Overexpression of Bud5p can suppress mutations in the Gsp1p guanine nucleotide exchange factor Prp20p in Saccharomyces cerevisiae

The gene product Prp20p, which is located in the nucleus. serves as the nuc leotide exchange factor (GEF) for the small nuclear G protein Gsp1p in Sacc haromyces cerevisiae, and catalyses the replacement of Gsp1-bound GDP by GT P. These proteins are involved in numerous cellular processes, including nu cleocytoplasmic trafficking of macromolecules, cell cycle progression, DNA replication and maintenance of chromosome structure/stability. It is believ ed that in order to complete a full GDP/GTP cycle, Gsp1p has to shuttle bet ween the nucleus and the cytoplasm, where its GTPase Activating Protein (GA P) Rna1p is located. Here, we report on the ability of Bud5p, the exchange factor for Rsr1p, to suppress conditional prp20 mutants when an extra copy of GSP1 is present. This suppression by BUDS can be reversed by simultaneou s overexpression of RNA I, and is not Rsr1p-dependent, nor allele-specific. We also show that Bud5p can physically interact with Gsp1p, both in vitro and in vivo. These findings raise the possibility that Bud5p could act as a cytoplasmic exchange factor for Gsp1p and, therefore, that a complete GDP/ GTP cycle could take place in the cytoplasm.