MITOTIC ACTIVATION OF C-SRC IS SUPPRESSED BY CSK

The kinase activity of the proto-oncogene product, c-Src, increases du ring mitosis through partial dephosphorylation of Tyr527, the negative regulatory site of c-Src. To examine whether or not Csk, a candidate kinase specific for Tyr527, is involved in this regulation, we develop ed a Balb/c 3T3 cell line overexpressing Csk and a Csk-deficient cell line. The overexpression of wild-type Csk caused significant suppressi on of the c-Src activity during mitosis. A membrane-targeted Csk, whic h has an amino-terminal myristylation signal of c-Src, exhibited an ef fective suppression of the c-Src activity, even though its expression level was lower than that of endogenous Csk. Concomitant with the supp ression of the c-Src activation, the level of tyrosine phosphorylation of a cortactin-related protein, a potential substrate of c-Src in viv o, was reduced. In contrast, the Csk-deficient cells exhibited constit utive activation of c-Src, which showed no significant change in its a ctivity during mitosis. These results suggest that Csk indeed particip ates in the regulation of the c-Src activity during mitosis.