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Mitosis exhibits astonishing evolutionary plasticity, with dividing eukaryotic cells differing in the organization of the mitotic spindle and the extent of nuclear envelope breakdown. A new study suggests that a multinucleated lifestyle may favor the evolution of closed nuclear division., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Schizosaccharomyces japonicus belongs to the single-genus class Schizosaccharomycetes, otherwise known as "fission yeasts." As part of a composite model system with its widely studied S. pombe sister species, S. japonicus has provided critical insights into the workings and the evolution of cell biological mechanisms. Furthermore, its divergent biology makes S. japonicus a valuable model organism in its own right. However, the currently available genome assembly contains gaps and has been unable to resolve centromeres and other repeat-rich chromosomal regions. Here we present a telomere-to-telomere long-read genome assembly of the S. japonicus genome. This includes the three megabase-length chromosomes, with centromeres hundreds of kilobases long, rich in 5S ribosomal RNA genes, transfer RNA genes, long terminal repeats, and short repeats. We identify a gene-sparse region on chromosome 2 that resembles a 331 kb centromeric duplication. We revise the genome size of S. japonicus to at least 16.6 Mb and possibly up to 18.12 Mb, at least 30% larger than previous estimates. Our whole genome assembly will support the growing S. japonicus research community and facilitate research in new directions, including centromere and DNA repeat evolution, and yeast comparative genomics., (© 2024 The Authors. Yeast published by John Wiley & Sons Ltd.)

The fission yeast species Schizosaccharomyces japonicus is currently divided into two varieties-S. japonicus var. japonicus and S. japonicus var. versatilis. Here we examine the var. versatilis isolate CBS5679. The CBS5679 genome shows 88% identity to the reference genome of S. japonicus var. japonicus at the coding sequence level, with phylogenetic analyses suggesting that it has split from the S. japonicus lineage 25 million years ago. The CBS5679 genome contains a reciprocal translocation between chromosomes 1 and 2, together with several large inversions. The products of genes linked to the major translocation are associated with 'metabolism' and 'cellular assembly' ontology terms. We further show that CBS5679 does not generate viable progeny with the reference strain of S. japonicus. Although CBS5679 shares closer similarity to the 'type' strain of var. versatilis as compared to S. japonicus, it is not identical to the type strain, suggesting population structure within var. versatilis. We recommend that the taxonomic status of S. japonicus var. versatilis is raised, with it being treated as a separate species, Schizosaccharomyces versatilis., (© 2023 The Authors. Yeast published by John Wiley & Sons Ltd.)

Standardized nomenclature for genes, gene products, and isoforms is crucial to prevent ambiguity and enable clear communication of scientific data, facilitating efficient biocuration and data sharing. Standardized genotype nomenclature, which describes alleles present in a specific strain that differ from those in the wild-type reference strain, is equally essential to maximize research impact and ensure that results linking genotypes to phenotypes are Findable, Accessible, Interoperable, and Reusable (FAIR). In this publication, we extend the fission yeast clade gene nomenclature guidelines to support the curation efforts at PomBase (www.pombase.org), the Schizosaccharomyces pombe Model Organism Database. This update introduces nomenclature guidelines for noncoding RNA genes, following those set forth by the Human Genome Organisation Gene Nomenclature Committee. Additionally, we provide a significant update to the allele and genotype nomenclature guidelines originally published in 1987, to standardize the diverse range of genetic modifications enabled by the fission yeast genetic toolbox. These updated guidelines reflect a community consensus between numerous fission yeast researchers. Adoption of these rules will improve consistency in gene and genotype nomenclature, and facilitate machine-readability and automated entity recognition of fission yeast genes and alleles in publications or datasets. In conclusion, our updated guidelines provide a valuable resource for the fission yeast research community, promoting consistency, clarity, and FAIRness in genetic data sharing and interpretation., Competing Interests: Conflicts of interest statement The author(s) declare no conflict of interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of The Genetics Society of America.)

Cellular metabolism relies on just a few redox cofactors. Selective compartmentalization may prevent competition between metabolic reactions requiring the same cofactor. Is such compartmentalization necessary for optimal cell function? Is there an optimal compartment size? Here we probe these fundamental questions using peroxisomal compartmentalization of the last steps of lysine and histidine biosynthesis in the fission yeast Schizosaccharomyces japonicus. We show that compartmentalization of these NAD + dependent reactions together with a dedicated NADH/NAD + recycling enzyme supports optimal growth when an increased demand for anabolic reactions taxes cellular redox balance. In turn, compartmentalization constrains the size of individual organelles, with larger peroxisomes accumulating all the required enzymes but unable to support both biosynthetic reactions at the same time. Our reengineering and physiological experiments indicate that compartmentalized biosynthetic reactions are sensitive to the size of the compartment, likely due to scaling-dependent changes within the system, such as enzyme packing density., (© 2023. Springer Nature Limited.)

