The centromeric protein Sgo1 is required to sense lack of tension on mitotic chromosomes

Molecular mechanism targeting condensin for chromosome condensation

Genomes are organised into DNA loops by the Structural Maintenance of Chromosomes (SMC) proteins. SMCs establish functional chromosomal sub-domains for DNA repair, gene expression and chromosome segregation, but how SMC activity is specifically targeted is unclear. Here, we define the molecular mechanism targeting the condensin SMC complex to specific chromosomal regions in budding yeast. A conserved pocket on the condensin HAWK subunit Ycg1 binds to chromosomal receptors carrying a related motif, CR1. In early mitosis, CR1 motifs in receptors Sgo1 and Lrs4 recruit condensin to pericentromeres and rDNA, to facilitate sister kinetochore biorientation and rDNA condensation, respectively. We additionally find that chromosome arm condensation begins as sister kinetochores come under tension, in a manner dependent on the Ycg1 pocket. We propose that multiple CR1-containing proteins recruit condensin to chromosomes and identify several additional candidates based on their sequence. Overall, we uncover the molecular mechanism that targets condensin to functionalise chromosomal domains to achieve accurate chromosome segregation during mitosis.

The dependence of shugoshin on Bub1-kinase activity is dispensable for the maintenance of spindle assembly checkpoint response in Cryptococcus neoformans

During chromosome segregation, the spindle assembly checkpoint (SAC) detects errors in kinetochore-microtubule attachments. Timely activation and maintenance of the SAC until defects are corrected is essential for genome stability. Here, we show that shugoshin (Sgo1), a conserved tension-sensing protein, ensures the maintenance of SAC signals in response to unattached kinetochores during mitosis in a basidiomycete budding yeast Cryptococcus neoformans. Sgo1 maintains optimum levels of Aurora B kinase Ipl1 and protein phosphatase 1 (PP1) at kinetochores. The absence of Sgo1 results in the loss of Aurora BIpl1 with a concomitant increase in PP1 levels at kinetochores. This leads to a premature reduction in the kinetochore-bound Bub1 levels and early termination of the SAC signals. Intriguingly, the kinase function of Bub1 is dispensable for shugoshin's subcellular localization. Sgo1 is predominantly localized to spindle pole bodies (SPBs) and along the mitotic spindle with a minor pool at kinetochores. In the absence of proper kinetochore-microtubule attachments, Sgo1 reinforces the Aurora B kinaseIpl1-PP1 phosphatase balance, which is critical for prolonged maintenance of the SAC response.

A conserved site on Ndc80 complex facilitates dynamic recruitment of Mps1 to yeast kinetochores to promote accurate chromosome segregation

Accurate chromosome segregation relies on kinetochores carrying out multiple functions, including establishing and maintaining microtubule attachments, forming precise bi-oriented attachments between sister chromatids, and activating the spindle assembly checkpoint. Central to these processes is the highly conserved Ndc80 complex. This kinetochore subcomplex interacts directly with microtubules but also serves as a critical platform for recruiting kinetochore-associated factors and as a key substrate for error correction kinases. The precise manner in which these kinetochore factors interact and regulate each other's function remains unknown, considerably hindering our understanding of how Ndc80 complex-dependent processes function together to orchestrate accurate chromosome segregation. Here, we aimed to uncover the role of Nuf2's CH domain, a component of the Ndc80 complex, in ensuring these processes. Through extensive mutational analysis, we identified a conserved interaction domain composed of two segments in Nuf2's CH domain that form the binding site for Mps1 within the yeast Ndc80 complex. Interestingly, this site also associates with the Dam1 complex, suggesting Mps1 recruitment may be subject to regulation by competitive binding with other factors. Mutants disrupting this "interaction hub" exhibit defects in spindle assembly checkpoint function and severe chromosome segregation errors. Significantly, specifically restoring Mps1-Ndc80 complex association rescues these defects. Our findings shed light on the intricate regulation of Ndc80 complex-dependent functions and highlight the essential role of Mps1 in kinetochore bi-orientation and accurate chromosome segregation.

