Recruitment of NIMA kinase shows that maturation of the S. pombe spindle-pole body occurs over consecutive cell cycles and reveals a role for NIMA in modulating SIN activity

Transient PP2A SIP complex localization to mitotic SPBs for SIN inhibition is mediated solely by the Csc1 FHA domain

Many organisms utilize an actin- and myosin-based cytokinetic ring (CR) to help complete cytokinesis. In Schizosaccharomyces pombe, the Septation Initiation Network (SIN) promotes proper CR function and stability. The SIN is a conserved and essential signaling network consisting of a GTPase and a cascade of kinases assembled at the spindle pole body (SPB). The PP2A SIN inhibitory phosphatase (SIP) complex related to the STRIPAK phosphatase complex is one inhibitor of SIN signaling. The SIP consists of Csc1, Csc2, Csc3, Csc4, Paa1, and the phosphatase subunit Ppa3. Here, we determine that the SIP is anchored at the SPB via the Csc1 FHA domain and that constitutive SPB localization of the SIP is lethal due to persistent SIN inhibition. Disrupting SIP docking at the SPB with a point mutation within the FHA domain or eliminating phosphatase activity by introducing a point mutation within Ppa3 resulted in intact SIP complexes without SIN inhibitory function. Lastly, we defined the unique features of Ppa3 that allow it, but not two other PP2A catalytic subunits, to incorporate into the SIP. Overall, we provide insight into how the SIP complex assembles, localizes, and functions to counteract the SIN with spatiotemporal precision during cytokinesis.

A role for the carbon source of the cell and protein kinase A in regulating the S. pombe septation initiation network

The septation initiation network (SIN) is a conserved signal transduction network, which is important for cytokinesis in Schizosaccharomyces pombe. The SIN component Etd1p is required for association of some SIN proteins with the spindle pole body (SPB) during anaphase and for contractile ring formation. We show that tethering of Cdc7p or Sid1p to the SIN scaffold Cdc11p at the SPB, rescues etd1-Δ. Analysis of a suppressor of the mutant etd1-M9 revealed that SIN signalling is influenced by the carbon source of the cell. Growth on a non-fermentable carbon source glycerol reduces the requirement for SIN signalling but does not bypass it. The decreased need for SIN signalling is mediated largely by reduction of protein kinase A activity, and it is phenocopied by deletion of pka1 on glucose medium. We conclude that protein kinase A is an important regulator of the SIN, and that SIN signalling is regulated by the carbon source of the cell.

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.

Quantitative analysis of nuclear pore complex organization in Schizosaccharomyces pombe

Characterization of nuclear pores in Schizosaccharomyces pombe identifies regions of heterogeneous NPC density and composition and shows that NPCs are excluded near the spindle pole body by Lem2-mediated centromere tethering.

The number, distribution, and composition of nuclear pore complexes (NPCs) in the nuclear envelope varies between cell types and changes during cellular differentiation and in disease. To understand how NPC density and organization are controlled, we analyzed the NPC number and distribution in the fission yeast Schizosaccharomyces pombe using structured illumination microscopy. The small size of yeast nuclei, genetic features of fungi, and our robust image analysis pipeline allowed us to study NPCs in intact nuclei under multiple conditions. Our data revealed that NPC density is maintained across a wide range of nuclear sizes. Regions of reduced NPC density are observed over the nucleolus and surrounding the spindle pole body (SPB). Lem2-mediated tethering of the centromeres to the SPB is required to maintain NPC exclusion near SPBs. These findings provide a quantitative understanding of NPC number and distribution in S. pombe and show that interactions between the centromere and the nuclear envelope influences local NPC distribution.

The phosphatase inhibitor Sds23 promotes symmetric spindle positioning in fission yeast

A hallmark of cell division in eukaryotic cells is the formation and elongation of a microtubule (MT)-based mitotic spindle. Proper positioning of the spindle is critical to ensure equal segregation of the genetic material to the resulting daughter cells. Both the timing of spindle elongation and constriction of the actomyosin contractile ring must be precisely coordinated to prevent missegregation or damage to the genetic material during cellular division. Here, we show that Sds23, an inhibitor of protein phosphatases, contributes to proper positioning of elongating spindles in fission yeast cells. We found that sds23∆ mutant cells exhibit asymmetric spindles that initially elongate asymmetrically toward one end of the dividing cell. Spindle asymmetry in sds23∆ cells results from a defect that is distinct from previously identified mechanisms, including MT protrusions and enlarged vacuoles. Combined with our previous work, this study demonstrates that Sds23, an inhibitor of PP2A-family protein phosphatases, promotes proper positioning of both the bipolar spindle and cytokinetic ring during fission yeast cell division. These two steps ensure the overall symmetry and fidelity of the cell division process.

MOB: Pivotal Conserved Proteins in Cytokinesis, Cell Architecture and Tissue Homeostasis

The MOB family proteins are constituted by highly conserved eukaryote kinase signal adaptors that are often essential both for cell and organism survival. Historically, MOB family proteins have been described as kinase activators participating in Hippo and Mitotic Exit Network/ Septation Initiation Network (MEN/SIN) signaling pathways that have central roles in regulating cytokinesis, cell polarity, cell proliferation and cell fate to control organ growth and regeneration. In metazoans, MOB proteins act as central signal adaptors of the core kinase module MST1/2, LATS1/2, and NDR1/2 kinases that phosphorylate the YAP/TAZ transcriptional co-activators, effectors of the Hippo signaling pathway. More recently, MOBs have been shown to also have non-kinase partners and to be involved in cilia biology, indicating that its activity and regulation is more diverse than expected. In this review, we explore the possible ancestral role of MEN/SIN pathways on the built-in nature of a more complex and functionally expanded Hippo pathway, by focusing on the most conserved components of these pathways, the MOB proteins. We discuss the current knowledge of MOBs-regulated signaling, with emphasis on its evolutionary history and role in morphogenesis, cytokinesis, and cell polarity from unicellular to multicellular organisms.

Microtubule nucleation promoters Mto1 and Mto2 regulate cytokinesis in fission yeast

Microtubules of the mitotic spindle direct cytokinesis in metazoans but this has not been documented in fungi. We report evidence that microtubule nucleators at the spindle pole body help coordinate cytokinetic furrow formation in fission yeast. The temperature-sensitive cps1-191 strain (Liu et al., 1999) with a D277N substitution in β-glucan synthase 1 (Cps1/Bgs1) was reported to arrest with an unconstricted contractile ring. We discovered that contractile rings in cps1-191 cells constrict slowly and that an mto2S338N mutation is required with the bgs1D277Nmutation to reproduce the cps1-191 phenotype. Complexes of Mto2 and Mto1 with γ-tubulin regulate microtubule assembly. Deletion of Mto1 along with the bgs1D277N mutation also gives the cps1-191 phenotype, which is not observed in mto2S338N or mto1Δ cells expressing bgs1+. Both mto2S338N and mto1Δ cells nucleate fewer astral microtubules than normal and have higher levels of Rho1-GTP at the division site than wild-type cells. We report multiple conditions that sensitize mto1Δ and mto2S338N cells to furrow ingression phenotypes.

