Systematic deletion and mitotic localization of the nuclear pore complex proteins of Aspergillus nidulans

Aspergillus SUMOylation mutants exhibit chromosome segregation defects including chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. Via forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO activation enzyme UbaB in the filamentous fungus Aspergillus nidulans. The ubaBQ247*, ΔubaB, and ΔsumO mutants all produce abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. The bridges are enclosed by nuclear membrane containing peripheral nuclear pore complex proteins that normally get dispersed during mitosis, and the bridges are also surrounded by cytoplasmic microtubules typical of interphase cells. Time-lapse sequences further indicate that most bridges persist through interphase prior to the next mitosis, and anaphase chromosome segregation can produce new bridges that persist into the next interphase. When the first mitosis happens at a higher temperature of 42°C, SUMOylation deficiency produces not only chromatin bridges but also many abnormally shaped single nuclei that fail to divide. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO targets being nuclear proteins. Finally, although the budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, loss of SUMOylation does not cause any obvious defect in dynein-mediated transport of nuclei and early endosomes, indicating that SUMOylation is unnecessary for dynein activation in A. nidulans.

Evolutionary, structural and functional insights in nuclear organisation and nucleocytoplasmic transport in trypanosomes

One of the remarkable features of eukaryotes is the nucleus, delimited by the nuclear envelope (NE), a complex structure and home to the nuclear lamina and nuclear pore complex (NPC). For decades, these structures were believed to be mainly architectural elements and, in the case of the NPC, simply facilitating nucleocytoplasmic trafficking. More recently, the critical roles of the lamina, NPC and other NE constituents in genome organisation, maintaining chromosomal domains and regulating gene expression have been recognised. Importantly, mutations in genes encoding lamina and NPC components lead to pathogenesis in humans, while pathogenic protozoa disrupt the progression of normal development and expression of pathogenesis-related genes. Here, we review features of the lamina and NPC across eukaryotes and discuss how these elements are structured in trypanosomes, protozoa of high medical and veterinary importance, highlighting lineage-specific and conserved aspects of nuclear organisation.

Distinct domains in Ndc1 mediate its interaction with the Nup84 complex and the nuclear membrane

Nuclear pore complexes (NPCs) are embedded in the nuclear envelope and built from ∼30 different nucleoporins (Nups) in multiple copies, few are integral membrane proteins. One of these transmembrane nucleoporins, Ndc1, is thought to function in NPC assembly at the fused inner and outer nuclear membranes. Here, we show a direct interaction of Ndc1's transmembrane domain with Nup120 and Nup133, members of the pore membrane coating Y-complex. We identify an amphipathic helix in Ndc1's C-terminal domain binding highly curved liposomes. Upon overexpression, this amphipathic motif is toxic and dramatically alters the intracellular membrane organization in yeast. Ndc1's amphipathic motif functionally interacts with related motifs in the C-terminus of the nucleoporins Nup53 and Nup59, important for pore membrane binding and interconnecting NPC modules. The essential function of Ndc1 can be suppressed by deleting the amphipathic helix from Nup53. Our data indicate that nuclear membrane and presumably NPC biogenesis depends on a balanced ratio between amphipathic motifs in diverse nucleoporins.

Aspergillus SUMOylation mutants have normal dynein function but exhibit chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. The budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, but dynein-pathway components were not identified as SUMO-targets in the filamentous fungus Aspergillus nidulans. Via A. nidulans forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO-activation enzyme UbaB. Colonies of the ubaBQ247*, ΔubaB and ΔsumO mutants looked similar and less healthy than the wild-type colony. In these mutants, about 10% of nuclei are connected by abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. Nuclei connected by chromatin bridges are mostly in interphase, suggesting that these bridges do not prevent cell-cycle progression. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO-targets being nuclear proteins, for example, topoisomerase II whose SUMOylation defect gives rise to chromatin bridges in mammalian cells. Unlike in mammalian cells, however, loss of SUMOylation in A. nidulans does not apparently affect the metaphase-to-anaphase transition, further highlighting differences in the requirements of SUMOylation in different cell types. Finally, loss of UbaB or SumO does not affect dynein- and LIS1-mediated early-endosome transport, indicating that SUMOylation is unnecessary for dynein or LIS1 function in A. nidulans.

Meiotic nuclear pore complex remodeling provides key insights into nuclear basket organization

Nuclear pore complexes (NPCs) are large proteinaceous assemblies that mediate nuclear compartmentalization. NPCs undergo large-scale structural rearrangements during mitosis in metazoans and some fungi. However, our understanding of NPC remodeling beyond mitosis remains limited. Using time-lapse fluorescence microscopy, we discovered that NPCs undergo two mechanistically separable remodeling events during budding yeast meiosis in which parts or all of the nuclear basket transiently dissociate from the NPC core during meiosis I and II, respectively. Meiosis I detachment, observed for Nup60 and Nup2, is driven by Polo kinase-mediated phosphorylation of Nup60 at its interface with the Y-complex. Subsequent reattachment of Nup60-Nup2 to the NPC core is facilitated by a lipid-binding amphipathic helix in Nup60. Preventing Nup60-Nup2 reattachment causes misorganization of the entire nuclear basket in gametes. Strikingly, meiotic nuclear basket remodeling also occurs in the distantly related fission yeast, Schizosaccharomyces pombe. Our study reveals a conserved and developmentally programmed aspect of NPC plasticity, providing key mechanistic insights into the nuclear basket organization.

Insights into the role of Nup62 and Nup93 in assembling cytoplasmic ring and central transport channel of the nuclear pore complex

The nuclear pore complex (NPC) is a highly modular assembly of 34 distinct nucleoporins (Nups) to form a versatile transport channel between the nucleus and the cytoplasm. Among them, Nup62 is known as an essential component for nuclear transport, Nup93 for proper nuclear envelope assembly. These Nups constitute various NPC subcomplexes such as the central transport channel (CTC), the cytoplasmic ring (CR), and the inner ring (IR). However, how they play their roles in NPC assembly and transport activity is not clear. Here we delineated the interacting regions and conducted biochemical reconstitution and structural characterization of the mammalian CR complex to reveal its intrinsic dynamic behavior and a distinct "4"-shaped architecture resembling the CTC complex. Our in vitro reconstitution data demonstrate that the Nup62 coiled-coil domain is critical to form both Nup62^322-525 •Nup88^517-742 and Nup62^322-525•Nup88^517-742•Nup214^693-926 heterotrimers and both can bind to Nup93^1-150. We therefore propose that Nup93 acts as a "sensor" to bind to Nup62 shared heterotrimers including the Nup62•Nup54 heterotrimer of the CTC, which was not shown previously to be an interacting partner. Altogether, our biochemical study suggests that Nup62 via its coiled-coil domain is central to form compositionally distinct yet structurally similar heterotrimers and Nup93 binds these diverse heterotrimers nonselectively.

