Interactions and three-dimensional localization of a group of nuclear pore complex proteins

From the trap to the basket: getting to the bottom of the nuclear pore complex

Nuclear pore complexes (NPCs) are large supramolecular assemblies that perforate the double-membraned nuclear envelope and serve as the sole gateways of molecular exchange between the cytoplasm and the nucleus in interphase cells. Combining novel specimen preparation regimes with innovative use of high-resolution scanning electron microscopy, Hans Ris produced in the late eighties stereo images of the NPC with unparalleled clarity and structural detail, thereby setting new standards in the field. Since that time, efforts undertaken to resolve the molecular structure and architecture, and the numerous interactions that occur between NPC proteins (nucleoporins), soluble transport receptors, and the small GTPase Ran, have led to a deeper understanding of the functional role of NPCs in nucleocytoplasmic transport. In spite of these breakthroughs, getting to the bottom of the actual cargo translocation mechanism through the NPC remains elusive and controversial. Here, we review recent insights into NPC function by correlating structural findings with biochemical data. By introducing new experimental and computational results, we reexamine how NPCs can discriminate between receptor-mediated and passive cargo to promote vectorial translocation in a highly regulated manner. Moreover, we comment on the importance and potential benefits of identifying and experimenting with individual key components implicated in the translocation mechanism. We conclude by dwelling on questions that we feel are pertinent to a more rational understanding of the physical aspects governing NPC mechanics. Last but not least, we substantiate these uncertainties by boldly suggesting a new direction in NPC research as a means to verify such novel concepts, for example, a de novo designed 'minimalist' NPC.

Nuclear pore complex structure and plasticity revealed by electron and atomic force microscopy

To study the ultrastructure of nuclear pore complexes (NPCs), a wide spectrum of different electron microscopy (EM) or atomic force microscopy (AFM) techniques can be employed. The combination of these methods can reveal new insights into the structural and functional organization of this important supramolecular machine through which nucleocytoplasmic transport occurs. Negative staining, quick freezing/freeze-drying/rotary metal shadowing, embedding and thin sectioning, cryoelectron microscopy and tomography, scanning electron microscopy, or combination with immunolabeling techniques are tools for collecting data and information about the three-dimensional structure and architecture of the NPCs. AFM enables investigation of the functional dynamics of native NPCs under physiological conditions.

Nucleoporins NPP-1, NPP-3, NPP-4, NPP-11 and NPP-13 are required for proper spindle orientation in C. elegans

Nucleoporins are components of the nuclear pore, which is required for nucleo-cytoplasmic transport. We report a role for a subclass of nucleoporins in orienting the mitotic spindle in C. elegans embryos. RNAi-mediated depletion of any of five putative nucleoporins npp-1, npp-3, npp-4, npp-11, and npp-13 leads to indistinguishable spindle orientation defects. Transgenic worms expressing NPP-1::GFP or NPP-11::GFP show GFP localization at the nuclear envelope, consistent with their predicted function. NPP-1 interacts with the other nucleoporins in yeast two-hybrid assays, suggesting that the proteins affect spindle orientation by a common process. The failed orientation phenotype of npp-1(RNAi) is at least partially epistatic to the ectopic spindle rotation in the AB blastomere of par-3 mutant embryos. This suggests that NPP-1 contributes to the mechanics of spindle orientation. However, NPP-1 is also required for PAR-6 asymmetry at the two-cell stage, indicating that nucleoporins may be required to define cortical domains in the germ line blastomere P1. Nuclear envelope structure is abnormal in npp-1(RNAi) embryos, but the envelope maintains its integrity, and most nuclear proteins we assayed accumulate normally. These findings raise the possibility that these nucleoporins may have direct roles in orienting the mitotic spindle and the maintenance of cell polarity.

Herpes simplex virus 1 envelopment follows two diverse pathways

Herpesvirus envelopment is assumed to follow an uneconomical pathway including primary envelopment at the inner nuclear membrane, de-envelopment at the outer nuclear membrane, and reenvelopment at the trans-Golgi network. In contrast to the hypothesis of de-envelopment by fusion of the primary envelope with the outer nuclear membrane, virions were demonstrated to be transported from the perinuclear space to rough endoplasmic reticulum (RER) cisternae. Here we show by high-resolution microscopy that herpes simplex virus 1 envelopment follows two diverse pathways. First, nuclear envelopment includes budding of capsids at the inner nuclear membrane into the perinuclear space whereby tegument and a thick electron dense envelope are acquired. The substance responsible for the dense envelope is speculated to enable intraluminal transportation of virions via RER into Golgi cisternae. Within Golgi cisternae, virions are packaged into transport vacuoles containing one or several virions. Second, for cytoplasmic envelopment, capsids gain direct access from the nucleus to the cytoplasm via impaired nuclear pores. Cytoplasmic capsids could bud at the outer nuclear membrane, at membranes of RER, Golgi cisternae, and large vacuoles, and at banana-shaped membranous entities that were found to continue into Golgi membranes. Envelopes originating by budding at the outer nuclear membrane and RER membrane also acquire a dense substance. Budding at Golgi stacks, designated wrapping, results in single virions within small vacuoles that contain electron-dense substances between envelope and vacuolar membranes.

Versatility at the nuclear pore complex: lessons learned from the nucleoporin Nup153

The vertebrate pore protein Nup153 plays pivotal roles in nuclear pore function. In addition to being important to pore architecture, Nup153 is a key participant in both import and export. The scope of Nup153 function also extends beyond the canonical view of the pore as a trafficking gateway. During the transition into mitosis, Nup153 directs proteins involved in membrane remodeling to the nuclear envelope. As cells exit mitosis, Nup153 is recruited to the chromosomal surface, where nuclear pores are formed anew in a complicated process still under much experimental scrutiny. In addition, Nup153 is targeted for protease cleavage during apoptosis and in response to certain viral infections, providing molecular insight into pore reconfiguration during cell response. Overall, the versatile nature of Nup153 underscores an emerging view of the nuclear pore at the nexus of many key cellular processes.

Nucleoporin domain topology is linked to the transport status of the nuclear pore complex

Nuclear pore complexes (NPCs) facilitate macromolecular exchange between the nucleus and cytoplasm of eukaryotic cells. The vertebrate NPC is composed of approximately 30 different proteins (nucleoporins), of which around one third contain phenylalanine-glycine (FG)-repeat domains that are thought to mediate the main interaction between the NPC and soluble transport receptors. We have recently shown that the FG-repeat domain of Nup153 is flexible within the NPC, although this nucleoporin is anchored to the nuclear side of the NPC. By using domain-specific antibodies, we have now mapped the domain topology of Nup214 in Xenopus oocytes and in human somatic cells by immuno-EM. We have found that whereas Nup214 is anchored to the cytoplasmic side of the NPC via its N-terminal and central domain, its FG-repeat domain appears flexible, residing on both sides of the NPC. Moreover, the spatial distribution of the FG-repeat domains of both Nup153 and Nup214 shifts in a transport-dependent manner, suggesting that the location of FG-repeat domains within the NPC correlates with cargo/receptor interactions and that they concomitantly move with cargo through the central pore of the NPC.