Involvement of the septation initiation network in events during cytokinesis in fission yeast

The septation initiation network (SIN), comprising a GTPase and a cascade of three protein kinases, regulates cell division in fission yeast Schizosaccharomyces pombe, but questions remain about its influence on cytokinesis. Here, we made quantitative measurements of the numbers of Cdc7p kinase molecules (a marker for SIN activity) on spindle pole bodies (SPBs), and on the timing of assembly, maturation and constriction of contractile rings via six different proteins tagged with fluorescent proteins. When SIN activity is low in spg1-106 mutant cells at 32°C, cytokinetic nodes formed contractile rings ∼3 min slower than wild-type cells. During the maturation period, these rings maintained normal levels of the myosin-II mEGFP-Myo2p but accumulated less of the F-BAR protein Cdc15p-GFP than in wild-type cells. The Cdc15p-GFP fluorescence then disintegrated into spots as mEGFP-Myo2p dissociated slowly. Some rings started to constrict at the normal time, but most failed to complete constriction. When high SIN activity persists far longer than normal on both SPBs in cdc16-116 mutant cells at 32°C, contractile rings assembled and constricted normally, but disassembled slowly, delaying cell separation.

A checkpoint-independent mechanism delays entry into mitosis after UV irradiation

When cells are exposed to stress they delay entry into mitosis. The most extensively studied mechanism behind this delay is the DNA-damage-induced G2/M checkpoint. Here, we show the existence of an additional stress-response pathway in Schizosaccharomyces pombe that is independent of the classic ATR/Rad3-dependent checkpoint. This novel mechanism delays entry mitosis independently of the spindle assembly checkpoint and the mitotic kinases Fin1, Ark1 and Plo1. The pathway delays activation of the mitotic cyclin-dependent kinase (CDK) Cdc2 after UV irradiation. Furthermore, we demonstrate that translation of the mitotic cyclin Cdc13 is selectively downregulated after UV irradiation, and we propose that this downregulation of Cdc13 contributes to the delayed activation of Cdc2 and the delayed mitosis.

Live Cell Imaging in Fission Yeast

Live cell imaging complements the array of biochemical and molecular genetic approaches to provide a comprehensive insight into functional dependencies and molecular interactions in fission yeast. Fluorescent proteins and vital dyes reveal dynamic changes in the spatial distribution of organelles and the proteome and how each alters in response to changes in environmental and genetic composition. This introduction discusses key issues and basic image analysis for live cell imaging of fission yeast.

Dialogue between centrosomal entrance and exit scaffold pathways regulates mitotic commitment

The fission yeast scaffold molecule Sid4 anchors the septum initiation network to the spindle pole body (SPB, centrosome equivalent) to control mitotic exit events. A second SPB-associated scaffold, Cut12, promotes SPB-associated Cdk1-cyclin B to drive mitotic commitment. Signals emanating from each scaffold have been assumed to operate independently to promote two distinct outcomes. We now find that signals from Sid4 contribute to the Cut12 mitotic commitment switch. Specifically, phosphorylation of Sid4 by NIMAFin1 reduces Sid4 affinity for its SPB anchor, Ppc89, while also enhancing Sid4's affinity for casein kinase 1δ (CK1δ). The resulting phosphorylation of Sid4 by the newly docked CK1δ recruits Chk2Cds1 to Sid4. Chk2Cds1 then expels the Cdk1-cyclin B antagonistic phosphatase Flp1/Clp1 from the SPB. Flp1/Clp1 departure can then support mitotic commitment when Cdk1-cyclin B activation at the SPB is compromised by reduction of Cut12 function. Such integration of signals emanating from neighboring scaffolds shows how centrosomes/SPBs can integrate inputs from multiple pathways to control cell fate.

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.

Analysis of the Schizosaccharomyces pombe Cell Cycle

Schizosaccharomyces pombe cells are rod shaped, and they grow by tip elongation. Growth ceases during mitosis and cell division; therefore, the length of a septated cell is a direct measure of the timing of mitotic commitment, and the length of a wild-type cell is an indicator of its position in the cell cycle. A large number of documented stage-specific changes can be used as landmarks to characterize cell cycle progression under specific experimental conditions. Conditional mutations can permanently or transiently block the cell cycle at almost any stage. Large, synchronously dividing cell populations, essential for the biochemical analysis of cell cycle events, can be generated by induction synchrony (arrest-release of a cell cycle mutant) or selection synchrony (centrifugal elutriation or lactose-gradient centrifugation). Schizosaccharomyces pombe cell cycle studies routinely combine particular markers, mutants, and synchronization procedures to manipulate the cycle. We describe these techniques and list key landmarks in the fission yeast mitotic cell division cycle.

Growth and the Environment of Schizosaccharomyces pombe

Here, we summarize the composition and uses of Schizosaccharomyces pombe media and discuss key issues for consideration in the generation of S. pombe cultures. We discuss the concept of "culture memory," in which the growth state and stress experienced by a strain during storage, propagation, and starter culture preparation can alter experimental outcomes at later stages. We also describe the triggers that are widely used to manipulate signaling through the environment sensing pathways.

An Extended, Boolean Model of the Septation Initiation Network in S.Pombe Provides Insights into Its Regulation

Cytokinesis in fission yeast is controlled by the Septation Initiation Network (SIN), a protein kinase signaling network using the spindle pole body as scaffold. In order to describe the qualitative behavior of the system and predict unknown mutant behaviors we decided to adopt a Boolean modeling approach. In this paper, we report the construction of an extended, Boolean model of the SIN, comprising most SIN components and regulators as individual, experimentally testable nodes. The model uses CDK activity levels as control nodes for the simulation of SIN related events in different stages of the cell cycle. The model was optimized using single knock-out experiments of known phenotypic effect as a training set, and was able to correctly predict a double knock-out test set. Moreover, the model has made in silico predictions that have been validated in vivo, providing new insights into the regulation and hierarchical organization of the SIN.

Pombe's thirteen - control of fission yeast cell division by the septation initiation network

The septation initiation network (SIN) regulates aspects of cell growth and division in Schizosaccharomyces pombe and is essential for cytokinesis. Insufficient signalling results in improper assembly of the contractile ring and failure of cytokinesis, generating multinucleated cells, whereas too much SIN signalling uncouples cytokinesis from the rest of the cell cycle. SIN signalling is therefore tightly controlled to coordinate cytokinesis with chromosome segregation. Signalling originates from the cytoplasmic face of the spindle pole body (SPB), and asymmetric localisation of some SIN proteins to one of the two SPBs during mitosis is important for regulation of the SIN. Recent studies have identified in vivo substrates of the SIN, which include components involved in mitotic control, those of the contractile ring and elements of the signalling pathway regulating polarised growth. The SIN is also required for spore formation following meiosis. This has provided insights into how the SIN performs its diverse functions in the cell cycle and shed new light on its regulation.

The centrosome and its duplication cycle

The centrosome was discovered in the late 19th century when mitosis was first described. Long recognized as a key organelle of the spindle pole, its core component, the centriole, was realized more than 50 or so years later also to comprise the basal body of the cilium. Here, we chart the more recent acquisition of a molecular understanding of centrosome structure and function. The strategies for gaining such knowledge were quickly developed in the yeasts to decipher the structure and function of their distinctive spindle pole bodies. Only within the past decade have studies with model eukaryotes and cultured cells brought a similar degree of sophistication to our understanding of the centrosome duplication cycle and the multiple roles of this organelle and its component parts in cell division and signaling. Now as we begin to understand these functions in the context of development, the way is being opened up for studies of the roles of centrosomes in human disease.