The Nuclear Pore Complex: Birth, Life, and Death of a Cellular Behemoth

Nuclear pore complexes (NPCs) are the only transport channels that cross the nuclear envelope. Constructed from ~500-1000 nucleoporin proteins each, they are among the largest macromolecular assemblies in eukaryotic cells. Thanks to advances in structural analysis approaches, the construction principles and architecture of the NPC have recently been revealed at submolecular resolution. Although the overall structure and inventory of nucleoporins are conserved, NPCs exhibit significant compositional and functional plasticity even within single cells and surprising variability in their assembly pathways. Once assembled, NPCs remain seemingly unexchangeable in post-mitotic cells. There are a number of as yet unresolved questions about how the versatility of NPC assembly and composition is established, how cells monitor the functional state of NPCs or how they could be renewed. Here, we review current progress in our understanding of the key aspects of NPC architecture and lifecycle.

Partial Disassembly of the Nuclear Pore Complex Proteins during Semi-Closed Mitosis in Dictyostelium discoideum

Dictyostelium cells undergo a semi-closed mitosis, during which the nuclear envelope (NE) persists; however, free diffusion between the cytoplasm and the nucleus takes place. To permit the formation of the mitotic spindle, the nuclear envelope must be permeabilized in order to allow diffusion of tubulin dimers and spindle assembly factors into the nucleus. In Aspergillus, free diffusion of proteins between the cytoplasm and the nucleus is achieved by a partial disassembly of the nuclear pore complexes (NPCs) prior to spindle assembly. In order to determine whether this is also the case in Dictyostelium, we analysed components of the NPC by immunofluorescence microscopy and live cell imaging and studied their behaviour during interphase and mitosis. We observed that the NPCs are absent from the contact area of the nucleoli and that some nucleoporins also localize to the centrosome and the spindle poles. In addition, we could show that, during mitosis, the central FG protein NUP62, two inner ring components and Gle1 depart from the NPCs, while all other tested NUPs remained at the NE. This leads to the conclusion that indeed a partial disassembly of the NPCs takes place, which contributes to permeabilisation of the NE during semi-closed mitosis.

Comprehensive structure and functional adaptations of the yeast nuclear pore complex

Nuclear pore complexes (NPCs) mediate the nucleocytoplasmic transport of macromolecules. Here we provide a structure of the isolated yeast NPC in which the inner ring is resolved by cryo-EM at sub-nanometer resolution to show how flexible connectors tie together different structural and functional layers. These connectors may be targets for phosphorylation and regulated disassembly in cells with an open mitosis. Moreover, some nucleoporin pairs and transport factors have similar interaction motifs, which suggests an evolutionary and mechanistic link between assembly and transport. We provide evidence for three major NPC variants that may foreshadow functional specializations at the nuclear periphery. Cryo-electron tomography extended these studies, providing a model of the in situ NPC with a radially expanded inner ring. Our comprehensive model reveals features of the nuclear basket and central transporter, suggests a role for the lumenal Pom152 ring in restricting dilation, and highlights structural plasticity that may be required for transport.

Protein Retargeting in Aspergillus nidulans to Study the Function of Nuclear Pore Complex Proteins

Targeting a protein of interest to a subcellular location by linking it to another protein is a commonly used approach to help determine function in many model systems. Such targeting strategies rely on the creation of functional protein-protein fusions followed by microscopic examination if one or both proteins have fluorescent tags. In this paper, using the model filamentous fungus Aspergillus nidulans, we describe methods to link GFP-tagged proteins to other proteins in the cell by fusing the latter with a GFP-Binding Protein (GBP) that has a high affinity for GFP. This method enables rapid generation of strains with linked proteins in filamentous fungi by sexual crossing or transformations. Additionally, if these two linked proteins stably associate with subcellular structures, it is possible to link the structures using this approach. For example, we used this method to link Nuclear Pore Complexes (NPCs) with mitotic chromatin in A. nidulans. This was done to show that the NPC protein Nup2, that uniquely transitions from NPC onto mitotic chromatin, couples NPC segregation with chromatin segregation by bridging these two structures. In the absence of Nup2, we used the described approach to show that an artificial NPC-chromatin bridge was sufficient for faithful NPC segregation.

Dunking into the Lipid Bilayer: How Direct Membrane Binding of Nucleoporins Can Contribute to Nuclear Pore Complex Structure and Assembly

Nuclear pore complexes (NPCs) mediate the selective and highly efficient transport between the cytoplasm and the nucleus. They are embedded in the two membrane structure of the nuclear envelope at sites where these two membranes are fused to pores. A few transmembrane proteins are an integral part of NPCs and thought to anchor these complexes in the nuclear envelope. In addition, a number of nucleoporins without membrane spanning domains interact with the pore membrane. Here we review our current knowledge of how these proteins interact with the membrane and how this interaction can contribute to NPC assembly, stability and function as well as shaping of the pore membrane.

The nucleoporin Nup50 activates the Ran guanine nucleotide exchange factor RCC1 to promote NPC assembly at the end of mitosis

During mitotic exit, thousands of nuclear pore complexes (NPCs) assemble concomitant with the nuclear envelope to build a transport-competent nucleus. Here, we show that Nup50 plays a crucial role in NPC assembly independent of its well-established function in nuclear transport. RNAi-mediated downregulation in cells or immunodepletion of Nup50 protein in Xenopus egg extracts interferes with NPC assembly. We define a conserved central region of 46 residues in Nup50 that is crucial for Nup153 and MEL28/ELYS binding, and for NPC interaction. Surprisingly, neither NPC interaction nor binding of Nup50 to importin α/β, the GTPase Ran, or chromatin is crucial for its function in the assembly process. Instead, an N-terminal fragment of Nup50 can stimulate the Ran GTPase guanine nucleotide exchange factor RCC1 and NPC assembly, indicating that Nup50 acts via the Ran system in NPC reformation at the end of mitosis. In support of this conclusion, Nup50 mutants defective in RCC1 binding and stimulation cannot replace the wild-type protein in in vitro NPC assembly assays, whereas excess RCC1 can compensate the loss of Nup50.

Spindle Dynamics during Meiotic Development of the Fungus Podospora anserina Requires the Endoplasmic Reticulum-Shaping Protein RTN1

The endoplasmic reticulum (ER) is an elaborate organelle composed of distinct structural and functional domains. ER structure and dynamics involve membrane-shaping proteins of the reticulon and Yop1/DP1 families, which promote membrane curvature and regulate ER shaping and remodeling. Here, we analyzed the function of the reticulon (RTN1) and Yop1 proteins (YOP1 and YOP2) of the model fungus Podospora anserina and their contribution to sexual development. We found that RTN1 and YOP2 localize to the peripheral ER and are enriched in the dynamic apical ER domains of the polarized growing hyphal region. We discovered that the formation of these domains is diminished in the absence of RTN1 or YOP2 and abolished in the absence of YOP1 and that hyphal growth is moderately reduced when YOP1 is deleted in combination with RTN1 and/or YOP2. In addition, we found that RTN1 associates with the Spitzenkörper. Moreover, RTN1 localization is regulated during meiotic development, where it accumulates at the apex of growing asci (meiocytes) during their differentiation and at their middle region during the subsequent meiotic progression. Furthermore, we discovered that loss of RTN1 affects ascospore (meiotic spore) formation, in a process that does not involve YOP1 or YOP2. Finally, we show that the defects in ascospore formation of rtn1 mutants are associated with defective nuclear segregation and spindle dynamics throughout meiotic development. Our results show that sexual development in P. anserina involves a developmental remodeling of the ER that implicates the reticulon RTN1, which is required for meiotic nucleus segregation.

