GEF Localization Affects Ras Activation

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Science's STKE  02 Sep 2003:
Vol. 2003, Issue 198, pp. tw339-TW339
DOI: 10.1126/stke.2003.198.tw339

Members of the Ras subfamily of small-molecule guanosine triphosphatases (GTPases) have been implicated in various signaling pathways affecting cell proliferation and differentiation. Although traditionally viewed as acting exclusively at the plasma membrane, Ras proteins located in intracellular compartments can also activate signaling pathways; moreover, the subcellular localization of Ras activation may affect signaling specificity. Caloca et al. investigated the subcellular localization of Ras protein activation by members of three families of Ras guanosine nucleotide exchange factors (GEFs)--the Son-of-sevenless (SOS), Ras guanosine nucleotide releasing factor (RasGRF), and Ras guanosine nucleotide releasing protein (RasGRP) families. Immunofluorescence analysis revealed that, in COS1 cells transfected with Ras GEFs, different GEFs localized to distinct compartments. Sos1.1 and RasGRP2 were cytosolic, Sos1.2 and RasGRF2 were found in cytoplasm and associated with microtubules and endoplasmic reticulum (ER), and RasGRP1 and RasGRP3 were predominantly localized to Golgi and ER. Localization of RasGRP1 was influenced by cell proliferative status and by treatment with phorbol myristate acetate. The authors used mutational analysis to show that localization of RasGRP1 to ER and Golgi depended on the zinc finger region. Experiments in which a fluorescent protein that bound activated Ras or artificially targeted versions of Ras were coexpressed with RasGEFs showed that, whereas most GEFs activated Ras at the plasma membrane or the ER, only RasGRP1 (but not RasGRP2, RasGRF2, or SOS proteins) stimulated Ras in the Golgi. The differential localization of RasGEFs, the sensitivity of localization to upstream signaling, and the possible effects of localization on downstream signaling add additional complexity to the signaling capacities of the Ras GTPases.

M. J. Caloca, J. L. Zugaza, X. Bustelo, Exchange factors of the RasGRP family mediate Ras activation in the Golgi. J. Biol. Chem. 278, 33465-33473 (2003). [Abstract] [Full Text]

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