A protein required for nuclear-protein import, Mog1p, directly interacts with GTP–Gsp1p, the Saccharomyces cerevisiae Ran homologue

We previously isolated 25 temperature-sensitivegsp1 alleles ofSaccharomyces cerevisiae Ran homologue, each of which possesses amino acid changes that differ from each other. We report here isolation of three multicopy suppressors—PDE2, NTF2, and a gene designatedMOG1 —all of which rescued a growth defect of thesegsp1 strains. Thegsp1 suppression occurred even in the absence ofGSP2 , anotherS. cerevisiae GSP1 -like gene. Previously,NTF2 was reported to suppressgsp1 but notPDE2 . Mog1p, with a calculated molecular mass of 24 kDa, was found to be encoded by the yeast ORF YJR074W. BothMOG1 andNTF2 suppressed a series ofgsp1 alleles with similar efficiency, and both suppressedgsp1 even with a single gene dose. Consistent with the high efficiency ofgsp1 suppression, Mog1p directly bound to GTP, but not to GDP-Gsp1p. The disruption ofMOG1 made yeast temperature-sensitive for growth. Δmog1, which was suppressed by overexpression ofNTF2 , was found to have a defect in both classic and nonclassic nuclear localization signal-dependent nuclear-protein imports, but not in mRNA export. Thus, Mog1p, which was localized in the nucleus, is a Gsp1p-binding protein involved in nuclear-protein import and that functionally interacts with Ntf2p. Furthermore, the finding thatPDE2 suppressed bothgsp1 andrna1–1 indicates that the Ran GTPase cycle is regulated by the Ras-cAMP pathway.