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

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

Eco1-dependent cohesin acetylation anchors chromatin loops and cohesion to define functional meiotic chromosome domains

Cohesin organizes the genome by forming intra-chromosomal loops and inter-sister chromatid linkages. During gamete formation by meiosis, chromosomes are reshaped to support crossover recombination and two consecutive rounds of chromosome segregation. Here we show that meiotic chromosomes are organised into functional domains by Eco1 acetyltransferase-dependent positioning of both chromatin loops and sister chromatid cohesion in budding yeast. Eco1 acetylates the Smc3 cohesin subunit in meiotic S phase to establish chromatin boundaries, independently of DNA replication. Boundary formation by Eco1 is critical for prophase exit and for the maintenance of cohesion until meiosis II, but is independent of the ability of Eco1 to antagonize the cohesin-release factor, Wpl1. Conversely, prevention of cohesin release by Wpl1 is essential for centromeric cohesion, kinetochore monoorientation and co-segregation of sister chromatids in meiosis I. Our findings establish Eco1 as a key determinant of chromatin boundaries and cohesion positioning, revealing how local chromosome structuring directs genome transmission into gametes.

Minichromosome maintenance proteins in eukaryotic chromosome segregation

Minichromosome maintenance (Mcm) proteins are well-known for their functions in DNA replication. However, their roles in chromosome segregation are yet to be reviewed in detail. Following the discovery in 1984, a group of Mcm proteins, known as the ARS-nonspecific group consisting of Mcm13, Mcm16-19, and Mcm21-22, were characterized as bonafide kinetochore proteins and were shown to play significant roles in the kinetochore assembly and high-fidelity chromosome segregation. This review focuses on the structure, function, and evolution of this group of Mcm proteins. Our in silico analysis of the physical interactors of these proteins reveals that they share non-overlapping functions despite being copurified in biochemically stable complexes. We have discussed the contrasting results reported in the literature and experimental strategies to address them. Taken together, this review focuses on the structure-function of the ARS-nonspecific Mcm proteins and their evolutionary flexibility to maintain genome stability in various organisms.

Shugoshin ensures maintenance of the spindle assembly checkpoint response and efficient spindle disassembly

Shugoshin proteins are evolutionarily conserved across eukaryotes, with some species-specific cellular functions, ensuring the fidelity of chromosome segregation. They act as adaptors at various subcellular locales to mediate several protein-protein interactions in a spatio-temporal manner. Here, we characterize shugoshin (Sgo1) in the human fungal pathogen Candida albicans. We observe that Sgo1 retains its centromeric localization and performs its conserved functions of regulating the sister chromatid biorientation, centromeric condensin localization, and maintenance of chromosomal passenger complex (CPC). We identify novel roles of Sgo1 as a spindle assembly checkpoint (SAC) component with functions in maintaining a prolonged SAC response by retaining Mad2 and Bub1 at the kinetochores in response to improper kinetochore-microtubule attachments. Strikingly, we discover the in vivo localization of Sgo1 along the length of the mitotic spindle. Our results indicate that Sgo1 performs a hitherto unknown function of facilitating timely disassembly of the mitotic spindle in C. albicans. To summarize, this study unravels a unique functional adaptation of shugoshin in maintaining genomic stability.

SUMOylation stabilizes sister kinetochore biorientation to allow timely anaphase

During mitosis, sister chromatids attach to microtubules from opposite poles, called biorientation. Sister chromatid cohesion resists microtubule forces, generating tension, which provides the signal that biorientation has occurred. How tension silences the surveillance pathways that prevent cell cycle progression and correct erroneous kinetochore-microtubule attachments remains unclear. Here we show that SUMOylation dampens error correction to allow stable sister kinetochore biorientation and timely anaphase onset. The Siz1/Siz2 SUMO ligases modify the pericentromere-localized shugoshin (Sgo1) protein before its tension-dependent release from chromatin. Sgo1 SUMOylation reduces its binding to protein phosphatase 2A (PP2A), and weakening of this interaction is important for stable biorientation. Unstable biorientation in SUMO-deficient cells is associated with persistence of the chromosome passenger complex (CPC) at centromeres, and SUMOylation of CPC subunit Bir1 also contributes to timely anaphase onset. We propose that SUMOylation acts in a combinatorial manner to facilitate dismantling of the error correction machinery within pericentromeres and thereby sharpen the metaphase-anaphase transition.

Ctf3/CENP-I provides a docking site for the desumoylase Ulp2 at the kinetochore

The step-by-step process of chromosome segregation defines the stages of the cell cycle. In eukaryotes, signals controlling these steps converge upon the kinetochore, a multiprotein assembly that connects spindle microtubules to chromosomal centromeres. Kinetochores control and adapt to major chromosomal transactions, including replication of centromeric DNA, biorientation of sister centromeres on the metaphase spindle, and transit of sister chromatids into daughter cells during anaphase. Although the mechanisms that ensure tight microtubule coupling at anaphase are at least partly understood, kinetochore adaptations that support other cell cycle transitions are not. We report here a mechanism that enables regulated control of kinetochore sumoylation. A conserved surface of the Ctf3/CENP-I kinetochore protein provides a binding site for Ulp2, the nuclear enzyme that removes SUMO chains from modified substrates. Ctf3 mutations that disable Ulp2 recruitment cause elevated inner kinetochore sumoylation and defective chromosome segregation. The location of the site within the assembled kinetochore suggests coordination between sumoylation and other cell cycle–regulated processes.

Deprotection of centromeric cohesin at meiosis II requires APC/C activity but not kinetochore tension

Genome haploidization involves sequential loss of cohesin from chromosome arms and centromeres during two meiotic divisions. At centromeres, cohesin's Rec8 subunit is protected from separase cleavage at meiosis I and then deprotected to allow its cleavage at meiosis II. Protection of centromeric cohesin by shugoshin-PP2A seems evolutionarily conserved. However, deprotection has been proposed to rely on spindle forces separating the Rec8 protector from cohesin at metaphase II in mammalian oocytes and on APC/C-dependent destruction of the protector at anaphase II in yeast. Here, we have activated APC/C in the absence of sister kinetochore biorientation at meiosis II in yeast and mouse oocytes, and find that bipolar spindle forces are dispensable for sister centromere separation in both systems. Furthermore, we show that at least in yeast, protection of Rec8 by shugoshin and inhibition of separase by securin are both required for the stability of centromeric cohesin at metaphase II. Our data imply that related mechanisms preserve the integrity of dyad chromosomes during the short metaphase II of yeast and the prolonged metaphase II arrest of mammalian oocytes.

