(1) Phospholipase C-gamma (PLC-gamma1), an isozyme of the phosphoinositide-specific phospholipase C family, which occupies a central role in hormonal signal transduction pathways, is an excellent substrate for the platelet-derived growth factor (PDGF) receptor tyrosine kinase. The role of tyrosine phosphorylation was investigated by substituting phenylalanine for tyrosine at PLC-gamma1, phosphorylation sites 771, 783, and 1254 and expressing the mutant enzymes in NIH 3T3 cells. Phenylalanine substitution at Tyr-783 completely blocked the activation of PLC by PDGF, whereas mutation at Tyr-1254 inhibited by 50% and mutation at Tyr-771 enhanced the response. These results provided direct evidence that growth factor receptors activated the function of intracellular protein by tyrosine phosphorylation. (2) The human T-cell line Jurkat was found to contain at least two immunologically distinct isoforms of PLC, PLC-beta1 and PLC-gamma1. Treatment of Jurkat cells with antibody to CD3 led to rapid and transient phosphorylation of PLC-gamma on both serine and tyrosine residues. Two- dimensional phosphopeptide map analysis revealed that the major sites of tyrosine and serine phosphorylation in PLC-gamma1 from activated T-cells are the same as those in PLC-gamma1 from PDGF-treated cells. This result suggests that activation of PLC-gamma1 is achieved by a nonreceptor tyrosine kinase coupled to the T-cell antigen receptors. (3) A 42 kDa G protein (a member of Gq class) had been purified by Exton's group on the basis of it capacity to activate PLC. Reconstitution of the 42 kDa Gq with PLC-beta1, but not PLC-gamma1 and PLC-delta1, markedly stimulated phosphatidyl 4,5-bisphosphate hydrolysis. This represents the first identification of PLC isoform which can be activated by a pertussis toxin-insensitive G protein.
