The core centromere and Sgo1 establish a 50-kb cohesin-protected domain around centromeres during meiosis I

Phosphorylation of H2A by Bub1 prevents chromosomal instability through localizing shugoshin

Bub1 is a multi-task protein kinase required for proper chromosome segregation in eukaryotes. Impairment of Bub1 in humans may lead to chromosomal instability (CIN) or tumorigenesis. Yet, the primary cellular substrate of Bub1 has remained elusive. Here, we show that Bub1 phosphorylates the conserved serine 121 of histone H2A in fission yeast Schizosaccharomyces pombe. The h2a-SA mutant, in which all cellular H2A-S121 is replaced by alanine, phenocopies the bub1 kinase-dead mutant (bub1-KD) in losing the centromeric localization of shugoshin proteins. Artificial tethering of shugoshin to centromeres largely restores the h2a-SA or bub1-KD-related CIN defects, a function that is evolutionally conserved. Thus, Bub1 kinase creates a mark for shugoshin localization and the correct partitioning of chromosomes.

Histone h3 exerts a key function in mitotic checkpoint control

It has been firmly established that many interphase nuclear functions, including transcriptional regulation, are regulated by chromatin and histones. How mitotic progression and quality control might be influenced by histones is less well characterized. We show that histone H3 plays a crucial role in activating the spindle assembly checkpoint in response to a defect in mitosis. Prior to anaphase, all chromosomes must attach to spindles emanating from the opposite spindle pole bodies. The tension between sister chromatids generated by the poleward pulling force is an integral part of chromosome biorientation. Lack of tension due to erroneous attachment activates the spindle assembly checkpoint, which corrects the mistakes and ensures segregation fidelity. A histone H3 mutation impairs the ability of yeast cells to activate the checkpoint in a tensionless crisis, leading to missegregation and aneuploidy. The defects in tension sensing result directly from an attenuated H3-Sgo1p interaction essential for pericentric recruitment of Sgo1p. Reinstating the pericentric enrichment of Sgo1p alleviates the mitotic defects. Histone H3, and hence the chromatin, is thus a key factor transmitting the tension status to the spindle assembly checkpoint.

The Scc2/Scc4 cohesin loader determines the distribution of cohesin on budding yeast chromosomes

Cohesins mediate sister chromatid cohesion and DNA repair and also function in gene regulation. Chromosomal cohesins are distributed nonrandomly, and their deposition requires the heterodimeric Scc2/Scc4 loader. Whether Scc2/Scc4 establishes nonrandom cohesin distributions on chromosomes is poorly characterized, however. To better understand the spatial regulation of cohesin association, we mapped budding yeast Scc2 and Scc4 chromosomal distributions. We find that Scc2/Scc4 resides at previously mapped cohesin-associated regions (CARs) in pericentromeric and arm regions, and that Scc2/Scc4-cohesin colocalization persists after the initial deposition of cohesins in G1/S phase. Pericentromeric Scc2/Scc4 enrichment is kinetochore-dependent, and both Scc2/Scc4 and cohesin associations are coordinately reduced in these regions following chromosome biorientation. Thus, these characteristics of Scc2/Scc4 binding closely recapitulate those of cohesin. Although present in G1, Scc2/Scc4 initially has a poor affinity for CARs, but its affinity increases as cells traverse the cell cycle. Scc2/Scc4 association with CARs is independent of cohesin, however. Taken together, these observations are inconsistent with a previous suggestion that cohesins are relocated by translocating RNA polymerases from separate loading sites to intergenic regions between convergently transcribed genes. Rather, our findings suggest that budding yeast cohesins are targeted to CARs largely by Scc2/Scc4 loader association at these locations.

