Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press The Nuclear Pore Complex and Nuclear Transport Susan R. Wente1 and Michael P. Rout2 1Department of Cell and Developmental Biology, Vanderbilt University Medical Center, Nashville, Tennessee 37232 2Laboratory of Cellular and Structural Biology, The Rockefeller University, New York, New York 10065 Correspondence: [email protected] and [email protected] Internal membrane bound structures sequester all genetic material in eukaryotic cells. The most prominent of these structures is the nucleus, which is bounded by a double membrane termed the nuclear envelope (NE). Though this NE separates the nucleoplasm and genetic material within the nucleus from the surrounding cytoplasm, it is studded throughout with portals called nuclear pore complexes (NPCs). The NPC is a highly selective, bidirectional transporter for a tremendous range of protein and ribonucleoprotein cargoes. All the while the NPC must prevent the passage of nonspecific macromolecules, yet allow the free diffu- sion of water, sugars, and ions. These many types of nuclear transport are regulated at mul- tiple stages, and the NPC carries binding sites for many of the proteins that modulate and modify the cargoes as they pass across the NE. Assembly, maintenance, and repair of the NPC must somehow occur while maintaining the integrity of the NE. Finally, the NPC appears to be an anchor for localization of many nuclear processes, including gene acti- vation and cell cycle regulation. All these requirements demonstrate the complex design of the NPC and the integral role it plays in key cellular processes. axonomically speaking, all life on earth falls eukaryotes allowed them to adopt a multicel- Tinto one of two fundamental groups, the lular lifestyle, as seen in the plants, fungi and prokaryotes and the eukaryotes. The prokar- animals of today (reviewed in Field and Dacks yotes, the first group to evolve, are single cell 2009). organisms bounded by a single membrane. Internal membrane bound structures se- About 1.5 billion years later, a series of evo- quester all genetic material in eukaryotic cells. lutionary innovations led to the emergence The most prominent of these structures, which of eukaryotes. Eukaryotes have multiple inner gives the eukaryotes their Greek-rooted name, is membrane structures that allow for compart- the nucleus—the central “kernel” (gr. “karyo-”) mentalization within the cell, and therefore dif- of the cell. The nucleus is bounded by a double ferentiation of the cell and regulation within it. membrane termed the nuclear envelope (NE), Ultimately, the greater cellular complexity of which separates the nucleoplasm and genetic Editors: Tom Misteli and David L. Spector Additional Perspectives on The Nucleus available at www.cshperspectives.org Copyright # 2010 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a000562 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a000562 1 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press S.R. Wente and M.P. Rout material from the surrounding cytoplasm. 100–150 nm in diameter and 50–70 nm in However the genetic material in the nucleus is thickness depending on the organism (reviewed not totally isolated from the rest of the cell. in Wente 2000; Lim et al. 2008). This overall ap- Studded throughout the NE are portals called pearance seems broadly conserved throughout nuclear pore complexes (NPCs). The NPC is a all eukaryotes. The two membranes of the NE, highly selective, bidirectional transporter for a the outer and inner membranes, join only in a tremendous range of cargoes. Going into the specialized, sharply curved piece of “pore nucleus, these cargoes include inner nuclear membrane” that forms a grommet in the NE membrane proteins and all the proteins in the within which the NPC sits. Within each NPC nucleoplasm. Going out are RNA-associated is a core structure containing eight spokes proteins that are assembled into ribosomal sub- surrounding a central tube. This central hole units or messenger ribonucleoproteins (mRNPs). (30 nm diameter and 50 nm long) is where Once transported, the NPC must ensure these the nucleoplasm connects to the cytoplasm cargos are retained in their respective nuclear and where macromolecular exchange occurs. and cytoplasmic compartments. All the while Peripheral filaments are attached to the core, the NPC must prevent the passage of nonspe- filling the central hole as well as emanating cific macromolecules, yet allow the free diffu- into the nucleoplasm and cytoplasm. These fil- sion of water, sugars, and ions. These many aments form a basket-like structure on the types of nuclear transport are regulated at mul- nuclear side of the NPC (Fig. 1). tiple stages, providing a powerful extra level of One can envision the NPC as being com- cellular control that is not necessary in prokar- prised of layers of interacting proteins, starting yotes. Assembly, maintenance, and repair of with the core structure, moving