An interaction hub on Ndc80 complex facilitates dynamic recruitment of Mps1 to yeast kinetochores to promote accurate chromosome segregation

Accurate chromosome segregation relies on kinetochores carrying out multiple functions, including establishing and maintaining microtubule attachments, forming precise bioriented attachments between sister chromatids, and activating the spindle assembly checkpoint. Central to these processes is the highly conserved Ndc80 complex. This kinetochore subcomplex interacts directly with microtubules, but also serves as a critical platform for recruiting kinetochore-associated factors and as a key substrate for error correction kinases. The precise manner in which these kinetochore factors interact, and regulate each other's function, remains unknown - considerably hindering our understanding of how Ndc80 complex-dependent processes function together to orchestrate accurate chromosome segregation. Here, we aimed to uncover the role of Nuf2's CH domain, a component of the Ndc80 complex, in ensuring accurate chromosome segregation. Through extensive mutational analysis, we identified a conserved "interaction hub" comprising two segments in Nuf2's CH domain, forming the binding site for Mps1 within the yeast Ndc80 complex. Intriguingly, the interaction between Mps1 and the Ndc80 complex seems to be subject to regulation by competitive binding with other factors. Mutants disrupting this interaction hub exhibit defects in spindle assembly checkpoint function and severe chromosome segregation errors. Significantly, specifically restoring Mps1-Ndc80 complex association rescues these defects. Our findings shed light on the intricate regulation of Ndc80 complex-dependent functions and highlight the essential role of Mps1 in kinetochore biorientation and accurate chromosome segregation.

A new layer of regulation of chromosomal passenger complex (CPC) translocation in budding yeast

The conserved chromosomal passenger complex (CPC) consists of Ipl1Aurora-B, Sli15INCENP, Bir1Survivin, and Nbl1Borealin, and localizes at the kinetochore/centromere to correct kinetochore attachment errors and to prevent checkpoint silencing. After anaphase entry, the CPC moves from the kinetochore/centromere to the spindle. In budding yeast, CPC subunit Sli15 is phosphorylated by both cyclin-dependent kinase (CDK) and Ipl1 kinase. Following anaphase onset, activated Cdc14 phosphatase reverses Sli15 phosphorylation imposed by CDK to promote CPC translocation. Although abolished Sli15 phosphorylation imposed by Ipl1 also causes CPC translocation, the regulation of Ipl1-imposed Sli15 phosphorylation remains unclear. In addition to Sli15, Cdc14 also dephosphorylates Fin1, a regulatory subunit of protein phosphatase 1 (PP1), to enable kinetochore localization of Fin1-PP1. Here, we present evidence supporting the notion that kinetochore-localized Fin1-PP1 likely reverses Ipl1-imposed Sli15 phosphorylation to promote CPC translocation from the kinetochore/centromere to the spindle. Importantly, premature Fin1 kinetochore localization or phospho-deficient sli15 mutation causes checkpoint defects in response to tensionless attachments, resulting in chromosome missegregation. In addition, our data indicate that reversion of CDK- and Ipl1-imposed Sli15 phosphorylation shows an additive effect on CPC translocation. Together, these results reveal a previously unidentified pathway to regulate CPC translocation, which is important for accurate chromosome segregation.

Yeast Kinesin-5 Motor Protein CIN8 Promotes Accurate Chromosome Segregation

Accurate chromosome segregation depends on bipolar chromosome–microtubule attachment and tension generation on chromosomes. Incorrect chromosome attachment results in chromosome missegregation, which contributes to genome instability. The kinetochore is a protein complex that localizes at the centromere region of a chromosome and mediates chromosome–microtubule interaction. Incorrect chromosome attachment leads to checkpoint activation to prevent anaphase onset. Kinetochore detachment activates the spindle assembly checkpoint (SAC), while tensionless kinetochore attachment relies on both the SAC and tension checkpoint. In budding yeast Saccharomyces cerevisiae, kinesin-5 motor proteins Cin8 and Kip1 are needed to separate spindle pole bodies for spindle assembly, and deletion of CIN8 causes lethality in the absence of SAC. To study the function of Cin8 and Kip1 in chromosome segregation, we constructed an auxin-inducible degron (AID) mutant, cin8-AID. With this conditional mutant, we first confirmed that cin8-AID kip1∆ double mutants were lethal when Cin8 is depleted in the presence of auxin. These cells arrested in metaphase with unseparated spindle pole bodies and kinetochores. We further showed that the absence of either the SAC or tension checkpoint was sufficient to abolish the cell-cycle delay in cin8-AID mutants, causing chromosome missegregation and viability loss. The tension checkpoint-dependent phenotype in cells with depleted Cin8 suggests the presence of tensionless chromosome attachment. We speculate that the failed spindle pole body separation in cin8 mutants could increase the chance of tensionless syntelic chromosome attachments, which depends on functional tension checkpoint for survival.

