Dimples, pores, star-rings, and thin rings on growing nuclear envelopes: evidence for structural intermediates in nuclear pore complex assembly

Assembly and preferential localization of Nup116p on the cytoplasmic face of the nuclear pore complex by interaction with Nup82p

The yeast Saccharomyces cerevisiae nucleoporin Nup116p serves as a docking site for both nuclear import and export factors. However, the mechanism for assembling Nup116p into the nuclear pore complex (NPC) has not been resolved. By conducting a two-hybrid screen with the carboxy (C)-terminal Nup116p region as bait, we identified Nup82p. The predicted coiled-coil region of Nup82p was not required for Nup116p interaction, making the binding requirements distinct from those for the Nsp1p-Nup82p-Nup159p subcomplex (N. Belgareh, C. Snay-Hodge, F. Pasteau, S. Dagher, C. N. Cole, and V. Doye, Mol. Biol. Cell 9:3475-3492, 1998). Immunoprecipitation experiments using yeast cell lysates resulted in the coisolation of a Nup116p-Nup82p subcomplex. Although the absence of Nup116p had no effect on the NPC localization of Nup82p, overexpression of C-terminal Nup116p in a nup116 null mutant resulted in Nup82p mislocalization. Moreover, NPC localization of Nup116p was specifically diminished in a nup82-Delta108 mutant after growth at 37 degrees C. Immunoelectron microscopy analysis showed Nup116p was localized on both the cytoplasmic and nuclear NPC faces. Its distribution was asymmetric with the majority at the cytoplasmic face. Taken together, these results suggest that Nup82p and Nup116p interact at the cytoplasmic NPC face, with nucleoplasmic Nup116p localization utilizing novel binding partners.

Ran alters nuclear pore complex conformation

Transport across the nuclear membranes occurs through the nuclear pore complex (NPC), and is mediated by soluble transport factors including Ran, a small GTPase that is generally GDP-bound during import and GTP-bound for export. The dynamic nature of the NPC structure suggests a possible active role for it in driving translocation. Here we show that RanGTP but not RanGDP causes alterations of NPC structure when injected into the cytoplasm of Xenopus oocytes, including compaction of the NPC and extension of the cytoplasmic filaments. RanGTP caused accumulation of nucleoplasmin-gold along the length of extended cytoplasmic filaments, whereas RanGDP caused accumulation around the cytoplasmic rim of the NPC. This suggests a possible role for Ran in altering the conformation of the cytoplasmic filaments during transport.

The nuclear pore complex: a protein machine bridging the nucleus and cytoplasm

Compositional analysis of nuclear pore complexes (NPCs) is nearing completion, and efforts are now focused on understanding how these protein machines work. Recent analysis of soluble transport factor interactions with NPC proteins reveals distinct and overlapping pathways for movement between the nucleus and cytoplasm. New fluorescence- and microscopy-based strategies have been used to monitor the pathway of NPC assembly and to reveal the dynamics of the NPC during transport.

The nuclear pore complex: mediator of translocation between nucleus and cytoplasm

The enclosure of nuclear contents in eukaryotes means that cells require sites in the boundary that mediate exchange of material between nucleus and cytoplasm. These sites, termed nuclear pore complexes (NPCs), number 100–200 in yeast, a few thousand in mammalian cells and approximately 50 million in the giant nuclei of amphibian oocytes. NPCs are large (125 MDa) macromolecular complexes that comprise 50–100 different proteins in vertebrates. In spite of their size and complex structure, NPCs undergo complete breakdown and reformation at cell division. Transport through NPCs can be rapid (estimated at several hundred molecules/pore/second) and accommodates both passive diffusion of relatively small molecules, and active transport of complexes up to several megadaltons in molecular mass. Each pore can facilitate both import and export. The two processes apparently involve multiple pathways for different cargoes, and their transport signals, transport receptors and adapters, and the molecules (and their regulators) that underpin the transport mechanisms. Over the past few years there has been an increasing interest in the pore complex: structural studies have been followed by elucidation of the biochemical aspects of nuclear import, and subsequent investigations into nuclear export. The current challenge is to understand the interactions between the structural elements of the pore complex and the mechanisms that drive the physical processes of translocation through it.