The functionally conserved nucleoporins Nup124p from fission yeast and the human Nup153 mediate nuclear import and activity of the Tf1 retrotransposon and HIV-1 Vpr

We report that the fission yeast nucleoporin Nup124p is required for the nuclear import of both, retrotransposon Tf1-Gag as well as the retroviral HIV-1 Vpr. Failure to import Tf1-Gag into the nucleus in a nup124 null mutant resulted in complete loss of Tf1 transposition. Similarly, nuclear import of HIV-1 Vpr was impaired in nup124 null mutant strains and cells became resistant to Vpr's cell-killing activity. On the basis of protein domain similarity, the human nucleoporin Nup153 was identified as a putative homolog of Nup124p. We demonstrate that in vitro-translated Nup124p and Nup153 coimmunoprecipitate Tf1-Gag or HIV-1 Vpr. Though full-length Nup153 was unable to complement the Tf1 transposition defect in a nup124 null mutant, we provide evidence that both nucleoporins share a unique N-terminal domain, Nup124p(AA264-454) and Nup153(AA448-634) that is absolutely essential for Tf1 transposition. Epigenetic overexpression of this domain in a wild-type (nup124(+)) background blocked Tf1 activity implying that sequences from Nup124p and the human Nup153 challenged the same pathway affecting Tf1 transposition. Our results establish a unique relationship between two analogous nucleoporins Nup124p and Nup153 wherein the function of a common domain in retrotransposition is conserved.

Carrier-independent nuclear import of the transcription factor PU.1 via RanGTP-stimulated binding to Nup153

PU.1 is a transcription factor of the Ets family with important functions in hematopoietic cell differentiation. Using green fluorescent protein-PU.1 fusions, we show that the Ets DNA binding domain of PU.1 is necessary and sufficient for its nuclear localization. Fluorescence and ultrastructural nuclear import assays showed that PU.1 nuclear import requires energy but not soluble carriers. PU.1 interacted directly with two nucleoporins, Nup62 and Nup153. The binding of PU.1 to Nup153, but not to Nup62, increased dramatically in the presence of RanGMPPNP, indicating the formation of a PU.1.RanGTP.Nup153 complex. The Ets domain accounted for the bulk of the interaction of PU.1 with Nup153 and RanGMPPNP. Because Nup62 is located close to the midplane of the nuclear pore complex whereas Nup153 is at its nuclear side, these findings suggest a model whereby RanGTP propels PU.1 toward the nuclear side of the nuclear pore complex by increasing its affinity for Nup153. This notion was confirmed by ultrastructural studies using gold-labeled PU.1 in permeabilized cells.

Bidirectional increase in permeability of nuclear envelope upon poliovirus infection and accompanying alterations of nuclear pores

Poliovirus and some other picornaviruses trigger relocation of certain nuclear proteins into the cytoplasm. Here, by using a protein changing its fluorescence color with time and containing a nuclear localization signal (NLS), we demonstrate that the poliovirus-triggered relocation is largely due to the exit of presynthesized nuclear protein into the cytoplasm. The leakiness of the nuclear envelope was also documented by the inability of nuclei from digitonin-permeabilized, virus-infected (but not mock-infected) cells to retain an NLS-containing derivative of green fluorescent protein (GFP). The cytoplasm-to-nucleus traffic was also facilitated during infection, as evidenced by experiments with GAPDH (glyceraldehyde-3-phosphate dehydrogenase), cyclin B1, and an NLS-lacking derivative of GFP, which are predominantly cytoplasmic in uninfected cells. Electron microscopy demonstrated that a bar-like barrier structure in the channel of the nuclear pores, seen in uninfected cells, was missing in the infected cells, giving the impression of fully open pores. Transient expression of poliovirus 2A protease also resulted in relocation of the nuclear proteins. Lysates from poliovirus-infected or 2A-expressing cells induced efflux of 3xEGFP-NLS from the nuclei of permeabilized uninfected cells. This activity was inhibited by the elastase inhibitors elastatinal and N-(methoxysuccinyl)-L-alanyl-L-alanyl-L-prolyl-L-valine chloromethylketone (drugs known also to be inhibitors of poliovirus protease 2A), a caspase inhibitor zVAD(OMe), fmk, and some other protease inhibitors. These data suggest that 2A elicited nuclear efflux, possibly in cooperation with a zVAD(OMe).fmk-sensitive protease. However, poliovirus infection facilitated nuclear protein efflux also in cells deficient in caspase-3 and caspase-9, suggesting that the efflux may occur without the involvement of these enzymes. The biological relevance of nucleocytoplasmic traffic alterations in infected cells is discussed.

Nucleoporins as components of the nuclear pore complex core structure and Tpr as the architectural element of the nuclear basket

The vertebrate nuclear pore complex (NPC) is a macromolecular assembly of protein subcomplexes forming a structure of eightfold radial symmetry. The NPC core consists of globular subunits sandwiched between two coaxial ring-like structures of which the ring facing the nuclear interior is capped by a fibrous structure called the nuclear basket. By postembedding immunoelectron microscopy, we have mapped the positions of several human NPC proteins relative to the NPC core and its associated basket, including Nup93, Nup96, Nup98, Nup107, Nup153, Nup205, and the coiled coil-dominated 267-kDa protein Tpr. To further assess their contributions to NPC and basket architecture, the genes encoding Nup93, Nup96, Nup107, and Nup205 were posttranscriptionally silenced by RNA interference (RNAi) in HeLa cells, complementing recent RNAi experiments on Nup153 and Tpr. We show that Nup96 and Nup107 are core elements of the NPC proper that are essential for NPC assembly and docking of Nup153 and Tpr to the NPC. Nup93 and Nup205 are other NPC core elements that are important for long-term maintenance of NPCs but initially dispensable for the anchoring of Nup153 and Tpr. Immunogold-labeling for Nup98 also results in preferential labeling of NPC core regions, whereas Nup153 is shown to bind via its amino-terminal domain to the nuclear coaxial ring linking the NPC core structures and Tpr. The position of Tpr in turn is shown to coincide with that of the nuclear basket, with different Tpr protein domains corresponding to distinct basket segments. We propose a model in which Tpr constitutes the central architectural element that forms the scaffold of the nuclear basket.

Autoantigens of the nuclear pore complex

The nuclear envelope (NE) is one of many intracellular targets of the autoimmune response in patients with autoimmune liver disease, systemic lupus erythematosus, and related conditions. In eukaryotic organisms the NE consists of five interconnected regions: an outer nuclear membrane (ONM) that is continuous with the endoplasmic reticulum, an intermembrane or perinuclear space, an inner nuclear membrane (INM) with a unique set of integral membrane proteins, the underlying nuclear lamina, and the pore domains that are regions where the ONM and INM come together. The pore domains are sites of regulated continuity between the cytoplasm and nucleus that are occupied by supramolecular structures, termed nuclear pore complexes (NPCs). Human autoantibodies identified to date bind to specific components in three of the five NE compartments. Autoantigen targets include the lamins A, B, and C of the nuclear lamina, gp210, p62 complex proteins, Nup153, and Tpr within the NPC, and LBR, MAN1, LAP1, and LAP2 that are integral proteins of the INM. Autoantibodies to these NE targets have been shown to be correlated with various autoimmune diseases such as primary biliary cirrhosis, other autoimmune liver diseases and systemic rheumatic diseases. Now that the proteome of the NE is more clearly defined, other autoantibodies to components in this cell compartment are likely to be defined.