Rec8 Cohesin: A Structural Platform for Shaping the Meiotic Chromosomes

Meiosis is critically different from mitosis in that during meiosis, pairing and segregation of homologous chromosomes occur. During meiosis, the morphology of sister chromatids changes drastically, forming a prominent axial structure in the synaptonemal complex. The meiosis-specific cohesin complex plays a central role in the regulation of the processes required for recombination. In particular, the Rec8 subunit of the meiotic cohesin complex, which is conserved in a wide range of eukaryotes, has been analyzed for its function in modulating chromosomal architecture during the pairing and recombination of homologous chromosomes in meiosis. Here, we review the current understanding of Rec8 cohesin as a structural platform for meiotic chromosomes.

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.

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.

Differential requirement for Bub1 and Bub3 in regulation of meiotic versus mitotic chromosome segregation

Accurate chromosome segregation depends on the proper attachment of kinetochores to spindle microtubules before anaphase onset. The Ipl1/Aurora B kinase corrects improper attachments by phosphorylating kinetochore components and so releasing aberrant kinetochore–microtubule interactions. The localization of Ipl1 to kinetochores in budding yeast depends upon multiple pathways, including the Bub1–Bub3 pathway. We show here that in meiosis, Bub3 is crucial for correction of attachment errors. Depletion of Bub3 results in reduced levels of kinetochore-localized Ipl1 and concomitant massive chromosome missegregation caused by incorrect chromosome–spindle attachments. Depletion of Bub3 also results in shorter metaphase I and metaphase II due to premature localization of protein phosphatase 1 (PP1) to kinetochores, which antagonizes Ipl1-mediated phosphorylation. We propose a new role for the Bub1–Bub3 pathway in maintaining the balance between kinetochore localization of Ipl1 and PP1, a balance that is essential for accurate meiotic chromosome segregation and timely anaphase onset.

Establishing correct kinetochore-microtubule attachments in mitosis and meiosis

Faithful chromosome segregation in mitosis and meiosis requires that chromosomes properly attach to spindle microtubules. Initial kinetochore-microtubule attachments are often incorrect and rely on error correction mechanisms to release improper attachments, allowing the formation of new attachments. Aurora B kinase and, in mammalian germ cells, Aurora C kinase function as the enzymatic component of the Chromosomal Passenger Complex (CPC), which localizes to the inner centromere/kinetochore and phosphorylates kinetochore proteins for microtubule release during error correction. In this review, we discuss recent findings of the molecular pathways that regulate the chromosomal localization of Aurora B and C kinases in human cell lines, mice, fission yeast, and budding yeast. We also discuss differences in the importance of localization pathways between mitosis and meiosis.

The regulation of Net1/Cdc14 by the Hog1 MAPK upon osmostress unravels a new mechanism regulating mitosis

During evolution, cells have developed a plethora of mechanisms to optimize survival in a changing and unpredictable environment. In this regard, they have evolved networks that include environmental sensors, signaling transduction molecules and response mechanisms. Hog1 (yeast) and p38 (mammals) stress-activated protein kinases (SAPKs) are activated upon stress and they drive a full collection of cell adaptive responses aimed to maximize survival. SAPKs are extensively used to learn about the mechanisms through which cells adapt to changing environments. In addition to regulating gene expression and metabolism, SAPKs control cell cycle progression. In this review, we will discuss the latest findings related to the SAPK-driven regulation of mitosis upon osmostress in yeast.

