ELYS/MEL-28 chromatin association coordinates nuclear pore complex assembly and replication licensing

Both Chromosome Decondensation and Condensation Are Dependent on DNA Replication in C. elegans Embryos

During cell division, chromatin alternates between a condensed state to facilitate chromosome segregation and a decondensed form when DNA replicates. In most tissues, S phase and mitosis are separated by defined G1 and G2 gap phases, but early embryogenesis involves rapid oscillations between replication and mitosis. Using Caenorhabditis elegans embryos as a model system, we show that chromosome condensation and condensin II concentration on chromosomal axes require replicated DNA. In addition, we found that, during late telophase, replication initiates on condensed chromosomes and promotes the rapid decondensation of the chromatin. Upon replication initiation, the CDC-45-MCM-GINS (CMG) DNA helicase drives the release of condensin I complexes from chromatin and the activation or displacement of inactive MCM-2-7 complexes, which together with the nucleoporin MEL-28/ELYS tethers condensed chromatin to the nuclear envelope, thereby promoting chromatin decondensation. Our results show how, in an early embryo, the chromosome-condensation cycle is functionally linked with DNA replication.

Intranuclear dynamics of the Nup107-160 complex

The Nup107-160 nuclear pore subcomplex (Y-complex) and the chromatin-binding nucleoporin Elys dynamically colocalize with Nup98 and the export factor CRM1 in nuclear GLFG bodies present in HeLa sublines. Thus, in addition to its structural role at the NPC and its mitotic functions, the Y-complex may also act inside the nucleus during interphase.

Nup98 is a glycine-leucine-phenylalanine-glycine (GLFG) repeat–containing nucleoporin that, in addition to nuclear transport, contributes to multiple aspects of gene regulation. Previous studies revealed its dynamic localization within intranuclear structures known as GLFG bodies. Here we show that the mammalian Nup107-160 complex (Y-complex), a major scaffold module of the nuclear pore, together with its partner Elys, colocalizes with Nup98 in GLFG bodies. The frequency and size of GLFG bodies vary among HeLa sublines, and we find that an increased level of Nup98 is associated with the presence of bodies. Recruitment of the Y-complex and Elys into GLFG bodies requires the C-terminal domain of Nup98. During cell division, Y-Nup–containing GLFG bodies are disassembled in mitotic prophase, significantly ahead of nuclear pore disassembly. FRAP studies revealed that, unlike at nuclear pores, the Y-complex shuttles into and out of GLFG bodies. Finally, we show that within the nucleoplasm, a fraction of Nup107, a key component of the Y-complex, displays reduced mobility, suggesting interaction with other nuclear components. Together our data uncover a previously neglected intranuclear pool of the Y-complex that may underscore a yet-uncharacterized function of these nucleoporins inside the nucleus, even in cells that contain no detectable GLFG bodies.

Nuclear pore proteins and the control of genome functions

Nuclear pore complexes (NPCs) are composed of several copies of ∼30 different proteins called nucleoporins (Nups). NPCs penetrate the nuclear envelope (NE) and regulate the nucleocytoplasmic trafficking of macromolecules. Beyond this vital role, NPC components influence genome functions in a transport-independent manner. Nups play an evolutionarily conserved role in gene expression regulation that, in metazoans, extends into the nuclear interior. Additionally, in proliferative cells, Nups play a crucial role in genome integrity maintenance and mitotic progression. Here we discuss genome-related functions of Nups and their impact on essential DNA metabolism processes such as transcription, chromosome duplication, and segregation.

Nuclear envelope and chromatin, lock and key of genome integrity

More than as an inert separation between the inside and outside of the nucleus, the nuclear envelope (NE) constitutes an active toll, which controls the import and export of molecules, and also a hub for a diversity of genomic processes, such as transcription, DNA repair, and chromatin dynamics. Proteins localized at the inner surface of the NE (such as lamins, nuclear pore proteins, lamin-associated proteins) interact with chromatin in a dynamic manner, contributing to the establishment of topological domains. In this review, we address the complex interplay between chromatin and NE. We discuss the divergence of this cross talk during evolution and comment both on the current established models and the most recent findings. In particular, we focus our attention on how the NE cooperates with chromatin in protecting the genome integrity.

Beyond ubiquitination: proteolytic and nonproteolytic roles of HOS1

The E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1) functions as a cold signaling attenuator by degrading the INDUCER OF CBF EXPRESSION 1 transcription factor, which is a key regulator of the cold-induced transcriptome and freezing tolerance in plants. Recent studies demonstrate that HOS1 also plays nonproteolytic roles in gene expression regulation. HOS1 acts as a chromatin remodeling factor that modulates FLOWERING LOCUS C chromatin in cold regulation of flowering time. It associates with the nuclear pore complex to facilitate nucleocytoplasmic mRNA export to maintain circadian periodicity over a range of light and temperature conditions. In this review, we summarize recent advances in molecular mechanisms underlying HOS1 function during plant development in response to fluctuating environmental conditions.

Nucleosomal regulation of chromatin composition and nuclear assembly revealed by histone depletion

Nucleosomes are the fundamental unit of chromatin, but analysis of transcription-independent nucleosome functions has been complicated by the gene-expression changes resulting from histone manipulation. Here we solve this dilemma by developing Xenopus laevis egg extracts deficient for nucleosome formation and by analyzing the proteomic landscape and behavior of nucleosomal chromatin and nucleosome-free DNA. We show that although nucleosome-free DNA can recruit nuclear-envelope membranes, nucleosomes are required for spindle assembly and for formation of the lamina and of nuclear pore complexes (NPCs). We show that, in addition to the Ran G-nucleotide exchange factor RCC1, ELYS, the initiator of NPC formation, fails to associate with naked DNA but directly binds histone H2A-H2B dimers and nucleosomes. Tethering ELYS and RCC1 to DNA bypasses the requirement for nucleosomes in NPC formation in a synergistic manner. Thus, the minimal essential function of nucleosomes in NPC formation is to recruit RCC1 and ELYS.