Analysis of S. pombe SIN protein association to the SPB reveals two genetically separable states of the SIN

The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis, and asymmetric association of SIN proteins with the mitotic spindle pole bodies (SPBs) is important for its regulation. Here, we have used semi-automated image analysis to study SIN proteins in large numbers of wild-type and mutant cells. Our principal conclusions are: first, that the association of Cdc7p with the SPBs in early mitosis is frequently asymmetric, with a bias in favour of the new SPB; second, that the early association of Cdc7p-GFP to the SPB depends on Plo1p but not Spg1p, and is unaffected by mutations that influence its asymmetry in anaphase; third, that Cdc7p asymmetry in anaphase B is delayed by Pom1p and by activation of the spindle assembly checkpoint, and is promoted by Rad24p; and fourth, that the length of the spindle, expressed as a fraction of the length of the cell, at which Cdc7p becomes asymmetric is similar in cells dividing at different sizes. These data reveal that multiple regulatory mechanisms control the SIN in mitosis and lead us to propose a two-state model to describe the SIN.

Kinesin-14 and kinesin-5 antagonistically regulate microtubule nucleation by γ-TuRC in yeast and human cells

Bipolar spindle assembly is a critical control point for initiation of mitosis through nucleation and organization of spindle microtubules and is regulated by kinesin-like proteins. In fission yeast, the kinesin-14 Pkl1 binds the γ-tubulin ring complex (γ-TuRC) microtubule-organizing centre at spindle poles and can alter its structure and function. Here we show that kinesin-14 blocks microtubule nucleation in yeast and reveal that this inhibition is countered by the kinesin-5 protein, Cut7. Furthermore, we demonstrate that Cut7 binding to γ-TuRC and the Cut7 BimC domain are both required for inhibition of Pkl1. We also demonstrate that a yeast kinesin-14 peptide blocks microtubule nucleation in two human breast cancer cell lines, suggesting that this mechanism is evolutionarily conserved. In conclusion, using genetic, biochemical and cell biology approaches we uncover antagonistic control of microtubule nucleation at γ-TuRC by two kinesin-like proteins, which may represent an attractive anti-mitotic target for cancer therapies.

Mitotic spindle assembly requires strict control of microtubule nucleation by γ-tubulin ring complexes. Olmsted et al. report that the kinesin-like proteins Pkl1 and Cut7 antagonistically regulate nucleation in fission yeast, and show that a Pkl1 peptide blocks spindle assembly in human cancer cells.

Tts1, the fission yeast homologue of the TMEM33 family, functions in NE remodeling during mitosis

The evolutionarily conserved eukaryotic TMEM33-family protein Tts1 functions in promoting mitotic spindle pole body–nuclear envelope insertion and modulating mitotic nuclear pore complex distribution in Schizosaccharomyces pombe. The structural features of Tts1 important for its function in distinct aspects of membrane remodeling are identified.

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.

Regulation of spindle pole body assembly and cytokinesis by the centrin-binding protein Sfi1 in fission yeast

A previous model suggested doubling of Sfi1 as the first step of SPB assembly. Here it is shown that Sfi1 is gradually recruited to SPBs throughout the cell cycle. Conserved tryptophans in Sfi1 are required for its equal partitioning during mitosis, and unequal partitioning of Sfi1 underlies SPB assembly and mitotic defects in the next cell cycle.

Centrosomes play critical roles in the cell division cycle and ciliogenesis. Sfi1 is a centrin-binding protein conserved from yeast to humans. Budding yeast Sfi1 is essential for the initiation of spindle pole body (SPB; yeast centrosome) duplication. However, the recruitment and partitioning of Sfi1 to centrosomal structures have never been fully investigated in any organism, and the presumed importance of the conserved tryptophans in the internal repeats of Sfi1 remains untested. Here we report that in fission yeast, instead of doubling abruptly at the initiation of SPB duplication and remaining at a constant level thereafter, Sfi1 is gradually recruited to SPBs throughout the cell cycle. Like an sfi1Δ mutant, a Trp-to-Arg mutant (sfi1-M46) forms monopolar spindles and exhibits mitosis and cytokinesis defects. Sfi1-M46 protein associates preferentially with one of the two daughter SPBs during mitosis, resulting in a failure of new SPB assembly in the SPB receiving insufficient Sfi1. Although all five conserved tryptophans tested are involved in Sfi1 partitioning, the importance of the individual repeats in Sfi1 differs. In summary, our results reveal a link between the conserved tryptophans and Sfi1 partitioning and suggest a revision of the model for SPB assembly.

The KASH protein Kms2 coordinates mitotic remodeling of the spindle pole body

Defects in the biogenesis of the spindle pole body (SPB), the yeast centrosome equivalent, can lead to monopolar spindles and mitotic catastrophe. The KASH domain protein Kms2 and the SUN domain protein Sad1 colocalize within the nuclear envelope at the site of SPB attachment during interphase and at the spindle poles during mitosis in Schizosaccharomyces pombe. We show that Kms2 interacts with the essential SPB components Cut12 and Pcp1 and the Polo kinase Plo1. Depletion of Kms2 delays mitotic entry and leads to defects in the insertion of the SPB into the nuclear envelope, disrupting stable bipolar spindle formation. These effects are mediated in part by a delay in the recruitment of Plo1 to the SPB at mitotic entry. Plo1 activity supports mitotic SPB remodeling by driving a burst of incorporation of Cut12 and Pcp1. Thus, a fission yeast SUN-KASH complex plays an important role in supporting the remodeling of the SPB at mitotic entry.

Cdk1 promotes cytokinesis in fission yeast through activation of the septation initiation network

In Schizosaccharomyces pombe, late mitotic events are coordinated with cytokinesis by the septation initiation network (SIN), an essential spindle pole body (SPB)-associated kinase cascade, which controls the formation, maintenance, and constriction of the cytokinetic ring. It is not fully understood how SIN initiation is temporally regulated, but it depends on the activation of the GTPase Spg1, which is inhibited during interphase by the essential bipartite GTPase-activating protein Byr4-Cdc16. Cells are particularly sensitive to the modulation of Byr4, which undergoes cell cycle-dependent phosphorylation presumed to regulate its function. Polo-like kinase, which promotes SIN activation, is partially responsible for Byr4 phosphorylation. Here we show that Byr4 is also controlled by cyclin-dependent kinase (Cdk1)-mediated phosphorylation. A Cdk1 nonphosphorylatable Byr4 phosphomutant displays severe cell division defects, including the formation of elongated, multinucleate cells, failure to maintain the cytokinetic ring, and compromised SPB association of the SIN kinase Cdc7. Our analyses show that Cdk1-mediated phosphoregulation of Byr4 facilitates complete removal of Byr4 from metaphase SPBs in concert with Plo1, revealing an unexpected role for Cdk1 in promoting cytokinesis through activation of the SIN pathway.