Killing two birds with one stone: how budding yeast Mps1 controls chromosome segregation and spindle assembly checkpoint through phosphorylation of a single kinetochore protein

During mitosis, the identical sister chromatids of each chromosome must attach through their kinetochores to microtubules emanating from opposite spindle poles. This process, referred to as chromosome biorientation, is essential for equal partitioning of the genetic information to the two daughter cells. Defects in chromosome biorientation can give rise to aneuploidy, a hallmark of cancer and genetic diseases. A conserved surveillance mechanism called spindle assembly checkpoint (SAC) prevents the onset of anaphase until biorientation is attained. Key to chromosome biorientation is an error correction mechanism that allows kinetochores to establish proper bipolar attachments by disengaging faulty kinetochore-microtubule connections. Error correction relies on the Aurora B and Mps1 kinases that also promote SAC signaling, raising the possibility that they are part of a single sensory device responding to improper attachments and concomitantly controlling both their disengagement and a temporary mitotic arrest. In budding yeast, Aurora B and Mps1 promote error correction independently from one another, but while the substrates of Aurora B in this process are at least partially known, the mechanism underlying the involvement of Mps1 in the error correction pathway is unknown. Through the characterization of a novel mps1 mutant and an unbiased genetic screen for extragenic suppressors, we recently gained evidence that a common mechanism based on Mps1-dependent phosphorylation of the Knl1/Spc105 kinetochore scaffold and subsequent recruitment of the Bub1 kinase is critical for the function of Mps1 in chromosome biorientation as well as for SAC activation (Benzi et al. EMBO Rep, 2020).

Checkpoint Proteins Bub1 and Bub3 Delay Anaphase Onset in Response to Low Tension Independent of Microtubule-Kinetochore Detachment

The spindle assembly checkpoint (SAC) delays anaphase onset until sister chromosomes are bound to microtubules from opposite spindle poles. Only then can dynamic microtubules produce tension across sister kinetochores. The interdependence of kinetochore attachment and tension has proved challenging to understanding SAC mechanisms. Whether the SAC responds simply to kinetochore attachment or to tension status remains obscure. Unlike higher eukaryotes, budding yeast kinetochores bind only one microtubule, simplifying the relation between attachment and tension. We developed a Taxol-sensitive yeast model to reduce tension in fully assembled spindles. Our results show that low tension on bipolar-attached kinetochores delays anaphase onset, independent of detachment. The delay is transient relative to that imposed by unattached kinetochores. Furthermore, it is mediated by Bub1 and Bub3, but not Mad1, Mad2, and Mad3 (BubR1). Our results demonstrate that reduced tension delays anaphase onset via a signal that is temporally and mechanistically distinct from that produced by unattached kinetochores.

Kinetochore attachment and tension are critical for proper chromosome segregation, but isolating the contribution of either stimulus has been challenging. Using a Taxol-sensitive yeast model, Proudfoot et al. show that reducing tension specifically produces a delay in mitotic progression that is temporally and mechanistically distinct from that produced by unattached kinetochores.

A common molecular mechanism underlies the role of Mps1 in chromosome biorientation and the spindle assembly checkpoint

The Mps1 kinase corrects improper kinetochore-microtubule attachments, thereby ensuring chromosome biorientation. Yet, its critical phosphorylation targets in this process remain largely elusive. Mps1 also controls the spindle assembly checkpoint (SAC), which halts chromosome segregation until biorientation is attained. Its role in SAC activation is antagonised by the PP1 phosphatase and involves phosphorylation of the kinetochore scaffold Knl1/Spc105, which in turn recruits the Bub1 kinase to promote assembly of SAC effector complexes. A crucial question is whether error correction and SAC activation are part of a single or separable pathways. Here, we isolate and characterise a new yeast mutant, mps1-3, that is severely defective in chromosome biorientation and SAC signalling. Through an unbiased screen for extragenic suppressors, we found that mutations lowering PP1 levels at Spc105 or forced association of Bub1 with Spc105 reinstate both chromosome biorientation and SAC signalling in mps1-3 cells. Our data argue that a common mechanism based on Knl1/Spc105 phosphorylation is critical for Mps1 function in error correction and SAC signalling, thus supporting the idea that a single sensory apparatus simultaneously elicits both pathways.