Functioning mechanisms of Shugoshin-1 in centromeric cohesion during mitosis

Proper regulation of centromeric cohesion is required for faithful chromosome segregation that prevents chromosomal instability. Extensive studies have identified and established the conserved protein Shugoshin (Sgo1/2) as an essential protector for centromeric cohesion. In this review, we summarize the current understanding of how Shugoshin-1 (Sgo1) protects centromeric cohesion at the molecular level. Targeting of Sgo1 to inner centromeres is required for its proper function of cohesion protection. We therefore discuss about the molecular mechanisms that install Sgo1 onto inner centromeres. At metaphase-to-anaphase transition, Sgo1 at inner centromeres needs to be disabled for the subsequent sister-chromatid segregation. A few recent studies suggest interesting models to explain how it is achieved. These models are discussed as well.

Meiosis I Kinase Regulators: Conserved Orchestrators of Reductional Chromosome Segregation

Research over the last two decades has identified a group of meiosis-specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister kinetochores face towards the same pole (mono-orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk.

Meiotic CENP-C is a shepherd: bridging the space between the centromere and the kinetochore in time and space

While many of the proteins involved in the mitotic centromere and kinetochore are conserved in meiosis, they often gain a novel function due to the unique needs of homolog segregation during meiosis I (MI). CENP-C is a critical component of the centromere for kinetochore assembly in mitosis. Recent work, however, has highlighted the unique features of meiotic CENP-C. Centromere establishment and stability require CENP-C loading at the centromere for CENP-A function. Pre-meiotic loading of proteins necessary for homolog recombination as well as cohesion also rely on CENP-C, as do the main scaffolding components of the kinetochore. Much of this work relies on new technologies that enable in vivo analysis of meiosis like never before. Here, we strive to highlight the unique role of this highly conserved centromere protein that loads on to centromeres prior to M-phase onset, but continues to perform critical functions through chromosome segregation. CENP-C is not merely a structural link between the centromere and the kinetochore, but also a functional one joining the processes of early prophase homolog synapsis to late metaphase kinetochore assembly and signaling.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN. Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach. Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13 Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13 Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8. Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13 Δ cells.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN.

Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach.

Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8.

Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13Δ cells.

Meiotic prophase-like pathway for cleavage-independent removal of cohesin for chromosome morphogenesis

Sister chromatid cohesion is essential for chromosome segregation both in mitosis and meiosis. Cohesion between two chromatids is mediated by a protein complex called cohesin. The loading and unloading of the cohesin are tightly regulated during the cell cycle. In vertebrate cells, cohesin is released from chromosomes by two distinct pathways. The best characterized pathway occurs at the onset of anaphase, when the kleisin component of the cohesin is destroyed by a protease, separase. The cleavage of the cohesin by separase releases entrapped sister chromatids allowing anaphase to commence. In addition, prior to the metaphase-anaphase transition, most of cohesin is removed from chromosomes in a cleavage-independent manner. This cohesin release is referred to as the prophase pathway. In meiotic cells, sister chromatid cohesion is essential for the segregation of homologous chromosomes during meiosis I. Thus, it was assumed that the prophase pathway for cohesin removal from chromosome arms would be suppressed during meiosis to avoid errors in chromosome segregation. However, recent studies revealed the presence of a meiosis-specific prophase-like pathway for cleavage-independent removal of cohesin during late prophase I in different organisms. In budding yeast, the cleavage-independent removal of cohesin is mediated through meiosis-specific phosphorylation of cohesin subunits, Rec8, the meiosis-specific kleisin, and the yeast Wapl ortholog, Rad61/Wpl1. This pathway plays a role in chromosome morphogenesis during late prophase I, promoting chromosome compaction. In this review, we give an overview of the prophase pathway for cohesin dynamics during meiosis, which has a complex regulation leading to differentially localized populations of cohesin along meiotic chromosomes.

Reductional Meiosis I Chromosome Segregation Is Established by Coordination of Key Meiotic Kinases

Meiosis produces gametes through a specialized, two-step cell division, which is highly error prone in humans. Reductional meiosis I, where maternal and paternal chromosomes (homologs) segregate, is followed by equational meiosis II, where sister chromatids separate. Uniquely during meiosis I, sister kinetochores are monooriented and pericentromeric cohesin is protected. Here, we demonstrate that these key adaptations for reductional chromosome segregation are achieved through separable control of multiple kinases by the meiosis-I-specific budding yeast Spo13 protein. Recruitment of Polo kinase to kinetochores directs monoorientation, while independently, cohesin protection is achieved by containing the effects of cohesin kinases. Therefore, reductional chromosome segregation, the defining feature of meiosis, is established by multifaceted kinase control by a master regulator. The recent identification of Spo13 orthologs, fission yeast Moa1 and mouse MEIKIN, suggests that kinase coordination by a meiosis I regulator may be a general feature in the establishment of reductional chromosome segregation.

An interplay between Shugoshin and Spo13 for centromeric cohesin protection and sister kinetochore mono-orientation during meiosis I in Saccharomyces cerevisiae

Meiosis is a specialized cell division process by which haploid gametes are produced from a diploid mother cell. Reductional chromosome segregation during meiosis I (MI) is achieved by two unique and conserved events: centromeric cohesin protection (CCP) and sister kinetochore mono-orientation (SKM). In Saccharomyces cerevisiae, a meiosis-specific protein Spo13 plays a role in both these centromere-specific events. Despite genome-wide association of Spo13, we failed to detect its function in global processes such as cohesin loading, cohesion establishment and homologs pairing. While Shugoshin (Sgo1) and protein phosphatase 2A (PP2ARts1) play a central role in CCP, it is not fully understood whether Spo13 functions in the process through a Sgo1- PP2ARts1-dependent or -independent mechanism. To delineate this and to find the relative contribution of each of these proteins in CCP and SKM, we meticulously observed the sister chromatid segregation pattern in the wild type, sgo1Δ, rts1Δ and spo13Δ single mutants and in their respective double mutants. We found that Spo13 protects centromeric cohesin through a Sgo1- PP2ARts1-independent mechanism. To our surprise, we observed a hitherto unknown role of Sgo1 in SKM. Further investigation revealed that Sgo1-mediated recruitment of aurora kinase Ipl1 to the centromere facilitates monopolin loading at the kinetochore during MI. Hence, this study uncovers the role of Sgo1 in SKM and demonstartes how the regulators (Sgo1, PP2ARts1, Spo13) work in a coordinated manner to achieve faithful chromosome segregation during meiosis, the failure of which leads to aneuploidy and birth defects.

