title: Importin-beta is a GDP-to-GTP exchange factor of Ran: implications for the mechanism of nuclear import.
abstract: Ran-GTP interacts strongly with importin-beta, and this interaction promotes the release of the importin-alpha-nuclear localization signal cargo from importin-beta. Ran-GDP also interacts with importin-beta, but this interaction is 4 orders of magnitude weaker than the Ran-GTP.importin-beta interaction. Here we use the yeast complement of nuclear import proteins to show that the interaction between Ran-GDP and importin-beta promotes the dissociation of GDP from Ran. The release of GDP from the Ran-GDP-importin-beta complex stabilizes the complex, which cannot be dissociated by importin-alpha. Although Ran has a higher affinity for GDP compared with GTP, Ran in complex with importin-beta has a higher affinity for GTP. This feature is responsible for the generation of Ran-GTP from Ran-GDP by importin-beta. Ran-binding protein-1 (RanBP1) activates this reaction by forming a trimeric complex with Ran-GDP and importin-beta. Importin-alpha inhibits the GDP exchange reaction by sequestering importin-beta, whereas RanBP1 restores the GDP nucleotide exchange by importin-beta by forming a tetrameric complex with importin-beta, Ran, and importin-alpha. The exchange is also inhibited by nuclear-transport factor-2 (NTF2). We suggest a mechanism for nuclear import, additional to the established RCC1 (Ran-guanine exchange factor)-dependent pathway that incorporates these results.
authors: Australian Research Council Centre of Excellence for Integrative Legume Research, School of Chemistry and Molecular Biosciences, University of Queensland, QLD 4072, St. Lucia, Australia. t.lonhienne@uq.edu.au
authors: Thierry G
authors: TG
authors: Lonhienne
authors: Jade K
authors: JK
authors: Forwood
authors: Mary
authors: M
authors: Marfori
authors: Gautier
authors: G
authors: Robin
authors: Bostjan
authors: B
authors: Kobe
authors: Bernard J
authors: BJ
authors: Carroll
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000235:genetics / Q000502:physiology*; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism; D011485:Protein Binding; D028901:alpha Karyopherins / Q000378:metabolism; D028961:beta Karyopherins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D009687:Nuclear Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; C091775:ran-binding protein 1; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020931:ran GTP-Binding Protein