ABSTRACT
For a eukaryotic cell to function, its genome has to communicate with its cytoplasm. The genome however is surrounded by a double membrane which is impermeable to the necessary molecules, which either regulate nuclear functions or are a result of nuclear activities. Assembly of a nuclear pore complex (NPC) causes the fusion of the inner and outer membranes of the nuclear envelope (NE) resulting in a route of passage across the membranes. This ‘hole’ is however rapidly plugged with a beautiful, highly symmetrical protein complex (Figure 1), consisting of about 30 different proteins in multiple copies (Cronshaw et al., 2002; Rout et al., 2000). The number of proteins is in fact rather smaller than expected considering the gargantuan size of the NPC which is 90-120 nm in diameter (Akey and Radermacher, 1993; Goldberg and Allen, 1996; Hinshaw et al., 1992, Kiseleva et al., 1998; Yang et al., 1998) and over 100 000 kDa in vertebrates (Reichelt et al., 1990). This relative compositional simplicity can be explained by the NPC’s symmetrical character as it appears to consist of eight identical radially arranged subunits. There is also symmetry within each subunit, something that is emphasized by immuno-gold electron microscopy, where the majority of nucleoporins (nuclear pore proteins) appear to be located on both nucleoplasmic and cytoplasmic sides of the NPC (Rout et al., 2000).
