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Traffic. 2009 Sep;10(9):1188-98. doi: 10.1111/j.1600-0854.2009.00937.x. Epub 2009 Apr 29.

Importins and beyond: non-conventional nuclear transport mechanisms.

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Nuclear Signalling Laboratory, Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia.


The movement of proteins between the cytoplasm and the nucleus conventionally involves the recognition of nuclear targeting signals by members of the importin (Imp) superfamily of nuclear transporters, followed by translocation through the nuclear envelope-embedded nuclear pore complexes (NPCs). It is becoming increasingly apparent, however, that distinct alternative pathways for nuclear transport exist and are relatively abundant. This review examines several of these novel pathways, including facilitation of Imp-dependent transport by microtubule motors, and Imp-independent pathways involving either other transport molecules such as the calcium-binding protein calmodulin or through direct binding to the components of the NPC. The existence of these pathways and the fact that many proteins appear to possess separate Imp-dependent and -independent nuclear import mechanisms ensure that the cell can function under conditions in which Imp-dependent transport is inhibited and/or modulate the efficiency of Imp-dependent transport itself, according to the need.

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