Virtual breakdown of the nuclear envelope in fission yeast meiosis

Asymmetric localization of Ran regulators (RanGAP1 and RanGEF/RCC1) produces a gradient of RanGTP across the nuclear envelope. In higher eukaryotes, the nuclear envelope breaks down as the cell enters mitosis (designated "open" mitosis). This nuclear envelope breakdown (NEBD) leads to collapse of the RanGTP gradient and the diffusion of nuclear and cytoplasmic macromolecules in the cell, resulting in irreversible progression of the cell cycle. On the other hand, in many fungi, chromosome segregation takes place without NEBD (designated "closed" mitosis). Here we report that in the fission yeast Schizosaccharomyces pombe, despite the nuclear envelope and the nuclear pore complex remaining intact throughout both the meiotic and mitotic cell cycles, nuclear proteins diffuse into the cytoplasm transiently for a few minutes at the onset of anaphase of meiosis II. We also found that nuclear protein diffusion into the cytoplasm occurred coincidently with nuclear localization of Rna1, an S. pombe RanGAP1 homolog that is usually localized in the cytoplasm. These results suggest that nuclear localization of RanGAP1 and depression of RanGTP activity in the nucleus may be mechanistically tied to meiosis-specific diffusion of nuclear proteins into the cytoplasm. This nucleocytoplasmic shuffling of RanGAP1 and nuclear proteins represents virtual breakdown of the nuclear envelope.     

Nucleocytoplasmic transport and nuclear envelope integrity in the fission yeast Schizosaccharomyces pombe

The nuclear envelope is essential for compartmentalizing the nucleus from the cytoplasm in all eukaryotic cells. There is a tremendous flux of both RNA and proteins across the nuclear envelope, which is intact throughout the entire cell cycle of yeasts but breaks down during mitosis of animal cells. Transport across the nuclear envelope requires the recognition of cargo molecules by receptors, docking at the nuclear pore, transit through the nuclear pore, and then dissociation of the cargo from the receptor. This process depends on the RanGTPase system, transport receptors, and the nuclear pore complex. We provide an overview of the nuclear transport process, with particular emphasis on the fission yeast Schizosaccharomyces pombe, including strategies for predicting and experimentally verifying the signals that determine the sub-cellular localization of a protein of interest. We also describe a variety of reagents and experimental strategies, including the use of mutants and chemical inhibitors, to study nuclear protein import, nuclear protein export, nucleocytoplasmic protein shuttling, and mRNA export in fission yeast. The RanGTPase and its regulators also play an essential transport independent role in nuclear envelope re-assembly after mitosis in animal cells and in the maintenance of nuclear envelope integrity at mitosis in S. pombe. Several experimental strategies and reagents for studying nuclear size, nuclear shape, the localization of nuclear pores, and the integrity of the nuclear envelope in living fission yeast cells are described.     

Formation of the Nuclear Envelope Permeability Barrier Studied by Sequential Photoswitching and Flux Analysis

In higher eukaryotes, the nuclear envelope breaks down during mitosis. It reforms during telophase, and nuclear import is reestablished within <10 min after anaphase onset. It is widely assumed that import functionality simultaneously leads to the exclusion of bulk cytoplasmic proteins. However, nuclear pore complex assembly is not fully completed when import capacity is regained, which raises the question of whether the transport and permeability barrier functions of the nuclear envelope are indeed coupled. In this study, we therefore analyzed the reestablishment of the permeability barrier of the nuclear envelope after mitosis in living cells by monitoring the flux of the reversibly photoswitchable fluorescent protein Dronpa from the cytoplasm into the nucleus after photoactivation. We performed many consecutive flux measurements in the same cell to directly monitor changes in nuclear envelope permeability. Our measurements at different time points after mitosis in individual cells show that contrary to the general view and despite the rapid reestablishment of facilitated nuclear import, the nuclear envelope remains relatively permeable for passive diffusion for the first 2 h after mitosis. Our data demonstrate that reformation of the permeability barrier of nuclear pore complexes occurs only gradually and is uncoupled from regaining active import functionality.     

