CD3 STIMULATION CAUSES PHOSPHORYLATION OF PHOSPHOLIPASE C-GAMMA-1 ON SERINE AND TYROSINE RESIDUES IN A HUMAN T-CELL LINE

The human T-cell line Jurkat was found to contain at least two immunologically distinct isoforms of inositol phospholipid-specific phospholipase C (PLC), PLC-beta-1 and PLC-gamma-1. Treatment of Jurkat cells with antibody to CD3 led to phosphorylation of PLC-gamma-1 but not of PLC-beta-1. The phosphorylation of PLC-gamma-1 occurred rapidly and transiently on both serine and tyrosine residues; tyrosine phosphorylation reached a maximum level less than 1 min after stimulation and decreased rapidly, both in the presence and in the absence of orthovanadate. Two-dimensional phosphopeptide map analysis revealed that the major sites of tyrosine and serine phosphorylation in PLC-gamma-1 from activated Jurkat cells are the same as those in PLC-gamma-1 from cells treated with peptide growth factors such as epidermal growth factor and platelet-derived growth factor. Previously, it has been shown that multiple phosphorylation of PLC-gamma-1 by the growth factor receptor tyrosine kinases leads to activation of PLC-gamma-1. Thus, the current data suggest that inositol phospholipid hydrolysis triggered by the T-cell antigen receptor-CD3 complex is due, at least in part, to activation of PLC-gamma-1 and that the mechanism by which this activation is achieved involves phosphorylation of multiple tyrosine residues on PLC-gamma-1 by a nonreceptor tyrosine kinase coupled to the T-cell antigen receptor-CD3 complex.