Establishment of cohesion at the pericentromere by the Ctf19 kinetochore subcomplex and the replication fork-associated factor, Csm3

The cohesin complex holds sister chromatids together from the time of their duplication in S phase until their separation during mitosis. Although cohesin is found along the length of chromosomes, it is most abundant at the centromere and surrounding region, the pericentromere. We show here that the budding yeast Ctf19 kinetochore subcomplex and the replication fork-associated factor, Csm3, are both important mediators of pericentromeric cohesion, but they act through distinct mechanisms. We show that components of the Ctf19 complex direct the increased association of cohesin with the pericentromere. In contrast, Csm3 is dispensable for cohesin enrichment in the pericentromere but is essential in ensuring its functionality in holding sister centromeres together. Consistently, cells lacking Csm3 show additive cohesion defects in combination with mutants in the Ctf19 complex. Furthermore, delaying DNA replication rescues the cohesion defect observed in cells lacking Ctf19 complex components, but not Csm3. We propose that the Ctf19 complex ensures additional loading of cohesin at centromeres prior to passage of the replication fork, thereby ensuring its incorporation into functional linkages through a process requiring Csm3.

During cell division, chromosomes must be distributed accurately to daughter cells to protect against aneuploidy, a state in which cells have too few or too many chromosomes, and which is associated with diseases such as cancer and birth defects. This process begins with the generation of an exact copy of each chromosome and the establishment of tight linkages that hold the newly duplicated sister chromosomes together. These linkages, generated by the cohesin complex, are essential to resist the pulling forces of the spindle, which will pull the sister chromosomes apart into the two new daughter cells. Here we examine the establishment of cohesin at the pericentromere, the region surrounding the site of spindle attachment and where its forces are strongest. We find that a dedicated pathway promotes cohesin establishment in this region through a two-step mechanism. In the first step, a group of proteins, known as the Ctf19 complex, promote the association of cohesin with this region. In the second step, the Csm3 protein, which is coupled to the DNA replication machinery, ensures its conversion into functional linkages. We demonstrate the importance of this process for accurate chromosome segregation during cell division.

The role of meiotic cohesin REC8 in chromosome segregation in gamma irradiation-induced endopolyploid tumour cells

Escape from mitotic catastrophe and generation of endopolyploid tumour cells (ETCs) represents a potential survival strategy of tumour cells in response to genotoxic treatments. ETCs that resume the mitotic cell cycle have reduced ploidy and are often resistant to these treatments. In search for a mechanism for genome reduction, we previously observed that ETCs express meiotic proteins among which REC8 (a meiotic cohesin component) is of particular interest, since it favours reductional cell division in meiosis. In the present investigation, we induced endopolyploidy in p53-dysfunctional human tumour cell lines (Namalwa, WI-L2-NS, HeLa) by gamma irradiation, and analysed the sub-cellular localisation of REC8 in the resulting ETCs. We observed by RT-PCR and Western blot that REC8 is constitutively expressed in these tumour cells, along with SGOL1 and SGOL2, and that REC8 becomes modified after irradiation. REC8 localised to paired sister centromeres in ETCs, the former co-segregating to opposite poles. Furthermore, REC8 localised to the centrosome of interphase ETCs and to the astral poles in anaphase cells where it colocalised with the microtubule-associated protein NuMA. Altogether, our observations indicate that radiation-induced ETCs express features of meiotic cell divisions and that these may facilitate chromosome segregation and genome reduction.

Dynamic reprogramming of transcription factors to and from the subtelomere

Transcription factors are most commonly thought of as proteins that regulate expression of specific genes, independently of the order of those genes along the chromosome. By screening genome-wide chromatin immunoprecipitation (ChIP) profiles in yeast, we find that more than 10% of DNA-binding transcription factors concentrate at the subtelomeric regions near to chromosome ends. None of the proteins identified were previously implicated in regulation at telomeres, yet genomic and proteomic studies reveal that a subset of factors show many interactions with established telomere binding complexes. For many factors, the subtelomeric binding pattern is dynamic and undergoes flux toward or away from the telomere as physiological conditions shift. We find that subtelomeric binding is dependent on environmental conditions and correlates with the induction of gene expression in response to stress. Taken together, these results underscore the importance of genome structure in understanding the regulatory dynamics of transcriptional networks.

