Sid4p is required to localize components of the septation initiation pathway to the spindle pole body in fission yeast

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.

The fission yeast cytokinetic ring component Fic1 promotes septum formation

In Schizosaccharomyces pombe, septum formation is coordinated with cytokinetic ring constriction but the mechanisms linking these events are unclear. In this study, we explored the role of the cytokinetic ring component Fic1, first identified by its interaction with the F-BAR protein Cdc15, in septum formation. We found that the fic1 phospho-ablating mutant, fic1-2A, is a gain-of-function allele that suppresses myo2-E1, the temperature-sensitive allele of the essential type-II myosin, myo2. This suppression is achieved by the promotion of septum formation and required Fic1's interaction with the F-BAR proteins Cdc15 and Imp2. Additionally, we found that Fic1 interacts with Cyk3 and that this interaction was likewise required for Fic1's role in septum formation. Fic1, Cdc15, Imp2, and Cyk3 are the orthologs of the Saccharomyces cerevisiae ingression progression complex, which stimulates the chitin synthase Chs2 to promote primary septum formation. However, our findings indicate that Fic1 promotes septum formation and cell abscission independently of the S. pombe Chs2 ortholog. Thus, while similar complexes exist in the two yeasts that each promote septation, they appear to have different downstream effectors.

The fission yeast cytokinetic ring component Fic1 promotes septum formation

In Schizosaccharomyces pombe septum formation is coordinated with cytokinetic ring constriction but the mechanisms linking these events are unclear. In this study, we explored the role of the cytokinetic ring component Fic1, first identified by its interaction with the F-BAR protein Cdc15, in septum formation. We found that the fic1 phospho-ablating mutant, fic1-2A , is a gain-of-function allele that suppresses myo2-E1 , the temperature-sensitive allele of the essential type-II myosin, myo2 . This suppression is achieved by the promotion of septum formation and required Fic1's interaction with the F-BAR proteins Cdc15 and Imp2. Additionally, we found that Fic1 interacts with Cyk3 and that this interaction was likewise required for Fic1's role in septum formation. Fic1, Cdc15, Imp2, and Cyk3 are the orthologs of the Saccharomyces cerevisiae ingression progression complex, which stimulates the chitin synthase Chs2 to promote primary septum formation. However, our findings indicate that Fic1 promotes septum formation and cell abscission independently of the S. pombe Chs2 ortholog. Thus, while similar complexes exist in the two yeasts that each promote septation, they appear to have different downstream effectors.

Summary Statement

The S. pombe cytokinetic ring protein Fic1 promotes septum formation in a manner dependent on interactions with the cytokinetic ring components Cdc15, Imp2, and Cyk3.

The kinesin-5 protein Cut7 moves bidirectionally on fission yeast spindles with activity that increases in anaphase

Kinesin-5 motors are essential to separate mitotic spindle poles and assemble a bipolar spindle in many organisms. These motors crosslink and slide apart antiparallel microtubules via microtubule plus-end-directed motility. However, kinesin-5 localization is enhanced away from antiparallel overlaps. Increasing evidence suggests this localization occurs due to bidirectional motility or trafficking. The purified fission-yeast kinesin-5 protein Cut7 moves bidirectionally, but bidirectionality has not been shown in cells, and the function of the minus-end-directed movement is unknown. Here, we characterized the motility of Cut7 on bipolar and monopolar spindles and observed movement toward both plus- and minus-ends of microtubules. Notably, the activity of the motor increased at anaphase B onset. Perturbations to microtubule dynamics only modestly changed Cut7 movement, whereas Cut7 mutation reduced movement. These results suggest that the directed motility of Cut7 contributes to the movement of the motor. Comparison of the Cut7 mutant and human Eg5 (also known as KIF11) localization suggest a new hypothesis for the function of minus-end-directed motility and spindle-pole localization of kinesin-5s.

Localization of the ubiquitin ligase Dma1 to the fission yeast contractile ring is modulated by phosphorylation

The timing of cytokinesis relative to other mitotic events in the fission yeast Schizosaccharomyces pombe is controlled by the septation initiation network (SIN). During a mitotic checkpoint, the SIN is inhibited by the E3 ubiquitin ligase Dma1 to prevent chromosome mis-segregation. Dma1 dynamically localizes to spindle pole bodies (SPBs) and the contractile ring (CR) during mitosis, though its role at the CR is unknown. Here, we examined whether Dma1 phosphorylation affects its localization or function. We found that preventing Dma1 phosphorylation by substituting the six phosphosites with alanines diminished its CR localization but did not affect its mitotic checkpoint function. These studies reinforce the conclusion that Dma1 localization to the SPB is key to its role in the mitotic checkpoint.

