Essay: Nuclear pore complexes

Introduction
Nuclear pore complexes (NPC) completes the vital role of channelling molecules between the cytoplasm and nucleus. NPC are aqueous channels formed from a complex network of evolutionarily conserved proteins known as nucleporins. They are roughly cylindrical in shape (Diameter ~120nm, height ~70nm) and embedded into the double membrane known as the nuclear envelope. Typically, a vertebrate somatic cell has between 1000-10,000 such pore complexes. There are a number of steps associated with regulating the movement of proteins and RNA to the nucleus, this short report will describe these steps and discuss how NPC can be targeted.
Structure of the Nuclear pore complex
To understand the nuclear pore complex fully we need to start with its structure. The NPC is made up of eights spokes that are edged by ‘nuclear cytoplasmic rings’ which surround a central channel structure (the cylindrical shape mentioned in the introduction) from these rings cytoplasmic and nuclear fibrils extend out on either side. The lengths of the fibrils are 35-50nm and 50-100nm long respectively. Notice the nuclear fibrils join together in a ‘basket like structure.’
Mechanisms of the nuclear pore complex
Ions and small proteins under 40 kD move through the pore via passive diffusion. However, larger molecules require energy dependent mechanisms.
These mechanisms can be split into three key stages; how the protein is recognised and carried though the pore, how the protein is released into the nucleus and then how the NPC resets ready for the next repetition.
Firstly, appropriate proteins, such as T antigen, are recognized due to nuclear localisation signals. These are one or two lines of amino acid sequences (as seen in Figure.2) which are recognised by nuclear transport receptors called Importin (which has two subunits α & β) the protein binds to the importin in a complex assembly.
After they have bound they move through the NP via sequential binding of the importin starting with the cytoplasmic filaments then onto interior pore proteins. After the nuclear filaments the complex is disrupted by another binding of Ran, ran carries at GTP molecule. This inturn causes the release of the cargo protein. This complex then travels back through the pore. When the pair reach the cytoplasmic filaments they are met by RAN GAP, which caused ran to hydrolise its GTP to GDP as a result importin is release back in to the cytoplasm for another active transfer of protein through the nuclear pore. N.B. ran GDP cannot split importin from its protein (on the cytoplasm side). Ran GDP now binds with NTF2 which transports it into the nucleus (its own import receptor) then Ran GEF (Guanine nucleotde exchance factor causes ran to release GDP and pick up GTP which resets the system and ran GTP can disrupt importin and its cargo protein. As a result of Ran GAP and Ran GEF remaining on the cytoplasmic and nuclear side respectively and steep gradient of Ran GTP and Ran GDP is maintained across the membrane of the nucleus.
The reverse sequence occours when proteins inside the nucleus have a different amino code which is reffered to as nuclear export signals which then join with exportin’s