A novel role of Dma1 in regulating forespore membrane assembly and sporulation in fission yeast

In fission yeast Schizosaccharomyces pombe, a diploid mother cell differentiates into an ascus containing four haploid ascospores following meiotic nuclear divisions, through a process called sporulation. Several meiosis-specific proteins of fission yeast have been identified to play essential roles in meiotic progression and sporulation. We report here an unexpected function of mitotic spindle checkpoint protein Dma1 in proper spore formation. Consistent with its function in sporulation, expression of dma1(+) is up-regulated during meiosis I and II. We showed that Dma1 localizes to the SPB during meiosis and the maintenance of this localization at meiosis II depends on septation initiation network (SIN) scaffold proteins Sid4 and Cdc11. Cells lacking Dma1 display defects associated with sporulation but not nuclear division, leading frequently to formation of asci with fewer spores. Our genetic analyses support the notion that Dma1 functions in parallel with the meiosis-specific Sid2-related protein kinase Slk1/Mug27 and the SIN signaling during sporulation, possibly through regulating proper forespore membrane assembly. Our studies therefore revealed a novel function of Dma1 in regulating sporulation in fission yeast.     

Nuclear membrane: nuclear envelope PORosity in fission yeast meiosis

The fission yeast Schizosaccharomyces pombe undergoes closed mitosis but 'virtual nuclear envelope breakdown' at anaphase of meiosis II, in which the nuclear envelope is structurally closed but functionally open.     

Localization of the 26S proteasome during mitosis and meiosis in fission yeast.

The 26S proteasome is a large multisubunit complex involved in degrading both cytoplasmic and nuclear proteins. We have investigated the localization of this complex in the fission yeast, Schizosaccharomyces pombe. Immunofluorescence microscopy shows a striking localization pattern whereby the proteasome is found predominantly at the nuclear periphery, both in interphase and throughout mitosis. Electron microscopic analysis revealed a concentration of label near the inner side of the nuclear envelope. The localization of green fluorescent protein (GFP)-tagged 26S proteasomes was analyzed in live cells during mitosis and meiosis. Throughout mitosis the proteasome remained predominantly at the nuclear periphery. During meiosis the proteasome was found to undergo dramatic changes in its localization. Throughout the first meiotic division, the signal is more dispersed over the nucleus. During meiosis II, there was a dramatic re-localization, and the signal became restricted to the area between the separating DNA until the end of meiosis when the signal dispersed before returning to the nuclear periphery during spore formation. These findings strongly imply that the nuclear periphery is a major site of protein degradation in fission yeast both in interphase and throughout mitosis. Furthermore they raise interesting questions as to the spatial organization of protein degradation during meiosis.     

A mutation in the gene involved in sister chromatid separation causes a defect in nuclear mRNA export in fission yeast

Fission yeast ptr4-1 is one of the mRNA transport mutants that accumulate poly(A)(+) RNA in the nuclei at the nonpermissive temperature. We cloned the ptr4(+) gene and found that it is identical with the cut1(+) gene essential for chromosome segregation during mitosis. ptr4/cut1 has no defects in nucleocytoplasmic transport of a protein, indicative of a specific blockage of mRNA export by this mutation. A mutant of Cut2p cooperating with Cut1p in sister chromatid separation also showed defective mRNA export at the nonpermissive temperature. Our results suggest a novel linkage between the cell division cycle and nuclear mRNA export in eukaryotic cells.     

Functional conservation and cell cycle localization of the Nhp2 core component of H + ACA snoRNPs in fission and budding yeasts.