The Putative RNA-Binding Protein Dri1 Promotes the Loading of Kinesin-14/Klp2 to the Mitotic Spindle and Is Sequestered into Heat-Induced Protein Aggregates in Fission Yeast

Cells form a bipolar spindle during mitosis to ensure accurate chromosome segregation. Proper spindle architecture is established by a set of kinesin motors and microtubule-associated proteins. In most eukaryotes, kinesin-5 motors are essential for this process, and genetic or chemical inhibition of their activity leads to the emergence of monopolar spindles and cell death. However, these deficiencies can be rescued by simultaneous inactivation of kinesin-14 motors, as they counteract kinesin-5. We conducted detailed genetic analyses in fission yeast to understand the mechanisms driving spindle assembly in the absence of kinesin-5. Here, we show that deletion of the dri1 gene, which encodes a putative RNA-binding protein, can rescue temperature sensitivity caused by cut7-22, a fission yeast kinesin-5 mutant. Interestingly, kinesin-14/Klp2 levels on the spindles in the cut7 mutants were significantly reduced by the dri1 deletion, although the total levels of Klp2 and the stability of spindle microtubules remained unaffected. Moreover, RNA-binding motifs of Dri1 are essential for its cytoplasmic localization and function. We have also found that a portion of Dri1 is spatially and functionally sequestered by chaperone-based protein aggregates upon mild heat stress and limits cell division at high temperatures. We propose that Dri1 might be involved in post-transcriptional regulation through its RNA-binding ability to promote the loading of Klp2 on the spindle microtubules.

Chiasmata and the kinetochore component Dam1 are crucial for elimination of erroneous chromosome attachments and centromere oscillation at meiosis I

Establishment of proper chromosome attachments to the spindle requires elimination of erroneous attachments, but the mechanism of this process is not fully understood. During meiosis I, sister chromatids attach to the same spindle pole (mono-oriented attachment), whereas homologous chromosomes attach to opposite poles (bi-oriented attachment), resulting in homologous chromosome segregation. Here, we show that chiasmata that link homologous chromosomes and kinetochore component Dam1 are crucial for elimination of erroneous attachments and oscillation of centromeres between the spindle poles at meiosis I in fission yeast. In chiasma-forming cells, Mad2 and Aurora B kinase, which provides time for attachment correction and destabilizes erroneous attachments, respectively, caused elimination of bi-oriented attachments of sister chromatids, whereas in chiasma-lacking cells, they caused elimination of mono-oriented attachments. In chiasma-forming cells, in addition, homologous centromere oscillation was coordinated. Furthermore, Dam1 contributed to attachment elimination in both chiasma-forming and chiasma-lacking cells, and drove centromere oscillation. These results demonstrate that chiasmata alter attachment correction patterns by enabling error correction factors to eliminate bi-oriented attachment of sister chromatids, and suggest that Dam1 induces elimination of erroneous attachments. The coincidental contribution of chiasmata and Dam1 to centromere oscillation also suggests a potential link between centromere oscillation and attachment elimination.

Pericentrin-mediated SAS-6 recruitment promotes centriole assembly

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

Kinesin-6 Klp9 plays motor-dependent and -independent roles in collaboration with Kinesin-5 Cut7 and the microtubule crosslinker Ase1 in fission yeast

Bipolar mitotic spindles play a critical part in accurate chromosome segregation. During late mitosis, spindle microtubules undergo drastic elongation in a process called anaphase B. Two kinesin motors, Kinesin-5 and Kinesin-6, are thought to generate outward forces to drive spindle elongation, and the microtubule crosslinker Ase1/PRC1 maintains structural integrity of antiparallel microtubules. However, how these three proteins orchestrate this process remains unknown. Here we explore the functional interplay among fission yeast Kinesin-5/Cut7, Kinesin-6/Klp9 and Ase1. Using total internal reflection fluorescence microscopy, we show that Klp9 forms homotetramers and that Klp9 is a processive plus end-directed motor. klp9Δase1Δ is synthetically lethal. Surprisingly, this lethality is not ascribable to the defective motor activity of Klp9; instead, it is dependent upon a nuclear localisation signal and coiled coil domains within the non-motor region. We isolated a cut7 mutant (cut7-122) that displays temperature sensitivity only in the absence of Klp9. Interestingly, cut7-122 alone is impaired in spindle elongation during anaphase B, and furthermore, cut7-122klp9Δ double mutants exhibit additive defects. We propose that Klp9 plays dual roles during anaphase B; one is motor-dependent that collaborates with Cut7 in force generation, while the other is motor-independent that ensures structural integrity of spindle microtubules together with Ase1.

