The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds

The core spindle pole body scaffold Ppc89 links the pericentrin orthologue Pcp1 to the fission yeast spindle pole body via an evolutionarily conserved interface

Centrosomes and spindle pole bodies (SPBs) are important for mitotic spindle formation and serve as cellular signaling platforms. Although centrosomes and SPBs differ in morphology, many mechanistic insights into centrosome function have been gleaned from SPB studies. In the fission yeast Schizosaccharomyces pombe, the α-helical protein Ppc89, identified based on its interaction with the septation initiation network scaffold Sid4, comprises the SPB core. High-resolution imaging has suggested that SPB proteins assemble on the Ppc89 core during SPB duplication, but such interactions are undefined. Here, we define a connection between Ppc89 and the essential pericentrin Pcp1. Specifically, we found that a predicted third helix within Ppc89 binds the Pcp1 pericentrin-AKAP450 centrosomal targeting (PACT) domain complexed with calmodulin. Ppc89 helix 3 contains similarity to present in the N-terminus of Cep57 (PINC) motifs found in the centrosomal proteins fly SAS-6 and human Cep57 and also to the S. cerevisiae SPB protein Spc42. These motifs bind pericentrin-calmodulin complexes and AlphaFold2 models suggest a homologous complex assembles in all four organisms. Mutational analysis of the S. pombe complex supports the importance of Ppc89-Pcp1 binding interface in vivo. Our studies provide insight into the core architecture of the S. pombe SPB and suggest an evolutionarily conserved mechanism of scaffolding pericentrin-calmodulin complexes for mitotic spindle formation.

Fungal microtubule organizing centers are evolutionarily unstable structures

For most Eukaryotic species the requirements of cilia formation dictate the structure of microtubule organizing centers (MTOCs). In this study we find that loss of cilia corresponds to loss of evolutionary stability for fungal MTOCs. We used iterative search algorithms to identify proteins homologous to those found in Saccharomyces cerevisiae, and Schizosaccharomyces pombe MTOCs, and calculated site-specific rates of change for those proteins that were broadly phylogenetically distributed. Our results indicate that both the protein composition of MTOCs as well as the sequence of MTOC proteins are poorly conserved throughout the fungal kingdom. To begin to reconcile this rapid evolutionary change with the rigid structure and essential function of the S. cerevisiae MTOC we further analyzed how structural interfaces among proteins influence the rates of change for specific residues within a protein. We find that a more stable protein may stabilize portions of an interacting partner where the two proteins are in contact. In summary, while the protein composition and sequences of the MTOC may be rapidly changing the proteins within the structure have a stabilizing effect on one another. Further exploration of fungal MTOCs will expand our understanding of how changes in the functional needs of a cell have affected physical structures, proteomes, and protein sequences throughout fungal evolution.

Production of codon-optimized Factor C fragment from Tachypleus gigas in the Pichia pastoris GS115 expression system for endotoxin detection

BACKGROUND

Factor C (FC) is widely used as a standard material for endotoxin testing. It functions as a zymogenic serine protease and serve as a biosensor that detects lipopolysaccharides. Prior investigations involving molecular docking and molecular dynamics simulations of FC demonstrated an interaction between the C-type lectin domain (CLECT) and the ligand lipopolysaccharide (lipid A). In this study, our aim was to assess the stability of the interaction between fragment FC and the lipid A ligand using protein modeling approaches, molecular docking, molecular dynamics simulation, and gene construction into the pPIC9K expression vector.

METHODS AND RESULTS

The FC structure was modelled by online tools. In this case, both molecular docking and MD simulations were applied to identify the interaction between protein and ligand (lipid A) including its complex stability. The FC structure model using three modeling websites has varied values, according to a Ramachandran plot study. When compared to other models, AlphaFold server modeling produced the best Ramachandran findings, with residues in the most advantageous area at 88.3%, followed by ERRAT values at 89.83% and 3D Verify at 71.93%. From the docking simulation of FC fragments with three ligands including diphosphoryl lipid A, FC-Core lipid A, and Kdo2 lipid A can be an activator of FC protein by binding to receptor regions to form ligand-receptor complexes. MD simulations were performed on all three complexes to assess their stability in water solvents showing that all complexes were stable during the simulation. The optimization of recombinant protein expression in Pichia pastoris was conducted by assessing the OD value and protease activity. Induction was carried out using 1% (v/v) methanol in BMMY media at 30°C for 72 h.

CONCLUSIONS

Protein fragments of Factor C has been proven to detect endotoxins and serve as a potential biomarker. Molecular docking simulation and MD simulation were employed to study the complex formation of protein fragments FC with ligands. The expression of FC fragments was successfully achieved through heterologous expression. We propose optimizing the expression of FC fragments by inducing them with 1% methanol at 30°C and incubating them for 72 h. These optimized conditions are well-suited for upscaling the production of recombinant FC fragments using a bioreactor.

Direct recruitment of Mis18 to interphase spindle pole bodies promotes CENP-A chromatin assembly

CENP-A chromatin specifies mammalian centromere identity, and its chaperone HJURP replenishes CENP-A when recruited by the Mis18 complex (Mis18C) via M18BP1/KNL2 to CENP-C at kinetochores during interphase. However, the Mis18C recruitment mechanism remains unresolved in species lacking M18BP1, such as fission yeast. Fission yeast centromeres cluster at G2 spindle pole bodies (SPBs) when CENP-ACnp1 is replenished and where Mis18C also localizes. We show that SPBs play an unexpected role in concentrating Mis18C near centromeres through the recruitment of Mis18 by direct binding to the major SPB linker of nucleoskeleton and cytoskeleton (LINC) component Sad1. Mis18C recruitment by Sad1 is important for CENP-ACnp1 chromatin establishment and acts in parallel with a CENP-C-mediated Mis18C recruitment pathway to maintain centromeric CENP-ACnp1 but operates independently of Sad1-mediated centromere clustering. SPBs therefore provide a non-chromosomal scaffold for both Mis18C recruitment and centromere clustering during G2. This centromere-independent Mis18-SPB recruitment provides a mechanism that governs de novo CENP-ACnp1 chromatin assembly by the proximity of appropriate sequences to SPBs and highlights how nuclear spatial organization influences centromere identity.