Nup84 persists within the nuclear envelope of the rice blast fungus, Magnaporthe oryzae, during mitosis

The arrangement of the nuclear envelope in the rice blast fungus, Magnaporthe oryzae, was previously undetermined. Here, we identified two conserved components of the nuclear envelope, a core nucleoporin, Nup84, and an inner nuclear membrane protein, Src1. Live-cell super-resolution structured illumination microscopy revealed that Nup84-tdTomato and Src1-EGFP colocalized within the nuclear envelope during interphase and that Nup84-tdTomato remained associated with the dividing nucleus. We also found that appressorium development involved a mitotic nuclear migration event through the germ tube.

The Hitchhiker's Guide to Nucleocytoplasmic Trafficking in Neurodegeneration

The widespread nature of nucleocytoplasmic trafficking defects and protein accumulation suggests distinct yet overlapping mechanisms in a variety of neurodegenerative diseases. Detailed understanding of the cellular pathways involved in nucleocytoplasmic transport and its dysregulation are essential for elucidating neurodegenerative pathogenesis and pinpointing potential areas for therapeutic intervention. The transport of cargos from the nucleus to the cytoplasm is generally regulated by the structure and function of the nuclear pore as well as the karyopherin α/β, importin, exportin, and mRNA export mechanisms. The disruption of these crucial transport mechanisms has been extensively described in the context of neurodegenerative diseases. One common theme in neurodegeneration is the cytoplasmic aggregation of proteins, including nuclear RNA binding proteins, repeat expansion associated gene products, and tau. These cytoplasmic aggregations are partly a consequence of failed nucleocytoplasmic transport machinery, but can also further disrupt transport, creating cyclical feed-forward mechanisms that exacerbate neurodegeneration. Here we describe the canonical mechanisms that regulate nucleocytoplasmic trafficking as well as how these mechanisms falter in neurodegenerative diseases.

Identification of the guanine nucleotide exchange factor for SAR1 in the filamentous fungal model Aspergillus nidulans

In spite of its basic and applied interest, the regulation of ER exit by filamentous fungi is insufficiently understood. In previous work we isolated a panel of conditional mutations in sarA encoding the master GTPase SarA^SAR1 in A. nidulans and demonstrated its key role in exocytosis and hyphal morphogenesis. However, the SAR1 guanine nucleotide exchange factor (GEF), Sec12, has not been characterized in any filamentous fungus, largely due to the fact that SEC12 homologues share little amino acid sequence identity beyond a GGGGxxxxGϕxN motif involved in guanine nucleotide exchange. Here we demonstrate that AN11127 encodes A. nidulans Sec12, which is an essential protein that localizes to the ER and that, when overexpressed, rescues the growth defect resulting from a hypomorphic sarA6^ts mutation at 37 °C. Using purified, bacterially expressed proteins we demonstrate that the product of AN11127 accelerates nucleotide exchange on SarA^SAR1, but not on its closely related GTPase ArfA^ARF1, as expected for a bona fide GEF. The unequivocal characterization of A. nidulans Sec12 paves the way for the tailored modification of ER exit in a model organism that is closely related to industrial species of filamentous fungi.

The mode of mitosis is dramatically modified by deletion of a single nuclear pore complex gene in Aspergillus nidulans

Nuclear pore complex (NPC) proteins (Nups) play multiple roles during mitosis. In this study we expand these roles and reveal that in Aspergillus nidulans, compromising the core Nup84-120 subcomplex of the NPC modifies the mitotic behavior of the nuclear envelope (NE). In wildtype cells, the NE undergoes simultaneous double pinching events to separate daughter nuclei during mitotic exit, whereas in Nup84-120 complex mutants, only one restriction of the NE is observed. Investigating the basis for this modified behavior of the NE in Nup deleted cells uncovered previously unrealized roles for core Nups in mitotic exit. During wildtype anaphase, the NE surrounds the two separating daughter DNA masses which typically flank the central nucleolus, to form three distinct nuclear compartments. In contrast, deletion of core Nups frequently results in early nucleolar eviction from the mitotic nucleus, in turn causing an uncharacteristic dumbbell-shaped NE morphology of anaphase nuclei with a nuclear membrane bridge connecting the two forming G1 nuclei. Importantly, the absence of the nucleolus between the separating daughter nuclei during anaphase delays chromosome segregation and progression into G1 as nuclei remain connected by chromatin bridges. Proteins localizing to late segregating chromosome arms are observed between forming daughter nuclei, and the mitotic spindle fails to resolve in a timely manner. These chromatin bridges are occupied by the Aurora kinase until nuclei have fully separated, suggesting involvement of Aurora in monitoring mitotic spindle and nuclear membrane resolution during mitotic exit. Our findings thus reveal a novel requirement for core Nups in mediating nucleolar positioning during mitosis, which dictates the pattern of NE fissions during karyokinesis and facilitates normal chromosome segregation. The findings additionally demonstrate that the mode of mitosis can be dramatically modified by deletion of a single NPC gene and reveals surprising fluidity in mitotic mechanisms.

Poring over chromosomes: mitotic nuclear pore complex segregation

Eukaryotic cells rely on flux of macromolecules between the nucleus and the cytoplasm for growth and survival. Bidirectional transport is achieved through Nuclear Pore Complexes (NPCs) embedded in the Nuclear Envelope (NE). NPC proteins perform other cellular functions during mitosis, chromatin organization, DNA repair and gene regulation. Dysregulation of NPC number, or defects in their structure and function, are linked to numerous diseases but how NPCs are faithfully inherited during mitosis is poorly understood. In this review, we discuss recent insights to mechanisms of mammalian mitotic NPC segregation and NPC assembly as well as mitotic NPC inheritance via the mitotic chromatin located NPC protein Nup2 in Aspergillus nidulans. We suggest mitotic Nup2 chromatin-based mechanisms could also operate in vertebrate cells.

Evolutionary divergence of the nuclear pore complex from fungi to metazoans

Nuclear pore complex (NPC) is the largest multimeric protein assembly of the eukaryotic cell, which mediates the nucleocytoplasmic transport. The constituent proteins of this assembly (nucleoporins) are present in varying copy numbers to give a size from ~ 60 MDa (yeast) to 112 MDa (human) and share common ancestry with other membrane‐associated complexes such as COPI/COPII and thus share the same structural folds. However, the nucleoporins across species exhibit very low percentage sequence similarity and this reflects in their distinct secondary structure and domain organization. We employed thorough sequence and phylogenetic analysis guided from structure‐based alignments of all the nucleoporins from fungi to metazoans to understand the evolution of NPC. Through evolutionary pressure analysis on various nucleoporins, we deduced that these proteins are under differential selection pressure and hence the homologous interacting partners do not complement each other in the in vitro pull‐down assay. The super tree analysis of all nucleoporins taken together illustrates divergent evolution of nucleoporins and notably, the degree of divergence is more apparent in higher order organisms as compared to lower species. Overall, our results support the hypothesis that the protein–protein interactions in such large multimeric assemblies are species specific in nature and hence their structure and function should also be studied in an organism‐specific manner.