Shugoshin prevents cohesin cleavage by PP2A(Cdc55)-dependent inhibition of separase

Chromosome segregation is triggered by separase, an enzyme that cleaves cohesin, the protein complex that holds sister chromatids together. Separase activation requires the destruction of its inhibitor, securin, which occurs only upon the correct attachment of chromosomes to the spindle. However, other mechanisms restrict separase activity to the appropriate window in the cell cycle because cohesin is cleaved in a timely manner in securin-deficient cells. We investigated the mechanism by which the protector protein Shugoshin counteracts cohesin cleavage in budding yeast. We show that Shugoshin can prevent separase activation independently of securin. Instead, PP2A(Cdc55) is essential for Shugoshin-mediated inhibition of separase. Loss of both securin and Cdc55 leads to premature sister chromatid separation, resulting in aneuploidy. We propose that Cdc55 is a separase inhibitor that acts downstream from Shugoshin under conditions where sister chromatids are not under tension.

Studies of meiosis disclose distinct roles of cohesion in the core centromere and pericentromeric regions

During meiosis, a single round of genome duplication is followed by two sequential rounds of chromosome segregation. Through this process, a diploid parent cell generates gametes with a haploid set of chromosomes. A characteristic of meiotic chromosome segregation is a stepwise loss of sister chromatid cohesion along chromosomal arms and at centromeres. Whereas arm cohesion plays an important role in ensuring homologue disjunction at meiosis I, persisting cohesion at pericentromeric regions throughout meiosis I is essential for the faithful equational segregation of sisters in the following meiosis II, similar to mitosis. A widely conserved pericentromeric protein called shugoshin, which associates with protein phosphatase 2A (PP2A), plays a critical role in this protection of cohesin. Another key aspect of meiosis I is the establishment of monopolar attachment of sister kinetochores to spindle microtubules. Cohesion or physical linkage at the core centromeres, where kinetochores assemble, may conjoin sister kinetochores, leading to monopolar attachment. A meiosis-specific kinetochore factor such as fission yeast Moa1 or budding yeast monopolin contributes to this regulation. We propose that cohesion at the core centromere and pericentromeric regions plays distinct roles, especially in defining the orientation of kinetochores.

Emerging roles for centromeres in meiosis I chromosome segregation

Centromeres are an essential and conserved feature of eukaryotic chromosomes, yet recent research indicates that we are just beginning to understand the numerous roles that centromeres have in chromosome segregation. During meiosis I, in particular, centromeres seem to function in many processes in addition to their canonical role in assembling kinetochores, the sites of microtubule attachment. Here we summarize recent advances that place centromeres at the centre of meiosis I, and discuss how these studies affect a variety of basic research fields and thus hold promise for increasing our understanding of human reproductive defects and disease states.

The cohesin complex and its roles in chromosome biology

Cohesin is a chromosome-associated multisubunit protein complex that is highly conserved in eukaryotes and has close homologs in bacteria. Cohesin mediates cohesion between replicated sister chromatids and is therefore essential for chromosome segregation in dividing cells. Cohesin is also required for efficient repair of damaged DNA and has important functions in regulating gene expression in both proliferating and post-mitotic cells. Here we discuss how cohesin associates with DNA, how these interactions are controlled during the cell cycle; how binding of cohesin to DNA may mediate sister chromatid cohesion, DNA repair, and gene regulation; and how defects in these processes can lead to human disease.

Shugoshin1 may play important roles in separation of homologous chromosomes and sister chromatids during mouse oocyte meiosis

Background

Homologous chromosomes separate in meiosis I and sister chromatids separate in meiosis II, generating haploid gametes. To address the question why sister chromatids do not separate in meiosis I, we explored the roles of Shogoshin1 (Sgo1) in chromosome separation during oocyte meiosis.

Methodology/Principal Findings

Sgo1 function was evaluated by exogenous overexpression to enhance its roles and RNAi to suppress its roles during two meioses of mouse oocytes. Immunocytochemistry and chromosome spread were used to evaluate phenotypes. The exogenous Sgo1 overexpression kept homologous chromosomes and sister chromatids not to separate in meiosis I and meiosis II, respectively, while the Sgo1 RNAi promoted premature separation of sister chromatids.

Conclusions

Our results reveal that prevention of premature separation of sister chromatids in meiosis I requires the retention of centromeric Sgo1, while normal separation of sister chromatids in meiosis II requires loss of centromeric Sgo1.

