The study of the molecular basis of cellular transformation by activated ras oncogenes in relation to the function of its proto- oncogenes has been the major emphasis of this project. The ras gene product, p21 (21,000-dalton protein), binds guanine nucleotides and exhibits quanosine triphosphate (GTPase) activity. The viral transforming and cellular ras proteins differ at positions 12 and 59. The point mutations at these positions affect the above activities. By analogy to other G-proteins the ras gene product, p21, may act as a regulatory component of the signal transduction. There have been reports on the involvement of active protein kinase C for the mitogenic activity of ras p21 and also ras protein increases the levels of diacylglycerol (DAG), a protein kinase C activator in some cells. Our laboratory earlier showed that the ras gene product was phosphorylated by phorbol esters in certain cell lines. We have further shown that ras p21 could be phosphorylated by protein kinase C and protein kinase A, both in vivo and in vitro, by peptide mapping, sequencing and oligonucleotide-directed mutagenesis, the phosphorylation site for protein kinases has been identified as 177 serine at the carboxyl terminus of p21. Our studies are continuing to understand the biological significance of this phosphorylation.