Tripartite Chromatin Localization of Budding Yeast Shugoshin Involves Higher-Ordered Architecture of Mitotic Chromosomes

The spindle assembly checkpoint (SAC) is key to faithful segregation of chromosomes. One requirement that satisfies SAC is appropriate tension between sister chromatids at the metaphase-anaphase juncture. Proper tension generated by poleward pulling of mitotic spindles signals biorientation of the underlying chromosome. In the budding yeast, the tension status is monitored by the conserved Shugoshin protein, Sgo1p, and the tension sensing motif (TSM) of histone H3. ChIP-seq reveals a unique TSM-dependent, tripartite domain of Sgo1p in each mitotic chromosome. This domain consists of one centromeric and two flanking peaks 3 - 4 kb away, present exclusively in mitosis. Strikingly, this trident motif coincides with cohesin localization, but only at the centromere and the two immediate adjacent loci, despite that cohesin is enriched at numerous regions throughout mitotic chromosomes. Chromosome conformation capture assays reveal apparent looping at the centromeric and pericentric regions. The TSM-Sgo1p-cohesin triad is therefore at the center stage of higher-ordered chromatin architecture for error-free segregation.

Budding yeast CENP-ACse4 interacts with the N-terminus of Sgo1 and regulates its association with centromeric chromatin

Shugoshin is an evolutionarily conserved protein, which is involved in tension sensing on mitotic chromosomes, kinetochore biorientation, and protection of centromeric (CEN) cohesin for faithful chromosome segregation. Interaction of the C-terminus of Sgo1 with phosphorylated histone H2A regulates its association with CEN and pericentromeric (peri-CEN) chromatin, whereas mutations in histone H3 selectively compromise the association of Sgo1 with peri-CEN but not CEN chromatin. Given that histone H3 is absent from CEN and is replaced by a histone H3 variant CENP-A^Cse4, we investigated if CENP-A^Cse4 interacts with Sgo1 and promotes its association with the CEN chromatin. In this study, we found that Sgo1 interacts with CENP-A^Cse4 in vivo and in vitro. The N-terminus coiled-coil domain of Sgo1 without the C-terminus (sgo1-NT) is sufficient for its interaction with CENP-A^Cse4, association with CEN but not the peri-CEN, and this CEN association is cell cycle dependent with maximum enrichment in mitosis. In agreement with the role of CENP-A^Cse4 in CEN maintenance of Sgo1, depletion of CENP-A^Cse4 results in the loss of Sgo1 and sgo1-NT from the CEN chromatin. The N-terminus of Sgo1 is required for genome stability as a mutant lacking the N-terminus (sgo1-CT) exhibits increased chromosome missegregation when compared to a sgo1-NT mutant. In summary, our results define a novel role for the N-terminus of Sgo1 in CENP-A^Cse4 mediated recruitment of Sgo1 to CEN chromatin for faithful chromosome segregation.

Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Cell division in mitosis and meiosis is governed by evolutionary highly conserved protein kinases and phosphatases, controlling the timely execution of key events such as nuclear envelope breakdown, spindle assembly, chromosome attachment to the spindle and chromosome segregation, and cell cycle exit. In mitosis, the spindle assembly checkpoint (SAC) controls the proper attachment to and alignment of chromosomes on the spindle. The SAC detects errors and induces a cell cycle arrest in metaphase, preventing chromatid separation. Once all chromosomes are properly attached, the SAC-dependent arrest is relieved and chromatids separate evenly into daughter cells. The signaling cascade leading to checkpoint arrest depends on several protein kinases that are conserved from yeast to man. In meiosis, haploid cells containing new genetic combinations are generated from a diploid cell through two specialized cell divisions. Though apparently less robust, SAC control also exists in meiosis. Recently, it has emerged that SAC kinases have additional roles in executing accurate chromosome segregation during the meiotic divisions. Here, we summarize the main differences between mitotic and meiotic cell divisions, and explain why meiotic divisions pose special challenges for correct chromosome segregation. The less-known meiotic roles of the SAC kinases are described, with a focus on two model systems: yeast and mouse oocytes. The meiotic roles of the canonical checkpoint kinases Bub1, Mps1, the pseudokinase BubR1 (Mad3), and Aurora B and C (Ipl1) will be discussed. Insights into the molecular signaling pathways that bring about the special chromosome segregation pattern during meiosis will help us understand why human oocytes are so frequently aneuploid.

A Functional Link Between Bir1 and the Saccharomyces cerevisiae Ctf19 Kinetochore Complex Revealed Through Quantitative Fitness Analysis

The chromosomal passenger complex (CPC) is a key regulator of eukaryotic cell division, consisting of the protein kinase Aurora B/Ipl1 in association with its activator (INCENP/Sli15) and two additional proteins (Survivin/Bir1 and Borealin/Nbl1). Here, we report a genome-wide genetic interaction screen in Saccharomyces cerevisiae using the bir1-17 mutant, identifying through quantitative fitness analysis deletion mutations that act as enhancers and suppressors. Gene knockouts affecting the Ctf19 kinetochore complex were identified as the strongest enhancers of bir1-17, while mutations affecting the large ribosomal subunit or the mRNA nonsense-mediated decay pathway caused strong phenotypic suppression. Thus, cells lacking a functional Ctf19 complex become highly dependent on Bir1 function and vice versa. The negative genetic interaction profiles of bir1-17 and the cohesin mutant mcd1-1 showed considerable overlap, underlining the strong functional connection between sister chromatid cohesion and chromosome biorientation. Loss of some Ctf19 components, such as Iml3 or Chl4, impacted differentially on bir1-17 compared with mutations affecting other CPC components: despite the synthetic lethality shown by either iml3∆ or chl4∆ in combination with bir1-17, neither gene knockout showed any genetic interaction with either ipl1-321 or sli15-3 Our data therefore imply a specific functional connection between the Ctf19 complex and Bir1 that is not shared with Ipl1.

Structure of centromere chromatin: from nucleosome to chromosomal architecture

The centromere is essential for the segregation of chromosomes, as it serves as attachment site for microtubules to mediate chromosome segregation during mitosis and meiosis. In most organisms, the centromere is restricted to one chromosomal region that appears as primary constriction on the condensed chromosome and is partitioned into two chromatin domains: The centromere core is characterized by the centromere-specific histone H3 variant CENP-A (also called cenH3) and is required for specifying the centromere and for building the kinetochore complex during mitosis. This core region is generally flanked by pericentric heterochromatin, characterized by nucleosomes containing H3 methylated on lysine 9 (H3K9me) that are bound by heterochromatin proteins. During mitosis, these two domains together form a three-dimensional structure that exposes CENP-A-containing chromatin to the surface for interaction with the kinetochore and microtubules. At the same time, this structure supports the tension generated during the segregation of sister chromatids to opposite poles. In this review, we discuss recent insight into the characteristics of the centromere, from the specialized chromatin structures at the centromere core and the pericentromere to the three-dimensional organization of these regions that make up the functional centromere.

