Importin-β is a GDP-to-GTP exchange factor of Ran. Implications for the mechanism of nuclear import

ArticleinJournal of Biological Chemistry 284(34):22549-58 · July 2009with12 Reads
Impact Factor: 4.57 · DOI: 10.1074/jbc.M109.019935 · Source: PubMed

    Abstract

    Ran-GTP interacts strongly with importin-β, and this interaction promotes the release of the importin-α-nuclear localization
    signal cargo from importin-β. Ran-GDP also interacts with importin-β, but this interaction is 4 orders of magnitude weaker
    than the Ran-GTP·importin-β interaction. Here we use the yeast complement of nuclear import proteins to show that the interaction
    between Ran-GDP and importin-β promotes the dissociation of GDP from Ran. The release of GDP from the Ran-GDP-importin-β complex
    stabilizes the complex, which cannot be dissociated by importin-α. Although Ran has a higher affinity for GDP compared with
    GTP, Ran in complex with importin-β has a higher affinity for GTP. This feature is responsible for the generation of Ran-GTP
    from Ran-GDP by importin-β. Ran-binding protein-1 (RanBP1) activates this reaction by forming a trimeric complex with Ran-GDP
    and importin-β. Importin-α inhibits the GDP exchange reaction by sequestering importin-β, whereas RanBP1 restores the GDP
    nucleotide exchange by importin-β by forming a tetrameric complex with importin-β, Ran, and importin-α. 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.