Yeast Importin-{alpha} (Srp1) Performs Distinct Roles in the Import of Nuclear Proteins and in Targeting Proteasomes to the Nucleus [Enzymology]
October 1st, 2014 by Chen, L., Madura, K.
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 if 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; srp1-55) successfully localized proteasomes to the nucleus. In contrast, srp1 mutants that did not target proteasomes to the nucleus (srp1-49; 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 (multiUb) 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 distinct mechanism from the Srp1-mediated import of nuclear proteins.