Nup2 performs diverse interphase functions in Aspergillus nidulans

The nuclear pore complex (NPC) protein Nup2 plays interphase nuclear transport roles and in Aspergillus nidulans also functions to bridge NPCs at mitotic chromatin for their faithful coinheritance to daughter G1 nuclei. In this study, we further investigate the interphase functions of Nup2 in A. nidulans. Although Nup2 is not required for nuclear import of all nuclear proteins after mitosis, it is required for normal G1 nuclear accumulation of the NPC nuclear basket–associated components Mad2 and Mlp1 as well as the THO complex protein Tho2. Targeting of Mlp1 to nuclei partially rescues the interphase delay seen in nup2 mutants indicating that some of the interphase defects in Nup2-deleted cells are due to Mlp1 mislocalization. Among the inner nuclear membrane proteins, Nup2 affects the localization of Ima1, orthologues of which are involved in nuclear movement. Interestingly, nup2 mutant G1 nuclei also exhibit an abnormally long period of extensive to-and-fro movement immediately after mitosis in a manner dependent on the microtubule cytoskeleton. This indicates that Nup2 is required to limit the transient postmitotic nuclear migration typical of many filamentous fungi. The findings reveal that Nup2 is a multifunctional protein that performs diverse functions during both interphase and mitosis in A. nidulans.

A nuclear contortionist: the mitotic migration of Magnaporthe oryzae nuclei during plant infection

Magnaporthe oryzae is a filamentous fungus, which causes significant destruction to cereal crops worldwide. To infect plant cells, the fungus develops specialised constricted structures such as the penetration peg and the invasive hyphal peg. Live-cell imaging of M. oryzae during plant infection reveals that nuclear migration occurs during intermediate mitosis, in which the nuclear envelope neither completely disassembles nor remains entirely intact. Remarkably, in M. oryzae, mitotic nuclei show incredible malleability while undergoing confined migration through the constricted penetration and invasive hyphal pegs. Here, we review early events in plant infection, discuss intermediate mitosis, and summarise current knowledge of intermediate mitotic nuclear migration in M. oryzae.

A self-inhibitory interaction within Nup155 and membrane binding are required for nuclear pore complex formation

Nuclear pore complexes (NPCs) are gateways through the nuclear envelope. How they form into a structure containing three rings and integrate into the nuclear envelope remains a challenging paradigm for coordinated assembly of macro-complexes. In vertebrates, the cytoplasmic and nucleoplasmic rings of NPCs are mostly formed by multiple copies of the Nup107-Nup160 complex, whereas the central, or inner ring is composed of Nup53, Nup93, Nup155 and the two paralogues Nup188 and Nup205. Inner ring assembly is only partially understood. Using in vitro nuclear assembly reactions, we show that direct pore membrane binding of Nup155 is crucial for NPC formation. Replacing full-length Nup155 with its N-terminal β-propeller allows assembly of the outer ring components to the NPC backbone that also contains Nup53. However, further assembly, especially recruitment of the Nup93 and Nup62 complexes, is blocked. Self-interaction between the N- and C-terminal domains of Nup155 has an auto-inhibitory function that prevents interaction between the N-terminus of Nup155 and the C-terminal region of Nup53. Nup93 can overcome this block by binding to Nup53, thereby promoting formation of the inner ring and the NPC.

Tools for retargeting proteins within Aspergillus nidulans

Endogenously tagging proteins with green fluorescent protein (GFP) enables the visualization of the tagged protein using live cell microscopy. GFP-tagging is widely utilized to study biological processes in model experimental organisms including filamentous fungi such as Aspergillus nidulans. Many strains of A. nidulans have therefore been generated with different proteins endogenously tagged with GFP. To further enhance experimental approaches based upon GFP-tagging, we have adapted the GFP Binding Protein (GBP) system for A. nidulans. GBP is a genetically encoded Llama single chain antibody against GFP which binds GFP with high affinity. Using gene replacement approaches, it is therefore possible to link GBP to anchor proteins, which will then retarget GFP-tagged proteins away from their normal location to the location of the anchor-GBP protein. To facilitate this approach in A. nidulans, we made four base plasmid cassettes that can be used to generate gene replacement GBP-tagging constructs by utilizing fusion PCR. Using these base cassettes, fusion PCR, and gene targeting approaches, we generated strains with SPA10-GBP and Tom20-GBP gene replacements. These strains enabled test targeting of GFP-tagged proteins to septa or to the surface of mitochondria respectively. SPA10-GBP is shown to effectively target GFP-tagged proteins to both forming and mature septa. Tom20-GBP has a higher capacity to retarget GFP-tagged proteins being able to relocate all Nup49-GFP from its location within nuclear pore complexes (NPCs) to the cytoplasm in association with mitochondria. Notably, removal of Nup49-GFP from NPCs causes cold sensitivity as does deletion of the nup49 gene. The cassette constructs described facilitate experimental approaches to generate precise protein-protein linkages in fungi. The A. nidulans SPA10-GBP and Tom20-GBP strains can be utilized to modulate other GFP-tagged proteins of interest.

A short linear motif in scaffold Nup145C connects Y-complex with pre-assembled outer ring Nup82 complex

Nucleocytoplasmic transport occurs through nuclear pore complexes (NPCs), which are formed from multiple copies of ~30 different nucleoporins (Nups) and inserted into the double nuclear membrane. Many of these Nups are organized into subcomplexes, of which the Y-shaped Nup84 complex is the major constituent of the nuclear and cytoplasmic rings. The Nup82–Nup159–Nsp1 complex is another module that, however, is only assembled into the cytoplasmic ring. By means of crosslinking mass spectrometry, biochemical reconstitution, and molecular modeling, we identified a short linear motif in the unstructured N-terminal region of Chaetomium thermophilum Nup145C, a subunit of the Y-complex, that is sufficient to recruit the Nup82 complex, but only in its assembled state. This finding points to a more general mechanism that short linear motifs in structural Nups can act as sensors to cooperatively connect pre-assembled NPC modules, thereby facilitating the formation and regulation of the higher-order NPC assembly.

The Nup82–Nup159–Nsp1 complex, which plays a key role in mRNA export, is recruited late during the process of nuclear pore complex (NPC) assembly. Here the authors combine crosslinking mass spectrometry, biochemical reconstitution and molecular modeling to gain insights into the mechanism of Nup82 recruitment to the NPC.

Mitotic nuclear pore complex segregation involves Nup2 in Aspergillus nidulans

Transport through nuclear pore complexes (NPCs) during interphase is facilitated by the nucleoporin Nup2 via its importin α- and Ran-binding domains. However, Aspergillus nidulans and vertebrate Nup2 also locate to chromatin during mitosis, suggestive of mitotic functions. In this study, we report that Nup2 is required for mitotic NPC inheritance in A. nidulans Interestingly, the role of Nup2 during mitotic NPC segregation is independent of its importin α- and Ran-binding domains but relies on a central targeting domain that is necessary for localization and viability. To test whether mitotic chromatin-associated Nup2 might function to bridge NPCs with chromatin during segregation, we provided an artificial link between NPCs and chromatin via Nup133 and histone H1. Using this approach, we bypassed the requirement of Nup2 for NPC segregation. This indicates that A. nidulans cells ensure accurate mitotic NPC segregation to daughter nuclei by linking mitotic DNA and NPC segregation via the mitotic specific chromatin association of Nup2.

