We previously isolated 25 temperature-sensitive gsp1 alleles of Saccharomyces 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 designated MOG1-all of which rescued a growth defect of these gsp1 strains. The gsp1 suppression occurred even in the absence of GSP2, another S. cerevisiae GSP1-like gene. Previously, NTF2 was reported to suppress gsp1 but not PDE2. Mog1p, with a calculated molecular mass of 24 kDa, was found to be encoded by the yeast ORF YJR074W. Both MOG1 and NTF2 suppressed a series of gsp1 alleles with similar efficiency, and both suppressed gsp1 even with a single gene dose. Consistent with the high efficiency of gsp1 suppression, Mog1p directly bound to GTP, but not to GDP-Gsp1p. The disruption of MOG1 made yeast temperature-sensitive for growth. Deltamog1, which was suppressed by overexpression of NTF2, 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 that PDE2 suppressed both gsp1 and rna1-1 indicates that the Ran GTPase cycle is regulated by the Ras-cAMP pathway.