A Lack of SUMO Conjugation Affects cNLS-dependent Nuclear Protein Import in Yeast

• Published in: The Journal of biological chemistry 2002-12, Vol.277 (51), p.49554-49561
• Main Authors: Stade, Katrin, Vogel, Frank, Schwienhorst, Ingrid, Meusser, Birgit, Volkwein, Corinna, Nentwig, Brigitte, Dohmen, R. Jürgen, Sommer, Thomas
• Format: Article
• Language: English
• Subjects: Active Transport, Cell Nucleus; alpha Karyopherins - metabolism; beta Karyopherins - metabolism; Cell Nucleus - metabolism; Cysteine Endopeptidases - genetics; Genes, Reporter; Green Fluorescent Proteins; Luminescent Proteins - metabolism; Mutation; Nuclear Localization Signals - chemistry; Plasmids - metabolism; Proteins - genetics; RNA, Messenger - metabolism; Small Ubiquitin-Related Modifier Proteins - metabolism; Temperature; Time Factors; Ubiquitin-Activating Enzymes; Yeasts - physiology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M207991200

Yeast SUMO (Smt3) and its mammalian ortholog SUMO-1 are ubiquitin-like proteins that can reversibly be conjugated to other proteins. Among the substrates for SUMO modification in vertebrates are RanGAP1 and RanBP2/Nup358, two proteins previously implicated in nucleocytoplasmic transport. Sumoylated RanGAP1 binds to the nuclear pore complex via RanBP2/Nup358, a giant nucleoporin, which was recently reported to act as a SUMO E3 ligase on some nuclear substrates. However, no direct evidence for a role of the SUMO system in nuclear transport has been obtained so far. By the use of conditional yeast mutants, we examined nuclear protein importin vivo. We show here that cNLS-dependent protein import is impaired in mutants with defective Ulp1 and Uba2, two enzymes involved in the SUMO conjugation reaction. In contrast, other transport pathways such as rgNLS-mediated protein import and mRNA export are not affected. Furthermore, we find that the yeast importin-α subunit Srp1 accumulates in the nucleus ofulp1 and uba2 strains but not the importin-β subunit Kap95, indicating that a lack of Srp1 export might impair cNLS import. In summary, our results provide evidence that SUMO modification in yeast, as has been suspected for vertebrates, plays an important role in nucleocytoplasmic trafficking.