The adoption of a twisted structure of importin-β is essential for the protein-protein interaction required for nuclear transport

Importin-β is a nuclear transport factor which mediates the nuclear import of various nuclear proteins. The N-terminal 1–449 residue fragment of mouse importin-β (impβ449) possesses the ability to bidirectionally translocate through the nuclear pore complex (NPC), and to bind RanGTP. The structure of the uncomplexed form of impβ449 has been solved at a 2.6 Å resolution by X-ray crystallography. It consists of ten copies of the tandemly arrayed HEAT repeat and exhibits conformational flexibility which is involved in protein-protein interaction for nuclear transport. The overall conformation of the HEAT repeats shows that a twisted motion produces a significantly varied superhelical architecture from the previously reported structure of RanGTP-bound importin-β. These conformational changes appear to be the sum of small conformational changes throughout the polypeptide. Such a flexibility, which resides in the stacked HEAT repeats, is essential for interaction with RanGTP or with NPCs. Furthermore, it was found that impβ449 has a structural similarity with another nuclear migrating protein, namely β-catenin, which is composed of another type of helix-repeated structure of ARM repeat. Interestingly, the essential regions for NPC translocation for both importin-β and β-catenin are spatially well overlapped with one another. This strongly indicates the importance of helix stacking of the HEAT or ARM repeats for NPC-passage.