UNFARNESYLATED TRANSFORMING RAS MUTANT INHIBITS THE RAS-SIGNALING PATHWAY BY FORMING A STABLE RAS-CENTER-DOT-RAF COMPLEX IN THE CYTOSOL

Farnesyltransferase inhibitors cause the growth arrest of ras-transfor med cells, but not that of normal cells. To elucidate the mechanism of this differential effect, we examined the effect of accumulation of u nfarnesylated Ras in the cytosol by using Ras(G12V,C186S) and Ras(C186 S), which mimic unfarnesylated form of the oncogenic and the normal Ra s, respectively. We found that Ras(G12,C186S) inhibited activation and membrane translocation of Raf by forming a stable complex with Raf in the cytosol. In contrast, Ras(C186S) showed inhibitory effect on neit her Raf activation nor Raf translocation. These results indicate that unfarnesylated oncogenic Ras interacts with Raf in the cytosol and inh ibits its membrane translocation, a crucial step for the Raf activatio n, while unfarnesylated normal Ras does not.