Structure and assembly of the Nup84p complex

The Nup84p complex consists of five nucleoporins (Nup84p, Nup85p, Nup120p, Nup145p-C, and Seh1p) and Sec13p, a bona fide subunit of the COPII coat complex. We show that a pool of green fluorescent protein-tagged Sec13p localizes to the nuclear pores in vivo, and identify sec13 mutant alleles that are synthetically lethal with nup85Delta and affect the localization of a green fluorescent protein-Nup49p reporter protein. In the electron microscope, sec13 mutants exhibit structural defects in nuclear pore complex (NPC) and nuclear envelope organization. For the assembly of the complex, Nup85p, Nup120p, and Nup145p-C are essential. A highly purified Nup84p complex was isolated from yeast under native conditions and its molecular mass was determined to be 375 kD by quantitative scanning transmission electron microscopy and analytical ultracentrifugation, consistent with a monomeric complex. Furthermore, the Nup84p complex exhibits a Y-shaped, triskelion-like morphology 25 nm in diameter in the transmission electron microscope. Thus, the Nup84p complex constitutes a paradigm of an NPC structural module with distinct composition, structure, and a role in nuclear mRNA export and NPC bio- genesis.

Live fluorescence imaging reveals early recruitment of emerin, LBR, RanBP2, and Nup153 to reforming functional nuclear envelopes

We determined the times when the nuclear membrane, nuclear pore complex (NPC) components, and nuclear import function were recovered during telophase in living HeLa cells. Simultaneous observation of fluorescently-labeled NLS-bearing proteins, lamin B receptor (LBR)-GFP, and Hoechst33342-stained chromosomes revealed that nuclear membranes reassembled around chromosomes by 5 minutes after the onset of anaphase (early telophase) whereas nuclear import function was recovered later, at 8 minutes. GFP-tagged emerin also accumulated on chromosomes 5 minutes after the onset of anaphase. Interestingly, emerin and LBR initially accumulated at distinct, separate locations, but then became uniform 8 minutes after the onset of anaphase, concurrent with the recovery of nuclear import function. We further determined the timing of NPC assembly by immunofluorescence staining of cells fixed at precise times after the onset of anaphase. Taken together, these results showed that emerin, LBR, and several NPC components (RanBP2, Nup153, p62), but not Tpr, reconstitute around chromosomes very early in telophase prior to the recovery of nuclear import activity.

PBC68: a nuclear pore complex protein that associates reversibly with the mitotic spindle

Using autoimmune antibodies from a patient with primary biliary cirrhosis we have identified a 68 kDa nuclear envelope protein, termed PBC68. This protein is co-precipitated with a 98 kDa and a 250 kDa polypeptide and is distinct from the nuclear lamins. Immunostaining of digitonin-permeabilized cells indicates that PBC68 is restricted to the inner (nucleoplasmic) face of the nuclear envelope, while indirect immunofluorescence and immunoelectron microscopy show that PBC68 is located on fibrillar structures emanating from the nuclear pore complex. The autoantigen is modified at early prophase and disassembles at prometaphase concurrently with the breakdown of the nuclear envelope. The disassembled material, instead of diffusing throughout the cytoplasm as other nucleoporins, is targeted to the mitotic spindle and remains stably bound to it until anaphase. At telophase PBC68 is released from the mitotic apparatus and reassembles late, after incorporation of LAP2B and B-type lamins, onto the reforming nuclear envelope. The partitioning of PBC68 in dividing cells supports the notion that subsets of nuclear envelope proteins are actively sorted during mitosis by transiently anchoring to spindle microtubules. Furthermore, the data suggest that specific constituents of pore complex are released in a stepwise fashion from their anchorage sites before becoming available for nuclear reassembly.

Targeting of the mammalian nucleoporin p62 to the nuclear envelope in the yeast Saccharomyces cerevisiae and HeLa cells

We have analyzed the sorting of the mammalian nucleoporin p62 in human culture cells and in the yeast Saccharomyces cerevisiae. To this end, gene fusions were generated that carry Aequorea victoria green fluorescence protein and defined portions of p62. Upon transient gene expression fluorescent fusion proteins were localized in HeLa cells. Likewise, fusion proteins were studied in S. cerevisiae using wild-type as well as mutant cells that cluster nuclear pore complexes. Our results demonstrate that evolutionarily distant organisms, such as humans and yeasts, recognize the same sequence elements of p62 for sorting to the nuclear envelope. Specifically, the entire sequence of p62 or its complete C-terminal domain targeted fusion proteins to the nuclear membranes. In contrast, truncations of the C-terminal domain or the N-terminal segment of p62 failed to associate with the nuclear envelope in either organism. In HeLa cells overexpression of several p62-containing fusion proteins resulted in nuclear fragmentation. The C-terminal domain of p62 caused this effect, and amino acid residues 477 to 525 were sufficient to induce aberrant nuclei. Thus, overexpression of 49 amino acid residues located at the C-terminal tail of p62 interferes with the nuclear integrity in human culture cells.Key words: nuclear pore complex, nuclear transport, nucleoporin, green fluorescent protein, transfection.