Actin- and protein-4.1-containing filaments link nuclear pore complexes to subnuclear organelles inXenopusoocyte nuclei

We imaged the interiors of relatively intact Xenopus oocyte nuclei by field emission scanning electron microscopy (feSEM) and visualized a network of filaments that attach to nuclear pore complexes and extend throughout the nucleus. Within the nucleus, these `pore-linked filaments' (PLFs) were embedded into spherical structures 100 nm to ∼5 μm in diameter. A subset of spheres was identified as Cajal bodies by immuno-gold labeling; the rest were inferred to be nucleoli and snurposomes both of which are abundant in Xenopus oocyte nuclei. Most PLFs were independent of chromatin. The thickness of a typical PLF was 40 nm (range, ∼12-100 nm), including the 4 nm chromium coat. PLFs located inside the nucleus merged, bundled and forked, suggesting architectural adaptability. The PLF network collapsed upon treatment with latrunculin A, which depolymerizes actin filaments. Jasplakinolide, which stabilizes actin filaments, produced PLFs with more open substructure including individual filaments with evenly-spaced rows of radially projecting short filaments. Immuno-gold labeling of untreated oocyte nuclei showed that actin and protein 4.1 each localized on PLFs. Protein 4.1-gold epitopes were spaced at ∼120 nm intervals along filaments, and were often paired (∼70 nm apart) at filament junctions. We suggest that protein 4.1 and actin contribute to the structure of a network of heterogeneous filaments that link nuclear pore complexes to subnuclear organelles, and discuss possible functions for PLFs in nuclear assembly and intranuclear traffic.

Nup358/RanBP2 attaches to the nuclear pore complex via association with Nup88 and Nup214/CAN and plays a supporting role in CRM1-mediated nuclear protein export

Nuclear pore complexes (NPCs) traverse the nuclear envelope (NE), providing a channel through which nucleocytoplasmic transport occurs. Nup358/RanBP2, Nup214/CAN, and Nup88 are components of the cytoplasmic face of the NPC. Here we show that Nup88 localizes midway between Nup358 and Nup214 and physically interacts with them. RNA interference of either Nup88 or Nup214 in human cells caused a strong reduction of Nup358 at the NE. Nup88 and Nup214 showed an interdependence at the NPC and were not affected by the absence of Nup358. These data indicate that Nup88 and Nup214 mediate the attachment of Nup358 to the NPC. We show that localization of the export receptor CRM1 at the cytoplasmic face of the NE is Nup358 dependent and represents its empty state. Also, removal of Nup358 causes a distinct reduction in nuclear export signal-dependent nuclear export. We propose that Nup358 provides both a platform for rapid disassembly of CRM1 export complexes and a binding site for empty CRM1 recycling into the nucleus.

Pores for thought: nuclear pore complex proteins

Nuclear pore complexes (NPCs) are enormous macromolecular structures that mediate the active exchange of proteins and RNPs between the nucleus and cytoplasm. Recent work has resulted in a windfall of identified NPC polypeptides, many with unique sequences. Several of the proteins have been shown to be part of extended cytoplasmic and nucleoplasmic NPC filaments. Biochemical, structural and genetic studies on NPC proteins are just beginning to allow an understanding of how they associate into a functional organelle.

The nuclear pore complex: nucleocytoplasmic transport and beyond

Over the past two years, it has become evident that there is an unexpected link between nuclear pore complex structure and dynamics, nucleocytoplasmic transport and chromosome segregation. In addition, a tomographic three-dimensional reconstruction of native nuclear pore complexes preserved in thick amorphous ice has unveiled a number of new structural features of this supramolecular machine. These data, together with some of the elementary physical principles that underlie nucleocytoplasmic transport, will be discussed in this review.

Nup358 integrates nuclear envelope breakdown with kinetochore assembly

Nuclear envelope breakdown (NEBD) and release of condensed chromosomes into the cytoplasm are key events in the early stages of mitosis in metazoans. NEBD involves the disassembly of all major structural elements of the nuclear envelope, including nuclear pore complexes (NPCs), and the dispersal of nuclear membrane components. The breakdown process is facilitated by microtubules of the mitotic spindle. After NEBD, engagement of spindle microtubules with chromosome-associated kinetochores leads to chromatid segregation. Several NPC subunits relocate to kinetochores after NEBD. siRNA-mediated depletion of one of these proteins, Nup358, reveals that it is essential for kinetochore function. In the absence of Nup358, chromosome congression and segregation are severely perturbed. At the same time, the assembly of other kinetochore components is strongly inhibited, leading to aberrant kinetochore structure. The implication is that Nup358 plays an essential role in integrating NEBD with kinetochore maturation and function. Mitotic arrest associated with Nup358 depletion further suggests that mitotic checkpoint complexes may remain active at nonkinetochore sites.

The COPI complex functions in nuclear envelope breakdown and is recruited by the nucleoporin Nup153

Nuclear envelope breakdown is a critical step in the cell cycle of higher eukaryotes. Although integral membrane proteins associated with the nuclear membrane have been observed to disperse into the endoplasmic reticulum at mitosis, the mechanisms involved in this reorganization remain to be fully elucidated. Here, using Xenopus extracts, we report a role for the COPI coatomer complex in nuclear envelope breakdown, implicating vesiculation as an important step. We have found that a nuclear pore protein, Nup153, plays a critical role in directing COPI to the nuclear membrane at mitosis and that this event provides feedback to other aspects of nuclear disassembly. These results provide insight into how key steps in nuclear division are orchestrated.

Importin beta-depending nuclear import pathways: role of the adapter proteins in the docking and releasing steps

Nuclear imports of uridine-rich small nuclear ribonucleoprotein (U1 snRNP) and proteins with classical nuclear localization signal (cNLS-protein) are mediated by importin beta. However, due to the presence of different import signals, the adapter protein of the imported molecules and importin beta is different for each pathway. Although the adapter for cNLS-protein is importin alpha, the adapter for U1 snRNP is snurportin1 (SPN1). Herein, we show that the use of distinct adapters by importin beta results in differences at the docking and releasing step for these two import pathways. Nuclear pore complex (NPC) docking of U1 snRNP but not of cNLS-protein was inhibited by an anti-CAN/Nup214 antibody. Thus, the initial NPC-binding site is different for each pathway. Pull-down assays between immobilized SPN1 and two truncated forms of importin beta documented that SPN1 and importin alpha have different binding sites on importin beta. Importin beta fragment 1-618, which binds to SPN1 but not to importin alpha, was able to support the nuclear import of U1 snRNPs. After the translocation through the NPC, both import complexes associated with the nuclear side of the NPC. However, we found that the nature of the importin beta-binding domain of the adapters influences the release of the cargo into the nucleoplasm.

