4Pi microscopy of the nuclear pore complex

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.

Combined alteration of lamin and nuclear morphology influences the localization of the tumor-associated factor AKTIP

Background

Lamins, key nuclear lamina components, have been proposed as candidate risk biomarkers in different types of cancer but their accuracy is still debated. AKTIP is a telomeric protein with the property of being enriched at the nuclear lamina. AKTIP has similarity with the tumor susceptibility gene TSG101. AKTIP deficiency generates genome instability and, in p53^−/− mice, the reduction of the mouse counterpart of AKTIP induces the exacerbation of lymphomas. Here, we asked whether the distribution of AKTIP is altered in cancer cells and whether this is associated with alterations of lamins.

Methods

We performed super-resolution imaging, quantification of lamin expression and nuclear morphology on HeLa, MCF7, and A549 tumor cells, and on non-transformed fibroblasts from healthy donor and HGPS (LMNA c.1824C > T p.Gly608Gly) and EDMD2 (LMNA c.775 T > G) patients. As proof of principle model combining a defined lamin alteration with a tumor cell setting, we produced HeLa cells exogenously expressing the HGPS lamin mutant progerin that alters nuclear morphology.

Results

In HeLa cells, AKTIP locates at less than 0.5 µm from the nuclear rim and co-localizes with lamin A/C. As compared to HeLa, there is a reduced co-localization of AKTIP with lamin A/C in both MCF7 and A549. Additionally, MCF7 display lower amounts of AKTIP at the rim. The analyses in non-transformed fibroblasts show that AKTIP mislocalizes in HGPS cells but not in EDMD2. The integrated analysis of lamin expression, nuclear morphology, and AKTIP topology shows that positioning of AKTIP is influenced not only by lamin expression, but also by nuclear morphology. This conclusion is validated by progerin-expressing HeLa cells in which nuclei are morphologically altered and AKTIP is mislocalized.

Conclusions

Our data show that the combined alteration of lamin and nuclear morphology influences the localization of the tumor-associated factor AKTIP. The results also point to the fact that lamin alterations per se are not predictive of AKTIP mislocalization, in both non-transformed and tumor cells. In more general terms, this study supports the thesis that a combined analytical approach should be preferred to predict lamin-associated changes in tumor cells. This paves the way of next translational evaluation to validate the use of this combined analytical approach as risk biomarker.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02480-5.

Physics of the Nuclear Pore Complex: Theory, Modeling and Experiment

The hallmark of eukaryotic cells is the nucleus that contains the genome, enclosed by a physical barrier known as the nuclear envelope (NE). On the one hand, this compartmentalization endows the eukaryotic cells with high regulatory complexity and flexibility. On the other hand, it poses a tremendous logistic and energetic problem of transporting millions of molecules per second across the nuclear envelope, to facilitate their biological function in all compartments of the cell. Therefore, eukaryotes have evolved a molecular "nanomachine" known as the Nuclear Pore Complex (NPC). Embedded in the nuclear envelope, NPCs control and regulate all the bi-directional transport between the cell nucleus and the cytoplasm. NPCs combine high molecular specificity of transport with high throughput and speed, and are highly robust with respect to molecular noise and structural perturbations. Remarkably, the functional mechanisms of NPC transport are highly conserved among eukaryotes, from yeast to humans, despite significant differences in the molecular components among various species. The NPC is the largest macromolecular complex in the cell. Yet, despite its significant complexity, it has become clear that its principles of operation can be largely understood based on fundamental physical concepts, as have emerged from a combination of experimental methods of molecular cell biology, biophysics, nanoscience and theoretical and computational modeling. Indeed, many aspects of NPC function can be recapitulated in artificial mimics with a drastically reduced complexity compared to biological pores. We review the current physical understanding of the NPC architecture and function, with the focus on the critical analysis of experimental studies in cells and artificial NPC mimics through the lens of theoretical and computational models. We also discuss the connections between the emerging concepts of NPC operation and other areas of biophysics and bionanotechnology.