Function and assembly of nuclear pore complex proteins

Nuclear pore complexes (NPCs) are extremely elaborate structures that mediate the bidirectional movement of macromolecules between the nucleus and cytoplasm. The current view of NPC organization features a massive symmetrical framework that is embedded in the double membranes of the nuclear envelope. It embraces a central channel of as yet ill-defined structure but which may accommodate particles with diameters up to 26 nm provided that they bear specific import/export signals. Attached to both faces of the central framework are peripheral structures, short cytoplasmic filaments, and a nuclear basket assembly, which interact with molecules transiting the NPC. The mechanisms of assembly and the nature of NPC structural intermediates are still poorly understood. However, mutagenesis and expression studies have revealed discrete sequences within certain NPC proteins that are necessary and sufficient for their appropriate targeting. In addition, some details are emerging from observations on cells undergoing mitosis where the nuclear envelope is disassembled and its components, including NPC subunits, are dispersed throughout the mitotic cytoplasm. At the end of mitosis, all of these components are reutilized to form nuclear envelopes in the two daughter cells. To date, it has been possible to define a time course of postmitotic assembly for a group of NPC components (CAN/Nup214, Nup153, POM121, p62 and Tpr) relative to the integral inner nuclear membrane protein LAP2 and the NPC membrane glycoprotein gp210. Nup153, a dynamic component of the nuclear basket, associates with chromatin towards the end of anaphase coincident with, although independent of, the inner nuclear membrane protein, LAP2. Assembly of the remaining proteins follows that of the nuclear membranes and occurs in the sequence POM121, p62, CAN/Nup214 and gp210/Tpr. Since p62 remains as a complex with three other NPC proteins (p58, p54, p45) during mitosis, and CAN/Nup214 maintains a similar interaction with its partner, Nup84, the relative timing of assembly of these additional four proteins may also be inferred. These observations suggest that there is a sequential association of NPC proteins with chromosomes during nuclear envelope reformation and the recruitment of at least eight of these precedes that of gp210. These findings support a model in which it is POM121 rather than gp210 that defines initial membrane-associated NPC assembly intermediates and which may therefore represent an essential component of the central framework of the NPC. Key words: nuclear pore complex, nucleoporin, mitosis, nuclear transport

In vitro nuclear assembly with affinity-purified nuclear envelope precursor vesicle fractions, PV1 and PV2

Nuclear envelope precursor vesicles were affinity purified from a Xenopus egg extract by a chromatin binding method. Vesicles bound to chromatin at 4 degrees C were dissociated with a high salt buffer and further fractionated into nuclear envelope precursor vesicle fractions 1 (PV1) and 2 (PV2) by differential centrifugation. PV1 contained larger vesicles. When chromatin was incubated in a Xenopus egg cytosol fraction supplemented with PV1, vesicles bound to chromatin, fused with each other, formed a bilayered nuclear envelope, and assembled into spherical small nuclei. However, the thus assembled nuclei did not grow to the normal size. Nuclear pore complexes were not found on the thus assembled nuclei. On the other hand, PV2 contained smaller vesicles. PV2 vesicles bound to chromatin, fused little with each other in the Xenopus egg cytosol fraction, and no nuclei were assembled. When PV1 supplemented with PV2 was used for the nuclear assembly reaction, the assembled nuclei grew to the normal size. Nuclear pore complexes existed in the thus assembled nuclear envelopes. These results suggested that 1) two vesicle populations, PV1 and PV2, are necessary for the assembly of normal sized nuclei, 2) PV1 contains a chromatin targeting molecule(s) and membrane fusion machinery, 3) PV2 contains a chromatin targeting molecule(s) and a molecule(s) necessary for nuclear pore complex assembly, and 4) PV1 has the ability to assemble a nuclear membrane, and PV2 is necessary for the assembly of nuclear pore complexes and for nuclei to grow to the normal size. An in vitro nuclear assembly system constituted with affinity-purified vesicle fractions, PV1 and PV2, was established.

