Architecture of the nuclear pore complex and its involvement in nucleocytoplasmic transport

Recent structural analyses of the nuclear pore complex (NPC) have described in some detail the numerous sub-domains which make up this supramolecular assembly. Three dimensional image analysis of detergent-extracted NPCs reveals that the NPC framework is made up of spoke units, each containing four major domains, arranged with 822 symmetry. As shown by freeze-drying/metal shadowing techniques, attached to this framework are several peripheral components including particles and fibers on the cytoplasmic face and a cage-like structure on the nucleoplasmic face. While it is known that traffic between cytoplasm and nucleus occurs exclusively through the nuclear pore, the specifics of NPC involvement in such traffic remain unknown at present. Proteins destined for the nucleus contain nuclear localization sequences (NLSs). Several proteins have been identified which bind to these NLSs and may act to direct these proteins to the NPC, either releasing them prior to or remaining attached during translocation through the NPC. These NLS-binding proteins have been localized to the cytoplasm, nucleus, nuclear envelope and nucleolus, suggesting some of these proteins transverse the NPC bound or unbound to NLS-containing proteins.