Extending the Schizosaccharomyces pombe molecular genetic toolbox

Targeted alteration of the genome lies at the heart of the exploitation of S. pombe as a model system. The rate of analysis is often determined by the efficiency with which a target locus can be manipulated. For most loci this is not a problem, however for some loci, such as fin1+, rates of gene targeting below 5% can limit the scope and scale of manipulations that are feasible within a reasonable time frame. We now describe a simple modification of transformation procedure for directing integration of genomic sequences that leads to a 5-fold increase in the transformation efficiency when antibiotic based dominant selection markers are used. We also show that removal of the pku70+ and pku80+ genes, which encode DNA end binding proteins required for the non-homologous end joining DNA repair pathway, increases the efficiency of gene targeting at fin1+ to around 75-80% (a 16-fold increase). We describe how a natMX6/rpl42+ cassette can be used for positive and negative selection for integration at a targeted locus. To facilitate the evaluation of the impact of a series of mutations on the function of a gene of interest we have generated three vector series that rely upon different selectable markers to direct the expression of tagged/untagged molecules from distinct genomic integration sites. pINTL and pINTK vectors use ura4+ selection to direct disruptive integration of leu1+ and lys1+ respectively, while pINTH vectors exploit nourseothricin resistance to detect the targeted disruption of a hygromycin B resistance conferring hphMX6 cassette that has been integrated on chromosome III. Finally, we have generated a series of multi-copy expression vectors that use resistance to nourseothricin or kanamycin/G418 to select for propagation in prototrophic hosts. Collectively these protocol modifications and vectors extend the versatility of this key model system.

Measuring affinities of fission yeast spindle pole body proteins in live cells across the cell cycle

Characterizing protein-protein interactions is essential for understanding molecular mechanisms, although reproducing cellular conditions in vitro is challenging and some proteins are difficult to purify. We developed a method to measure binding to cellular structures using fission yeast cells as reaction vessels. We varied the concentrations of Sid2p and Mob1p (proteins of the septation initiation network) and measured their binding to spindle pole bodies (SPBs), the centrosome equivalent of yeast. From our measurements we infer that Sid2p and Mob1p both exist as monomeric, heterodimeric, and homodimeric species throughout the cell cycle. During interphase these species have widely different affinities for their common receptor Cdc11p on the SPB. The data support a model with a subset of Cdc11p binding the heterodimeric species with a Kd < 0.1 μM when Sid2p binds Mob1p-Cdc11p and Kd in the micromolar range when Mob1p binds Sid2p-Cdc11p. During mitosis an additional species presumed to be the phosphorylated Sid2p-Mob1p heterodimer binds SPBs with a lower affinity. Homodimers of Sid2p or Mob1p bind to the rest of Cdc11p at SPBs with lower affinity: Kds > 10 μM during interphase and somewhat stronger during mitosis. These measurements allowed us to account for the fluctuations in Sid2p binding to SPBs throughout the cell cycle.

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

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

Dma1-dependent degradation of Septation Initiation Network proteins during meiosis in Schizosaccharomyces pombe

The Schizosaccharomyces pombe septation initiation network (SIN) is required for cytokinesis during vegetative growth and spore formation during meiosis. Regulation of the SIN during mitosis has been studied extensively, but less is known about its meiotic regulation. Here, we show that several aspects of the SIN regulation differ between mitosis and meiosis. First, the presence of GTP-bound spg1p is not the main determinant of the timing of cdc7p and sid1p association with the SPB during meiosis. Second, the localisation dependencies of SIN proteins differ from those in mitotic cells, suggesting a modified functional organisation of the SIN during meiosis. Third, there is stage-specific degradation of SIN components in meiosis; byr4p is degraded after meiosis I, while the degradation of cdc7p, cdc11p and sid4p occurs after the second meiotic division and depends upon the ubiquitin ligase dma1p. Finally, dma1p-dependent degradation is not restricted to the SIN, for we show that dma1p is needed for the degradation of mcp6p/hrs1p in meiosis I. Together, these data suggest that stage-specific targetted proteolysis will play an important role in regulating meiotic progression.

Fission yeast does not age under favorable conditions, but does so after stress

BACKGROUND

Many unicellular organisms age: as time passes, they divide more slowly and ultimately die. In budding yeast, asymmetric segregation of cellular damage results in aging mother cells and rejuvenated daughters. We hypothesize that the organisms in which this asymmetry is lacking, or can be modulated, may not undergo aging.

RESULTS

We performed a complete pedigree analysis of microcolonies of the fission yeast Schizosaccharomyces pombe growing from a single cell. When cells were grown under favorable conditions, none of the lineages exhibited aging, which is defined as a consecutive increase in division time and increased death probability. Under favorable conditions, few cells died, and their death was random and sudden rather than following a gradual increase in division time. Cell death correlated with the inheritance of Hsp104-associated protein aggregates. After stress, the cells that inherited large aggregates aged, showing a consecutive increase in division time and an increased death probability. Their sisters, who inherited little or no aggregates, did not age.

CONCLUSIONS

We conclude that S. pombe does not age under favorable growth conditions, but does so under stress. This transition appears to be passive rather than active and results from the formation of a single large aggregate, which segregates asymmetrically at the subsequent cell division. We argue that this damage-induced asymmetric segregation has evolved to sacrifice some cells so that others may survive unscathed after severe environmental stresses.

Dynamics of SIN asymmetry establishment

Timing of cell division is coordinated by the Septation Initiation Network (SIN) in fission yeast. SIN activation is initiated at the two spindle pole bodies (SPB) of the cell in metaphase, but only one of these SPBs contains an active SIN in anaphase, while SIN is inactivated in the other by the Cdc16-Byr4 GAP complex. Most of the factors that are needed for such asymmetry establishment have been already characterized, but we lack the molecular details that drive such quick asymmetric distribution of molecules at the two SPBs. Here we investigate the problem by computational modeling and, after establishing a minimal system with two antagonists that can drive reliable asymmetry establishment, we incorporate the current knowledge on the basic SIN regulators into an extended model with molecular details of the key regulators. The model can capture several peculiar earlier experimental findings and also predicts the behavior of double and triple SIN mutants. We experimentally tested one prediction, that phosphorylation of the scaffold protein Cdc11 by a SIN kinase and the core cell cycle regulatory Cyclin dependent kinase (Cdk) can compensate for mutations in the SIN inhibitor Cdc16 with different efficiencies. One aspect of the prediction failed, highlighting a potential hole in our current knowledge. Further experimental tests revealed that SIN induced Cdc11 phosphorylation might have two separate effects. We conclude that SIN asymmetry is established by the antagonistic interactions between SIN and its inhibitor Cdc16-Byr4, partially through the regulation of Cdc11 phosphorylation states.

Cell cycle regulation by the NEK family of protein kinases

Genetic screens for cell division cycle mutants in the filamentous fungus Aspergillus nidulans led to the discovery of never-in-mitosis A (NIMA), a serine/threonine kinase that is required for mitotic entry. Since that discovery, NIMA-related kinases, or NEKs, have been identified in most eukaryotes, including humans where eleven genetically distinct proteins named NEK1 to NEK11 are expressed. Although there is no evidence that human NEKs are essential for mitotic entry, it is clear that several NEK family members have important roles in cell cycle control. In particular, NEK2, NEK6, NEK7 and NEK9 contribute to the establishment of the microtubule-based mitotic spindle, whereas NEK1, NEK10 and NEK11 have been implicated in the DNA damage response. Roles for NEKs in other aspects of mitotic progression, such as chromatin condensation, nuclear envelope breakdown, spindle assembly checkpoint signalling and cytokinesis have also been proposed. Interestingly, NEK1 and NEK8 also function within cilia, the microtubule-based structures that are nucleated from basal bodies. This has led to the current hypothesis that NEKs have evolved to coordinate microtubule-dependent processes in both dividing and non-dividing cells. Here, we review the functions of the human NEKs, with particular emphasis on those family members that are involved in cell cycle control, and consider their potential as therapeutic targets in cancer.