Nucleosome assembly is required for nuclear pore complex assembly in mouse zygotes

Packaging of DNA into nucleosomes not only helps to store genetic information but also creates diverse means for regulating DNA-templated processes. Attempts to reveal additional functions of the nucleosome have been unsuccessful, owing to cell lethality caused by nucleosome deletion. Taking advantage of the mammalian fertilization process, in which sperm DNA assembles into nucleosomes de novo, we generated nucleosome-depleted (ND) paternal pronuclei by depleting maternal histone H3.3 or its chaperone HIRA in mouse zygotes. We found that the ND pronucleus forms a nuclear envelope devoid of nuclear pore complexes (NPCs). Loss of NPCs is accompanied by defective localization of ELYS, a nucleoporin essential for NPC assembly, to the nuclear rim. Interestingly, tethering ELYS to the nuclear rim of the ND nucleus rescues NPC assembly. Our study thus demonstrates that nucleosome assembly is a prerequisite for NPC assembly during paternal pronuclear formation.

Xenopus Cdc7 executes its essential function early in S phase and is counteracted by checkpoint-regulated protein phosphatase 1

The initiation of DNA replication requires two protein kinases: cyclin-dependent kinase (Cdk) and Cdc7. Although S phase Cdk activity has been intensively studied, relatively little is known about how Cdc7 regulates progression through S phase. We have used a Cdc7 inhibitor, PHA-767491, to dissect the role of Cdc7 in Xenopus egg extracts. We show that hyperphosphorylation of mini-chromosome maintenance (MCM) proteins by Cdc7 is required for the initiation, but not for the elongation, of replication forks. Unlike Cdks, we demonstrate that Cdc7 executes its essential functions by phosphorylating MCM proteins at virtually all replication origins early in S phase and is not limiting for progression through the Xenopus replication timing programme. We demonstrate that protein phosphatase 1 (PP1) is recruited to chromatin and rapidly reverses Cdc7-mediated MCM hyperphosphorylation. Checkpoint kinases induced by DNA damage or replication inhibition promote the association of PP1 with chromatin and increase the rate of MCM dephosphorylation, thereby counteracting the previously completed Cdc7 functions and inhibiting replication initiation. This novel mechanism for regulating Cdc7 function provides an explanation for previous contradictory results concerning the control of Cdc7 by checkpoint kinases and has implications for the use of Cdc7 inhibitors as anti-cancer agents.

NETs and cell cycle regulation

There are many ways that the nuclear envelope can influence the cell cycle. In addition to roles of lamins in regulating the master cell cycle regulator pRb and nuclear envelope breakdown in mitosis, many other nuclear envelope proteins influence the cell cycle through regulatory or structural functions. Of particular note among these are the nuclear envelope transmembrane proteins (NETs) that appear to influence cell cycle regulation through multiple separate mechanisms. Some NETs and other nuclear envelope proteins accumulate on the mitotic spindle, suggesting functional or structural roles in the cell cycle. In interphase exogenous overexpression of some NETs promotes an increase in G1 populations, while others promote an increase in G2/M populations, sometimes associated with the induction of senescence. Intriguingly, most of the NETs linked to the cell cycle are highly restricted in their tissue expression; thus, their misregulation in cancer could contribute to the many tissue-specific types of cancer.

Ran GTPase in nuclear envelope formation and cancer metastasis

Ran is a small ras-related GTPase that controls the nucleocytoplasmic exchange of macromolecules across the nuclear envelope. It binds to chromatin early during nuclear formation and has important roles during the eukaryotic cell cycle, where it regulates mitotic spindle assembly, nuclear envelope formation and cell cycle checkpoint control. Like other GTPases, Ran relies on the cycling between GTP-bound and GDP-bound conformations to interact with effector proteins and regulate these processes. In nucleocytoplasmic transport, Ran shuttles across the nuclear envelope through nuclear pores. It is concentrated in the nucleus by an active import mechanism where it generates a high concentration of RanGTP by nucleotide exchange. It controls the assembly and disassembly of a range of complexes that are formed between Ran-binding proteins and cellular cargo to maintain rapid nuclear transport. Ran also has been identified as an essential protein in nuclear envelope formation in eukaryotes. This mechanism is dependent on importin-β, which regulates the assembly of further complexes important in this process, such as Nup107-Nup160. A strong body of evidence is emerging implicating Ran as a key protein in the metastatic progression of cancer. Ran is overexpressed in a range of tumors, such as breast and renal, and these perturbed levels are associated with local invasion, metastasis and reduced patient survival. Furthermore, tumors with oncogenic KRAS or PIK3CA mutations are addicted to Ran expression, which yields exciting future therapeutic opportunities.

Interaction of Nup53 with Ndc1 and Nup155 is required for nuclear pore complex assembly

Nuclear pore complexes (NPCs) are the gateways for nucleocytoplasmic exchange. The ordered assembly of these huge complexes from several hundred individual components into an intricate protein interaction network which deforms the two membranes of the nuclear envelope into a pore is only rudimentarily understood. Here, we show that the interaction between Nup53 and the integral pore membrane protein Ndc1 is essential for vertebrate NPC assembly. The Ndc1 binding site on Nup53 overlaps with a region that induces membrane bending and is specifically required to modulate this activity, suggesting that the membrane-deforming capability of Nup53 is adjusted during the NPC assembly process. We further demonstrate that the interaction of Nup53 and Nup155 has a crucial role in NPC formation as the main determinant of recruitment of Nup155 to the assembling pore. Overall, our results pinpoint the diversity of interaction modes accomplished by Nup53, highlighting this protein as an essential link between the pore membrane and the NPC, and as a crucial factor in the formation of the pore membrane.

Replisome stall events have shaped the distribution of replication origins in the genomes of yeasts

During S phase, the entire genome must be precisely duplicated, with no sections of DNA left unreplicated. Here, we develop a simple mathematical model to describe the probability of replication failing due to the irreversible stalling of replication forks. We show that the probability of complete genome replication is maximized if replication origins are evenly spaced, the largest inter-origin distances are minimized, and the end-most origins are positioned close to chromosome ends. We show that origin positions in the yeast Saccharomyces cerevisiae genome conform to all three predictions thereby maximizing the probability of complete replication if replication forks stall. Origin positions in four other yeasts-Kluyveromyces lactis, Lachancea kluyveri, Lachancea waltii and Schizosaccharomyces pombe-also conform to these predictions. Equating failure rates at chromosome ends with those in chromosome interiors gives a mean per nucleotide fork stall rate of ∼5 × 10(-8), which is consistent with experimental estimates. Using this value in our theoretical predictions gives replication failure rates that are consistent with data from replication origin knockout experiments. Our theory also predicts that significantly larger genomes, such as those of mammals, will experience a much greater probability of replication failure genome-wide, and therefore will likely require additional compensatory mechanisms.

