Live analysis of lagging chromosomes during anaphase and their effect on spindle elongation rate in fission yeast

The role of heterochromatin in centromere function

Chromatin at centromeres is distinct from the chromatin in which the remainder of the genome is assembled. Two features consistently distinguish centromeres: the presence of the histone H3 variant CENP-A and, in most organisms, the presence of heterochromatin. In fission yeast, domains of silent "heterochromatin" flank the CENP-A chromatin domain that forms a platform upon which the kinetochore is assembled. Thus, fission yeast centromeres resemble their metazoan counterparts where the kinetochore is embedded in centromeric heterochromatin. The centromeric outer repeat chromatin is underacetylated on histones H3 and H4, and methylated on lysine 9 of histone H3, which provides a binding site for the chromodomain protein Swi6 (orthologue of Heterochromatin Protein 1, HP1). The remarkable demonstration that the assembly of repressive heterochromatin is dependent on the RNA interference machinery provokes many questions about the mechanisms of this process that may be tractable in fission yeast. Heterochromatin ensures that a high density of cohesin is recruited to centromeric regions, but it could have additional roles in centromere architecture and the prevention of merotely, and it might also act as a trigger for kinetochore assembly. In addition, we discuss an epigenetic model for ensuring that CENP-A is targeted and replenished at the kinetochore domain.

Anaphase Spindle Mechanics Prevent Mis-Segregation of Merotelically Oriented Chromosomes

Merotelic kinetochore orientation is a kinetochore misattachment in which a single kinetochore is attached to microtubules from both spindle poles instead of just one. It can be favored in specific circumstances, is not detected by the mitotic checkpoint, and induces lagging chromosomes in anaphase. In mammalian cells, it occurs at high frequency in early mitosis, but few anaphase cells show lagging chromosomes. We developed live-cell imaging methods to determine whether and how the mitotic spindle prevents merotelic kinetochores from producing lagging chromosomes. We found that merotelic kinetochores entering anaphase never lost attachment to the spindle poles; they remained attached to both microtubule bundles, but this did not prevent them from segregating correctly. The two microtubule bundles usually showed different fluorescence intensities, the brighter bundle connecting the merotelic kinetochore to the correct pole. During anaphase, the dimmer bundle lengthened much more than the brighter bundle as spindle elongation occurred. This resulted in correct segregation of the merotelically oriented chromosome. We propose a model based on the ratios of microtubules to the correct versus incorrect pole for how anaphase spindle dynamics and microtubule polymerization at kinetochores prevent potential segregation errors deriving from merotelic kinetochore orientation.

Microtubule nucleation at non-spindle pole body microtubule-organizing centers requires fission yeast centrosomin-related protein mod20p

BACKGROUND

Many types of differentiated eukaryotic cells display microtubule distributions consistent with nucleation from noncentrosomal intracellular microtubule organizing centers (MTOCs), although such structures remain poorly characterized. In fission yeast, two types of MTOCs exist in addition to the spindle pole body, the yeast centrosome equivalent. These are the equatorial MTOC, which nucleates microtubules from the cell division site at the end of mitosis, and interphase MTOCs, which nucleate microtubules from multiple sites near the cell nucleus during interphase.

RESULTS

From an insertional mutagenesis screen we identified a novel gene, mod20+, which is required for microtubule nucleation from non-spindle pole body MTOCs in fission yeast. Mod20p is not required for intranuclear mitotic spindle assembly, although it is required for cytoplasmic astral microtubule growth during mitosis. Mod20p localizes to MTOCs throughout the cell cycle and is also dynamically distributed along microtubules themselves. We find that mod20p is required for the localization of components of the gamma-tubulin complex to non-spindle pole body MTOCs and physically interacts with the gamma-tubulin complex in vivo. Database searches reveal a family of eukaryotic proteins distantly related to mod20p; these are found in organisms ranging from fungi to mammals and include Drosophila centrosomin.

CONCLUSIONS

Mod20p appears to act by recruiting components of the gamma-tubulin complex to non-spindle pole body MTOCs. The identification of mod20p-related proteins in higher eukaryotes suggests that this may represent a general mechanism for the organization of noncentrosomal MTOCs in eukaryotic cells.

Fission yeast Sap1 protein is essential for chromosome stability

Sap1 is a dimeric sequence-specific DNA binding-protein, initially identified for its role in mating-type switching of the fission yeast Schizosaccharomyces pombe. The protein is relatively abundant, around 10,000 dimers/cell, and is localized in the nucleus. sap1+ is essential for viability, and transient overexpression is accompanied by rapid cell death, without an apparent checkpoint response and independently of mating-type switching. Time lapse video microscopy of living cells revealed that the loss of viability is accompanied by abnormal mitosis and chromosome fragmentation. Overexpression of the C terminus of Sap1 induces minichromosome loss associated with the "cut" phenotype (uncoupling mitosis and cytokinesis). These phenotypes are favored when the C terminus of Sap1 is overexpressed during DNA replication. Fluorescence in situ hybridization experiments demonstrated that the cut phenotype is related to precocious centromere separation, a typical marker for loss of cohesion. We propose that Sap1 is an architectural chromatin-associated protein, required for chromosome organization.