Super-Resolution Microscopy of Chromatin

Since the advent of super-resolution microscopy, countless approaches and studies have been published contributing significantly to our understanding of cellular processes. With the aid of chromatin-specific fluorescence labeling techniques, we are gaining increasing insight into gene regulation and chromatin organization. Combined with super-resolution imaging and data analysis, these labeling techniques enable direct assessment not only of chromatin interactions but also of the function of specific chromatin conformational states.

Intrinsic refractive index matched 3D dSTORM with two objectives: Comparison of detection techniques

We have built a setup for 3D single molecule localisation microscopy (SMLM) where a very high resolution is achieved by, firstly, the use of two objectives instead of one and, secondly, minimizing optical aberrations by refractive index matching with a glycerol-water mixture as immersion medium in conjunction with glycerol-immersion objectives. Multiple optical paths of the microscope allow to switch between astigmatic and interferometric localisation along the optical axis, thus enabling a direct comparison of the performance of these localisation methods.

Nuclear pore complex plasticity during developmental process as revealed by super-resolution microscopy

Nuclear Pore Complex (NPC) is of paramount importance for cellular processes since it is the unique gateway for molecular exchange through the nucleus. Unraveling the modifications of the NPC structure in response to physiological cues, also called nuclear pore plasticity, is key to the understanding of the selectivity of this molecular machinery. As a step towards this goal, we use the optical super-resolution microscopy method called direct Stochastic Optical Reconstruction Microscopy (dSTORM), to analyze oocyte development impact on the internal structure and large-scale organization of the NPC. Staining of the FG-Nups proteins and the gp210 proteins allowed us to pinpoint a decrease of the global diameter by measuring the mean diameter of the central channel and the luminal ring of the NPC via autocorrelation image processing. Moreover, by using an angular and radial density function we show that development of the Xenopus laevis oocyte is correlated with a progressive decrease of the density of NPC and an ordering on a square lattice.

Super-Resolution Microscopy in Studying the Structure and Function of the Cell Nucleus

In recent decades, novel microscopic methods commonly referred to as super- resolution microscopy have been developed. These methods enable the visualization of a cell with a resolution of up to 10 nm. The application of these methods is of great interest in studying the structure and function of the cell nucleus. The review describes the main achievements in this field.

Single-Molecule Real-Time 3D Imaging of the Transcription Cycle by Modulation Interferometry

Many essential cellular processes, such as gene control, employ elaborate mechanisms involving the coordination of large, multi-component molecular assemblies. Few structural biology tools presently have the combined spatial-temporal resolution and molecular specificity required to capture the movement, conformational changes, and subunit association-dissociation kinetics, three fundamental elements of how such intricate molecular machines work. Here, we report a 3D single-molecule super-resolution imaging study using modulation interferometry and phase-sensitive detection that achieves <2 nm axial localization precision, well below the few-nanometer-sized individual protein components. To illustrate the capability of this technique in probing the dynamics of complex macromolecular machines, we visualize the movement of individual multi-subunit E. coli RNA polymerases through the complete transcription cycle, dissect the kinetics of the initiation-elongation transition, and determine the fate of σ⁷⁰ initiation factors during promoter escape. Modulation interferometry sets the stage for single-molecule studies of several hitherto difficult-to-investigate multi-molecular transactions that underlie genome regulation.

Super-resolution microscopy approaches to nuclear nanostructure imaging

The human genome has been decoded, but we are still far from understanding the regulation of all gene activities. A largely unexplained role in these regulatory mechanisms is played by the spatial organization of the genome in the cell nucleus which has far-reaching functional consequences for gene regulation. Until recently, it appeared to be impossible to study this problem on the nanoscale by light microscopy. However, novel developments in optical imaging technology have radically surpassed the limited resolution of conventional far-field fluorescence microscopy (ca. 200nm). After a brief review of available super-resolution microscopy (SRM) methods, we focus on a specific SRM approach to study nuclear genome structure at the single cell/single molecule level, Spectral Precision Distance/Position Determination Microscopy (SPDM). SPDM, a variant of localization microscopy, makes use of conventional fluorescent proteins or single standard organic fluorophores in combination with standard (or only slightly modified) specimen preparation conditions; in its actual realization mode, the same laser frequency can be used for both photoswitching and fluorescence read out. Presently, the SPDM method allows us to image nuclear genome organization in individual cells down to few tens of nanometer (nm) of structural resolution, and to perform quantitative analyses of individual small chromatin domains; of the nanoscale distribution of histones, chromatin remodeling proteins, and transcription, splicing and repair related factors. As a biomedical research application, using dual-color SPDM, it became possible to monitor in mouse cardiomyocyte cells quantitatively the effects of ischemia conditions on the chromatin nanostructure (DNA). These novel "molecular optics" approaches open an avenue to study the nuclear landscape directly in individual cells down to the single molecule level and thus to test models of functional genome architecture at unprecedented resolution.