Location and functional analysis of the Aspergillus nidulans Aurora kinase confirm mitotic functions and suggest non-mitotic roles

Filamentous fungi have devastating negative impacts as pathogens and agents of food spoilage but also have critical ecological importance and are utilized for industrial applications. The characteristic multinucleate nature of filamentous fungi is facilitated by limiting if, when and where septation, the fungal equivalent of cytokinesis, occurs. In the model filamentous fungus Aspergillus nidulans septation does not occur immediately after mitosis and is an incomplete process resulting in the formation of a septal pore whose permeability is cell cycle regulated. How mitotic regulators, such as the Aurora kinase, contribute to the often unique biology of filamentous fungi is not well understood. The Aurora B kinase has not previously been investigated in any detail during hyphal growth. Here we demonstrate for the first time that Aurora displays cell cycle dependent locations to the region of forming septa, the septal pore and mature septa as well as the mitotic apparatus. To functionally analyze Aurora, we generated a temperature sensitive allele revealing essential mitotic and spindle assembly checkpoint functions consistent with its location to the kinetochore region and spindle midzone. Our analysis also reveals that cellular and kinetochore Aurora levels increase during a mitotic spindle assembly checkpoint arrest and we propose that this could be important for checkpoint inactivation when spindle formation is prevented. We demonstrate that Aurora accumulation at mature septa following mitotic entry does not require mitotic progression but is dependent upon a timing mechanism. Surprisingly we also find that Aurora inactivation leads to cellular swelling and lysis indicating an unexpected function for Aurora in fungal cell growth. Thus in addition to its conserved mitotic functions our data suggest that Aurora has the capacity to be an important regulator of septal biology and cell growth in filamentous fungi.

Compositionally distinct nuclear pore complexes of functionally distinct dimorphic nuclei in the ciliate Tetrahymena

The nuclear pore complex (NPC), a gateway for nucleocytoplasmic trafficking, is composed of ∼30 different proteins called nucleoporins. It remains unknown whether the NPCs within a species are homogeneous or vary depending on the cell type or physiological condition. Here, we present evidence for compositionally distinct NPCs that form within a single cell in a binucleated ciliate. In Tetrahymena thermophila, each cell contains both a transcriptionally active macronucleus (MAC) and a germline micronucleus (MIC). By combining in silico analysis, mass spectrometry analysis for immuno-isolated proteins and subcellular localization analysis of GFP-fused proteins, we identified numerous novel components of MAC and MIC NPCs. Core members of the Nup107-Nup160 scaffold complex were enriched in MIC NPCs. Strikingly, two paralogs of Nup214 and of Nup153 localized exclusively to either the MAC or MIC NPCs. Furthermore, the transmembrane components Pom121 and Pom82 localize exclusively to MAC and MIC NPCs, respectively. Our results argue that functional nuclear dimorphism in ciliates is likely to depend on the compositional and structural specificity of NPCs.

A mitotic nuclear envelope tether for Gle1 also impacts nuclear and nucleolar architecture

During Aspergillus nidulans mitosis peripheral nuclear pore complex (NPC) proteins (Nups) disperse from the core NPC structure. Unexpectedly, one predicted peripheral Nup, Gle1, remains at the mitotic NE via an unknown mechanism. Gle1 affinity purification identified MtgA ( M: itotic T: ether for G: le1), which tethers Gle1 to the NE during mitosis, but not during interphase when Gle1 is at NPCs. MtgA is the ortholog of the Schizosaccharomyces pombe telomere-anchoring inner nuclear membrane protein Bqt4. Like Bqt4, MtgA has meiotic roles but is functionally distinct from Bqt4 as MtgA is not required for tethering telomeres to the NE. Domain analyses revealed MtgA targeting to the NE requires its C-terminal transmembrane domain and a nuclear localization signal. Importantly, MtgA functions beyond Gle1 mitotic targeting and meiosis and impacts nuclear and nucleolar architecture when deleted or overexpressed. Deletion of MtgA generates small, round nuclei whereas overexpressing MtgA generates larger nuclei with altered nuclear compartmentalization resulting from NE expansion around the nucleolus. The accumulation of MtgA around the nucleolus promotes a similar accumulation of the endoplasmic reticulum (ER) protein Erg24 lowering its levels in the ER. This study extends the functions of Bqt4-like proteins to include mitotic Gle1 targeting and modulation of nuclear and nucleolar architecture.

Mixing and matching nuclear envelope remodeling and spindle assembly strategies in the evolution of mitosis

In eukaryotes, cellular genome is enclosed inside a membrane-bound organelle called the nucleus. The nucleus compartmentalizes genome replication, repair and expression, keeping these activities separated from protein synthesis and other metabolic processes. Each proliferative division, the duplicated chromosomes must be equipartitioned between the daughter cells and this requires precise coordination between assembly of the microtubule-based mitotic spindle and nuclear remodeling. Here we review a surprising variety of strategies used by modern eukaryotes to manage these processes and discuss possible mechanisms that might have led to the emergence of this diversity in evolution.

Interactome Mapping Reveals the Evolutionary History of the Nuclear Pore Complex

The nuclear pore complex (NPC) is responsible for nucleocytoplasmic transport and constitutes a hub for control of gene expression. The components of NPCs from several eukaryotic lineages have been determined, but only the yeast and vertebrate NPCs have been extensively characterized at the quaternary level. Significantly, recent evidence indicates that compositional similarity does not necessarily correspond to homologous architecture between NPCs from different taxa. To address this, we describe the interactome of the trypanosome NPC, a representative, highly divergent eukaryote. We identify numerous new NPC components and report an exhaustive interactome, allowing assignment of trypanosome nucleoporins to discrete NPC substructures. Remarkably, despite retaining similar protein composition, there are exceptional architectural dissimilarities between opisthokont (yeast and vertebrates) and excavate (trypanosomes) NPCs. Whilst elements of the inner core are conserved, numerous peripheral structures are highly divergent, perhaps reflecting requirements to interface with divergent nuclear and cytoplasmic functions. Moreover, the trypanosome NPC has almost complete nucleocytoplasmic symmetry, in contrast to the opisthokont NPC; this may reflect divergence in RNA export processes at the NPC cytoplasmic face, as we find evidence supporting Ran-dependent mRNA export in trypanosomes, similar to protein transport. We propose a model of stepwise acquisition of nucleocytoplasmic mechanistic complexity and demonstrate that detailed dissection of macromolecular complexes provides fuller understanding of evolutionary processes.

Dissection of the nuclear pore complex—an ancient eukaryotic molecular machine—exposes a fundamental divergence in structure and function between yeast and humans versus trypanosomes and provides insights into the evolution of the nucleus.