Regulation of Mitotic Exit by Cell Cycle Checkpoints: Lessons From Saccharomyces cerevisiae

In order to preserve genome integrity and their ploidy, cells must ensure that the duplicated genome has been faithfully replicated and evenly distributed before they complete their division by mitosis. To this end, cells have developed highly elaborated checkpoints that halt mitotic progression when problems in DNA integrity or chromosome segregation arise, providing them with time to fix these issues before advancing further into the cell cycle. Remarkably, exit from mitosis constitutes a key cell cycle transition that is targeted by the main mitotic checkpoints, despite these surveillance mechanisms being activated by specific intracellular signals and acting at different stages of cell division. Focusing primarily on research carried out using Saccharomyces cerevisiae as a model organism, the aim of this review is to provide a general overview of the molecular mechanisms by which the major cell cycle checkpoints control mitotic exit and to highlight the importance of the proper regulation of this process for the maintenance of genome stability during the distribution of the duplicated chromosomes between the dividing cells.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

Accurate chromosome segregation during cell division is essential to maintain genome integrity in all eukaryotic cells, and chromosome missegregation leads to aneuploidy and therefore represents a hallmark of many cancers. Accurate segregation requires sister kinetochores to attach to microtubules emanating from opposite spindle poles, known as bipolar attachment or biorientation. Recent studies have uncovered several mechanisms critical to chromosome bipolar attachment. First, a mechanism exists to ensure that the conformation of sister centromeres is biased toward bipolar attachment. Second, the phosphorylation of some kinetochore proteins destabilizes kinetochore attachment to facilitate error correction, but a protein phosphatase reverses this phosphorylation. Moreover, the activity of the spindle assembly checkpoint is regulated by kinases and phosphatases at the kinetochore, and this checkpoint prevents anaphase entry in response to faulty kinetochore attachment. The fine-tuned kinase/phosphatase balance at kinetochores is crucial for faithful chromosome segregation during both mitosis and meiosis. Here, we discuss the function and regulation of protein phosphatases in the establishment of chromosome bipolar attachment with a focus on the model organism budding yeast.

The expanding phenotypes of cohesinopathies: one ring to rule them all!

