Figure 12-11. Possible paths for free diffusion through the nuclear pore complex.

Figure 12-11Possible paths for free diffusion through the nuclear pore complex

This drawing shows a hypothetical diaphragm (gray) inserted into the pore to restrict the size of the open channel to 9 nm, the pore size estimated from diffusion measurements. Nine nanometers is a much smaller diameter than that of the central opening apparent on the images of the nuclear pore complex derived from electron micrographs. It is also smaller than the opening estimated during active transport, when the pore dilates to allow the transport of particles of up to 26 nm in diameter (arrow). Thus, it is likely that some pore components are lost during the preparation of specimens for electron microscopy, and that these normally restrict free diffusion through the central opening. Such components may form a diaphragm (or plug) that opens and closes to allow the passage of large objects during active transport, which depends on sorting signals (discussed below). Although plugs can be seen in some preparations, it is not clear whether they are components of the pore complex or material that is being transported through it. Three-dimensional computer reconstructions suggest that the channels permitting free diffusion might be located near the rim of the pore complex, between the column subunits, rather than at its center (see Figure 12-10A); this would mean that passive diffusion and active transport take place through different parts of the complex.

From: The Transport of Molecules between the Nucleus and the Cytosol

Cover of Molecular Biology of the Cell
Molecular Biology of the Cell. 4th edition.
Alberts B, Johnson A, Lewis J, et al.
New York: Garland Science; 2002.
Copyright © 2002, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter; Copyright © 1983, 1989, 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson .

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