Nuclear import and export pathways

Macromolecules enter or leave the nucleus by using nuclear localization signals (NLS) or nuclear export signals (NES), respectively. Different types of NLS and NES are recognized directly or indirectly via adapters, by transport receptors. All transport receptors identified thus far are members of the same family and share an ability to shuttle between the nucleus and the cytoplasm, and to interact with the small GTPase Ran and with nucleoporins at the nuclear pore complex (NPC). The GTPase Ran regulates the interaction of transport receptors with either cargoes, or adapters, or nucleoporins and is crucial in providing directionality to nuclear import and export. Surprisingly, GTP hydrolysis by Ran is not required for translocation of same receptor/cargo complexes through the NPC. One of the challenges for the future will be to establish the mechanisms of translocation through the NPC of different transport receptors together with their cargoes. (C) 1999 Wiley-Liss, inc.