ATP-Induced shape change of nuclear pores visualized with the atomic force microscope

Unveiling the complexity: assessing models describing the structure and function of the nuclear pore complex

The nuclear pore complex (NPC) serves as a pivotal subcellular structure, acting as a gateway that orchestrates nucleocytoplasmic transport through a selectively permeable barrier. Nucleoporins (Nups), particularly those containing phenylalanine-glycine (FG) motifs, play indispensable roles within this barrier. Recent advancements in technology have significantly deepened our understanding of the NPC's architecture and operational intricacies, owing to comprehensive investigations. Nevertheless, the conspicuous presence of intrinsically disordered regions within FG-Nups continues to present a formidable challenge to conventional static characterization techniques. Historically, a multitude of strategies have been employed to unravel the intricate organization and behavior of FG-Nups within the NPC. These endeavors have given rise to multiple models that strive to elucidate the structural layout and functional significance of FG-Nups. Within this exhaustive review, we present a comprehensive overview of these prominent models, underscoring their proposed dynamic and structural attributes, supported by pertinent research. Through a comparative analysis, we endeavor to shed light on the distinct characteristics and contributions inherent in each model. Simultaneously, it remains crucial to acknowledge the scarcity of unequivocal validation for any of these models, as substantiated by empirical evidence.

Nuclear pores dilate and constrict in cellulo

[Figure: see text].

An Ancient, Unified Mechanism for Metformin Growth Inhibition in C. elegans and Cancer

Metformin has utility in cancer prevention and treatment, though the mechanisms for these effects remain elusive. Through genetic screening in C. elegans, we uncover two metformin response elements: the nuclear pore complex (NPC) and acyl-CoA dehydrogenase family member-10 (ACAD10). We demonstrate that biguanides inhibit growth by inhibiting mitochondrial respiratory capacity, which restrains transit of the RagA-RagC GTPase heterodimer through the NPC. Nuclear exclusion renders RagC incapable of gaining the GDP-bound state necessary to stimulate mTORC1. Biguanide-induced inactivation of mTORC1 subsequently inhibits growth through transcriptional induction of ACAD10. This ancient metformin response pathway is conserved from worms to humans. Both restricted nuclear pore transit and upregulation of ACAD10 are required for biguanides to reduce viability in melanoma and pancreatic cancer cells, and to extend C. elegans lifespan. This pathway provides a unified mechanism by which metformin kills cancer cells and extends lifespan, and illuminates potential cancer targets. PAPERCLIP.

Calcium regulation of nucleocytoplasmic transport

Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calciumregulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule superresolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.

Exceptional structural and mechanical flexibility of the nuclear pore complex

Nuclear pore complexes (NPCs) mediate all transport between the cytosol and the nucleus and therefore take centre stage in physiology. While transport through NPCs has been extensively investigated little is known about their structural and barley anything about their mechanical flexibility. Structural and mechanical flexibility of NPCs, however, are presumably of key importance. Like the cell and the cell nucleus, NPCs themselves are regularly exposed to physiological mechanical forces. Besides, NPCs reveal striking transport properties which are likely to require fairly high structural flexibility. The NPC transports up to 1,000 molecules per second through a physically 9 nm wide channel which repeatedly opens to accommodate macromolecules significantly larger than its physical diameter. We hypothesised that NPCs possess remarkable structural and mechanical stability. Here, we tested this hypothesis at the single NPC level using the nano-imaging and probing approach atomic force microscopy (AFM). AFM presents the NPC as a highly flexible structure. The NPC channel dilates by striking 35% on exposure to trans-cyclohexane-1,2-diol (TCHD), which is known to transiently collapse the hydrophobic phase in the NPC channel like receptor-cargo complexes do in transit. It constricts again to its initial size after TCHD removal. AFM-based nano-indentation measurements show that the 50 nm long NPC basket can astonishingly be squeezed completely into the NPC channel on exposure to incremental mechanical loads but recovers its original vertical position within the nuclear envelope plane when relieved. We conclude that the NPC possesses exceptional structural and mechanical flexibility which is important to fulfilling its functions.

Nuclear pore complex: biochemistry and biophysics of nucleocytoplasmic transport in health and disease

Nuclear pore complexes (NPCs) are the gateways connecting the nucleoplasm and cytoplasm. This structures are composed of over 30 different proteins and 60-125 MDa of mass depending on type of species. NPCs are bilateral pathways that selectively control the passage of macromolecules into and out of the nucleus. Molecules smaller than 40 kDa diffuse through the NPC passively while larger molecules require facilitated transport provided by their attachment to karyopherins. Kinetic studies have shown that approximately 1000 translocations occur per second per NPC. Maintaining its high selectivity while allowing for rapid translocation makes the NPC an efficient chemical nanomachine. In this review, we approach the NPC function via a structural viewpoint. Putting together different pieces of this puzzle, this chapter confers an overall insight into what molecular processes are engaged in import/export of active cargos across the NPC and how different transporters regulate nucleocytoplasmic transport. In the end, the correlation of several diseases and disorders with the NPC structural defects and dysfunctions is discussed.

