| Research Abstract |
Small G protein superfamily consists of more than 50 members such as Ras, Ral, and Rac, and regulates important cellular functions including proliferation, differentiation, motion, and vesicle transport. Ras has been shown to possess multiple effector proteins including Raf and phosphatidylinositol-3 kinase. We have found that Ral GDP dissociation stimulator (RalGDS), a GDP/GTP exchange protein of Ral, is an effector protein of Ras and proposed that there is a Ras-signaling pathway through RalGDS.RalGDS interacted with Ras in response to EGF.Protein kinase A inhibited EGF-dependent Raf activation but not interaction of RalGDS and Ras. Furthermore, RalGDS bound to a Ras effector loop mutant with which Raf did not interact, and RalGDS synergized with Raf to stimulate c-fos promoter activity. These results indicate that RalGDS acts downstream of Ras in response to extracellular signal and that it constitutes a Ras-signaling pathway distinct from Raf. It is known that small G proteins undergo post-translational lipid modifications and that the modifications are important for their membrane localization and functions. The modified form of Ras bound to RalGDS more effectively than the unmodified form, and induced the translocation of RalGDS from the cytosol to membrane fractions. The modification of Ral enhanced the RalGDS action. Furthermore, the modified form of Ral bound to RalBP1, a putative effector protein of Ral, more effectively than the unmodified form, and induced the translocation of RalBP1 from the cytosol to membrane fractions. The modification of Rac enhanced the GAP activity of RalBP1 for Rac. Taken together with the observations that Ras, Ral, and Rac are localized to the membranes through their modification, these results suggest that the post-translational modifications of small G proteins play roles for transmitting the signal effectively on the membranes in the signal transduction pathway of Ras/RalGDS/Ral/RalBP1/Rac.
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