Sequential recruitment of NPC proteins to the nuclear periphery at the end of mitosis

Nuclear pore complexes (NPCs) are extremely elaborate structures that mediate the bidirectional movement of macromolecules between the nucleus and cytoplasm. With a mass of about 125 MDa, NPCs are thought to be composed of 50 or more distinct protein subunits, each present in multiple copies. During mitosis in higher cells the nuclear envelope is disassembled and its components, including NPC subunits, are dispersed throughout the mitotic cytoplasm. At the end of mitosis, all of these components are reutilized. Using both conventional and digital confocal immunofluorescence microscopy we have been able to define a time course of post-mitotic assembly for a group of NPC components (CAN/Nup214, Nup153, POM121, p62 and Tpr) relative to the integral nuclear membrane protein LAP2 and the NPC membrane glycoprotein gp210. Nup153, a component of the nuclear basket, associates with chromatin towards the end of anaphase, in parallel with the inner nuclear membrane protein, LAP2. However, immunogold labeling suggests that the initial Nup153 chromatin association is membrane-independent. Assembly of the remaining proteins follows that of the nuclear membranes and occurs in the sequence POM121, p62, CAN/Nup214 and gp210/Tpr. Since p62 remains as a complex with three other NPC proteins (p58, 54, 45) during mitosis and CAN/Nup214 maintains a similar interaction with its partner, Nup84, the relative timing of assembly of these additional four proteins may also be inferred. These observations suggest that there is a sequential association of NPC proteins with chromosomes during nuclear envelope reformation and the recruitment of at least eight of these precedes that of gp210. These findings support a model in which it is POM121 rather than gp210 that defines initial membrane-associated NPC assembly intermediates.

Calcium-mediated structural changes of native nuclear pore complexes monitored by time-lapse atomic force microscopy

Nuclear pore complexes (NPCs) are large macromolecular assemblies embedded in the double membrane nuclear envelope. They are the major gateways mediating transport of ions, small molecules, proteins, RNAs, and ribonucleoprotein particles in and out of the nucleus in interphase cells. Understanding structural changes at the level of individual pores will be a prerequisite to eventually correlate the molecular architecture of the NPC with its distinct functional states during nucleocytoplasmic transport. Toward this goal, we have employed time-lapse atomic force microscopy of native NPCs kept in buffer, and recorded calcium-mediated structural changes such as the opening (i.e. +Ca2+) and closing (i.e. -Ca2+) of individual nuclear baskets. Most likely, this structural change of the nuclear basket involves its distal ring which may act as an iris-like diaphragm. In order to directly correlate distinct structural features with corresponding functional states and dynamic aspects, we also addressed the question of whether the "central plug" or "transporter" actually represents a calcium-sensitive component of the NPC involved in mediating nucleocytoplasmic transport. Our data indicate that in the absence of ATP, cytoplasmic plugging/unplugging of the NPC is insensitive to calcium.

Identification and characterization of nuclear pore subcomplexes in mitotic extract of human somatic cells

In higher eukaryotic cells, it is generally thought that nuclear pore complex disassembles at the beginning of mitosis and reassembles at the end. Using a monoclonal antibody that recognizes nuclear pore antigens, we found that, at mitosis of mammalian somatic Hela cells, the nuclear pore complex disassembles into at least three subcomplexes, termed subcomplexes A, B and C (molecular mass; 2 MDa<, approximately 660 kDa, and approximately 440 kDa, respectively). The direct partial amino acid sequencing of the components of these subcomplexes indicates that the A subcomplex contains CAN/Nup214/p250 and p62 and the B subcomplex also contains p62, indicating that p62 is contained in two different subcomplexes. Subcomplex C was shown to consist of Nup98 and human RAE1, a human homolog of yeast Rae1p/Gle2p. Since Nup98 and Rae1p/Gle2p have been reported to be involved in mRNA export, this suggests that some of the mitotic subcomplex may be formed by functionally related proteins.