Meiotic recombination at the ends of chromosomes in Saccharomyces cerevisiae

Meiotic reciprocal recombination (crossing over) was examined in the outermost 60-80 kb of almost all Saccharomyces cerevisiae chromosomes. These sequences included both repetitive gene-poor subtelomeric heterochromatin-like regions and their adjacent unique gene-rich euchromatin-like regions. Subtelomeric sequences underwent very little crossing over, exhibiting approximately two- to threefold fewer crossovers per kilobase of DNA than the genomic average. Surprisingly, the adjacent euchromatic regions underwent crossing over at twice the average genomic rate and contained at least nine new recombination "hot spots." These results prompted an analysis of existing genetic mapping data, which showed that meiotic reciprocal recombination rates were on average greater near chromosome ends exclusive of the subtelomeres. Thus, the distribution of crossovers in S. cerevisiae appears to resemble that found in several higher eukaryotes where the outermost chromosomal regions show increased crossing over.

Mapping of meiotic single-stranded DNA reveals double-stranded-break hotspots near centromeres and telomeres

BACKGROUND

Every chromosome requires at least one crossover to be faithfully segregated during meiosis. At least two levels of regulation govern crossover distribution: where the initiating DNA double-strand breaks (DSBs) occur and whether those DSBs are repaired as crossovers.

RESULTS

We mapped meiotic DSBs in budding yeast by identifying sites of DSB-associated single-stranded DNA (ssDNA) accumulation. These analyses revealed substantial DSB activity in pericentrometric regions, in which crossover formation is largely absent. Our data suggest that centromeric suppression of recombination occurs at the level of break repair rather than DSB formation. Additionally, we found an enrichment of DSBs within a approximately 100 kb region near the ends of all chromosomes. Introduction of new telomeres was sufficient for inducing large ectopic regions of increased DSB formation, thereby revealing a remarkable long-range effect of telomeres on DSB formation. The concentration of DSBs close to chromosome ends increases the relative DSB density on small chromosomes, providing an interference-independent mechanism that ensures that all chromosomes receive at least one crossover per homolog pair.

CONCLUSIONS

Together, our results indicate that selective DSB repair accounts for crossover suppression near centromeres and suggest a simple telomere-guided mechanism that ensures sufficient DSB activity on all chromosomes.

Mechanisms of sister chromatid pairing

The continuance of life through cell division requires high fidelity DNA replication and chromosome segregation. During DNA replication, each parental chromosome is duplicated exactly and one time only. At the same time, the resulting chromosomes (called sister chromatids) become tightly paired along their length. This S-phase pairing, or cohesion, identifies chromatids as sisters over time. During mitosis in most eukaryotes, sister chromatids bi-orient to the mitotic spindle. After each chromosome pair is properly oriented, the cohesion established during S phase is inactivated in a tightly regulated fashion, allowing sister chromatids to segregate away from each other. Recent findings of cohesin structure and enzymology provide new insights into cohesion, while many critical facets of cohesion (how cohesins tether together sister chromatids and how those tethers are established) remain actively debated.

Shugoshin promotes sister kinetochore biorientation in Saccharomyces cerevisiae

Chromosome segregation must be executed accurately during both mitotic and meiotic cell divisions. Sgo1 plays a key role in ensuring faithful chromosome segregation in at least two ways. During meiosis this protein regulates the removal of cohesins, the proteins that hold sister chromatids together, from chromosomes. During mitosis, Sgo1 is required for sensing the absence of tension caused by sister kinetochores not being attached to microtubules emanating from opposite poles. Here we describe a differential requirement for Sgo1 in the segregation of homologous chromosomes and sister chromatids. Sgo1 plays only a minor role in segregating homologous chromosomes at meiosis I. In contrast, Sgo1 is important to bias sister kinetochores toward biorientation. We suggest that Sgo1 acts at sister kinetochores to promote their biorientation.

