Nuclear Pore

Nuclear Pore

Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are about an average of 2000 nuclear pore complexes (NPCs), in the nuclear envelope of a vertebrate cell, but it varies depending on cell type and the stage in the life cycle. The proteins that make up the nuclear pore complex are known as nucleoporins. About half of the nucleoporins typically contain either an alpha solenoid or a beta-propeller fold, or in some cases both as separate structural domains. Each NPC contains at least 456 individual protein molecules and is composed of 30 distinct proteins (nucleoporins). The other half show structural characteristics typical of "natively unfolded" proteins, i.e. they are highly flexible proteins that lack ordered secondary structure. These disordered proteins are the FG nucleoporins, so called because their amino-acid sequence contains many repeats of the peptide phenylalanine—glycine.

Nuclear pore complexes allow the transport of water-soluble molecules across the nuclear envelope. This transport includes RNA and ribosomes moving from nucleus to the cytoplasm and proteins (such as DNA polymerase and lamins), carbohydrates, signaling molecules and lipids moving into the nucleus. It is notable that the nuclear pore complex (NPC) can actively conduct 1000 translocations per complex per second. 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 nucleoporins into or out of the nucleus. This is known as the RAN cycle. 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 contain a plug-like structure. It is yet unknown whether this corresponds to an actual plug or is merely cargo caught in transit.