We report the identification of a novel nucleolar protein from fission yeast, p17(nhp2), which is homologous to the recently identified Nhp2p core component of H+ACA snoRNPs in Saccharomyces cerevisiae. We show that the fission yeast p17(nhp2) localizes to the nucleolus in live S. cerevisiae or Schizosaccharomyces pombe cells and is functionally conserved since the fission yeast gene can complement a deletion of the NHP2 gene in budding yeast. Analysis of p17(nhp2) during the mitotic cell cycles of living fission and budding yeast cells shows that this protein, and by implication H+ACA snoRNPs, remains localized with nucleolar material during mitosis, although the gross organization of partitioning of p17(nhp2) during anaphase is different in a comparison of the two yeasts. During anaphase in S. pombe p17(nhp2) trails segregating chromatin, while in S. cerevisiae the protein segregates alongside bulk chromatin. The pattern of segregation comparing haploid and diploid S. cerevisiae cells suggests that p17(nhp2) is closely associated with the rDNA during nuclear division.     

A novel Cdc20-related WD-repeat protein, Fzr1, is required for spore formation in Schizosaccharomyces pombe

Ste9/Srw1 which shows sequence homology to Hctl from budding yeast, is an activator of the anaphase-promoting complex (APC) in the fission yeast Schizosaccharomyces pombe. By homology search of the S. pombe genome, we identified the gene fr1+, which encodes the protein with the highest homology to Ste9 among five Cdc20-like proteins. Like Ste9, Fzr1 contains seven WD-repeats in its C-terminal region. In spite of this structural similarity, however, overproduction of either of these proteins cannot complement mutants lacking the other. fzr1+ is transcribed exclusively during meiosis and sporulation, suggesting that it plays a role in these processes. In fact, the fzr1 disruptant formed aberrant asci, which contained only one or two mature spores, though meiotic nuclear divisions proceeded with kinetics similar to wild type, and meiotic segregation of chromosomes was normal. Structural alteration of spindle pole bodies, which is a prerequisite for the formation of the forespore membrane, occurred normally in fzr1delta during the second meiotic division. Localization of spore rim marker proteins fused to green fluorescent protein showed that nascent prespores were irregularly shaped, small in size and few in number in fzr1delta cells compared to wild-type cells. Furthermore, electron microscopy revealed that the outer layer of the spore walls was often missing in fzr1delta spores. These results show that Fzr1 is specifically involved in the assembly of the spore envelope and also in spore maturation. Fzr1, a structural homolog of the APC regulator, therefore plays an important role in spore morphogenesis.     

Shugoshin prevents dissociation of cohesin from centromeres during mitosis in vertebrate cells

Cohesion between sister chromatids is essential for their bi-orientation on mitotic spindles. It is mediated by a multisubunit complex called cohesin. In yeast, proteolytic cleavage of cohesin's alpha kleisin subunit at the onset of anaphase removes cohesin from both centromeres and chromosome arms and thus triggers sister chromatid separation. In animal cells, most cohesin is removed from chromosome arms during prophase via a separase-independent pathway involving phosphorylation of its Scc3-SA1/2 subunits. Cohesin at centromeres is refractory to this process and persists until metaphase, whereupon its alpha kleisin subunit is cleaved by separase, which is thought to trigger anaphase. What protects centromeric cohesin from the prophase pathway? Potential candidates are proteins, known as shugoshins, that are homologous to Drosophila MEI-S332 and yeast Sgo1 proteins, which prevent removal of meiotic cohesin complexes from centromeres at the first meiotic division. A vertebrate shugoshin-like protein associates with centromeres during prophase and disappears at the onset of anaphase. Its depletion by RNA interference causes HeLa cells to arrest in mitosis. Most chromosomes bi-orient on a metaphase plate, but precocious loss of centromeric cohesin from chromosomes is accompanied by loss of all sister chromatid cohesion, the departure of individual chromatids from the metaphase plate, and a permanent cell cycle arrest, presumably due to activation of the spindle checkpoint. Remarkably, expression of a version of Scc3-SA2 whose mitotic phosphorylation sites have been mutated to alanine alleviates the precocious loss of sister chromatid cohesion and the mitotic arrest of cells lacking shugoshin. These data suggest that shugoshin prevents phosphorylation of cohesin's Scc3-SA2 subunit at centromeres during mitosis. This ensures that cohesin persists at centromeres until activation of separase causes cleavage of its alpha kleisin subunit. Centromeric cohesion is one of the hallmarks of mitotic chromosomes. Our results imply that it is not an intrinsically stable property, because it can easily be destroyed by mitotic kinases, which are kept in check by shugoshin.     