APC/C-Cdc20 mediates deprotection of centromeric cohesin at meiosis II in yeast

Cells undergoing meiosis produce haploid gametes through one round of DNA replication followed by 2 rounds of chromosome segregation. This requires that cohesin complexes, which establish sister chromatid cohesion during S phase, are removed in a stepwise manner. At meiosis I, the separase protease triggers the segregation of homologous chromosomes by cleaving cohesin's Rec8 subunit on chromosome arms. Cohesin persists at centromeres because the PP2A phosphatase, recruited by the shugoshin protein, dephosphorylates Rec8 and thereby protects it from cleavage. While chromatids disjoin upon cleavage of centromeric Rec8 at meiosis II, it was unclear how and when centromeric Rec8 is liberated from its protector PP2A. One proposal is that bipolar spindle forces separate PP2A from Rec8 as cells enter metaphase II. We show here that sister centromere biorientation is not sufficient to "deprotect" Rec8 at meiosis II in yeast. Instead, our data suggest that the ubiquitin-ligase APC/C^Cdc20 removes PP2A from centromeres by targeting for degradation the shugoshin Sgo1 and the kinase Mps1. This implies that Rec8 remains protected until entry into anaphase II when it is phosphorylated concurrently with the activation of separase. Here, we provide further support for this model and speculate on its relevance to mammalian oocytes.

Drosophila Dalmatian combines sororin and shugoshin roles in establishment and protection of cohesion

Sister chromatid cohesion is crucial to ensure chromosome bi-orientation and equal chromosome segregation. Cohesin removal via mitotic kinases and Wapl has to be prevented in pericentromeric regions in order to protect cohesion until metaphase, but the mechanisms of mitotic cohesion protection remain elusive in Drosophila Here, we show that dalmatian (Dmt), an ortholog of the vertebrate cohesin-associated protein sororin, is required for protection of mitotic cohesion in flies. Dmt is essential for cohesion establishment during interphase and is enriched on pericentromeric heterochromatin. Dmt is recruited through direct association with heterochromatin protein-1 (HP1), and this interaction is required for cohesion. During mitosis, Dmt interdependently recruits protein phosphatase 2A (PP2A) to pericentromeric regions, and PP2A binding is required for Dmt to protect cohesion. Intriguingly, Dmt is sufficient to protect cohesion upon heterologous expression in human cells. Our findings of a hybrid system, in which Dmt exerts both sororin-like establishment functions and shugoshin-like heterochromatin-based protection roles, provide clues to the evolutionary modulation of eukaryotic cohesion regulation systems.

Premature Silencing of the Spindle Assembly Checkpoint Is Prevented by the Bub1-H2A-Sgo1-PP2A Axis in Saccharomyces cerevisiae

The spindle assembly checkpoint (SAC) monitors mistakes in kinetochore-microtubule interaction and its activation prevents anaphase entry. The SAC remains active until all chromosomes have achieved bipolar attachment which applies tension on kinetochores. Our previous data in budding yeast Saccharomyces cerevisiae show that Ipl1/Aurora B kinase and a centromere-associated protein, Sgo1, are required to prevent SAC silencing prior to tension generation, but we believe that this regulatory network is incomplete. Bub1 kinase is one of the SAC components, and Bub1-dependent H2A phosphorylation triggers centromere recruitment of Sgo1 by H2A in yeast and human cells. Although yeast cells lacking the kinase domain of Bub1 show competent SAC activation, we found that the mutant cells fail to maintain a prolonged checkpoint arrest in the presence of tensionless attachment. Mutation of the Bub1 phosphorylation site in H2A also results in premature SAC silencing in yeast cells. Previous data indicate that Sgo1 protein binds to PP2A^Rts1, and we found that rts1Δ mutants exhibited premature SAC silencing as well. We further revealed that sgo1 mutants with abolished binding to H2A or PP2A^Rts1 displayed premature SAC silencing. Together, our results suggest that, in budding yeast S. cerevisiae, the Bub1-H2A-Sgo1-PP2A^Rts1 axis prevents SAC silencing and helps prolonged checkpoint arrest prior to tension establishment at kinetochores.

Identification of Tension Sensing Motif of Histone H3 in Saccharomyces cerevisiae and Its Regulation by Histone Modifying Enzymes

To ensure genome stability during cell division, all chromosomes must attach to spindles emanating from the opposite spindle pole bodies before segregation. The tension between sister chromatids generated by the poleward pulling force is an integral part of chromosome biorientation. In budding yeast, the residue Gly44 of histone H3 is critical for retaining the conserved Shugoshin protein Sgo1p at the pericentromeres for monitoring the tension status during mitosis. Studies carried out in this work showed that Lys42, Gly44, and Thr45 of H3 form the core of a tension sensing motif (TSM). Similar to the previously reported G44S mutant, K42A, G44A, and T45A alleles all rendered cells unable to respond to erroneous spindle attachment, a phenotype suppressed by Sgo1p overexpression. TSM functions by physically recruiting or retaining Sgo1p at pericentromeres as evidenced by chromatin immunoprecipitation and by in vitro pulldown experiments. Intriguingly, the function of TSM is likely regulated by multiple histone modifying enzymes, including the histone acetyltransferase Gcn5p, and deacetylases Rpd3p and Hos2p Defects caused by TSM mutations can be suppressed by the expression of a catalytically inactive mutant of Gcn5p Conversely, G44S mutant cells exhibit prominent chromatin instability phenotype in the absence of RPD3 Importantly, the gcn5^- suppressor restores the tension sensing function in tsm^- background in a fashion that bypasses the need of stably associating Sgo1p with chromatin. These results demonstrate that the TSM of histone H3 is a key component of a mechanism that ensures faithful segregation, and that interaction with chromatin modifying enzymes may be an important part of the mitotic quality control process.

The Aurora B Kinase in Chromosome Bi-Orientation and Spindle Checkpoint Signaling

Aurora B, a member of the Aurora family of serine/threonine protein kinases, is a key player in chromosome segregation. As part of a macromolecular complex known as the chromosome passenger complex, Aurora B concentrates early during mitosis in the proximity of centromeres and kinetochores, the sites of attachment of chromosomes to spindle microtubules. There, it contributes to a number of processes that impart fidelity to cell division, including kinetochore stabilization, kinetochore–microtubule attachment, and the regulation of a surveillance mechanism named the spindle assembly checkpoint. In the regulation of these processes, Aurora B is the fulcrum of a remarkably complex network of interactions that feed back on its localization and activation state. In this review, we discuss the multiple roles of Aurora B during mitosis, focusing in particular on its role at centromeres and kinetochores. Many details of the network of interactions at these locations remain poorly understood, and we focus here on several crucial outstanding questions.

