ACTIVATION AND SERINE PHOSPHORYLATION OF THE P56(LCK) PROTEIN-TYROSINE KINASE IN RESPONSE TO ANTIGEN RECEPTOR CROSS-LINKING IN B-LYMPHOCYTES

We show that cross-linking the B cell AgR with anti-lg Abs activates p 56(lck)(Lck) in both the immature B cell line WEHI-231 and mature rest ing B cells from mouse spleen. Anti-lg-stimulated Lck activity peaked after 1 to 2 min, but remained elevated for at least 15 min. Consisten t with the proposed role for src family tyrosine kinases in AgR signal ing, we found that Lck could phosphorylate the cytoplasmic tails of th e Ig-cr and Ig-P components of the B cell AgR in vitro. Lck phosphoryl ated both of the tyrosines in the ig-P AgR homology motif and one of t he two tyrosines in the Ig-cr AgR homology motif. Finally, we show tha t AgR ligation in B cells caused a significant portion of the Lck to m igrate with an apparent molecular mass of 60 kDa on SDS-PAGE gels. Con version of p56(lck) to p6O(lck) was maximal at 5 to 15 min, at which t imes Lck activity in the cells was decreasing. This Lck ''band shift'' has been observed previously in activated T cells and correlates with phosphorylation of Lck at serine 59. We show that the 60-kDa form of Lck induced by AgR cross-linking in B cells is also phosphorylated at serine 59. Phosphorylation of Lck at this site in vitro decreases its activity. Thus, in B cells, AgR cross-linking activates Lck and subseq uently activates a kinase that phosphorylates Lck at serine 59, a pote ntial negative regulatory site.