Structure of the nuclear transport complex karyopherin-β2-Ran•GppNHp

Transport factors in the karyopherin-beta (also called importin-beta) family mediate the movement of macromolecules in nuclear-cytoplasmic transport pathways. Karyopherin-beta 2 (transportin) binds a cognate import substrate and targets It to the nuclear pore complex. In the nucleus, Ran GTP binds karyopherin-beta 2 and dissociates the substrate. Here we present the 3.0 Angstrom structure of the karyopherin-beta 2-Ran GppNHp complex where GppNHp is a non-hydrolysable GTP analogue. Karyopherin-beta 2 contains eighteen HEAT repeats arranged into two continuous orthogonal arches. Ran is clamped in the amino-terminal arch and substrate-binding activity Is mapped to the carboxy-terminal arch. A large loop In HEAT repeat 7 spans both arches. Interactions of the loop with Ran and the C-terminal arch implicate it in GTPase-mediated dissociation of the import-substrate. Ran GppNHp In the complex shows extensive structural rearrangement, compared to Ran GDP, in regions contacting karyopherin-beta 2. This provides a structural basis for the specificity of the karyopherin-beta family for the GTP-bound state of Ran, as well as a rationale for interactions of the karyopherin-Ran complex with the regulatory proteins ranGAP, ranGEF and ranBP1.