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 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 actively transported 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.

Nuclear pore. Top and side view.
1. Nuclear envelope.
2. Outer ring.
3. Spokes.
4. Plug.

(Drawing is based on ER images.)

The whole pore complex has a diameter of about 150 nm, and the actual pore is about 10 nm wide.