Rate and extent of protein localization is controlled by peptide-binding domain association kinetics and morphology

Protein localization is an important regulatory mechanism in many cell signaling pathways such as cytoskeletal organization and genetic regulation. The specific mechanism of protein localization determines the kinetics and morphological constraints of protein translocation, and thus affects the rate and extent of localization. To investigate the affect of localization kinetics and morphology on protein localization, we designed a protein localization system based on Ca(2+)-calmodulin and Src homology 3 domain binding peptides that can translocate between specific localizations in response to a Ca(2+) signal. We used a stochastic biomolecular simulator to predict that such a protein localization system will exhibit slower and less complete translocations when the association kinetics of a binding domain and peptide are reduced. As well, we predicted that increasing the diffusion resistance by manipulating the morphology of the system would similarly impair translocation speed and completeness. We then constructed a network of synthetic fusion proteins and showed that these predictions could be qualitatively confirmed in vitro. This work provides a basis for explaining the different characteristics (rate and extent) of protein transport and localization in cells as a consequence of the kinetics and morphology of the transport mechanism.

The role of the nuclear transport system in cell differentiation

The eukaryotic cell nuclear transport system selectively mediates molecular trafficking to facilitate the regulation of cellular processes. The components of this system include diverse transport factors such as importins and nuclear pore components that are precisely organized to coordinate cellular events. A number of studies have demonstrated that the nuclear transport system is indispensible in many types of cellular responses. In particular, the nuclear transport machinery has been shown to be an important regulator of development, organogenesis, and tissue formation, wherein altered nuclear transport of key transcription factors can lead to disease. Importantly, precise switching between distinct forms of importin alpha is central to neural lineage specification, consistent with the hypothesis that importin expression can be a key mediator of cell differentiation.

Structure, dynamics and function of nuclear pore complexes

Nuclear pore complexes are large aqueous channels that penetrate the nuclear envelope, thereby connecting the nuclear interior with the cytoplasm. Until recently, these macromolecular complexes were viewed as static structures, the only function of which was to control the molecular trafficking between the two compartments. It has now become evident that this simplistic scenario is inaccurate and that nuclear pore complexes are highly dynamic multiprotein assemblies involved in diverse cellular processes ranging from the organization of the cytoskeleton to gene expression. In this review, we discuss the most recent developments in the nuclear-pore-complex field, focusing on the assembly, disassembly, maintenance and function of this macromolecular structure.

On the octagonal structure of the nuclear pore complex: insights from coarse-grained models

The basic structure of the nuclear pore complex (NPC), conserved across almost all organisms from yeast to humans, persists in featuring an octagonal symmetry involving the nucleoporins that constitute the NPC ring. In this article, we seek to understand and evaluate the potential biomechanical reasons for this eightfold symmetry. Our analytical investigation shows that the eightfold symmetry maximizes the bending stiffness of each of the eight NPC spokes while our computational analyses identify the most likely deformation modes, frequencies, and associated kinetic energies of the NPC. These modes have energies close to other published findings using membrane analysis of the nuclear membrane pore opening, and deformation states in agreement with experimental observations. A better understanding of NPC mechanics is essential for characterizing the nucleocytoplasmic transport, which has a central importance in cell biology.

Changes in nucleoporin domain topology in response to chemical effectors

Nucleoporins represent the molecular building blocks of nuclear pore complexes (NPCs), which mediate facilitated macromolecular trafficking between the cytoplasm and nucleus of eukaryotic cells. Phenylalanine-glycine (FG) repeat motifs are found in about one-third of the nucleoporins, and they provide major binding or docking sites for soluble transport receptors. We have shown recently that localization of the FG-repeat domains of vertebrate nucleoporins Nup153 and Nup214 within the NPC is influenced by its transport state. To test whether chemical effectors, such as calcium and ATP, influence the localization of the FG-repeat domains of Nup153 and Nup214 within the NPC, we performed immuno-electron microscopy of Xenopus oocyte nuclei using domain-specific antibodies against Nup153 and Nup214, respectively. Ca2+ and ATP are known to induce conformational changes in the NPC architecture, especially at the cytoplasmic face, but also at the nuclear basket of the NPC. We have found concentrations of calcium in the micromolar range or 1 mM ATP in the surrounding buffer leaves the spatial distribution of the FG-repeat of Nup153 and Nup214 largely unchanged. In contrast, ATP depletion, calcium store depletion by EGTA or thapsigargin, and high concentrations of divalent cation (i.e. 2 mM Ca2+ and 2 mM Mg2+) constrain the distribution of the FG-repeats of Nup153 and Nup214. Our data suggest that the location of the FG-repeat domains of Nup153 and Nup214 is sensitive to chemical changes within the near-field environment of the NPC.

