THE PRODUCT OF THE SACCHAROMYCES-CEREVISIAE RSS1 GENE, IDENTIFIED AS A HIGH-COPY SUPPRESSOR OF THE RAT7-1 TEMPERATURE-SENSITIVE ALLELE OF THE RAT7 NUP159 NUCLEOPORIN, IS REQUIRED FOR EFFICIENT MESSENGER-RNA EXPORT/

RAT7/NUP159 was identified previously in a screen for genes whose prod ucts are important for nucleocytoplasmic export of poly(A)(+) RNA and encodes an essential nucleoporin. We report here the identification of RSS1 (Rat Seven Suppressor) as a high-copy extragenic suppressor of t he rat7-1 temperature-sensitive allele. Rss1p encodes a novel essentia l protein of 538 amino acids, which contains an extended predicted coi led-coil domain and is located both at nuclear pore complexes (NPCs) a nd in the cytoplasm. RSS1 is the first reported high-copy extragenic s uppressor of a mutant nucleoporin. Overexpression of Rss1p partially s uppresses the defects in nucleocytoplasmic export of poly(A)(+) RNA, r RNA synthesis and processing, and nucleolar morphology seen in rat7-1 cells shifted to the nonpermissive temperature of 37 degrees C and, th us, restores these processes to levels adequate for growth at a rate a pproximately one-half that of wild-type cells. After a shift to 37 deg rees C, the mutant Rat7-1p/Nup159-1p is lost from the nuclear rim of r at7-1 cells and NPCs, which are clustered together in these cells grow n under permissive conditions become substantially less clustered. Ove rexpression of Rss1p did not result in retention of the mutant Rat7-1p /Nup159-1p in NPCs, but it did result in partial maintenance of the NP C-clustering phenotype seen in mutant cells. Depletion of Rss1p by pla cing the RSS1 open reading frame (ORF) under control of the GAL1 promo ter led to cessation of growth and nuclear accumulation of poly(A)(+) RNA without affecting nuclear protein import or nuclear pore complex d istribution, suggesting that RSS1 is directly involved in mRNA export. Because both rat7-1 cells and cells depleted for Rss1p are defective in mRNA export, our data are consistent with both gene products playin g essential roles in the process of mRNA export and suggest that Rss1p overexpression suppresses the growth defect of rat7-1 cells at 37 deg rees C by acting to maintain mRNA export.