4Pi microscopy of the nuclear pore complex

4Pi microscopy is a far-field fluorescence microscopy technique, in which the wave fronts of two opposing illuminating beams are adjusted to constructively interfere in a common focus. This yields a diffraction pattern in the direction of the optical axis, which essentially consists of a main focal spot accompanied by two smaller side lobes. At optimal conditions, the main peak of this so-called point spread function has a full width at half maximum: fixed phrase of 100 nm in the direction of the optical axis, and thus is 6-7-fold smaller than that of a confocal microscope. In this chapter, we describe the basic features of 4Pi microscopy and its application to cell biology using the example of the nuclear pore complex, a large protein assembly spanning the nuclear envelope.

'Go with the flow ': a review of methods and advancements in blood flow imaging

Physics has delivered extraordinary developments in almost every facet of modern life. From the humble thermometer and stethoscope to X-Ray, CT, MRI, ultrasound, PET and radiotherapy, our health has been transformed by these advances yielding both morphological and functional metrics. Recently high resolution label-free imaging of the microcirculation at clinically relevant depths has become available in the research domain. In this paper, we present a comprehensive review on current imaging techniques, state-of-the-art advancements and applications, and general perspectives on the prospects for these modalities in the clinical realm.

4Pi Microscopy

Optical microscopy has become a key technology in the life sciences today. Its noninvasive nature provides access to the interior of intact and even living cells, where specific molecules can be precisely localized by fluorescent tagging. However, the attainable 3D resolution of an optical microscope has long been hampered by a comparatively poor resolution along the optic axis. By coherent focusing through two objective lenses, 4Pi microscopy improves the axial resolution by three- to fivefold. This primer is intended as a starting point for the design and operation of a 4Pi microscope of type A.

Fluorescence in nanobiotechnology: sophisticated fluorophores for novel applications

Nanobiotechnology is one of the fastest growing and broadest-ranged interdisciplinary subfields of the nanosciences. Countless hybrid bio-inorganic composites are currently being pursued for various uses, including sensors for medical and diagnostic applications, light- and energy-harvesting devices, along with multifunctional architectures for electronics and advanced drug-delivery. Although many disparate biological and nanoscale materials will ultimately be utilized as the functional building blocks to create these devices, a common element found among a large proportion is that they exert or interact with light. Clearly continuing development will rely heavily on incorporating many different types of fluorophores into these composite materials. This review covers the growing utility of different classes of fluorophores in nanobiotechnology, from both a photophysical and a chemical perspective. For each major structural or functional class of fluorescent probe, several representative applications are provided, and the necessary technological background for acquiring the desired nano-bioanalytical information are presented.

Superlocalization spectral imaging microscopy of a multicolor quantum dot complex

The key factor of realizing super-resolution optical microscopy at the single-molecule level is to separately position two adjacent molecules. An opportunity to independently localize target molecules is provided by the intermittency (blinking) in fluorescence of a quantum dot (QD) under the condition that the blinking of each emitter can be recorded and identified. Herein we develop a spectral imaging based color nanoscopy which is capable of determining which QD is blinking in the multicolor QD complex through tracking the first-order spectrum, and thus, the distance at tens of nanometers between two QDs is measured. Three complementary oligonucleotides with lengths of 15, 30, and 45 bp are constructed as calibration rulers. QD585 and QD655 are each linked at one end. The measured average distances are in good agreement with the calculated lengths with a precision of 6 nm, and the intracellular dual-color QDs within a diffraction-limited spot are distinguished.