Most eukaryotes respire oxygen, using it to generate biomass and energy. However, a few organisms have lost the capacity to respire. Understanding how they manage biomass and energy production may illuminate the critical points at which respiration feeds into central carbon metabolism and explain possible routes to its optimization. Here, we use two related fission yeasts, Schizosaccharomyces pombe and Schizosaccharomyces japonicus, as a comparative model system. We show that although S. japonicus does not respire oxygen, unlike S. pombe, it is capable of efficient NADH oxidation, amino acid synthesis, and ATP generation. We probe possible optimization strategies through the use of stable isotope tracing metabolomics, mass isotopologue distribution analysis, genetics, and physiological experiments. S. japonicus appears to have optimized cytosolic NADH oxidation via glycerol-3-phosphate synthesis. It runs a fully bifurcated TCA pathway, sustaining amino acid production. Finally, we propose that it has optimized glycolysis to maintain high ATP/ADP ratio, in part by using the pentose phosphate pathway as a glycolytic shunt, reducing allosteric inhibition of glycolysis and supporting biomass generation. By comparing two related organisms with vastly different metabolic strategies, our work highlights the versatility and plasticity of central carbon metabolism in eukaryotes, illuminating critical adaptations supporting the preferential use of glycolysis over oxidative phosphorylation., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The Gene Ontology (GO) knowledgebase (http://geneontology.org) is a comprehensive resource concerning the functions of genes and gene products (proteins and noncoding RNAs). GO annotations cover genes from organisms across the tree of life as well as viruses, though most gene function knowledge currently derives from experiments carried out in a relatively small number of model organisms. Here, we provide an updated overview of the GO knowledgebase, as well as the efforts of the broad, international consortium of scientists that develops, maintains, and updates the GO knowledgebase. The GO knowledgebase consists of three components: (1) the GO-a computational knowledge structure describing the functional characteristics of genes; (2) GO annotations-evidence-supported statements asserting that a specific gene product has a particular functional characteristic; and (3) GO Causal Activity Models (GO-CAMs)-mechanistic models of molecular "pathways" (GO biological processes) created by linking multiple GO annotations using defined relations. Each of these components is continually expanded, revised, and updated in response to newly published discoveries and receives extensive QA checks, reviews, and user feedback. For each of these components, we provide a description of the current contents, recent developments to keep the knowledgebase up to date with new discoveries, and guidance on how users can best make use of the data that we provide. We conclude with future directions for the project., Competing Interests: Conflicts of interest The authors declare no conflicts of interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of The Genetics Society of America.)

Nuclear envelope (NE) expansion must be controlled to maintain nuclear shape and function. The nuclear membrane expands massively during closed mitosis, enabling chromosome segregation within an intact NE. Phosphatidic acid (PA) and diacylglycerol (DG) can both serve as biosynthetic precursors for membrane lipid synthesis. How they are regulated in time and space and what the implications are of changes in their flux for mitotic fidelity are largely unknown. Using genetically encoded PA and DG probes, we show that DG is depleted from the inner nuclear membrane during mitosis in the fission yeast Schizosaccharomyces pombe, but PA does not accumulate, indicating that it is rerouted to membrane synthesis. We demonstrate that DG-to-PA conversion catalyzed by the diacylglycerol kinase Dgk1 (also known as Ptp4) and direct glycerophospholipid synthesis from DG by diacylglycerol cholinephosphotransferase/ethanolaminephosphotransferase Ept1 reinforce NE expansion. We conclude that DG consumption through both the de novo pathway and the Kennedy pathway fuels a spike in glycerophospholipid biosynthesis, controlling NE expansion and, ultimately, mitotic fidelity., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Tropomyosins are structurally conserved α-helical coiled-coil proteins that bind along the length of filamentous actin (F-actin) in fungi and animals. Tropomyosins play essential roles in the stability of actin filaments and in regulating myosin II contractility. Despite the crucial role of tropomyosin in actin cytoskeletal regulation, in vivo investigations of tropomyosin are limited, mainly due to the suboptimal live-cell imaging tools currently available. Here, we report on an mNeonGreen (mNG)-tagged tropomyosin, with native promoter and linker length configuration, that clearly reports tropomyosin dynamics in Schizosaccharomyces pombe (Cdc8), Schizosaccharomyces japonicus (Cdc8) and Saccharomyces cerevisiae (Tpm1 and Tpm2). We also describe a fluorescent probe to visualize mammalian tropomyosin (TPM2 isoform). Finally, we generated a camelid nanobody against S. pombe Cdc8, which mimics the localization of mNG-Cdc8 in vivo. Using these tools, we report the presence of tropomyosin in previously unappreciated patch-like structures in fission and budding yeasts, show flow of tropomyosin (F-actin) cables to the cytokinetic actomyosin ring and identify rearrangements of the actin cytoskeleton during mating. These powerful tools and strategies will aid better analyses of tropomyosin and F-actin cables in vivo., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

The fission yeast Schizosaccharomyces japonicus has recently emerged as a powerful system for studying the evolution of essential cellular processes, drawing on similarities as well as key differences between S. japonicus and the related, well-established model Schizosaccharomyces pombe. We have deployed the open-source, modular code and tools originally developed for PomBase, the S. pombe model organism database (MOD), to create JaponicusDB (www.japonicusdb.org), a new MOD dedicated to S. japonicus. By providing a central resource with ready access to a growing body of experimental data, ontology-based curation, seamless browsing and querying, and the ability to integrate new data with existing knowledge, JaponicusDB supports fission yeast biologists to a far greater extent than any other source of S. japonicus data. JaponicusDB thus enables S. japonicus researchers to realize the full potential of studying a newly emerging model species and illustrates the widely applicable power and utility of harnessing reusable PomBase code to build a comprehensive, community-maintainable repository of species-relevant knowledge., (© The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America.)