Preservation and development of life depend on the adequate segregation of sister chromatids during mitosis and meiosis. This process is ensured by the cohesin multi-subunit complex. Mutations in this complex have been associated with an increasing number of diseases, termed cohesinopathies. The best characterized cohesinopathy is Cornelia de Lange syndrome (CdLS), in which intellectual and growth retardations are the main phenotypic manifestations. Despite some overlap, the clinical manifestations of cohesinopathies vary considerably. Novel roles of the cohesin complex have emerged during the past decades, suggesting that important cell cycle regulators exert important biological effects through non-cohesion-related functions and broadening the potential pathomechanisms involved in cohesinopathies. This review focuses on non-cohesion-related functions of the cohesin complex, gene dosage effect, epigenetic regulation and TGF-β in cohesinopathy context, especially in comparison to Chronic Atrial and Intestinal Dysrhythmia (CAID) syndrome, a very distinct cohesinopathy caused by a homozygous Shugoshin-1 (SGO1) mutation (K23E) and characterized by pacemaker failure in both heart (sick sinus syndrome followed by atrial flutter) and gut (chronic intestinal pseudo-obstruction) with no intellectual or growth delay. We discuss the possible impact of SGO1 alterations in human pathologies and the potential impact of the SGO1 K23E mutation in the sinus node and gut development and functions. We suggest that the human phenotypes observed in CdLS, CAID syndrome and other cohesinopathies can inform future studies into the less well-known non-cohesion-related functions of cohesin complex genes. Abbreviations: AD: Alzheimer Disease; AFF4: AF4/FMR2 Family Member 4; ANKRD11: Ankyrin Repeat Domain 11; APC: Anaphase Promoter Complex; ASD: Atrial Septal Defect; ATRX: ATRX Chromatin Remodeler; ATRX: Alpha Thalassemia X-linked intellectual disability syndrome; BIRC5: Baculoviral IAP Repeat Containing 5; BMP: Bone Morphogenetic Protein; BRD4: Bromodomain Containing 4; BUB1: BUB1 Mitotic Checkpoint Serine/Threonine Kinase; CAID: Chronic Atrial and Intestinal Dysrhythmia; CDK1: Cyclin Dependent Kinase 1; CdLS: Cornelia de Lange Syndrome; CHD: Congenital Heart Disease; CHOPS: Cognitive impairment, coarse facies, Heart defects, Obesity, Pulmonary involvement, Short stature, and skeletal dysplasia; CIPO: Chronic Intestinal Pseudo-Obstruction; c-kit: KIT Proto-Oncogene Receptor Tyrosine Kinase; CoATs: Cohesin Acetyltransferases; CTCF: CCCTC-Binding Factor; DDX11: DEAD/H-Box Helicase 11; ERG: Transcriptional Regulator ERG; ESCO2: Establishment of Sister Chromatid Cohesion N-Acetyltransferase 2; GJC1: Gap Junction Protein Gamma 1; H2A: Histone H2A; H3K4: Histone H3 Lysine 4; H3K9: Histone H3 Lysine 9; HCN4: Hyperpolarization Activated Cyclic Nucleotide Gated Potassium and Sodium Channel 4;p HDAC8: Histone deacetylases 8; HP1: Heterochromatin Protein 1; ICC: Interstitial Cells of Cajal; ICC-MP: Myenteric Plexus Interstitial cells of Cajal; ICC-DMP: Deep Muscular Plexus Interstitial cells of Cajal; If: Pacemaker Funny Current; IP3: Inositol trisphosphate; JNK: C-Jun N-Terminal Kinase; LDS: Loeys-Dietz Syndrome; LOAD: Late-Onset Alzheimer Disease; MAPK: Mitogen-Activated Protein Kinase; MAU: MAU Sister Chromatid Cohesion Factor; MFS: Marfan Syndrome; NIPBL: NIPBL, Cohesin Loading Factor; OCT4: Octamer-Binding Protein 4; P38: P38 MAP Kinase; PDA: Patent Ductus Arteriosus; PDS5: PDS5 Cohesin Associated Factor; P-H3: Phospho Histone H3; PLK1: Polo Like Kinase 1; POPDC1: Popeye Domain Containing 1; POPDC2: Popeye Domain Containing 2; PP2A: Protein Phosphatase 2; RAD21: RAD21 Cohesin Complex Component; RBS: Roberts Syndrome; REC8: REC8 Meiotic Recombination Protein; RNAP2: RNA polymerase II; SAN: Sinoatrial node; SCN5A: Sodium Voltage-Gated Channel Alpha Subunit 5; SEC: Super Elongation Complex; SGO1: Shogoshin-1; SMAD: SMAD Family Member; SMC1A: Structural Maintenance of Chromosomes 1A; SMC3: Structural Maintenance of Chromosomes 3; SNV: Single Nucleotide Variant; SOX2: SRY-Box 2; SOX17: SRY-Box 17; SSS: Sick Sinus Syndrome; STAG2: Cohesin Subunit SA-2; TADs: Topology Associated Domains; TBX: T-box transcription factors; TGF-β: Transforming Growth Factor β; TGFBR: Transforming Growth Factor β receptor; TOF: Tetralogy of Fallot; TREK1: TREK-1 K(+) Channel Subunit; VSD: Ventricular Septal Defect; WABS: Warsaw Breakage Syndrome; WAPL: WAPL Cohesin Release Factor.