An ESCRT grommet cooperates with a diffusion barrier to maintain nuclear integrity

The molecular mechanisms by which the endosomal sorting complexes required for transport (ESCRT) proteins contribute to the integrity of the nuclear envelope (NE) barrier are not fully defined. We leveraged the single NE hole generated by mitotic extrusion of the Schizosaccharomyces pombe spindle pole body to reveal two modes of ESCRT function executed by distinct complements of ESCRT-III proteins, both dependent on CHMP7/Cmp7. A grommet-like function is required to restrict the NE hole in anaphase B, whereas replacement of Cmp7 by a sealing module ultimately closes the NE in interphase. Without Cmp7, nucleocytoplasmic compartmentalization remains intact despite NE discontinuities of up to 540 nm, suggesting mechanisms to limit diffusion through these holes. We implicate spindle pole body proteins as key components of a diffusion barrier acting with Cmp7 in anaphase B. Thus, NE remodelling mechanisms cooperate with proteinaceous diffusion barriers beyond nuclear pore complexes to maintain the nuclear compartment.

Microtubule competition and cell growth recenter the nucleus after anaphase in fission yeast

Cells actively position their nuclei based on their activity. In fission yeast, microtubule-dependent nuclear centering is critical for symmetrical cell division. After spindle disassembly at the end of anaphase, the nucleus recenters over an ∼90-min period, approximately half of the duration of the cell cycle. Live-cell and simulation experiments support the cooperation of two distinct microtubule competition mechanisms in the slow recentering of the nucleus. First, a push-push mechanism acts from spindle disassembly to septation and involves the opposing actions of the mitotic spindle pole body microtubules that push the nucleus away from the ends of the cell, while a postanaphase array of microtubules baskets the nucleus and limits its migration toward the division plane. Second, a slow-and-grow mechanism slowly centers the nucleus in the newborn cell by a combination of microtubule competition and asymmetric cell growth. Our work underlines how intrinsic properties of microtubules differently impact nuclear positioning according to microtubule network organization and cell size.

The many faces of the bouquet centrosome MTOC in meiosis and germ cell development

Meiotic chromosomal pairing is facilitated by a conserved cytoskeletal organization. Telomeres associate with perinuclear microtubules via Sun/KASH complexes on the nuclear envelope (NE) and dynein. Telomere sliding on perinuclear microtubules contributes to chromosome homology searches and is essential for meiosis. Telomeres ultimately cluster on the NE, facing the centrosome, in a configuration called the chromosomal bouquet. Here, we discuss novel components and functions of the bouquet microtubule organizing center (MTOC) in meiosis, but also broadly in gamete development. The cellular mechanics of chromosome movements and the bouquet MTOC dynamics are striking. The newly identified zygotene cilium mechanically anchors the bouquet centrosome and completes the bouquet MTOC machinery in zebrafish and mice. We hypothesize that various centrosome anchoring strategies evolved in different species. Evidence suggests that the bouquet MTOC machinery is a cellular organizer, linking meiotic mechanisms with gamete development and morphogenesis. We highlight this cytoskeletal organization as a new platform for creating a holistic understanding of early gametogenesis, with direct implications to fertility and reproduction.

Microtubule competition and cell growth recenter the nucleus after anaphase in fission yeast

Cells actively position their nucleus based on their activity. In fission yeast, microtubule-dependent nuclear centering is critical for symmetrical cell division. After spindle disassembly at the end of anaphase, the nucleus recenters over a ~90 min period, approximately half of the duration of the cell cycle. Live cell and simulation experiments support the cooperation of two distinct mechanisms in the slow recentering of the nucleus. First, a push-push mechanism acts from spindle disassembly to septation and involves the opposing actions of the mitotic Spindle Pole Body microtubules that push the nucleus away from the ends of the cell while post-anaphase array of microtubules basket the nucleus and limit its migration toward the division plane. Second, a slow-and-grow mechanism finalizes nuclear centering in the newborn cell. In this mechanism, microtubule competition stalls the nucleus while asymmetric cell growth slowly centers it. Our work underlines how intrinsic properties of microtubules differently impact nuclear positioning according to microtubule network organization and cell size.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

The Rabl chromosome configuration masks a kinetochore reassembly mechanism in yeast mitosis

During cell cycle progression in metazoans, the kinetochore is assembled at mitotic onset and disassembled during mitotic exit. Once assembled, the kinetochore complex attached to centromeres interacts directly with the spindle microtubules, the vehicle of chromosome segregation. This reassembly program is assumed to be absent in budding and fission yeast, because most kinetochore proteins are stably maintained at the centromeres throughout the entire cell cycle. Here, we show that the reassembly program of the outer kinetochore at mitotic onset is unexpectedly conserved in the fission yeast Schizosaccharomyces pombe. We identified this behavior by removing the Rabl chromosome configuration, in which centromeres are permanently associated with the nuclear envelope beneath the spindle pole body during interphase. In addition to having evolutionary implications for kinetochore reassembly, our results aid the understanding of the molecular processes responsible for kinetochore disassembly and assembly during mitotic entry.

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

Distribution of γ-tubulin ring complex at the Schizosaccharomyces pombe spindle pole

Microtubule nucleation is mediated by the conserved γ-tubulin ring complex (γ-TuRC). Using super-resolution microscopy, we investigate the distribution of γ-TuRC components at the spindle pole body (SPB) in wild-type Schizosaccharomyces pombe . We observed asymmetric distribution of γ-TuRC on its nuclear and cytoplasmic surfaces, consistent with the uneven distribution of microtubules. Examination of deletion mutants in the three non-essential γ-TuRC subunits showed defects in γ-TuRC accumulation on the old and new SPB, particularly in cells lacking alp16+ (the Gcp6 ortholog) that may explain the monopolar spindles observed in this mutant upon mitotic entry.

Loss of kinesin-8 improves the robustness of the self-assembled spindle in Schizosaccharomyces pombe

Chromosome segregation in female meiosis in many metazoans is mediated by acentrosomal spindles, the existence of which implies that microtubule spindles self-assemble without the participation of the centrosomes. Although it is thought that acentrosomal meiosis is not conserved in fungi, we recently reported the formation of self-assembled microtubule arrays, which were able to segregate chromosomes, in fission yeast mutants, in which the contribution of the spindle pole body (SPB; the centrosome equivalent in yeast) was specifically blocked during meiosis. Here, we demonstrate that this unexpected microtubule formation represents a bona fide type of acentrosomal spindle. Moreover, a comparative analysis of these self-assembled spindles and the canonical SPB-dependent spindle reveals similarities and differences; for example, both spindles have a similar polarity, but the location of the γ-tubulin complex differs. We also show that the robustness of self-assembled spindles can be reinforced by eliminating kinesin-8 family members, whereas kinesin-8 mutants have an adverse impact on SPB-dependent spindles. Hence, we consider that reinforced self-assembled spindles in yeast will help to clarify the molecular mechanisms behind acentrosomal meiosis, a crucial step towards better understanding gametogenesis.