From equator to pole: splitting chromosomes in mitosis and meiosis

During eukaryotic cell division, chromosomes must be precisely partitioned to daughter cells. This relies on a mechanism to move chromosomes in defined directions within the parental cell. While sister chromatids are segregated from one another in mitosis and meiosis II, specific adaptations enable the segregation of homologous chromosomes during meiosis I to reduce ploidy for gamete production. Many of the factors that drive these directed chromosome movements are known, and their molecular mechanism has started to be uncovered. Here we review the mechanisms of eukaryotic chromosome segregation, with a particular emphasis on the modifications that ensure the segregation of homologous chromosomes during meiosis I.

Role of Ctf3 and COMA subcomplexes in meiosis: Implication in maintaining Cse4 at the centromere and numeric spindle poles

During mitosis and meiosis, kinetochore, a conserved multi-protein complex, connects microtubule with the centromere and promotes segregation of the chromosomes. In budding yeast, central kinetochore complex named Ctf19 has been implicated in various functions and is believed to be made up of three biochemically distinct subcomplexes: COMA, Ctf3 and Iml3-Chl4. In this study, we aimed to identify whether Ctf3 and COMA subcomplexes have any unshared function at the kinetochore. Our data suggests that both these subcomplexes may work as a single functional unit without any unique functions, which we tested. Analysis of severity of the defects in the mutants suggests that COMA is epistatic to Ctf3 subcomplex. Interestingly, we noticed that these subcomplexes affect the organization of mitotic and meiotic kinetochores with subtle differences and they promote maintenance of Cse4 at the centromeres specifically during meiosis which is similar to the role of Mis6 (Ctf3 homolog) in fission yeast during mitosis. Interestingly, analysis of ctf3Δ and ctf19Δ mutants revealed a novel role of Ctf19 complex in regulation of SPB cohesion and duplication in meiosis.

Shugoshins: tension-sensitive pericentromeric adaptors safeguarding chromosome segregation

The shugoshin/Mei-S332 family are proteins that associate with the chromosomal region surrounding the centromere (the pericentromere) and that play multiple and distinct roles in ensuring the accuracy of chromosome segregation during both mitosis and meiosis. The underlying role of shugoshins appears to be to serve as pericentromeric adaptor proteins that recruit several different effectors to this region of the chromosome to regulate processes critical for chromosome segregation. Crucially, shugoshins undergo changes in their localization in response to the tension that is exerted on sister chromosomes by the forces of the spindle that will pull them apart. This has led to the idea that shugoshins provide a platform for activities required at the pericentromere only when sister chromosomes lack tension. Conversely, disassembly of the shugoshin pericentromeric platform may provide a signal that sister chromosomes are under tension. Here the functions and regulation of these important tension-sensitive pericentromeric proteins are discussed.

The centromere: chromatin foundation for the kinetochore machinery

Since discovery of the centromere-specific histone H3 variant CENP-A, centromeres have come to be defined as chromatin structures that establish the assembly site for the complex kinetochore machinery. In most organisms, centromere activity is defined epigenetically, rather than by specific DNA sequences. In this review, we describe selected classic work and recent progress in studies of centromeric chromatin with a focus on vertebrates. We consider possible roles for repetitive DNA sequences found at most centromeres, chromatin factors and modifications that assemble and activate CENP-A chromatin for kinetochore assembly, plus the use of artificial chromosomes and kinetochores to study centromere function.

Sgo1 regulates both condensin and Ipl1/Aurora B to promote chromosome biorientation

Correct chromosome segregation is essential in order to prevent aneuploidy. To segregate sister chromatids equally to daughter cells, the sisters must attach to microtubules emanating from opposite spindle poles. This so-called biorientation manifests itself by increased tension and conformational changes across kinetochores and pericentric chromatin. Tensionless attachments are dissolved by the activity of the conserved mitotic kinase Aurora B/Ipl1, thereby promoting the formation of correctly attached chromosomes. Recruitment of the conserved centromeric protein shugoshin is essential for biorientation, but its exact role has been enigmatic. Here, we identify a novel function of shugoshin (Sgo1 in budding yeast) that together with the protein phosphatase PP2A-Rts1 ensures localization of condensin to the centromeric chromatin in yeast Saccharomyces cerevisiae. Failure to recruit condensin results in an abnormal conformation of the pericentric region and impairs the correction of tensionless chromosome attachments. Moreover, we found that shugoshin is required for maintaining Aurora B/Ipl1 localization on kinetochores during metaphase. Thus, shugoshin has a dual function in promoting biorientation in budding yeast: first, by its ability to facilitate condensin recruitment it modulates the conformation of the pericentric chromatin. Second, shugoshin contributes to the maintenance of Aurora B/Ipl1 at the kinetochore during gradual establishment of bipolarity in budding yeast mitosis. Our findings identify shugoshin as a versatile molecular adaptor that governs chromosome biorientation.

Accurate chromosome segregation is required for the equal distribution of genetic information to progeny. Failure to equally segregate chromosomes leads to aneuploidy, cell death or cancer. Proteins of the conserved shugoshin family contribute to accurate chromosome segregation in both meiosis and mitosis. The role of shugoshin in protection of centromeric cohesion during meiosis is well understood, but only little is known about shugoshin's function during mitosis. We show that Sgo1 mediates localization of the heterotrimeric phosphatase PP2A-Rts1 to the centromere and that this is in turn important for the efficient recruitment of condensin to the centromere. The failure to load centromeric condensin results in a defect during correction of improper microtubule-kinetochore attachments. Moreover, Sgo1 facilitates the maintenance of a centromeric pool of Aurora B/Ipl1, a conserved mitotic kinase essential for the correction of faulty microtubule-kinetochore attachments. Our results show that Sgo1 operates as a multifunctional hub that coordinates two centromeric functions essential for correct chromosome segregation.

Tension-dependent removal of pericentromeric shugoshin is an indicator of sister chromosome biorientation

During mitosis and meiosis, sister chromatid cohesion resists the pulling forces of microtubules, enabling the generation of tension at kinetochores upon chromosome biorientation. How tension is read to signal the bioriented state remains unclear. Shugoshins form a pericentromeric platform that integrates multiple functions to ensure proper chromosome biorientation. Here we show that budding yeast shugoshin Sgo1 dissociates from the pericentromere reversibly in response to tension. The antagonistic activities of the kinetochore-associated Bub1 kinase and the Sgo1-bound phosphatase protein phosphatase 2A (PP2A)-Rts1 underlie a tension-dependent circuitry that enables Sgo1 removal upon sister kinetochore biorientation. Sgo1 dissociation from the pericentromere triggers dissociation of condensin and Aurora B from the centromere, thereby stabilizing the bioriented state. Conversely, forcing sister kinetochores to be under tension during meiosis I leads to premature Sgo1 removal and precocious loss of pericentromeric cohesion. Overall, we show that the pivotal role of shugoshin is to build a platform at the pericentromere that attracts activities that respond to the absence of tension between sister kinetochores. Disassembly of this platform in response to intersister kinetochore tension signals the bioriented state. Therefore, tension sensing by shugoshin is a central mechanism by which the bioriented state is read.