Spindle pole body movement is affected by glucose and ammonium chloride in fission yeast

The complexity of chromatin dynamics is orchestrated by several active processes. In fission yeast, the centromeres are clustered around the spindle pole body (SPB) and oscillate in a microtubule- and adenosine triphosphate (ATP)-dependent manner. However, whether and how SPB oscillation are affected by different environmental conditions remain poorly understood. In this study, we quantitated movements of the SPB component, which colocalizes with the centromere in fission yeast. We found that SPB movement was significantly reduced at low glucose concentrations. Movement of the SPB was also affected by the presence of ammonium chloride. Power spectral analysis revealed that periodic movement of the SPB is disrupted by low glucose concentrations. Measurement of ATP levels in living cells by quantitative single-cell imaging suggests that ATP levels are not the only determinant of SPB movement. Our results provide novel insight into how SPB movement is regulated by cellular energy status and additional factors such as the medium nutritional composition.

Relief of the Dma1-mediated checkpoint requires Dma1 autoubiquitination and dynamic localization

Chromosome segregation and cell division are coupled to prevent aneuploidy and cell death. In the fission yeast Schizosaccharomyces pombe, the septation initiation network (SIN) promotes cytokinesis, but upon mitotic checkpoint activation, the SIN is actively inhibited to prevent cytokinesis from occurring before chromosomes have safely segregated. SIN inhibition during the mitotic checkpoint is mediated by the E3 ubiquitin ligase Dma1. Dma1 binds to the CK1-phosphorylated SIN scaffold protein Sid4 at the spindle pole body (SPB), and ubiquitinates it. Sid4 ubiquitination antagonizes the SPB localization of the Pololike kinase Plo1, the major SIN activator, so that SIN signaling is delayed. How this checkpoint is silenced once spindle defects are resolved has not been clear. Here we establish that Dma1 transiently leaves SPBs during anaphase B due to extensive autoubiquitination. The SIN is required for Dma1 to return to SPBs later in anaphase. Blocking Dma1 removal from SPBs by permanently tethering it to Sid4 prevents SIN activation and cytokinesis. Therefore, controlling Dma1’s SPB dynamics in anaphase is an essential step in S. pombe cell division and the silencing of the Dma1-dependent mitotic checkpoint.

Centrosome Remodelling in Evolution

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

Dependency relationships within the fission yeast polarity network

The ability to regulate polarised cell growth is crucial to maintain the viability of cells. Growth is modulated to facilitate essential cell functions and respond to the external environment. Failure to do so can lead to numerous developmental and disease states, including cancer. We have undertaken a detailed analysis of the regulatory interplay between molecules involved in the regulation and maintenance of polarised cell growth within fission yeast. Internally controlled live cell imaging was used to examine interactions between 10 key polarity proteins. Analysis reveals interplay between the microtubule and actin cytoskeletons, as well as multiple novel dependency pathways and feedback networks between groups of proteins. This study provides important insights into the conserved regulation of polarised cell growth within eukaryotes.

The kinase domain of CK1 enzymes contains the localization cue essential for compartmentalized signaling at the spindle pole

CK1 protein kinases contribute to multiple biological processes, but how they are tailored to function in compartmentalized signaling events is largely unknown. Hhp1 and Hhp2 (Hhp1/2) are the soluble CK1 family members in Schizosaccharomyces pombe. One of their functions is to inhibit the septation initiation network (SIN) during a mitotic checkpoint arrest. The SIN is assembled by Sid4 at spindle pole bodies (SPBs), and though Hhp1/2 colocalize there, it is not known how they are targeted there or whether their SPB localization is required for SIN inhibition. Here, we establish that Hhp1/2 localize throughout the cell cycle to SPBs, as well as to the nucleus, cell tips, and division site. We find that their catalytic domains but not their enzymatic function are used for SPB targeting and that this targeting strategy is conserved in human CK1δ/ε localization to centrosomes. Further, we pinpoint amino acids in the Hhp1 catalytic domain required for SPB interaction; mutation of these residues disrupts Hhp1 association with the core SPB protein Ppc89, and the inhibition of cytokinesis in the setting of spindle stress. Taken together, these data have enabled us to define a molecular mechanism used by CK1 enzymes to target a specific cellular locale for compartmentalized signaling.

Distinct 'safe zones' at the nuclear envelope ensure robust replication of heterochromatic chromosome regions

Chromosome replication and transcription occur within a complex nuclear milieu whose functional subdomains are beginning to be mapped out. Here we delineate distinct domains of the fission yeast nuclear envelope (NE), focusing on regions enriched for the inner NE protein, Bqt4, or the lamin interacting domain protein, Lem2. Bqt4 is relatively mobile around the NE and acts in two capacities. First, Bqt4 tethers chromosome termini and the mat locus to the NE specifically while these regions are replicating. This positioning is required for accurate heterochromatin replication. Second, Bqt4 mobilizes a subset of Lem2 molecules around the NE to promote pericentric heterochromatin maintenance. Opposing Bqt4-dependent Lem2 mobility are factors that stabilize Lem2 beneath the centrosome, where Lem2 plays a crucial role in kinetochore maintenance. Our data prompt a model in which Bqt4-rich nuclear subdomains are 'safe zones' in which collisions between transcription and replication are averted and heterochromatin is reassembled faithfully.

