Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are about 3,000-4,000 nuclear pore complexes in the nuclear envelope of an animal cell, depending on the number of transcriptions.

Nuclear pores 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, signal molecules and lipids moving into the nucleus. 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 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

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Small particles (< 50 kDa) 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 negligible rates. Efficient passage through the complex requires several protein factors. Karyopherins, which may act as importins or exportins are part of the Importin-β super-family which all share a similar three-dimensional structure.

Import of proteins

Any cargo with a Nuclear Localization Signal (NLS) exposed will be destined for quick and efficient transport through the pore. There are several kinds of NLS sequences but they are usually conserved polypeptide sequence with basic residues such as PKKKRKV. Any material with an NLS will be taken up by importins to the nucleus. The classical scheme of NLS-protein import is as this : Importin-α binds first to the NLS sequence, and acts as a bridge for Importin-β to attach. The complex importinβ-importinα-cargo is then directed towards the nuclear pore and diffuses through it. Once the complex is in the nucleus, RanGTP binds to Importin-β and displaces it from the complex. Then CAS (Cellular Apoptosis Susceptibility) protein, an exportin which in the nucleus is bound to RanGTP, displaces Importin-α from the cargo. The NLS-protein is thus free in the nucleoplasm. The Importinβ-RanGTP and Importinα-CAS-RanGTP complex diffuses back to the cytoplasm where GTPs are hydrolysed to GDP leading to the release of Importinβ and Importinα which become available for a new NLS-protein import round.

Although cargo passes through the pore with the assistance of chaperone proteins, the translocation through the pore itself is not energy dependent. However, the whole import cycle needs the hydrolysis of 2 GTPs and is thus energy dependent and has to be considered as active transport. The import cycle is powered by the nucleo-cytoplasmic RanGTP gradient. This gradient arises from the exclusive nuclear localization of RanGEFs, proteins that exchage GDP to GTP on Ran molecules. Thus there is an elevated RanGTP concentration in the nucleus compared to the cytoplasm.

Export of proteins

Some nuclear proteins have to be exported to the cytoplasma sometimes. Pre-ribosomal particules and mRNAs as well. There is therefore an export mechanism similar to the import. In the classical export scheme, proteins with an NES (Nuclear Export Sequence) can bind in the nucleus to form a heterotrimeric complex with an exportin and RanGTP (for example the exportin CRM1). The complex can then diffuse to the cytoplasm where GTP is hydrolysed and the NES-protein is released. CRM1-RanGDP diffuses back to the nucleus where GDP is exchanged to GTP by RanGEFs. This process is also energy dependent as it consumes one GTP. Export with the exportin CRM1 can be inhibited by Leptomycin B.

cell biology | Nuklea poro | pore nucléaire | Kernpore