Shugoshin biases chromosomes for biorientation through condensin recruitment to the pericentromere

To protect against aneuploidy, chromosomes must attach to microtubules from opposite poles (‘biorientation’) prior to their segregation during mitosis. Biorientation relies on the correction of erroneous attachments by the aurora B kinase, which destabilizes kinetochore-microtubule attachments that lack tension. Incorrect attachments are also avoided because sister kinetochores are intrinsically biased towards capture by microtubules from opposite poles. Here, we show that shugoshin acts as a pericentromeric adaptor that plays dual roles in biorientation in budding yeast. Shugoshin maintains the aurora B kinase at kinetochores that lack tension, thereby engaging the error correction machinery. Shugoshin also recruits the chromosome-organizing complex, condensin, to the pericentromere. Pericentromeric condensin biases sister kinetochores towards capture by microtubules from opposite poles. Our findings uncover the molecular basis of the bias to sister kinetochore capture and expose shugoshin as a pericentromeric hub controlling chromosome biorientation.

DOI:http://dx.doi.org/10.7554/eLife.01374.001

When a cell divides to create two new daughter cells, it must produce a copy of each of its chromosomes. It is important that each daughter cell gets just one copy of each chromosome. If an error occurs and one cell gets two copies of a single chromosome, it can lead to cancer or birth defects. Fortunately, there are multiple checks to ensure that this does not happen.

During cell division the chromosomes line up in a way that increases the likelihood that each daughter cell will have one copy of each chromosome. After this process—which is called biorientation—is completed, microtubules pull the chromosomes to opposite ends of the cell, which then divides.

Proteins called shugoshin proteins are known to be involved in biorientation in many organisms. These proteins are found in a region called the pericentromere, which surrounds the area on the chromosomes that the microtubules attach to, but the details of their involvement in biorientation are not fully understood. Now Verzijlbergen et al. have exploited sophisticated genetic techniques in yeast to explore how shugoshin proteins work.

These experiments showed that the shugoshin protein helps to recruit condensin—a protein that keeps the DNA organized within the chromosome—to the pericentromere to assist with biorientation. It also keeps aurora B kinase—one of the enzymes that helps to correct errors during cell division—in the pericentromere when a microtubule attaches to the wrong chromosome. These results help us understand how a ‘hub’ in the pericentromere ensures biorientation. The next challenge will be to understand how this hub is disassembled after biorientation to allow error-free cell division to proceed. As shugoshins have been found to be damaged in some cancers, understanding the workings of this hub could also shed new light on how they contribute to disease.

DOI:http://dx.doi.org/10.7554/eLife.01374.002

Chromosome segregation in budding yeast: sister chromatid cohesion and related mechanisms

Studies on budding yeast have exposed the highly conserved mechanisms by which duplicated chromosomes are evenly distributed to daughter cells at the metaphase-anaphase transition. The establishment of proteinaceous bridges between sister chromatids, a function provided by a ring-shaped complex known as cohesin, is central to accurate segregation. It is the destruction of this cohesin that triggers the segregation of chromosomes following their proper attachment to microtubules. Since it is irreversible, this process must be tightly controlled and driven to completion. Furthermore, during meiosis, modifications must be put in place to allow the segregation of maternal and paternal chromosomes in the first division for gamete formation. Here, I review the pioneering work from budding yeast that has led to a molecular understanding of the establishment and destruction of cohesion.

Large-scale periodicity of nucleosome positioning signal in pericentric regions of chromosomes (Drosophila melanogaster)

Nucleosome positioning signal (NPS) in heterochromatin is not uniform. We suggest the analysis of its heterogeneity by correlation with periodic function (analog of Furrier analysis). It was established the periodical repetition of the nucleosome clusters of large size in pericentric regions in a discontinuous manner. In the 3L pericentric region, it was revealed the domination of 78-85 kbp wavelength in the correlation coefficient profile and also strong presentation of 50 kbp signal. In further to centromere position, the 69 kbp value strongly dominates as well as the 50 kbp value in the closest proximity. In addition to the long wavelength signals, there are plenty of short wavelengths signals especially in the closest vicinity to centromere. In some positions throughout pericentric region of 2L chromosome, there are two sizes of repeated intermingled correlation signals (50, and 75 kbp) with dominating value of 75 kbp in proximity and 50 kbp distantly to centromere, the situation for 2R is analogous. Some genes with long introns support these quantitative characteristics of NPSs and to some extent their dominating character in each region. The characteristic repeat periods for 3L pericentric region coincide with the distances between heterochromatin epigenetic mark clusters and their distribution throughout this region for fly embryos, larvae, and some cell lines.

Global linkage map connects meiotic centromere function to chromosome size in budding yeast

Synthetic genetic array (SGA) analysis automates yeast genetics, enabling high-throughput construction of ordered arrays of double mutants. Quantitative colony sizes derived from SGA analysis can be used to measure cellular fitness and score for genetic interactions, such as synthetic lethality. Here we show that SGA colony sizes also can be used to obtain global maps of meiotic recombination because recombination frequency affects double-mutant formation for gene pairs located on the same chromosome and therefore influences the size of the resultant double-mutant colony. We obtained quantitative colony size data for ~1.2 million double mutants located on the same chromosome and constructed a genome-scale genetic linkage map at ~5 kb resolution. We found that our linkage map is reproducible and consistent with previous global studies of meiotic recombination. In particular, we confirmed that the total number of crossovers per chromosome tends to follow a simple linear model that depends on chromosome size. In addition, we observed a previously unappreciated relationship between the size of linkage regions surrounding each centromere and chromosome size, suggesting that crossovers tend to occur farther away from the centromere on larger chromosomes. The pericentric regions of larger chromosomes also appeared to load larger clusters of meiotic cohesin Rec8, and acquire fewer Spo11-catalyzed DNA double-strand breaks. Given that crossovers too near or too far from centromeres are detrimental to homolog disjunction and increase the incidence of aneuploidy, our data suggest that chromosome size may have a direct role in regulating the fidelity of chromosome segregation during meiosis.