Functional repurposing revealed by comparing S. pombe and S. cerevisiae genetic interactions

We present a genetic interaction map of pairwise measures including ∼40% of nonessential S. pombe genes. By comparing interaction maps for fission and budding yeast, we confirmed widespread conservation of genetic relationships within and between complexes and pathways. However, we identified an important subset of orthologous complexes that have undergone functional "repurposing": the evolution of divergent functions and partnerships. We validated three functional repurposing events in S. pombe and mammalian cells and discovered that (1) two lumenal sensors of misfolded ER proteins, the kinase/nuclease Ire1 and the glucosyltransferase Gpt1, act together to mount an ER stress response; (2) ESCRT factors regulate spindle-pole-body duplication; and (3) a membrane-protein phosphatase and kinase complex, the STRIPAK complex, bridges the cis-Golgi, the centrosome, and the outer nuclear membrane to direct mitotic progression. Each discovery opens new areas of inquiry and-together-have implications for model organism-based research and the evolution of genetic systems.

Polar opposites: Fine-tuning cytokinesis through SIN asymmetry

Mitotic exit and cell division must be spatially and temporally integrated to facilitate equal division of genetic material between daughter cells. In the fission yeast, Schizosaccharomyces pombe, a spindle pole body (SPB) localized signaling cascade termed the septation initiation network (SIN) couples mitotic exit with cytokinesis. The SIN is controlled at many levels to ensure that cytokinesis is executed once per cell cycle and only after cells segregate their DNA. An interesting facet of the SIN is that its activity is asymmetric on the two SPBs during anaphase; however, how and why the SIN is asymmetric has remained elusive. Many key factors controlling SIN asymmetry have now been identified, shedding light on the significance of SIN asymmetry in regulating cytokinesis. In this review, we highlight recent advances in our understanding of SIN regulation, with an emphasis on how SIN asymmetry is achieved and how this aspect of SIN regulation fine-tunes cytokinesis.

The S. pombe cytokinesis NDR kinase Sid2 activates Fin1 NIMA kinase to control mitotic commitment through Pom1/Wee1

Mitotic exit integrates the reversal of the phosphorylation events initiated by mitotic kinases with a controlled cytokinesis event that cleaves the cell in two. The Mitotic Exit Network (MEN) of budding yeast regulates both processes, while the fission yeast equivalent, the Septum Initiation Network (SIN), only controls the execution of cytokinesis. The components and architecture of the SIN and MEN are highly conserved¹. It is currently assumed that the functions of the core SIN/MEN components are restricted to their characterised roles at the end of mitosis. We now show that the NDR kinase component of the fission yeast SIN, Sid2/Mob1, acts independently of the other known SIN components in G2 phase of the cell cycle to control the timing of mitotic commitment. Sid2/Mob1 promotes mitotic commitment by directly activating the NIMA related kinase Fin1. Fin1’s activation promotes its own destruction, thereby making Fin1 activation a transient feature of G2 phase. This spike of Fin1 activation modulates the activity of the Pom1/Cdr1/Cdr2 geometry network towards Wee1.

The fission yeast septation initiation network (SIN) kinase, Sid2, is required for SIN asymmetry and regulates the SIN scaffold, Cdc11

Some components of the fission yeast septation initiation network (SIN) localize asymmetrically to spindle pole bodies during anaphase. Symmetric localization of these proteins correlates with cytokinesis defects. It is shown that the SIN-kinase Sid2 mediates SIN asymmetry, in part via the scaffold Cdc11, revealing a previously unknown feedback loop operating to generate SIN asymmetry.

The Schizosaccharomyces pombe septation initiation network (SIN) is an Spg1-GTPase–mediated protein kinase cascade that triggers actomyosin ring constriction, septation, and cell division. The SIN is assembled at the spindle pole body (SPB) on the scaffold proteins Cdc11 and Sid4, with Cdc11 binding directly to SIN signaling components. Proficient SIN activity requires the asymmetric distribution of its signaling components to one of the two SPBs during anaphase, and Cdc11 hyperphosphorylation correlates with proficient SIN activity. In this paper, we show that the last protein kinase in the signaling cascade, Sid2, feeds back to phosphorylate Cdc11 during mitosis. The characterization of Cdc11 phosphomutants provides evidence that Sid2-mediated Cdc11 phosphorylation promotes the association of the SIN kinase, Cdc7, with the SPB and maximum SIN signaling during anaphase. We also show that Sid2 is crucial for the establishment of SIN asymmetry, indicating a positive-feedback loop is an important element of the SIN.

H₂O₂ stress-specific regulation of S. pombe MAPK Sty1 by mitochondrial protein phosphatase Ptc4

In fission yeast, the stress-activated MAP kinase, Sty1, is activated via phosphorylation upon exposure to stress and orchestrates an appropriate response. Its activity is attenuated by either serine/threonine PP2C or tyrosine phosphatases. Here, we found that the PP2C phosphatase, Ptc4, plays an important role in inactivating Sty1 specifically upon oxidative stress. Sty1 activity remains high in a ptc4 deletion mutant upon H(2)O(2) but not under other types of stress. Surprisingly, Ptc4 localizes to the mitochondria and is targeted there by an N-terminal mitochondrial targeting sequence (MTS), which is cleaved upon import. A fraction of Sty1 also localizes to the mitochondria suggesting that Ptc4 attenuates the activity of a mitochondrial pool of this MAPK. Cleavage of the Ptc4 MTS is greatly reduced specifically upon H(2)O(2), resulting in the full-length form of the phosphatase; this displays a stronger interaction with Sty1, thus suggesting a novel mechanism by which the negative regulation of MAPK signalling is controlled and providing an explanation for the oxidative stress-specific nature of the regulation of Sty1 by Ptc4.

Characterization of ypa1 and ypa2, the Schizosaccharomyces pombe orthologs of the peptidyl proyl isomerases that activate PP2A, reveals a role for Ypa2p in the regulation of cytokinesis

The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis. Cdc7p is the first kinase in the core SIN; we have screened genetically for SIN regulators by isolating cold-sensitive suppressors of cdc7-24. Our screen yielded a mutant in SPAC1782.05, one of the two fission yeast orthologs of mammalian phosphotyrosyl phosphatase activator. We have characterized this gene and its ortholog SPAC4F10.04, which we have named ypa2 and ypa1, respectively. We find that Ypa2p is the major form of protein phosphatase type 2A activator in S. pombe. A double ypa1-Δ ypa2-Δ null mutant is inviable, indicating that the two gene products have at least one essential overlapping function. Individually, the ypa1 and ypa2 genes are essential for survival only at low temperatures. The ypa2-Δ mutant divides at a reduced cell size and displays aberrant cell morphology and cytokinesis. Genetic analysis implicates Ypa2p as an inhibitor of the septation initiation network. We also isolated a cold-sensitive allele of ppa2, the major protein phosphatase type 2A catalytic subunit, implicating this enzyme as a regulator of the septation initiation network.