Molecular model of fission yeast centrosome assembly determined by superresolution imaging

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

Facile manipulation of protein localization in fission yeast through binding of GFP-binding protein to GFP

GFP-binding protein (or GBP) has been recently developed in various systems and organisms as an efficient tool to purify GFP-fusion proteins. Due to the high affinity between GBP and GFP or GFP variants, this GBP-based approach is also ideally suited to alter the localization of functional proteins in live cells. In order to facilitate the wide use of the GBP-targeting approach in the fission yeast Schizosaccharomyces pombe, we developed a set of pFA6a-, pJK148- and pUC119-based vectors containing GBP- or GBP-mCherry-coding sequences and variants of inducible nmt1 or constitutive adh1 promoters that result in different levels of expression. The GBP or GBP-mCherry fragments can serve as cassettes for N- or C-terminal genomic tagging of genes of interest. We illustrated the application of these vectors in the construction of yeast strains with Dma1 or Cdc7 tagged with GBP-mCherry and efficient targeting of Dma1- or Cdc7-GBP-mCherry to the spindle pole body by Sid4-GFP. This series of vectors should help to facilitate the application of the GBP-targeting approach in manipulating protein localization and the analysis of gene function in fission yeast, at the level of single genes, as well as at a systematic scale.

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.

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.

Loss of kinesin-14 results in aneuploidy via kinesin-5-dependent microtubule protrusions leading to chromosome cut

Aneuploidy – chromosome instability leading to incorrect chromosome number in dividing cells – can arise from defects in centrosome duplication, bipolar spindle formation, kinetochore-microtubule attachment, chromatid cohesion, mitotic checkpoint monitoring, or cytokinesis. As most tumors show some degree of aneuploidy, mechanistic understanding of these pathways has been an intense area of research to provide potential therapeutics. Here, we present a mechanism for aneuploidy in fission yeast based on spindle pole microtubule defocusing by loss of kinesin-14 Pkl1, leading to kinesin-5 Cut7-dependent aberrant long spindle microtubule minus end protrusions that push the properly segregated chromosomes to the site of cell division, resulting in chromosome cut at cytokinesis. Pkl1 localization and function at the spindle pole is mutually dependent on spindle pole-associated protein Msd1. This mechanism of aneuploidy bypasses the known spindle assembly checkpoint that monitors chromosome segregation.

Monitoring SPB biogenesis in fission yeast with high resolution and quantitative fluorescent microscopy

Like centrosomes, yeast spindle pole bodies (SPBs) undergo a tightly controlled duplication cycle in order to restrict their number to one or two per cell and promote the assembly of a bipolar spindle at mitotic entry. This conservative duplication cycle is tightly coordinated with cell cycle progression although the mechanisms that ensure this coordination remain largely unknown. In this chapter, we describe simple high resolution microscopy- and quantitative light microscopy-based methods that allow to monitor SPB biogenesis in fission yeast and may be useful to study the molecular pathways controlling the successive phases of the duplication cycle.

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.

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.

The Cdc15 and Imp2 SH3 domains cooperatively scaffold a network of proteins that redundantly ensure efficient cell division in fission yeast

The fission yeast F-BAR proteins Cdc15 and Imp2 and their combined SH3-domain partners appear to act as “molecular glue” to stabilize the interaction between the plasma membrane and a complex network of proteins at the division site that mediates cell division.

Schizosaccharomyces pombe cdc15 homology (PCH) family members participate in numerous biological processes, including cytokinesis, typically by bridging the plasma membrane via their F-BAR domains to the actin cytoskeleton. Two SH3 domain–containing PCH family members, Cdc15 and Imp2, play critical roles in S. pombe cytokinesis. Although both proteins localize to the contractile ring, with Cdc15 preceding Imp2, only cdc15 is an essential gene. Despite these distinct roles, the SH3 domains of Cdc15 and Imp2 cooperate in the essential process of recruiting other proteins to stabilize the contractile ring. To better understand the connectivity of this SH3 domain–based protein network at the CR and its function, we used a biochemical approach coupled to proteomics to identify additional proteins (Rgf3, Art1, Spa2, and Pos1) that are integrated into this network. Cell biological and genetic analyses of these SH3 partners implicate them in a range of activities that ensure the fidelity of cell division, including promoting cell wall metabolism and influencing cell morphogenesis.