Plugs in nuclear pores: transcripts in early oocyte development identified with nanotechniques

Throughout oogenesis, huge amounts of RNA are produced that are needed for early development. Early stages of oocyte development are characterized by high transcriptional activity whereas translation of maternal RNA dominates late stages. Nuclear pore complexes (NPCs), located in the nuclear envelope (NE), mediate bidirectional macromolecule exchange between the nuclear and cytosolic compartments including RNA export. Here, we report on structural correlates of this transport pathway at single NPC level. Using atomic force microscopy (AFM), we imaged the nucleoplasmic ("inner") surface of the NE of Xenopus laevis oocytes in different stages of development. We found that NPC frequency per nucleus increases with maturation. However, individual NPCs are more active in immature stages. In early stages, known for high transcriptional activity, we found nearly 10% of NPC central channels plugged with a 400-800 kDa mass. In contrast, the incidence of plugged NPCs was below 1% in late oocyte stages. On-site RNA digestion led to a change in plug shape from prominent to flat while plug mass decreased by almost 20%. Quantitative AFM analysis revealed that RNase exposure reduced total nucleoplasmic NPC mass by about 58 and 25% in early and late stage oocytes, respectively. We conclude: (i) NPCs of immature oocytes are more active in RNA transport, (ii) Plugs identified at the nucleoplasmic entrance of NPC central channels represent ribonucleoproteins exiting the nucleus, (iii) RNA is a structural component of the NPC nanomachine.

The role of nuclear envelope calcium in modifying nuclear pore complex structureThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Some of the most important trafficking processes in cells involve transport across the nuclear envelope. Whether it is the import of transcription factors or the export of RNA, the only known portal across the double lipid bilayer that forms the nuclear envelope are the macromolecular pores known as nuclear pore complexes (NPCs). Understanding how signals influence the conformation of the NPC is important for testing models of, and perhaps modifying, transport across the nuclear envelope. Here we summarize high-resolution atomic force microscopy studies of NPC structure following manipulation of nuclear envelope calcium stores of nuclei from Xenopus laevis oocytes. The results show that the release of calcium from these stores through the specific activation of inositol 1,4,5-trisphosphate receptors leads to changes in NPC structure observable from both sides of the nuclear envelope. The diameter of the NPC is also sensitive to these calcium stores and increases upon calcium release. Western blot analysis reveals the presence of ryanodine receptors in the nuclear envelope of X. laevis oocytes, although in low abundance. Activation of these calcium channels also leads to the displacement of the central mass and changes in NPC diameter. This change in structure may involve a displacement of the cytoplasmic and nuclear rings of the NPC towards each other, leading to the apparent emergence of the central mass from both sides of the NPC. The changes in conformation and diameter of the NPC may alter cargo access and binding to phenylalanine-glycine repeats lining the pore, thus altering transport.

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.

Relationship between RNA synthesis and the Ca2+-filled state of the nuclear envelope store

RNA synthesis and ATP-dependent (45)Ca(2+) uptake were measured simultaneously in isolated nuclear fraction of rat liver nuclei. Maximal level of RNA synthesis was obtained under ATP-dependent (45)Ca(2+)-uptake conditions (1 microM free [Ca(2+)] and 1 mM ATP in the bathing solution). This experimental condition was defined as "stimulated nuclei" condition. ATP-dependent (45)Ca(2+) uptake was inhibited using different strategies including: (a) eliminating Ca(2+) (1 mM EGTA); (b) lowering the ATP concentration; (c) modifying nuclear envelope membranes Ca(2+) permeability (Ca(2+) ionophores); or (d) inhibiting the nuclear Ca(2+) pump (thapsigargin and 3',3'',5',5''-tetraiodophenolsulfonephthalein). Under all the above conditions, RNA synthesis was lower than in "stimulated nuclei" condition. In the presence of ionomycin, RNA synthesis was significantly higher at 500 nM free [Ca(2+)], as compared with RNA synthesis in a Ca(2+)-free medium or at 1muM free [Ca(2+)]. However, even in such condition (500 nM free [Ca(2+)]), RNA synthesis was lower than RNA synthesis obtained in "stimulated nuclei" condition. We suggest two components for the effect of Ca(2+) on RNA synthesis: (A) a direct effect of nucleoplasmic [Ca(2+)]; and (B) an effect dependent on the accumulation of Ca(2+) in the nuclear envelope store mediated by the SERCA nuclear Ca(2+) pump.

Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophrys sol. Upon addition of Ca2+ ([Ca2+]free = 2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5+/-1.7 microm to 11.0+/-1.3 microm. The threshold level of [Ca2+]free for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus.

Steroids dilate nuclear pores imaged with atomic force microscopy

Macromolecules that act in the cell nucleus must overcome the nuclear envelope (NE). This barrier between cytosol and the nucleus is perforated by nuclear pore complexes (NPCs) that serve as translocation machineries. We visualized the translocation process at the NE surface, applying a nanotechnical approach using atomic force microscopy (AFM). In order to initiate protein targeting to NPCs, dexamethasone (dex) was injected into Xenopus laevis oocytes. Dex is a synthetic steroid of great therapeutic relevance that specifically binds to glucocorticoid receptors and thus triggers an intracellular signal cascade involving the cell nucleus. Ninety and 180 sec after dex injection cell nuclei were isolated, the NEs spread on glass and scanned with AFM. With single molecule resolution we observed that dex initiated proteins (DIPs) first bind to NPC-free areas of the outer nuclear membrane. This causes NPCs to dilate. Then, in a second step, DIPs attach directly to NPCs and enter the dilated central channels. DIPs accumulation and NPC conformational changes were blocked by RU486, a specific glucocorticoid receptor antagonist. In conclusion, dex exposure induces NPC dilation. NPCs change conformation already prior to transport. The NPC dilation signal is most likely transmitted through NPC associated filaments or yet unknown structures in the NE outer membrane. NPC dilation could have significant impact on nuclear targeting of therapeutic macromolecules.

