Calcium induces tyrosine phosphorylation of a novel p120GAP-associated protein of 65 kDa.

Several tyrosine-phosphorylated proteins have been identified that associate with p120GAP, the GTPase activating protein of p21ras. In keratinocytes, calcium induced the tyrosine phosphorylation of a 65 kDa p120GAP-associated protein (p65Ca). This protein did not comigrate with two previously reported p120GAP-associated proteins, i.e. a 68 kDa protein 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 antibodies against p68. In addition, a monoclonal antibody directed to p60(2C4) did not recognize p65Ca. From these results we conclude that p65Ca is different from p68 and p60(2C4) and thus, a novel p120GAP-associated protein. Since calcium has an important, tyrosine kinase dependent, role in the differentiation of keratinocytes, phosphorylation of p65Ca may be important for this differentiation process. However, surprisingly, calcium induced the phosphorylation of a similar-sized p120GAP-associated 65 kDa protein in fibroblast cell lines.