An Iml3-Chl4 heterodimer links the core centromere to factors required for accurate chromosome segregation

Accurate segregation of genetic material in eukaryotes relies on the kinetochore, a multiprotein complex that connects centromeric DNA with microtubules. In yeast and humans, two proteins-Mif2/CENP-C and Chl4/CNEP-N-interact with specialized centromeric nucleosomes and establish distinct but cross-connecting axes of chromatin-microtubule linkage. Proteins recruited by Chl4/CENP-N include a subset that regulates chromosome transmission fidelity. We show that Chl4 and a conserved member of this subset, Iml3, both from Saccharomyces cerevisiae, form a stable protein complex that interacts with Mif2 and Sgo1. We have determined the structures of an Iml3 homodimer and an Iml3-Chl4 heterodimer, which suggest a mechanism for regulating the assembly of this functional axis of the kinetochore. We propose that at the core centromere, the Chl4-Iml3 complex participates in recruiting factors, such as Sgo1, that influence sister chromatid cohesion and encourage sister kinetochore biorientation.

Kinetochore function and chromosome segregation rely on critical residues in histones H3 and H4 in budding yeast

Accurate chromosome segregation requires that sister kinetochores biorient and attach to microtubules from opposite poles. Kinetochore biorientation relies on the underlying centromeric chromatin, which provides a platform to assemble the kinetochore and to recruit the regulatory factors that ensure the high fidelity of this process. To identify the centromeric chromatin determinants that contribute to chromosome segregation, we performed two complementary unbiased genetic screens using a library of budding yeast mutants in every residue of histone H3 and H4. In one screen, we identified mutants that lead to increased loss of a nonessential chromosome. In the second screen, we isolated mutants whose viability depends on a key regulator of biorientation, the Aurora B protein kinase. Nine mutants were common to both screens and exhibited kinetochore biorientation defects. Four of the mutants map near the unstructured nucleosome entry site, and their genetic interaction with reduced IPL1 can be suppressed by increasing the dosage of SGO1, a key regulator of biorientation. In addition, the composition of purified kinetochores was altered in six of the mutants. Together, this work identifies previously unknown histone residues involved in chromosome segregation and lays the foundation for future studies on the role of the underlying chromatin structure in chromosome segregation.

Cohesin-dependent association of scc2/4 with the centromere initiates pericentromeric cohesion establishment

Cohesin is a conserved ring-shaped multiprotein complex that participates in chromosome segregation, DNA repair, and transcriptional regulation [1, 2]. Cohesin loading onto chromosomes universally requires the Scc2/4 "loader" complex (also called NippedBL/Mau2), mutations in which cause the developmental disorder Cornelia de Lange syndrome in humans [1-9]. Cohesin is most concentrated in the pericentromere, the region surrounding the centromere [10-15]. Enriched pericentromeric cohesin requires the Ctf19 kinetochore subcomplex in budding yeast [16-18]. Here, we uncover the spatial and temporal determinants for Scc2/4 centromere association. We demonstrate that the critical role of the Ctf19 complex is to enable Scc2/4 association with centromeres, through which cohesin loads and spreads onto the adjacent pericentromere. We show that, unexpectedly, Scc2 association with centromeres depends on cohesin itself. The absence of the Scc1/Mcd1/Rad21 cohesin subunit precludes Scc2 association with centromeres from anaphase until late G1. Expression of SCC1 is both necessary and sufficient for the binding of cohesin to its loader, the association of Scc2 with centromeres, and cohesin loading. We propose that cohesin triggers its own loading by enabling Scc2/4 to connect with chromosomal landmarks, which at centromeres are specified by the Ctf19 complex. Overall, our findings provide a paradigm for the spatial and temporal control of cohesin loading.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

Geometry and force behind kinetochore orientation: lessons from meiosis

During mitosis, replicated chromosomes (sister chromatids) become attached at the kinetochore by spindle microtubules emanating from opposite poles and segregate equationally. In the first division of meiosis, however, sister chromatids become attached from the same pole and co-segregate, whereas homologous chromosomes connected by chiasmata segregate to opposite poles. Disorder in this specialized chromosome attachment in meiosis is the leading cause of miscarriage in humans. Recent studies have elucidated the molecular mechanisms determining chromosome orientation, and consequently segregation, in meiosis. Comparative studies of meiosis and mitosis have led to the general principle that kinetochore geometry and tension exerted by microtubules synergistically generate chromosome orientation.

Iron-responsive transcription factor Aft1 interacts with kinetochore protein Iml3 and promotes pericentromeric cohesin

The Saccharomyces cerevisiae iron-responsive transcription factor, Aft1, has a well established role in regulating iron homeostasis through the transcriptional induction of iron-regulon genes. However, recent studies have implicated Aft1 in other cellular processes independent of iron regulation such as chromosome stability. In addition, chromosome spreads and two-hybrid data suggest that Aft1 interacts with and co-localizes with kinetochore proteins; however, the cellular implications of this have not been established. Here, we demonstrate that Aft1 associates with the kinetochore complex through Iml3. Furthermore, like Iml3, Aft1 is required for the increased association of cohesin with pericentric chromatin, which is required to resist microtubule tension, and aft1Δ cells display chromosome segregation defects in meiosis. Our work defines a new role for Aft1 in chromosome stability and transmission.

OsSGO1 maintains synaptonemal complex stabilization in addition to protecting centromeric cohesion during rice meiosis

Shugoshin is a conserved protein in eukaryotes that protects the centromeric cohesin of sister chromatids from cleavage by separase during meiosis. In this study, we identify the rice (Oryza sativa, 2n=2x=24) homolog of ZmSGO1 in maize (Zea mays), named OsSGO1. During both mitosis and meiosis, OsSGO1 is recruited from nucleoli onto centromeres at the onset of prophase. In the Tos17-insertional Ossgo1-1 mutant, centromeres of sister chromatids separate precociously from each other from metaphase I, which causes unequal chromosome segregation during meiosis II. Moreover, the release of OsSGO1 from nucleoli is completely blocked in Ossgo1-1, which leads to the absence of OsSGO1 in centromeric regions after the onset of mitosis and meiosis. Furthermore, the timely assembly and maintenance of synaptonemal complexes during early prophase I are affected in Ossgo1 mutants. Finally, we found that the centromeric localization of OsSGO1 depends on OsAM1, not other meiotic proteins such as OsREC8, PAIR2, OsMER3, or ZEP1.