Bub1 kinase targets Sgo1 to ensure efficient chromosome biorientation in budding yeast mitosis

During cell division all chromosomes must be segregated accurately to each daughter cell. Errors in this process give rise to aneuploidy, which leads to birth defects and is implicated in cancer progression. The spindle checkpoint is a surveillance mechanism that ensures high fidelity of chromosome segregation by inhibiting anaphase until all kinetochores have established bipolar attachments to spindle microtubules. Bub1 kinase is a core component of the spindle checkpoint, and cells lacking Bub1 fail to arrest in response to microtubule drugs and precociously segregate their DNA. The mitotic role(s) of Bub1 kinase activity remain elusive, and it is controversial whether this C-terminal domain of Bub1p is required for spindle checkpoint arrest. Here we make a detailed analysis of budding yeast cells lacking the kinase domain (bub1DeltaK). We show that despite being able to arrest in response to microtubule depolymerisation and kinetochore-microtubule attachment defects, bub1DeltaK cells are sensitive to microtubule drugs. This is because bub1DeltaK cells display significant chromosome mis-segregation upon release from nocodazole arrest. bub1DeltaK cells mislocalise Sgo1p, and we demonstrate that both the Bub1 kinase domain and Sgo1p are required for accurate chromosome biorientation after nocodazole treatment. We propose that Bub1 kinase and Sgo1p act together to ensure efficient biorientation of sister chromatids during mitosis.

The spindle checkpoint rescues the meiotic segregation of chromosomes whose crossovers are far from the centromere

Improper meiotic chromosome segregation causes conditions such as Down's syndrome. Recombination promotes proper chromosome segregation in meiosis I; chromosomes without crossovers near the centromere are more likely to segregate to the same spindle pole (nondisjoin). Here we have used budding yeast to determine whether the spindle checkpoint promotes segregation of such chromosomes. In checkpoint-defective mad2Delta cells, properly segregating chromosomes have more crossovers near the centromere than their wild-type counterparts, and an artificial tether that holds chromosomes together suppresses nondisjunction as long as the tether is near the centromere. The tether partially rescues the segregation of chromosomes that lack crossovers.

The Aurora kinase Ipl1 maintains the centromeric localization of PP2A to protect cohesin during meiosis

Homologue segregation during the first meiotic division requires the proper spatial regulation of sister chromatid cohesion and its dissolution along chromosome arms, but its protection at centromeric regions. This protection requires the conserved MEI-S332/Sgo1 proteins that localize to centromeric regions and also recruit the PP2A phosphatase by binding its regulatory subunit, Rts1. Centromeric Rts1/PP2A then locally prevents cohesion dissolution possibly by dephosphorylating the protein complex cohesin. We show that Aurora B kinase in Saccharomyces cerevisiae (Ipl1) is also essential for the protection of meiotic centromeric cohesion. Coupled with a previous study in Drosophila melanogaster, this meiotic function of Aurora B kinase appears to be conserved among eukaryotes. Furthermore, we show that Sgo1 recruits Ipl1 to centromeric regions. In the absence of Ipl1, Rts1 can initially bind to centromeric regions but disappears from these regions after anaphase I onset. We suggest that centromeric Ipl1 ensures the continued centromeric presence of active Rts1/PP2A, which in turn locally protects cohesin and cohesion.

Genome-wide redistribution of meiotic double-strand breaks in Saccharomyces cerevisiae

Meiotic recombination is initiated by the formation of programmed DNA double-strand breaks (DSBs) catalyzed by the Spo11 protein. DSBs are not randomly distributed along chromosomes. To better understand factors that control the distribution of DSBs in budding yeast, we have examined the genome-wide binding and cleavage properties of the Gal4 DNA binding domain (Gal4BD)-Spo11 fusion protein. We found that Gal4BD-Spo11 cleaves only a subset of its binding sites, indicating that the association of Spo11 with chromatin is not sufficient for DSB formation. In centromere-associated regions, the centromere itself prevents DSB cleavage by tethered Gal4BD-Spo11 since its displacement restores targeted DSB formation. In addition, we observed that new DSBs introduced by Gal4BD-Spo11 inhibit surrounding DSB formation over long distances (up to 60 kb), keeping constant the number of DSBs per chromosomal region. Together, these results demonstrate that the targeting of Spo11 to new chromosomal locations leads to both local stimulation and genome-wide redistribution of recombination initiation and that some chromosomal regions are inherently cold regardless of the presence of Spo11.

