Nuclear Pore Complex
              (Dan Stoffler @ Scripps)
NUCLEAR PORES... are large supramolecular assemblies that are embedded in the double-membraned nuclear envelope. Nuclear pores are openings in the nuclear envelope at sites where the two membranes are in contact. The pores are octagonal orifices about 60 nm in diameter. They do not freely communicate, but rather are plugged by a cylinder of protein material that is called the annulus.
     The nuclear envelope regulates the passage of ions and small molecules and the pore complex may mediate the selective transport of
nuclear proteins, RNAs, and ribonucleoprotein particles (RNP)between the nucleus and the cytoplasm.
     Nuclear import is an energy dependent process and is mediated by soluble import factors. Nuclear import is a signal mediated process. The signal is a polypeptide sequence in the encoded protein known as the Nuclear Localization Signal (NLS). There are several unique modular polypeptide sequences that import protein through a specific nuclear import pathway and there exist protein sequences that can signal the export of a protein from the nucleus to the cytoplasm, as well - Nuclear Export Signals (NES)