The nuclear envelope is studded with pores which allow regulated transport of macromolecules into and out of the nucleus. Nuclear pores form through the double membrane of the nuclear envelope, and NPCs are assembled at the site of the pore. Higher eukaryotes undergo an "open mitosis," in which the nuclear envelope and NPCs break down during mitosis and later reassemble. Yeast, however, undergo a "closed mitosis" in which the nuclear envelope and NPCs remain intact during cell division. In each case, however, new pores and pore complexes must continually form in the nuclear envelope to maintain a consistent number of NPCs throughout the cell cycle. The yeast Saccharomyces cerevisiae NPC is composed of multiple copies of ~30 proteins. Because of this complexity, NPC assembly is predicted to be a highly ordered process. However, basic questions regarding membrane fusion, nucleation of NPCs, and the ordered addition of factors or substructures have not been answered.
Adapted from Suntharalingam and Wente, 2003
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