Cryo-electron tomography provides novel insights into nuclear pore architecture: implications for nucleocytoplasmic transport

To go beyond the current structural consensus model of the nuclear pore complex (NPC), we performed cryo-electron tomography of fully native NPCs from Xenopus oocyte nuclear envelopes (NEs). The cytoplasmic face of the NPC revealed distinct anchoring sites for the cytoplasmic filaments, whereas the nuclear face was topped with a massive distal ring positioned above the central pore with indications of the anchoring sites for the nuclear basket filaments and putative intranuclear filaments. The rather "spongy" central framework of the NPC was perforated by an elaborate channel and void system, and at the membrane pore interface it exhibited distinct "handles" protruding into the lumen of the NE. The most variable structural moiety of the NPC was a rather tenuous central plug partially obstructing the central pore. Its mobile character was documented by time-lapse atomic force microscopy. Taken together, the new insights we gained into NPC structure support the notion that the NPC acts as a constrained diffusion pore for molecules and particles without retention signal and as an affinity gate for signal-bearing cargoes.

Pre-M phase-promoting factor associates with annulate lamellae in Xenopus oocytes and egg extracts

We have used complementary biochemical and in vivo approaches to study the compartmentalization of M phase-promoting factor (MPF) in prophase Xenopus eggs and oocytes. We first examined the distribution of MPF (Cdc2/CyclinB2) and membranous organelles in high-speed extracts of Xenopus eggs made during mitotic prophase. These extracts were found to lack mitochondria, Golgi membranes, and most endoplasmic reticulum (ER) but to contain the bulk of the pre-MPF pool. This pre-MPF could be pelleted by further centrifugation along with components necessary to activate it. On activation, Cdc2/CyclinB2 moved into the soluble fraction. Electron microscopy and Western blot analysis showed that the pre-MPF pellet contained a specific ER subdomain comprising "annulate lamellae" (AL): stacked ER membranes highly enriched in nuclear pores. Colocalization of pre-MPF with AL was demonstrated by anti-CyclinB2 immunofluorescence in prophase oocytes, in which AL are positioned close to the vegetal surface. Green fluorescent protein-CyclinB2 expressed in oocytes also localized at AL. These data suggest that inactive MPF associates with nuclear envelope components just before activation. This association may explain why nuclei and centrosomes stimulate MPF activation and provide a mechanism for targeting of MPF to some of its key substrates.

Domain-specific antibodies reveal multiple-site topology of Nup153 within the nuclear pore complex

Nup153, one of the best characterized nuclear pore complex proteins (nucleoporins), plays a critical role in the import of proteins into the nucleus as well as in the export of RNAs and proteins from the nucleus. Initially an epitope of Nup153 was found to reside at the distal ring of the NPC, whereas more recently another epitope was localized to the nuclear ring moiety of the NPC. In an effort to more definitively determine the location of Nup153 within the 3-D architecture of the NPC we have generated domain-specific antibodies against distinct domains of Xenopus Nup153. With this approach we have found that the N-terminal domain is exposed at the nuclear ring of the NPC, whereas the zinc-finger domain of Nup153 is exposed at the distal ring of the NPC. In contrast, the C-terminal domain of Nup153 is not restricted to one particular subdomain of the NPC but rather appears to be highly flexible. Exogenous epitope-tagged hNup153 incorporated into Xenopus oocyte NPCs further underscored these findings. Our data illustrate that multiple domain-specific antibodies are essential to understanding the topology of a nucleoporin within the context of the NPC. Moreover, this approach has revealed new clues to the mechanisms by which Nup153 may contribute to nucleocytoplasmic transport.

Inhibition of nuclear import and alteration of nuclear pore complex composition by rhinovirus

Nucleocytoplasmic trafficking pathways and the status of nuclear pore complex (NPC) components were examined in cells infected with rhinovirus type 14. A variety of shuttling and nonshuttling nuclear proteins, using multiple nuclear import pathways, accumulated in the cytoplasm of cells infected with rhinovirus. An in vitro nuclear import assay with semipermeabilized infected cells confirmed that nuclear import was inhibited and that docking of nuclear import receptor-cargo complexes at the cytoplasmic face of the NPC was prevented in rhinovirus-infected cells. The relocation of cellular proteins and inhibition of nuclear import correlated with the degradation of two NPC components, Nup153 and p62. The degradation of Nup153 and p62 was not due to induction of apoptosis, because p62 was not proteolyzed in apoptotic HeLa cells, and Nup153 was cleaved to produce a 130-kDa cleavage product that was not observed in cells infected with poliovirus or rhinovirus. The finding that both poliovirus and rhinovirus cause inhibition of nuclear import and degradation of NPC components suggests that this may be a common feature of the replicative cycle of picornaviruses. Inhibition of nuclear import is predicted to result in the cytoplasmic accumulation of a large number of nuclear proteins that could have functions in viral translation, RNA synthesis, packaging, or assembly. Additionally, inhibition of nuclear import also presents a novel strategy whereby cytoplasmic RNA viruses can evade host immune defenses by preventing signal transduction into the nucleus.

Steady-state nuclear localization of exportin-t involves RanGTP binding and two distinct nuclear pore complex interaction domains

Vertebrate tRNA export receptor exportin-t (Xpo-t) binds to RanGTP and mature tRNAs cooperatively to form a nuclear export complex. Xpo-t shuttles bidirectionally through nuclear pore complexes (NPCs) but is mainly nuclear at steady state. The steady-state distribution of Xpo-t is shown to depend on its interaction with RanGTP. Two distinct Xpo-t NPC interaction domains that bind differentially to peripherally localized nucleoporins in vitro are identified. The N terminus binds to both Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner, while the C terminus binds to CAN/Nup214 independently of Ran. We propose that these interactions increase the concentration of tRNA export complexes and of empty Xpo-t in the vicinity of NPCs and thus increase the efficiency of the Xpo-t transport cycle.

The cytoplasmic filaments of the nuclear pore complex are dispensable for selective nuclear protein import

The nuclear pore complex (NPC) mediates bidirectional macromolecular traffic between the nucleus and cytoplasm in eukaryotic cells. Eight filaments project from the NPC into the cytoplasm and are proposed to function in nuclear import. We investigated the localization and function of two nucleoporins on the cytoplasmic face of the NPC, CAN/Nup214 and RanBP2/Nup358. Consistent with previous data, RanBP2 was localized at the cytoplasmic filaments. In contrast, CAN was localized near the cytoplasmic coaxial ring. Unexpectedly, extensive blocking of RanBP2 with gold-conjugated antibodies failed to inhibit nuclear import. Therefore, RanBP2-deficient NPCs were generated by in vitro nuclear assembly in RanBP2-depleted Xenopus egg extracts. NPCs were formed that lacked cytoplasmic filaments, but that retained CAN. These nuclei efficiently imported nuclear localization sequence (NLS) or M9 substrates. NPCs lacking CAN retained RanBP2 and cytoplasmic filaments, and showed a minor NLS import defect. NPCs deficient in both CAN and RanBP2 displayed no cytoplasmic filaments and had a strikingly immature cytoplasmic appearance. However, they showed only a slight reduction in NLS-mediated import, no change in M9-mediated import, and were normal in growth and DNA replication. We conclude that RanBP2 is the major nucleoporin component of the cytoplasmic filaments of the NPC, and that these filaments do not have an essential role in importin alpha/beta- or transportin-dependent import.