Nuclear pore complexes in the maintenance of genome integrity

Maintaining genome integrity is crucial for successful organismal propagation and for cell and tissue homeostasis. Several processes contribute to safeguarding the genomic information of cells. These include accurate replication of genetic information, detection and repair of DNA damage, efficient segregation of chromosomes, protection of chromosome ends, and proper organization of genome architecture. Interestingly, recent evidence shows that nuclear pore complexes, the channels connecting the nucleus with the cytoplasm, play important roles in these processes suggesting that these multiprotein platforms are key regulators of genome integrity.

Structural and functional studies of the 252 kDa nucleoporin ELYS reveal distinct roles for its three tethered domains

In metazoa, the nuclear envelope (NE), together with the embedded nuclear pore complexes (NPCs), breaks down and reassembles during cell division. It is suggested that ELYS, a nucleoporin, binds to chromatin in an initial step of postmitotic NPC assembly and subsequently recruits the essential Y-subcomplex, the major scaffolding unit of the NPC. Here, we show that ELYS contains three domains: an N-terminal β-propeller domain, a central α-helical domain, and a C-terminal disordered region. While the disordered region is responsible for the interactions with chromatin, the two preceding domains synergistically mediate tethering to the NPC. We present the crystal structure of the seven-bladed β-propeller domain at 1.9 Å resolution. Analysis of the β-propeller surface reveals the regions that are required for NPC anchorage. We discuss the possible roles of ELYS in the context of the NPC scaffold architecture.

Regulation and coordination of nuclear envelope and nuclear pore complex assembly

In metazoans with “open” mitosis, cells undergo structural changes involving the complete disassembly of the nuclear envelope (NE). In post-mitosis, the dividing cell faces the difficulty to reassemble NE structures in a highly regulated fashion around separated chromosomes. The de novo formation of nuclear pore complexes (NPCs), which are gateways between the cytoplasm and nucleoplasm across the nuclear membrane, is an archetype of macromolecular assembly and is therefore of special interest. The reformation of a functional NE further involves the reassembly and organization of other NE components, the nuclear membrane and NE proteins, around chromosomes in late mitosis.

 

Here, we discuss the function of NE components, such as lamins and INM proteins, in NE reformation and highlight recent results on coordination of NPC and NE assembly.

Building a nuclear envelope at the end of mitosis: coordinating membrane reorganization, nuclear pore complex assembly, and chromatin de-condensation

The metazoan nucleus is disassembled and re-built at every mitotic cell division. The nuclear envelope, including nuclear pore complexes, breaks down at the beginning of mitosis to accommodate the capture of massively condensed chromosomes by the spindle apparatus. At the end of mitosis, a nuclear envelope is newly formed around each set of segregating and de-condensing chromatin. We review the current understanding of the membrane restructuring events involved in the formation of the nuclear membrane sheets of the envelope, the mechanisms governing nuclear pore complex assembly and integration in the nascent nuclear membranes, and the regulated coordination of these events with chromatin de-condensation.

Dormant origins, the licensing checkpoint, and the response to replicative stresses

Only ∼10% of replication origins that are licensed by loading minichromosome maintenance 2-7 (MCM2-7) complexes are normally used, with the majority remaining dormant. If replication fork progression is inhibited, nearby dormant origins initiate to ensure that all of the chromosomal DNA is replicated. At the same time, DNA damage-response kinases are activated, which preferentially suppress the assembly of new replication factories. This diverts initiation events away from completely new areas of the genome toward regions experiencing replicative stress. Mice hypomorphic for MCM2-7, which activate fewer dormant origins in response to replication inhibition, are cancer-prone and are genetically unstable. The licensing checkpoint delays entry into S phase if an insufficient number of origins have been licensed. In contrast, humans with Meier-Gorlin syndrome have mutations in pre-RC proteins and show defects in cell proliferation that may be a consequence of chronic activation of the licensing checkpoint.

Chromatin-bound NLS proteins recruit membrane vesicles and nucleoporins for nuclear envelope assembly via importin-α/β

The mechanism for nuclear envelope (NE) assembly is not fully understood. Importin-β and the small GTPase Ran have been implicated in the spatial regulation of NE assembly process. Here we report that chromatin-bound NLS (nuclear localization sequence) proteins provide docking sites for the NE precursor membrane vesicles and nucleoporins via importin-α and -β during NE assembly in Xenopus egg extracts. We show that along with the fast recruitment of the abundant NLS proteins such as nucleoplasmin and histones to the demembranated sperm chromatin in the extracts, importin-α binds the chromatin NLS proteins rapidly. Meanwhile, importin-β binds cytoplasmic NE precursor membrane vesicles and nucleoporins. Through interacting with importin-α on the chromatin NLS proteins, importin-β targets the membrane vesicles and nucleoporins to the chromatin surface. Once encountering Ran-GTP on the chromatin generated by RCC1, importin-β preferentially binds Ran-GTP and releases the membrane vesicles and nucleoporins for NE assembly. NE assembly is disrupted by blocking the interaction between importin-α and NLS proteins with excess soluble NLS proteins or by depletion of importin-β from the extract. Our findings reveal a novel molecular mechanism for NE assembly in Xenopus egg extracts.

Preserving the genome by regulating chromatin association with the nuclear envelope

The nuclear envelope compartmentalizes chromatin within eukaryotic cells and influences diverse cellular functions by controlling nucleocytoplasmic trafficking. Recent evidence has revealed the importance of interactions between chromatin and nuclear envelope components in the maintenance of genome integrity. Nuclear pore complexes (NPCs), traditionally regarded as transport gateways, have emerged as specialized hubs involved in organizing genome architecture, influencing DNA topology, and modulating DNA repair. Here, we review the interplay between the nuclear envelope, chromatin and DNA damage checkpoint pathways, and discuss the physiological and pathological implications of these associations.