Nuclear transporters in a multinucleated organism: functional and localization analyses in Aspergillus nidulans

Nuclear transporters mediate bidirectional macromolecule traffic through the nuclear pore complex (NPC), thus participating in vital processes of eukaryotic cells. A systematic functional analysis in Aspergillus nidulans permitted the identification of 4 essential nuclear transport pathways of a hypothetical number of 14. The absence of phenotypes for most deletants indicates redundant roles for these nuclear receptors. Subcellular distribution studies of these carriers show three main distributions: nuclear, nucleocytoplasmic, and in association with the nuclear envelope. These locations are not specific to predicted roles as exportins or importins but indicate that bidirectional transport may occur coordinately in all nuclei of a syncytium. Coinciding with mitotic NPC rearrangements, transporters dynamically modified their localizations, suggesting supplementary roles to nucleocytoplasmic transport specifically during mitosis. Loss of transportin-SR and Mex/TAP from the nuclear envelope indicates absence of RNA transport during the partially open mitosis of Aspergillus, whereas nucleolar accumulation of Kap121 and Kap123 homologues suggests a role in nucleolar disassembly. This work provides new insight into the roles of nuclear transporters and opens an avenue for future studies of the molecular mechanisms of transport among nuclei within a common cytoplasm, using A. nidulans as a model organism.     

MITOSIS AND CELL DIVISION IN HYDRURUS FOETIDUS (CHRYSOPHYCEAE)1

Mitosis and cell division have been examined ultrastructurally in the vegetative cells of Hydrurus foetidus (Vill) Trev. and found to resemble that of Ochromonas in two important aspects. First, the rhizoplast acts as the spindle organizing body and second, the spindle elongates considerably during anaphase. It differs from Ochromonas in that there is no movement of the basal bodies and flagella towards the poles. Moreover, the nuclear envelope remains relatively intact throughout early stages of mitosis, with gaps developing at the poles during prophase to permit entry of spindle microtubules. Disruption of the nuclear envelope does not occur in the equatorial plane until late anaphase. The spindle persists into telophase and is bent towards the posterior of the cell by the ingrowing edge of the cleavage furrow. Persistence of the spindle and lack of Ochromoms-type cell elongation may be related to the constricting presence of the sheath during cell division—a completely different strategy to that adopted by the green algae under conditions of similar constraint.     

A novel complex of membrane proteins required for formation of a spherical nucleus.

Two membrane proteins were identified through their genetic interaction with the nucleoporin Nup84p and shown to participate in nuclear envelope morphogenesis in yeast. One component is a known sporulation factor Spo7p, and the other, Nem1p, a novel protein whose C-terminal domain is conserved during eukaryotic evolution. Spo7p and Nem1p localize to the nuclear/ER membrane and behave biochemically as integral membrane proteins. Nem1p binds to Spo7p via its conserved C-terminal domain. Although cells without Spo7p or Nem1p are viable, they exhibit a drastically altered nuclear morphology with long, pore-containing double nuclear membrane extensions. These protrusions emanate from a core nucleus which contains the DNA, and penetrate deeply into the cytoplasm. Interestingly, not only Spo7(-) and Nem1(-), but also several nucleoporin mutants are defective in sporulation. Thus, Spo7p and Nem1p, which exhibit a strong genetic link to nucleoporins of the Nup84p complex, fulfil an essential role in formation of a spherical nucleus and meiotic division.     