Cellular dimensions profoundly influence cellular physiology. For unicellular organisms, this has direct bearing on their ecology and evolution. The morphology of a cell is governed by scaling rules. As it grows, the ratio of its surface area to volume is expected to decrease. Similarly, if environmental conditions force proliferating cells to settle on different size optima, cells of the same type may exhibit size-dependent variation in cellular processes. In fungi, algae and plants where cells are surrounded by a rigid wall, division at smaller size often produces immediate changes in geometry, decreasing cell fitness. Here, we discuss how cells interpret their size, buffer against changes in shape and, if necessary, scale their polarity to maintain optimal shape at different cell volumes., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

At the end of mitosis, cells must remodel their nuclear envelope to produce two identical daughter nuclei. Two new studies using Schizosaccharomyces pombe provide insight into how compartmentalized nuclear pore complex disassembly allows cells that undergo closed mitosis to achieve nuclear division., (Copyright © 2020 Elsevier Inc. All rights reserved.)

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., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Membrane function is fundamental to life. Each species explores membrane lipid diversity within a genetically predefined range of possibilities. How membrane lipid composition in turn defines the functional space available for evolution of membrane-centered processes remains largely unknown. We address this fundamental question using related fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. We show that, unlike S. pombe that generates membranes where both glycerophospholipid acyl tails are predominantly 16-18 carbons long, S. japonicus synthesizes unusual "asymmetrical" glycerophospholipids where the tails differ in length by 6-8 carbons. This results in stiffer bilayers with distinct lipid packing properties. Retroengineered S. pombe synthesizing the S.-japonicus-type phospholipids exhibits unfolded protein response and downregulates secretion. Importantly, our protein sequence comparisons and domain swap experiments support the hypothesis that transmembrane helices co-evolve with membranes, suggesting that, on the evolutionary scale, changes in membrane lipid composition may necessitate extensive adaptation of the membrane-associated proteome., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2019 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Cells of a specific cell type may divide within a certain size range. Yet, functionally optimal cellular organization is typically maintained across different cell sizes, a phenomenon known as scaling. The mechanisms underlying scaling and its physiological significance remain elusive. Here we approach this problem by interfering with scaling in the rod-shaped fission yeast Schizosaccharomyces japonicus that relies on cellular geometry cues to position the division site. We show that S. japonicus uses the Cdc42 polarity module to adjust its geometry to changes in the cell size. When scaling is prevented resulting in abnormal cellular length-to-width aspect ratio, cells exhibit severe division site placement defects. We further show that despite the generally accepted view, a similar scaling phenomenon can occur in the sister species, Schizosaccharomyces pombe. Our results demonstrate that scaling is required for normal cell function and delineate possible rules for cellular geometry maintenance in populations of proliferating cells.

A long-appreciated variation in fundamental cell biological processes between different species is becoming increasingly tractable due to recent breakthroughs in whole-genome analyses and genome editing techniques. However, the bulk of our mechanistic understanding in cell biology continues to come from just a few well-established models. In this Review, I use the highly diverse strategies of chromosome segregation in eukaryotes as an instrument for a more general discussion on phenotypic variation, possible rules underlying its emergence and its utility in understanding conserved functional relationships underlying this process. Such a comparative approach, supported by modern molecular biology tools, might provide a wider, holistic view of biology that is difficult to achieve when concentrating on a single experimental system., Competing Interests: Competing interestsI declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Cytokinesis in many eukaryotes involves a tension-generating actomyosin-based contractile ring. Many components of actomyosin rings turn over during contraction, although the significance of this turnover has remained enigmatic. Here, using Schizosaccharomyces japonicus , we investigate the role of turnover of actin and myosin II in its contraction. Actomyosin ring components self-organize into ∼1-µm-spaced clusters instead of undergoing full-ring contraction in the absence of continuous actin polymerization. This effect is reversed when actin filaments are stabilized. We tested the idea that the function of turnover is to ensure actin filament homeostasis in a synthetic system, in which we abolished turnover by fixing rings in cell ghosts with formaldehyde. We found that these rings contracted fully upon exogenous addition of a vertebrate myosin. We conclude that actin turnover is required to maintain actin filament homeostasis during ring contraction and that the requirement for turnover can be bypassed if homeostasis is achieved artificially., (© 2017 Chew et al.)

Model organisms are widely used in research as accessible and convenient systems to study a particular area or question in biology. Traditionally only a handful of organisms have been widely studied, but modern research tools are enabling researchers to extend the set of model organisms to include less-studied and more unusual systems. This Forum highlights a range of 'non-model model organisms' as emerging systems for tackling questions across the whole spectrum of biology (and beyond), the opportunities and challenges, and the outlook for the future.

