Mitotic Activation of c‐Src Is Suppressed by Csk

The kinase activity of the proto‐oncogene product, c‐Src, increases during mitosis through partial dephosphorylation of Tyr527, the negative regulatory site of c‐Src. To examine whether or not Csk, a candidate kinasc specific for Tyr527, is involved in this regulation, we developed a BalbJc 3T3 cell line overexpressing Csk and a Csk‐deficient cell line. The overexpression of wild‐type Csk caused significant suppression of the c‐Src activity during mitosis. A membrane‐targeted Csk, which has an ammo‐terminal myristylation signal of c‐Src, exhibited an effective suppression of the c‐Src activity, even though its expression level was lower than that of endogenous Csk. Concomitant with the suppression of the c‐Src activation, the level of tyrosine phosphorylation of a cortactin‐related protein, a potential substrate of c‐Src in vivo, was reduced. In contrast, the Csk‐deficient cells exhibited constitutive activation of c‐Src, which showed no significant change in its activity during mitosis. These results suggest that Csk indeed participates in the regulation of the c‐Src activity during mitosis.