Cdc55 coordinates spindle assembly and chromosome disjunction during meiosis

During meiosis, two consecutive nuclear divisions follow a single round of deoxyribonucleic acid replication. In meiosis I, homologues are segregated, whereas in meiosis II, sister chromatids are segregated. This requires that the sequential assembly and dissolution of specialized chromosomal factors are coordinated with two rounds of spindle assembly and disassembly. How these events are coupled is unknown. In this paper, we show, in budding yeast, that the protein phosphatase 2A regulatory subunit Cdc55 couples the loss of linkages between chromosomes with nuclear division by restraining two other phosphatases, Cdc14 and PP2A(Rts1). Cdc55 maintains Cdc14 sequestration in the nucleolus during early meiosis, and this is essential for the assembly of the meiosis I spindle but not for chromosomes to separate. Cdc55 also limits the formation of PP2A holocomplexes containing the alternative regulatory subunit Rts1, which is crucial for the timely dissolution of sister chromatid cohesion. Therefore, Cdc55 orders passage through the meiotic divisions by ensuring a balance of phosphatases.

Angelika Amon: conquering the divide. Interview by Ben Short

Amon studies how cells segregate their chromosomes and what happens when they get it wrong.

Bub1, Sgo1, and Mps1 mediate a distinct pathway for chromosome biorientation in budding yeast

The conserved mitotic kinase Bub1 performs multiple functions that are only partially characterized. Besides its role in the spindle assembly checkpoint and chromosome alignment, Bub1 is crucial for the kinetochore recruitment of multiple proteins, among them Sgo1. Both Bub1 and Sgo1 are dispensable for growth of haploid and diploid budding yeast, but they become essential in cells with higher ploidy. We find that overexpression of SGO1 partially corrects the chromosome segregation defect of bub1Δ haploid cells and restores viability to bub1Δ tetraploid cells. Using an unbiased high-copy suppressor screen, we identified two members of the chromosomal passenger complex (CPC), BIR1 (survivin) and SLI15 (INCENP, inner centromere protein), as suppressors of the growth defect of both bub1Δ and sgo1Δ tetraploids, suggesting that these mutants die due to defects in chromosome biorientation. Overexpression of BIR1 or SLI15 also complements the benomyl sensitivity of haploid bub1Δ and sgo1Δ cells. Mutants lacking SGO1 fail to biorient sister chromatids attached to the same spindle pole (syntelic attachment) after nocodazole treatment. Moreover, the sgo1Δ cells accumulate syntelic attachments in unperturbed mitoses, a defect that is partially corrected by BIR1 or SLI15 overexpression. We show that in budding yeast neither Bub1 nor Sgo1 is required for CPC localization or affects Aurora B activity. Instead we identify Sgo1 as a possible partner of Mps1, a mitotic kinase suggested to have an Aurora B-independent function in establishment of biorientation. We found that Sgo1 overexpression rescues defects caused by metaphase inactivation of Mps1 and that Mps1 is required for Sgo1 localization to the kinetochore. We propose that Bub1, Sgo1, and Mps1 facilitate chromosome biorientation independently of the Aurora B-mediated pathway at the budding yeast kinetochore and that both pathways are required for the efficient turnover of syntelic attachments.

The role of shugoshin in meiotic chromosome segregation

During meiosis, DNA replication is followed by 2 successive chromosome segregation events, resulting in the production of gametes with a haploid number of chromosomes from a diploid precursor cell. Faithful chromosome segregation in meiosis requires that sister chromatid cohesion is lost from chromosome arms during meiosis I, but retained at centromeric regions until meiosis II. Recent studies have begun to uncover the mechanisms underlying this stepwise loss of cohesion in meiosis and the role of a conserved protein, shugoshin, in regulating this process.

Two histone marks establish the inner centromere and chromosome bi-orientation

For proper partitioning of chromosomes in mitosis, the chromosomal passenger complex (CPC) including Aurora B and survivin must be localized at the center of paired kinetochores, at the site called the inner centromere. It is largely unknown what defines the inner centromere and how the CPC is targeted to this site. Here, we show that the phosphorylation of histone H3-threonine 3 (H3-pT3) mediated by Haspin cooperates with Bub1-mediated histone 2A-serine 121 (H2A-S121) phosphorylation in targeting the CPC to the inner centromere in fission yeast and human cells. H3-pT3 promotes nucleosome binding of survivin, whereas phosphorylated H2A-S121 facilitates the binding of shugoshin, the centromeric CPC adaptor. Haspin colocalizes with cohesin by associating with Pds5, whereas Bub1 localizes at kinetochores. Thus, the inner centromere is defined by intersection of two histone kinases.

Sgo1 establishes the centromeric cohesion protection mechanism in G2 before subsequent Bub1-dependent recruitment in mitosis

Bub1 was one of the first protein kinases identified as a component of the spindle-assembly checkpoint, a surveillance mechanism that delays anaphase onset until all chromosomes are stably attached to spindle microtubules. Whereas the kinase activity of Bub1 is not required for checkpoint function in yeast, its requirement in mammalian cells is still unclear. Using a complementation assay with bona fide BUB1-null mouse embryonic fibroblasts, we show that the kinase activity of Bub1 is not required for checkpoint function or chromosome alignment. Its activity is, however, required for centromeric localisation of Sgo1, a known protector of centromeric cohesion. Despite the absence of Sgo1 from mitotic centromeres in cells devoid of Bub1 activity, centromeric cohesion is still maintained until anaphase. An explanation for this comes from observations showing that Sgo1 is first recruited to centromeric heterochromatin in G2, but then becomes diffusely localised throughout the nucleus in early prophase, before returning to centromeres later in prophase. Importantly, whereas centromeric localisation of Sgo1 in prophase is dependent on the kinase activity of Bub1, its recruitment to centromeric heterochromatin in G2 is not. Rather, the localisation of Sgo1 in G2 is abolished when heterochromatin protein 1 is not bound to centromeric heterochromatin. Thus, it seems that Sgo1 sets up the centromeric protection mechanism in G2, but that its Bub1-dependent localisation to centromeres during mitosis is not required to maintain cohesion.

Regulators of the cohesin network

Chromosome cohesion is the term used to describe the cellular process in which sister chromatids are held together from the time of their replication until the time of separation at the metaphase to anaphase transition. In this capacity, chromosome cohesion, especially at centromeric regions, is essential for chromosome segregation. However, cohesion of noncentromeric DNA sequences has been shown to occur during double-strand break (DSB) repair and the transcriptional regulation of genes. Cohesion for the purposes of accurate chromosome segregation, DSB repair, and gene regulation are all achieved through a similar network of proteins, but cohesion for each purpose may be regulated differently. In this review, we focus on recent developments regarding the regulation of this multipurpose network for tying DNA sequences together. In particular, regulation via effectors and posttranslational modifications are reviewed. A picture is emerging in which complex regulatory networks are capable of differential regulation of cohesion in various contexts.