Many eukaryotes assemble a ring-shaped actomyosin network that contracts to drive cytokinesis. Unlike actomyosin in sarcomeres, which cycles through contraction and relaxation, the cytokinetic ring disassembles during contraction through an unknown mechanism. Here we find in Schizosaccharomyces japonicus and Schizosaccharomyces pombe that, during actomyosin ring contraction, actin filaments associated with actomyosin rings are expelled as micron-scale bundles containing multiple actomyosin ring proteins. Using functional isolated actomyosin rings we show that expulsion of actin bundles does not require continuous presence of cytoplasm. Strikingly, mechanical compression of actomyosin rings results in expulsion of bundles predominantly at regions of high curvature. Our work unprecedentedly reveals that the increased curvature of the ring itself promotes its disassembly. It is likely that such a curvature-induced mechanism may operate in disassembly of other contractile networks., Competing Interests: MKB: Reviewing editor, eLife. The other authors declare that no competing interests exist.

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., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Eukaryotes remodel the nucleus during mitosis using a variety of mechanisms that differ in the timing and the extent of nuclear envelope (NE) breakdown. Here, we probe the principles enabling this functional diversity by exploiting the natural divergence in NE management strategies between the related fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus [1-3]. We show that inactivation of Ned1, the phosphatidic acid phosphatase of the lipin family, by CDK phosphorylation is both necessary and sufficient to promote NE expansion required for "closed" mitosis in S. pombe. In contrast, Ned1 is not regulated during division in S. japonicus, thus limiting membrane availability and necessitating NE breakage. Interspecies gene swaps result in phenotypically normal divisions with the S. japonicus lipin acquiring an S. pombe-like mitotic phosphorylation pattern. Our results provide experimental evidence for the mitotic regulation of phosphatidic acid flux and suggest that the regulatory networks governing lipin activity diverged in evolution to give rise to strikingly dissimilar mitotic programs., (Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Cytokinesis must be regulated in time and space in order to preserve genome integrity during cell proliferation and to allow daughter cells to adopt distinct fates and geometries during differentiation. The fission yeast Schizosaccharomyces pombe has been a popular model organism for understanding spatiotemporal regulation of cytokinesis in a symmetrically dividing cell. Recent work on another member of the same genus, Schisozaccharomyces japonicus, suggests that S. pombe may have evolved an unusual division site placement mechanism based on a recently duplicated anillin paralog. Here we discuss an extraordinary evolutionary plasticity of cytokinesis within the fission yeast clade and argue that the comparative cell biology approach may provide functional insights beyond those afforded by scrutinizing individual model species., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Strategies to position the division apparatus exhibit a bewildering diversity [1], but how these mechanisms evolve remains virtually unknown. Here, we explore the plasticity of division site positioning in fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. We demonstrate that, whereas both species divide in the middle, only S. pombe uses the anillin Mid1 as a primary nucleus-derived cue to assemble the actomyosin ring at the equatorial cortex. We trace this variance to the divergence in subcellular targeting of Mid1 and show that duplication of an ancestral anillin early in the Schizosaccharomyces lineage may have led to subfunctionalization of the Mid1 orthologs. In contrast to S. pombe, medial assembly of the actomyosin ring in mitotic S. japonicus relies on the cortical anchor protein Cdc15 regulated by the tip-localized kinase Pom1. Our data suggest that division site placement is determined by cortical positioning of the actomyosin-plasma membrane linkers and that both identity of the linker and control of its subcellular targeting are highly modular., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Excess fatty acids and sterols are stored as triacylglycerols and sterol esters in specialized cellular organelles, called lipid droplets. Understanding what determines the cellular amount of neutral lipids and their packaging into lipid droplets is of fundamental and applied interest. Using two species of fission yeast, we show that cycling cells deficient in the function of the ER-resident CDP-DG synthase Cds1 exhibit markedly increased triacylglycerol content and assemble large lipid droplets closely associated with the ER membranes. We demonstrate that these unusual structures recruit the triacylglycerol synthesis machinery and grow by expansion rather than by fusion. Our results suggest that interfering with the CDP-DG route of phosphatidic acid utilization rewires cellular metabolism to adopt a triacylglycerol-rich lifestyle reliant on the Kennedy pathway., (© 2014 He, Yam, et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

The fission yeast Schizosaccharomyces pombe undergoes "closed" mitosis in which the nuclear envelope (NE) stays intact throughout chromosome segregation. Here we show that Tts1, the fission yeast TMEM33 protein that was previously implicated in organizing the peripheral endoplasmic reticulum (ER), also functions in remodeling the NE during mitosis. Tts1 promotes insertion of spindle pole bodies (SPBs) in the NE at the onset of mitosis and modulates distribution of the nuclear pore complexes (NPCs) during mitotic NE expansion. Structural features that drive partitioning of Tts1 to the high-curvature ER domains are crucial for both aspects of its function. An amphipathic helix located at the C-terminus of Tts1 is important for ER shaping and modulating the mitotic NPC distribution. Of interest, the evolutionarily conserved residues at the luminal interface of the third transmembrane region function specifically in promoting SPB-NE insertion. Our data illuminate cellular requirements for remodeling the NE during "closed" nuclear division and provide insight into the structure and functions of the eukaryotic TMEM33 family., (© 2014 Zhang and Oliferenko. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