Centromere-proximal crossovers are associated with precocious separation of sister chromatids during meiosis in Saccharomyces cerevisiae

In most organisms, meiotic chromosome segregation is dependent on crossovers (COs), which enable pairs of homologous chromosomes to segregate to opposite poles at meiosis I. In mammals, the majority of meiotic chromosome segregation errors result from a lack of COs between homologs. Observations in Homo sapiens and Drosophila melanogaster have revealed a second class of exceptional events in which a CO occurred near the centromere of the missegregated chromosome. We show that in wild-type strains of Saccharomyces cerevisiae, most spore inviability is due to precocious separation of sister chromatids (PSSC) and that PSSC is often associated with centromere-proximal crossing over. COs, as opposed to nonreciprocal recombination events (NCOs), are preferentially associated with missegregation. Strains mutant for the RecQ homolog, SGS1, display reduced spore viability and increased crossing over. Much of the spore inviability in sgs1 results from PSSC, and these events are often associated with centromere-proximal COs, just as in wild type. When crossing over in sgs1 is reduced by the introduction of a nonnull allele of SPO11, spore viability is improved, suggesting that the increased PSSC is due to increased crossing over. We present a model for PSSC in which a centromere-proximal CO promotes local loss of sister-chromatid cohesion.

DNA microarray technologies for measuring protein-DNA interactions

DNA-binding proteins have key roles in many cellular processes, including transcriptional regulation and replication. Microarray-based technologies permit the high-throughput identification of binding sites and enable the functional roles of these binding proteins to be elucidated. In particular, microarray readout either of chromatin immunoprecipitated DNA-bound proteins (ChIP-chip) or of DNA adenine methyltransferase fusion proteins (DamID) enables the identification of in vivo genomic target sites of proteins. A complementary approach to analyse the in vitro binding of proteins directly to double-stranded DNA microarrays (protein binding microarrays; PBMs), permits rapid characterization of their DNA binding site sequence specificities. Recent advances in DNA microarray synthesis technologies have facilitated the definition of DNA-binding sites at much higher resolution and coverage, and advances in these and emerging technologies will further increase the efficiencies of these exciting new approaches.

Rec8 phosphorylation and recombination promote the step-wise loss of cohesins in meiosis

During meiosis, cohesins--protein complexes that hold sister chromatids together--are lost from chromosomes in a step-wise manner. Loss of cohesins from chromosome arms is necessary for homologous chromosomes to segregate during meiosis I. Retention of cohesins around centromeres until meiosis II is required for the accurate segregation of sister chromatids. Here we show that phosphorylation of the cohesin subunit Rec8 contributes to step-wise cohesin removal. Our data further implicate two other key regulators of meiotic chromosome segregation, the cohesin protector Sgo1 and meiotic recombination in bringing about the step-wise loss of cohesins and thus the establishment of the meiotic chromosome segregation pattern. Understanding the interplay between these processes should provide insight into the events underlying meiotic chromosome mis-segregation, the leading cause of miscarriages and mental retardation in humans.

Shugoshin collaborates with protein phosphatase 2A to protect cohesin

Sister chromatid cohesion, mediated by a complex called cohesin, is crucial--particularly at centromeres--for proper chromosome segregation in mitosis and meiosis. In animal mitotic cells, phosphorylation of cohesin promotes its dissociation from chromosomes, but centromeric cohesin is protected by shugoshin until kinetochores are properly captured by the spindle microtubules. However, the mechanism of shugoshin-dependent protection of cohesin is unknown. Here we find a specific subtype of serine/threonine protein phosphatase 2A (PP2A) associating with human shugoshin. PP2A colocalizes with shugoshin at centromeres and is required for centromeric protection. Purified shugoshin complex has an ability to reverse the phosphorylation of cohesin in vitro, suggesting that dephosphorylation of cohesin is the mechanism of protection at centromeres. Meiotic shugoshin of fission yeast also associates with PP2A, with both proteins collaboratively protecting Rec8-containing cohesin at centromeres. Thus, we have revealed a conserved mechanism of centromeric protection of eukaryotic chromosomes in mitosis and meiosis.