Karyopherins: from nuclear-transport mediators to nuclear-function regulators.

The karyopherin beta (or importin beta) family comprises soluble transport factors that mediate the movement of proteins and RNAs between the nucleus and cytoplasm. Recent studies have extended the role of karyopherins to regulating assembly of the nuclear pore complex (NPC), assembly of the nuclear envelope, mitosis and replication. New data also address how karyopherins specifically recognize and transport many distinct cargoes and traverse the NPC. These data raise the possibility that, although there might be a universal mechanism for nuclear transport, specific interactions between karyopherins and components of the NPC might function to regulate differentially the ability of the different karyopherins to cross the NPC.