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The surface immunoglobulin M (sIgM)‐associated src family protein tyrosine kinases (PTKs) p55blk, p59fyn, and p53/56lynbecome activated in B cells within seconds following sIgM cross‐linking. Studies using protein tyrosine kinase (PTK) inhibitors have demonstrated that PTK activity is crucial for downstream events such as calcium flux, inositol phospholipid hydrolysis, and cell cycle entry. The roles that the individual src family PTKs play in sIgM signaling are largely unknown, however. In order to determine whether p59fynplays a distinct role in sIgM signal transduction, the signaling capabilities of B cells isolated from fyn “knockout” mice were evaluated. We observed that in the absence of p59fyn, there was no demonstrable compromise of the slgM‐coupled signaling events measured (tyrosine phosphorylation, inositol phospholipid hydrolysis, and Ca2+flux). We propose that either p59fynis not involved in coupling sIgM to these specific signaling pathways or that other PTKs are able to compensate for the absence of p59fyn, indicating redundancy in the sIgM signaling pathways. J. Leukoc. Biol.56: 812–816; 1994.

Phospholipase Cγ-catalyzed inositol phospholipid hydrolysis, a critical step in B cell antigen receptor signaling leading to second messenger generation and proliferation, depends upon tyrosine kinase activation. The B cell antigen receptor-associated tyrosine kinases p53/56lyn, p59fyn, p55blk, and p72sykare assumed to participate in receptor-initiated signaling. It is unknown, however, which of these kinases is involved in the tyrosine phosphorylation and resulting activation of phospholipase Cγin response to antigen receptor cross-linking. We have used a fusion protein containing the tandem src homology-2 (SH2) domains of phospholipase Cγ1 (PLCγ1) to identify B cell kinases which associate with PLCγ1. Using an in vitrokinase assay, we demonstrate SH2-dependent association of tyrosine kinase activity from anti-μ-stimulated B cells. The PLCγ1 SH2 domains associate with a prominent 70-72-kDa tyrosine phosphoprotein from anti-μ-stimulated, but not resting, B cells. Immunoblotting and secondary immunoprecipitation studies definitively identify this protein as p72syk. These results imply a physical interaction between PLCγ1 and p72sykin antigen receptor-stimulated B cells. This conclusion is confirmed by our ability to co-immunoprecipitate p72sykand PLCγ1 from lysates of anti-μ-stimulated B cells. These results implicate p72sykin the activation of phospholipase Cγ1 during B cell antigen receptor signaling.