Genome architecture and stability in the Saccharomyces cerevisiae knockout collection

Despite major progress in defining the functional roles of genes, a complete understanding of their influences is far from being realized, even in relatively simple organisms. A major milestone in this direction arose via the completion of the yeast Saccharomyces cerevisiae gene-knockout collection (YKOC), which has enabled high-throughput reverse genetics, phenotypic screenings and analyses of synthetic-genetic interactions1-3. Ensuing experimental work has also highlighted some inconsistencies and mistakes in the YKOC, or genome instability events that rebalance the effects of specific knockouts4-6, but a complete overview of these is lacking. The identification and analysis of genes that are required for maintaining genomic stability have traditionally relied on reporter assays and on the study of deletions of individual genes, but whole-genome-sequencing technologies now enable-in principle-the direct observation of genome instability globally and at scale. To exploit this opportunity, we sequenced the whole genomes of nearly all of the 4,732 strains comprising the homozygous diploid YKOC. Here, by extracting information on copy-number variation of tandem and interspersed repetitive DNA elements, we describe-for almost every single non-essential gene-the genomic alterations that are induced by its loss. Analysis of this dataset reveals genes that affect the maintenance of various genomic elements, highlights cross-talks between nuclear and mitochondrial genome stability, and shows how strains have genetically adapted to life in the absence of individual non-essential genes.

Aurora A Protein Kinase: To the Centrosome and Beyond

Accurate chromosome segregation requires the perfect spatiotemporal rearrangement of the cellular cytoskeleton. Isolated more than two decades ago from Drosophila, Aurora A is a widespread protein kinase that plays key roles during cell division. Numerous studies have described the localisation of Aurora A at centrosomes, the mitotic spindle, and, more recently, at mitotic centromeres. In this review, we will summarise the cytoskeletal rearrangements regulated by Aurora A during cell division. We will also discuss the recent discoveries showing that Aurora A also controls not only the dynamics of the cortical proteins but also regulates the centromeric proteins, revealing new roles for this kinase during cell division.

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.

Role of Kip2 during early mitosis - impact on spindle pole body separation and chromosome capture

Mitotic spindle dynamics are regulated during the cell cycle by microtubule motor proteins. In Saccharomyces cerevisiae, one such protein is Kip2p, a plus-end motor that regulates the polymerization and stability of cytoplasmic microtubules (cMTs). Kip2p levels are regulated during the cell cycle, and its overexpression leads to the formation of hyper-elongated cMTs. To investigate the significance of varying Kip2p levels during the cell cycle and the hyper-elongated cMTs, we overexpressed KIP2 in the G1 phase and examined the effects on the separation of spindle pole bodies (SPBs) and chromosome segregation. Our results show that failure to regulate the cMT lengths during G1-S phase prevents the separation of SPBs. This, in turn, affects chromosome capture and leads to the activation of spindle assembly checkpoint (SAC) and causes mitotic arrest. These defects could be rescued by either the inactivation of checkpoint components or by co-overexpression of CIN8, which encodes a motor protein that elongates inter-polar microtubules (ipMTs). Hence, we propose that the maintenance of Kip2p level and cMT lengths during early cell division is important to ensure coordination between SPB separation and chromosome capture by kinetochore microtubules (kMTs).

Critical roles of Shugoshin and histones as tension sensors during mitosis

Biorientation of paired sister chromosomes is required to maintain mitotic fidelity. A critical signal indicative of bipolar attachment is tension between cohesion-linked sister chromatids. Key components of the tension signaling apparatus include the Shugoshin family of proteins and the tension sensing motif of histone H3. Shugoshin proteins are recruited to chromatin to create discrete domains integral to tension sensing. Many factors involved in the chromatin association of Shugoshin proteins are well established, most strikingly through modifications found directly on centromeric and pericentric chromatin. It has been well established that phosphorylation at the centromere is essential to nucleating Shugoshin recruitment, but recent evidence revealed a role for pericentric histones and acetylation in modulating Shugoshin recruitment and activity. These data demonstrate that chromatins are not simply passive cargo during mitosis, but are instead actively involved in their segregation.

Aurora A-dependent CENP-A phosphorylation at inner centromeres protects bioriented chromosomes against cohesion fatigue

Sustained spindle tension applied to sister centromeres during mitosis eventually leads to uncoordinated loss of sister chromatid cohesion, a phenomenon known as “cohesion fatigue.” We report that Aurora A-dependent phosphorylation of serine 7 of the centromere histone variant CENP-A (p-CENP-AS7) protects bioriented chromosomes against cohesion fatigue. Expression of a non-phosphorylatable version of CENP-A (CENP-AS7A) weakens sister chromatid cohesion only when sister centromeres are under tension, providing the first evidence of a regulated mechanism involved in protection against passive cohesion loss. Consistent with this observation, p-CENP-AS7 is detected at the inner centromere where it forms a discrete domain. The depletion or inhibition of Aurora A phenocopies the expression of CENP-AS7A and we show that Aurora A is recruited to centromeres in a Bub1-dependent manner. We propose that Aurora A-dependent phosphorylation of CENP-A at the inner centromere protects chromosomes against tension-induced cohesion fatigue until the last kinetochore is attached to spindle microtubules.

