The human Nup107-160 nuclear pore subcomplex contributes to proper kinetochore functions

Structural and functional studies of Nup107/Nup133 interaction and its implications for the architecture of the nuclear pore complex

Nuclear pore complexes (NPCs) are 40-60 MDa protein assemblies embedded in the nuclear envelope of eukaryotic cells. NPCs exclusively mediate all transport between cytoplasm and nucleus. The nucleoporins that build the NPC are arranged in a stable core of module-like subcomplexes with eight-fold rotational symmetry. To gain insight into the intricate assembly of the NPC, we have solved the crystal structure of a protein complex between two nucleoporins, human Nup107 and Nup133. Both proteins form elongated structures that interact tightly via a compact interface in tail-to-tail fashion. Additional experiments using structure-guided mutants show that Nup107 is the critical anchor for Nup133 to the NPC, positioning Nup133 at the periphery of the NPC. The significant topological differences between Nup107 and Nup133 suggest that *-helical nucleoporin domains of the NPC scaffold fall in different classes and fulfill largely nonredundant functions.

Nuclear pore composition regulates neural stem/progenitor cell differentiation in the mouse embryo

Serving as the primary conduit for communication between the nucleus and the cytoplasm, nuclear pore complexes (NPCs) impact nearly every cellular process. The extent to which NPC composition varies and the functional significance of such variation in mammalian development has not been investigated. Here we report that a null allele of mouse nucleoporin Nup133, a structural subunit of the NPC, disrupts neural differentiation. We find that expression of Nup133 is cell type and developmental stage restricted, with prominent expression in dividing progenitors. Nup133-deficient epiblast and ES cells abnormally maintain features of pluripotency and differentiate inefficiently along the neural lineage. Neural progenitors achieve correct spatial patterning in mutant embryos; however, they are impaired in generating terminally differentiated neurons, as are Nup133 null ES cells. Our results reveal a role for structural nucleoporins in coordinating cell differentiation events in the developing embryo.

In vivo dynamics of Drosophila nuclear envelope components

Nuclear pore complexes (NPCs) are multisubunit protein entities embedded into the nuclear envelope (NE). Here, we examine the in vivo dynamics of the essential Drosophila nucleoporin Nup107 and several other NE-associated proteins during NE and NPCs disassembly and reassembly that take place within each mitosis. During both the rapid mitosis of syncytial embryos and the more conventional mitosis of larval neuroblasts, Nup107 is gradually released from the NE, but it remains partially confined to the nuclear (spindle) region up to late prometaphase, in contrast to nucleoporins detected by wheat germ agglutinin and lamins. We provide evidence that in all Drosophila cells, a structure derived from the NE persists throughout metaphase and early anaphase. Finally, we examined the dynamics of the spindle checkpoint proteins Mad2 and Mad1. During mitotic exit, Mad2 and Mad1 are actively imported back from the cytoplasm into the nucleus after the NE and NPCs have reformed, but they reassociate with the NE only later in G1, concomitantly with the recruitment of the basket nucleoporin Mtor (the Drosophila orthologue of vertebrate Tpr). Surprisingly, Drosophila Nup107 shows no evidence of localization to kinetochores, despite the demonstrated importance of this association in mammalian cells.

The RanGTP gradient - a GPS for the mitotic spindle

The GTPase Ran has a key role in nuclear import and export, mitotic spindle assembly and nuclear envelope formation. The cycling of Ran between its GTP- and GDP-bound forms is catalyzed by the chromatin-bound guanine nucleotide exchange factor RCC1 and the cytoplasmic Ran GTPase-activating protein RanGAP. The result is an intracellular concentration gradient of RanGTP that equips eukaryotic cells with a ;genome-positioning system' (GPS). The binding of RanGTP to nuclear transport receptors (NTRs) of the importin beta superfamily mediates the effects of the gradient and generates further downstream gradients, which have been elucidated by fluorescence resonance energy transfer (FRET) imaging and computational modeling. The Ran-dependent GPS spatially directs many functions required for genome segregation by the mitotic spindle during mitosis. Through exportin 1, RanGTP recruits essential centrosome and kinetochore components, whereas the RanGTP-induced release of spindle assembly factors (SAFs) from importins activates SAFs to nucleate, bind and organize nascent spindle microtubules. Although a considerable fraction of cytoplasmic SAFs is active and RanGTP induces only partial further activation near chromatin, bipolar spindle assembly is robustly induced by cooperativity and positive-feedback mechanisms within the network of Ran-activated SAFs. The RanGTP gradient is conserved, although its roles vary among different cell types and species, and much remains to be learned regarding its functions.