During cellular proliferation, the mother nucleus divides into two daughters, each carrying a full complement of chromosomes and capable of driving the next cell cycle. Mitotic chromosome segregation must be coordinated with remodeling of other nuclear components, including the nuclear envelope (NE) and the non-membrane-bound nucleolus. Here, we show that in the fission yeast Schizosaccharomyces japonicus, which breaks and reforms the NE during mitosis, the evolutionary conserved LEM-domain protein Man1 promotes equal partitioning of the nuclear membrane, nucleolar remodeling, and efficient inheritance of the nuclear pore complexes (NPCs) by the daughter nuclei. Analyses of man1 mutants and results obtained using an artificial chromatin-nuclear pore tether suggest that Man1 may exert its function by linking the NPCs to segregating chromatin. By integrating the partitioning of the nuclear membrane and nucleolar material with chromosome segregation, cells build two nearly identical copies of the original nucleus., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The mitotic spindle assembly and chromosome segregation in eukaryotes must be coordinated with the nuclear envelope (NE) remodeling. In a so-called 'open' mitosis the envelope of the mother nucleus is dismantled allowing the cytoplasmic spindle microtubules to capture the chromosomes. Alternatively, cells undergoing 'closed' mitosis assemble the intranuclear spindle and divide the nucleus without ever losing the nucleocytoplasmic compartmentalization. Here we focus on the mechanisms underlying mitotic NE dynamics in unicellular eukaryotes undergoing a closed nuclear division, paying specific attention to the emerging roles of the lipid biosynthesis machinery in this process. We argue that lessons learned in these organisms may be generally relevant to understanding the NE remodeling and the evolution of mitotic mechanisms throughout the eukaryotic domain., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

How the molecular mechanisms of stress response are integrated at the cellular level remains obscure. Here we show that the cellular polarity machinery in the fission yeast Schizosaccharomyces pombe undergoes dynamic adaptation to thermal stress resulting in a period of decreased Cdc42 activity and altered, monopolar growth. Cells where the heat stress-associated transcription was genetically upregulated exhibit similar growth patterning in the absence of temperature insults. We identify the Ssa2-Mas5/Hsp70-Hsp40 chaperone complex as repressor of the heat shock transcription factor Hsf1. Cells lacking this chaperone activity constitutively activate the heat-stress-associated transcriptional program. Interestingly, they also exhibit intermittent monopolar growth within a physiological temperature range and are unable to adapt to heat stress. We propose that by negatively regulating the heat stress-associated transcription, the Ssa2-Mas5 chaperone system could optimize cellular growth under different temperature regiments., Competing Interests: The authors have declared that no competing interests exist.

The cortical endoplasmic reticulum (ER) is an intricate network of tubules and cisternae tightly associated with the plasma membrane (PM) in plants, yeast, and the excitable cell types in metazoans [1-5]. How the ER is attached to the cell cortex and what necessitates its highly reticulated architecture remain largely unknown. Here, we identify the integral ER vesicle-associated membrane protein-associated proteins (VAPs), previously shown to control the composition of phosphoinositides at the ER-PM contact sites [6, 7], as major players in sustaining the ER-PM tethering in fission yeast. We show that genetic conversion of the reticulated ER structure to the cisternal morphology shields large areas of the PM, preventing the actomyosin division ring assembly at the equatorial cortex. Using a combination of VAP mutants where the cortical ER is detached from the PM and a set of artificial ER-PM tethers suppressing this phenotype, we demonstrate that the PM footprint of the cortical ER is functionally insulated from the cytosol. In cells with prominent ER-PM contacts, fine reticulation of the ER network may have emerged as a critical adaptation enabling a uniform access of peripheral protein complexes to the inner surface of the plasma membrane., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Studies of budding yeast arrested in mitosis outline a set of rules for nuclear envelope expansion during closed nuclear division., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The aim of mitosis is to produce two daughter nuclei, each containing a chromosome complement identical to that of the mother nucleus. This can be accomplished through a variety of strategies, with "open" and "closed" modes of mitosis positioned at the opposite ends of the spectrum and a range of intermediate patterns in between. In the "closed" mitosis, the nuclear envelope remains intact throughout the nuclear division. In the "open" division type, the envelope of the original nucleus breaks down early in mitosis and reassembles around the segregated daughter genomes. In any case, the nuclear membrane has to remodel to accommodate the mitotic spindle assembly, chromosome segregation and formation of the daughter nuclei. We have recently shown that within the fission yeast clade, the mitotic control of the nuclear surface area may determine the choice between the nuclear envelope breakdown and a fully "closed" division. Here we discuss our data and argue that comparative cell biology studies using two fission yeast species, Schizosaccharomyces pombe and Schizosaccharomyces japonicus, could provide unprecedented insights into physiology and evolution of mitosis.