High-resolution near-field optical imaging of single nuclear pore complexes under physiological conditions

Scanning near-field optical microscopy (SNOM) circumvents the diffraction limit of conventional light microscopy and is able to achieve optical resolutions substantially below 100 nm. However, in the field of cell biology SNOM has been rarely applied, probably because previous techniques for sample-distance control are less sensitive in liquid than in air. Recently we developed a distance control based on a tuning fork in tapping mode, which is also well-suited for imaging in solution. Here we show that this approach can be used to visualize single membrane protein complexes kept in physiological media throughout. Nuclear envelopes were isolated from Xenopus laevis oocytes at conditions shown recently to conserve the transport functions of the nuclear pore complex (NPC). Isolated nuclear envelopes were fluorescently labeled by antibodies against specific proteins of the NPC (NUP153 and p62) and imaged at a resolution of approximately 60 nm. The lateral distribution of epitopes within the supramolecular NPC could be inferred from an analysis of the intensity distribution of the fluorescence spots. The different number densities of p62- and NUP153-labeled NPCs are determined and discussed. Thus we show that SNOM opens up new possibilities for directly visualizing the transport of single particles through single NPCs and other transporters.

Nuclear envelope barrier leak induced by dexamethasone

Nuclear pore complexes (NPCs) are multiprotein channels that span the nuclear envelope. They strongly limit the efficiency of gene transfection by restriction of nuclear delivery of exogenously applied therapeutic macromolecules. NPC dilation could significantly increase this efficiency. Recently, it was shown in oocytes of Xenopus laevis that NPCs dilate from about 82 to 110 nm within min after injection of the glucocorticoid analog dexamethasone (dex). In the present paper we analyzed by means of atomic force microscopy the structural details of NPC dilation and correlated them with functional changes in nuclear envelope permeability. 5-11 min after Dex injection NPC dilation was found at its maximum (approximately 140 nm). In addition, a yet unknown configuration, so-called giant pore, up to 300 nm in diameter, was visualized. Giant pore formation was paralleled by an increase in nuclear envelope permeability tested by electrophysiology and confocal fluorescence microscopy. Even large macromolecules lacking any nuclear localization signal (77 kDa FITC-dextran, molecule diameter up to 36 nm) could gain access to the nucleus. We conclude that dex transiently opens unspecific pathways for large macromolecules. Dex treatment could be potentially useful for improving the efficiency of nuclear gene transfection.

Passive transport of macromolecules through Xenopus laevis nuclear envelope

Although nuclear pore complexes (NPC) are considered to be key structures in gene expression, little is known about their regulatory control. In order to explore the regulatory mechanism of passive transport of small macromolecules we examined the influence of different factors on the diffusional pathway of NPCs in isolated Xenopus laevis oocyte nuclei. Diffusion of fluorescence-labeled 10-kD dextran was measured across the nuclear envelope with confocal fluorescence microscopy. Surprisingly, the filling state of the perinuclear Ca(2+) store had no influence on passive transport of 10-kD dextran. Furthermore, nuclear envelope permeability was independent of cytoplasmic pH (pH range 8.3-6.3). In contrast, nuclear swelling, induced by omission of the endogenous cytosolic macromolecules, clearly increased nuclear permeability. An antibody against the glycoprotein gp62, located at the central channel entrance, reduced macromolecule diffusion. In addition, nuclei from transcriptionally active, early developmental stages (stage II) were less permeable compared to transcriptionally inactive, late-developmental-stage (stage VI) nuclei. In stage II nuclei, atomic force microscopy disclosed NPC central channels with plugs that most likely were ribonucleoproteins exiting the nucleus. In conclusion, the difference between macromolecule permeability and previous measurements of electrical resistance strongly indicates separate routes for macromolecules and ions across the nuclear envelope.

Lipid rafts: feeling is believing

In the late 1990s, accumulated evidence led to the proposal that biological membranes are composed of microdomains of different lipids, which form functional "rafts." Recent work using atomic force microscopy has given us new insights into the factors influencing the formation and behavior of these physiological microenvironments

Modulation of [Ca2+]i signaling dynamics and metabolism by perinuclear mitochondria in mouse parotid acinar cells

