Nuclear protein import is mediated mainly by the transport factor importin-β that binds cytoplasmic cargo, most often via the importin-α adaptor, and then transports it through nuclear pore complexes. This active transport is driven by disassembly of the import complex by nuclear RanGTP1,2,3,4. The switch I and II loops of Ran change conformation with nucleotide state5,6,7, and regulate its interactions with nuclear trafficking components. Importin-β consists of 19 HEAT repeats that are based on a pair of antiparallel α-helices (referred to as the A- and B-helices). The HEAT repeats stack to yield two C-shaped arches, linked together to form a helicoidal molecule that has considerable conformational flexibility1,8,9,10,11,12. Here we present the structure of full-length yeast importin-β (Kap95p or karyopherin-β) complexed with RanGTP, which provides a basis for understanding the crucial cargo-release step of nuclear import. We identify a key interaction site where the RanGTP switch I loop binds to the carboxy-terminal arch of Kap95p. This interaction produces a change in helicoidal pitch that locks Kap95p in a conformation that cannot bind importin-α or cargo. We suggest an allosteric mechanism for nuclear import complex disassembly by RanGTP.
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We thank M. Allen and M. Canci for data collection, Y. Yoneda and T. Sekimoto for plasmids, and our colleagues in Cambridge, especially N. Marshall, R. Grant, R. A. Crowther, R. Henderson, K. Nagai and N. Unwin, for their assistance, comments and suggestions. S.J.L. holds an EMBO Long-Term Fellowship and S.M.L holds an MRC studentship. This work was supported in part by a Human Frontier Research Program grant.
The authors declare that they have no competing financial interests.
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Lee, S., Matsuura, Y., Liu, S. et al. Structural basis for nuclear import complex dissociation by RanGTP. Nature 435, 693–696 (2005). https://doi.org/10.1038/nature03578
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