T-cell antigen receptor ligation induces tyrosine phosphorylation of phospholipase C-gamma 1.

Ligand-mediated perturbation of the T-cell antigen receptor (TCR) triggers a rapid increase in phosphoinositide-specific phospholipase C (PLC) activity in resting T-cells. Although the mechanism by which TCR ligation regulates PLC activity is unknown, recent studies suggest that coupling of this receptor complex to PLC activity is dependent on an intermediate protein tyrosine phosphorylation event(s). In the present study, we demonstrate that antibody-mediated TCR cross-linkage results in the tyrosine phosphorylation of PLC-gamma 1. Stimulation of the TCR for 30 s induced a 4-5-fold increase in the level of PLC activity recovered in anti-phosphotyrosine (Tyr(P)) antibody immunoprecipitates from stimulated Jurkat cells. The appearance of PLC activity in the immunoprecipitates preceded the onset of phosphoinositide hydrolysis in vivo, which began 30-60 s after TCR ligation. Furthermore, the TCR-mediated increase in anti-Tyr(P) antibody-bound PLC activity was inhibited by staurosporine at drug concentrations identical with those required for in vivo inhibition of TCR-dependent phosphoinositide breakdown. Immunoblot analyses demonstrated that TCR ligation dramatically increased the level of tyrosine-phosphorylated PLC-gamma 1 present in anti-Tyr(P) antibody immunoprecipitates from stimulated Jurkat cells. These results strongly suggest that the TCR complex expressed by Jurkat cells is functionally coupled to the phosphoinositide-dependent signaling pathway through the tyrosine phosphorylation of PLC-gamma 1.