Vesicle-like biomechanics governs important aspects of nuclear geometry in fission yeast

It has long been known that during the closed mitosis of many unicellular eukaryotes, including the fission yeast (Schizosaccharomyces pombe), the nuclear envelope remains intact while the nucleus undergoes a remarkable sequence of shape transformations driven by elongation of an intranuclear mitotic spindle whose ends are capped by spindle pole bodies embedded in the nuclear envelope. However, the mechanical basis of these normal cell cycle transformations, and abnormal nuclear shapes caused by intranuclear elongation of microtubules lacking spindle pole bodies, remain unknown. Although there are models describing the shapes of lipid vesicles deformed by elongation of microtubule bundles, there are no models describing normal or abnormal shape changes in the nucleus. We describe here a novel biophysical model of interphase nuclear geometry in fission yeast that accounts for critical aspects of the mechanics of the fission yeast nucleus, including the biophysical properties of lipid bilayers, forces exerted on the nuclear envelope by elongating microtubules, and access to a lipid reservoir, essential for the large increase in nuclear surface area during the cell cycle. We present experimental confirmation of the novel and non-trivial geometries predicted by our model, which has no free parameters. We also use the model to provide insight into the mechanical basis of previously described defects in nuclear division, including abnormal nuclear shapes and loss of nuclear envelope integrity. The model predicts that (i) despite differences in structure and composition, fission yeast nuclei and vesicles with fluid lipid bilayers have common mechanical properties; (ii) the S. pombe nucleus is not lined with any structure with shear resistance, comparable to the nuclear lamina of higher eukaryotes. We validate the model and its predictions by analyzing wild type cells in which ned1 gene overexpression causes elongation of an intranuclear microtubule bundle that deforms the nucleus of interphase cells.     

核孔复合体的结构及其功能

核孔复合体(Nuclear pore complexes, NPCs)镶嵌在核膜上,是细胞核与细胞质之间的唯一通道。冷冻电子X射线断层扫描将环状NPCs分为三个环,分别称为胞质环、内环和核质环,胞质环上附有胞质纤丝,核质环上附有核篮。由于物种不同,NPCs由30～50多种不同的核孔蛋白(nucleoporins, Nups)组成,但结构和功能高度保守。根据其结构、氨基酸序列,NPCs定位和功能,Nups被分为跨膜Nups、屏障Nups、骨架Nups、胞质纤丝Nups和核篮Nups。相互间作用稳定、紧密连接的数个Nups可组成亚复合体。为了应对不同生理需要,NPCs处于高度动态变化中,间期和有丝分裂期均可通过组装和去组装改变核孔数量和功能。NPCs的主要功能是调控核质转运,小分子物质可自由扩散,大分子物质则需在核转位信号和转运载体的介导下以主动运输的方式进行转运。除了核质转运这一主要功能外,Nups还能以一个独立于转运的方式影响基因组功能。通过影响染色质结构和影响转录调控元件对靶基因的访问,Nups促进或抑制转录。在酵母,Nups介导的基因调控主要由位于NPCs中的Nups执行;在多细胞生物,不仅NPCs中的Nups,核质内游离的Nups也具有基因调控功能。此外,Nups还能通过参与形成染色质边界和形成转录记忆对基因进行调控。在增殖细胞, Nups通过与DNA修复机器相互作用,参与DNA损伤修复,保护基因组完整性。有丝分裂时,Nups协助核膜解体和中心体迁移,并通过作用于着丝粒来控制有丝分裂组件的空间定位与活性,稳定它们与微管之间的相互作用,保证纺锤体正常组装和染色体准确分离。总之,NPCs与生物分子的核质转运、基因表达和细胞周期密切相关,它的结构和功能的稳定是真核细胞生长、增殖、分化等生命活动的基本保证。     