The kinetochore protein Kis1/Eic1/Mis19 ensures the integrity of mitotic spindles through maintenance of kinetochore factors Mis6/CENP-I and CENP-A

Microtubules play multiple roles in a wide range of cellular phenomena, including cell polarity establishment and chromosome segregation. A number of microtubule regulators have been identified, including microtubule-associated proteins and kinases, and knowledge of these factors has contributed to our molecular understanding of microtubule regulation of each relevant cellular process. The known regulators, however, are insufficient to explain how those processes are linked to one another, underscoring the need to identify additional regulators. To find such novel mechanisms and microtubule regulators, we performed a screen that combined genetics and microscopy for fission yeast mutants defective in microtubule organization. We isolated approximately 900 mutants showing defects in either microtubule organization or the nuclear envelope, and these mutants were classified into 12 categories. We particularly focused on one mutant, kis1, which displayed spindle defects in early mitosis. The kis1 mutant frequently failed to assemble a normal bipolar spindle. The responsible gene encoded a kinetochore protein, Mis19 (also known as Eic1), which localized to the interface of kinetochores and spindle poles. We also found that the inner kinetochore proteins Mis6/CENP-I and Cnp1/CENP-A were delocalized from kinetochores in the kis1 cells and that kinetochore-microtubule attachment was defective. Another mutant, mis6, also displayed similar spindle defects. We conclude that Kis1 is required for inner kinetochore organization, through which Kis1 ensures kinetochore-microtubule attachment and spindle integrity. Thus, we propose an unexpected relationship between inner kinetochore organization and spindle integrity.

csi2p modulates microtubule dynamics and organizes the bipolar spindle for chromosome segregation

Proper chromosome segregation is of paramount importance for proper genetic inheritance. Defects in chromosome segregation can lead to aneuploidy, which is a hallmark of cancer cells. Eukaryotic chromosome segregation is accomplished by the bipolar spindle. Additional mechanisms, such as the spindle assembly checkpoint and centromere positioning, further help to ensure complete segregation fidelity. Here we present the fission yeast csi2+. csi2p localizes to the spindle poles, where it regulates mitotic microtubule dynamics, bipolar spindle formation, and subsequent chromosome segregation. csi2 deletion (csi2Δ) results in abnormally long mitotic microtubules, high rate of transient monopolar spindles, and subsequent high rate of chromosome segregation defects. Because csi2Δ has multiple phenotypes, it enables estimates of the relative contribution of the different mechanisms to the overall chromosome segregation process. Centromere positioning, microtubule dynamics, and bipolar spindle formation can all contribute to chromosome segregation. However, the major determinant of chromosome segregation defects in fission yeast may be microtubule dynamic defects.

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.

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.

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 MOZART1/Mzt1 is an essential γ-tubulin complex component required for complex recruitment to the microtubule organizing center, but not its assembly

γ-Tubulin plays a universal role in microtubule nucleation from microtubule organizing centers (MTOCs) such as the animal centrosome and fungal spindle pole body (SPB). γ-Tubulin functions as a multiprotein complex called the γ-tubulin complex (γ-TuC), consisting of GCP1-6 (GCP1 is γ-tubulin). In fungi and flies, it has been shown that GCP1-3 are core components, as they are indispensable for γ-TuC complex assembly and cell division, whereas the other three GCPs are not. Recently a novel conserved component, MOZART1, was identified in humans and plants, but its precise functions remain to be determined. In this paper, we characterize the fission yeast homologue Mzt1, showing that it is essential for cell viability. Mzt1 is present in approximately equal stoichiometry with Alp4/GCP2 and localizes to all the MTOCs, including the SPB and interphase and equatorial MTOCs. Temperature-sensitive mzt1 mutants display varying degrees of compromised microtubule organization, exhibiting multiple defects during both interphase and mitosis. Mzt1 is required for γ-TuC recruitment, but not sufficient to localize to the SPB, which depends on γ-TuC integrity. Intriguingly, the core γ-TuC assembles in the absence of Mzt1. Mzt1 therefore plays a unique role within the γ-TuC components in attachment of this complex to the major MTOC site.

Cooperation between Rho-GEF Gef2 and its binding partner Nod1 in the regulation of fission yeast cytokinesis

Previous results showed that putative Rho-GEF Gef2 regulates division-site positioning during early cytokinesis in fission yeast. Here Nod1 is identified as a binding partner of Gef2. The two proteins form a complex to regulate division-site positioning and contractile-ring maintenance. In addition, Gef2 binds to GTPases Rho1, Rho4, and Rho5 in vitro.

Cytokinesis is the last step of the cell-division cycle, which requires precise spatial and temporal regulation to ensure genetic stability. Rho guanine nucleotide exchange factors (Rho GEFs) and Rho GTPases are among the key regulators of cytokinesis. We previously found that putative Rho-GEF Gef2 coordinates with Polo kinase Plo1 to control the medial cortical localization of anillin-like protein Mid1 in fission yeast. Here we show that an adaptor protein, Nod1, colocalizes with Gef2 in the contractile ring and its precursor cortical nodes. Like gef2∆, nod1∆ has strong genetic interactions with various cytokinesis mutants involved in division-site positioning, suggesting a role of Nod1 in early cytokinesis. We find that Nod1 and Gef2 interact through the C-termini, which is important for their localization. The contractile-ring localization of Nod1 and Gef2 also depends on the interaction between Nod1 and the F-BAR protein Cdc15, where the Nod1/Gef2 complex plays a role in contractile-ring maintenance and affects the septation initiation network. Moreover, Gef2 binds to purified GTPases Rho1, Rho4, and Rho5 in vitro. Taken together, our data indicate that Nod1 and Gef2 function cooperatively in a protein complex to regulate fission yeast cytokinesis.