The adenomatous polyposis coli protein contributes to normal compaction of mitotic chromatin

The tumour suppressor Adenomatous Polyposis Coli (APC) is required for proper mitosis; however, the exact role of APC in mitosis is not understood. Using demembranated sperm chromatin exposed to meiotic Xenopus egg extract and HeLa cells expressing fluorescently labelled histones, we established that APC contributes to chromatin compaction. Sperm chromatin in APC-depleted Xenopus egg extract frequently formed tight round or elongated structures. Such abnormally compacted chromatin predominantly formed spindles with low microtubule content. Furthermore, in mitotic HeLa cells expressing GFP- and mCherry-labelled H2B histones, depletion of APC caused a decrease in the donor fluorescence lifetime of neighbouring fluorophores, indicative of excessive chromatin compaction. Profiling the chromatin-associated proteome of sperm chromatin incubated with Xenopus egg extracts revealed temporal APC-dependent changes in the abundance of histones, closely mirrored by chromatin-associated Topoisomerase IIa, condensin I complex and Kif4. In the absence of APC these factors initially accumulated on chromatin, but then decreased faster than in controls. We also found and validated significant APC-dependent changes in chromatin modifiers Set-a and Rbbp7. Both were decreased on chromatin in APC-depleted extract; in addition, the kinetics of association of Set-a with chromatin was altered in the absence of APC.

Preparation and use of Xenopus egg extracts to study DNA replication and chromatin associated proteins

The use of cell-free extracts prepared from eggs of the South African clawed toad, Xenopus laevis, has led to many important discoveries in cell cycle research. These egg extracts recapitulate the key nuclear transitions of the eukaryotic cell cycle in vitro under apparently the same controls that exist in vivo. DNA added to the extract is first assembled into a nucleus and is then efficiently replicated. Progression of the extract into mitosis then allows the separation of paired sister chromatids. The Xenopus cell-free system is therefore uniquely suited to the study of the mechanisms, dynamics and integration of cell cycle regulated processes at a biochemical level. In this article we describe methods currently in use in our laboratory for the preparation of Xenopus egg extracts and demembranated sperm nuclei for the study of DNA replication in vitro. We also detail how DNA replication can be quantified in this system. In addition, we describe methods for isolating chromatin and chromatin-bound protein complexes from egg extracts. These recently developed and revised techniques provide a practical starting point for investigating the function of proteins involved in DNA replication.

Nuclear distributions of NUP62 and NUP214 suggest architectural diversity and spatial patterning among nuclear pore complexes

The shape of nuclei in many adherent cultured cells approximates an oblate ellipsoid, with contralateral flattened surfaces facing the culture plate or the medium. Observations of cultured cell nuclei from orthogonal perspectives revealed that nucleoporin p62 (NUP62) and nucleoporin 214 (NUP214) are differentially distributed between nuclear pore complexes on the flattened surfaces and peripheral rim of the nucleus. High resolution stimulated emission depletion (STED) immunofluorescence microscopy resolved individual NPCs, and suggested both heterogeneity and microheterogeneity in NUP62 and NUP214 immunolabeling among in NPC populations. Similar to nuclear domains and interphase chromosome territories, architectural diversity and spatial patterning of NPCs may be an intrinsic property of the nucleus that is linked to the functions and organization of underlying chromatin.

The nucleoporin ELYS/Mel28 regulates nuclear envelope subdomain formation in HeLa cells

In open mitosis the nuclear envelope (NE) reassembles at the end of each mitosis. This process involves the reformation of the nuclear pore complex (NPC), the inner and outer nuclear membranes, and the nuclear lamina. In human cells cell cycle-dependent NE subdomains exist, characterized as A-type lamin-rich/NPC-free or B-type lamin-rich/NPC-rich, which are initially formed as core or noncore regions on mitotic chromosomes, respectively. Although postmitotic NE formation has been extensively studied, little is known about the coordination of NPC and NE assembly. Here, we report that the nucleoporin ELYS/Mel28, which is crucial for postmitotic NPC formation, is essential for recruiting the lamin B receptor (LBR) to the chromosomal noncore region. Furthermore, ELYS/Mel28 is responsible for focusing of A-type lamin-binding proteins like emerin, Lap2α and the barrier-to-autointegration factor (BAF) at the chromosomal core region. ELYS/Mel28 biochemically interacts with the LBR in a phosphorylation-dependent manner. Recruitment of the LBR depends on the nucleoporin Nup107, which interacts with ELYS/Mel28 but not on nucleoporin Pom121, suggesting that the specific molecular interactions with ELYS/Mel28 are involved in the NE assembly at the noncore region. The depletion of the LBR affected neither the behavior of emerin nor Lap2α indicating that the recruitment of the LBR to mitotic chromosomes is not involved in formation of the core region. The depletion of ELYS/Mel28 also accelerates the entry into cytokinesis after recruitment of emerin to chromosomes. Our data show that ELYS/Mel28 plays a role in NE subdomain formation in late mitosis.

Post-transcriptional homeostasis and regulation of MCM2-7 in mammalian cells

The MiniChromosome Maintenance 2-7 (MCM2-7) complex provides essential replicative helicase function. Insufficient MCMs impair the cell cycle and cause genomic instability (GIN), leading to cancer and developmental defects in mice. Remarkably, depletion or mutation of one Mcm can decrease all Mcm levels. Here, we use mice and cells bearing a GIN-causing hypomophic allele of Mcm4 (Chaos3), in conjunction with disruption alleles of other Mcms, to reveal two new mechanisms that regulate MCM protein levels and pre-RC formation. First, the Mcm4^Chaos3 allele, which disrupts MCM4:MCM6 interaction, triggers a Dicer1 and Drosha-dependent ∼40% reduction in Mcm2–7 mRNAs. The decreases in Mcm mRNAs coincide with up-regulation of the miR-34 family of microRNAs, which is known to be Trp53-regulated and target Mcms. Second, MCM3 acts as a negative regulator of the MCM2–7 helicase in vivo by complexing with MCM5 in a manner dependent upon a nuclear-export signal-like domain, blocking the recruitment of MCMs onto chromatin. Therefore, the stoichiometry of MCM components and their localization is controlled post-transcriptionally at both the mRNA and protein levels. Alterations to these pathways cause significant defects in cell growth reflected by disease phenotypes in mice.

