The NPCs allow passive diffusion of ions and small molecules. The central pore is often plugged with a distinct particle, called central plug or transporter, of highly variable appearance whose molecular architecture and functional significance remain to be established.
Surface renderings of the nuclear pore complex architecture.
In support of this possibility, Nup153, a nucleoporin being a constituent of the distal ring and consisting of three distinct domains, was shown to play a role in protein import as well as in mRNA export. It is conceivable that Nup153, which forms an octameric complex, represents part of a central framework or scaffold of the distal ring. In contrast, the cytoplasmic NPC topography appeared rather insensitive to calcium. The arrow highlights a well preserved NPC whose nuclear basket and distal ring is clearly visible. Imaging reversible calcium-mediated structural changes of native NPCs by AFM in contact mode.