A structure/function analysis of Rat7p/Nup159p, an essential nucleoporin of Saccharomyces cerevisiae

Rat7p/Nup159p is an essential nucleoporin of Sac-charomyces cerevisiae originally isolated in a genetic screen designed to identify yeast temperature-sensitive mutants defective in mRNA export. Here we describe a detailed structural-functional analysis of Rat7p/Nup159p. The mutation in the rat7-1 ts allele, isolated in the original genetic screen, was found to be a single base pair change that created a stop codon approximately 100 amino acids upstream of the actual stop codon of this 1,460 amino acid polypeptide, thus eliminating one of the two predicted coiled-coil regions located near the carboxyl terminus of the protein. These coiled-coil regions are essential since an allele lacking both coiled-coil regions was unable to support growth under any conditions. In contrast, no other region of the protein was absolutely required. The SAFG/PSFG repeat region in the central third of the protein was completely dispensable for growth at temperatures between 16 degrees C and 37 degrees C and cells expressing this mutant allele were indistinguishable from wild type. Deletion of the amino-terminal third of the protein, upstream from the repeat region, or the portion between the repeat region and the coiled-coils resulted in temperature-sensitivity, but the two alleles showed distinct phenotypes with respect to the behavior of nuclear pore complexes (NPCs). Taken together, our data suggest that Rat7p/Nup159p is anchored within the NPC through its coiled-coil region and adjacent sequences. In addition, we postulate that the N-terminal third of Rat7p/Nup159p plays an important role in mRNA export.

Transformation of a continuous rat embryo fibroblast cell line requires three separate domains of simian virus 40 large T antigen

Mouse C3H 10T1/2 cells and the established rat embryo fibroblast cell line REF-52 are two cell lines widely used in studies of viral transformation. Studies have shown that transformation of 10T1/2 cells requires only the amino-terminal 121 amino acids of simian virus 40 (SV40) large T antigen, while transformation of REF-52 cells requires considerably more of large T antigen, extending from near the N terminus to beyond residue 600. The ability of a large set of linker insertion, small deletion, and point mutants of SV40 T antigen to transform these two cell lines and to bind p105Rb was determined. Transformation of 10T1/2 cells was greatly reduced by mutations within the first exon of the gene for large T antigen but was only modestly affected by mutations affecting the p105Rb binding site or the p53 binding region. All mutants defective for transformation of 10T1/2 cells were also defective for transformation of REF-52 cells. In addition, mutants whose T antigens had alterations in the Rb binding site showed a substantial reduction in transformation of REF-52 cells, and the degree of this reduction could be correlated with the ability of the mutant T antigens to bind p105Rb. There was a tight correlation between the ability of mutants to transform REF-52 cells and the ability of their T antigens to bind p53. These results demonstrate that multiple regions of large T antigen are required for full transformation by SV40.