| Crosslinking of cell surface IgM led to rapid activation of both tyrosine and serine / threonine protein kinase activities that resulted in phosphorylation in vitro of proteins coprecipitating with Syk in particular , a serine / threonine phosphorylated protein 120 kDa in size ( pp 120 ) . ^^^ The 120 kDa serine / threonine phosphorylated protein that coprecipitated with PLC gamma 1 resembled the Syk associated pp 120 by several criteria . ^^^ Thus , pp 120 may serve as a link between Syk and PLC gamma 1 , coupling the B cell antigen receptor to the phosphatidylinositol pathway . . ^^^ |