The nuclear pore complex: mediator of translocation between nucleus and cytoplasm

The enclosure of nuclear contents in eukaryotes means that cells require si tes in the boundary that mediate exchange of material between nucleus and c ytoplasm, These sites, termed nuclear pore complexes (NPCs), number 100-200 in yeast, a few thousand in mammalian cells and similar to 50 million in t he giant nuclei of amphibian oocytes. NPCs are large (125 MDa) macromolecul ar complexes that comprise 50-100 different proteins in vertebrates, In spi te of their size and complex structure, NPCs undergo complete breakdown and reformation at cell division. Transport through NPCs can be rapid (estimat ed at several hundred molecules/pore/second) and accommodates both passive diffusion of relatively small molecules, and active transport of complexes up to several megadaltons in molecular mass. Each pore can facilitate both import and export. The two processes apparently involve multiple pathways f or different cargoes, and their transport signals, transport receptors and adapters, and the molecules (and their regulators) that underpin the transp ort mechanisms, Over the past few years there has been an increasing intere st in the pore complex: structural studies have been followed by elucidatio n of the biochemical aspects of nuclear import, and subsequent investigatio ns into nuclear export. The current challenge is to understand the interact ions between the structural elements of the pore complex and the mechanisms that drive the physical processes of translocation through it.