Parotid acinar cells exhibit rapid cytosolic calcium signals ([Ca2+]i) that initiate in the apical region but rapidly become global in nature. These characteristic [Ca2+]i signals are important for effective fluid secretion, which critically depends on a synchronized activation of spatially separated ion fluxes. Apically restricted [Ca2+]i signals were never observed in parotid acinar cells. This is in marked contrast to the related pancreatic acinar cells, where the distribution of mitochondria has been suggested to contribute to restricting [Ca2+]i signals to the apical region. Therefore, the aim of this study was to determine the mitochondrial distribution and the role of mitochondrial Ca2+ uptake in shaping the spatial and temporal properties of [Ca2+]i signaling in parotid acinar cells. Confocal imaging of cells stained with MitoTracker dyes (MitoTracker Green FM or MitoTracker CMXRos) and SYTO dyes (SYTO-16 and SYTO-61) revealed that a majority of mitochondria is localized around the nucleus. Carbachol (CCh) and caged inositol 1,4,5-trisphosphate-evoked [Ca2+]i signals were delayed as they propagated through the nucleus. This delay in the CCh-evoked nuclear [Ca2+]i signal was abolished by inhibition of mitochondrial Ca2+ uptake with ruthenium red and Ru360. Likewise, simultaneous measurement of [Ca2+]i with mitochondrial [Ca2+] ([Ca2+]m), using fura-2 and rhod-FF, respectively, revealed that mitochondrial Ca2+ uptake was also inhibited by ruthenium red and Ru360. Finally, at concentrations of agonist that evoke[Ca2+]i oscillations, mitochondrial Ca2+ uptake, and a nuclear [Ca2+] delay, CCh also evoked a substantial increase in NADH autofluorescence. This autofluorescence exhibited a predominant perinuclear localization that was also sensitive to mitochondrial inhibitors. These data provide evidence that perinuclear mitochondria and mitochondrial Ca2+ uptake may differentially shape nuclear [Ca2+] signals but more importantly drive mitochondrial metabolism to generate ATP close to the nucleus. These effects may profoundly affect a variety of nuclear processes in parotid acinar cells while facilitating efficient fluid secretion.

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.

Actin-filled nuclear invaginations indicate degree of cell de-differentiation

For years the existence of nuclear actin has been heavily debated, but recent data have clearly demonstrated that actin, as well as actin-binding proteins (ABPs), are located in the nucleus. We examined live EGFP-actin-expressing cells using confocal microscopy and saw the presence of structures strongly resembling actin filaments in the nuclei of MDA-MB-231 human mammary epithelial tumor cells. Many nuclei had more than one of these filamentous structures, some of which appeared to cross the entire nucleus. Extensive analysis, including fluorescence recovery after photobleaching (FRAP), showed that all EGFP-actin in the nucleus is monomeric (G-actin) rather than filamentous (F-actin) and that the apparent filaments seen in the nucleus are invaginations of cytoplasmic monomeric actin. Immunolocalization of nuclear pore complex proteins shows that similar invaginations are seen in cells that are not overexpressing EGFP-actin. To determine whether there is a correlation between increased levels of invagination in the cell nuclei and the state of de-differentiation of the cell, we examined a variety of cell types, including live Xenopus embryonic cells. Cells that were highly de-differentiated, or cancerous, had an increased incidence of invagination, while cells that were differentiated had few nuclear invaginations. The nuclei of embryonic cells that were not yet differentiated underwent multiple shape changes throughout interphase, and demonstrated numerous transient invaginations of varying sizes and shapes. Although the function of these actin-filled invaginations remains speculative, their presence correlates with cells that have increased levels of nuclear activity.

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.

Nuclear pore complexes exceeding eightfold rotational symmetry

Nuclear pore complexes are rotationally symmetric structures that span the nuclear envelope and provide channels for nucleocytoplasmic traffic. These large complexes normally consist of eight spokes arranged around a central channel, although, occasionally, 9- and 10-fold nuclear pore complexes are found in preparations of Xenopus oocyte macronuclei. Here we examine these unusual nuclear pore complexes by negative stain electron microscopy and image analysis and compare the results with data previously obtained from 8-fold structures. The details in two-dimensional and three-dimensional maps indicate that the substructure of the spoke is the same in 8-, 9- and 10-fold nuclear pore complexes: therefore, the spoke is likely an immutable structural component. In all three variant forms, the spacing between adjacent annular subunits, which surround the central channel, is identical. Distances between spokes at higher radius decrease in the 9- and 10-fold nuclear pore complexes. These data imply that the most important connections holding the nuclear pore complex together are those between adjacent annular subunits and that these interactions may play a predominant role in nuclear pore complex assembly. Circumferential connections mediated by ring subunits and radial arms presumably further stabilize the structure and are flexible enough to accommodate additional spokes.

Ca2+-dependent contractility of isolated and demembranated macronuclei in the hypotrichous ciliate Euplotes aediculatus

The hypotrichous ciliated protozoan Euplotes aediculatus possesses a characteristic C-shaped somatic nucleus (macronucleus) within the cytoplasm, which shows dynamic shape change during the cell cycle. It is shown that isolated macronuclei possess Ca(2+)-dependent contractility. Macronuclei were isolated, stuck fast on the glass surface, and subjected to different concentrations of Ca(2+) in a Ca(2+)-EGTA buffer. The nuclei became expanded at [Ca(2+)]<10(-7)M, and they contracted on subsequent addition of higher concentrations of Ca(2+). Cycles of expansion and contraction of the nucleus could be repeated many times by alternate addition of EGTA and Ca(2+), indicating that the size of isolated nuclei can be regulated by [Ca(2+)] alone. The nuclear contraction was observed in all phases of the cell cycle, but contractility was less evident around replication bands in the S phase. In addition to the hypotrichous ciliate Euplotes, similar Ca(2+)-dependent nuclear contractility was found to exist in other cell types, including protozoans of different taxa (a heliozoon Actinophrys sol and a peniculine ciliate Paramecium bursaria), and also mammalian culture cells (HeLa cells). Our findings suggest a possibility that Ca(2+)-dependent nuclear contractility may be shared among diverse eukaryotic organisms.

