EPIDERMAL GROWTH-FACTOR REGULATES THE EXCHANGE-RATE OF GUANINE-NUCLEOTIDES ON P21RAS IN FIBROBLASTS

Treatment of intact Rat-I fibroblasts with epidermal growth factor (EGF) leads to rapid activation of cellular ras-encoded proteins. By using the bacterial toxin streptolysin 0 to permeabilize these cells, it was shown that the low basal rate at which guanine nucleotides bind to, and dissociate from, ras-encoded protein in quiescent fibroblasts was greatly accelerated by EGF treatment. Nucleotide binding to other proteins was not affected. Stimulation of nucleotide exchange op ras-encoded protein required tyrosine kinase but not phospholipase activity. EGF had no effect on total GTPase-activating protein activity. Regulation of ras-encoded protein in Rat-1 fibroblasts is therefore mediated by stimulation, either directly or indirectly, of ras-encoded protein-specific guanine nucleotide exchange factors by the EGF receptor tyrosine kinase.