The RNA binding protein Sbp1p is involved in mRNA export in Saccharomyces cerevisiae

The nuclear export of messenger mRNA (mRNA) is an essential step in eukaryotic gene expression. Despite its importance much remains unknown about the mechanism of this process. All nucleocytoplasmic trafficking of macromolecules in eukaryotic cells occurs through complex proteinacious structures known as nuclear pore complexes (NPCs). Nuclear import and export of proteins and ribonucleoproteins across the NPCs are tightly regulated processes that are largely dependent upon the components of the NPC, nucleoporins, and the transportin family of proteins. Gle1p is a nucleoporin that has been implicated in mRNA export in Saccharomyces cerevisiae.; We performed a genetic screen in yeast to identify additional factors that may act along the Gle1p mRNA export pathway. We identified SBP1 , a gene encoding a GAR domain-containing mRNA binding protein, as an extragenic high-copy suppressor of mutant alleles of GLE1. We showed by in situ hybridization that SBP1 specifically suppresses the mRNA export defect of gle1-4 cells. We created a series of deletion mutations in Sbp1p and demonstrated that suppression of the growth phenotype of gle1-4 cells is dependent upon the GAR domain of Sbp1p. We have also shown that high-copy SBP1 is able to partially suppress the temperature-sensitive growth phenotype of rat7-1 cells, which carry a mutation in NUP159, a nucleoporin required for mRNA export. By indirect immunofluorescence we have demonstrated that Sbp1p is nuclear and cytoplasmic in localization. We observed Sbp1p interactions with a subset of nucleoporins implicated in mRNA export and with the transportin family nuclear import factor, Kap104p in the two hybrid system.; Based on these data, we propose that contrary to previous reports describing Sbp1p as mostly nucleolar in localization, Sbp1p is a shuttling RNA binding protein that contributes to mRNA export in S. cerevisiae .