In vivo dynamics of Swi6 in yeast: evidence for a stochastic model of heterochromatin

The mechanism for transcriptional silencing of pericentric heterochromatin is conserved from fission yeast to mammals. Silenced genome regions are marked by epigenetic methylation of histone H3, which serves as a binding site for structural heterochromatin proteins. In the fission yeast Schizosaccharomyces pombe, the major structural heterochromatin protein is Swi6. To gain insight into Swi6 function in vivo, we have studied its dynamics in the nucleus of living yeast. We demonstrate that, in contrast to mammalian cells, yeast heterochromatin domains undergo rapid, large-scale motions within the nucleus. Similar to the situation in mammalian cells, Swi6 does not permanently associate with these chromatin domains but binds only transiently to euchromatin and heterochromatin. Swi6 binding dynamics are dependent on growth status and on the silencing factors Clr4 and Rik1, but not Clr1, Clr2, or Clr3. By comparing the kinetics of mutant Swi6 proteins in swi6(-) and swi6(+) strains, we demonstrate that homotypic protein-protein interactions via the chromoshadow domain stabilize Swi6 binding to chromatin in vivo. Kinetic modeling allowed quantitative estimation of residence times and indicated the existence of at least two kinetically distinct populations of Swi6 in heterochromatin. The observed dynamics of Swi6 binding are consistent with a stochastic model of heterochromatin and indicate evolutionary conservation of heterochromatin protein binding properties from mammals to yeast.

Individual microtubule dynamics contribute to the function of mitotic and cytoplasmic arrays in fission yeast

Schizosaccharomyces pombe is an excellent organism for studying microtubule dynamics owing to the presence of well-defined microtubule arrays that undergo dramatic rearrangements during various stages of the cell cycle. Using sensitive time-lapse video microscopy and kymographic analysis, we have determined the polymerization/depolymerization kinetics of individual microtubules within these arrays throughout the fission yeast cell cycle. Interphase bundles are composed of 4-7 microtubules that act autonomously, demonstrating that individual microtubules are responsible for mediating the functions ascribed to these arrays. The nucleation and growth of cytoplasmic microtubules is inhibited upon cellular transition into mitosis, leading to their gradual disappearance. At the onset of mitosis, microtubules form on the nuclear face of the spindle pole body and exhibit dramatically increased dynamics. The presence of these intra-nuclear astral microtubules (INA) is reminiscent of spindle assembly and the search and chromosome capture mechanism observed in metazoan cells. Consistent with other in vivo studies, we do not observe microtubule flux in the anaphase B spindle. Finally, the depolymerization of individual microtubules alternates between each half-spindle, resulting in spindle collapse during telophase. On the basis of these observations, we conclude that microtubules in these diverse cytoskeletal arrays have autonomous behaviors that are an essential component of any model describing cell-cycle-dependent changes in the behavior and function of microtubule arrays.

Hsk1-Dfp1 is required for heterochromatin-mediated cohesion at centromeres

Heterochromatin performs a central role in chromosome segregation and stability by promoting cohesion at centromeres. Establishment of both heterochromatin-mediated silencing and cohesion requires passage through S phase, although the mechanism is unknown. Here we demonstrate that Schizosaccharomyces pombe Hsk1 (CDC7), a conserved Dbf4-dependent protein kinase (DDK) that regulates replication initiation, interacts with and phosphorylates the heterochromatin protein 1 (HP1) equivalent Swi6 (ref. 6). Hsk1 and its regulatory subunit Dfp1 function downstream of Swi6 localization to promote heterochromatin function and cohesion specifically at centromeres. This role for Hsk1-Dfp1 is separable from its replication initiation activity, providing a temporal link between S phase and centromere cohesion that is mediated by heterochromatin.

