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 impair ed formation of neural tube. In these embryos, an accumulation of tyro sine phosphorylated proteins was observed as a consequence of constitu tive activation of Src family kinases. In order to identify those tyro sine phosphorylated proteins, we established a Csk-deficient cell line from embryos lacking both Csk and the anti-oncogene product p53. On s urveying several proteins known as Src substrates, we found that phosp horylation level of p80/85 (cortactin) was markedly elevated in the Cs k-deficient cells. Enhancement of cortactin phosphorylation was also s een in Csk-deficient embryos. Furthermore, immunoprecipitated Src was able to directly phosphorylate cortactin in vitro. Thus, we suggest th at cortactin is a good substrate of activated Src family kinases ill v ivo and may play important roles in signaling pathways mediated by Src family kinases.