THE NUCLEAR PORE COMPLEX: CONDUIT, PUMP OR TURNSTILE?

T.D. Allen, S.A. Rutherford, G.R. Bennion, E. Kiseleva and M.W. Goldberg
CRC, Dept. of Structural Cell Biology, Paterson Institute, Christie Hospital, Manchester, M20 9BX, UK

Genetic information in eucaryotic cells is enclosed by a double membrane, the nuclear envelope, which provides an effective barrier between nuclear contents and the surrounding cytoplasm. Bidirectional transport of material between nucleus and cytoplasm takes place at specific sites (nuclear pore complexes) which allow passive diffusion of small molecules, but has an energy requirement for the transport of larger proteins. In the case of DNA precursors, the precise amount of nuclear import is controlled by the pore complexes, which become impermeable to further precursor import once the DNA has been precisely duplicated prior to cell division.

Each nuclear pore complex is a major molecular structure, measuring 100 nms in diameter, and a molecular mass of 125 million daltons, involving up to a hundred different proteins of which only around a dozen are currently characterized. Despite its enormous structure, each pore complex is effectively dismantled and rebuilt within the 1 hr time span of mitotic division in eucaryotic cells. Nuclear pore complexes also change their numbers and position throughout the cell cycle, and their transport rates and gate sizes in differing biological circumstances. Rates of transport are rapid (up to 100 molecules per minute in ribosomal synthesis alone), and although the biochemical aspects of transport are currently under intense investigation, sites of reaction in the pore and molecular mechanics of transport remain obscure. This communication will report our current contribution to molecular structure and function in the nuclear pore complex.