Nucleo cytoplasmic trafficking
a. Nuclear Transport Basics
A hallmark of eukaryotes is the compartmentalization of the cell. One of the gained compartments, the nucleus, enables the cell to separate nuclear transcription from the cytoplasmic translation. This offers a multitude of additional control mechanisms for these processes. The downside of the spatial separation is the need to transport macromolecules, like proteins and ribonucleic acid (RNA) across this selective membrane (Fig.1). This process is essential for cellular function and requires an active nuclear-cytoplasmic transport machinery. This machinery is based on soluble transport receptors, which recognize and bind cargoes in one compartment, mediate their transport through so called nuclear pore complexes (NPCs) which are embedded in the double membrane of the nucleus (Fig. 2) and release the cargoes in the target compartment after traverse. NPC are large macromolecular machines of around 1.2 MDa composed of up to 30 different proteins. They come in multiples of eight, due to the eightfold symmetry of the NPC.
Fig.1. Exchange of molecules across the nuclear envelope.
mRNA has to be exported into the cytoplasm in order to be translated, tRNA and ribosomes have to be exported as well, whereas regulatory proteins as well as histones and ribosomal proteins have to be imported into the nucleus.