The nuclear envelope as an integrator of nuclear and cytoplasmic architecture

Initially perceived as little more than a container for the genome, our view of the nuclear envelope (NE) and its role in defining global nuclear architecture has evolved significantly in recent years. The recognition that certain human diseases arise from defects in NE components has provided new insight into its structural and regulatory functions. In particular, NE defects associated with striated muscle disease have been shown to cause structural perturbations not just of the nucleus itself but also of the cytoplasm. It is now becoming increasingly apparent that these two compartments display co-dependent mechanical properties. The identification of cytoskeletal binding complexes that localize to the NE now reveals a molecular framework that can seamlessly integrate nuclear and cytoplasmic architecture.

Sumoylating and desumoylating enzymes at nuclear pores: underpinning their unexpected duties?

Modulation of protein activities by SUMO-dependent modification has emerged as a key feature of cellular regulation. Evidence of the localization of different enzymes of the sumoylation-desumoylation cycle at nuclear pore complexes (NPCs), and of its biological relevance, has steadily accumulated over the past ten years. Recent findings indicate that, beyond nucleocytoplasmic transport, sumoylation processes underpin newly emerging, and initially unexpected, roles for NPCs in a broad array of biological functions. These include cell division, DNA repair, DNA replication and mRNA quality control. Most of these functions were initially discovered through genetic studies in budding yeast, but the localization of SUMO-proteases at NPCs in higher eukaryotes suggests that at least some of these mechanisms might have been conserved during evolution.

Molecular architecture of the kinetochore-microtubule interface

Segregation of the replicated genome during cell division in eukaryotes requires the kinetochore to link centromeric DNA to spindle microtubules. The kinetochore is composed of a number of conserved protein complexes that direct its specification and assembly, bind to spindle microtubules and regulate chromosome segregation. Recent studies have identified more than 80 kinetochore components, and are revealing how these proteins are organized into the higher order kinetochore structure, as well as how they function to achieve proper chromosome segregation.

Dynamic rearrangement of nucleoporins during fungal "open" mitosis

Mitosis in animals starts with the disassembly of the nuclear pore complexes and the breakdown of the nuclear envelope. In contrast to many fungi, the corn smut fungus Ustilago maydis also removes the nuclear envelope. Here, we report on the dynamic behavior of the nucleoporins Nup214, Pom152, Nup133, and Nup107 in this "open" fungal mitosis. In prophase, the nuclear pore complexes disassembled and Nup214 and Pom152 dispersed in the cytoplasm and in the endoplasmic reticulum, respectively. Nup107 and Nup133 initially spread throughout the cytoplasm, but in metaphase and early anaphase occurred on the chromosomes. In anaphase, the Nup107-subcomplex redistributed to the edge of the chromosome masses, where the new envelope was reconstituted. Subsequently, Nup214 and Pom152 are recruited to the nuclear pores and protein import starts. Recruitment of nucleoporins and protein import reached a steady state in G2 phase. Formation of the nuclear envelope and assembly of nuclear pores occurred in the absence of microtubules or F-actin, but not if both were disrupted. Thus, the basic principles of nuclear pore complex dynamics seem to be conserved in organisms displaying open mitosis.