Summary: We report a comparative analysis of self-assembled spindles and canonical centrosomal spindles in fission yeast, which could clarify the mechanisms underlying acentrosomal meiosis.

Redistribution of centrosomal proteins by centromeres and Polo kinase controls partial nuclear envelope breakdown in fission yeast

Proper mitotic progression in Schizosaccharomyces pombe requires partial nuclear envelope breakdown (NEBD) and insertion of the spindle pole body (SPB—yeast centrosome) to build the mitotic spindle. Linkage of the centromere to the SPB is vital to this process, but why that linkage is important is not well understood. Utilizing high-resolution structured illumination microscopy, we show that the conserved Sad1-UNC-84 homology-domain protein Sad1 and other SPB proteins redistribute during mitosis to form a ring complex around SPBs, which is a precursor for localized NEBD and spindle formation. Although the Polo kinase Plo1 is not necessary for Sad1 redistribution, it localizes to the SPB region connected to the centromere, and its activity is vital for redistribution of other SPB ring proteins and for complete NEBD at the SPB to allow for SPB insertion. Our results lead to a model in which centromere linkage to the SPB drives redistribution of Sad1 and Plo1 activation that in turn facilitate partial NEBD and spindle formation through building of a SPB ring structure.

Potential roles of condensin in genome organization and beyond in fission yeast

The genome is highly organized hierarchically by the function of structural maintenance of chromosomes (SMC) complex proteins such as condensin and cohesin from bacteria to humans. Although the roles of SMC complex proteins have been well characterized, their specialized roles in nuclear processes remain unclear. Condensin and cohesin have distinct binding sites and mediate long-range and short-range genomic associations, respectively, to form cell cycle-specific genome organization. Condensin can be recruited to highly expressed genes as well as dispersed repeat genetic elements, such as Pol III-transcribed genes, LTR retrotransposon, and rDNA repeat. In particular, mitotic transcription factors Ace2 and Ams2 recruit condensin to their target genes, forming centromeric clustering during mitosis. Condensin is potentially involved in various chromosomal processes such as the mobility of chromosomes, chromosome territories, DNA reannealing, and transcription factories. The current knowledge of condensin in fission yeast summarized in this review can help us understand how condensin mediates genome organization and participates in chromosomal processes in other organisms.

Tell the Difference Between Mitosis and Meiosis: Interplay Between Chromosomes, Cytoskeleton, and Cell Cycle Regulation

Meiosis is a specialized style of cell division conserved in eukaryotes, particularly designed for the production of gametes. A huge number of studies to date have demonstrated how chromosomes behave and how meiotic events are controlled. Yeast substantially contributed to the understanding of the molecular mechanisms of meiosis in the past decades. Recently, evidence began to accumulate to draw a perspective landscape showing that chromosomes and microtubules are mutually influenced: microtubules regulate chromosomes, whereas chromosomes also regulate microtubule behaviors. Here we focus on lessons from recent advancement in genetical and cytological studies of the fission yeast Schizosaccharomyces pombe, revealing how chromosomes, cytoskeleton, and cell cycle progression are organized and particularly how these are differentiated in mitosis and meiosis. These studies illuminate that meiosis is strategically designed to fulfill two missions: faithful segregation of genetic materials and production of genetic diversity in descendants through elaboration by meiosis-specific factors in collaboration with general factors.

Anatomy of the fungal microtubule organizing center, the spindle pole body

The fungal kingdom is large and diverse, representing extremes of ecology, life cycles and morphology. At a cellular level, the diversity among fungi is particularly apparent at the spindle pole body (SPB). This nuclear envelope embedded structure, which is essential for microtubule nucleation, shows dramatically different morphologies between different fungi. However, despite phenotypic diversity, many SPB components are conserved, suggesting commonalities in structure, function and duplication. Here, I review the organization of the most well-studied SPBs and describe how advances in genomics, genetics and cell biology have accelerated knowledge of SPB architecture in other fungi, providing insights into microtubule nucleation and other processes conserved across eukaryotes.

Lem2 and Lnp1 maintain the membrane boundary between the nuclear envelope and endoplasmic reticulum

The nuclear envelope (NE) continues to the endoplasmic reticulum (ER). Proper partitioning of NE and ER is crucial for cellular activity, but the key factors maintaining the boundary between NE and ER remain to be elucidated. Here we show that the conserved membrane proteins Lem2 and Lnp1 cooperatively play a crucial role in maintaining the NE-ER membrane boundary in fission yeast Schizosaccharomyces pombe. Cells lacking both Lem2 and Lnp1 caused severe growth defects associated with aberrant expansion of the NE/ER membranes, abnormal leakage of nuclear proteins, and abnormal formation of vacuolar-like structures in the nucleus. Overexpression of the ER membrane protein Apq12 rescued the growth defect associated with membrane disorder caused by the loss of Lem2 and Lnp1. Genetic analysis showed that Apq12 had overlapping functions with Lnp1. We propose that a membrane protein network with Lem2 and Lnp1 acts as a critical factor to maintain the NE-ER boundary.

Factors promoting nuclear envelope assembly independent of the canonical ESCRT pathway

The nuclear envelope (NE) undergoes dynamic remodeling to maintain NE integrity, a process involving the inner nuclear membrane protein LEM2 recruiting CHMP7/Cmp7 and then ESCRT-III. However, prior work has hinted at CHMP7/ESCRT-independent mechanisms. To identify such mechanisms, we studied NE assembly in Schizosaccharomyces japonicus, a fission yeast that undergoes partial mitotic NE breakdown and reassembly. S. japonicus cells lacking Cmp7 have compromised NE sealing after mitosis but are viable. A genetic screen identified mutations that promote NE integrity in cmp7Δ cells. Unexpectedly, loss of Lem2 or its interacting partner Nur1 suppressed cmp7Δ defects. In the absence of Cmp7, Lem2 formed aggregates that appear to interfere with ESCRT-independent NE sealing. A gain-of-function mutation implicated a membrane and ESCRT-III regulator, Alx1, in this alternate pathway. Additional results suggest a potentially general role for unsaturated fatty acids in NE integrity. These findings establish the existence of mechanisms for NE sealing independent of the canonical ESCRT pathway.

