title: Yeast importin-α (Srp1) performs distinct roles in the import of nuclear proteins and in targeting proteasomes to the nucleus.
abstract: Srp1 (importin-α) can translocate proteins that contain a nuclear localization signal (NLS) into the nucleus. The loss of Srp1 is lethal, although several temperature-sensitive mutants have been described. Among these mutants, srp1-31 displays the characteristic nuclear import defect of importin-α mutants, whereas srp1-49 shows a defect in protein degradation. We characterized these and additional srp1 mutants to determine whether distinct mechanisms were required for intracellular proteolysis and the import of NLS-containing proteins. We determined that srp1 mutants that failed to import NLS-containing proteins (srp1-31 and srp1-55) successfully localized proteasomes to the nucleus. In contrast, srp1 mutants that did not target proteasomes to the nucleus (srp1-49 and srp1-E402Q) were able to import NLS-containing proteins. The proteasome targeting defect of specific srp1 mutants caused stabilization of nuclear substrates and overall accumulation of multiubiquitylated proteins. Co-expression of a member of each class of srp1 mutants corrected both the proteasome localization defect and the import of NLS-containing proteins. These findings indicate that the targeting of proteasomes to the nucleus occurs by a mechanism distinct from the Srp1-mediated import of nuclear proteins.
authors.affiliation: Department of Pharmacology, Robert Wood Johnson Medical School, Rutgers University, Piscataway, New Jersey 08854.
authors.forename: Li
authors.initials: L
authors.lastname: Chen
authors.affiliation: Department of Pharmacology, Robert Wood Johnson Medical School, Rutgers University, Piscataway, New Jersey 08854. Electronic address: maduraki@rwjms.rutgers.edu.
authors.forename: Kiran
authors.initials: K
authors.lastname: Madura
mesh_terms: D021581:Active Transport, Cell Nucleus; D000483:Alleles; D002467:Cell Nucleus / Q000378:metabolism*; D003600:Cytosol / Q000201:enzymology / Q000378:metabolism; D005816:Genetic Complementation Test; D049452:Green Fluorescent Proteins / Q000378:metabolism; D028884:Karyopherins / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D009154:Mutation; D019913:Nuclear Localization Signals / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D013696:Temperature; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D028884:Karyopherins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; D029701:Saccharomyces cerevisiae Proteins; C538902:Srp1 protein, S cerevisiae; D025801:Ubiquitin; D049452:Green Fluorescent Proteins; D046988:Proteasome Endopeptidase Complex
keywords: Proteasome; Protein Sorting; Protein Stability; Proteolysis; Ubiquitin-dependent Protease