Dynamic interactions of high Cdt1 and geminin levels regulate S phase in early Xenopus embryos

Cdt1 plays a key role in licensing DNA for replication. In the somatic cells of metazoans, both Cdt1 and its natural inhibitor geminin show reciprocal fluctuations in their protein levels owing to cell cycle-dependent proteolysis. Here, we show that the protein levels of Cdt1 and geminin are persistently high during the rapid cell cycles of the early Xenopus embryo. Immunoprecipitation of Cdt1 and geminin complexes, together with their cell cycle spatiotemporal dynamics, strongly supports the hypothesis that Cdt1 licensing activity is regulated by periodic interaction with geminin rather than its proteolysis. Overexpression of ectopic geminin slows down, but neither arrests early embryonic cell cycles nor affects endogenous geminin levels; apparent embryonic lethality is observed around 3-4 hours after mid-blastula transition. However, functional knockdown of geminin by ΔCdt1_193-447, which lacks licensing activity and degradation sequences, causes cell cycle arrest and DNA damage in affected cells. This contributes to subsequent developmental defects in treated embryos. Our results clearly show that rapidly proliferating early Xenopus embryonic cells are able to regulate replication licensing in the persistent presence of high levels of licensing proteins by relying on changing interactions between Cdt1 and geminin during the cell cycle, but not their degradation.

A dominant-negative form of POM121 binds chromatin and disrupts the two separate modes of nuclear pore assembly

Nuclear pore complexes (NPCs) are formed during two separate stages of the metazoan cell cycle. They are assembled into the re-forming nuclear envelope (NE) at the exit from mitosis and into an intact, expanding NE during interphase. Here, we show that a soluble internal fragment of the membrane nucleoporin POM121 has a dominant-negative effect on both modes of assembly in a cell-free reconstitution system. The soluble POM121 fragment binds chromatin at sites that are distinct from ELYS-Nup107-160 'seeding' sites and prevents membrane enclosure and NPC formation. Importin-β negatively regulates chromatin binding by the POM121 fragment through a conserved NLS motif and is also shown to affect the recruitment of the endogenous membrane protein to chromatin in the full assembly system. When an intact NE is present before the addition of the dominant-negative fragment, NPCs are inserted into the NE but membrane expansion is inhibited. This results in densely packed NPCs with no intervening membrane patches, as visualized by scanning electron microscopy. We conclude that POM121 plays an important role in both modes of assembly and links nuclear membrane formation and expansion to nuclear pore biogenesis.

Formation of the postmitotic nuclear envelope from extended ER cisternae precedes nuclear pore assembly

Live-cell imaging and electron tomography show that nuclear pore complexes only assemble on a previously formed nuclear envelope.

During mitosis, the nuclear envelope merges with the endoplasmic reticulum (ER), and nuclear pore complexes are disassembled. In a current model for reassembly after mitosis, the nuclear envelope forms by a reshaping of ER tubules. For the assembly of pores, two major models have been proposed. In the insertion model, nuclear pore complexes are embedded in the nuclear envelope after their formation. In the prepore model, nucleoporins assemble on the chromatin as an intermediate nuclear pore complex before nuclear envelope formation. Using live-cell imaging and electron microscope tomography, we find that the mitotic assembly of the nuclear envelope primarily originates from ER cisternae. Moreover, the nuclear pore complexes assemble only on the already formed nuclear envelope. Indeed, all the chromatin-associated Nup107–160 complexes are in single units instead of assembled prepores. We therefore propose that the postmitotic nuclear envelope assembles directly from ER cisternae followed by membrane-dependent insertion of nuclear pore complexes.

Nucleoporins: leaving the nuclear pore complex for a successful mitosis

The nuclear envelope (NE) separates the cytoplasm and the cell nucleus of interphase eukaryotic cells and nuclear pore complexes (NPCs) mediate the macromolecular exchange between these two compartments. The NE and the NPCs of vertebrate cells disassemble during prophase and the nuclear pore proteins (nucleoporins) are distributed within the mitotic cytoplasm. For an increasing number of them active mitotic functions have been assigned over the past few years. Nucleoporins are participating in spindle assembly, kinetochore organisation, and the spindle assembly checkpoint, all processes that control chromosome segregation and are important for maintenance of genome integrity. But nucleoporins are also engaged in early and late mitotic events, such as centrosome positioning and cytokinesis. Here we will highlight recent progress in deciphering the roles for nucleoporins in the distinct steps of mitosis.

The nuclear envelope in the plant cell cycle: structure, function and regulation

BACKGROUND

Higher plants are, like animals, organisms in which successful completion of the cell cycle requires the breakdown and reformation of the nuclear envelope in a highly controlled manner. Interestingly, however, while the structures and processes appear similar, there are remarkable differences in protein composition and function between plants and animals.

SCOPE

Recent characterization of integral and associated components of the plant nuclear envelope has been instrumental in understanding its functions and behaviour. It is clear that protein interactions at the nuclear envelope are central to many processes in interphase and dividing cells and that the nuclear envelope has a key role in structural and regulatory events.

CONCLUSION

Dissecting the mechanisms of nuclear envelope breakdown and reformation in plants is necessary before a better understanding of the functions of nuclear envelope components during the cell cycle can be gained.

The nuclear pore complex protein Elys is required for genome stability in mouse intestinal epithelial progenitor cells

BACKGROUND & AIMS

Elys is a conserved protein that directs nuclear pore complex (NPC) assembly in mammalian cell lines and developing worms and zebrafish. Related studies in these systems indicate a role for Elys in DNA replication and repair. Intestinal epithelial progenitors of zebrafish elys mutants undergo apoptosis early in development. However, it is not known whether loss of Elys has a similar effect in the mammalian intestine or whether the NPC and DNA repair defects each contribute to the overall phenotype.

METHODS

We developed mice in which a conditional Elys allele was inactivated in the developing intestinal epithelium and during preimplantation development. Phenotypes of conditional mutant mice were determined using immunohistochemical analysis for nuclear pore proteins, electron microscopy, and immunoblot analysis of DNA replication and repair proteins.

RESULTS

Conditional inactivation of the Elys locus in the developing mouse intestinal epithelium led to a reversible delay in growth in juvenile mice that was associated with epithelial architecture distortion and crypt cell apoptosis. The phenotype was reduced in adult mutant mice, which were otherwise indistinguishable from wild-type mice. All mice had activated DNA damage responses but no evidence of NPC assembly defects.

CONCLUSIONS

In mice, Elys maintains genome stability in intestinal epithelial progenitor cells, independent of its role in NPC assembly in zebrafish.

