Structural basis for nuclear import complex dissociation by RanGTP

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.