Nuclear pore. Top and side view. 1. Nuclear envelope. 2. Outer ring. 3. Spokes. 4. Plug. (Drawing is based on electron microscopy images)

Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are about 3,000-5,000 nuclear pore complexes in the nuclear envelope of an animal cell.

Nuclear pores allow the transport of water-soluble molecules across the nuclear envelope. This transport is restricted to either RNA moving out or proteins moving into the nucleous. Although smaller molecules simply diffuse through the pores, larger molecules may be recognized by specific signal sequences and then be diffused with the help of the chaperone proteins into or out of the nucleus. Each of the eight protein subunits surrounding the actual pore (the outer ring) projects a spoke-shaped protein into the pore channel. The center of the pore often appears to contains a plug-like structure. It is yet unknown whether this corresponds to an actual plug or is merely cargo caught in transit.

The whole pore complex has a diameter of about 150 nm, and the diameter of the opening is about 50 nm wide.

Transport through the nuclear pore complex

Small particles are able to pass through the nuclear pore complex by passive diffusion. Larger particles are also able to pass through the large diameter of the pore but at almost negligable rates. Efficient passage through the complex requires several protein factors. Importins and exportins are part of the Importin-β super-family which all share a similar three-dimensional structure.

Any cargo with a Nuclear Localization Signal (NLS) exposed will be destined for quick and efficient transport through the pore. The NLS is a conserved polypeptide sequence with basic residues: PKKKRKV. Any material with the NLS will be taken up by importins to the nucleus. When the NLS is recognized, Importin-α binds to the sequence and acts as a bridge for Importin-β to attach. The Importin-bound cargo with exposed NLS is then directed towards the nuclear pore and the whole complex is diffused through the pore. Although cargo passes through the pore with the assistance of chaperone proteins, the translocation through the pore does not use energy.

Once the cargo is through the pore, RanGTP displaces Importin-β and CAS, an exportin, displaces Importin-α from the cargo.