Structural plasticity of the nuclear pore complex.

Abstract

The nuclear pore complex (NPC) is strategically located at continuous junctions of the inner and outer nuclear membranes to catalyze macromolecular transport, without impending the diffusion of small molecules. In this paper, the structural plasticity of 4412 NPCs in isolated nuclear envelopes has been evaluated, utilizing correspondence analysis, classification and difference mapping. The data are grouped into seven clusters comprising two major groups, based on the degree of radial compaction within spokes and the symmetry of the inner spoke ring. The results have been correlated with differences in spoke domain packing observed in two published three-dimensional maps suggesting that symmetrical detergent-extracted NPCs are similar, but not identical to the most probable in vivo structure. A model is proposed in which spoke architecture is responsive to changes in the turgor pressure of the nuclear envelope. For example, detergent extraction may allow the outward facing domains of each spoke to adopt a radially-extended configuration while osmotic swelling may induce an inwards displacement, resulting in a radially compact spoke. Difference maps between approximately 822 symmetric projections of NPCs in membranes and after detergent-extraction have localized the nuclear envelope border. The data place limits on the radial and circumferential dimensions of diffusion channels (approximately 0 to 20 A x 190 A), proposed to reside at the pore periphery. The results confirm the observation that each spoke penetrates the nuclear envelope, linking up with the radial arms to form a "lumenal ring". Finally, putative closed, open and in-transit forms of the transporter are found with the same relative frequency in membrane-associated NPCs with radially compact or extended spokes; hence spoke deformations in isolated envelopes may be induced by experimental factors. However, concerted movements of the spoke domains (if reversible) may be utilized in the biological function of the NPC and some examples are given.