EVIDENCE FOR A PREFORMED TRANSDUCER COMPLEX ORGANIZED BY THE B-CELL ANTIGEN RECEPTOR

The B cell antigen receptor (BCR) consists of the membrane-bound immun oglobulin (mIg) molecule and the Ig-alpha/Ig-beta heterodimer, which f unctions as signaling subunit of the receptor. Stimulation of the BCR activates protein tyrosine kinases (PTKs) that phosphorylate a number of substrate proteins, including the Ig-alpha/Ig-beta heterodimer of t he BCR itself. How the PTKs become activated after BCR engagement is n ot known at present. Here, we show that BCR-negative J558L cells treat ed with the protein tyrosine phosphatase inhibitor pervanadate/H2O2 di splay only a weak substrate phosphorylation. However, in BCR-positive transfectants of J558L, treatment with pervanadate/H2O2 induces a stro ng phosphorylation of several substrate proteins. Treatment with perva nadate/H2O2 does not result in receptor crosslinking, yet the pattern of protein phosphorylation is similar to that observed after BCR stimu lation by antigen. The response requires cellular integrity because ty rosine phosphorylation of most substrates is not visible in cell lysat es. Cells that express a BCR containing an Ig-alpha subunit with a mut ated immunoreceptor tyrosine-based activation motif display a delayed response. The data suggest that, once expressed on the surface, the BC R organizes protein tyrosine phosphatases, PTKs, and their substrates into a transducer complex that can be activated by pervanadate/H2O2 in the absence of BCR crosslinking. Assembly of this preformed complex s eems to be a prerequisite for BCR-mediated signal transduction.