Multiscale dynamics in nucleocytoplasmic transport

The nuclear pore complex (NPC) has long been viewed as a point-like entry and exit channel between the nucleus and the cytoplasm. New data support a different view whereby the complex displays distinct spatial dynamics of variable duration ranging from milliseconds to events spanning the entire cell cycle. Discrete interaction sites outside the central channel become apparent, and transport regulation at these sites seems to be of greater importance than currently thought. Nuclear pore components are highly active outside the NPC or impact the fate of cargo transport away from the nuclear pore. The NPC is a highly dynamic, crowded environment-constantly loaded with cargo while providing selectivity based on unfolded proteins. Taken together, this comprises a new paradigm in how we view import/export dynamics and emphasizes the multiscale nature of NPC-mediated cellular transport.

A view of the chromatin landscape

The microscope has been indispensable to the last century of chromatin structure research. Microscopy techniques have revealed that the three-dimensional location of chromatin is not random but represents a further manifestation of a highly compartmentalized cell nucleus. Moreover, the structure and location of genetic loci display cell type-specific differences and relate directly to the state of differentiation. Advances to bridge imaging with genetic, molecular and biochemical approaches have greatly enhanced our understanding of the interdependence of chromatin structure and nuclear function in mammalian cells. In this review we discuss the current state of chromatin structure research in relationship to the variety of microscopy techniques that have contributed to this field.

Single-molecule studies of nucleocytoplasmic transport: from one dimension to three dimensions

In eukaryotic cells, the bidirectional trafficking of proteins and genetic materials across the double-membrane nuclear envelope is mediated by nuclear pore complexes (NPCs). A highly selective barrier formed by the phenylalanine-glycine (FG)-nucleoporin (Nup) in the NPC allows for two transport modes: passive diffusion and transport receptor-facilitated translocation. Strict regulation of nucleocytoplasmic transport is crucial for cell survival, differentiation, growth and other essential activities. However, due to the limited knowledge of the native configuration of the FG-Nup barrier and the interactions between the transiting molecules and the barrier in the NPC, the precise nucleocytoplasmic transport mechanism remains unresolved. To refine the transport mechanism, single-molecule fluorescence microscopy methods have been employed to obtain the transport kinetics of individual fluorescent molecules through the NPC and to map the interactions between transiting molecules and the FG-Nup barrier. Important characteristics of nucleocytoplasmic transport, such as transport time, transport efficiency and spatial distribution of single transiting molecules in the NPC, have been obtained that could not be measured by either ensemble average methods or conventional electron microscopy. In this critical review, we discuss the development of various single-molecule techniques and their application to nucleocytoplasmic transport in vitro and in vivo. In particular, we highlight a recent advance from one-dimensional to three-dimensional single-molecule characterization of transport through the NPC and present a comprehensive understanding of the nucleocytoplasmic transport mechanism obtained by this new technical development (105 references).

Nuclear export dynamics of RNA-protein complexes

The central dogma of molecular biology - DNA makes RNA makes proteins - is a flow of information that in eukaryotes encounters a physical barrier: the nuclear envelope, which encapsulates, organizes and protects the genome. Nuclear-pore complexes, embedded in the nuclear envelope, regulate the passage of molecules to and from the nucleus, including the poorly understood process of the export of RNAs from the nucleus. Recent imaging approaches focusing on single molecules have provided unexpected insight into this crucial step in the information flow. This review addresses the latest studies of RNA export and presents some models for how this complex process may work.

A readily retrievable pool of synaptic vesicles

Although clathrin-mediated endocytosis is thought to be the predominant mechanism of synaptic vesicle recycling, it seems to be too slow for fast recycling. Therefore, it was suggested that a presorted and preassembled pool of synaptic vesicle proteins on the presynaptic membrane might support a first wave of fast clathrin-mediated endocytosis. In this study we monitored the temporal dynamics of such a 'readily retrievable pool' of synaptic vesicle proteins in rat hippocampal neurons using a new type of probe. By applying cypHer5E, a new cyanine dye-based pH-sensitive exogenous marker, coupled to antibodies to luminal domains of synaptic vesicle proteins, we could reliably monitor synaptic vesicle recycling and demonstrate the preferential recruitment of a surface pool of synaptic vesicle proteins upon stimulated endocytosis. By using fluorescence nanoscopy of surface-labeled synaptotagmin 1, we could resolve the spatial distribution of the surface pool at the periactive zone in hippocampal boutons, which represent putative sites of endocytosis.