Brr6 drives the Schizosaccharomyces pombe spindle pole body nuclear envelope insertion/extrusion cycle

Insertion into and release of the cytoplasmic domain of the Schizosaccharomyces pombe spindle pole body from a nuclear envelope fenestra during mitosis requires Brr6.

The fission yeast interphase spindle pole body (SPB) is a bipartite structure in which a bulky cytoplasmic domain is separated from a nuclear component by the nuclear envelope. During mitosis, the SPB is incorporated into a fenestra that forms within the envelope during mitotic commitment. Closure of this fenestra during anaphase B/mitotic exit returns the cytoplasmic component to the cytoplasmic face of an intact interphase nuclear envelope. Here we show that Brr6 is transiently recruited to SPBs at both SPB insertion and extrusion. Brr6 is required for both SPB insertion and nuclear envelope integrity during anaphase B/mitotic exit. Genetic interactions with apq12 and defective sterol assimilation suggest that Brr6 may alter envelope composition at SPBs to promote SPB insertion and extrusion. The restriction of the Brr6 domain to eukaryotes that use a polar fenestra in an otherwise closed mitosis suggests a conserved role in fenestration to enable a single microtubule organizing center to nucleate both cytoplasmic and nuclear microtubules on opposing sides of the nuclear envelope.

Fission yeast Lem2 and Man1 perform fundamental functions of the animal cell nuclear lamina

In animal cells the nuclear lamina, which consists of lamins and lamin-associated proteins, serves several functions: it provides a structural scaffold for the nuclear envelope and tethers proteins and heterochromatin to the nuclear periphery. In yeast, proteins and large heterochromatic domains including telomeres are also peripherally localized, but there is no evidence that yeast have lamins or a fibrous nuclear envelope scaffold. Nonetheless, we found that the Lem2 and Man1 proteins of the fission yeast Schizosaccharomyces pombe, evolutionarily distant relatives of the Lap2/Emerin/Man1 (LEM) sub-family of animal cell lamin-associated proteins, perform fundamental functions of the animal cell lamina. These integral inner nuclear membrane localized proteins, with nuclear localized DNA binding Helix-Extension-Helix (HEH) domains, impact nuclear envelope structure and integrity, are essential for the enrichment of telomeres at the nuclear periphery and by means of their HEH domains anchor chromatin, most likely transcriptionally repressed heterochromatin, to the nuclear periphery. These data indicate that the core functions of the nuclear lamina are conserved between fungi and animal cells and can be performed in fission yeast, without lamins or other intermediate filament proteins.

Programmed fluctuations in sense/antisense transcript ratios drive sexual differentiation in S. pombe

Strand-specific RNA sequencing of S. pombe reveals a highly structured programme of ncRNA expression at over 600 loci. Functional investigations show that this extensive ncRNA landscape controls the complex programme of sexual differentiation in S. pombe.

The model eukaryote S. pombe features substantial numbers of ncRNAs many of which are antisense regulatory transcripts (ARTs), ncRNAs expressed on the opposing strand to coding sequences.

Individual ARTs are generated during the mitotic cycle, or at discrete stages of sexual differentiation to downregulate the levels of proteins that drive and coordinate sexual differentiation.

Antisense transcription occurring from events such as bidirectional transcription is not simply artefactual ‘chatter', it performs a critical role in regulating gene expression.

Regulation of the RNA profile is a principal control driving sexual differentiation in the fission yeast Schizosaccharomyces pombe. Before transcription, RNAi-mediated formation of heterochromatin is used to suppress expression, while post-transcription, regulation is achieved via the active stabilisation or destruction of transcripts, and through at least two distinct types of splicing control (Mata et al, 2002; Shimoseki and Shimoda, 2001; Averbeck et al, 2005; Mata and Bähler, 2006; Xue-Franzen et al, 2006; Moldon et al, 2008; Djupedal et al, 2009; Amorim et al, 2010; Grewal, 2010; Cremona et al, 2011).

Around 94% of the S. pombe genome is transcribed (Wilhelm et al, 2008). While many of these transcripts encode proteins (Wood et al, 2002; Bitton et al, 2011), the majority have no known function. We used a strand-specific protocol to sequence total RNA extracts taken from vegetatively growing cells, and at different points during a time course of sexual differentiation. The resulting data redefined existing gene coordinates and identified additional transcribed loci. The frequency of reads at each of these was used to monitor transcript abundance.

Transcript levels at 6599 loci changed in at least one sample (G-statistic; False Discovery Rate <5%). 4231 (72.3%), of which 4011 map to protein-coding genes, while 809 loci were antisense to a known gene. Comparisons between haploid and diploid strains identified changes in transcript levels at over 1000 loci.

At 354 loci, greater antisense abundance was observed relative to sense, in at least one sample (putative antisense regulatory transcripts—ARTs). Since antisense mechanisms are known to modulate sense transcript expression through a variety of inhibitory mechanisms (Faghihi and Wahlestedt, 2009), we postulated that the waves of antisense expression activated at different stages during meiosis might be regulating protein expression.

To ask whether transcription factors that drive sense-transcript levels influenced ART production, we performed RNA-seq of a pat1.114 diploid meiosis in the absence of the transcription factors Atf21 and Atf31 (responsible for late meiotic transcription; Mata et al, 2002). Transcript levels at 185 ncRNA loci showed significant changes in the knockout backgrounds. Although meiotic progression is largely unaffected by removal of Atf21 and Atf31, viability of the resulting spores was significantly diminished, indicating that Atf21- and Atf31-mediated events are critical to efficient sexual differentiation.

If changes to relative antisense/sense transcript levels during a particular phase of sexual differentiation were to regulate protein expression, then the continued presence of the antisense at points in the differentiation programme where it would normally be absent should abolish protein function during this phase. We tested this hypothesis at four loci representing the three means of antisense production: convergent gene expression, improper termination and nascent transcription from an independent locus. Induction of the natural antisense transcripts that opposed spo4⁺, spo6⁺ and dis1⁺ (Figures 3 and 7) in trans from a heterologous locus phenocopied a loss of function of the target protein. ART overexpression decreased Dis1 protein levels. Antisense transcription opposing spk1⁺ originated from improper termination of the sense ups1⁺ transcript on the opposite strand (Figure 3B, left locus). Expression of either the natural full-length ups1⁺ transcript or a truncated version, restricted to the portion of ups1⁺ overlapping spk1⁺ (Figure 3, orange transcripts) in trans from a heterologous locus phenocopied the spk1.Δ differentiation deficiency. Convergent transcription from a neighbouring gene on the opposing strand is, therefore, an effective mechanism to generate RNAi-mediated (below) silencing in fission yeast. Further analysis of the data revealed, for many loci, substantial changes in UTR length over the course of meiosis, suggesting that UTR dynamics may have an active role in regulating gene expression by controlling the transcriptional overlap between convergent adjacent gene pairs.

The RNAi machinery (Grewal, 2010) was required for antisense suppression at each of the dis1, spk1, spo4 and spo6 loci, as antisense to each locus had no impact in ago1.Δ, dcr1.Δ and rdp1.Δ backgrounds. We conclude that RNAi control has a key role in maintaining the fidelity of sexual differentiation in fission yeast. The histone H3 methyl transferase Clr4 was required for antisense control from a heterologous locus.

