TYROSINE DEPHOSPHORYLATION OF PP60(C-SRC) IS STIMULATED BY A SERINE THREONINE PHOSPHATASE INHIBITOR/

Incubation of NIH3T3-derived c-src overexpressor cells with okadaic ac id, a specific serine/threonine phosphatase inhibitor, stimulates pp60 (c-src) kinase activity about 2-3-fold. Activation is blocked if cells are simultaneously treated with orthovanadate, a tyrosine phosphatase inhibitor. Furthermore, okadaic acid treatment induces a small decrea se in Tyr 527 phosphorylation of wild-type pp60(c-src) and a large dec rease in Tyr 527 phosphorylation of kinase-defective pp60(c-src)(lys 2 95--> Arg). These results suggest that the activation is mediated by o kadaic acid-induced changes in tyrosine phosphorylation of pp60(c-src) involving 'cross-over' from serine/threonine to tyrosine signal trans duction pathways. Stimulation of pp60(c-src) activity and Tyr 527 deph osphorylation do not require changes in serine/threonine phosphorylati on of pp60(c-src), suggesting that these changes result from modulatio n of an upstream Tyr 527 phosphatase or kinase which is itself regulat ed by altered serine/threonine phosphorylation. Since okadaic acid ind uces a pseudo-mitotic phenotype in rodent cells (K. Yamashita, H. Yasu da, J. Pines, K. Yasumoto, H. Nishitani, M. Ohtsubo, T. Hunter, T. Sug imura and T. Nishimoto, EMBO J., 9: 4331-4338, 1990), it is possible t hat these phenomena are induced by a biochemical mechanism similar to that which causes transient tyrosine dephosphorylation of pp60(c-src) during mitosis.