THE ACTIVATED FORM OF THE LCK TYROSINE PROTEIN-KINASE IN CELLS EXPOSED TO HYDROGEN-PEROXIDE IS PHOSPHORYLATED AT BOTH TYR-394 AND TYR-505

Members of the Src family of non-receptor tyrosine protein kinases are known to be inhibited by the intramolecular association between a pho sphorylated carboxyl-terminal tyrosine residue and the SH2 domain. We have previously shown that exposure of cells to H2O2 strongly activate s Lck, a lymphocyte-specific Src family kinase, by inducing phosphoryl ation on Tyr-394, an absolutely conserved residue within the activatio n loop of the catalytic domain. Here we show that Lck that has been ac tivated by H2O2 is simultaneously phosphorylated at both the carboxyl- terminal tyrosine (Tyr-505) and Tyr-394. Thus, dephosphorylation of Ty r-505 is not a prerequisite for either phosphorylation of Lck at Tyr-3 94 or catalytic activation of the kinase. These results indicate that activation of Lck by phosphorylation of Tyr-394 is dominant over any i nhibition induced by phosphorylation of Tyr-505. We propose that these results may be extended to all Src family members.