Association of the human SUMO-1 protease SENP2 with the nuclear pore

SUMO-1 is a small ubiquitin-like protein that can be covalently conjugated to other proteins. A family of proteases catalyzes deconjugation of SUMO-1-containing species. Members of this family also process newly synthesized SUMO-1 into its conjugatable form. To understand these enzymes better, we have examined the localization and behavior of the human SUMO-1 protease SENP2. Here we have shown that SENP2 associates with the nuclear face of nuclear pores and that this association requires protein sequences near the N terminus of SENP2. We have also shown that SENP2 binds to Nup153, a nucleoporin that is localized to the nucleoplasmic face of the pore. Nup153 binding requires the same domain of SENP2 that mediates its targeting in vivo. Removal of the Nup153-interacting region of SENP2 results in a significant change in the spectrum of SUMO-1 conjugates within the cell. Our results suggest that association with the pore plays an important negative role in the regulation of SENP2, perhaps by restricting its activity to a subset of the conjugated proteins within the nucleus.

Tpr is localized within the nuclear basket of the pore complex and has a role in nuclear protein export

Tpr is a coiled-coil protein found near the nucleoplasmic side of the pore complex. Since neither the precise localization of Tpr nor its functions are well defined, we generated antibodies to three regions of Tpr to clarify these issues. Using light and EM immunolocalization, we determined that mammalian Tpr is concentrated within the nuclear basket of the pore complex in a distribution similar to Nup153 and Nup98. Antibody localization together with imaging of GFP-Tpr in living cells revealed that Tpr is in discrete foci inside the nucleus similar to several other nucleoporins but is not present in intranuclear filamentous networks (Zimowska et al., 1997) or in long filaments extending from the pore complex (Cordes et al., 1997) as proposed. Injection of anti-Tpr antibodies into mitotic cells resulted in depletion of Tpr from the nuclear envelope without loss of other pore complex basket proteins. Whereas nuclear import mediated by a basic amino acid signal was unaffected, nuclear export mediated by a leucine-rich signal was retarded significantly. Nuclear injection of anti-Tpr antibodies in interphase cells similarly yielded inhibition of protein export but not import. These results indicate that Tpr is a nucleoporin of the nuclear basket with a role in nuclear protein export.

Modular self-assembly of a Y-shaped multiprotein complex from seven nucleoporins

Now that it is likely that all yeast nucleoporins are known, one of the ultimate goals is the in vitro assembly of the entire nuclear pore complex from its approximately 30 individual components. Here, we report the reconstitution of seven proteins (Nup133p, Nup145p-C, Nup120p, Nup85p, Nup84p, Seh1p and Sec13p) into a heptameric 0.5 MDa nuclear pore subcomplex. We found that double plasmid transformation combined with bi-cistronic mRNA translation allow the expression and assembly of distinct subcomplexes of up to five nucleoporins in a single Escherichia coli cell. During the sequential reconstitution of the Nup84p complex, smaller assembly intermediates can be isolated, which exhibit modular structures determined by electron microscopy that finally make up the whole Y-shaped Nup84p complex. Importantly, a seventh subunit, Nup133p, was incorporated into the complex through its interaction with Nup84p, thereby elongating one arm of the Y-shaped assembly to an approximately 40 nm long stalk. Taken together, our data document that the Nup84p-Nup133p complex self-assembles in a modular concept from distinct smaller nucleoporin construction sets.

Import of adenovirus DNA involves the nuclear pore complex receptor CAN/Nup214 and histone H1

Adenovirus type 2 (Ad2) imports its DNA genome through the nuclear pore complex (NPC) of cells in interphase for viral production. Here we identify the NPC-filament protein CAN/Nup214 as a docking site for incoming Ad2 capsids. Binding to CAN is independent of cytosolic factors. Capsids disassemble at NPCs to free their DNA for import. This process requires binding of nuclear histone H1 to the stably docked capsids and involves H1-import factors, restricting this irreversible process to the proximity of the nucleus. Our results provide a molecular mechanism for disassembly of Ad2 and reveal an unexpected function of histone H1 in virus-mediated DNA import.

RNA association defines a functionally conserved domain in the nuclear pore protein Nup153

Traffic between the nucleus and cytoplasm takes place through a macromolecular structure termed the nuclear pore complex. To understand how the vital process of nucleocytoplasmic transport occurs, the contribution of individual pore proteins must be elucidated. One such protein, the nucleoporin Nup153, is localized to the nuclear basket of the pore complex and has been shown to be a central component of the nuclear transport machinery. Perturbation of Nup153 function was demonstrated previously to block the export of several classes of RNA cargo. Moreover, these studies also showed that Nup153 can stably associate with RNA in vitro. In this study, we have mapped a domain within Nup153, encompassing amino acids 250-400 in human Nup153, that is responsible for RNA association. After cloning this region of Xenopus Nup153, we performed a cross-species analysis. Despite variation in sequence conservation between Drosophila, Xenopus, and human, this domain of Nup153 displayed robust RNA binding activity in each case, indicating that this property is a hallmark feature of Nup153 and pointing toward a subset of amino acid residues that are key to conferring this ability. We have further determined that a recombinant fragment of Nup153 can bind directly to RNA and that this fragment can interact with endogenous RNA targets. Our findings identify a functionally conserved domain in Nup153 and suggest a role for RNA binding in Nup153 function at the nuclear pore.

In vitro analysis of nuclear transport mediated by the C-terminal shuttle domain of Tap

The Tap protein of higher eukaryotes is implicated in the nuclear export of type D retroviral mRNA and some cellular mRNAs. Here we have developed an in vitro assay to study nuclear export mediated by the C-terminal shuttle domain of Tap involving the rapamycin-induced attachment of this transport domain to a nuclear green fluorescent protein-containing reporter. We found that export by the Tap transport domain does not involve cytosolic transport factors including the GTPase Ran. The transport domain directly binds to several nucleoporins positioned in different regions of the nuclear pore complex. These results argue that a direct interaction of the Tap transport domain with nucleoporins is responsible for its nucleocytoplasmic translocation. We found that the karyopherin beta-related export receptor CRM1 competes with the Tap transport domain for binding to Nup214 but not for binding to Nup62 or Nup153, suggesting that the Tap and CRM1 nuclear export pathways converge at the cytoplasmic periphery of the nuclear pore complex. Because the rates of in vitro nuclear import and export by the Tap transport domain are very similar, the directionality of mRNA export mediated by Tap probably is determined by mechanisms other than simple binding of the Tap transport domain to nucleoporins.