Distinct centromere domain structures with separate functions demonstrated in live fission yeast cells

Fission yeast (Saccharomyces pombe) centromere DNA is organized in a central core region flanked on either side by a region of outer repeat (otr) sequences. The otr region is known to be heterochromatic and bound by the Swi6 protein whereas the central core region contains an unusual chromatin structure involving the histone H3 variant Cnp1 (S. pombe CENP-A). The central core is the base for formation of the kinetochore structure whereas the flanking region is important for sister centromere cohesion. We have previously shown that the ultrastructural domain structure of S. pombe centromeres in interphase is similar to that of human centromeres. Here we demonstrate that S. pombe centromeres are organized in cytologically distinct domains even in mitosis. Fluorescence in situ hybridization of fixed metaphase cells revealed that the otr regions of the centromere were still held together by cohesion even after the sister kinetochores had separated. In live cells, the central cores and kinetochores of sister chromosomes could be distinguished from one another when they were subjected to mitotic tension. The function of the different centromeric domains was addressed. Transacting mutations affecting the kinetochore (nuf2) central core domain (mis6) and the heterochromatin domain (rik1) were analyzed in live cells. In interphase, both nuf2 and mis6 caused declustering of centromeres from the spindle pole body whereas centromere clustering was normal in rik1 despite an apparent decondensation defect. The declustering of centromeres in mis6 cells correlated with loss the Ndc80 kinetochore marker protein from the centromeres. Interestingly the declustered centromeres were still restricted to the nuclear periphery thus revealing a kinetochore-independent peripheral localization mechanism for heterochromatin. Time-lapse microscopy of live mis6 and nuf2-1 mutant cells in mitosis showed similar severe misaggregation phenotypes whereas the rik1 mutants showed a mild cohesion defect. Thus, S. pombe centromeres have two distinguishable domains even during mitosis, and our functional analyses support the previous observations that the kinetochore/central core and the heterochromatin domains have distinct functions both in interphase and mitosis.

Centromere Silencing and Function in Fission Yeast Is Governed by the Amino Terminus of Histone H3

BACKGROUND

Centromeric domains often consist of repetitive elements that are assembled in specialized chromatin, characterized by hypoacetylation of histones H3 and H4 and methylation of lysine 9 of histone H3 (K9-MeH3). Perturbation of this underacetylated state by transient treatment with histone deacetylase inhibitors leads to defective centromere function, correlating with delocalization of the heterochromatin protein Swi6/HP1. Likewise, deletion of the K9-MeH3 methyltransferase Clr4/Suvar39 causes defective chromosome segregation. Here, we create fission yeast strains retaining one histone H3 and H4 gene; the creation of these strains allows mutation of specific N-terminal tail residues and their role in centromeric silencing and chromosome stability to be investigated.

RESULTS

Reduction of H3/H4 gene dosage to one-third does not affect cell viability or heterochromatin formation. Mutation of lysines 9 or 14 or serine 10 within the amino terminus of histone H3 impairs centromere function, leading to defective chromosome segregation and Swi6 delocalization. Surprisingly, silent centromeric chromatin does not require the conserved lysine 8 and 16 residues of histone H4.

CONCLUSIONS

To date, mutation of conserved N-terminal residues in endogenous histone genes has only been performed in budding yeast, which lacks the Clr4/Suvar39 histone methyltransferase and Swi6/HP1. We demonstrate the importance of conserved residues within the histone H3 N terminus for the maintenance of centromeric heterochromatin in fission yeast. In sharp contrast, mutation of two conserved lysines within the histone H4 tail has no impact on the integrity of centromeric heterochromatin. Our data highlight the striking divergence between the histone tail requirements for the fission yeast and budding yeast silencing pathways.

Fission yeast Cdc23/Mcm10 functions after pre-replicative complex formation to promote Cdc45 chromatin binding

Using a cytological assay to monitor the successive chromatin association of replication proteins leading to replication initiation, we have investigated the function of fission yeast Cdc23/Mcm10 in DNA replication. Inactivation of Cdc23 before replication initiation using tight degron mutations has no effect on Mcm2 chromatin association, and thus pre-replicative complex (pre-RC) formation, although Cdc45 chromatin binding is blocked. Inactivating Cdc23 during an S phase block after Cdc45 has bound causes a small reduction in Cdc45 chromatin binding, and replication does not terminate in the absence of Mcm10 function. These observations show that Cdc23/Mcm10 function is conserved between fission yeast and Xenopus, where in vitro analysis has indicated a similar requirement for Cdc45 binding, but apparently not compared with Saccharomyces cerevisiae, where Mcm10 is needed for Mcm2 chromatin binding. However, unlike the situation in Xenopus, where Mcm10 chromatin binding is dependent on Mcm2-7, we show that the fission yeast protein is bound to chromatin throughout the cell cycle in growing cells, and only displaced from chromatin during quiescence. On return to growth, Cdc23 chromatin binding is rapidly reestablished independently from pre-RC formation, suggesting that chromatin association of Cdc23 provides a link between proliferation and competence to execute DNA replication.