Eukaryotes segregate chromosomes in "open" or "closed" mitosis, depending on whether their nuclear envelopes (NEs) break down or remain intact. Here we show that the control of the nuclear surface area may determine the choice between these two modes. The dividing nucleus does not expand its surface in the fission yeast Schizosaccharomyces japonicus, confining the mitotic spindle and causing it to buckle. The NE ruptures in anaphase, releasing the compressive stress and allowing chromosome segregation. Blocking the NE expansion in the related species Schizosaccharomyces pombe that undergoes closed mitosis induces spindle buckling and collapse in the absence of an intrinsic NE rupture mechanism. We propose that scaling considerations could have shaped the evolution of eukaryotic mitosis by necessitating either nuclear surface expansion or the NE breakdown., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Microtubule arrays effect cell polarisation by directing cellular cues for cortical remodelling and growth. Their function depends crucially on the intrinsic dynamic properties of constituent microtubules. Microtubule dynamicity is restricted to a certain range within the confines of a cellular geometry. Thus it is of great interest to determine whether rescaling of dynamic properties of microtubules has consequences for cell polarity. We constructed fission yeast strains exhibiting depressed microtubule dynamics by mutating the β-tubulin gene, nda3. This interfered with efficient accumulation of a polarity factor Tea1 at cell tips. Interestingly, the polarity machinery in the mutant cells was highly susceptible to perturbations. Simulations of growth zone formation followed by imaging of actin distribution showed a significantly delayed onset of bipolar growth. We propose that there exists a threshold of microtubule dynamicity that allows robust cellular polarisation., (Copyright © 2011 Wiley-Liss, Inc.)

Mitosis in eukaryotic cells employs spindle microtubules to drive accurate chromosome segregation at cell division. Cells lacking spindle microtubules arrest in mitosis due to a spindle checkpoint that delays mitotic progression until all chromosomes have achieved stable bipolar attachment to spindle microtubules. In fission yeast, mitosis occurs within an intact nuclear membrane with the mitotic spindle elongating between the spindle pole bodies. We show here that in fission yeast interference with mitotic spindle formation delays mitosis only briefly and cells proceed to an unusual nuclear division process we term nuclear fission, during which cells perform some chromosome segregation and efficiently enter S-phase of the next cell cycle. Nuclear fission is blocked if spindle pole body maturation or sister chromatid separation cannot take place or if actin polymerization is inhibited. We suggest that this process exhibits vestiges of a primitive nuclear division process independent of spindle microtubules, possibly reflecting an evolutionary intermediate state between bacterial and Archeal chromosome segregation where the nucleoid divides without a spindle and a microtubule spindle-based eukaryotic mitosis., Competing Interests: The authors have declared that no competing interests exist.

Precise positioning of the cellular division plane is important for accurate segregation of genetic material and determination of daughter cell fates. Here we report a surprising connection between division site positioning and the organization of the cortical endoplasmic reticulum (ER). The cortical ER is an interconnected network of flat cisternae and highly curved tubules sharing a continuous lumen. Stabilization of high curvature by reticulon and DP1 family proteins contributes to formation of tubules. We show that in the fission yeast Schizosaccharomyces pombe, the ER network is maintained by a set of three membrane proteins: reticulon/Rtn1p, DP1/Yop1p, and a newly identified evolutionarily conserved protein, Tts1p. Cells lacking the ER domain sustained by these proteins exhibit severe defects in division plane positioning as a result of abnormal dispersion of a key regulator of division site selection, Mid1p, along the cell cortex. This triggers delocalized assembly of actomyosin cables and compromises their compaction into a single medially positioned ring. We propose that the cortical ER network restricts the lateral motion of Mid1p and hence generates a permissive zone for actomyosin ring assembly precisely at the cell equator., (2010 Elsevier Ltd. All rights reserved.)

Microtubule (MT) arrays are mechanistic effectors of polarity specification and cell division. Linear bundles in which MTs are bridged laterally are dynamically assembled in systems ranging from differentiated metazoan cells to fungi in a process that remains poorly understood. Often, bundled MTs slide with respect to each other via molecular motors. In interphase cells of the fission yeast Schizosaccharomyces pombe, MT nucleation frequently occurs at preexisting arrays. As the nascent MT lengthens, stable antiparallel MT overlaps are thought to form through competition between motion of the minus-end-directed kinesin Klp2p and braking force exerted by the accumulating lateral crosslinker Ase1p. Here we show that Mia1p/Alp7p, a transforming acidic coiled-coil (TACC) protein, functions as a length-independent MT crosslinker. In cells lacking Mia1p MT-bundling activity, linear arrays frequently disassemble, accompanied by a marked increase in Ase1p off rate and erratic motion of sliding MTs. We propose that the combined action of lateral length-dependent (Ase1p) and terminal length-independent (Mia1p) crosslinkers is crucial for robust assembly and stability of linear MT arrays. Such use of qualitatively distinct crosslinking mechanisms in tandem may point to a general design principle in the engineering of stable cytoskeletal assemblies.

