Nuclear pore

Nuclear pores are large protein complexes which cross the nuclear envelope, the membrane surrounding the eucaryotic cell nucleus. There are about 3.000 nuclear pore complexes on the nuclear envelope of an animal cell. They allow the transport of water-soluble molecules across the nuclear envelope. 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 contains a plug protein of yet unknown structure.

Nuclear pore. Top and side view.
(1) Nuclear envelope. (2) Outer ring. (3) Spokes. (4) Plug.
The whole pore complex has a diameter of about 150 nm.
The actual pore is about 10 nm wide.
(Drawing is based on ER images.)
Image from Nupedia server