Chromosomes of the budding yeast Saccharomyces cerevisiae

The mitotic chromosomes of the baker's yeast, Saccharomyces cerevisiae, cannot be visualized by standard cytological methods. Only the study of meiotic bivalents and the synaptonemal complex and the visualization of chromosome-sized DNA molecules on pulsed-field gels have provided some insight into chromosome structure and behavior. More recently, advanced techniques such as in situ hybridization, the illumination of chromosomal loci by GFP-tagged DNA-binding proteins, and immunostaining of chromosomal proteins have promoted our knowledge about yeast chromosomes. These novel cytological approaches in combination with the yeast's advanced biochemistry and genetics have produced a great wealth of information on the interplay between molecular and cytological processes and have strengthened the role of yeast as a leading cell biological model organism. Recent cytological studies have revealed much about the chromosomal organization in interphase nuclei and have contributed significantly to our current understanding of chromosome condensation, sister chromatid cohesion, and centromere orientation in mitosis. Moreover, important details about the biochemistry and ultrastructure of meiotic pairing and recombination have been revealed by combined cytological and molecular approaches. This article covers several aspects of yeast chromosome structure, including their organization within interphase nuclei and their behavior during mitosis and meiosis.

Chromosome segregation: clamping down on deviant orientations

Chromosome segregation depends on proper orientation of sister kinetochores. The protein Csm1 is required for mono-orientation of sister kinetochores at meiosis I in budding yeast. Surprisingly, its homologue in fission yeast appears instead of clamp micro-tubule binding sites together on single mitotic kinetochores so that they all face one spindle pole.

Sim4: a novel fission yeast kinetochore protein required for centromeric silencing and chromosome segregation

Fission yeast centromeres are composed of two domains: the central core and the outer repeats. Although both regions are required for full centromere function, the central core has a distinct chromatin structure and is likely to underlie the kinetochore itself, as it is associated with centromere-specific proteins. Genes placed within either region are transcriptionally silenced, reflecting the formation of a functional kinetochore complex and flanking centromeric heterochromatin. Here, transcriptional silencing was exploited to identify components involved in central core silencing and kinetochore assembly or structure. The resulting sim (silencing in the middle of the centromere) mutants display severe chromosome segregation defects. sim2+ encodes a known kinetochore protein, the centromere-specific histone H3 variant Cnp1CENP-A. sim4+ encodes a novel essential coiled-coil protein, which is specifically associated with the central core region and is required for the unusual chromatin structure of this region. Sim4 coimmunoprecipitates with the central core component Mis6 and, like Mis6, affects Cnp1CENP-A association with the central domain. Functional Mis6 is required for Sim4 localization at the kinetochore. Our analyses illustrate the fundamental link between silencing, chromatin structure, and kinetochore function, and establish defective silencing as a powerful approach for identifying proteins required to build a functional kinetochore.

Stretching it: putting the CEN(P-A) in centromere

The centromere is the locus responsible for the segregation of chromosomes during mitosis and meiosis. The number of newly characterised centromere-associated proteins continues to increase. The kinetochore complex assembles at this site and in many organisms is visible as the primary constriction. In several systems the location of the site of kinetochore assembly is known to vary and the site is not specified by a strict cis-acting primary sequence. It is proposed that tension between bioriented sister centromeres may act to imprint the site.

Nonrandom homolog segregation at meiosis I in Schizosaccharomyces pombe mutants lacking recombination

Physical connection between homologous chromosomes is normally required for their proper segregation to opposite poles at the first meiotic division (MI). This connection is generally provided by the combination of reciprocal recombination and sister-chromatid cohesion. In the absence of meiotic recombination, homologs are predicted to segregate randomly at MI. Here we demonstrate that in rec12 mutants of the fission yeast Schizosaccharomyces pombe, which are devoid of meiosis-induced recombination, homologs segregate to opposite poles at MI 63% of the time. Residual, Rec12-independent recombination appears insufficient to account for the observed nonrandom homolog segregation. Dyad asci are frequently produced by rec12 mutants. More than half of these dyad asci contain two viable homozygous-diploid spores, the products of a single reductional division. This set of phenotypes is shared by other S. pombe mutants that lack meiotic recombination, suggesting that nonrandom MI segregation and dyad formation are a general feature of meiosis in the absence of recombination and are not peculiar to rec12 mutants. Rec8, a meiosis-specific sister-chromatid cohesin, is required for the segregation phenotypes displayed by rec12 mutants. We propose that S. pombe possesses a system independent of recombination that promotes homolog segregation and discuss possible mechanisms.

The mal2p protein is an essential component of the fission yeast centromere

Precise segregation of chromosomes requires the activity of a specialized chromatin region, the centromere, that assembles the kinetochore complex to mediate the association with spindle microtubules. We show here that Mal2p, previously identified as a protein required for genome stability, is an essential component of the fission yeast centromere. Loss of functional Mal2p leads to extreme missegregation of chromosomes due to nondisjunction of sister chromatids and results in inviable cells. Mal2p associates specifically with the central region of the complex fission yeast centromere, where it is required for the specialized chromatin architecture as well as for transcriptional silencing of this region. Genetic evidence indicates that mal2(+) interacts with mis12(+), encoding another component of the inner centromere core complex. In addition, Mal2p is required for correct metaphase spindle length. Our data imply that the Mal2p protein is required to build up a functional fission yeast centromere.