ESCRT-III/Vps4 Controls Heterochromatin-Nuclear Envelope Attachments

Eukaryotic genomes are organized within the nucleus through interactions with inner nuclear membrane (INM) proteins. How chromatin tethering to the INM is controlled in interphase and how this process contributes to subsequent mitotic nuclear envelope (NE) remodeling remains unclear. We have probed these fundamental questions using the fission yeast Schizosaccharomyces japonicus, which breaks and reforms the NE during mitosis. We show that attachments between heterochromatin and the transmembrane Lem2-Nur1 complex at the INM are remodeled in interphase by the ESCRT-III/Vps4 machinery. Failure of ESCRT-III/Vps4 to release Lem2-Nur1 from heterochromatin leads to persistent association of chromosomes with the INM throughout mitosis. At mitotic exit, such trapping of Lem2-Nur1 on heterochromatin prevents it from re-establishing nucleocytoplasmic compartmentalization. Our work identifies the Lem2-Nur1 complex as a substrate for the nuclear ESCRT machinery and explains how the dynamic tethering of chromosomes to the INM is linked to the establishment of nuclear compartmentalization.

Generation of temperature sensitive mutations with error-prone PCR in a gene encoding a component of the spindle pole body in fission yeast

Temperature-sensitive (ts) mutants provide powerful tools for investigation of cellular functions of essential genes. We report here asimple procedure to generate ts mutations using error-prone PCR within pcp1 that encodes aspindle pole body (SPB) component in Schizosaccharomyces pombe. This manipulation is not restricted to pcp1, and can be suited to any essential genes involved in other processes.

Spatiotemporal control of spindle disassembly in fission yeast

Maintenance of genomic stability during cell division is one of the most important cellular tasks, and it critically depends on the faithful replication of the genetic material and its equal partitioning into daughter cells, gametes, or spores in the case of yeasts. Defective mitotic spindle assembly and disassembly both result in changes in cellular ploidy that ultimately impinge proliferation fitness and might increase tumor malignancy. Although a great progress has been made in understanding how spindles are assembled to orchestrate chromosome segregation, much less is known about how they are disassembled once completed their function. Here, we review two recently uncovered mechanisms of spindle disassembly that operate at different stages of the fission yeast life cycle.

Pericentrin-mediated SAS-6 recruitment promotes centriole assembly

The centrosome is composed of two centrioles surrounded by a microtubule-nucleating pericentriolar material (PCM). Although centrioles are known to regulate PCM assembly, it is less known whether and how the PCM contributes to centriole assembly. Here we investigate the interaction between centriole components and the PCM by taking advantage of fission yeast, which has a centriole-free, PCM-containing centrosome, the SPB. Surprisingly, we observed that several ectopically-expressed animal centriole components such as SAS-6 are recruited to the SPB. We revealed that a conserved PCM component, Pcp1/pericentrin, interacts with and recruits SAS-6. This interaction is conserved and important for centriole assembly, particularly its elongation. We further explored how yeasts kept this interaction even after centriole loss and showed that the conserved calmodulin-binding region of Pcp1/pericentrin is critical for SAS-6 interaction. Our work suggests that the PCM not only recruits and concentrates microtubule-nucleators, but also the centriole assembly machinery, promoting biogenesis close by.

Spindle assembly without spindle pole body insertion into the nuclear envelope in fission yeast meiosis

Centrosomes represent the major microtubule organizing center (MTOC) in eukaryotic cells and are responsible for nucleation of the spindle, the vehicle of chromosome segregation. In human female meiosis, however, spindle assembly occurs in the absence of centrosomes or other MTOCs and microtubules are nucleated around chromosomes. In yeast, spindle formation in mitosis and meiosis depends on the activity of spindle pole bodies (SPBs), the functional equivalents of centrosomes; thus, SPBs and centrosomes use similar machineries to assemble spindles. Here, we develop a system to explore the molecular mechanisms supporting acentrosomal spindle formation using fission yeast meiosis as a model scenario. We achieve this situation by removing access of the SPBs to the nucleus after their duplication. Under these conditions, we observe self-assembly-based spindle formation in the nuclear environment, conferring an ability to segregate chromosomes independently of the SPBs. Our results open the possibility to utilize the experimental advantages of fission yeast for insights into the molecular basis of acentrosomal spindle formation in meiosis.

Pivoting of microtubules driven by minus-end-directed motors leads to spindle assembly

BACKGROUND

At the beginning of mitosis, the cell forms a spindle made of microtubules and associated proteins to segregate chromosomes. An important part of spindle architecture is a set of antiparallel microtubule bundles connecting the spindle poles. A key question is how microtubules extending at arbitrary angles form an antiparallel interpolar bundle.

RESULTS

Here, we show in fission yeast that microtubules meet at an oblique angle and subsequently rotate into antiparallel alignment. Our live-cell imaging approach provides a direct observation of interpolar bundle formation. By combining experiments with theory, we show that microtubules from each pole search for those from the opposite pole by performing random angular movement. Upon contact, two microtubules slide sideways along each other in a directed manner towards the antiparallel configuration. We introduce the contour length of microtubules as a measure of activity of motors that drive microtubule sliding, which we used together with observation of Cut7/kinesin-5 motors and our theory to reveal the minus-end-directed motility of this motor in vivo.

CONCLUSION

Random rotational motion helps microtubules from the opposite poles to find each other and subsequent accumulation of motors allows them to generate forces that drive interpolar bundle formation.

Association of mitochondria with microtubules inhibits mitochondrial fission by precluding assembly of the fission protein Dnm1

Mitochondria are organized as tubular networks in the cell and undergo fission and fusion. Although several of the molecular players involved in mediating mitochondrial dynamics have been identified, the precise cellular cues that initiate mitochondrial fission or fusion remain largely unknown. In fission yeast (Schizosaccharomyces pombe), mitochondria are organized along microtubule bundles. Here, we employed deletions of kinesin-like proteins to perturb microtubule dynamics and used high-resolution and time-lapse fluorescence microscopy, revealing that mitochondrial lengths mimic microtubule lengths. Furthermore, we determined that compared with WT cells, mutant cells with long microtubules exhibit fewer mitochondria, and mutant cells with short microtubules have an increased number of mitochondria because of reduced mitochondrial fission in the former and elevated fission in the latter. Correspondingly, upon onset of closed mitosis in fission yeast, wherein interphase microtubules assemble to form the spindle within the nucleus, we observed increased mitochondrial fission. We found that the consequent rise in the mitochondrial copy number is necessary to reduce partitioning errors during independent segregation of mitochondria between daughter cells. We also discovered that the association of mitochondria with microtubules physically impedes the assembly of the fission protein Dnm1 around mitochondria, resulting in inhibition of mitochondrial fission. Taken together, we demonstrate a mechanism for the regulation of mitochondrial fission that is dictated by the interaction between mitochondria and the microtubule cytoskeleton.

