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 exchange factors mSos1, GRF1 and GRP, by using a modified Saccharomyces cerevisiae whose growth is dependent on activation of a mammalian Ras protein by its activator. All four mammalian Ras proteins were activated with similar 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 defective for activation by all factors tested, suggesting that the integrity of this residue is necessary for catalyzed exchange. However, when other H-Ras mutants were studied, some distinct sensitivities to the exchange factors were observed. GRP-mediated, but not mSos1-mediated, exchange was blocked in additional mutants, suggesting different structural requirements for GRP. Analysis of Ras-mediated gene activation in murine fibroblasts confirmed these results.