MCM2-7 form double hexamers at licensed origins in Xenopus egg extract

In late mitosis and G1, Mcm2-7 are assembled onto replication origins to license them for initiation in the upcoming S phase. After initiation, Mcm2-7 provide helicase activity to unwind DNA at the replication fork. Here we examine the structure of Mcm2-7 on chromatin in Xenopus egg extracts. We show that prior to replication initiation, Mcm2-7 is present at licensed replication origins in a complex with a molecular mass close to double that of the Mcm2-7 hexamer. This complex has approximately stoichiometric quantities of the 6 Mcm2-7 proteins and we conclude that it consists of a double heterohexamer. This provides a configuration potentially capable of initiating a pair of bidirectional replication forks in S phase. We also show that after initiation, Mcm2-7 associate with Cdc45 and GINS to form a relatively stable CMG (Cdc45-MCM-GINS) complex. The CMG proteins also associate less strongly with other replication proteins, consistent with the idea that a single CMG complex forms the core of the replisome.

Lumenal interactions in nuclear pore complex assembly and stability

A mechanism of nuclear pore complex assembly into intact nuclear envelopes remains elusive. We explore roles of conserved inner nuclear membrane proteins, Heh1p and Heh2p, in this process. The data support the existence of a lumenal bridge between Heh1p and the nucleoporin Pom152p, which might facilitate early nuclear pore complex assembly events.

Nuclear pore complexes (NPCs) provide a gateway for the selective transport of macromolecules across the nuclear envelope (NE). Although we have a solid understanding of NPC composition and structure, we do not have a clear grasp of the mechanism of NPC assembly. Here, we demonstrate specific defects in nucleoporin distribution in strains lacking Heh1p and Heh2p—two conserved members of the LEM (Lap2, emerin, MAN1) family of integral inner nuclear membrane proteins. These effects on nucleoporin localization are likely of functional importance as we have defined specific genetic interaction networks between HEH1 and HEH2, and genes encoding nucleoporins in the membrane, inner, and outer ring complexes of the NPC. Interestingly, expression of a domain of Heh1p that resides in the NE lumen is sufficient to suppress both the nucleoporin mislocalization and growth defects in heh1Δpom34Δ strains. We further demonstrate a specific physical interaction between the Heh1p lumenal domain and the massive cadherin-like lumenal domain of the membrane nucleoporin Pom152p. These findings support a role for Heh1p in the assembly or stability of the NPC, potentially through the formation of a lumenal bridge with Pom152p.

Identification and characterization of nuclear pore complex components in Arabidopsis thaliana

The nuclear pore complex (NPC) facilitates nucleocytoplasmic transport, a crucial process for various cellular activities. The NPC comprises ~30 nucleoporins and is well characterized in vertebrates and yeast. However, only eight plant nucleoporins have been identified, and little information is available about the complete molecular structure of plant NPCs. In this study, an interactive proteomic approach was used to identify Arabidopsis thaliana nucleoporins. A series of five cycles of interactive proteomic analysis was performed using green fluorescent protein (GFP)-tagged nucleoporins. The identified nucleoporins were then cloned and subcellular localization analyses were performed. We found that the plant NPC contains at least 30 nucleoporins, 22 of which had not been previously annotated. Surprisingly, plant nucleoporins shared a similar domain organization to their vertebrate (human) and yeast (Saccharomyces cerevisiae) counterparts. Moreover, the plant nucleoporins exhibited higher sequence homology to vertebrate nucleoporins than to yeast nucleoporins. Plant NPCs lacked seven components (NUCLEOPORIN358 [Nup358], Nup188, Nup153, Nup45, Nup37, NUCLEAR DIVISION CYCLE1, and PORE MEMBRANE PROTEIN OF 121 kD) that were present in vertebrate NPCs. However, plants possessed a nucleoporin, Nup136/Nup1, that contained Phe-Gly repeats, and sequence analysis failed to identify a vertebrate homolog for this protein. Interestingly, Nup136-GFP showed greater mobility on the nuclear envelope than did other nucleoporins, and a Nup136/Nup1 deficiency caused various defects in plant development. These findings provide valuable new information about plant NPC structure and function.

Pom121 links two essential subcomplexes of the nuclear pore complex core to the membrane

Pom121 anchors core structures of the NPC to the membrane through its binding to the β-propeller domains of Nup155 and Nup160.

Nuclear pore complexes (NPCs) control the movement of molecules across the nuclear envelope (NE). We investigated the molecular interactions that exist at the interface between the NPC scaffold and the pore membrane. We show that key players mediating these interactions in mammalian cells are the nucleoporins Nup155 and Nup160. Nup155 depletion massively alters NE structure, causing a dramatic decrease in NPC numbers and the improper targeting of membrane proteins to the inner nuclear membrane. The role of Nup155 in assembly is likely closely linked to events at the membrane as we show that Nup155 interacts with pore membrane proteins Pom121 and NDC1. Furthermore, we demonstrate that the N terminus of Pom121 directly binds the β-propeller regions of Nup155 and Nup160. We propose a model in which the interactions of Pom121 with Nup155 and Nup160 are predicted to assist in the formation of the nuclear pore and the anchoring of the NPC to the pore membrane.

Inner/Outer nuclear membrane fusion in nuclear pore assembly: biochemical demonstration and molecular analysis

Nuclear pore complexes (NPCs) are large proteinaceous channels embedded in double nuclear membranes, which carry out nucleocytoplasmic exchange. The mechanism of nuclear pore assembly involves a unique challenge, as it requires creation of a long-lived membrane-lined channel connecting the inner and outer nuclear membranes. This stabilized membrane channel has little evolutionary precedent. Here we mapped inner/outer nuclear membrane fusion in NPC assembly biochemically by using novel assembly intermediates and membrane fusion inhibitors. Incubation of a Xenopus in vitro nuclear assembly system at 14°C revealed an early pore intermediate where nucleoporin subunits POM121 and the Nup107-160 complex were organized in a punctate pattern on the inner nuclear membrane. With time, this intermediate progressed to diffusion channel formation and finally to complete nuclear pore assembly. Correct channel formation was blocked by the hemifusion inhibitor lysophosphatidylcholine (LPC), but not if a complementary-shaped lipid, oleic acid (OA), was simultaneously added, as determined with a novel fluorescent dextran-quenching assay. Importantly, recruitment of the bulk of FG nucleoporins, characteristic of mature nuclear pores, was not observed before diffusion channel formation and was prevented by LPC or OA, but not by LPC+OA. These results map the crucial inner/outer nuclear membrane fusion event of NPC assembly downstream of POM121/Nup107-160 complex interaction and upstream or at the time of FG nucleoporin recruitment.