Segregating sister genomes: the molecular biology of chromosome separation

During cell division, each daughter cell inherits one copy of every chromosome. Accurate transmission of chromosomes requires that the sister DNA molecules created during DNA replication are disentangled and then pulled to opposite poles of the cell before division. Defects in chromosome segregation produce cells that are aneuploid (containing an abnormal number of chromosomes)-a situation that can have dire consequences. Aneuploidy is a leading cause of spontaneous miscarriages in humans and is also a hallmark of many human cancer cells. Recent work with yeast, Xenopus, and other model systems has provided new information about the proteins that control chromosome segregation during cell division and how the activities of these proteins are coordinated with the cell cycle.

The endoplasmic reticulum cation P-type ATPase Cta4p is required for control of cell shape and microtubule dynamics

Here we describe the phenotypic characterization of the cta4+ gene, encoding a novel member of the P4 family of P-type ATPases of fission yeast. The cta4Delta mutant is temperature sensitive and cold sensitive lethal and displays several morphological defects in cell polarity and cytokinesis. Microtubules are generally destabilized in cells lacking Cta4p. The microtubule length is decreased, and the number of microtubules per cell is increased. This is concomitant with an increase in the number of microtubule catastrophe events in the midzone of the cell. These defects are likely due to a general imbalance in cation homeostasis. Immunofluorescence microscopy and membrane fractionation experiments revealed that green fluorescent protein-tagged Cta4 localizes to the ER. Fluorescence resonance energy transfer experiments in living cells using the yellow cameleon indicator for Ca2+ indicated that Cta4p regulates the cellular Ca2+ concentration. Thus, our results reveal a link between cation homeostasis and the control of cell shape, microtubule dynamics, and cytokinesis, and appoint Ca2+ as a key ion in controlling these processes.

Bi-orienting chromosomes on the mitotic spindle

For the proper segregation of sister chromatids before cell division, each sister kinetochore must attach to microtubules that extend to opposite spindle poles. This process is called bipolar microtubule attachment or chromosome bi-orientation. The mechanism for chromosome bi-orientation lies at the heart of chromosome segregation, but is still poorly understood. Recent studies suggest that cells can promote bi-orientation by re-orienting kinetochore-spindle pole connections.

Two kinesin-like Kin I family proteins in fission yeast regulate the establishment of metaphase and the onset of anaphase A

BACKGROUND

Metaphase is thought to be a force-equilibrium state of "tug of war," in which poleward forces are pulling kinetochores and counteracting the cohesive forces between the centromeres. Unlike conventional kinesins, members of the Kin I family are microtubule-depolymerizing enzymes, which are expected to be molecules that could generate poleward forces.

RESULTS

We have characterized mitotic roles of two Kin I homologs, Klp5 and Klp6, in fission yeast. Klp5 and Klp6 colocalize to the mitotic kinetochores and the spindle midzone. These two proteins form a heterocomplex, but not a homocomplex. Albeit not essential, both proteins are required for accurate chromosome segregation and normal morphology of interphase microtubules. Time-lapse live analysis using GFP-alpha-tubulin indicates that these mutants spend a much longer time (2-fold) in mitosis before the initiation of anaphase B. Further observation using kinetochore and centromere markers shows that, in these mutants, sister centromeres move back and forth between the two poles, indicating that entry into anaphase A is delayed. This is supported by live image analysis showing that Cut2 securin is retained during the prolonged mitosis. Furthermore, the mitotic extension is dependent upon the Mad2 spindle checkpoint.

CONCLUSIONS

We discuss two models of Kin I function in fission yeast. One proposes that Klp5 and Klp6 are required for efficient capturing of kinetochores by the spindles, while the other proposes that they are required to generate tension upon kinetochore capturing. Kin I, therefore, plays a fundamental role in the establishment of metaphase, probably by generating poleward forces at the kinetochores.