Thus, a significant portion of the impact of ncRNA upon sexual differentiation arises from antisense gene silencing. Importantly, in contrast to the extensively characterised ability of the RNAi machinery to operate in cis at a target locus in S. pombe (Grewal, 2010), each case of gene silencing generated here could be achieved in trans by expression of the antisense transcript from a single heterologous locus elsewhere in the genome.

Integration of an antibiotic marker gene immediately downstream of the dis1⁺ locus instigated antisense control in an orientation-dependent manner. PCR-based gene tagging approaches are widely used to fuse the coding sequences of epitope or protein tags to a gene of interest. Not only do these tagging approaches disrupt normal 3′UTR controls, but the insertion of a heterologous marker gene immediately downstream of an ORF can clearly have a significant impact upon transcriptional control of the resulting fusion protein. Thus, PCR tagging approaches can no longer be viewed as benign manipulations of a locus that only result in the production of a tagged protein product.

Repression of Dis1 function by gene deletion or antisense control revealed a key role this conserved microtubule regulator in driving the horsetail nuclear migrations that promote recombination during meiotic prophase.

Non-coding transcripts have often been viewed as simple ‘chatter', maintained solely because evolutionary pressures have not been strong enough to force their elimination from the system. Our data show that phenomena such as improper termination and bidirectional transcription are not simply interesting artifacts arising from the complexities of transcription or genome history, but have a critical role in regulating gene expression in the current genome. Given the widespread use of RNAi, it is reasonable to anticipate that future analyses will establish ARTs to have equal importance in other organisms, including vertebrates.

These data highlight the need to modify our concept of a gene from that of a spatially distinct locus. This view is becoming increasingly untenable. Not only are the 5′ and 3′ ends of many genes indistinct, but that this lack of a hard and fast boundary is actively used by cells to control the transcription of adjacent and overlapping loci, and thus to regulate critical events in the life of a cell.

Strand-specific RNA sequencing of S. pombe revealed a highly structured programme of ncRNA expression at over 600 loci. Waves of antisense transcription accompanied sexual differentiation. A substantial proportion of ncRNA arose from mechanisms previously considered to be largely artefactual, including improper 3′ termination and bidirectional transcription. Constitutive induction of the entire spk1⁺, spo4⁺, dis1⁺ and spo6⁺ antisense transcripts from an integrated, ectopic, locus disrupted their respective meiotic functions. This ability of antisense transcripts to disrupt gene function when expressed in trans suggests that cis production at native loci during sexual differentiation may also control gene function. Consistently, insertion of a marker gene adjacent to the dis1⁺ antisense start site mimicked ectopic antisense expression in reducing the levels of this microtubule regulator and abolishing the microtubule-dependent ‘horsetail' stage of meiosis. Antisense production had no impact at any of these loci when the RNA interference (RNAi) machinery was removed. Thus, far from being simply ‘genome chatter', this extensive ncRNA landscape constitutes a fundamental component in the controls that drive the complex programme of sexual differentiation in S. pombe.

SIN-inhibitory phosphatase complex promotes Cdc11p dephosphorylation and propagates SIN asymmetry in fission yeast

BACKGROUND

Cytokinesis in many eukaryotes involves the function of an actomyosin-based contractile ring. In fission yeast, actomyosin ring maturation and stability require a conserved signaling pathway termed the SIN (septation initiation network). The SIN consists of a GTPase (Spg1p) and three protein kinases, all of which localize to the mitotic spindle pole bodies (SPBs). Two of the SIN kinases, Cdc7p and Sid1p, localize asymmetrically to the newly duplicated SPB in late anaphase. How this asymmetry is achieved is not understood, although it is known that their symmetric localization impairs cytokinesis.

RESULTS

Here we characterize a new Forkhead-domain-associated protein, Csc1p, and identify SIN-inhibitory PP2A complex (SIP), which is crucial for the establishment of SIN asymmetry. Csc1p localizes to both SPBs early in mitosis, is lost from the SPB that accumulates Cdc7p, and instead accumulates at the SPB lacking Cdc7p. Csc1p is required for the dephosphorylation of the SIN scaffolding protein Cdc11p and is thereby required for the recruitment of Byr4p, a component of the GTPase-activating subunit for Spg1p, to the SPB.

CONCLUSIONS

Because Cdc7p does not bind to GDP-Spg1p, we propose that the SIP-mediated Cdc11p dephosphorylation and the resulting recruitment of Byr4p are among the earliest steps in the establishment of SIN asymmetry.

Dividing the spoils of growth and the cell cycle: The fission yeast as a model for the study of cytokinesis

Cytokinesis is the final stage of the cell cycle, and ensures completion of both genome segregation and organelle distribution to the daughter cells. Cytokinesis requires the cell to solve a spatial problem (to divide in the correct place, orthogonally to the plane of chromosome segregation) and a temporal problem (to coordinate cytokinesis with mitosis). Defects in the spatiotemporal control of cytokinesis may cause cell death, or increase the risk of tumor formation [Fujiwara et al., 2005 (Fujiwara T, Bandi M, Nitta M, Ivanova EV, Bronson RT, Pellman D. 2005. Cytokinesis failure generating tetraploids promotes tumorigenesis in p53-null cells. Nature 437:1043–1047); reviewed by Ganem et al., 2007 (Ganem NJ, Storchova Z, Pellman D. 2007. Tetraploidy, aneuploidy and cancer. Curr Opin Genet Dev 17:157–162.)]. Asymmetric cytokinesis, which permits the generation of two daughter cells that differ in their shape, size and properties, is important both during development, and for cellular homeostasis in multicellular organisms [reviewed by Li, 2007 (Li R. 2007. Cytokinesis in development and disease: variations on a common theme. Cell Mol Life Sci 64:3044–3058)]. The principal focus of this review will be the mechanisms of cytokinesis in the mitotic cycle of the yeast Schizosaccharomyces pombe. This simple model has contributed significantly to our understanding of how the cell cycle is regulated, and serves as an excellent model for studying aspects of cytokinesis. Here we will discuss the state of our knowledge of how the contractile ring is assembled and disassembled, how it contracts, and what we know of the regulatory mechanisms that control these events and assure their coordination with chromosome segregation.

Dma1 ubiquitinates the SIN scaffold, Sid4, to impede the mitotic localization of Plo1 kinase

Proper cell division requires strict coordination between mitotic exit and cytokinesis. In the event of a mitotic error, cytokinesis must be inhibited to ensure equal partitioning of genetic material. In the fission yeast, Schizosaccharomyces pombe, the checkpoint protein and E3 ubiquitin ligase, Dma1, delays cytokinesis by inhibiting the septation initiation network (SIN) when chromosomes are not attached to the mitotic spindle. To elucidate the mechanism by which Dma1 inhibits the SIN, we screened all SIN components as potential Dma1 substrates and found that the SIN scaffold protein, Sid4, is ubiquitinated in vivo in a Dma1-dependent manner. To investigate the role of Sid4 ubiquitination in checkpoint function, a ubiquitination deficient sid4 allele was generated and our data indicate that Sid4 ubiquitination by Dma1 is required to prevent cytokinesis during a mitotic checkpoint arrest. Furthermore, Sid4 ubiquitination delays recruitment of the Polo-like kinase and SIN activator, Plo1, to spindle pole bodies (SPBs), while at the same time prolonging residence of the SIN inhibitor, Byr4, providing a mechanistic link between Dma1 activity and cytokinesis inhibition.