Live imaging of single nuclear pores reveals unique assembly kinetics and mechanism in interphase

Recruitment of nuclear pore complex (NPC) components during interphase occurs in a different order and with slower kinetics than during postmitotic NPC assembly, suggesting the two processes are regulated by distinct mechanisms.

In metazoa, new nuclear pore complexes (NPCs) form at two different cell cycle stages: at the end of mitosis concomitant with the reformation of the nuclear envelope and during interphase. However, the mechanisms of these assembly processes may differ. In this study, we apply high resolution live cell microscopy to analyze the dynamics of single NPCs in living mammalian cells during interphase. We show that nuclear growth and NPC assembly are correlated and occur at a constant rate throughout interphase. By analyzing the kinetics of individual NPC assembly events, we demonstrate that they are initiated by slow accumulation of the membrane nucleoporin Pom121 followed by the more rapid association of the soluble NPC subcomplex Nup107–160. This inverse order of recruitment and the overall much slower kinetics compared with postmitotic NPC assembly support the conclusion that the two processes occur by distinct molecular mechanisms.

Nuclear transport and the mitotic apparatus: an evolving relationship

The trafficking of macromolecules between the cytoplasm and the nucleus is controlled by the nuclear pore complexes (NPCs) and various transport factors that facilitate the movement of cargos through the NPCs and their accumulation in the target compartment. While their functions in transport are well established, an ever-growing number of observations have also linked components of the nuclear transport machinery to processes that control chromosome segregation during mitosis, including spindle assembly, kinetochore function, and the spindle assembly checkpoint. In this review, we will discuss this evolving area of study and emerging hypotheses that propose key roles for components of the nuclear transport apparatus in mitotic progression.

Cell cycle-dependent differences in nuclear pore complex assembly in metazoa

In metazoa, nuclear pore complexes (NPCs) assemble from disassembled precursors into a reforming nuclear envelope (NE) at the end of mitosis and into growing intact NEs during interphase. Here, we show via RNAi-mediated knockdown that ELYS, a nucleoporin critical for the recruitment of the essential Nup107/160 complex to chromatin, is required for NPC assembly at the end of mitosis but not during interphase. Conversely, the transmembrane nucleoporin POM121 is critical for the incorporation of the Nup107/160 complex into new assembly sites specifically during interphase. Strikingly, recruitment of the Nup107/160 complex to an intact NE involves a membrane curvature-sensing domain of its constituent Nup133, which is not required for postmitotic NPC formation. Our results suggest that in organisms with open mitosis, NPCs assemble via two distinct mechanisms to accommodate cell cycle-dependent differences in NE topology.

The zebrafish flotte lotte mutant reveals that the local retinal environment promotes the differentiation of proliferating precursors emerging from their stem cell niche

It is currently unclear how intrinsic and extrinsic mechanisms cooperate to control the progression from self-renewing to neurogenic divisions in retinal precursor cells. Here, we use the zebrafish flotte lotte (flo) mutant, which carries a mutation in the elys (ahctf1) gene, to study the relationship between cell cycle progression and neuronal differentiation by investigating how proliferating progenitor cells transition towards differentiation in a retinal stem cell niche termed the ciliary marginal zone (CMZ). In zebrafish embryos without Elys, CMZ cells retain the capacity to proliferate but lose the ability to enter their final neurogenic divisions to differentiate as neurons. However, mosaic retinae composed of wild-type and flo cells show that despite inherent cell cycle defects, flo mutant cells progress from proliferation to differentiation when in the vicinity of wild-type retinal neurons. We propose that the differentiated retinal environment limits the proliferation of precursors emerging from the CMZ in a manner that explains the spatial organisation of cells in the CMZ and ensures that proliferative retinal progenitors are driven towards differentiation.

Members of the RSC chromatin-remodeling complex are required for maintaining proper nuclear envelope structure and pore complex localization

Genome-wide screening approaches were employed to identify factors required for nuclear pore complex structure and distribution in Saccharomyces cerevisiae. Roles were found for multiple components of the RSC complex, revealing a functional connection between proper chromatin remodeling and nuclear envelope/nuclear pore complex structure.

The assembly, distribution, and functional integrity of nuclear pore complexes (NPCs) in the nuclear envelope (NE) are key determinants in the nuclear periphery architecture. However, the mechanisms controlling proper NPC and NE structure are not fully defined. We used two different genetic screening approaches to identify Saccharomyces cerevisiae mutants with defects in NPC localization. The first approach examined green fluorescent protein (GFP)-Nic96 in 531 strains from the yeast Tet-promoters Hughes Collection with individual essential genes expressed from a doxycycline-regulated promoter (TetO₇-orf). Under repressive conditions, depletion of the protein encoded by 44 TetO₇-orf strains resulted in mislocalized GFP-Nic96. These included STH1, RSC4, RSC8, RSC9, RSC58, ARP7, and ARP9, each encoding components of the RSC chromatin remodeling complex. Second, a temperature-sensitive sth1-F793S (npa18-1) mutant was identified in an independent genetic screen for NPC assembly (npa) mutants. NPC mislocalization in the RSC mutants required new protein synthesis and ongoing transcription, confirming that lack of global transcription did not underlie the phenotypes. Electron microscopy studies showed significantly altered NEs and nuclear morphology, with coincident cytoplasmic membrane sheet accumulation. Strikingly, increasing membrane fluidity with benzyl alcohol treatment prevented the sth1-F793S NE structural defects and NPC mislocalization. We speculate that NE structure is functionally linked to proper chromatin architecture.

Border control at the nucleus: biogenesis and organization of the nuclear membrane and pore complexes

Over the last decade, the nuclear envelope (NE) has emerged as a key component in the organization and function of the nuclear genome. As many as 100 different proteins are thought to specifically localize to this double membrane that separates the cytoplasm and the nucleoplasm of eukaryotic cells. Selective portals through the NE are formed at sites where the inner and outer nuclear membranes are fused, and the coincident assembly of approximately 30 proteins into nuclear pore complexes occurs. These nuclear pore complexes are essential for the control of nucleocytoplasmic exchange. Many of the NE and nuclear pore proteins are thought to play crucial roles in gene regulation and thus are increasingly linked to human diseases.

