Figure 1:Molecular trafficking between the nucleus and the cytoplasm of interphase cells via the nuclear pore complexes (NPCs). The NPCs allow passive diffusion of ions and small molecules. Nuclear proteins, RNAs, and ribonucleoprotein (RNP) particles larger than ~9 nm are selectively and actively transported through NPCs by a signal-mediated and energy-dependent mechanism

The structure of the NPC has been extensively investigated by electron microscopy (EM), and a consensus model of its central framework has emerged (Fig. 2). Accordingly, the vertebrate NPC exhibits an 8-fold symmetric (i.e., perpendicular to the plane of the NE) tripartite architecture with a total mass of ~125 MDa. Its ~55 MDa central framework is a ring-like assembly built of eight multi-domain spokes consisting of two roughly identical halves each so that its asymmetric unit (i.e., one half-spoke) represents one 16th of its mass or roughly the size of a ribosome. This central framework is sandwiched between a ~32 MDa cytoplasmic ring and a ~21 MDa nuclear ring. From the cytoplasmic ring eight short, kinky fibrils emanate, whereas the nuclear ring anchors a basket (or fishtrap), assembled from eight thin, ~50 nm long filaments joined distally by a 30- to 50-nm-diameter ring. The ring-like, ~822-symmetric central framework embraces the central pore of the NPC which acts as a gated channel. 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.

Figure 2: Surface renderings of the nuclear pore complex architecture. A 3-D reconstruction of detergent extracted and negatively stained nuclear pore complexes (NPCs; Hinshaw et al., 1992) revealing the central framework which exhibits strong 822 symmetry and thus consists of two identical halves relative to the central plane of the nuclear envelope (NE).

Employing time-lapse atomic force microscopy (AFM) of native Xenopus oocyte NEs in buffer solution, the repeated opening and closing of the nuclear baskets in response to adding and removing micromolar amounts of calcium was monitored (Fig. 3), an event most likely involving the basket's distal ring acting as a calcium-sensitive iris-like diaphragm (Stoffler et al., 1999). 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.

Figure 3: top:cryo-electron microscopy of thick-ice embedded (i.e., 250 nm), completely unfixed and unstained NEs revealing individual nuclear pore complexes. The arrow highlights a well preserved NPC whose nuclear basket and distal ring is clearly visible. bottom: Imaging reversible calcium-mediated structural changes of native NPCs by AFM in contact mode. The distal ring (see arrow) might act as an iris-like diaphragm, opening upon addition of millimolar amounts of calcium and closing upon removal of calcium.

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