Fission yeast mutants affecting telomere clustering and meiosis-specific spindle pole body integrity

In meiotic prophase of many eukaryotic organisms, telomeres attach to the nuclear envelope and form a polarized configuration called the bouquet. Bouquet formation is hypothesized to facilitate homologous chromosome pairing. In fission yeast, bouquet formation and telomere clustering occurs in karyogamy and persists throughout the horsetail stage. Here we report the isolation and characterization of six mutants from our screen for meiotic mutants. These mutants show defective telomere clustering as demonstrated by mislocalization of Swi6::GFP, a heterochromatin-binding protein, and Taz1p::GFP, a telomere-specific protein. These mutants define four complementation groups and are named dot1 to dot4-defective organization of telomeres. dot3 and dot4 are allelic to mat1-Mm and mei4, respectively. Immunolocalization of Sad1, a protein associated with the spindle pole body (SPB), in dot mutants showed an elevated frequency of multiple Sad1-nuclei signals relative to wild type. Many of these Sad1 foci were colocalized with Taz1::GFP. Impaired SPB structure and function were further demonstrated by failure of spore wall formation in dot1, by multiple Pcp1::GFP signals (an SPB component) in dot2, and by abnormal microtubule organizations during meiosis in dot mutants. The coincidence of impaired SPB functions with defective telomere clustering suggests a link between the SPB and the telomere cluster.

Kinesinsklp5+ andklp6+ are required for normal chromosome movement in mitosis

Proper mitotic chromosome segregation requires dynamic interactions between spindle microtubules and kinetochores. Here we demonstrate that two related fission yeast kinesins, klp5+ and klp6+, are required for normal chromosome segregation in mitosis. Null mutants frequently lack a normal metaphase chromosome alignment. Chromosome pairs move back and forth along the spindle for an extended period prior to sister chromatid separation, a phenotype reminiscent of the loss of CENP-E in metazoans. Ultimately, sister chromatids segregate, regardless of chromosome position along the spindle, and viable daughter cells are usually produced. The initiation of anaphase B is sometimes delayed, but the rate of spindle elongation is similar to wildtype. Despite a delay, anaphase B often begins before anaphase A is completed. The klp5Δ and klp6Δ null mutants are synthetically lethal with a deletion of the spindle assembly checkpoint gene, bub1+, several mutants in components of the anaphase promoting complex, and a cold sensitive allele of the kinetochore and microtubule-binding protein, Dis1p. Klp5p-GFP and Klp6p-GFP localize to kinetochores from prophase to the onset of anaphase A, but relocalize to the spindle midzone during anaphase B. These data indicate that Klp5p and Klp6p are kinetochore kinesins required for normal chromosome movement in prometaphase.

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

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

Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest

Sister chromatid cohesion, which is established during the S phase of the eukaryotic cell cycle and persists until the onset of anaphase, is essential for the maintenance of genomic integrity. Cohesion requires the multi-protein complex cohesin, as well as a number of accessory proteins including Pds5/BIMD/Spo76. In the budding yeast Saccharomyces cerevisiae Pds5 is an essential protein that localises to chromosomes in a cohesin-dependent manner. Here we describe the characterisation in the fission yeast Schizosaccharomyces pombe of pds5(+), a novel, non-essential orthologue of S. cerevisiae PDS5. The S. pombe Pds5 protein was localised to punctate nuclear foci in a manner that was dependent on the Rad21 cohesin component. This, together with additional genetic evidence, points towards an involvement of S. pombe Pds5 in sister chromatid cohesion. S. pombe pds5 mutants were hypersensitive to DNA damage and to mitotic metaphase delay, but this sensitivity was apparently not due to precocious loss of sister chromatid cohesion. These cells also suffered increased spontaneous chromosome loss and meiotic defects and their viability was dependent on the spindle checkpoint protein Bub1. Thus, while S. pombe Pds5 has an important cohesin-related role, this differs significantly from that of the equivalent budding yeast protein.

Merotelic kinetochore orientation versus chromosome mono-orientation in the origin of lagging chromosomes in human primary cells

Defects in chromosome segregation play a critical role in producing genomic instability and aneuploidy, which are associated with congenital diseases and carcinogenesis. We recently provided evidence from immunofluorescence and electron microscopy studies that merotelic kinetochore orientation is a major mechanism for lagging chromosomes during mitosis in PtK1 cells. Here we investigate whether human primary fibroblasts exhibit similar errors in chromosome segregation and if at least part of lagging chromosomes may arise in cells entering anaphase in the presence of mono-oriented chromosomes. By using in situ hybridization with alphoid probes to chromosome 7 and 11 we showed that loss of a single sister is much more frequent than loss of both sisters from the same chromosome in anatelophases from human primary fibroblasts released from a nocodazole-induced mitotic arrest, as predicted from merotelic orientation of single kinetochores. Furthermore, the lagging of pairs of separated sisters was higher than expected from random chance indicating that merotelic orientation of one sister may promote merotelic orientation of the other. Kinetochores of lagging chromosomes in anaphase human cells were found to be devoid of the mitotic checkpoint phosphoepitopes recognized by the 3F3/2 antibody, suggesting that they attached kinetochore microtubules prior to anaphase onset. Live cell imaging of H2B histone-GFP-transfected cells showed that cells with mono-oriented chromosomes never enter anaphase and that lagging chromosomes appear during anaphase after chromosome alignment occurs during metaphase. Thus, our results demonstrate that the mitotic checkpoint efficiently prevents the possible aneuploid burden due to mono-oriented chromosomes and that merotelic kinetochore orientation is a major limitation for accurate chromosome segregation and a potentially important mechanism of aneuploidy in human cells.

