Transport between the cell nucleus and the cytoplasm

The compartmentation of eukaryotic cells requires all nuclear proteins to b e imported from the cytoplasm, whereas, for example, transfer RNAs, messeng er RNAs, and ribosomes are made in the nucleus and need to be exported to t he cytoplasm. Nuclear import and export proceed through nuclear pore comple xes and can occur along a great number of distinct pathways, many of which are mediated by importin beta-related nuclear transport receptors. These re ceptors shuttle between nucleus and cytoplasm, and they bind transport subs trates either directly or via adapter molecules. They all cooperate with th e RanGTPase system to regulate the interactions with their cargoes. Another focus of our review is nuclear export of messenger RNA, which apparently l argely relies on export mediators distinct from importin beta-related facto rs. We discuss mechanistic aspects and the energetics of transport receptor function and describe a number of pathways in detail.