Nucleocytoplasmic transport occurs through nuclear pore complexes (NPC), macromolecular structures which span the nuclear envelope (1 ). NPCs contain aqueous channels with a diameter of ~9 nm, allowing ions, metabolites, and small proteins to passively diffuse between the nucleus and the cytoplasm (2 ). Most proteins and RNAs, however, exceed the ~60-kDa diffusion limit and are transported through the NPC by mechanisms that are saturable (3 ), energy-dependent (4 ), and signal-mediated (6 ).