Fission yeast F-box protein Pof3 is required for genome integrity and telomere function

The Skp1-Cullin-1/Cdc53-F-box protein (SCF) ubiquitin ligase plays an important role in various biological processes. In this enzyme complex, a variety of F-box proteins act as receptors that recruit substrates. We have identified a fission yeast gene encoding a novel F-box protein Pof3, which contains, in addition to the F-box, a tetratricopeptide repeat motif in its N terminus and a leucine-rich-repeat motif in the C terminus, two ubiquitous protein-protein interaction domains. Pof3 forms a complex with Skp1 and Pcu1 (fission yeast cullin-1), suggesting that Pof3 functions as an adaptor for specific substrates. In the absence of Pof3, cells exhibit a number of phenotypes reminiscent of genome integrity defects. These include G2 cell cycle delay, hypersensitivity to UV, appearance of lagging chromosomes, and a high rate of chromosome loss. pof3 deletion strains are viable because the DNA damage checkpoint is continuously activated in the mutant, and this leads to G2 cell cycle delay, thereby preventing the mutant from committing lethal mitosis. Pof3 localizes to the nucleus during the cell cycle. Molecular analysis reveals that in this mutant the telomere is substantially shortened and furthermore transcriptional silencing at the telomere is alleviated. The results highlight a role of the SCF(Pof3) ubiquitin ligase in genome integrity via maintaining chromatin structures.

Requirement of heterochromatin for cohesion at centromeres

Centromeres are heterochromatic in many organisms, but the mitotic function of this silent chromatin remains unknown. During cell division, newly replicated sister chromatids must cohere until anaphase when Scc1/Rad21-mediated cohesion is destroyed. In metazoans, chromosome arm cohesins dissociate during prophase, leaving centromeres as the only linkage before anaphase. It is not known what distinguishes centromere cohesion from arm cohesion. Fission yeast Swi6 (a Heterochromatin protein 1 counterpart) is a component of silent heterochromatin. Here we show that this heterochromatin is specifically required for cohesion between sister centromeres. Swi6 is required for association of Rad21-cohesin with centromeres but not along chromosome arms and, thus, acts to distinguish centromere from arm cohesion. Therefore, one function of centromeric heterochromatin is to attract cohesin, thereby ensuring sister centromere cohesion and proper chromosome segregation.

Chromodomain protein Swi6-mediated role of DNA polymerase alpha in establishment of silencing in fission Yeast

Although DNA replication has been thought to play an important role in the silencing of mating type loci in Saccharomyces cerevisiae, recent studies indicate that silencing can be decoupled from replication. In Schizosaccharomyces pombe, mating type silencing is brought about by the trans-acting proteins, namely Swi6, Clr1-Clr4, and Rhp6, in cooperation with the cis-acting silencers. The latter contain an autonomous replication sequence, suggesting that DNA replication may be critical for silencing in S. pombe. To investigate the connection between DNA replication and silencing in S. pombe, we analyzed several temperature-sensitive mutants of DNA polymerase alpha. We find that one such mutant, swi7H4, exhibits silencing defects at mat, centromere, and telomere loci. This effect is independent of the checkpoint and replication defects of the mutant. Interestingly, the extent of the silencing defect in the swi7H4 mutant at the silent mat2 locus is further enhanced in absence of the cis-acting, centromere-proximal silencer. The chromodomain protein Swi6, which is required for silencing and is localized to mat and other heterochromatin loci, interacts with DNA polymerase alpha in vivo and in vitro in wild type cells. However, it does not interact with the mutant pol alpha and is delocalized away from the silent mat loci in the mutant. Our results demonstrate a role of DNA polymerase alpha in the establishment of silencing. We propose a recruitment model for the coupling of DNA replication with the establishment of silencing by the chromodomain protein Swi6, which may be applicable to higher eukaryotes.

A journey into space

The fission yeast, Schizosaccharomyces pombe, has been used as a model eukaryote to study processes such as the cell cycle and cell morphology. In this single-celled organism, growing in a straight line and maintaining the nucleus in the centre of the cell depend on intracellular positional information. Microtubules and microtubular transport are important for generating positional information within the fission yeast cell, and these molecular mechanisms are also probably relevant for generating positional information in other eukaryotic cells.