Structural protein 4.1R is integrally involved in nuclear envelope protein localization, centrosome-nucleus association and transcriptional signaling

The multifunctional structural protein 4.1R is required for assembly and maintenance of functional nuclei but its nuclear roles are unidentified. 4.1R localizes within nuclei, at the nuclear envelope, and in cytoplasm. Here we show that 4.1R, the nuclear envelope protein emerin and the intermediate filament protein lamin A/C co-immunoprecipitate, and that 4.1R-specific depletion in human cells by RNA interference produces nuclear dysmorphology and selective mislocalization of proteins from several nuclear subcompartments. Such 4.1R-deficiency causes emerin to partially redistribute into the cytoplasm, whereas lamin A/C is disorganized at nuclear rims and displaced from nucleoplasmic foci. The nuclear envelope protein MAN1, nuclear pore proteins Tpr and Nup62, and nucleoplasmic proteins NuMA and LAP2α also have aberrant distributions, but lamin B and LAP2β have normal localizations. 4.1R-deficient mouse embryonic fibroblasts show a similar phenotype. We determined the functional effects of 4.1R-deficiency that reflect disruption of the association of 4.1R with emerin and A-type lamin: increased nucleus-centrosome distances, increased β-catenin signaling, and relocalization of β-catenin from the plasma membrane to the nucleus. Furthermore, emerin- and lamin-A/C-null cells have decreased nuclear 4.1R. Our data provide evidence that 4.1R has important functional interactions with emerin and A-type lamin that impact upon nuclear architecture, centrosome-nuclear envelope association and the regulation of β-catenin transcriptional co-activator activity that is dependent on β-catenin nuclear export.

A guide to super-resolution fluorescence microscopy

For centuries, cell biology has been based on light microscopy and at the same time been limited by its optical resolution. However, several new technologies have been developed recently that bypass this limit. These new super-resolution technologies are either based on tailored illumination, nonlinear fluorophore responses, or the precise localization of single molecules. Overall, these new approaches have created unprecedented new possibilities to investigate the structure and function of cells.

Protein Tpr is required for establishing nuclear pore-associated zones of heterochromatin exclusion

Amassments of heterochromatin in somatic cells occur in close contact with the nuclear envelope (NE) but are gapped by channel- and cone-like zones that appear largely free of heterochromatin and associated with the nuclear pore complexes (NPCs). To identify proteins involved in forming such heterochromatin exclusion zones (HEZs), we used a cell culture model in which chromatin condensation induced by poliovirus (PV) infection revealed HEZs resembling those in normal tissue cells. HEZ occurrence depended on the NPC-associated protein Tpr and its large coiled coil-forming domain. RNAi-mediated loss of Tpr allowed condensing chromatin to occur all along the NE's nuclear surface, resulting in HEZs no longer being established and NPCs covered by heterochromatin. These results assign a central function to Tpr as a determinant of perinuclear organization, with a direct role in forming a morphologically distinct nuclear sub-compartment and delimiting heterochromatin distribution.

AP-1/sigma1B-adaptin mediates endosomal synaptic vesicle recycling, learning and memory

Synaptic vesicle recycling involves AP-2/clathrin-mediated endocytosis, but it is not known whether the endosomal pathway is also required. Mice deficient in the tissue-specific AP-1-sigma1B complex have impaired synaptic vesicle recycling in hippocampal synapses. The ubiquitously expressed AP-1-sigma1A complex mediates protein sorting between the trans-Golgi network and early endosomes. Vertebrates express three sigma1 subunit isoforms: A, B and C. The expressions of sigma1A and sigma1B are highest in the brain. Synaptic vesicle reformation in cultured neurons from sigma1B-deficient mice is reduced upon stimulation, and large endosomal intermediates accumulate. The sigma1B-deficient mice have reduced motor coordination and severely impaired long-term spatial memory. These data reveal a molecular mechanism for a severe human X-chromosome-linked mental retardation.

