On a snailroad to the nucleus

Abstract

Nucleocytoplasmic transport of proteins and RNA is mediated by receptors that shuttle their substrates across the nuclear pore complex (NPC). Most of these receptors belong to the rapidly expanding importin-β family (also called karyopherin-β family) and are commonly referred to as either importins or exportins depending on the direction of transport. In the case of nuclear import, substrate release in the nucleus is triggered by the binding of the GTP-bound form of the small GTPase Ran to importin-β. The Ran–GTP–importin-β complex is then exported back into the cytoplasm and disassembled in order to be available for another round of transport. So, how does binding of Ran–GTP cause importin-β to release its substrate?