Nuclear transport kinetics in microarrays of nuclear envelope patches

Optical Single Transporter Recording (OSTR) is a technique for analyzing membrane transport kinetics at high sensitivity, selectivity, and spatial resolution. Cellular membranes are firmly attached to microarrays of small test compartments (TCs) with diameters between approximately 0.1 and 100 microm and depths between approximately 10 and 100 microm. This permits to generate either "small" membrane patches containing few transporters or "large" patches containing many transporters. Transport of substrates across membrane patches is recorded by confocal microscopy. The present article reviews recent applications of OSTR to the nuclear pore complex (NPC). The results show that the transport functions of the NPC, previously studied almost exclusively in intact and permeabilized cells, are conserved in isolated nuclei and can be fully reconstituted in purified nuclear envelopes by addition of recombinant transport factors. This opens new avenues to the analysis of nuclear transport including the export of nucleic-acid-protein and ribosomal particles.

Regulation of nuclear pore complex conformation by IP(3) receptor activation

In recent years, both the molecular architecture and functional dynamics of nuclear pore complexes (NPCs) have been revealed with increasing detail. These large, supramolecular assemblages of proteins form channels that span the nuclear envelope of cells, acting as crucial regulators of nuclear import and export. From the cytoplasmic face of the nuclear envelope, nuclear pore complexes exhibit an eightfold symmetric ring structure encompassing a central lumen. The lumen often appears occupied by an additional structure alternatively referred to as the central granule, nuclear transport complex, or nuclear plug. Previous studies have suggested that the central granule may play a role in mediating calcium-dependent regulation of diffusion across the nuclear envelope for intermediate sized molecules (10-40 kDa). Using atomic force microscopy to measure the surface topography of chemically fixed Xenopus laevis oocyte nuclear envelopes, we present measurements of the relative position of the central granule within the NPC lumen under a variety of conditions known to modify nuclear Ca(2+) stores. These measurements reveal a large, approximately 9-nm displacement of the central granule toward the cytoplasmic face of the nuclear envelope under calcium depleting conditions. Additionally, activation of nuclear inositol triphosphate (IP(3)) receptors by the specific agonist, adenophostin A, results in a concentration-dependent displacement of central granule position with an EC(50) of ~1.2 nM. The displacement of the central granule within the NPC is observed on both the cytoplasmic and nucleoplasmic faces of the nuclear envelope. The displacement is blocked upon treatment with xestospongin C, a specific inhibitor of IP(3) receptor activation. These results extend previous models of NPC conformational dynamics linking central granule position to depletion of IP(3) sensitive nuclear envelope calcium stores.

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.

Aldosterone signaling pathway across the nuclear envelope

We describe the route by which aldosterone-triggered macromolecules enter and exit the cell nucleus of Xenopus laevis oocyte. Oocytes were microinjected with 50 fmol aldosterone and then enucleated 2-30 min after injection. After isolation, nuclear envelope electrical resistance (NEER) was measured in the intact cell nuclei by using the nuclear hourglass technique. We observed three NEER stages: an early peak 2 min after injection, a sustained depression after 5-15 min, and a final late peak 20 min after injection. Because NEER reflects the passive electrical permeability of nuclear pores, we investigated with atomic force microscopy aldosterone-induced conformational changes of individual nuclear pore complexes (NPCs). At the early peak we observed small ( congruent with 100 kDa) molecules (flags) attached to the NPC surface. At the sustained depression NPCs were found free of flags. At the late peak large ( congruent with 800 kDa) molecules (plugs) were detected inside the central channels. Ribonuclease or actinomycin D treatment prevented the late NEER peak. Coinjection of aldosterone (50 fmol) and its competitive inhibitor spironolactone (500 fmol) eliminated the electrical changes as well as flag and plug formation. We conclude: (i) The genomic response of aldosterone can be electrically measured in intact oocyte nuclei. (ii) Flags represent aldosterone receptors on their way into the cell nucleus whereas plugs represent ribonucleoproteins carrying aldosterone-induced mRNA from the nucleoplasm into the cytoplasm. (iii) Because plugs can be mechanically harvested with the atomic force microscopy stylus, oocytes could serve as a bioassay system for identifying aldosterone-induced early genes.

