The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth

Abstract

Nuclear pore complexes (NPCs) are the only transport channels that cross the nuclear envelope. Constructed from ~500-1000 nucleoporin proteins each, they are among the largest macromolecular assemblies in eukaryotic cells. Thanks to advances in structural analysis approaches, the construction principles and architecture of the NPC have recently been revealed at submolecular resolution. Although the overall structure and inventory of nucleoporins are conserved, NPCs exhibit significant compositional and functional plasticity even within single cells and surprising variability in their assembly pathways. Once assembled, NPCs remain seemingly unexchangeable in post-mitotic cells. There are a number of as yet unresolved questions about how the versatility of NPC assembly and composition is established, how cells monitor the functional state of NPCs or how they could be renewed. Here, we review current progress in our understanding of the key aspects of NPC architecture and lifecycle.

Keywords: Brl1; FG repeats; NPC; Ran; ageing; aggregation; amphipathic helix; assembly factor; autophagy; lipids; membrane fusion; neurodegneration; nuclear pore complex; nuclear transport receptor; nucleoporin.

Figure 1

Inventory of the budding yeast and human nuclear pore complex. The nuclear pore complex (NPC) forms a channel connecting the nucleoplasm (bottom) with the cytoplasm (top) and is built of three concentric rings: the cytoplasmic outer ring, the inner ring, and the nucleoplasmic outer ring. The basic building blocks of the NPC are nucleoporins (NUPs), which are organized into several subcomplexes (boxes) and largely composed of only a few structural motifs (center bottom). The rigid subcomplexes are connected by disordered linkers. They contain short linear interaction motifs (SLiMs), which flexibly tie the NUPs and subcomplexes together. Multiple NUPs contain a lipid-binding amphipathic helix (AH) that helps tether the NPC to the lipid membrane. See text for details.

Figure 2

The lifecycle of the NPC. At the end of open mitosis, NUP recruitment to chromatin and membrane association is promoted through high local concentration of RanGTP. The NUPs seed the formation of NPCs by interacting with the re-forming NE. NPC assembly in interphase occurs “inside-out”—by inserting NPCs from the nuclear side into the sealed NE—and relies on the import of newly synthesized NUPs. It requires fusion of the inner and outer nuclear membranes by a poorly understood mechanism. The membrane fusion might involve phosphatidic acid (PA) rich membranes and the transmembrane proteins Brl1, Brr6, and Apq12 in budding yeast or torsin AAA+ ATPases in vertebrates. Cytoplasmic NUPs can join and complete NPC assembly only after successful membrane fusion. Failure in NPC assembly leads to stalled NPC intermediates (herniations) in the inner nuclear membrane enclosed by the NE and deprived of cytoplasmic NUPs. NPCs mature into compositionally and functionally different sub-populations, e.g., the budding yeast NPC can vary in the content of nuclear basket proteins or the number of nuclear Y rings. Damaged NPCs can be repaired in a “piecemeal” manner by proteasomal degradation of individual NUPs without the requirement for complete NPC disassembly. Entire NPCs can be degraded by the autophagy machinery. NPCs can accumulate damage in old age or disease, such as oxidative damage, loss of NUPs or phase-transition of FG NUPs in the cytoplasm, which leads to NPC malfunction and impaired transport. See text for details.