A novel fluorescence-based genetic strategy identifies mutants of Saccharomyces cerevisiae defective for nuclear pore complex assembly

Nuclear pore complexes (NPCs) are large proteinaceous portals for exchanging macromolecules between the nucleus and the cytoplasm. Revealing how this transport apparatus is assembled will be critical for understanding the nuclear transport mechanism. To address this issue and to identify factors that regulate NPC formation and dynamics, a novel fluorescence-based strategy was used. This approach is based on the functional tagging of NPC proteins with the green fluorescent protein (GFP), and the hypothesis that NPC assembly mutants will have distinct GFP-NPC signals as compared with wild-type (wt) cells. By fluorescence-activated cell sorting for cells with low GFP signal from a population of mutagenized cells expressing GFP-Nup49p, three complementation groups were identified: two correspond to mutant nup120 and gle2 alleles that result in clusters of NPCs. Interestingly, a third group was a novel temperature-sensitive allele of nup57. The lowered GFP-Nup49p incorporation in the nup57-E17 cells resulted in a decreased fluorescence level, which was due in part to a sharply diminished interaction between the carboxy-terminal truncated nup57pE17 and wt Nup49p. Interestingly, the nup57-E17 mutant also affected the incorporation of a specific subset of other nucleoporins into the NPC. Decreased levels of NPC-associated Nsp1p and Nup116p were observed. In contrast, the localizations of Nic96p, Nup82p, Nup159p, Nup145p, and Pom152p were not markedly diminished. Coincidentally, nuclear import capacity was inhibited. Taken together, the identification of such mutants with specific perturbations of NPC structure validates this fluorescence-based strategy as a powerful approach for providing insight into the mechanism of NPC biogenesis.

Nuclear envelope and nuclear pore assembly: analysis of assembly intermediates by electron microscopy

At mitosis, the nucleus of higher eukaryotic cells disassemblies into components which subsequently reform functional nuclear envelopes in the two daughter cells. The molecular mechanisms underlying this remarkable morphological reorganization are the focus of active investigation. Recent electron microscopy techniques have provided intriguing glimpses of intermediate structures in both nuclear envelope and nuclear pore complex reassembly.

The integral membrane protein snl1p is genetically linked to yeast nuclear pore complex function

Integral membrane proteins are predicted to play key roles in the biogenesis and function of nuclear pore complexes (NPCs). Revealing how the transport apparatus is assembled will be critical for understanding the mechanism of nucleocytoplasmic transport. We observed that expression of the carboxyl-terminal 200 amino acids of the nucleoporin Nup116p had no effect on wild-type yeast cells, but it rendered the nup116 null strain inviable at all temperatures and coincidentally resulted in the formation of nuclear membrane herniations at 23 degrees C. To identify factors related to NPC function, a genetic screen for high-copy suppressors of this lethal nup116-C phenotype was conducted. One gene (designated SNL1 for suppressor of nup116-C lethal) was identified whose expression was necessary and sufficient for rescuing growth. Snl1p has a predicted molecular mass of 18.3 kDa, a putative transmembrane domain, and limited sequence similarity to Pom152p, the only previously identified yeast NPC-associated integral membrane protein. By both indirect immunofluorescence microscopy and subcellular fractionation studies, Snl1p was localized to both the nuclear envelope and the endoplasmic reticulum. Membrane extraction and topology assays suggested that Snl1p was an integral membrane protein, with its carboxyl-terminal region exposed to the cytosol. With regard to genetic specificity, the nup116-C lethality was also suppressed by high-copy GLE2 and NIC96. Moreover, high-copy SNL1 suppressed the temperature sensitivity of gle2-1 and nic96-G3 mutant cells. The nic96-G3 allele was identified in a synthetic lethal genetic screen with a null allele of the closely related nucleoporin nup100. Gle2p physically associated with Nup116p in vitro, and the interaction required the N-terminal region of Nup116p. Therefore, genetic links between the role of Snl1p and at least three NPC-associated proteins were established. We suggest that Snl1p plays a stabilizing role in NPC structure and function.

TPEN, a Zn2+/Fe2+ chelator with low affinity for Ca2+, inhibits lamin assembly, destabilizes nuclear architecture and may independently protect nuclei from apoptosis in vitro

