatidylinositol hydrolysis and Ca2+ mobilization are clearly important intermediary events in receptor-mediated B cell activation. However, many details of the cellular signal transduction pathways utilized by this receptor are not resolved. Recent studies that demonstrated co-capping of mIg and the proto-oncoprotein p21ras suggested that this low m.w. GTP-binding protein may function in mIg-mediated signal transduction. p21ras has been implicated in some but not all protein tyrosine kinase/phospholipase C involving signaling pathways. To explore the potential role of p21ras in B cell Ag receptor-mediated signaling, we assessed the effect of Ag receptor ligation on the proportion of p21ras in the active GTP-bound state. We present evidence that p21ras is activated by mIgM and mIgG cross-linking by anti-receptor antibodies as well as by Ag. Depending upon the stimulus employed, this response is detectable within 1 min and occurs with similar kinetics as inductive protein tyrosine phosphorylation and Ca2+ mobilization. Ag dose dependence of this response is similar to that of inductive protein tyrosine phosphorylation. In these cells p21ras is also activated by PMA suggesting that p21ras activation after receptor cross-linking may be mediated by an effector molecule that functions downstream from protein kinase C (PKC). However, the kinetics of p21ras activation after mIg cross-linking are inconsistent with the possibility that PKC functions as the sole mediator of p21ras activation in this system. Finally, under conditions in which the PKC inhibitor calphostin C blocks PMA-induced p21ras activation, it does not inhibit Ag-induced p21ras activation. These data suggest that PKC effector mechanisms play a negligible role in p21ras activation during mIg-mediated signaling.

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