Yeast Ran-binding protein Yrb1p is required for efficient proteolysis of cell cycle regulatory proteins Pds1p and Sic1p

Ubiquitin-dependent proteolysis of specific target proteins is required for several important steps during the cell cycle. Degradation of such proteins is strictly cell cycle-regulated and triggered by two large ubiquitin ligases, termed anaphase-promoting complex (APC) and Skp1/Cullin/F-box complex (SCF). Here we show that yeast Ran-binding protein 1 (Yrb1p), a predominantly cytoplasmic protein implicated in nucleocytoplasmic transport, is required for cell cycle regulated protein degradation. Depletion of Yrb1p results in the accumulation of unbudded G(1) cells and of cells arrested in mitosis implying a function of Yrb1p in the G(1)/S transition and in the progression through mitosis. Temperature-sensitive yrb1-51 mutants are defective in APC-mediated degradation of the anaphase inhibitor protein Pds1p and in degradation of the cyclin-dependent kinase inhibitor Sic1p, a target of SCF. Thus, Yrb1p is crucial for efficient APC- and SCF-mediated proteolysis of important cell cycle regulatory proteins. We have identified the UBS1 gene as a multicopy suppressor of yrb1-51 mutants. Ubs1p is a nuclear protein, and its deletion is synthetic lethal with a yrb1-51 mutation. Interestingly, UBS1 was previously identified as a multicopy suppressor of cdc34-2 mutants, which are defective in SCF activity. We suggest that Ubs1p may represent a link between nucleocytoplasmic transport and ubiquitin ligase activity.     

Ran GTPase: a master regulator of nuclear structure and function during the eukaryotic cell division cycle?

Ran is an abundant GTPase that is highly conserved in eukaryotic cells and has been implicated in many aspects of nuclear structure and function, especially determining the directionality of nucleocytoplasmic transport during interphase. However, cell-free systems have recently shown that Ran plays distinct roles in mitotic spindle assembly and nuclear envelope (NE) formation in vitro. During spindle assembly, Ran controls the formation of complexes with importins, the same effectors that control nucleocytoplasmic transport. Here, we review these advances and discuss a general model for Ran in the coordination of nuclear processes throughout the cell division cycle via common biochemical mechanisms.     

Fission yeast cells undergo nuclear division in the absence of spindle microtubules

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.     

Schizosaccharomyces pombe Bir1p, a nuclear protein that localizes to kinetochores and the spindle midzone, is essential for chromosome condensation and spindle elongation during mitosis

The inhibitor of apoptosis (IAP) family of proteins contains a subset of members characterized by the presence of highly conserved baculoviral IAP repeat (BIR) domains. Recent work has shown that some of these BIR-domain proteins play a prominent role in the regulation of cell division, in particular at the stage of chromosome segregation and cytokinesis. We and others have shown that the Schizosaccharomyces pombe BIR-domain protein, Bir1p/Pbh1p/Cut17p, is important for the regulation of mitosis. Here we further characterize S. pombe Bir1p using methods of cell biology and genetics. We show that Bir1p is dispersed throughout the nucleus during the cell cycle. In addition, a significant part of Bir1p is also detected at the kinetochores and the spindle midzone during mitosis and meiosis. Time-lapse microscopy studies suggest that Bir1p relocates from the kinetochores to the spindle at the end of anaphase A. Bir1p colocalizes with the S. pombe Aurora kinase homolog Aim1p, a protein essential for mitosis, at the kinetochores as well as the spindle midzone during mitosis, and functional Bir1p is essential for localization of Aim1p to the kinetochores and the spindle midzone. Analyses of bir1 conditional mutants revealed that Bir1p is essential for chromosome condensation during mitosis. In addition, anaphase cells show the presence of lagging chromosomes and a defect in spindle elongation. We conclude that Bir1p is important for multiple processes that occur during mitosis in S. pombe.