Centrosome Remodelling in Evolution

The centrosome is the major microtubule organizing centre (MTOC) in animal cells. The canonical centrosome is composed of two centrioles surrounded by a pericentriolar matrix (PCM). In contrast, yeasts and amoebozoa have lost centrioles and possess acentriolar centrosomes—called the spindle pole body (SPB) and the nucleus-associated body (NAB), respectively. Despite the difference in their structures, centriolar centrosomes and SPBs not only share components but also common biogenesis regulators. In this review, we focus on the SPB and speculate how its structures evolved from the ancestral centrosome. Phylogenetic distribution of molecular components suggests that yeasts gained specific SPB components upon loss of centrioles but maintained PCM components associated with the structure. It is possible that the PCM structure remained even after centrosome remodelling due to its indispensable function to nucleate microtubules. We propose that the yeast SPB has been formed by a step-wise process; (1) an SPB-like precursor structure appeared on the ancestral centriolar centrosome; (2) it interacted with the PCM and the nuclear envelope; and (3) it replaced the roles of centrioles. Acentriolar centrosomes should continue to be a great model to understand how centrosomes evolved and how centrosome biogenesis is regulated.

The fission yeast SPB component Dms1 is required to initiate forespore membrane formation and maintain meiotic SPB components

The spindle pole body (SPB) plays a central role in spore plasma membrane formation in addition to its recognized role in microtubule organization. During meiosis, a biomembrane called the forespore membrane (FSM) is newly formed at the SPB. Although several SPB proteins essential for the initiation of FSM formation (meiotic SPB components) have been identified, the molecular mechanism is still unknown. Here, we report the isolation and functional characterization of Dms1 as a component of the SPB. We show that FSM formation does not initiate in dms1Δ cells. Dms1 protein is constitutively expressed throughout the life cycle and localizes to the SPB and the nuclear envelope. The predicted Dms1 protein has a transmembrane domain, which is required for correct localization at the SPB. Dms1 is essential for the proper localization of three meiotic SPB components, Spo15, Spo2, and Spo13, but these components do not affect localization of Dms1. Collectively, these results suggest that Dms1 anchors these meiotic SPB components to the SPB, thereby facilitating the initiation of FSM formation.

Specific detection of fission yeast primary septum reveals septum and cleavage furrow ingression during early anaphase independent of mitosis completion

It is widely accepted in eukaryotes that the cleavage furrow only initiates after mitosis completion. In fission yeast, cytokinesis requires the synthesis of a septum tightly coupled to cleavage furrow ingression. The current cytokinesis model establishes that simultaneous septation and furrow ingression only initiate after spindle breakage and mitosis exit. Thus, this model considers that although Cdk1 is inactivated at early-anaphase, septation onset requires the long elapsed time until mitosis completion and full activation of the Hippo-like SIN pathway. Here, we studied the precise timing of septation onset regarding mitosis by exploiting both the septum-specific detection with the fluorochrome calcofluor and the high-resolution electron microscopy during anaphase and telophase. Contrarily to the existing model, we found that both septum and cleavage furrow start to ingress at early anaphase B, long before spindle breakage, with a slow ingression rate during anaphase B, and greatly increasing after telophase onset. This shows that mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. We found that the timing of septation during early anaphase correlates with the cell size and is regulated by the corresponding levels of SIN Etd1 and Rho1. Cdk1 inactivation was directly required for timely septation in early anaphase. Strikingly the reduced SIN activity present after Cdk1 loss was enough to trigger septation by immediately inducing the medial recruitment of the SIN kinase complex Sid2-Mob1. On the other hand, septation onset did not depend on the SIN asymmetry establishment, which is considered a hallmark for SIN activation. These results recalibrate the timing of key cytokinetic events in fission yeast; and unveil a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression to safeguard the proper chromosome segregation during cell division.

Fission yeast cytokinesis requires the invagination of the equatorial plasma membrane (cleavage furrow ingression) coupled to the synthesis of a special wall structure named septum (septation). Despite Cdk1 kinase is inactivated in early anaphase, it is believed that cleavage furrow ingression and septation onset require anaphase progression and mitosis completion, only initiating after the complete activation of the Hippo-like septation initiation network (SIN) after telophase onset. Here, we studied the precise timing of septation start with respect to mitosis through specific septum-staining and electron microscopy. We found that septum and cleavage furrow ingression initiate in early anaphase, showing first a slow ingression rate during anaphase B, and increasing to a fast ingression rate after telophase onset. Thus, mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. The timing of septation correlated with cell size and depended on the level of cytoplasmic activators like SIN Etd1 and Rho1. We further analyzed the mitotic mechanisms that control the septation onset during early anaphase. Cdk1 directly regulated the timing of septation onset during early anaphase, and the low SIN activity present after Cdk1 inactivation was enough to trigger septation. Globally, these results recalibrate the timing of the main cytokinetic events of fission yeast and reveal a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression.

Duplication and Nuclear Envelope Insertion of the Yeast Microtubule Organizing Centre, the Spindle Pole Body

The main microtubule organizing centre in the unicellular model organisms Saccharomyces cerevisiae and Schizosaccharomyces pompe is the spindle pole body (SPB). The SPB is a multilayer structure, which duplicates exactly once per cell cycle. Unlike higher eukaryotic cells, both yeast model organisms undergo mitosis without breakdown of the nuclear envelope (NE), a so-called closed mitosis. Therefore, in order to simultaneously nucleate nuclear and cytoplasmic MTs, it is vital to embed the SPB into the NE at least during mitosis, similarly to the nuclear pore complex (NPC). This review aims to embrace the current knowledge of the SPB duplication cycle with special emphasis on the critical step of the insertion of the new SPB into the NE.

Introduction to Fission Yeast as a Model System

Here, we briefly outline the history of fission yeast, its life cycle, and aspects of its biology that make it a useful model organism for studying problems of eukaryotic molecular and cell biology.

Brr6 and Brl1 locate to nuclear pore complex assembly sites to promote their biogenesis

The conserved paralogous Brr6 and Brl1 promote NPC biogenesis in an unclear manner. Here, Zhang et al. show that both transmembrane proteins transiently associate with NPC assembly intermediates and directly promote NPC biogenesis.