Novel vertebrate nucleoporins Nup133 and Nup160 play a role in mRNA export

RNA undergoing nuclear export first encounters the basket of the nuclear pore. Two basket proteins, Nup98 and Nup153, are essential for mRNA export, but their molecular partners within the pore are largely unknown. Because the mechanism of RNA export will be in question as long as significant vertebrate pore proteins remain undiscovered, we set out to find their partners. Fragments of Nup98 and Nup153 were used for pulldown experiments from Xenopus egg extracts, which contain abundant disassembled nuclear pores. Strikingly, Nup98 and Nup153 each bound the same four large proteins. Purification and sequence analysis revealed that two are the known vertebrate nucleoporins, Nup96 and Nup107, whereas two mapped to ORFs of unknown function. The genes encoding the novel proteins were cloned, and antibodies were produced. Immunofluorescence reveals them to be new nucleoporins, designated Nup160 and Nup133, which are accessible on the basket side of the pore. Nucleoporins Nup160, Nup133, Nup107, and Nup96 exist as a complex in Xenopus egg extracts and in assembled pores, now termed the Nup160 complex. Sec13 is prominent in Nup98 and Nup153 pulldowns, and we find it to be a member of the Nup160 complex. We have mapped the sites that are required for binding the Nup160 subcomplex, and have found that in Nup98, the binding site is used to tether Nup98 to the nucleus; in Nup153, the binding site targets Nup153 to the nuclear pore. With transfection and in vivo transport assays, we find that specific Nup160 and Nup133 fragments block poly[A]+ RNA export, but not protein import or export. These results demonstrate that two novel vertebrate nucleoporins, Nup160 and Nup133, not only interact with Nup98 and Nup153, but themselves play a role in mRNA export.

The nucleoporin Nup153 is required for nuclear pore basket formation, nuclear pore complex anchoring and import of a subset of nuclear proteins

The nuclear pore complex (NPC) is a large proteinaceous structure through which bidirectional transport of macromolecules across the nuclear envelope (NE) takes place. Nup153 is a peripheral NPC component that has been implicated in protein and RNP transport and in the interaction of NPCs with the nuclear lamina. Here, Nup153 is localized by immunogold electron microscopy to a position on the nuclear ring of the NPC. Nuclear reconstitution is used to investigate the role of Nup153 in nucleo- cytoplasmic transport and NPC architecture. NPCs assembled in the absence of Nup153 lacked several nuclear basket components, were unevenly distributed in the NE and, unlike wild-type NPCs, were mobile within the NE. Importin alpha/beta-mediated protein import into the nucleus was strongly reduced in the absence of Nup153, while transportin-mediated import was unaffected. This was due to a reduction in import complex translocation rather than to defective receptor recycling. Our results therefore reveal functions for Nup153 in NPC assembly, in anchoring NPCs within the NE and in mediating specific nuclear import events.

Nuclear envelope dynamics

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.Key words: nuclear envelope, lamin, nuclear pore complex, nuclear membranes, mitosis.

Vaccinia virus DNA replication occurs in endoplasmic reticulum-enclosed cytoplasmic mini-nuclei

Vaccinia virus (vv), a member of the poxvirus family, is unique among most DNA viruses in that its replication occurs in the cytoplasm of the infected host cell. Although this viral process is known to occur in distinct cytoplasmic sites, little is known about its organization and in particular its relation with cellular membranes. The present study shows by electron microscopy (EM) that soon after initial vv DNA synthesis at 2 h postinfection, the sites become entirely surrounded by membranes of the endoplasmic reticulum (ER). Complete wrapping requires ~45 min and persists until virion assembly is initiated at 6 h postinfection, and the ER dissociates from the replication sites. [(3)H]Thymidine incorporation at different infection times shows that efficient vv DNA synthesis coincides with complete ER wrapping, suggesting that the ER facilitates viral replication. Proteins known to be associated with the nuclear envelope in interphase cells are not targeted to these DNA-surrounding ER membranes, ruling out a role for these molecules in the wrapping process. By random green fluorescent protein-tagging of vv early genes of unknown function with a putative transmembrane domain, a novel vv protein, the gene product of E8R, was identified that is targeted to the ER around the DNA sites. Antibodies raised against this vv early membrane protein showed, by immunofluorescence microscopy, a characteristic ring-like pattern around the replication site. By electron microscopy quantitation the protein concentrated in the ER surrounding the DNA site and was preferentially targeted to membrane facing the inside of this site. These combined data are discussed in relation to nuclear envelope assembly/disassembly as it occurs during the cell cycle.

Caspases mediate nucleoporin cleavage, but not early redistribution of nuclear transport factors and modulation of nuclear permeability in apoptosis

In eukaryotic cells, both soluble transport factors and components of the nuclear pore complex mediate protein and RNA trafficking between the nucleus and the cytoplasm. Here, we investigated whether caspases, the major execution system in apoptosis, target the nuclear pore or components of the nuclear transport machinery. Four nucleoporins, Nup153, RanBP2, Nup214 and Tpr are cleaved by caspases during apoptosis. In contrast, the nuclear transport factors, Ran, importin alpha and importin beta are not proteolytically processed, but redistribute across the nuclear envelope independently and prior to caspase activation. Also, mRNA accumulates into the nucleus before caspases become active. Microinjection experiments further revealed that early in apoptosis, the nucleus becomes permeable to dextran molecules of 70 kD molecular weight. Redistribution of import factors and mRNA, as well as nuclear permeabilisation, occur prior to caspase-mediated nucleoporin cleavage. Our findings suggest that the apoptotic programme includes modifications in the machinery responsible for nucleocytoplasmic transport, which are independent from caspase-mediated degradation of nuclear proteins.

Investigation of nucleo-cytoplasmic transport using UV-guided microinjection

Active nucleo-cytoplasmic transport is mediated by dynamic signal-mediated pathways. We investigated the effects of transcription inhibitors or fluorescent lectins on nuclear import mediated by nuclear localization signals (NLSs). Therefore, a novel experimental approach that allows the controlled sequential introduction of fluorescent substances into living cells was established. A microinjection system equipped with an UV-source enabled us to identify fluorescent-labeled cells for the subsequent introduction of additional fluorescent compounds, in order to study their interactions in vivo. Cells were initially labeled either by expression of autofluorescent proteins or by microinjection of fluorescent substances. Transcription inhibitors did not affect nuclear transport mediated by classical NLSs but inhibited import mediated by the M9-domain of hnRNPA1. Comparison of a mono- and bipartite NLS revealed that the bipartite signal was more active in import. Sequential injection of differentially labeled nuclear import and export substrates allowed monitoring of import and export simultaneously in the same living cell. The introduced experimental approach will also be useful to analyze a variety of biological processes in living mammalian cells.