We used Xenopus egg extracts to examine the effects of TPEN, a chelator with strong affinities for Zn2+, Fe2+, and Mn2+, on nuclear assembly in vitro. At concentrations above 1 mM, TPEN blocked the assembly of the nuclear lamina and produced nuclei that were profoundly sensitive to stress-induced balloon-like 'shedding' of nuclear membranes away from chromatin-associated membranes. TPEN-arrested nuclei were also defective for DNA replication, which could be explained as secondary to the lack of a lamina. Imaging of TPEN-arrested nuclei by field emission in-lens scanning electron microscopy (FEISEM) revealed clustered, structurally-perturbed nuclear pore complexes. TPEN-arrested nuclei were defective in the accumulation of fluorescent karyophilic proteins. All detectable effects caused by TPEN were downstream of the effects of BAPTA, a Ca2+/Zn2+ chelator that blocks pore complex assembly at two distinct early stages. Surprisingly, TPEN-arrested nuclei, but not control nuclei, remained active for replication in apoptotic extracts, as assayed by [32P]-dCTP incorporation into high molecular weight DNA, suggesting that TPEN blocks a metal-binding protein(s) required for nuclear destruction during programmed cell death.

Nuclear assembly

We review old and new insights into the structure of the nuclear envelope and the components responsible for its dynamic reassembly during mitosis. New information is coming to light about several of the proteins that mediate nuclear reassembly. These proteins include the lamins and their emerging relationship with proteins such as otefin and the MAN antigens: peripheral proteins that might participate in lamina structure. There are four identified proteins localized to the inner nuclear membrane: the lamina-associated proteins LAP1 and LAP2, emerin, and the lamin B receptor (LBR). LBR can interact independently with lamin B and a chromodomain protein, Hp1, and appears to be a central player in targeting nuclear membranes to chromatin. Intermediates in the assembly of nuclear pore complexes (NPCs) can now be studied biochemically and visualized by high resolution scanning electron microscopy. We discuss the possibility that the filament-forming proteins Tpr/p270, NuMA, and perhaps actin may have roles in nuclear assembly.

Active nuclear pore complexes in Chironomus: visualization of transporter configurations related to mRNP export

The Nuclear Pore Complex (NPC) regulates nucleocytoplasmic transport by providing small channels for passive diffusion and multiple docking surfaces that lead to a central translocation channel for active transport. In this study we have investigated by high resolution scanning and transmission electron microscopy the dynamics of NPC structure in salivary gland nuclei from Chironomus during Balbiani ring (BR) mRNP translocation, and present evidence of rearrangement of the transporter related to mRNP export. Analysis of the individual NPC components verified a strong evolutionary conservation of NPC structure between vertebrates and invertebrates. The transporter is an integral part of the NPC and is composed of a central short double cylinder that is retained within the inner spoke ring, and two peripheral globular assemblies which are tethered to the cytoplasmic and nucleoplasmic coaxial rings by eight conserved internal ring filaments. Distinct stages of BR mRNP nuclear export through the individual NPC components were directly visualized and placed in a linear transport sequence. The BR mRNP first binds to the NPC basket, which forms an expanded distal basket ring. In this communication we present stages of BR mRNP transport through the nucleoplasmic, central and cytoplasmic transporter subunits, which change their conformation during mRNP translocation, and the emergence of mRNP into the cytoplasm. We propose that the reorganization of the basket may be driven, in part, by an active translocation process at the transporter. Furthermore, the images provide dramatic evidence that the transporter functions as a central translocation channel with transiently open discrete gates in its globular assemblies. A model of NPC transporter reorganization accompanied with mRNP translocation is discussed.

Nuclear pore complex structure in birds

The nuclear envelope consists of two parallel membranes enclosing an aqueous lumen. In places there are pores in both membranes at which the two membranes are joined. Within these pores reside the nuclear pore complexes. The current structural models of the nuclear pore complex have been derived from a number of studies using different electron microscopical techniques. Recently, using surface imaging techniques such as field emission in-lens scanning electron microscopy, novel structures have been identified, particularly at the periphery of the structure, most notably the nucleoplasmic basket. One limitation of the current models is that they are based almost entirely on nuclear envelopes isolated from amphibian oocytes and a pressing question is whether this structure is the same in other organisms and tissues. Here we have studied the structure of nuclear envelopes isolated from bird oocytes. We show that the overall structure is remarkably conserved. In particular, recently discovered peripheral structures appear very similar. We see variations in basket conformation but believe that this is related to the functional states of individual pore complexes.

From nucleoporins to nuclear pore complexes

One of the largest supramolecular assemblies in the eukaryotic cell, the nuclear pore complex, is now being dissected into its numerous molecular constituents. The combined use of biochemistry and genetics in yeast has made this rapid development possible. Although less is known about vertebrate nucleoporins, the first clues are now emerging about their in vivo function also. Much remains to be learned about nuclear pore complex assembly and function, however.