The paralogous Brr6 and Brl1 are conserved integral membrane proteins of the nuclear envelope (NE) with an unclear role in nuclear pore complex (NPC) biogenesis. Here, we analyzed double-degron mutants of Brr6/Brl1 to understand this function. Depletion of Brr6 and Brl1 caused defects in NPC biogenesis, whereas the already assembled NPCs remained unaffected. This NPC biogenesis defect was not accompanied by a change in lipid composition. However, Brl1 interacted with Ndc1 and Nup188 by immunoprecipitation, and with transmembrane and outer and inner ring NPC components by split yellow fluorescent protein analysis, indicating a direct role in NPC biogenesis. Consistently, we found that Brr6 and Brl1 associated with a subpopulation of NPCs and emerging NPC assembly sites. Moreover, BRL1 overexpression affected NE morphology without a change in lipid composition and completely suppressed the nuclear pore biogenesis defect of nup116Δ and gle2Δ cells. We propose that Brr6 and Brl1 transiently associate with NPC assembly sites where they promote NPC biogenesis.

The Yeast Genomes in Three Dimensions: Mechanisms and Functions

The three-dimensional (3D) genome structure is highly ordered by a hierarchy of organizing events ranging from enhancer-promoter or gene-gene contacts to chromosomal territorial arrangement. It is becoming clear that the cohesin and condensin complexes are key molecular machines that organize the 3D genome structure. These complexes are highly conserved from simple systems, e.g., yeast cells, to the much more complex human system. Therefore, knowledge from the budding and fission yeast systems illuminates highly conserved molecular mechanisms of how cohesin and condensin establish the functional 3D genome structures. Here I discuss how these complexes are recruited across the yeast genomes, mediate distinct genome-organizing events such as gene contacts and topological domain formation, and participate in important nuclear activities including transcriptional regulation and chromosomal dynamics.

The telomere bouquet facilitates meiotic prophase progression and exit in fission yeast

During meiotic prophase, chromosome arrangement and oscillation promote the pairing of homologous chromosomes for meiotic recombination. This dramatic movement involves clustering of telomeres at the nuclear membrane to form the so-called telomere bouquet. In fission yeast, the telomere bouquet is formed near the spindle pole body (SPB), which is the microtubule organising centre, functionally equivalent to the metazoan centrosome. Disruption of bouquet configuration impedes homologous chromosome pairing, meiotic recombination and spindle formation. Here, we demonstrate that the bouquet is maintained throughout meiotic prophase and promotes timely prophase exit in fission yeast. Persistent DNA damages, induced during meiotic recombination, activate the Rad3 and Chk1 DNA damage checkpoint kinases and extend the bouquet stage beyond the chromosome oscillation period. The auxin-inducible degron system demonstrated that premature termination of the bouquet stage leads to severe extension of prophase and consequently spindle formation defects. However, this delayed exit from meiotic prophase was not caused by residual DNA damage. Rather, loss of chromosome contact with the SPB caused delayed accumulation of CDK1-cyclin B at the SPB, which correlated with impaired SPB separation. In the absence of the bouquet, CDK1-cyclin B localised near the telomeres but not at the SPB at the later stage of meiotic prophase. Thus, bouquet configuration is maintained throughout meiotic prophase, by which this spatial organisation may facilitate local and timely activation of CDK1 near the SPB. Our findings illustrate that chromosome contact with the nuclear membrane synchronises meiotic progression of the nucleoplasmic chromosomes with that of the cytoplasmic SPB.

Function of nuclear membrane proteins in shaping the nuclear envelope integrity during closed mitosis

The nuclear envelope (NE) not only protects the genome from being directly accessed by detrimental agents but also regulates genome organization. Breaches in NE integrity threaten genome stability and impede cellular function. Nonetheless, the NE constantly remodels, and NE integrity is endangered in dividing or differentiating cells. Specifically, in unicellular eukaryotes undergoing closed mitosis, the NE expands instead of breaking down during chromosome segregation. The newly assembling nuclear pore complexes (NPCs) penetrate the existing NE in interphase. A peculiar example of NE remodelling during nuclear differentiation in Tetrahymena involves formation of the redundant NE and clustered NPCs. Even under these conditions, the NE remains intact. Many recent studies on unicellular organisms have revealed that nuclear membrane proteins, such as LEM-domain proteins, play a role in maintaining NE integrity. This review summarizes and discusses how nuclear membrane proteins participate in NE integrity.

Big Lessons from Little Yeast: Budding and Fission Yeast Centrosome Structure, Duplication, and Function

Centrosomes are a functionally conserved feature of eukaryotic cells that play an important role in cell division. The conserved γ-tubulin complex organizes spindle and astral microtubules, which, in turn, separate replicated chromosomes accurately into daughter cells. Like DNA, centrosomes are duplicated once each cell cycle. Although in some cell types it is possible for cell division to occur in the absence of centrosomes, these divisions typically result in defects in chromosome number and stability. In single-celled organisms such as fungi, centrosomes [known as spindle pole bodies (SPBs)] are essential for cell division. SPBs also must be inserted into the membrane because fungi undergo a closed mitosis in which the nuclear envelope (NE) remains intact. This poorly understood process involves events similar or identical to those needed for de novo nuclear pore complex assembly. Here, we review how analysis of fungal SPBs has advanced our understanding of centrosomes and NE events.

Molecular model of fission yeast centrosome assembly determined by superresolution imaging

Microtubule-organizing centers (MTOCs), known as centrosomes in animals and spindle pole bodies (SPBs) in fungi, are important for the faithful distribution of chromosomes between daughter cells during mitosis as well as for other cellular functions. The cytoplasmic duplication cycle and regulation of the Schizosaccharomyces pombe SPB is analogous to centrosomes, making it an ideal model to study MTOC assembly. Here, we use superresolution structured illumination microscopy with single-particle averaging to localize 14 S. pombe SPB components and regulators, determining both the relationship of proteins to each other within the SPB and how each protein is assembled into a new structure during SPB duplication. These data enabled us to build the first comprehensive molecular model of the S. pombe SPB, resulting in structural and functional insights not ascertained through investigations of individual subunits, including functional similarities between Ppc89 and the budding yeast SPB scaffold Spc42, distribution of Sad1 to a ring-like structure and multiple modes of Mto1 recruitment.

