ACTIVATION OF R-RAS BY RAS-GUANINE NUCLEOTIDE-RELEASING FACTOR

Ras-GRF/CDC25(Mm), mSos, and C3G have been identified as guanine nucle otide releasing factors for Ras family proteins. We investigated in th is study the guanine nucleotide-releasing activities of Ras-GRF, mSos, and C3G toward R-Ras, which shows high sequence similarity to Ras. Ra s-GRF markedly stimulated the dissociation of GDP from R-Ras, and C3G also promoted the release of R-Ras-bound GDP. Under the same condition s, mSos little affected the reaction. When Ras GRF and R-Ras were coex pressed in COS7 cells, the remarkable accumulation of the active GTP b ound form of R-Ras was observed. C3G also increased active R-Ras in CO S7 cells, while mSos did not give any effect. These results indicated that Ras-GRF and C3G could activate R-Ras. Furthermore, the activation of R-Ras by Ras-GRF was enhanced when cells were treated with ionomyc in, which is known to increase the intracellular calcium concentration . The examination of tissue distribution of R-Ras, Ras-GRF, and mSos b y the reverse transcription-polymerase chain reaction revealed that Ra s GRF was expressed only in brain and testis, whereas R-Ras, C3G, and mSos were expressed rather ubiquitously. These findings raise the poss ibility that R-Ras is activated by Ras-GRF in brain and testis, and by C3G in other tissues, respectively.