Double duty for nuclear proteins--the price of more open forms of mitosis

During cell division, eukaryotic cells pass on their genetic material to the next generation by undergoing mitosis, which segregates their chromosomes. During mitosis, the nuclear envelope, nuclear pore complexes and nucleolus must also be segregated. Cells achieve this in a range of different forms of mitosis, from closed, in which these nuclear structures remain intact, to open, in which these nuclear structures are disassembled. In between lies a smorgasbord of intermediate forms of mitosis, displaying varying degrees of nuclear disassembly. Gathering evidence is revealing links between the extent of nuclear disassembly and the evolution of new roles for nuclear proteins during mitosis. We propose that proteins with such double duties help coordinate reassembly of the nucleus with chromosomal segregation.

A genome-wide siRNA screen reveals diverse cellular processes and pathways that mediate genome stability

Signaling pathways that respond to DNA damage are essential for the maintenance of genome stability and are linked to many diseases, including cancer. Here, a genome-wide siRNA screen was employed to identify additional genes involved in genome stabilization by monitoring phosphorylation of the histone variant H2AX, an early mark of DNA damage. We identified hundreds of genes whose downregulation led to elevated levels of H2AX phosphorylation (gammaH2AX) and revealed links to cellular complexes and to genes with unclassified functions. We demonstrate a widespread role for mRNA-processing factors in preventing DNA damage, which in some cases is caused by aberrant RNA-DNA structures. Furthermore, we connect increased gammaH2AX levels to the neurological disorder Charcot-Marie-Tooth (CMT) syndrome, and we find a role for several CMT proteins in the DNA-damage response. These data indicate that preservation of genome stability is mediated by a larger network of biological processes than previously appreciated.

Transportin regulates major mitotic assembly events: from spindle to nuclear pore assembly

Mitosis in higher eukaryotes is marked by the sequential assembly of two massive structures: the mitotic spindle and the nucleus. Nuclear assembly itself requires the precise formation of both nuclear membranes and nuclear pore complexes. Previously, importin alpha/beta and RanGTP were shown to act as dueling regulators to ensure that these assembly processes occur only in the vicinity of the mitotic chromosomes. We now find that the distantly related karyopherin, transportin, negatively regulates nuclear envelope fusion and nuclear pore assembly in Xenopus egg extracts. We show that transportin-and importin beta-initiate their regulation as early as the first known step of nuclear pore assembly: recruitment of the critical pore-targeting nucleoporin ELYS/MEL-28 to chromatin. Indeed, each karyopherin can interact directly with ELYS. We further define the nucleoporin subunit targets for transportin and importin beta and find them to be largely the same: ELYS, the Nup107/160 complex, Nup53, and the FG nucleoporins. Equally importantly, we find that transportin negatively regulates mitotic spindle assembly. These negative regulatory events are counteracted by RanGTP. We conclude that the interplay of the two negative regulators, transportin and importin beta, along with the positive regulator RanGTP, allows precise choreography of multiple cell cycle assembly events.

Importin beta regulates the seeding of chromatin with initiation sites for nuclear pore assembly

The nuclear envelope of higher eukaryotic cells reforms at the exit from mitosis, in concert with the assembly of nuclear pore complexes (NPCs). The first step in postmitotic NPC assembly involves the "seeding" of chromatin with ELYS and the Nup107-160 complex. Subsequent steps in the assembly process are poorly understood and different mechanistic models have been proposed to explain the formation of the full supramolecular structure. Here, we show that the initial step of chromatin seeding is negatively regulated by importin beta. Direct imaging of the chromatin attachment sites reveals single sites situated predominantly on the highest substructures of chromatin surface and lacking any sign of annular structures or oligomerized pre-NPCs. Surprisingly, the inhibition by importin beta is only partially reversed by RanGTP. Importin beta forms a high-molecular-weight complex with both ELYS and the Nup107-160 complex in cytosol. We suggest that initiation sites for NPC assembly contain single copies of chromatin-bound ELYS/Nup107-160 and that the lateral oligomerization of these subunits depends on the recruitment of membrane components. We predict that additional regulators, besides importin beta and Ran, may be involved in coordinating the initial seeding of chromatin with subsequent steps in the NPC assembly pathway.

The structure of the scaffold nucleoporin Nup120 reveals a new and unexpected domain architecture

Nucleocytoplasmic transport is mediated by nuclear pore complexes (NPCs), enormous protein assemblies residing in circular openings in the nuclear envelope. The NPC is modular, with transient and stable components. The stable core is essentially built from two multiprotein complexes, the Y-shaped heptameric Nup84 complex and the Nic96 complex, arranged around an eightfold axis. We present the crystal structure of Nup120(1-757), one of the two short arms of the Y-shaped Nup84 complex. The protein adopts a compact oval shape built around a novel bipartite alpha-helical domain intimately integrated with a beta-propeller domain. The domain arrangement is substantially different from the Nup85*Seh1 complex, which forms the other short arm of the Y. With the data presented here, we establish that all three branches of the Y-shaped Nup84 complex are tightly connected by helical interactions and that the beta-propellers likely form interaction site(s) to neighboring complexes.

Nuclear pore complex biogenesis

Nuclear pore complexes (NPCs) are the sole mediators of transport between the nucleus and the cytoplasm. NPCs have a life cycle: they assemble, disassemble, turnover, and age. The molecular mechanisms governing these different vital steps are beginning to emerge, suggesting key roles for the core structural scaffold of the NPC and auxiliary factors in the assembly of this large macromolecular complex, and connections between NPC maintenance, NPC turnover, and aging of the cell.

The nucleoporins Nup170p and Nup157p are essential for nuclear pore complex assembly

We have established that two homologous nucleoporins, Nup170p and Nup157p, play an essential role in the formation of nuclear pore complexes (NPCs) in Saccharomyces cerevisiae. By regulating their synthesis, we showed that the loss of these nucleoporins triggers a decrease in NPCs caused by a halt in new NPC assembly. Preexisting NPCs are ultimately lost by dilution as cells grow, causing the inhibition of nuclear transport and the loss of viability. Significantly, the loss of Nup170p/Nup157p had distinct effects on the assembly of different architectural components of the NPC. Nucleoporins (nups) positioned on the cytoplasmic face of the NPC rapidly accumulated in cytoplasmic foci. These nup complexes could be recruited into new NPCs after reinitiation of Nup170p synthesis, and may represent a physiological intermediate. Loss of Nup170p/Nup157p also caused core and nucleoplasmically positioned nups to accumulate in NPC-like structures adjacent to the inner nuclear membrane, which suggests that these nucleoporins are required for formation of the pore membrane and the incorporation of cytoplasmic nups into forming NPCs.