Nuclear export complexes in the frame.

Protein and RNA molecules are exchanged between the nucleus and cytoplasm by members of the karyopherin beta family of transport factors. Karyopherins adopt a modular HEAT-repeat architecture and are regulated by the GTPase Ran. RanGTP acts as a signal for the nuclear compartment, dissociating molecular cargo from karyopherins that mediate nuclear import and promoting cargo uptake on those mediating nuclear export. After unraveling the mechanisms of nuclear import factors, structural studies have recently provided tremendous insights into nuclear export. The impact of RanGTP binding on the karyopherins ranges from large, global conformational changes to local, allosteric effects. A theme emerges where cargo recognition provides a molecular surveillance mechanism to prevent the transport of macromolecules in an inappropriate state.