Mitotic checkpoint proteins HsMAD1 and HsMAD2 are associated with nuclear pore complexes in interphase

Mad1 was first identified in budding yeast as an essential component of the checkpoint system that monitors spindle assembly in mitosis and prevents premature anaphase onset. Using antibodies to the human homologue of Mad1 (HsMAD1), we have begun to characterize this protein in mammalian cells. HsMad1 is found localized at kinetochores in mitosis. The labeling is brightest in prometaphase and is absent from kinetochores at metaphase and anaphase. In cells where most chromosomes have reached the metaphase plate, those aligned at the plate show no labeling while remaining, unaligned chromosomes are still brightly labeled. We find HsMad1 associated with HsMad2. Association with p55CDC, a protein previously shown to bind HsMad2, was not detected. Surprisingly, unlike any other known mitotic checkpoint proteins, HsMad1 and HsMAD2 were found localized at nuclear pores throughout interphase. This was confirmed by co-labeling with an antibody to known nuclear pore complex proteins and by their co-purification with enriched nuclear envelope fractions. HsMad1 was identified serendipitously by its binding to a viral protein, HTLV-1 Tax, which affects transcription of viral and human proteins. The localization of HsMad1 to nuclear pore complexes suggests an alternate, non-mitotic role for the Mad1/Tax interaction in the viral transformation of cells.

Effects of poliovirus infection on nucleo-cytoplasmic trafficking and nuclear pore complex composition

Infection of eukaryotic cells with lytic RNA viruses results in extensive interactions of viral gene products with macromolecular pathways of the host, ultimately leading to death of the infected cells. We show here that infection of cells with poliovirus results in the cytoplasmic accumulation of a variety of shuttling and non-shuttling nuclear proteins that use multiple nuclear import pathways. In vitro nuclear import assays using semi-permeabilized infected cells confirmed that nuclear import was blocked and demonstrated that docking of nuclear import receptor-cargo complexes at the cytoplasmic face of the nuclear pore complex (NPC) was prevented. Analysis of components of the NPC revealed that two proteins, Nup153 and p62, were proteolyzed during poliovirus infection. These results suggest that the cytoplasmic relocalization of numerous cellular proteins is caused by the inhibition of multiple nuclear import pathways via alterations in NPC composition in poliovirus-infected cells. Blocking of nuclear import points to a novel strategy by which cytoplasmic RNA viruses can evade host immune defenses, by preventing signal transduction to the nucleus.

Purification of the vertebrate nuclear pore complex by biochemical criteria

The nuclear pore is a large and complex biological machine, mediating all signal-directed transport between the nucleus and the cytoplasm. The vertebrate pore has a mass of approximately 120 million daltons or 30 times the size of a ribosome. The large size of the pore, coupled to its tight integration in the nuclear lamina, has hampered the isolation of pore complexes from vertebrate sources. We have now developed a strategy for the purification of nuclear pores from in vitro assembled annulate lamellae (AL), a cytoplasmic mimic of the nuclear envelope that lacks a lamina, nuclear matrix, and chromatin-associated proteins. We find that purified pore complexes from annulate lamellae contain every nuclear pore protein tested. In addition, immunoblotting reveals the presence of soluble transport receptors and factors known to play important roles in the transport of macromolecules through the pore. While transport factors such as Ran and NTF2 show only transient interaction with the pores, a number of soluble transport receptors, including importin beta, show a tight association with the purified pores. In summary, we report that we have purified the vertebrate pore by biochemical criteria; silver staining reveals approximately 40-50 distinct protein bands.

Identification of a new vertebrate nucleoporin, Nup188, with the use of a novel organelle trap assay

The study of the nuclear pore in vertebrates would benefit from a strategy to directly identify new nucleoporins and interactions between those nucleoporins. We have developed a novel two-step "organelle trap" assay involving affinity selection and in vitro pore assembly. In the first step, soluble proteins derived from Xenopus egg extracts are applied to a column containing a ligand of interest. The bound proteins are then tagged by biotinylation and eluted. In the second step, potential nucleoporins are selected for by virtue of their ability to assemble into annulate lamellae, a cytoplasmic mimic of nuclear pores. The incorporated proteins are then recognized by their biotin tag. Here we use the lectin wheat germ agglutinin (WGA) as ligand; WGA inhibits nuclear transport and has been shown to directly bind three known nucleoporins from Xenopus extract, Nup62, Nup98, and Nup214, all of which contain N-acetylglucosamine residues. Under reduced-stringency conditions, three additional proteins bind to WGA-Sepharose and are revealed by the organelle trap assay. We identified all three as partner nucleoporins. Two were discovered to be Xenopus Nup93 and Nup205. The third is a novel vertebrate nucleoporin, Nup188. This new vertebrate protein, Xenopus Nup188, exists in a complex with xNup93 and xNup205. The Nup93-Nup188-Nup205 complex does not bind directly to WGA but binds indirectly via the N-acetylglucosamine-modified nucleoporins. A gene encoding human Nup188 was also identified. The discovery of vertebrate Nup188, related to a yeast nucleoporin, and its novel protein-protein interactions illustrates the power of the two-step organelle trap assay and identifies new building blocks for constructing the nuclear pore.

Monitoring biomolecular interactions by time-lapse atomic force microscopy

The atomic force microscope (AFM) is a unique imaging tool that enables the tracking of single macromolecule events in response to physiological effectors and pharmacological stimuli. Direct correlation can therefore be made between structural and functional states of individual biomolecules in an aqueous environment. This review explores how time-lapse AFM has been used to learn more about normal and disease-associated biological processes. Three specific examples have been chosen to illustrate the capabilities of this technique. In the cell, actin polymerizes into filaments, depolymerizes, and undergoes interactions with numerous effector molecules (i.e., severing, capping, depolymerizing, bundling, and cross-linking proteins) in response to many different stimuli. Such events are critical for the function and maintenance of the molecular machinery of muscle contraction and the dynamic organization of the cytoskeleton. One goal is to use time-lapse AFM to examine and manipulate some of these events in vitro, in order to learn more about how these processes occur in the cell. Aberrant protein polymerization into amyloid fibrils occurs in a multitude of diseases, including Alzheimer's and type 2 diabetes. Local amyloid deposits may cause organ dysfunction and cell death; hence, it is of interest to learn how to interfere with fibril formation. One application of time-lapse AFM in this area has been the direct visualization of amyloid fibril growth in vitro. This experimental approach holds promise for the future testing of potential therapeutic drugs, for example, by directly visualizing at which level of fibril assembly (i.e., nucleation, elongation, branching, or lateral association of protofibrils) a given active compound will interfere. Nuclear pore complexes (NPCs) are large supramolecular assemblies embedded in the nuclear envelope. Transport of ions, small molecules, proteins, RNAs, and RNP particles in and out of the nucleus occurs via NPCs. Time-lapse AFM has been used to structurally visualize the response of individual NPC particles to various chemical and physical effectors known to interfere with nucleocytoplasmic transport. Taken together, such time-lapse AFM studies could provide novel insights into the molecular mechanisms of fundamental biological processes under both normal and pathological conditions at the single molecule level.

