Mammalian ras is an essential component of receptor-mediated signal transduction pathways regulating cell growth and differentiation. Following the identification of ras guanine nucleotide exchange factor (rGEF), which stimulates GDP/GTP exchange on ras, our research efforts have been made to establish the physiological function of rGEF as a potential initiator of ras activation. We studied the physiological response of rGEF to growth factors in various biological systems. In differentiation process, NGF was found to enhance both rGEF and GAP (ras GTPase activating protein) activities in pheochromocytoma PC12 cells. This stimulation was potentiated by the overexpression of trk tyrosine kinase (a high affinity NGF receptor) in PC12 cells. From these data, we speculate that the NGF receptor system activates both rGEF and GAP and that the balance in activity between the two regulatory proteins could cause the activation of ras. In cell proliferation process, similar activation of rGEF was observed in EGF- and PDGF-treated cells and in the cells transformed by oncogenic tyrosine kinases such as src and erbB2. This suggests a model in which receptor or non-receptor tyrosine kinases upstream of ras, either directly or indirectly, modulate the activity of rGEF. Since rGEF but not GAP was stimulated by EGF and PDGF, rGEF may play a key role in activation of ras in growth factor-stimulated cell proliferation. Taken together, these findings provide a new insight into the regulatory function of rGEF and GAP for the activation of ras. In another approach, we purified rGEF from bovine brain in order to set the stage for the molecular cloning of cDNA encoding rGEF. Amino acid sequencing of the purified protein and preparation of monoclonal antibodies directed to rGEF are in progress.