Much of the core architecture of the eukaryotic cell was established over one billion years ago. Significantly, many cellular systems possess lineage-specific features, and architectural and compositional variation of complexes and pathways that are likely keyed to specific functional adaptations. The nuclear pore complex (NPC) contributes to many processes, including nucleocytoplasmic transport, interactions with the nuclear lamina, and mRNA processing. We exploited trypanosome parasites to investigate NPC evolution and conservation at the level of protein–protein interactions and composition. We unambiguously assigned NPC components to specific substructures and found that the NPC structural scaffold is generally conserved, albeit with lineage-specific elements. However, there is significant variation in pore membrane proteins and an absence of critical components involved in mRNA export in fungi and animals (opisthokonts). This is reflected by the completely symmetric localization of all trypanosome nucleoporins, with the exception of the nuclear basket. This architecture is highly distinct from opisthokonts. We also identify features that suggest a Ran-dependent system for mRNA export in trypanosomes, a system that may presage distinct mechanisms of protein and mRNA transport in animals and fungi. Our study highlights that shared composition of macromolecular assemblies does not necessarily equate to shared architecture. Identification of lineage-specific features within the trypanosome NPC significantly advances our understanding of mechanisms of nuclear transport, gene expression, and evolution of the nucleus.

Virtual Nuclear Envelope Breakdown and Its Regulators in Fission Yeast Meiosis

Ran, a small GTPase, is required for the spindle formation and nuclear envelope (NE) formation. After NE breakdown (NEBD) during mitosis in metazoan cells, the Ran-GTP gradient across the NE is lost and Ran-GTP becomes concentrated around chromatin, thus affecting the stability of microtubules and promoting the assembly of spindle microtubules and segregation of chromosomes. Mitosis in which chromosomes are segregated subsequent to NEBD is called “open mitosis.” In contrast, many fungi undergo a process termed “closed mitosis” in which chromosome segregation and spindle formation occur without NEBD. Although the fission yeast Schizosaccharomyces pombe undergoes a closed mitosis, it exhibits a short period during meiosis (anaphase of the second meiosis; called “anaphase II”) when nuclear and cytoplasmic proteins are mixed in the presence of intact NE and nuclear pore complexes (NPC). This “virtual” nuclear envelope breakdown (vNEBD) involves changes in the localization of RanGAP1, an activator of Ran-GTP hydrolysis. Recently, Nup132, a component of the structural core Nup107-160 subcomplex of the NPC, has been shown to be involved in the maintenance of the nuclear cytoplasmic barrier in yeast meiosis. In this review, we highlight the possible roles of RanGAP1 and Nup132 in vNEBD and discuss the biological significance of vNEBD in S. pombe meiosis.

Complex Commingling: Nucleoporins and the Spindle Assembly Checkpoint

The segregation of the chromosomes during mitosis is an important process, in which the replicated DNA content is properly allocated into two daughter cells. To ensure their genomic integrity, cells present an essential surveillance mechanism known as the spindle assembly checkpoint (SAC), which monitors the bipolar attachment of the mitotic spindle to chromosomes to prevent errors that would result in chromosome mis-segregation and aneuploidy. Multiple components of the nuclear pore complex (NPC), a gigantic protein complex that forms a channel through the nuclear envelope to allow nucleocytoplasmic exchange of macromolecules, were shown to be critical for faithful cell division and implicated in the regulation of different steps of the mitotic process, including kinetochore and spindle assembly as well as the SAC. In this review, we will describe current knowledge about the interconnection between the NPC and the SAC in an evolutional perspective, which primarily relies on the two mitotic checkpoint regulators, Mad1 and Mad2. We will further discuss the role of NPC constituents, the nucleoporins, in kinetochore and spindle assembly and the formation of the mitotic checkpoint complex during mitosis and interphase.

Nuclear autonomy in multinucleate fungi

Within many fungal syncytia, nuclei behave independently despite sharing a common cytoplasm. Creation of independent nuclear zones of control in one cell is paradoxical considering random protein synthesis sites, predicted rapid diffusion rates, and well-mixed cytosol. In studying the surprising fungal nuclear autonomy, new principles of cellular organization are emerging. We discuss the current understanding of nuclear autonomy, focusing on asynchronous cell cycle progression where most work has been directed. Mechanisms underlying nuclear autonomy are diverse including mRNA localization, ploidy variability, and nuclear spacing control. With the challenges fungal syncytia face due to cytoplasmic size and shape, they serve as powerful models for uncovering new subcellular organization modes, variability sources among isogenic uninucleate cells, and the evolution of multicellularity.

The Inner Nuclear Membrane Protein Src1 Is Required for Stable Post-Mitotic Progression into G1 in Aspergillus nidulans

How membranes and associated proteins of the nuclear envelope (NE) are assembled specifically and inclusively around segregated genomes during exit from mitosis is incompletely understood. Inner nuclear membrane (INM) proteins play key roles by providing links between DNA and the NE. In this study we have investigated the highly conserved INM protein Src1 in Aspergillus nidulans and have uncovered a novel cell cycle response during post mitotic formation of G1 nuclei. Live cell imaging indicates Src1 could have roles during mitotic exit as it preferentially locates to the NE abscission points during nucleokinesis and to the NE surrounding forming daughter G1 nuclei. Deletion analysis further supported this idea revealing that although Src1 is not required for interphase progression or mitosis it is required for stable post-mitotic G1 nuclear formation. This conclusion is based upon the observation that in the absence of Src1 newly formed G1 nuclei are structurally unstable and immediately undergo architectural modifications typical of mitosis. These changes include NPC modifications that stop nuclear transport as well as disassembly of nucleoli. More intriguingly, the newly generated G1 nuclei then cycle between mitotic- and interphase-like states. The findings indicate that defects in post-mitotic G1 nuclear formation caused by lack of Src1 promote repeated failed attempts to generate stable G1 nuclei. To explain this unexpected phenotype we suggest a type of regulation that promotes repetition of defective cell cycle transitions rather than preventing progression past the defective cell cycle transition. We suggest the term “reboot regulation” to define this mode of cell cycle regulation. The findings are discussed in relationship to recent studies showing the Cdk1 master oscillator can entrain subservient oscillators that when uncoupled cause cell cycle transitions to be repeated.

Structural Characterization of the Chaetomium thermophilum TREX-2 Complex and its Interaction with the mRNA Nuclear Export Factor Mex67:Mtr2

The TREX-2 complex integrates mRNA nuclear export into the gene expression pathway and is based on a Sac3 scaffold to which Thp1, Sem1, Sus1, and Cdc31 bind. TREX-2 also binds the mRNA nuclear export factor, Mex67:Mtr2, through the Sac3 N-terminal region (Sac3N). Here, we characterize Chaetomium thermophilum TREX-2, show that the in vitro reconstituted complex has an annular structure, and define the structural basis for interactions between Sac3, Sus1, Cdc31, and Mex67:Mtr2. Crystal structures show that the binding of C. thermophilum Sac3N to the Mex67 NTF2-like domain (Mex67^NTF2L) is mediated primarily through phenylalanine residues present in a series of repeating sequence motifs that resemble those seen in many nucleoporins, and Mlp1 also binds Mex67:Mtr2 using a similar motif. Deletion of Sac3N generated growth and mRNA export defects in Saccharomyces cerevisiae, and we propose TREX-2 and Mlp1 function to facilitate export by concentrating mature messenger ribonucleoparticles at the nuclear pore entrance.

