DIFFERENTIAL RESPONSE OF THE RAS EXCHANGE FACTOR, RAS-GRF TO TYROSINEKINASE AND G-PROTEIN MEDIATED SIGNALS

Ras-GRF, a guanine-nucleotide exchange factor that activates Ras p21, was tested for its ability to couple to either tyrosine kinase or hete rotrimeric G protein signal transduction pathways. Ras-GRF failed to b ind the SH2 and SH3 containing adaptor protein Grb2, either in vitro o r in vivo. Furthermore, Ras-GRF did not form a stable complex with act ivated EGF receptor. However, as has been shown previously (Cen et al. , 1994), the presence of Ras-GRF in NIH3T3 cells enhanced the activati on of Ras induced by serum stimulation. A similar effect was not obser ved with PDGF stimulation. Moreover, serum stimulation lead to the hyp erphosphorylation of Ras-GRF. Both the serum induced super-activation of Ras, and the hyperphosphorylation of Ras-GRF were blocked by pretre atment of cells with the G(i,o) inhibitor pertussis toxin, but not by pretreatment with the tyrosine kinase inhibitor genistein. These resul ts suggest that Ras-GRF has the capacity to mediate Ras activation ini tiated by signals using heterotrimeric G proteins.