B-CELL ANTIGEN RECEPTOR CROSS-LINKING TRIGGERS RAPID PROTEIN-KINASE-CINDEPENDENT ACTIVATION OF P21(RAS1)

Membrane Ig (mIg) functions in binding and internalization of Ag for s ubsequent processing and presentation to T cells, as well as in transm embrane transduction of signals that lead to cell activation, prolifer ation, and differentiation. Tyrosine kinase activation and subsequent phosphatidylinositol hydrolysis and Ca2+ mobilization are clearly impo rtant intermediary events in receptor-mediated B cell activation. Howe ver, many details of the cellular signal transduction pathways utilize d 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 tran sduction. p21ras has been implicated in some but not all protein tyros ine kinase/phospholipase C involving signaling pathways. To explore th e potential role of p21ras in B cell Ag receptor-mediated signaling, w e assessed the effect of Ag receptor ligation on the proportion of p21 ras in the active GTP-bound state. We present evidence that p21ras is activated by mIgM and mIgG cross-linking by anti-receptor antibodies a s well as by Ag. Depending upon the stimulus employed, this response i s detectable within 1 min and occurs with similar kinetics as inductiv e protein tyrosine phosphorylation and Ca2+ mobilization. Ag dose depe ndence of this response is similar to that of inductive protein tyrosi ne phosphorylation. In these cells p21ras is also activated by PMA sug gesting that p21ras activation after receptor cross-linking may be med iated by an effector molecule that functions downstream from protein k inase C (PKC). However, the kinetics of p21ras activation after mIg cr oss-linking are inconsistent with the possibility that PKC functions a s the sole mediator of p21ras activation in this system. Finally, unde r conditions in which the PKC inhibitor calphostin C blocks PMA-induce d p21ras activation, it does not inhibit Ag-induced p21ras activation. These data suggest that PKC effector mechanisms play a negligible rol e in p21ras activation during mIg-mediated signaling.