Microtubule arrays are remodeled as cells proceed through the cell cycle. It is important to understand how remodeling is regulated in time and space. In fission yeast, the conserved microtubule associated TACC/TOG complex plays an important role in organizing microtubules throughout the cell cycle. Here we show that this complex undergoes nucleocytoplasmic shuttling through the nuclear import and export signals located in the TACC protein Mia1p/Alp7p. When the Crm1p-dependent nuclear export signal of Mia1p is disabled, Mia1p accumulates in the nucleus while its partner protein Alp14p/TOG is restricted to the cytoplasm. This leads to defects in assembly of both interphase arrays and the mitotic spindle. Artificial targeting of Alp14p to the nucleus partially rescues the mitotic spindle defects caused by lack of Mia1p nuclear export. Interestingly, the nuclear export sequence of Mia1p appears to overlap with the Alp14p binding site. We propose that intricate regulation of the subcellular distribution of TACC/TOG complexes drives microtubule array remodeling as cells progress through the cell cycle.

Cytokinesis is the terminal step of the cell cycle during which a mother cell divides into daughter cells. Often, the machinery of cytokinesis is positioned in such a way that daughter cells are born roughly equal in size. However, in many specialized cell types or under certain environmental conditions, the cell division machinery is placed at nonmedial positions to produce daughter cells of different sizes and in many cases of different fates. Here we review the different mechanisms that position the division machinery in prokaryotic and eukaryotic cell types. We also describe cytokinesis-positioning mechanisms that are not adequately explained by studies in model organisms and model cell types.

Recent studies of actomyosin-ring assembly in fission yeast have suggested that an intricate web of membrane-bound nodes containing myosin and the actin nucleator formin is pulled together into a tight ring through a 'search-and-capture' mechanism.

The ultimate goal of cytokinesis is to establish a membrane barrier between daughter cells. The fission yeast Schizosaccharomyces pombe utilizes an actomyosin-based division ring that is thought to provide physical force for the plasma membrane invagination. Ring constriction occurs concomitantly with the assembly of a division septum that is eventually cleaved. Membrane trafficking events such as targeting of secretory vesicles to the division site require a functional actomyosin ring suggesting that it serves as a spatial landmark. However, the extent of polarization of the secretion apparatus to the division site is presently unknown. We performed a survey of dynamics of several fluorophore-tagged proteins that served as markers for various compartments of the secretory pathway. These included markers for the endoplasmic reticulum, the COPII sites, and the early and late Golgi. The secretion machinery exhibited a marked polarization to the division site. Specifically, we observed an enrichment of the transitional endoplasmic reticulum (tER) accompanied by Golgi cisternae biogenesis. These processes required actomyosin ring assembly and the function of the EFC-domain protein Cdc15p. Cdc15p overexpression was sufficient to induce tER polarization in interphase. Thus, fission yeast polarizes its entire secretory machinery to the cell division site by utilizing molecular cues provided by the actomyosin ring.

Background: The nucleus and the centrosomes (spindle pole bodies; SPBs in yeast) are believed to play key roles in the organization of various cellular structures, such as the actomyosin ring and microtubules. The ability to generate cells lacking nuclei and centrosomes (SPBs) is key to the elucidation of the role of these structures in various cellular processes., Methodology/principal Findings: Here we describe a genetic method, using the Schizosaccharomyces pombe cdc16-116 mutant, to reliably and efficiently generate fission yeast cells lacking nuclei and SPBs. We use this approach to show that the assembly of microtubules does not require nuclear associated microtubule organizing centers and SPBs. We also show that actomyosin rings can assemble albeit inefficiently in the absence of nuclei and SPBs., Conclusion: We conclude that key cytoskeletal elements can be assembled in the absence of nuclei and SPBs. In addition, the approach we describe, taken together with physical approaches such as centrifugation, should facilitate the investigation of the role of the nucleus and SPBs in the assembly and inheritance of various cellular structures and organelles.

Many organisms divide chromosomes within the confines of the nuclear envelope (NE) in a process known as closed mitosis. Thus, they must ensure coordination between segregation of the genetic material and division of the NE itself. Although many years of work have led to a reasonably clear understanding of mitotic spindle function in chromosome segregation, the NE division mechanism remains obscure. Here, we show that fission yeast cells overexpressing the transforming acid coiled coil (TACC)-related protein, Mia1p/Alp7p, failed to separate the spindle pole bodies (SPBs) at the onset of mitosis, but could assemble acentrosomal bipolar and antiparallel spindle structures. Most of these cells arrested in anaphase with fully extended spindles and nonsegregated chromosomes. Spindle poles that lacked the SPBs did not lead the division of the NE during spindle elongation, but deformed it, trapping the chromosomes within. When the SPBs were severed by laser microsurgery in wild-type cells, we observed analogous deformations of the NE by elongating spindle remnants, resulting in NE division failure. Analysis of dis1Delta cells that elongate spindles despite unattached kinetochores indicated that the SPBs were required for maintaining nuclear shape at anaphase onset. Strikingly, when the NE was disassembled by utilizing a temperature-sensitive allele of the Ran GEF, Pim1p, the abnormal spindles induced by Mia1p overexpression were capable of segregating sister chromatids to daughter cells, suggesting that the failure to divide the NE prevents chromosome partitioning. Our results imply that the SPBs preclude deformation of the NE during spindle elongation and thus serve as specialized structures enabling nuclear division during closed mitosis in fission yeast.