outwards the NPC must somehow occur while maintain- through its peripheral filaments, and then to ing the integrity of the NE. Finally, the NPC ap- associating clouds of soluble transport factors pears to be an anchor for localization of many and peripherally associating protein complexes nuclear processes, including gene activation in the nucleus and cytoplasm (Rout and Aitch- and cell cycle regulation (reviewed in Ahmed ison 2001). These protein interactions can oc- and Brickner 2007; Hetzer and Wente 2009). cur on radically different time scales. Some All these requirements demonstrate the com- proteins form relatively permanent associations plex design of the NPC and the integral role it with the core structure, and so are termed nu- plays in key cellular processes. clear pore complex components or “nucleo- porins” (“Nups”). Other proteins associate transiently with the NPC, either constantly STRUCTURE OF THE NPC: SET UP OF THE cycling on and off or attaching only at particular MACHINE times in the cell’s life cycle. The NPC is covered The specifications of the NPC’s transport mac- in binding sites for these transiently associating hinery represent a huge engineering challenge proteins. Because the NPC is neither a motor for evolution. No transitional forms of this elab- nor an enzyme, the interactions provided by orate transport system have yet been found in its binding sites wholly define the function of modern day organisms to reveal how it evolved. the NPC. However, recent clues show that the NPC itself Recent work, mainly in the yeast Saccharo- retains in its core a fossil of its ancient origins, myces cerevisiae and in vertebrates, has begun indicating that the same mechanism that gen- to elucidate the molecular architecture of erated the internal membranes of eukaryotes the NPC (Rout et al. 2000; Cronshaw et al. might also have been responsible for the NPCs 2002; Alber et al. 2007b). Given its large size, and the transport machinery. the main body of the NPC comprises a surpris- In the electron microscope, the NPC ap- ingly small number of 30 unique proteins pears as a complex cylindrical structure with (Table 1). However, because of the NPC’s eight- strong octagonal symmetry, measuring some fold symmetry, these Nups are each present in 2 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a000562 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Nuclear Pore Complexes and Nuclear Transport Central Cytoplasmic tube filaments Cytoplasmic FG nups Core Outer ring Symmetric scaffold Inner ring FG nups Lumenal ring Nuclear envelope Pore membrane Nucleoplasmic Linker FG nups nups Nuclear basket Figure 1. Major structural features of the NPC (based on the architectural map of Alber et al. (2007b); see Table1 and main text for details). multiple copies (usually 16 per NPC) resulting To understand its evolutionary origins, the in around 400 polypeptides for each NPC in NPC of the highly divergent Trypanosoma was every eukaryote (Rout et al. 2000; Cronshaw recently characterized (DeGrasse et al. 2009). et al. 2002; DeGrasse et al. 2009). Further redun- Despite significant divergence in primary struc- dancy is evident from the recent mapping of the ture, the Trypanosome NPC consists mainly of yeast NPC. Indeed, the NPC’s structure is mod- the motifs and domains found in vertebrate ular, consisting of a few highly repetitive protein and yeast NPCs, indicating on a molecular fold types (Devos et al. 2006; Alber et al. 2007b; level that the basic structural components of DeGrasse et al. 2009). This suggests that the the NPC are conserved across all eukaryotes. bulk of the NPC’s structure has evolved through Importantly, this also strongly implies that the multiple duplications of a small precursor set of last common eukaryotic ancestor had many fea- genes encoding just a handful of progenitor tures in common with contemporary NPCs, Nups. and perhaps provided a key adaptive advantage Table 1. Nucleoporin homologs of yeast and vertebrates NPC substructure Yeast components Vertebrate components Outer Ring Nup84 subcomplex (Nup84, Nup107-160 complex (Nup160, Nup85, Nup120, Nup133, Nup133, Nup107, Nup96, Nup75, Nup145C, Sec13, Seh1) Seh1, Sec13, Aladin, Nup43, Nup37) Inner Ring Nup170 subcomplex (Nup170, Nup155 subcomplex (Nup155, Nup205, Nup157, Nup188, Nup192, Nup188, Nup35) Nup59, Nup53) Cytoplasmic FG Nups Nup159, Nup42 Nup358, Nup214, Nlp1 and Filaments Lumenal Ring Ndc1, Pom152, Pom34 Gp210, Ndc1, Pom121 Symmetric FG Nups Nsp1, Nup57, Nup49, Nup145N, Nup62, Nup58/45, Nup54, Nup98 Nup116, Nup100 Linker Nups Nup82, Nic96 Nup88, Nup93 Nucleoplasmic FG Nups Nup1, Nup60, Mlp1, Mlp2 Nup153, Tpr and Filaments Cite this article as Cold Spring Harb Perspect Biol 2010;2:a000562 3 Downloaded from http://cshperspectives.cshlp.org/ on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press S.R. Wente and M.P. Rout for this organism that has been retained, little 2009), although nearly 2 billion years of evolu- changed, ever since.