The domain structure of centromeres is conserved from fission yeast to humans

The centromeric DNA of fission yeast is arranged with a central core flanked by repeated sequences. The centromere-associated proteins, Mis6p and Cnp1p (SpCENP-A), associate exclusively with central core DNA, whereas the Swi6 protein binds the surrounding repeats. Here, electron microscopy and immunofluorescence light microscopy reveal that the central core and flanking regions occupy distinct positions within a heterochromatic domain. An "anchor" structure containing the Ndc80 protein resides between this heterochromatic domain and the spindle pole body. The organization of centromere-associated proteins in fission yeast is reminiscent of the multilayered structures of human kinetochores, indicating that such domain structure is conserved in eukaryotes.

Bir1/Cut17 moving from chromosome to spindle upon the loss of cohesion is required for condensation, spindle elongation and repair

BACKGROUND

In mammals, proteins containing BIR domains (IAPs and survivin) are implicated in inhibiting apoptosis and sister chromatid separation. In the nematode, Bir1 is required for a proper localization of aurora kinase, which moves from the mitotic chromosome in metaphase to the spindle midzone in anaphase as a passenger. Fission yeast Bir1/Pbh1 is essential for normal mitosis.

RESULTS

A temperature sensitive mutant cut17-275 exhibits the defect in condensation and spindle elongation at 36 degrees C, while securin is degraded. Gene cloning shows that the cut17+ gene is identical to bir1+/pbh1+. At 26 degrees C, cut17-275 is UV sensitive as the repair of DNA damage is severely compromised. Bir1/Cut17 is a nuclear protein in interphase, which is then required for recruiting condensin to the mitotic nucleus, and concentrates to form a discrete number of dots from prometaphase to metaphase. Once the chromatids are separated, Bir1/Cut17 no longer binds to kinetochores and instead moves to the middle of spindle. Chromatin immunoprecipitation suggested that Bir1/Cut17 associates with the outer repetitious centromere region in metaphase. Following the initiation of anaphase the protein switches from being a chromosomal protein to a spindle protein. This transit is stringently regulated by the state of sister chromatid cohesion proteins Mis4 and Rad21. Ark1, is an aurora kinase homologue whose mitotic distribution is identical to, and under the control of Bir1/Cut17.

CONCLUSIONS

Bir1/Cut17 and Ark1 act as "passengers" but they may play a main role as a recruitment factor, essential for condensation, spindle elongation and DNA repair. Bir1/Cut17 should have roles both in mitotic and in interphase chromosome. The proper location of Ark1 requires Bir1/Cut17, and the mitotic localization of Bir1/Cut17 requires sister cohesion.

Male meiotic spindle lengths in normal and mutant arabidopsis cells

Spindle elongation is crucial to normal chromosome separation in eukaryotes; in particular, it is required for or associated with the extension of distance between spindle poles and the further moving apart of the already separated chromosomes. However, little is known about the relationship between spindle elongation and the status of chromosome separation, and it is unknown whether spindle elongation in different organisms shares any quantitative feature. The Arabidopsis ask1-1 mutant might be a unique material for addressing these questions because it appears to have functional spindles, but a severe defect in homolog separation at male anaphase I (M. Yang, Y. Hu, M. Lodhi, W.R. McCombie, H Ma [1999] Proc Natl Acad Sci USA 96: 11416-11421). We have characterized male meiotic spindle lengths in wild-type and the ask1-1 mutant plants. We observed that during meiosis I some ask1-1 cells had spindles that were similar in length to fully elongated normal spindles, but the chromosomes in these cells did not show appreciable movement from the equator. Furthermore, greater movement of chromosomes from the equator was usually found in the ask1-1 cells that had longer than normal spindles. These results suggest that additional elongation of ask1-1 spindles occurred; one possible reason for the extra-long spindles may be that it is a consequence of chromosome non-separation. We also found that normal and ask1-1 spindle lengths are clustered at discrete values, and their differences are of multiples of 0.7 microm. A search of the literature revealed that in each of several organisms, spindle lengths also differ by multiples of 0.7 microm. These findings strongly suggest that the spindle elongates in response to status of chromosome separation, and perhaps there are conserved mechanisms controlling the extent of spindle elongation.

Establishment of a cellular axis in fission yeast

Recent studies in fission yeast Schizosaccharomyces pombe reveal how cells establish a cellular axis that specifies domains as the functional 'ends' and 'middle' of the cell. During interphase, dynamic microtubules position the nucleus at the middle of the cell and orientate microtubule 'plus' ends towards the ends of the cell. At the cell ends, the microtubule plus ends might establish a zone of polarized cell growth and actin assembly by depositing factors such as Tea1p. At the cell middle, the nucleus might specify the position of the actin contractile ring and the future cell division site by positioning cytokinesis factors such as Mid1p.