CALCIUM INDUCES TYROSINE PHOSPHORYLATION OF A NOVEL P120GAP-ASSOCIATED PROTEIN OF 65 KDA

Several tyrosine-phosphorylated proteins have been identified that ass ociate with p120GAP, the GTPase activating protein of p21ras. In kerat inocytes, calcium induced the tyrosine phosphorylation of a 65 kDa p12 0GAP-associated protein (p65Ca). This protein did not comigrate with t wo previously reported p120GAP-associated proteins, i.e. a 68 kDa prot ein from src-transformed cells (p68) and an insulin-induced protein of 60 kDa (p60(2C4)). P65Ca was neither recognized by poly(U)-sepharose, which efficiently precipitates p68, nor did it crossreact with antibo dies against p68. In addition, a monoclonal antibody directed to p60(2 C4) did not recognize p65Ca. From these results we conclude that p65Ca is different from p68 and p60(2C4) and thus, a novel p120GAP-associat ed protein. Since calcium has an important, tyrosine kinase dependent, role in the differentiation of keratinocytes, phosphorylation of p65C a may be important for this differentiation process. However, surprisi ngly, calcium induced the phosphorylation of a similar-sized p120GAP-a ssociated 65 kDa protein in fibroblast cell lines.