Abstract
A cellular gradient of the GTPase Ran orchestrates the movement of import and export complexes through the Nuclear Pore Complex (NPC). Ran-GTP modulates two essential activities of importin β for nuclear import. On one hand, it reduces the avidity of importin β for phenylalanine-glycine-rich nucleoporins (FG-nups), facilitating the passage of import complexes through the permeability barrier; on the other hand, it disassembles import complexes, releasing the import cargo into the nucleus. The precise mechanisms by which Ran-GTP modulates importin β activities remain hypothetical. Leveraging cryogenic electron microscopy (cryo-EM) single particle analysis, in this paper, we describe four distinct conformational states of importin β in complex with binding effectors encountered during an import reaction, specifically IBB-cargos, FG-repeats, Ran-GTP, and Ran-GTP:RanBP1. Comparing these four states enables us to decipher the conformational landscape of importin β without interference from crystallization agents and lattice forces. By correlating structural data with biochemical activities, we find that Ran-GTP constrains the solenoid structure of importin β, closing four high-affinity FG-binding pockets and displacing import cargos through allosteric crosstalk between the concave and convex surfaces. We propose that this allosteric mechanism is relevant to other β-karyopherins involved in nuclear import.