Electron tomography reveals novel microtubule lattice and microtubule organizing centre defects in +TIP mutants

Mal3p and Tip1p are the fission yeast (Schizosaccharomyces pombe) homologues of EB1 and CLIP-170, two conserved microtubule plus end tracking proteins (+TIPs). These proteins are crucial regulators of microtubule dynamics. Using electron tomography, we carried out a high-resolution analysis of the phenotypes caused by mal3 and tip1 deletions. We describe the 3-dimensional microtubule organization, quantify microtubule end structures and uncover novel defects of the microtubule lattices. We also reveal unexpected structural modifications of the spindle pole bodies (SPBs), the yeast microtubule organizing centers. In both mutants we observe an increased SPB volume and a reduced number of MT/SPB attachments. The discovered defects alter previous interpretations of the mutant phenotypes and provide new insights into the molecular functions of the two protein families.

Removal of centrosomal PP1 by NIMA kinase unlocks the MPF feedback loop to promote mitotic commitment in S. pombe

BACKGROUND

Activation of the Cdk1/cyclin B complex, also known as mitosis-promoting factor (MPF), drives commitment to mitosis. Interphase MPF is inhibited through phosphorylation of Cdk1 by Wee1-related kinases. Because Cdc25 phosphatases remove this phosphate, Cdc25 activity is an essential part of the switch that drives cells into mitosis. The generation of a critical "trigger" of active MPF promotes a positive feedback loop that employs Polo kinase to boost Cdc25 activity and inhibit Wee1, thereby ensuring that mitotic commitment is a bistable switch. Mutations in the spindle pole body (SPB) component Cut12 suppress otherwise lethal deficiencies in Cdc25.

RESULTS

Cut12 harbors a bipartite protein phosphatase 1 (PP1) docking domain. Mutation of either element alone suppressed the temperature-dependent lethality of cdc25.22, whereas simultaneous ablation of both allowed cells to divide in the complete absence of Cdc25. Late G2 phase phosphorylation between the two elements by MPF and the NIMA kinase Fin1 blocked PP1(Dis2) recruitment, thereby promoting recruitment of Polo to Cut12 and the SPB and elevating global Polo kinase activity throughout the cell.

CONCLUSIONS

PP1 recruitment to Cut12 sets a threshold for Polo's feedback-loop activity that locks the cell in interphase until Cdc25 pushes MPF activity through this barrier to initiate mitosis. We propose that events on the SPB (and, by inference, the centrosome) integrate inputs from diverse signaling networks to generate a coherent decision to divide that is appropriate for the particular environmental context of each cell. PP1 recruitment sets one or more critical thresholds for single or multiple local events within this switch.

Comprehensive proteomics analysis reveals new substrates and regulators of the fission yeast clp1/cdc14 phosphatase

The conserved family of Cdc14 phosphatases targets cyclin-dependent kinase substrates in yeast, mediating late mitotic signaling events. To discover substrates and regulators of the Schizosaccharomyces pombe Cdc14 phosphatase Clp1, TAP-tagged Clp1, and a substrate trapping mutant (Clp1-C286S) were purified from asynchronous and mitotic (prometaphase and anaphase) cells and binding partners were identified by 2D-LC-MS/MS. Over 100 Clp1-interacting proteins were consistently identified, over 70 of these were enriched in Clp1-C286S-TAP (potential substrates) and we and others detected Cdk1 phosphorylation sites in over half (44/73) of these potential substrates. According to GO annotations, Clp1-interacting proteins are involved in many essential cellular processes including mitosis, cytokinesis, ribosome biogenesis, transcription, and trafficking among others. We confirmed association and dephosphorylation of multiple candidate substrates, including a key scaffolding component of the septation initiation network called Cdc11, an essential kinase of the conserved morphogenesis-related NDR kinase network named Shk1, and multiple Mlu1-binding factor transcriptional regulators. In addition, we identified Sal3, a nuclear β-importin, as the sole karyopherin required for Clp1 nucleoplasmic shuttling, a key mode of Cdc14 phosphatase regulation. Finally, a handful of proteins were more abundant in wild type Clp1-TAP versus Clp1-C286S-TAP, suggesting that they may directly regulate Clp1 signaling or serve as scaffolding platforms to localize Clp1 activity.

Csi1 links centromeres to the nuclear envelope for centromere clustering

Csi1 promotes centromere clustering by linking centromeres to the SUN domain protein Sad1 in the nuclear envelope.

