ASSOCIATION OF P72(SYK) WITH THE SRC HOMOLOGY-G (SH2) DOMAINS OF PLC-GAMMA-1 IN B-LYMPHOCYTES

Phospholipase C gamma-catalyzed inositol phospholipid hydrolysis, a cr itical step in B cell antigen receptor signaling leading to second mes senger generation and proliferation, depends upon tyrosine kinase acti vation. The B cell antigen receptor-associated tyrosine kinases p53/ 5 6(lyn), p59(fyn), p55(blk), and p72(syk) are assumed to participate in receptor-initiated signaling. It is unknown, however, which of these kinases is involved in the tyrosine phosphorylation and resulting acti vation of phospholipase C gamma in response to antigen receptor cross- linking. We have used a fusion protein containing the tandem src homol ogy-2 (SH2) domains of phospholipase C gamma 1 (PLC gamma 1) to identi fy B cell kinases which associate with PLC gamma 1. Using an in vitro kinase assay, we demonstrate SH2-dependent association of tyrosine kin ase activity from anti-mu-stimulated B cells. The PLC gamma 1 SH2 doma ins associate with a prominent 70-72-kDa tyrosine phosphoprotein from anti-mu-stimulated, but not resting, B cells. Immunoblotting and secon dary immunoprecipitation studies definitively identify this protein as p72(syk). These results imply a physical interaction between PLC gamm a 1 and p72(syk) in antigen receptor-stimulated B cells. This conclusi on is confirmed by our ability to co-immunoprecipitate p72(syk) and PL C gamma 1 from lysates of anti-mu-stimulated B cells. These results im plicate p72(syk) in the activation of phospholipase C gamma 1 during B cell antigen receptor signaling.