Shape Transformation of the Nuclear Envelope during Closed Mitosis

The nuclear envelope (NE) in lower eukaryotes such as Schizosaccharomyces pombe undergoes large morphology changes during closed mitosis. However, which physical parameters are important in governing the shape evolution of the NE, and how defects in the dividing chromosomes/microtubules are reflected in those parameters, are fundamental questions that remain unresolved. In this study, we show that improper separation of chromosomes in genetically deficient cells leads to membrane tethering or asymmetric division in contrast to the formation of two equal-sized daughter nuclei in wild-type cells. We hypothesize that the poleward force is transmitted to the nuclear membrane through its physical contact with the separated sister chromatids at the two spindle poles. A theoretical model is developed to predict the morphology evolution of the NE where key factors such as the work done by the poleward force and bending and surface energies stored in the membrane have been taken into account. Interestingly, the predicted phase diagram, summarizing the dependence of nuclear shape on the size of the load transmission regions, and the pole-to-pole distance versus surface area relationship all quantitatively agree well with our experimental observations, suggesting that this model captures the essential physics involved in closed mitosis.

Position matters: multiple functions of LINC-dependent chromosome positioning during meiosis

Chromosome positioning is crucial for multiple chromosomal events, including DNA replication, repair, and recombination. The linker of nucleoskeleton and cytoskeleton (LINC) complexes, which consist of conserved nuclear membrane proteins, were shown to control chromosome positioning and facilitate various biological processes by interacting with the cytoskeleton. However, the precise functions and regulation of LINC-dependent chromosome positioning are not fully understood. During meiosis, the LINC complexes induce clustering of telomeres, forming the bouquet chromosome arrangement, which promotes homologous chromosome pairing. In fission yeast, the bouquet forms through LINC-dependent clustering of telomeres at the spindle pole body (SPB, the centrosome equivalent in fungi) and detachment of centromeres from the SPB-localized LINC. It was recently found that, in fission yeast, the bouquet contributes to formation of the spindle and meiotic centromeres, in addition to homologous chromosome pairing, and that centromere detachment is linked to telomere clustering, which is crucial for proper spindle formation. Here, we summarize these findings and show that the bouquet chromosome arrangement also contributes to nuclear fusion during karyogamy. The available evidence suggests that these functions are universal among eukaryotes. The findings demonstrate that LINC-dependent chromosome positioning performs multiple functions and controls non-chromosomal as well as chromosomal events, and that the chromosome positioning is stringently regulated for its functions. Thus, chromosome positioning plays a much broader role and is more strictly regulated than previously thought.

LEM2 recruits CHMP7 for ESCRT-mediated nuclear envelope closure in fission yeast and human cells

The molecular mechanism for sealing newly formed nuclear envelopes was unclear until the recent discovery that endosomal sorting complexes required for transport III (ESCRT-III) proteins mediate this process. Cmp7p (CHMP7), in particular, was identified as an early acting factor that recruits other ESCRT-III proteins to the nuclear envelope. A fundamental aspect of the varied roles of ESCRT factors is their recruitment by site-specific adaptors, yet the central question of how the ESCRT machinery is targeted to nuclear membranes has remained outstanding. Our study identifies the inner nuclear membrane protein LEM2 as a key, conserved factor that recruits CHMP7 and downstream ESCRT-III proteins to breaches in the nuclear envelope.

Endosomal sorting complexes required for transport III (ESCRT-III) proteins have been implicated in sealing the nuclear envelope in mammals, spindle pole body dynamics in fission yeast, and surveillance of defective nuclear pore complexes in budding yeast. Here, we report that Lem2p (LEM2), a member of the LEM (Lap2-Emerin-Man1) family of inner nuclear membrane proteins, and the ESCRT-II/ESCRT-III hybrid protein Cmp7p (CHMP7), work together to recruit additional ESCRT-III proteins to holes in the nuclear membrane. In Schizosaccharomyces pombe, deletion of the ATPase vps4 leads to severe defects in nuclear morphology and integrity. These phenotypes are suppressed by loss-of-function mutations that arise spontaneously in lem2 or cmp7, implying that these proteins may function upstream in the same pathway. Building on these genetic interactions, we explored the role of LEM2 during nuclear envelope reformation in human cells. We found that CHMP7 and LEM2 enrich at the same region of the chromatin disk periphery during this window of cell division and that CHMP7 can bind directly to the C-terminal domain of LEM2 in vitro. We further found that, during nuclear envelope formation, recruitment of the ESCRT factors CHMP7, CHMP2A, and IST1/CHMP8 all depend on LEM2 in human cells. We conclude that Lem2p/LEM2 is a conserved nuclear site-specific adaptor that recruits Cmp7p/CHMP7 and downstream ESCRT factors to the nuclear envelope.

Chromosomes Orchestrate Their Own Liberation: Nuclear Envelope Disassembly

The mammalian nuclear division cycle is coordinated with nuclear envelope breakdown (NEBD), in which the entire nuclear envelope (NE) is dissolved to allow chromosomes to access their segregation vehicle, the spindle. In other eukaryotes, complete NEBD is replaced by localized disassembly or remodeling of the NE. Although the molecular mechanisms controlling NE disassembly are incompletely understood, coordinated cycles of modification of specific NE components drive breakdown. Here, we review the current state of knowledge regarding NE disassembly and argue for a role of chromosome-NE contacts in triggering initiation of NE disassembly and thereby, cell division.

Mitotic Nuclear Envelope Breakdown and Spindle Nucleation Are Controlled by Interphase Contacts between Centromeres and the Nuclear Envelope

Faithful genome propagation requires coordination between nuclear envelope (NE) breakdown, spindle formation, and chromosomal events. The conserved linker of nucleoskeleton and cytoskeleton (LINC) complex connects fission yeast centromeres and the centrosome, across the NE, during interphase. During meiosis, LINC connects the centrosome with telomeres rather than centromeres. We previously showed that loss of telomere-LINC contacts compromises meiotic spindle formation. Here, we define the precise events regulated by telomere-LINC contacts and address the analogous possibility that centromeres regulate mitotic spindle formation. We develop conditionally inactivated LINC complexes in which the conserved SUN-domain protein Sad1 remains stable but severs interphase centromere-LINC contacts. Strikingly, the loss of such contacts abolishes spindle formation. We pinpoint the defect to a failure in the partial NE breakdown required for centrosome insertion into the NE, a step analogous to mammalian NE breakdown. Thus, interphase chromosome-LINC contacts constitute a cell-cycle control device linking nucleoplasmic and cytoplasmic events.

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.

