The NPC is a highly modular and complex structure, organized into discrete Nup subcomplexes that form the cytoplasmic filaments, central scaffold, and nuclear basket. New NPCs must continually form in the nuclear envelope to maintain a consistent number of NPCs throughout the cell cycle. Correct assembly and function of the NPC are critical to regulated gene expression and nuclear periphery architecture.
NPCs are formed in two processes: post-mitotic biogenesis and de novo biogenesis. An ordered mechanism of post-mitotic assembly has been described. First, the NE breaks down and is absorbed by the ER to allow for proper cell division. Specific Nups bind directly to chromatin during this time to form a pre-assembly complex. Once the nucleus has divided, the NE reforms from cisternal ER and sites of NPC assembly are seeded on the NE by the pre-assembly complex. At these sites of assembly, membrane components of the NPC, such as Ndc1 and other pore membrane (POM) proteins, are recruited to provide an anchor upon which the remaining Nup subcomplexes assemble, ultimately forming a mature NPC.
During interphase in higher eukarotes, and in organisms undergoing a closed mitosis, NPC biogenesis requires stepwise de novo assembly of NPC components into an intact nuclear envelope. First, the intact double membrane of the NE must fuse to allow the formation of a pore. Exactly how the double lipid bilayer is fused to form a nascent pore is not understood. One theory proposes that Poms mediate fusion of lipid bilayers through juxtapositioning of membranes and induction of curvature. Lumenal proteins or lumenal interactions of the Poms could further mediate membrane deformation. The membranes of both nascent pores and fully formed NPCs contain positive and negative curvature which must be stabilized. We uncovered a role at this step for Rtn1 and Yop1, two hairpin shaped ER membrane-bending proteins that are essential to formation of tubular ER. Similarly, coatamer-like proteins (such as the Nup84 subcomplex) and lipid composition could provide curvature to pore membranes. Following pore formation, the fused membrane is stabilized by structural NPC components that form a stable scaffold upon which the peripheral Nups assemble to generate a mature NPC.