Evidence of a novel redox-linked activation mechanism for the Src kinase which is independent of tyrosine 527-mediated regulation.

The kinase activity of p60c-src has been shown to be basically regulated through phosphorylation and dephosphorylation of Y527. We found that catalytic activity of the immunoprecipitated c-Src kinase from NIH3T3 cells was elevated several folds by exposure to 0.5-50 microM of sulfhydryl-reactive Hg2+. Vmax of the kinase was increased whereas Km was decreased. N-acetylcysteine neutralized this Hg2+ effect, suggesting a critical role of the Hg2+-mediated sulfhydryl modification of the kinase in the mechanism. Addition of protein tyrosine phosphatase inhibitor Na3VO4 into the reaction mixture did not inhibit the Hg2+-mediated activation. Further study revealed that Hg2+ was capable of activating the v-Src kinase lacking Y527 and the c-Src kinase from mutant cells defective of the Y527-phosphorylating Csk kinase. Cyanogen bromide cleavage maps of radiolabeled Src proteins showed that Hg2+ selectively promoted the autophosphorylation at Y416 and that the previously in vivo radiolabeled phosphorous on Y527 was not deleted during the promotion of Y416 autophosphorylation by Hg2+. Phosphoamino acid analysis demonstrated selective promotion of phosphorylation at tyrosine but not at serine/threonine. Not like bivalent Hg2+, monovalent p-chloromercuribenzenesulfonic acid was incapable of activating c-Src kinase. These results suggest a novel Y416 phosphorylation-linked activation pathway for Src kinases which is initially triggered independent of Y527-mediated or serine/threonine phosphorylation-linked regulation, possibly through sulfhydryl-based protein structural modification for functional alteration.