Identification of different roles for RanGDP and RanGTP in nuclear protein import.
The importin-alpha/beta heterodimer and the GTPase Ran play key roles in nuclear protein import. Importin binds the nuclear localization signal (NLS). Translocation of the resulting import ligand complex through the nuclear pore complex (NPC) requires Ran and is terminated at the nucleoplasmic side by its disassembly. The principal GTP exchange factor for Ran is the nuclear protein RCC1, whereas the major RanGAP is cytoplasmic, predicting that nuclear Ran is mainly in the GTP form and cytoplasmic Ran is in the GDP-bound form. Here, we show that nuclear import depends on cytoplasmic RanGDP and free GTP, and that RanGDP binds to the NPC. Therefore, import might involve nucleotide exchange and GTP hydrolysis on NPC-bound Ran. RanGDP binding to the NPC is not mediated by the Ran binding sites of importin-beta, suggesting that translocation is not driven from these sites. Consistently, a mutant importin-beta deficient in Ran binding can deliver its cargo up to the nucleoplasmic side of the NPC. However, the mutant is unable to release the import substrate into the nucleoplasm. Thus, binding of nucleoplasmic RanGTP to importin-beta probably triggers termination, i.e. the dissociation of importin-alpha from importin-beta and the subsequent release of the import substrate into the nucleoplasm.
Zentrum für Molekulare Biologie der Universität Heidelberg, Germany.
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Panté
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Kutay
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Aebi
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Bischoff
D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D020690:GTPase-Activating Proteins; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008845:Microinjections; D009687:Nuclear Proteins / Q000378:metabolism*; D009707:Nucleosomes / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D014981:Xenopus; D029867:Xenopus Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein
D002352:Carrier Proteins; D020690:GTPase-Activating Proteins; D009687:Nuclear Proteins; D009707:Nucleosomes; C092317:RANGAP1 protein, human; C060997:RNA1 protein, S cerevisiae; C513268:RanGAP1 protein, Xenopus; D029701:Saccharomyces cerevisiae Proteins; D029867:Xenopus Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D020931:ran GTP-Binding Protein