Nitric oxide and differential effects of ATP on mitochondrial permeability transition

The mitochondrial permeability transition pore (PTP) undergoes a calcium-dependent transition (MPT) that disrupts membrane potential and releases apoptogenic proteins. Because PTP opening is enhanced by oxidation of thiols at the so-called "S-site," we hypothesized that nitrogen monoxide (NO*) could enhance the open probability of the PTP, e.g., by S-nitrosylation or S-thiolation. At low NO donor concentrations (1 to 20 microM), PTP opening in succinate-energized liver mitochondria at nonlimiting calcium was delayed or unaffected, while it was accelerated by NO donors at 20 to 100 microM. At low donor concentrations, PTP opening was facilitated twofold by adenosine triphosphate (ATP), which normally delays PTP opening. Among NO donors, the oxatriazole GEA 3162, with an activation constant (Ka) of 1.9 microM at 500 microM ATP was more effective at enhancing pore transition than SIN-1 or SNAP. NO donor effects were superseded by diamide, which induces disulfide formation, but independent of SH-adduct formation by alkylation. NO-related changes in PTP function were accompanied by protein mixed disulfide formation, inhibited by dithiothreitol (DTT), and reversed by DTT after donor addition. PTP opening was stimulated in the presence of ATP by L-arginine-dependent NO production, i.e., mitochondrial NOS activity. ATP-facilitated pore opening was sensitive to atractyloside and depended on nucleotide interactions but not on hydrolysis, because specific nonhydrolyzable ATP analogs accelerated pore opening. These data indicate NO can influence pore transition by oxidation of thiols that produce conformational changes governing the ATP interaction at the adenine nucleotide transporter.

Characterization of AC mode scanning ion-conductance microscopy

A scanning ion-conductance microscope (SICM) with a vibrating probe has been recently developed (vSICM). In this system, the amplitude of the AC ionic current is detected by using a lock-in amplifier locked to the vibration frequency of the probe. Such a scheme allows for a better control of the tip position because the AC ionic current is more sensitive to the probe-surface distance than the DC ionic current used previously. In this paper, we demonstrate the utility of this technique to the imaging of topographically rough specimens and high-resolution imaging over selected small areas. We also show that it is possible to record the DC ionic current simultaneously during the scan, which can reveal additional information not apparent in the images obtained with the AC ionic current.

Cell cycling determines integrin-mediated adhesion in osteoblastic ROS 17/2.8 cells exposed to space-related conditions

Six days of microgravity (Bion10 mission) induced dramatic shape changes in ROS 17/2.8 osteoblasts (7). During the Foton 11 and 12 space flights, we studied the kinetics (0-4 days) of ROS 17/2.8 morphology and adhesion, the relationships between adhesion and cell cycle progression after 4 days in space, and osteoblastic growth and activity after 6 days in space. Quantitative analysis of high-resolution adhesion [focal adhesion area imaged by total interference reflection fluorescent microscopy (TIRFM)] and integrin-dependent adhesion (imaged on confocal microscope by vinculin and phosphotyrosine staining) as well as cell cycle phase classification [Ki-67 staining, S-G2, mitotic cells and G1 (postmitotic cells)] were performed using programs validated in parabolic flight and clinostat. We observed disorganization of the cytoskeleton associated with disassembling of vinculin spots and phosphorylated proteins within focal contacts with no major change in TIRFM adhesion after 2 and 4 days of microgravity. Postmitotic cells, alone, accounted for the differences observed in the whole population. They are characterized by immature peripheral contacts with complete loss of central spots and decreased spreading. Osteocalcin, P1CP and alkaline phosphatase, and proliferation were similar in flight cells and 1 g centrifuge and ground controls after 6 days. In conclusion, microgravity substantially affected osteoblastic integrin-mediated cell adhesion. ROS17/2.8 cells responded differently, whether or not they were cycling by reorganizing adhesion plaque topography or morphology. In ROS 17/2.8, this reorganization did not impair osteoblastic phenotype.

Evidence for Ca2+- and ATP-sensitive peripheral channels in nuclear pore complexes

In eukaryotic cells the nuclear envelope (NE) serves as a functional barrier between cytosol and nucleoplasm perforated by nuclear pore complexes (NPCs). Both active and passive transport of ions and macromolecules are thought to be mediated by the centrally located large NPC channel. However, 3-dimensional imaging of NPCs based on electron microscopy indicates the existence of additional small channels of unknown function located in the NPC periphery. By means of the recently developed nuclear hourglass technique that measures NE electrical conductance, we evaluated passive electrically driven transport through NPCs. In isolated Xenopus laevis oocyte nuclei, we varied ambient Ca2+ and ATP in the cytosolic solution and/or chelated Ca2+ in the perinuclear stores in order to assess the role of Ca2+ in regulating passive ion transport. We noticed that NE electrical conductance is large under conditions where macromolecule permeability is known to be low. In addition, atomic force microscopy applied to native NPCs detects multiple small pores in the NPC periphery consistent with channel openings. Peripheral pores were detectable only in the presence of ATP. We conclude that NPC transport of ions and macromolecules occurs through different routes. We present a model in which NE ion flux does not occur through the central NPC channel but rather through Ca2+- and ATP-activated peripheral channels of individual NPCs.

