ACTIVATION OF THE LCK TYROSINE PROTEIN-KINASE BY HYDROGEN-PEROXIDE REQUIRES THE PHOSPHORYLATION OF TYR-394

Exposure of cells to H2O2 mimics many of the effects of treatment of cells with extracellular ligands. Among these is the stimulation of tyrosine phosphorylation. In this study, we show that exposure of cells to H2O2 increases the catalytic activity of the lymphocyte-specific tyrosine protein kinase p56(lck) (Lck) and induces tyrosine phosphorylation of Lck at Tyr-394, the autophosphorylation site. Using mutant forms df Lck, we found that Tyr-394 is required for H2O2-induced activation of Lck, suggesting that phosphorylation of this site may activate Lck In addition, H2O2 treatment induced phosphorylation at Tyr-394 in a catalytically inactive mutant of Lck in tells that do not express endogenous Lck. This demonstrates that a kinase other than Lck itself is capable of phosphorylating Lck at the so-called autophosphorylation site and raises the possibility that this as yet unidentified tyrosine protein kinase functions as an activator of Lck Such an activating enzyme could play an important role in signal transduction in T cells.