The role of Schizosaccharomyces pombe dma1 in spore formation during meiosis

Meiosis is a specialised form of the cell cycle that gives rise to haploid gametes. In Schizosaccharomyces pombe, the products of meiosis are four spores, which are formed by encapsulation of the four meiosis II nuclei within the cytoplasm of the zygote produced by fusion of the mating cells. The S. pombe spindle pole body is remodelled during meiosis II and membrane vesicles are then recruited there to form the forespore membrane, which encapsulates the haploid nucleus to form a prespore. Spore wall material is then deposited, giving rise to the mature spore. The septation initiation network is required to coordinate cytokinesis and mitosis in the vegetative cycle and for spore formation in the meiotic cycle. We have investigated the role of the SIN regulator dma1p in meiosis; we find that although both meiotic divisions occur in the absence of dma1p, asci frequently contain fewer than four spores, which are larger than in wild-type meiosis. Our data indicate that dma1p acts in parallel to the leading-edge proteins and septins to assure proper formation for the forespore membrane. Dma1p also contributes to the temporal regulation of the abundance of the meiosis-specific SIN component mug27p.

Cytokinesis: Closure resets your SIN

A new study of fission yeast cell division has revealed a coupling between cytoplasmic partitioning and the turning-off of cytokinesis signalling that may be mediated by asymmetric protein distribution.

Fission yeast Pcp1 links polo kinase-mediated mitotic entry to gamma-tubulin-dependent spindle formation

The centrosomal pericentrin-related proteins play pivotal roles in various aspects of cell division; however their underlying mechanisms remain largely elusive. Here we show that fission-yeast pericentrin-like Pcp1 regulates multiple functions of the spindle pole body (SPB) through recruiting two critical factors, the gamma-tubulin complex (gamma-TuC) and polo kinase (Plo1). We isolated two pcp1 mutants (pcp1-15 and pcp1-18) that display similar abnormal spindles, but with remarkably different molecular defects. Both mutants exhibit defective monopolar spindle microtubules that emanate from the mother SPB. However, while pcp1-15 fails to localise the gamma-TuC to the mitotic SPB, pcp1-18 is specifically defective in recruiting Plo1. Consistently Pcp1 forms a complex with both gamma-TuC and Plo1 in the cell. pcp1-18 is further defective in the mitotic-specific reorganisation of the nuclear envelope (NE), leading to impairment of SPB insertion into the NE. Moreover pcp1-18, but not pcp1-15, is rescued by overproducing nuclear pore components or advancing mitotic onset. The central role for Pcp1 in orchestrating these processes provides mechanistic insight into how the centrosome regulates multiple cellular pathways.

Proper timing of cytokinesis is regulated by Schizosaccharomyces pombe Etd1

Spatial cues regulate cytokinesis: fully elongated spindles initiate cytokinesis in late anaphase, and the resulting cellular asymmetry triggers the process to end.

Cytokinesis must be initiated only after chromosomes have been segregated in anaphase and must be terminated once cleavage is completed. We show that the fission yeast protein Etd1 plays a central role in both of these processes. Etd1 activates the guanosine triphosphatase (GTPase) Spg1 to trigger signaling through the septum initiation network (SIN) pathway and onset of cytokinesis. Spg1 is activated in late anaphase when spindle elongation brings spindle pole body (SPB)–localized Spg1 into proximity with its activator Etd1 at cell tips, ensuring that cytokinesis is only initiated when the spindle is fully elongated. Spg1 is active at just one of the two SPBs during cytokinesis. When the actomyosin ring finishes constriction, the SIN triggers disappearance of Etd1 from the half of the cell with active Spg1, which then triggers Spg1 inactivation. Asymmetric activation of Spg1 is crucial for timely inactivation of the SIN. Together, these results suggest a mechanism whereby cell asymmetry is used to monitor cytoplasmic partitioning to turn off cytokinesis signaling.

Nuclear shape, growth and integrity in the closed mitosis of fission yeast depend on the Ran-GTPase system, the spindle pole body and the endoplasmic reticulum

The double lipid bilayer of the nuclear envelope (NE) remains intact during closed mitosis. In the fission yeast Schizosaccharomyces pombe, the intranuclear mitotic spindle has envelope-embedded spindle pole bodies (SPB) at its ends. As the spindle elongates and the nucleus divides symmetrically, nuclear volume remains constant but nuclear area rapidly increases by 26%. When Ran-GTPase function is compromised in S. pombe, nuclear division is strikingly asymmetrical and the newly synthesized SPB is preferentially associated with the smaller nucleus, indicative of a Ran-dependent SPB defect that interferes with symmetrical nuclear division. A second defect, which specifically influences the NE, results in breakage of the NE upon spindle elongation. This defect, but not asymmetric nuclear division, is partially rescued by slowing spindle elongation, stimulating endoplasmic reticulum (ER) proliferation or changing conformation of the ER membrane. We propose that redistribution of lipid within the ER-NE network is crucial for mitosis-specific NE changes in both open and closed mitosis.

The S. pombe mitotic regulator Cut12 promotes spindle pole body activation and integration into the nuclear envelope

The fission yeast spindle pole body (SPB) comprises a cytoplasmic structure that is separated from an ill-defined nuclear component by the nuclear envelope. Upon mitotic commitment, the nuclear envelope separating these domains disperses as the two SPBs integrate into a hole that forms in the nuclear envelope. The SPB component Cut12 is linked to cell cycle control, as dominant cut12.s11 mutations suppress the mitotic commitment defect of cdc25.22 cells and elevated Cdc25 levels suppress the monopolar spindle phenotype of cut12.1 loss of function mutations. We show that the cut12.1 monopolar phenotype arises from a failure to activate and integrate the new SPB into the nuclear envelope. The activation of the old SPB was frequently delayed, and its integration into the nuclear envelope was defective, resulting in leakage of the nucleoplasm into the cytoplasm through large gaps in the nuclear envelope. We propose that these activation/integration defects arise from a local deficiency in mitosis-promoting factor activation at the new SPB.

The Nek6 and Nek7 protein kinases are required for robust mitotic spindle formation and cytokinesis

Nek6 and Nek7 are members of the NIMA-related serine/threonine kinase family. Previous work showed that they contribute to mitotic progression downstream of another NIMA-related kinase, Nek9, although the roles of these different kinases remain to be defined. Here, we carried out a comprehensive analysis of the regulation and function of Nek6 and Nek7 in human cells. By generating specific antibodies, we show that both Nek6 and Nek7 are activated in mitosis and that interfering with their activity by either depletion or expression of reduced-activity mutants leads to mitotic arrest and apoptosis. Interestingly, while completely inactive mutants and small interfering RNA-mediated depletion delay cells at metaphase with fragile mitotic spindles, hypomorphic mutants or RNA interference treatment combined with a spindle assembly checkpoint inhibitor delays cells at cytokinesis. Importantly, depletion of either Nek6 or Nek7 leads to defective mitotic progression, indicating that although highly similar, they are not redundant. Indeed, while both kinases localize to spindle poles, only Nek6 obviously localizes to spindle microtubules in metaphase and anaphase and to the midbody during cytokinesis. Together, these data lead us to propose that Nek6 and Nek7 play independent roles not only in robust mitotic spindle formation but also potentially in cytokinesis.