In the fission yeast Schizosaccharomyces pombe, the centromeres of each chromosome are clustered together and attached to the nuclear envelope near the site of the spindle pole body during interphase. The mechanism and functional importance of this arrangement of chromosomes are poorly understood. In this paper, we identified a novel nuclear protein, Csi1, that localized to the site of centromere attachment and interacted with both the inner nuclear envelope SUN domain protein Sad1 and centromeres. Both Csi1 and Sad1 mutants exhibited centromere clustering defects in a high percentage of cells. Csi1 mutants also displayed a high rate of chromosome loss during mitosis, significant mitotic delays, and sensitivity to perturbations in microtubule–kinetochore interactions and chromosome numbers. These studies thus define a molecular link between the centromere and nuclear envelope that is responsible for centromere clustering.

Cytokinesis-based constraints on polarized cell growth in fission yeast

The rod-shaped fission yeast Schizosaccharomyces pombe, which undergoes cycles of monopolar-to-bipolar tip growth, is an attractive organism for studying cell-cycle regulation of polarity establishment. While previous research has described factors mediating this process from interphase cell tips, we found that division site signaling also impacts the re-establishment of bipolar cell growth in the ensuing cell cycle. Complete loss or targeted disruption of the non-essential cytokinesis protein Fic1 at the division site, but not at interphase cell tips, resulted in many cells failing to grow at new ends created by cell division. This appeared due to faulty disassembly and abnormal persistence of the cell division machinery at new ends of fic1Δ cells. Moreover, additional mutants defective in the final stages of cytokinesis exhibited analogous growth polarity defects, supporting that robust completion of cell division contributes to new end-growth competency. To test this model, we genetically manipulated S. pombe cells to undergo new end take-off immediately after cell division. Intriguingly, such cells elongated constitutively at new ends unless cytokinesis was perturbed. Thus, cell division imposes constraints that partially override positive controls on growth. We posit that such constraints facilitate invasive fungal growth, as cytokinesis mutants displaying bipolar growth defects formed numerous pseudohyphae. Collectively, these data highlight a role for previous cell cycles in defining a cell's capacity to polarize at specific sites, and they additionally provide insight into how a unicellular yeast can transition into a quasi-multicellular state.

Dnt1 acts as a mitotic inhibitor of the spindle checkpoint protein dma1 in fission yeast

The interaction between Dma1 and Dnt1 in fission yeast is characterized. The results show that, similar to its homologue Chfr in higher eukaryotes, Dma1 in fission yeast can also affect factors required for microtubule nucleation and spindle formation at early mitosis.

The Schizosaccharomyces pombe checkpoint protein Dma1 couples mitotic progression with cytokinesis and is important in delaying mitotic exit and cytokinesis when kinetochores are not properly attached to the mitotic spindle. Dma1 is a ubiquitin ligase and potential functional relative of the human tumor suppressor Chfr. Dma1 delays mitotic exit and cytokinesis by ubiquitinating a scaffold protein (Sid4) of the septation initiation network, which, in turn, antagonizes the ability of the Polo-like kinase Plo1 to promote cell division. Here we identify Dnt1 as a Dma1-binding protein. Several lines of evidence indicate that Dnt1 inhibits Dma1 function during metaphase. First, Dnt1 interacts preferentially with Dma1 during metaphase. Second, Dma1 ubiquitin ligase activity and Sid4 ubiquitination are elevated in dnt1∆ cells. Third, the enhanced mitotic defects in dnt1Δ plo1 double mutants are partially rescued by deletion of dma1⁺, suggesting that the defects in dnt1∆ plo1 double mutants are attributable to excess Dma1 activity. Taken together, these data show that Dnt1 acts to restrain Dma1 activity in early mitosis to allow normal mitotic progression.

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.

Spindle pole body components are reorganized during fission yeast meiosis

We show that spindle pole body (SPB) remodeling during meiosis in fission yeast is essential for meiosis. Many SPB components disappear during meiotic prophase and return to the SPBs at meiosis I onset. We found novel functions for Polo kinase/Plo1 and centrin/Cdc31 in the meiotic reorganization of SPB components.

During meiosis, the centrosome/spindle pole body (SPB) must be regulated in a manner distinct from that of mitosis to achieve a specialized cell division that will produce gametes. In this paper, we demonstrate that several SPB components are localized to SPBs in a meiosis-specific manner in the fission yeast Schizosaccharomyces pombe. SPB components, such as Cut12, Pcp1, and Spo15, which stay on the SPB during the mitotic cell cycle, disassociate from the SPB during meiotic prophase and then return to the SPB immediately before the onset of meiosis I. Interestingly, the polo kinase Plo1, which normally localizes to the SPB during mitosis, is excluded from them in meiotic prophase, when meiosis-specific, horse-tail nuclear movement occurs. We found that exclusion of Plo1 during this period was essential to properly remodel SPBs, because artificial targeting of Plo1 to SPBs resulted in an overduplication of SPBs. We also found that the centrin Cdc31 was required for meiotic SPB remodeling. Thus Plo1 and a centrin play central roles in the meiotic SPB remodeling, which is essential for generating the proper number of meiotic SPBs and, thereby provide unique characteristics to meiotic divisions.