Duplication of the Yeast Spindle Pole Body Once per Cell Cycle

The yeast spindle pole body (SPB) is the functional equivalent of the mammalian centrosome. Centrosomes and SPBs duplicate exactly once per cell cycle by mechanisms that use the mother structure as a platform for the assembly of the daughter. The conserved Sfi1 and centrin proteins are essential components of the SPB duplication process. Sfi1 is an elongated molecule that has, in its center, 20 to 23 binding sites for the Ca(2+)-binding protein centrin. In the yeastSaccharomyces cerevisiae, all Sfi1 N termini are in contact with the mother SPB whereas the free C termini are distal to it. During S phase and early mitosis, cyclin-dependent kinase 1 (Cdk1) phosphorylation of mainly serine residues in the Sfi1 C termini blocks the initiation of SPB duplication ("off" state). Upon anaphase onset, the phosphatase Cdc14 dephosphorylates Sfi1 ("on" state) to promote antiparallel and shifted incorporation of cytoplasmic Sfi1 molecules into the half-bridge layer, which thereby elongates into the bridge. The Sfi1 C termini of the two Sfi1 layers localize in the bridge center, whereas the N termini of the newly assembled Sfi1 molecules are distal to the mother SPB. These free Sfi1 N termini then assemble the new SPB in G1phase. Recruitment of Sfi1 molecules into the anaphase SPB and bridge formation were also observed inSchizosaccharomyces pombe, suggesting that the Sfi1 bridge cycle is conserved between the two organisms. Thus, restricting SPB duplication to one event per cell cycle requires only an oscillation between Cdk1 kinase and Cdc14 phosphatase activities. This clockwork regulates the "on"/"off" state of the Sfi1-centrin receiver.

Chromosome domain architecture and dynamic organization of the fission yeast genome

Advanced techniques including the chromosome conformation capture (3C) methodology and its derivatives are complementing microscopy approaches to study genome organization, and are revealing new details of three-dimensional (3D) genome architecture at increasing resolution. The fission yeast Schizosaccharomyces pombe (S. pombe) comprises a small genome featuring organizational elements of more complex eukaryotic systems, including conserved heterochromatin assembly machinery. Here we review key insights into genome organization revealed in this model system through a variety of techniques. We discuss the predominant role of Rabl-like configuration for interphase chromosome organization and the dynamic changes that occur during mitosis and meiosis. High resolution Hi-C studies have also revealed the presence of locally crumpled chromatin regions called "globules" along chromosome arms, and implicated a critical role for pericentromeric heterochromatin in imposing fundamental constraints on the genome to maintain chromosome territoriality and stability. These findings have shed new light on the connections between genome organization and function. It is likely that insights gained from the S. pombe system will also broadly apply to higher eukaryotes.

Dynamics of re-constitution of the human nuclear proteome after cell division is regulated by NLS-adjacent phosphorylation

Phosphorylation by the cyclin-dependent kinase 1 (Cdk1) adjacent to nuclear localization signals (NLSs) is an important mechanism of regulation of nucleocytoplasmic transport. However, no systematic survey has yet been performed in human cells to analyze this regulatory process, and the corresponding cell-cycle dynamics have not yet been investigated. Here, we focused on the human proteome and found that numerous proteins, previously not identified in this context, are associated with Cdk1-dependent phosphorylation sites adjacent to their NLSs. Interestingly, these proteins are involved in key regulatory events of DNA repair, epigenetics, or RNA editing and splicing. This finding indicates that cell-cycle dependent events of genome editing and gene expression profiling may be controlled by nucleocytoplasmic trafficking. For in-depth investigations, we selected a number of these proteins and analyzed how point mutations, expected to modify the phosphorylation ability of the NLS segments, perturb nucleocytoplasmic localization. In each case, we found that mutations mimicking hyper-phosphorylation abolish nuclear import processes. To understand the mechanism underlying these phenomena, we performed a video microscopy-based kinetic analysis to obtain information on cell-cycle dynamics on a model protein, dUTPase. We show that the NLS-adjacent phosphorylation by Cdk1 of human dUTPase, an enzyme essential for genomic integrity, results in dynamic cell cycle-dependent distribution of the protein. Non-phosphorylatable mutants have drastically altered protein re-import characteristics into the nucleus during the G1 phase. Our results suggest a dynamic Cdk1-driven mechanism of regulation of the nuclear proteome composition during the cell cycle.

Nucleocytoplasmic transport in the midzone membrane domain controls yeast mitotic spindle disassembly

During anaphase B, Imp1-mediated transport of the AAA-ATPase Cdc48 protein at the membrane domain surrounding the mitotic spindle midzone promotes spindle midzone dissolution in fission yeast.

During each cell cycle, the mitotic spindle is efficiently assembled to achieve chromosome segregation and then rapidly disassembled as cells enter cytokinesis. Although much has been learned about assembly, how spindles disassemble at the end of mitosis remains unclear. Here we demonstrate that nucleocytoplasmic transport at the membrane domain surrounding the mitotic spindle midzone, here named the midzone membrane domain (MMD), is essential for spindle disassembly in Schizosaccharomyces pombe cells. We show that, during anaphase B, Imp1-mediated transport of the AAA-ATPase Cdc48 protein at the MMD allows this disassembly factor to localize at the spindle midzone, thereby promoting spindle midzone dissolution. Our findings illustrate how a separate membrane compartment supports spindle disassembly in the closed mitosis of fission yeast.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

The tethering of chromatin to the nuclear envelope supports nuclear mechanics

The nuclear lamina is thought to be the primary mechanical defence of the nucleus. However, the lamina is integrated within a network of lipids, proteins and chromatin; the interdependence of this network poses a challenge to defining the individual mechanical contributions of these components. Here, we isolate the role of chromatin in nuclear mechanics by using a system lacking lamins. Using novel imaging analyses, we observe that untethering chromatin from the inner nuclear membrane results in highly deformable nuclei in vivo, particularly in response to cytoskeletal forces. Using optical tweezers, we find that isolated nuclei lacking inner nuclear membrane tethers are less stiff than wild-type nuclei and exhibit increased chromatin flow, particularly in frequency ranges that recapitulate the kinetics of cytoskeletal dynamics. We suggest that modulating chromatin flow can define both transient and long-lived changes in nuclear shape that are biologically important and may be altered in disease.

The mechanical properties of the metazoan nucleus can be influenced by the nuclear lamina. Here, Schreiner et al. show that untethering chromatin from the inner nuclear membrane results in highly deformable, softer nuclei, revealing an important role for chromatin in modulating nuclear mechanics.