Sensitivity of wild type and mutant ras alleles to Ras specific exchange factors: Identification of factor specific requirements

We have investigated the productive interaction between the four mammalian Ras proteins (H-, N-, KA- and KB-Ras) and their activators, the mammalian e xchange factors mSos1, GRF1 and GRP, by using a modified Saccharomyces cere visiae whose growth is dependent on activation of a mammalian Ras protein b y its activator. All four mammalian Ras proteins were activated with simila r efficiencies by the individual exchange factors. The H-Ras mutant V103E, which is competent for membrane localization, nucleotide binding, intrinsic and stimulated GTPase activity as well as intrinsic exchange, was defectiv e for activation by all factors tested, suggesting that the integrity of th is residue is necessary for catalyzed exchange. However, when other H-Ras m utants were studied, some distinct sensitivities to the exchange factors we re observed. GRP-mediated, but not mSos1-mediated, exchange was blocked in additional mutants, suggesting different structural requirements for GRP, A nalysis of Ras-mediated gene activation in murine fibroblasts confirmed the se results.