The nuclear pores are large protein complexes of the nuclear envelope that regulate the transport of substances between the cytoplasm and the cell nucleus, such as proteins, RNA, hormones. On average, there are about 2000 nuclear pores in the nuclear envelope of a vertebrate cell, but can vary depending on the cell type and the stage of the cell cycle.
Nuclear Pore structure
The nuclear pore complex, with a diameter of 120 nanometers (in vertebrates), is probably the largest protein complex of eukaryotic cells. It has a cylindrical shape and contains more than 30 different types of proteins, known as nucleoporins. The central channel, with a diameter of 9 nanometers, is formed by 8 subunits surrounding the actual pore, and two protein rings that anchor the channel to the nuclear envelope. From the outer side of the pore complex there are protein filaments that protrude towards the cytoplasm and are connected with the cytoskeleton. In the inner side of the pore, which faces the nucleoplasm, there is a basket-shaped structure that participates in the nuclear transport mechanism.
The transport through the nuclear pores can be active or passive: ions and small molecules are able to pass through the channel by passive diffusion, while bulkier molecules are selectively transported by specific carriers that interact with the pore complex and cause the widening of the channel (up to a diameter of 26 nm) that allows the passage of the cargo. The import and export of the cargo across the nuclear envelope is performed by different carriers, known as Importins and Exportins, which recognize specific signal sequences in the cargo, called Nuclear Localization Signal (NLS) for the import and Nuclear Export Signal (NES) for the export. Importins and Exportin work together with a small G-Protein, called RAN, and other co-factor that assist the dispatch and release of the cargo and allow the return of Importins in the cytoplasm and Exportins in the cell nucleus.