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Chaetomium thermophilum TREX-2 has an annular structure resembling the letter Q

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Structure of interfaces between TREX-2 components Sac3, Sus1, and Cdc31 defined

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Sac3N phenylalanines dominate C. thermophilum TREX-2 binding to Mex67 NTF2L domain

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TREX-2 facilitates mRNA export by concentrating mature mRNPs at nuclear pores

Spatial regulation of the spindle assembly checkpoint and anaphase-promoting complex in Aspergillus nidulans

The spindle assembly checkpoint (SAC) plays a critical role in preventing mitotic errors by inhibiting anaphase until all kinetochores are correctly attached to spindle microtubules. In spite of the economic and medical importance of filamentous fungi, relatively little is known about the behavior of SAC proteins in these organisms. In our efforts to understand the role of γ-tubulin in cell cycle regulation, we have created functional fluorescent protein fusions of four SAC proteins in Aspergillus nidulans, the homologs of Mad2, Mps1, Bub1/BubR1 and Bub3. Time-lapse imaging reveals that SAC proteins are in distinct compartments of the cell until early mitosis when they co-localize at the spindle pole body. SAC activity is, thus, spatially regulated in A. nidulans. Likewise, Cdc20, an activator of the anaphase-promoting complex/cyclosome, is excluded from interphase nuclei, but enters nuclei at mitotic onset and accumulates to a higher level in mitotic nuclei than in the surrounding nucleoplasm before leaving in anaphase/telophase. The activity of this critical cell cycle regulatory complex is likely regulated by the location of Cdc20. Finally, the γ-tubulin mutation mipAD159 causes a nuclear-specific failure of nuclear localization of Mps1 and Bub1/R1 but not of Cdc20, Bub3 or Mad2.

Nup2 requires a highly divergent partner, NupA, to fulfill functions at nuclear pore complexes and the mitotic chromatin region

Among nuclear pore proteins, Nup2 is unique because it transfers to the mitotic chromatin region to fulfill unknown functions. Analysis of Nup2 and a novel targeting partner, NupA, shows that they are required for normal anaphase and nucleokinesis. Their functions also involve an import pathway for Mad1 but apparently not general nuclear protein import.

Chromatin and nuclear pore complexes (NPCs) undergo dramatic changes during mitosis, which in vertebrates and Aspergillus nidulans involves movement of Nup2 from NPCs to the chromatin region to fulfill unknown functions. This transition is shown to require the Cdk1 mitotic kinase and be promoted prematurely by ectopic expression of the NIMA kinase. Nup2 localizes with a copurifying partner termed NupA, a highly divergent yet essential NPC protein. NupA and Nup2 locate throughout the chromatin region during prophase but during anaphase move to surround segregating DNA. NupA function is shown to involve targeting Nup2 to its interphase and mitotic locations. Deletion of either Nup2 or NupA causes identical mitotic defects that initiate a spindle assembly checkpoint (SAC)–dependent mitotic delay and also cause defects in karyokinesis. These mitotic problems are not caused by overall defects in mitotic NPC disassembly–reassembly or general nuclear import. However, without Nup2 or NupA, although the SAC protein Mad1 locates to its mitotic locations, it fails to locate to NPCs normally in G1 after mitosis. Collectively the study provides new insight into the roles of Nup2 and NupA during mitosis and in a surveillance mechanism that regulates nucleokinesis when mitotic defects occur after SAC fulfillment.

The diverse roles of the Nup93/Nic96 complex proteins - structural scaffolds of the nuclear pore complex with additional cellular functions

Nuclear pore complexes mediate the transport between the cell nucleoplasm and cytoplasm. These 125 MDa structures are among the largest assemblies found in eukaryotes, built from proteins organized in distinct subcomplexes that act as building blocks during nuclear pore complex biogenesis. In this review, we focus on one of these subcomplexes, the Nup93 complex in metazoa and its yeast counterpart, the Nic96 complex. We discuss its essential function in nuclear pore complex assembly as a linker between the nuclear membrane and the central part of the pore and its various roles in nuclear transport processes and beyond.

Restraint of the G2/M transition by the SR/RRM family mRNA shuttling binding protein SNXAHRB1 in Aspergillus nidulans

Control of the eukaryotic G2/M transition by CDC2/CYCLINB is tightly regulated by protein-protein interactions, protein phosphorylations, and nuclear localization of CDC2/CYCLINB. We previously reported a screen, in Aspergillus nidulans, for extragenic suppressors of nimX2(cdc2) that resulted in the identification of the cold-sensitive snxA1 mutation. We demonstrate here that snxA1 suppresses defects in regulators of the CDK1 mitotic induction pathway, including nimX2(cdc) (2), nimE6(cyclinB), and nimT23(cdc) (25), but does not suppress G2-arresting nimA1/nimA5 mutations, the S-arresting nimE10(cyclinB) mutation, or three other G1/S phase mutations. snxA encodes the A. nidulans homolog of Saccharomyces cerevisiae Hrb1/Gbp2; nonessential shuttling messenger RNA (mRNA)-binding proteins belonging to the serine-arginine-rich (SR) and RNA recognition motif (RRM) protein family; and human heterogeneous ribonucleoprotein-M, a spliceosomal component involved in pre-mRNA processing and alternative splicing. snxA(Hrb) (1) is nonessential, its deletion phenocopies the snxA1 mutation, and its overexpression rescues snxA1 and ΔsnxA mutant phenotypes. snxA1 and a second allele isolated in this study, snxA2, are hypomorphic mutations that result from decreased transcript and protein levels, suggesting that snxA acts normally to restrain cell cycle progression. SNXA(HRB1) is predominantly nuclear, but is not retained in the nucleus during the partially closed mitosis of A. nidulans. We show that the snxA1 mutation does not suppress nimX2 by altering NIMX2(CDC2)/NIME(CYCLINB) kinase activity and that snxA1 or ΔsnxA alter localization patterns of NIME(CYCLINB) at the restrictive temperatures for snxA1 and nimX2. Together, these findings suggest a novel and previously unreported role of an SR/RRM family protein in cell cycle regulation, specifically in control of the CDK1 mitotic induction pathway.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

Characterization of nuclear pore complex components in fission yeast Schizosaccharomyces pombe

The nuclear pore complex (NPC) is an enormous proteinaceous complex composed of multiple copies of about 30 different proteins called nucleoporins. In this study, we analyzed the composition of the NPC in the model organism Schizosaccharomyces pombe using strains in which individual nucleoporins were tagged with GFP. We identified 31 proteins as nucleoporins by their localization to the nuclear periphery. Gene disruption analysis in previous studies coupled with gene disruption analysis in the present study indicates that 15 of these nucleoporins are essential for vegetative cell growth and the other 16 nucleoporins are non-essential. Among the 16 non-essential nucleoporins, 11 are required for normal progression through meiosis and their disruption caused abnormal spore formation or poor spore viability. Based on fluorescence measurements of GFP-fused nucleoporins, we estimated the composition of the NPC in S. pombe and found that the organization of the S. pombe NPC is largely similar to that of other organisms; a single NPC was estimated as being 45.8–47.8 MDa in size. We also used fluorescence measurements of single NPCs and quantitative western blotting to analyze the composition of the Nup107-Nup160 subcomplex, which plays an indispensable role in NPC organization and function. Our analysis revealed low amounts of Nup107 and Nup131 and high amounts of Nup132 in the Nup107-Nup160 subcomplex, suggesting that the composition of this complex in S. pombe may differ from that in S. cerevisiae and humans. Comparative analysis of NPCs in various organisms will lead to a comprehensive understanding of the functional architecture of the NPC.

