HUMAN SPLEEN TYROSINE KINASE P72SYK ASSOCIATES WITH THE SRC-FAMILY KINASE P53 56LYN AND A 120-KDA PHOSPHOPROTEIN/

The 72-kDa spleen tyrosine kinase (Syk) and Src-family kinase p53/56Ly n (Lyn) contribute to signaling via the B-cell antigen receptor comple x. Here we show that Syk and Lyn from human B lymphocytes can interact directly. Syk and Lyn coimmunoprecipitated from mature and activated B-cell lines, and gel-purified Syk and Lyn reassociated in vitro, demo nstrating their direct interaction. This Syk-Lyn interaction may be de pendent on the stage of B-cell differentiation, since Syk-Lyn associat ions were not detected in pre-B and myeloma cell lines and Syk from an immature B-cell line did not reassociate with Lyn in vitro. Serine/th reonine kinase activity was also associated with Syk. Crosslinking of cell surface IgM led to rapid activation of both tyrosine and serine/t hreonine protein kinase activities that resulted in phosphorylation in vitro of proteins coprecipitating with Syk-in particular, a serine/th reonine phosphorylated protein 120 kDa in size (pp120). Several phosph oproteins, including one of 72 kDa and one of 120 kDa, coprecipitated with phospholipase C-gamma 1 (PLC gamma 1). Sequential immunoprecipita tion identified the 72-kDa protein associated with PLC gamma 1 as Syk The 120-kDa serine/threonine phosphorylated protein that coprecipitate d with PLC gamma 1 resembled the Syk-associated pp120 by several crite ria. Thus, pp120 may serve as a link between Syk and PLC gamma 1, coup ling the B-cell antigen receptor to the phosphatidylinositol pathway.