Incorporation of the nuclear pore basket protein nup153 into nuclear pore structures is dependent upon lamina assembly: evidence from cell-free extracts of Xenopus eggs

In cell-free extracts of Xenopus eggs that support the assembly of replication-competent nuclei, we found that lamin B(3) specifically associates with four polypeptides (termed SLAPs, soluble lamin associated proteins). Here, one SLAP is identified as the nuclear pore complex protein Nup153, one member of the F/GXFG motif-containing nucleoporins. In vitro translated Nup153 and lamin B(3) co-immunoprecipitate, and lamin B(3) interacts specifically with the C-terminal domain of Nup153. During nuclear envelope assembly, other F/GXFG-containing nucleoporins are incorporated into the nuclear envelope preceding lamina assembly. Incorporation of Nup153 occurs at the same time as lamina assembly. When lamina assembly is prevented using the dominant-negative mutant XlaminB delta 2+, Nup153 does not appear at the nuclear envelope, while other F/GXFG-containing nucleoporins and Nup93 are recruited normally. When the lamina of pre-assembled nuclei is disrupted using the same dominant-negative mutant, the distribution of other nucleoporins is unaffected. However, Nup153 recruitment at the nuclear envelope is lost. Our results indicate that both the recruitment and maintenance of Nup153 at the pore are dependent upon the integrity of the lamina.

Nup50, a nucleoplasmically oriented nucleoporin with a role in nuclear protein export

We present here a detailed analysis of a rat polypeptide termed Nup50 (formerly NPAP60) that was previously found to be associated with the nuclear pore complex (F. Fan et al., Genomics 40:444-453, 1997). We have found that Nup50 (and/or a related 70-kDa polypeptide) is present in numerous rat cells and tissues. By immunofluorescence microscopy, Nup50 was found to be highly concentrated at the nuclear envelope of rat liver nuclei, whereas in cultured NRK cells it also is abundant in intranuclear regions. On the basis of immunogold electron microscopy of both rat liver nuclear envelopes and NRK cells, we determined that Nup50 is specifically localized in the nucleoplasmic fibrils of the pore complex. Microinjection of anti-Nup50 antibodies into the nucleus of NRK cells resulted in strong inhibition of nuclear export of a protein containing a leucine-rich nuclear export sequence, whereas nuclear import of a protein containing a classical nuclear localization sequence was unaffected. Correspondingly, CRM1, the export receptor for leucine-rich export sequences, directly bound to a fragment of Nup50 in vitro, whereas several other import and export receptors did not significantly interact with this fragment. Taken together, our data indicate that Nup50 has a direct role in nuclear protein export and probably serves as a binding site on the nuclear side of the pore complex for export receptor-cargo complexes.

Phosphorylation of the nuclear transport machinery down-regulates nuclear protein import in vitro

We have examined whether signal-mediated nucleocytoplasmic transport can be regulated by phosphorylation of the nuclear transport machinery. Using digitonin-permeabilized cell assays to measure nuclear import and export, we found that the phosphatase inhibitors okadaic acid and microcystin inhibit transport mediated by the import receptors importin beta and transportin, but not by the export receptor CRM1. Several lines of evidence, including the finding that transport inhibition is partially reversed by the broad specificity protein kinase inhibitor staurosporine, indicate that transport inhibition is due to elevated phosphorylation of a component of the nuclear transport machinery. The kinases and phosphatases involved in this regulation are present in the permeabilized cells. A phosphorylation-sensitive component of the nuclear transport machinery also is present in permeabilized cells and is most likely a component of the nuclear pore complex. Substrate binding by the importin alpha.beta complex and the association of the complex with the nucleoporins Nup358/RanBP2 and Nup153 are not affected by phosphatase inhibitors, suggesting that transport inhibition by protein phosphorylation does not involve these steps. These results suggest that cells have mechanisms to negatively regulate entire nuclear transport pathways, thus providing a means to globally control cellular activity through effects on nucleocytoplasmic trafficking.

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.

Recombinant Nup153 incorporates in vivo into Xenopus oocyte nuclear pore complexes

Nup153 is a molecular constituent of the nuclear basket of the nuclear pore complex (NPC) that plays a critical role in nuclear export of RNAs and proteins. In an effort to map this nucleoporin more precisely within the nuclear basket we have developed an experimental approach for localizing Nup153 expressed and incorporated in vivo into Xenopus oocyte NPCs. This approach involves the microinjection into the cytoplasm of Xenopus oocytes of in vitro synthesized mRNA from a vector encoding an epitope-tagged cDNA. Here we present results obtained by Western blots, fluorescence microscopy, and immuno-electron microscopy, which clearly document that the heterologous protein is properly expressed, targeted, and incorporated into preexisting Xenopus NPCs. This new approach for localizing nucleoporins within the structure of the NPC overcomes limitations of previous techniques and allows for greater specificity and resolution than have been possible with previous methods.

Comparative spatial localization of protein-A-tagged and authentic yeast nuclear pore complex proteins by immunogold electron microscopy

The nuclear pore complex (NPC) mediates protein and RNP import in and RNA and RNP export out of the nucleus of eukaryotic cells. Due to its genetic tractability, yeast offers a versatile system for investigating the chemical composition and molecular architecture of the NPC. In this context, protein A tagging is a commonly used tool for characterizing and localizing yeast NPC proteins (nucleoporins). By preembedding anti-protein A immunogold electron microscopy (immunogold EM), we have localized two yeast nucleoporins, Nsp1p and Nic96p, in mutant yeast strains recombinantly expressing these nucleoporins tagged with four (Nsp1p) or two (Nic96p) IgG binding domains of protein A (i.e., ProtA-Nsp1p and ProtA-Nic96p). We have compared the location of the recombinant fusion proteins ProtA-Nsp1p and ProtA-Nic96p (i.e., as specified by their protein A tag) to the location of authentic Nsp1p and Nic96p (i.e., as defined by the epitopes recognized by corresponding nucleoporin antibodies) and found all of them to reside at the same three NPC sites. Hence, recombinant expression and protein A tagging of the nucleoporins Nsp1p and Nic96p have not caused any significant mislocation of the fusion proteins and thus enabled mapping of these two yeast nucleoporins at the ultrastructural level in a faithful manner.

P granules in the germ cells of Caenorhabditis elegans adults are associated with clusters of nuclear pores and contain RNA

The germ cells, and germ cell precursors, in the nematode Caenorhabditis elegans contain distinctive granules called P granules. During early embryogenesis, P granules are segregated asymmetrically into those blastomeres that eventually produce the germ line. Because of the correlation between P granule distribution and the development of the germ line, P granules are widely thought to function in some aspect of germ line specification or differentiation. Most of the analysis of P granule structure and localization has focused on the early embryo, when P granules are located in the cytoplasm. However, during most of development P granules are associated with germ cell nuclei. We report here an ultrastructural analysis of the nuclear-associated P granules in the germ cells of the adult hermaphrodite gonad. We show that P granules are tightly associated with nuclear pores and that the positions of certain structures within the P granules correspond to the positions of pores on the nuclear envelope. We present immunocytochemical and ultrastructural data suggesting that P granules can associate, or remain associated, with pore-like structures even after they detach from the nuclear envelope during oogenesis. Finally, we show that nuclear-associated P granules in the gonad contain RNA, complementing previous studies showing that cytoplasmic P granules in embryos contain RNA.