Application of a new dual localization-affinity purification tag reveals novel aspects of protein kinase biology in Aspergillus nidulans

Filamentous fungi occupy critical environmental niches and have numerous beneficial industrial applications but devastating effects as pathogens and agents of food spoilage. As regulators of essentially all biological processes protein kinases have been intensively studied but how they regulate the often unique biology of filamentous fungi is not completely understood. Significant understanding of filamentous fungal biology has come from the study of the model organism Aspergillus nidulans using a combination of molecular genetics, biochemistry, cell biology and genomic approaches. Here we describe dual localization-affinity purification (DLAP) tags enabling endogenous N or C-terminal protein tagging for localization and biochemical studies in A. nidulans. To establish DLAP tag utility we endogenously tagged 17 protein kinases for analysis by live cell imaging and affinity purification. Proteomic analysis of purifications by mass spectrometry confirmed association of the CotA and NimX^Cdk1 kinases with known binding partners and verified a predicted interaction of the SldA^Bub1/R1 spindle assembly checkpoint kinase with SldB^Bub3. We demonstrate that the single TOR kinase of A. nidulans locates to vacuoles and vesicles, suggesting that the function of endomembranes as major TOR cellular hubs is conserved in filamentous fungi. Comparative analysis revealed 7 kinases with mitotic specific locations including An-Cdc7 which unexpectedly located to mitotic spindle pole bodies (SPBs), the first such localization described for this family of DNA replication kinases. We show that the SepH septation kinase locates to SPBs specifically in the basal region of apical cells in a biphasic manner during mitosis and again during septation. This results in gradients of SepH between G1 SPBs which shift along hyphae as each septum forms. We propose that SepH regulates the septation initiation network (SIN) specifically at SPBs in the basal region of G1 cells and that localized gradients of SIN activity promote asymmetric septation.

Multiple nuclear localization signals mediate nuclear localization of the GATA transcription factor AreA

The Aspergillus nidulans GATA transcription factor AreA activates transcription of nitrogen metabolic genes in response to nitrogen limitation and is known to accumulate in the nucleus during nitrogen starvation. Sequence analysis of AreA revealed multiple nuclear localization signals (NLSs), five putative classical NLSs conserved in fungal AreA orthologs but not in the Saccharomyces cerevisiae functional orthologs Gln3p and Gat1p, and one putative noncanonical RRX33RXR bipartite NLS within the DNA-binding domain. In order to identify the functional NLSs in AreA, we constructed areA mutants with mutations in individual putative NLSs or combinations of putative NLSs and strains expressing green fluorescent protein (GFP)-AreA NLS fusion genes. Deletion of all five classical NLSs individually or collectively did not affect utilization of nitrogen sources or AreA-dependent gene expression and did not prevent AreA nuclear localization. Mutation of the bipartite NLS conferred the inability to utilize alternative nitrogen sources and abolished AreA-dependent gene expression likely due to effects on DNA binding but did not prevent AreA nuclear localization. Mutation of all six NLSs simultaneously prevented AreA nuclear accumulation. The bipartite NLS alone strongly directed GFP to the nucleus, whereas the classical NLSs collaborated to direct GFP to the nucleus. Therefore, AreA contains multiple conserved NLSs, which show redundancy and together function to mediate nuclear import. The noncanonical bipartite NLS is conserved in GATA factors from Aspergillus, yeast, and mammals, indicating an ancient origin.

The SUN protein Mps3 controls Ndc1 distribution and function on the nuclear membrane

Ndc1–Mps3 interaction is important for controlling the distribution of Ndc1 between the nuclear pore complex and spindle pole body to ensure proper nuclear envelope insertion of both complexes.

In closed mitotic systems such as Saccharomyces cerevisiae, nuclear pore complexes (NPCs) and the spindle pole body (SPB) must assemble into an intact nuclear envelope (NE). Ndc1 is a highly conserved integral membrane protein involved in insertion of both complexes. In this study, we show that Ndc1 interacts with the SUN domain–containing protein Mps3 on the NE in live yeast cells using fluorescence cross-correlation spectroscopy. Genetic and molecular analysis of a series of new ndc1 alleles allowed us to understand the role of Ndc1–Mps3 binding at the NE. We show that the ndc1-L562S allele is unable to associate specifically with Mps3 and find that this mutant is lethal due to a defect in SPB duplication. Unlike other ndc1 alleles, the growth and Mps3 binding defect of ndc1-L562S is fully suppressed by deletion of POM152, which encodes a NPC component. Based on our data we propose that the Ndc1–Mps3 interaction is important for controlling the distribution of Ndc1 between the NPC and SPB.

Mitotic regulation of fungal cell-to-cell connectivity through septal pores involves the NIMA kinase

Septal pores—the intercellular bridges of fungi—are open during interphase but closed at mitosis. The NIMA kinase mitotically regulates septal pore closing and opening potentially via mechanisms analogous to how it regulates mitotic nuclear pores. The findings explain how and why physically connected Aspergillus cells can maintain mitotic autonomy.

Intercellular bridges are a conserved feature of multicellular organisms. In multicellular fungi, cells are connected directly via intercellular bridges called septal pores. Using Aspergillus nidulans, we demonstrate for the first time that septal pores are regulated to be opened during interphase but closed during mitosis. Septal pore–associated proteins display dynamic cell cycle–regulated locations at mature septa. Of importance, the mitotic NIMA kinase locates to forming septa and surprisingly then remains at septa throughout interphase. However, during mitosis, when NIMA transiently locates to nuclei to promote mitosis, its levels at septa drop. A model is proposed in which NIMA helps keep septal pores open during interphase and then closed when it is removed from them during mitosis. In support of this hypothesis, NIMA inactivation is shown to promote interphase septal pore closing. Because NIMA triggers nuclear pore complex opening during mitosis, our findings suggest that common cell cycle regulatory mechanisms might control septal pores and nuclear pores such that they are opened and closed out of phase to each other during cell cycle progression. The study provides insights into how and why cytoplasmically connected Aspergillus cells maintain mitotic autonomy.

Enriching the pore: splendid complexity from humble origins

The nucleus is the defining intracellular organelle of eukaryotic cells and represents a major structural innovation that differentiates the eukaryotic and prokaryotic cellular form. The presence of a nuclear envelope (NE) encapsulating the nucleus necessitates a mechanism for interchange between the contents of the nuclear interior and the cytoplasm, which is mediated via the nuclear pore complex (NPC), a large protein assembly residing in nuclear pores in the NE. Recent advances have begun to map the structure and functions of the NPC in multiple organisms, and to allow reconstruction of some of the evolutionary events that underpin the modern NPC form, highlighting common and differential NPC features across the eukaryotes. Here we discuss some of these advances and the questions being pursued, consider how the evolution of the NPC has been constrained, and finally propose a model for how the NPC evolved.