Centrin 2 localizes to the vertebrate nuclear pore and plays a role in mRNA and protein export

Centrins in vertebrates have traditionally been associated with microtubule-nucleating centers such as the centrosome. Unexpectedly, we found centrin 2 to associate biochemically with nucleoporins, including the Xenopus laevis Nup107-160 complex, a critical subunit of the vertebrate nuclear pore in interphase and of the kinetochores and spindle poles in mitosis. Immunofluorescence of Xenopus cells and in vitro reconstituted nuclei indeed revealed centrin 2 localized at the nuclear pores. Use of the mild detergent digitonin in immunofluorescence also allowed centrin 2 to be clearly visualized at the nuclear pores of human cells. Disruption of nuclear pores using RNA interference of the pore assembly protein ELYS/MEL-28 resulted in a specific decrease of centrin 2 at the nuclear rim of HeLa cells. Functionally, excess expression of either the N- or C-terminal calcium-binding domains of human centrin 2 caused a dominant-negative effect on both mRNA and protein export, leaving protein import intact. The mRNA effect mirrors that found for the Saccharomyes cerevisiae centrin Cdc31p at the yeast nuclear pore, a role until now thought to be unique to yeast. We conclude that in vertebrates, centrin 2 interacts with major subunits of the nuclear pore, exhibits nuclear pore localization, and plays a functional role in multiple nuclear export pathways.

Biology and biophysics of the nuclear pore complex and its components

Nucleocytoplasmic exchange of proteins and ribonucleoprotein particles occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope (NE). Significant progress has been made during the past few years in obtaining better structural resolution of the three-dimensional architecture of NPC with the help of cryo-electron tomography and atomic structures of domains from nuclear pore proteins (nucleoporins). Biophysical and imaging approaches have helped elucidate how nucleoporins act as a selective barrier in nucleocytoplasmic transport. Nucleoporins act not only in trafficking of macromolecules but also in proper microtubule attachment to kinetochores, in the regulation of gene expression and signaling events associated with, for example, innate and adaptive immunity, development and neurodegenerative disorders. Recent research has also been focused on the dynamic processes of NPC assembly and disassembly that occur with each cell cycle. Here we review emerging results aimed at understanding the molecular arrangement of the NPC and how it is achieved, defining the roles of individual nucleoporins both at the NPC and at other sites within the cell, and finally deciphering how the NPC serves as both a barrier and a conduit of active transport.

The nuclear pore comes to the fore

The nuclear pore complex is the gateway of macromolecular trafficking between the nucleus and the cytoplasm. Although its composition is well characterized in yeast and mammalian systems, little is known about the plant nuclear pore. Several recent reports describe complex whole-organism phenotypes based on mutations in plant nucleoporins. The pathways affected include plant-microbe interactions, auxin response, cold-stress tolerance and flowering-time regulation. The effects are probably based, at least in part, on changes in protein import and/or RNA export (including regulatory small RNAs). Here, we review these new findings while comparing and contrasting them with what is known about nucleoporin functions from non-plant organisms, including nucleoporin activities not linked to nucleocytoplasmic transport.

From live-cell imaging to scanning electron microscopy (SEM): the use of green fluorescent protein (GFP) as a common label

The identification and characterization of many biological substructures at high resolution requires the use of electron microscopy (EM) technologies. Scanning electron microscopy (SEM) allows the resolution of cellular structures to approximately 3 nm and has facilitated the direct visualization of macromolecular structures, such as nuclear pore complexes (NPCs), which are essential for nucleo-cytoplasmic molecular trafficking. However, SEM generates only static images of fixed samples and therefore cannot give unambiguous information about protein dynamics. The investigation of active processes and analysis of protein dynamics has greatly benefited from the development of molecular biology techniques whereby vectors can be generated and transfected into tissue culture cells for the expression of specific proteins tagged with a fluorescent moiety for real-time light microscopy visualization. As light microscopy is limited in its powers of resolution relative to electron microscopy, it has been important to adapt a protocol for the processing of samples for real-time imaging by conventional light microscopy with protein labels that can also be identified by SEM. This allows correlation of dynamic events with high resolution molecular and structural identification. This method describes the use of GFP for tracking the dynamic distribution of NPC components in real-time throughout the cell cycle and for high resolution immuno-SEM labeling to determine localization at the nanometer level.