What is a nuclear pore?

I spoke earlier about how important the nuclear pore is - and this is true. It has an essential function to carry out - import and export. Everything that is needed to manufacture DNA and RNA has to get into the nucleus, and each required thing enters through one of around 4000 nuclear pores. Similarly, every molecular of RNA produced to manufature proteins has to get out, and does so through one of the many proteins.

The nuclear pore is a kind of hole in the side of the nucleus which allows entrance of chemicals required in DNA and RNA manufacture (molecules such as ATP and the proteins required to make ribosome subunits), and the exit of RNA molecules and ribosomes subunits from the nucleus. Although we know a lot about what it needs to do, and what it does, we don't know a lot about how it does it! The nucleus is small enough as it is - if you get a light microscope (the kind found in science class at school), you can barely see the nucleus, and even a powerful light microscope will not make out the difference between heterochromatin and euchromatin. To see what's going on at a nuclear pore, then, is incredibly difficult, so what we do know is quite impressive!

From what we can make out, a nuclear pore is made up of about 100 subunits of proteins. This includes eight on the nucleoplasm side, and eight on the cytoplasm side, known as the inner and outer rings respectively. These are basically subunits of proteins which are on the inside and outside of the nuclear envelope. Protruding towards the centre of the pore from each of these subunits is something called a spoke, which is a mixture of yet more protein subunits. In the centre of these spokes is a plug, seen in orange in the images in this section. The spokes and plug mean that a pore, which can be over 100nm in diameter, will not let anything through if it is greater than 9nm. Another measure is weight - that is, anything heavier than around 60,000 Daltons will not get through easily.

That may sound very heavy, but 60,000 Daltons is not actually that much; most proteins and RNA molecules will weigh a lot more than that! So how do the RNA molecules get out? Well, anything that weighs less than 60,000 will happily get through through simple, passive diffusion. Anything above that weight (or bigger than 9nm) needs to have some help getting across. So, if a protein wants to get into the nucleus, it needs to have a nuclear import signal or nuclear localisation sequence.

When proteins are produced in the cell, they will all have a specific target - some go to the nucleus, some go to the endoplasmic reticulum, some want to leave the cell entirely etc. If something needs to go to the nucleus, it will be synthesised with a special sequence: this nuclear localisation sequence, which is something recognised by molecules on on the outside of the nuclear pore. These molecules bind to the protein entering the nucleus, and guide it through, using energy from the hydrolysis of ATP to get it through the small gap.

It may seem quite complicated, but the amount we understand is really quite limited. For example, we don't quite know how things get out of the nucleus - though we think the molecules which guide things into it can also guide things out. Still, we know what needs to go on at a nuclear pore, and we know that they're a very important part of the nucleus.