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.

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.

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.

A novel fission yeast mei4 mutant that allows efficient synchronization of telomere dispersal and the first meiotic division

The progression of meiosis is controlled by a number of gene-expression systems in the fission yeast Schizosaccharomyces pombe. A forkhead-type transcription factor Mei4 activates a number of genes essential for progression from the middle to late stages of meiosis, which include meiosis I, meiosis II and sporulation. The mei4-deletion mutant (mei4Δ) arrests after meiotic prophase and does not enter meiosis I. To further analyse the Mei4 function, we isolated novel temperature-sensitive mei4 alleles. The two alleles isolated in the initial screen turned out to contain a substitution at N136 in the forkhead DNA-binding domain. Among site-directed mutants that carried a point mutation at this position, the mei4-N136A mutant showed the most severe temperature sensitivity. The mei4-N136A mutant arrested before meiosis I at the restrictive temperature, as did the mei4Δ mutant. In fission yeast, the telomeres are clustered at the spindle pole body (SPB) in meiotic prophase and disperse from it at the onset of meiosis I. The mei4Δ mutant was found to arrest with its telomeres clustered at the SPB, demonstrating a role for Mei4 in telomere dispersion. The mei4-N136A mutant also arrested with clustered telomeres at the restrictive temperature, and the clustering was synchronously resolved after a temperature down-shift, indicating that mei4-N136A is a reversible allele. Hence, the mei4-N136A mutant will be a unique tool to synchronize the meiotic cell cycle from meiosis I onwards and may facilitate analyses of cellular activities occurring during meiosis I.

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.

Spatiotemporal regulations of Wee1 at the G2/M transition

Wee1 is highly dynamic at the SPB during the G2/M transition. Wee1 accumulates at the nuclear face of the SPB when cyclin B–Cdc2 peaks at the SPB and disappears from the SPB during spindle assembly. This dynamic behavior of Wee1 at the SPB is important for regulation of cyclin B–Cdc2 activity and proper mitotic entry and progression.

Wee1 is a protein kinase that negatively regulates mitotic entry in G2 phase by suppressing cyclin B–Cdc2 activity, but its spatiotemporal regulations remain to be elucidated. We observe the dynamic behavior of Wee1 in Schizosaccharomyces pombe cells and manipulate its localization and kinase activity to study its function. At late G2, nuclear Wee1 efficiently suppresses cyclin B–Cdc2 around the spindle pole body (SPB). During the G2/M transition when cyclin B–Cdc2 is highly enriched at the SPB, Wee1 temporally accumulates at the nuclear face of the SPB in a cyclin B–Cdc2-dependent manner and locally suppresses both cyclin B–Cdc2 activity and spindle assembly to counteract a Polo kinase–dependent positive feedback loop. Then Wee1 disappears from the SPB during spindle assembly. We propose that regulation of Wee1 localization around the SPB during the G2/M transition is important for proper mitotic entry and progression.

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

By characterizing the fission yeast Dma1's function during meiosis, we revealed that Dma1 is required for spore formation, while it is dispensable for fidelity of nuclear divisions. We also found that Dma1 is functionally related to SIN pathway and meiosis-specific kinase Slk1 during sporulation.

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.

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.

A guaninine nucleotide exchange factor is a component of the meiotic spindle pole body in Schizosaccharomyces pombe

Spore morphogenesis in yeast is driven by the formation of membrane compartments that initiate growth at the spindle poles during meiosis II and grow to encapsulate daughter nuclei. Vesicle docking complexes, called meiosis II outer plaques (MOPs), form on each meiosis II spindle pole body (SPB) and serve as sites of membrane nucleation. How the MOP stimulates membrane assembly is not known. Here, we report that SpSpo13, a component of the MOP in Schizosaccharomyces pombe, shares homology with the guanine nucleotide exchange factor (GEF) domain of the Saccharomyces cerevisiae Sec2 protein. ScSec2 acts as a GEF for the small Rab GTPase ScSec4, which regulates vesicle trafficking from the late-Golgi to the plasma membrane. A chimeric protein in which the ScSec2-GEF domain is replaced with SpSpo13 is capable of supporting the growth of a sec2Delta mutant. SpSpo13 binds preferentially to the nucleotide-free form of ScSec4 and facilitates nucleotide exchange in vitro. In vivo, a Spspo13 mutant defective in GEF activity fails to support membrane assembly. In vitro specificity experiments suggest that SpYpt2 is the physiological substrate of SpSpo13. These results demonstrate that stimulation of Rab-GTPase activity is a property of the S. pombe MOP essential for the initiation of membrane formation.