Three-dimensional organization of promyelocytic leukemia nuclear bodies

Promyelocytic leukemia nuclear bodies (PML-NBs) are mobile subnuclear organelles formed by PML and Sp100 protein. They have been reported to have a role in transcription, DNA replication and repair, telomere lengthening, cell cycle control and tumor suppression. We have conducted high-resolution 4Pi fluorescence laser-scanning microscopy studies complemented with correlative electron microscopy and investigations of the accessibility of the PML-NB subcompartment. During interphase PML-NBs adopt a spherical organization characterized by the assembly of PML and Sp100 proteins into patches within a 50- to 100-nm-thick shell. This spherical shell of PML and Sp100 imposes little constraint to the exchange of components between the PML-NB interior and the nucleoplasm. Post-translational SUMO modifications, telomere repeats and heterochromatin protein 1 were found to localize in characteristic patterns with respect to PML and Sp100. From our findings, we derived a model that explains how the three-dimensional organization of PML-NBs serves to concentrate different biological activities while allowing for an efficient exchange of components.

Binding site distribution of nuclear transport receptors and transport complexes in single nuclear pore complexes

Transport through the nuclear pore complex (NPC) involves a large channel and an abundance of binding sites for nuclear transport receptors (NTRs). However, the mechanistically important distribution of NTR-binding sites along the channel is vividly debated. In this study, we visualized binding site distributions directly by two complementary optical super-resolution methods, single-molecule microscopy and 4Pi microscopy. First, we analyzed the distribution of RanGDP because this important nuclear transport substrate has two types of binding sites at the NPC, direct and indirect, NTR-mediated sites. We found that the direct binding sites had a maximum at approximately -30 nm with regard to the NPC center, whereas the indirect transport-relevant binding sites peaked at approximately -10 nm. The 20 nm-shift could be only resolved by 4Pi microscopy because of a two to threefold improved localization precision as compared with single-molecule microscopy. Then we analyzed the distribution of the NTR Kapbeta1 and a Kapbeta1-based transport complex and found them to have also binding maxima at approximately -10 nm. These observations support transport models in which NTR binding sites are distributed all along the transport channel and argue against models in which the cytoplasmic entrance of the channel is surrounded by a large cloud of binding sites.

Correlation of 4Pi and electron microscopy to study transport through single Golgi stacks in living cells with super resolution

Two problems have hampered the use of light microscopy for structural studies of cellular organelles for a long time: the limited resolution and the difficulty of obtaining true structural boundaries from complex intensity curves. The advent of modern high-resolution light microscopy techniques and their combination with objective image segmentation now provide us with the means to bridge the gap between light and electronmicroscopy in cell biology applications. In this study, we provide the first comparative correlative analysis of three-dimensional structures obtained by 4Pi microscopy and segmented by a zero-crossing procedure with those of transmission electron microscopy (TEM). The distribution within the cisternae of isolated Golgi stacks of the cargo protein procollagen 3 was mapped by both 4Pi microscopy and TEM for a detailed comparative analysis of their imaging capabilities. A high correlation was seen for the structures, indicating the particular accuracy of the 4Pi microscopy. Furthermore, for the first time, transport of a cargo molecule (vesicular stomatitis virus G protein-pEGFP) through individual Golgi stacks (labeled by galactosyl transferase-venus-YFP) was visualized by 4Pi microscopy. Following the procedures validated by the correlative analysis, our transport experiments show that (i) VSVG-pEGFP rapidly enter/exit individual Golgi stacks, (ii) VSVG-pEGFP never fills the GalT-venusYFP compartments completely and (iii) the GalT-venusYFP compartment volume increases upon VSVG-pEGFP arrival. This morphological evidence supports some previous TEM-based observations of intra-Golgi transport of VSVG-pEGFP and provides new insights toward a better understanding of protein progression across Golgi stacks. Our study thus demonstrates the general applicability of super resolution fluorescence microscopy, coupled with the zero-crossing segmentation procedure, for structural studies of suborganelle protein distributions under living cell conditions.

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.