Identification of major tyrosine-phosphorylated proteins in Csk-deficient cells.

Csk is a non-receptor protein-tyrosine kinase that acts as a negative regulator of Src family tyrosine kinases. Csk-deficient mouse embryos exhibited developmental defects including inability to turn and impaired formation of neural tube. In these embryos, an accumulation of tyrosine phosphorylated proteins was observed as a consequence of constitutive activation of Src family kinases. In order to identify those tyrosine phosphorylated proteins, we established a Csk-deficient cell line from embryos lacking both Csk and the anti-oncogene product p53. On surveying several proteins known as Src substrates, we found that phosphorylation level of p80/85 (cortactin) was markedly elevated in the Csk-deficient cells. Enhancement of cortactin phosphorylation was also seen in Csk-deficient embryos. Furthermore, immunoprecipitated Src was able to directly phosphorylate cortactin in vitro. Thus, we suggest that cortactin is a good substrate of activated Src family kinases in vivo and may play important roles in signaling pathways mediated by Src family kinases.