R-RAS IS REGULATED BY ACTIVATORS AND EFFECTORS DISTINCT FROM THOSE THAT CONTROL RAS FUNCTION

Like Ras, constitutively activated mutants of the Ras-related protein R-Ras cause tumorigenic transformation of NIH3T3 cells. However, since R-Ras causes a transformed phenotype distinct from that induced by Ra s, it is likely that R-Ras controls signaling pathways and cellular pr ocesses distinct from those regulated by Ras. To address this possibil ity, we determined if R-Ras is regulated by activators and effecters d istinct from those that regulate Ras function, We observed that Ras gu anine nucleotide exchange factors failed to activate R-Ras in vivo, in dicating that R-Ras is activated by distinct GEFs. Consistent with thi s, mutants of R-Ras with mutations analogous to the Ras(15A)/(17N) dom inant negative proteins did not antagonize Ras GEF function and lacked the growth inhibitory activity seen with these mutant Ras proteins, T hus, R-Ras, but not Ras, is dispensable for the viability of MIH3T3 ce lls. Finally, whereas constitutively activated Ras can overcome the gr owth inhibitory action of the Ras(17N) dominant negative protein via R af-dependent and -independent activities, transforming mutants of R-Ra s failed to do so. This inability was consistent with our observation that Ras-, but not R-Ras-transformed, NIH3T3 cells possessed constitut ively upregulated Raf kinase activities. Thus, R-Ras and Ras are regul ators of distinct signaling pathways and cellular processes.