Microtubule-organizing centers (MTOCs) concentrate microtubule nucleation, attachment and bundling factors and thus restrict formation of microtubule arrays in spatial and temporal manner. How MTOCs occur remains an exciting question in cell biology. Here, we show that the transforming acidic coiled coil-related protein Mia1p/Alp7p functions in emergence of large MTOCs in interphase fission yeast cells. We found that Mia1p was a microtubule-binding protein that preferentially localized to the minus ends of microtubules and was associated with the sites of microtubule attachment to the nuclear envelope. Cells lacking Mia1p exhibited less microtubule bundles. Microtubules could be nucleated and bundled but were frequently released from the nucleation sites in mia1delta cells. Mia1p was required for stability of microtubule bundles and persistent use of nucleation sites both in interphase and postanaphase array dynamics. The gamma-tubulin-rich material was not organized in large perinuclear or microtubule-associated structures in mia1delta cells. Interestingly, absence of microtubules in dividing wild-type cells prevented appearance of large gamma-tubulin-rich MTOC structures in daughters. When microtubule polymerization was allowed, MTOCs were efficiently assembled de novo. We propose a model where MTOC emergence is a self-organizing process requiring the continuous association of microtubules with nucleation sites.

The spindle orientation checkpoint (SOC) in fission yeast has been proposed to delay metaphase-to-anaphase transition when the spindle poles are misaligned with respect to the long axis of the cell. This checkpoint is activated in the absence of either an actomyosin division ring or astral microtubules. Although the SOC could be overridden in the absence of the transcription factor Atf1p, its mechanistic nature remained unclear. Here, we show that the SOC-triggered metaphase delay depends on a subset of the spindle assembly checkpoint (SAC) components Mph1p and Bub1p. Based on this finding and a detailed imaging of the spindle orientation process, we hypothesized that the spindle pole might contain proteins capable of sensing the achievement of spindle alignment. We identified the kendrin-like spindle pole body resident Pcp1p as a candidate molecule. A targeted mutation in its central domain specifically triggered the SOC in spite of the presence of oriented spindles, causing a metaphase delay that could be relieved in the absence of Mph1p, Bub1p, and Atf1p. Thus, Pcp1p might provide a link between the mechanical process of spindle alignment and the signal transduction that initiates anaphase.

The cell division apparatus is assembled at different stages of the cell cycle in different eukaryotic organisms. Mechanisms exist in all organisms, however, to ensure that the cell division apparatus and the mitotic spindle are aligned perpendicular to each other. Such an alignment ensures that each daughter cell receives a nucleus and that the cell division apparatus does not cleave and destroy the genetic material. The interaction(s) of astral microtubules with the cell cortex appears to play an important role in establishing perpendicularity between chromosome segregation and cell division machinery.

Segregating genetic material along the longest axis of the cell ensures that there is a sufficient distance between daughter chromosomes at the point of cytokinesis. Monitoring the orientation of the mitotic spindle can be subjected to cell cycle controls. In the fission yeast Schizosaccharomyces pombe, the existence of such a cell-cycle checkpoint has been proposed to delay the metaphase to anaphase transition when spindle poles are not properly oriented with respect to the actomyosin ring. Here we show, by using a fission yeast mutant compromised in its assembly of astral microtubules, that in the absence of astral microtubules short metaphase spindles are unable to orient themselves with respect to the long axis of the cell and are delayed in spindle elongation. This astral defect engages a spindle orientation checkpoint because deletion of the transcription factor Atf1, which is involved in maintaining this checkpoint, allows misaligned asterless metaphase spindles to elongate. We propose that astral microtubules are involved directly in monitoring orientation of the metaphase spindle and in controlling the timing of elongation in fission yeast.

Schizosaccharomyces pombe cells divide by medial fission through the use of an actomyosin-based contractile ring. Constriction of the actomyosin ring is accompanied by the centripetal addition of new membranes and cell wall material. In this article, we characterize the mechanism responsible for the localization of Cps1p, a septum-synthesizing 1,3-beta-glucan synthase, to the division site during cytokinesis. We show that Cps1p is an integral membrane protein that localizes to the cell division site late in anaphase. Neither F-actin nor microtubules are essential for the initial assembly of Cps1p to the medial division site. F-actin, but not microtubules, is however important for the eventual incorporation of Cps1p into the actomyosin ring. Assembly of Cps1p into the cell division ring is also dependent on the septation-inducing network (SIN) proteins that regulate division septum formation after assembly of the actomyosin ring. Fluorescence-recovery after-photobleaching experiments reveal that Cps1p does not diffuse appreciably within the plasma membrane and is retained at the division site by a mechanism that does not depend on an intact F-actin cytoskeleton. We conclude that the actomyosin ring serves as a spatial cue for Cps1p localization, whereas the maintenance of Cps1p at the division site occurs by a novel F-actin- and microtubule-independent mechanism. Furthermore, we propose that the SIN proteins ensure localization of Cps1p at the appropriate point in the cell cycle.

Despite their conserved structures, protein phosphatases of the budding yeast Cdc14p family appear to perform distinct physiological roles in controlling late events of the cell cycle in different organisms.