GUANINE-NUCLEOTIDE EXCHANGE FACTORS - ACTIVATORS OF RAS SUPERFAMILY PROTEINS

Members of the Ras superfamily of proteins function as regulated GDP/G TP switches that cycle between active GTP-complexed and inactive GDP-c omplexed states. Guanine nucleotide exchange factors (GEFs) stimulate formation of the GTP-bound state, whereas GTPase activating proteins ( GAPs) catalyze the formation of the GDP-bound state. We describe three studies that evaluate the mechanism of action of GEFs for Ras (SOS1 a nd RasGRF/CDC25) or Ras-related Rho (Dbl and Vav) proteins. Growth fac tor-mediated activation of Ras is believed to be mediated by activatio n of Ras GEFs (CDC25/GRF and SOS1/2). Although the mechanisms of Ras G EF regulation are unclear, recent studies suggest that translocation o f SOS1 to the plasma membrane, where Ras is located, might be responsi ble for Ras activation. Our observation that the addition of the Ras p lasma membrane-targeting sequence to the catalytic domains of CDC25 an d SOS1 greatly enhanced their transforming and transactivation activit ies (10-50 fold and 5-10 fold, respectively) suggests that membrane tr anslocation alone issufficient to potentiate GEF activation of Ras. We have determined that two Ras-related proteins, designated R-Ras and R -Ras2/C21, can trigger the malignant transformation of NIH 3T3 cells v ia activation of the Ras signal transduction pathway. Furthermore, lik e Ras and R-Ras, we observed that TC21 GTPase activity was stimulated by Ras GAPs. However, we observed that both SOS1 and CDC25 were activa tors of normal TC21, but not R-Ras, transforming activities. Therefore , TC21, but not R-Ras, may be activated by the same extracellular sign aling events that activate Ras proteins. Dbl family proteins are belie ved to function as GEFs and activators of the Ras-related Rho family o f proteins. However, one Dbl family oncogene, designated Vav, has been reported to be a GEF for Ras proteins. Therefore we were interested i n determining whether Dbl family oncogenes cause transformation by tri ggering the constitutive activation of Rho or Ras proteins. Our result s suggest that Dbl oncogenes cause transformation via a Ras-independen t activation of MAP kinases and Rho family proteins. (C) 1995 Wiley-Li ss, Inc.