Directed inhibition of nuclear import in cellular hypertrophy

Each nuclear pore is responsible for both nuclear import and export with a finite capacity for bidirectional transport across the nuclear envelope. It remains poorly understood how the nuclear transport pathway responds to increased demands for nucleocytoplasmic communication. A case in point is cellular hypertrophy in which increased amounts of genetic material need to be transported from the nucleus to the cytosol. Here, we report an adaptive down-regulation of nuclear import supporting such an increased demand for nuclear export. The induction of cardiac cell hypertrophy by phenylephrine or angiotensin II inhibited the nuclear translocation of H1 histones. The removal of hypertrophic stimuli reversed the hypertrophic phenotype and restored nuclear import. Moreover, the inhibition of nuclear export by leptomycin B rescued import. Hypertrophic reprogramming increased the intracellular GTP/GDP ratio and promoted the nuclear redistribution of the GTP-binding transport factor Ran, favoring export over import. Further, in hypertrophy, the reduced creatine kinase and adenylate kinase activities limited energy delivery to the nuclear pore. The reduction of activities was associated with the closure of the cytoplasmic phase of the nuclear pore preventing import at the translocation step. Thus, to overcome the limited capacity for nucleocytoplasmic transport, cells requiring increased nuclear export regulate the nuclear transport pathway by undergoing a metabolic and structural restriction of nuclear import.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

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.

Pushing, pulling, dragging, and vibrating renal epithelia by using atomic force microscopy

Renal physiologists focus on events that take place on and around the surfaces of cells. Various techniques have been developed that follow transport functions at the molecular level, but until recently none of these techniques has been capable of making the behavior of molecular structures visible under physiological conditions. This apparent gap may be filled in the future by the application of atomic force microscopy. This technique produces an image not by optical means, but by "feeling" its way across a surface. Atomic force microscopy can, however, be modified in a number of ways, which means that besides producing a high-resolution image, it is possible to obtain several types of data on the interactions between the ultrastructural components of cell membranes (such as proteins) and other biologically active molecules (such as ATP). In this review we describe the recent use of the atomic force microscope in renal physiology, ranging from experiments in intact cells to those in isolated renal transport protein molecules, include examples of these extended applications of the technique, and point to uses that the microscope has recently found in other areas of biology that should prove fruitful in renal physiology in the near future.

Calcium, ATP and nuclear pore channel gating

Nuclear envelope (NE) cisternal Ca2+ and cytosolic ATP are required for nuclear-pore-complex-(NPC-) mediated transport of DNAs, RNAs, transcription factors and other large molecules. Isolated cardiomyocyte nuclei, capable of macromolecular transport (MMT), have intrinsic NPC ion channel behavior. The large ion conductance (gamma) activity of the NPC channel (NPCC) is blocked by the NPC monoclonal antibody mAb414, known to block MMT, and is also silenced during periods of MMT. In cardiomyocytes, neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. To test the role of Ca2+ and ATP in NPCC activity, we carried out the present patch-clamp study with the pipette attached to the outer NE membrane of nuclei isolated from cultured Dunning G prostate cancer cells. Our investigations demonstrate that in these isolated nuclei neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. However, when simultaneously applied to the bath and pipette, they transiently silence NPCC activity through stimulation of MMT by raising the Ca2+ concentration in the NE cisterna ([Ca2+]NE). Our fluorescence microscopy observations with nuclear-targeted macromolecular fluorochromes (B-phycoerythrin and plasmid for the enhanced green fluorescence protein EGFP, pEGFP-C1) and with FITC-labeled RNA support the view that channel silence accompanies MMT. Repeated Ca2+ loading of the NE with Ca2+ and ATP, after unloading with 1-5 microM inositol 1,4,5-trisphosphate (IP3), thapsigargin (TSG) or 5 mM BAPTA or EGTA, failed to affect channel gating. This result indicates that other factors are involved in this phenomenon and that they are exhausted during the first cycle of NE Ca2+ loading/unloading--in agreement with current theories of NPC-mediated MMT. The results explain how Ca2+ and IP3 waves may convert the NE into an effective Ca2+ barrier and, consequently, affect the regulation of gene activity and expression through their feedback on MMT and NPCC gating. Thus, [Ca2+]NE regulation by intracellular messengers is an effective mechanism for synchronizing gene activity and expression to the cellular rhythm.

Distinct energy requirement for nuclear import and export of importin beta in living cells

Importin beta can shuttle between the nucleus and cytoplasm through the nuclear pore complex (NPC). This study deals with the issue of how the energy is utilized during the NPC passage of importin beta. In chilled or ATP-depleted cells, importin beta was transported into the nucleus, while the nuclear export of importin beta was inhibited. Further, it was found that the nuclear export inhibition of importin beta is not due to nuclear retention via binding to nucleoporins or nuclear importin alpha. These data show that the nuclear export of importin beta involves energy-requiring step(s) in living cells.

Identification of microphases in mixed alpha- and omega-gliadin protein films investigated by atomic force microscopy

Pure and mixed films of alpha- and omega-gliadins were studied by tapping mode atomic force microscopy (AFM). The technique was sensitive to the chemistry of the surface properties of the films, allowing imaging of the mixed gliadin phases at different ratios. In addition to the study of the phases at the micrometer level, higher resolution images allowed visualization of the protein films at the molecular level. These studies may have relevance to the formation of phases in developing protein bodies in grain, where gliadins and glutenins are deposited together. It has been assumed that the protein bodies consist of a random network of proteins; these studies indicate that microphases could be present in protein bodies. The technique provides novel methods for studying mixed biopolymer systems.

Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 +/- 0.34 S/cm(2)). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 x 10(6) NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically "open" state with a mean single NPC electrical conductance of 1.7 +/- 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.