title: Yeast importin-β is required for nuclear import of the Mig2 repressor.
abstract: BACKGROUND
  Mig2 has been described as a transcriptional factor that in the absence of Mig1 protein is required for glucose repression of the SUC2 gene. Recently it has been reported that Mig2 has two different subcellular localizations. In high-glucose conditions it is a nuclear modulator of several Mig1-regulated genes, but in low-glucose most of the Mig2 protein accumulates in mitochondria. Thus, the Mig2 protein enters and leaves the nucleus in a glucose regulated manner. However, the mechanism by which Mig2 enters into the nucleus was unknown until now.
  
  
  RESULTS
  Here, we report that the Mig2 protein is an import substrate of the carrier Kap95 (importin-β). The Mig2 nuclear import mechanism bypasses the requirement for Kap60 (importin-α) as an adaptor protein, since Mig2 directly binds to Kap95 in the presence of Gsp1(GDP). We also show that the Mig2 nuclear import and the binding of Mig2 with Kap95 are not glucose-dependent processes and require a basic NLS motif, located between lysine-32 and arginine-37. Mig2 interaction with Kap95 was assessed in vitro using purified proteins, demonstrating that importin-β, together with the GTP-binding protein Gsp1, is able to mediate efficient Mig2-Kap95 interaction in the absence of the importin-α (Kap60). It was also demonstrated, that the directionality of Mig2 transport is regulated by association with the small GTPase Gsp1 in the GDP- or GTP-bound forms, which promote cargo recognition and release, respectively.
  
  
  CONCLUSIONS
  The Mig2 protein accumulates in the nucleus through a Kap95 and NLS-dependent nuclear import pathway, which is independent of importin-α in Saccharomyces cerevisiae.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oviedo, 33006, Oviedo, Spain.
authors.forename: Alejandra
authors.initials: A
authors.lastname: Fernández-Cid
authors.forename: Montserrat
authors.initials: M
authors.lastname: Vega
authors.forename: Pilar
authors.initials: P
authors.lastname: Herrero
authors.forename: Fernando
authors.initials: F
authors.lastname: Moreno
mesh_terms: D021581:Active Transport, Cell Nucleus; D020816:Amino Acid Motifs; D002467:Cell Nucleus / Q000378:metabolism; D028884:Karyopherins / Q000378:metabolism; D020559:Monomeric GTP-Binding Proteins / Q000378:metabolism; D009154:Mutation; D009687:Nuclear Proteins / Q000378:metabolism; D011485:Protein Binding; D012097:Repressor Proteins / Q000737:chemistry / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000378:metabolism*; D028901:alpha Karyopherins / Q000235:genetics / Q000378:metabolism; D028961:beta Karyopherins / Q000235:genetics / Q000378:metabolism*
chemical_list: C080324:GSP1 protein, S cerevisiae; D028884:Karyopherins; C401306:MSN5 protein, S cerevisiae; C539385:Mig2 protein, S cerevisiae; D009687:Nuclear Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; D020559:Monomeric GTP-Binding Proteins