Sustained spindle tension applied to sister centromeres during mitosis leads to loss of sister chromatid cohesion which is known as cohesion fatigue. Here the authors show that Aurora A-dependent phosphorylation of CENP-A at the inner centromeres protects bioriented chromosomes against cohesion fatigue.

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.

The Ins and Outs of Aurora B Inner Centromere Localization

Error-free chromosome segregation is essential for the maintenance of genomic integrity during cell division. Aurora B, the enzymatic subunit of the Chromosomal Passenger Complex (CPC), plays a crucial role in this process. In early mitosis Aurora B localizes predominantly to the inner centromere, a specialized region of chromatin that lies at the crossroads between the inter-kinetochore and inter-sister chromatid axes. Two evolutionarily conserved histone kinases, Haspin and Bub1, control the positioning of the CPC at the inner centromere and this location is thought to be crucial for the CPC to function. However, recent studies sketch a subtler picture, in which not all functions of the CPC require strict confinement to the inner centromere. In this review we discuss the molecular pathways that direct Aurora B to the inner centromere and deliberate if and why this specific localization is important for Aurora B function.

A Failsafe for Sensing Chromatid Tension in Mitosis with the Histone H3 Tail in Saccharomyces cerevisiae

Mitotic fidelity is ensured by achieving biorientation on all paired chromosomes. The key signal for proper chromosome alignment is the tension between sister chromatids created by opposing poleward force from the spindles. In the budding yeast, the tension-sensing function requires that the Shugoshin protein, Shugoshin 1, be recruited to the centromeres and the neighboring pericentric regions. Concerted actions integrating proteins at centromeres and pericentromeres create highly specific Shugoshin 1 domains on mitotic chromosomes. We have previously reported that an important regulatory region on histone H3, termed the tension-sensing motif (TSM), is responsible for retaining Shugoshin 1 at pericentromeres. The TSM is negatively regulated by the acetyltransferase Gcn5p, but the underlying mechanism was elusive. In this work, we provide evidence that, when the TSM function is impaired, the histone H3 tail adopts a role that complements the damaged TSM to ensure faithful mitosis. This novel function of the H3 tail is controlled by Gcn5p, which targets selective lysine residues. Mutations to K14 and K23 ameliorate the mitotic defects resulting from TSM mutations. The restoration of faithful segregation is accompanied by regaining Shugoshin 1 access to the pericentric regions. Our data reveal a novel pathway for mitotic Shugoshin 1 recruitment and further reinforce the active role played by chromatins during their segregation in mitosis.

Mapping the Synthetic Dosage Lethality Network of CDK1/CDC28

Cdk1 (Cdc28 in yeast) is a cyclin-dependent kinase (CDK) essential for cell cycle progression and cell division in normal cells. However, CDK activity also underpins proliferation of tumor cells, making it a relevant study subject. While numerous targets and processes regulated by Cdc28 have been identified, the exact functions of Cdc28 are only partially understood. To further explore the functions of Cdc28, we systematically overexpressed ∼4800 genes in wild-type (WT) cells and in cells with artificially reduced Cdc28 activity. This screen identified 366 genes that, when overexpressed, specifically compromised cell viability under conditions of reduced Cdc28 activity. Consistent with the crucial functions of Cdc28 in cell cycle regulation and chromosome metabolism, most of these genes have functions in the cell cycle, DNA replication, and transcription. However, a substantial number of genes control processes not directly associated with the cell cycle, indicating that Cdc28 may also regulate these processes. Finally, because the dataset was enriched for direct Cdc28 targets, the results from this screen will aid in identifying novel targets and process regulated by Cdc28.

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.

Casein Kinase 1 Coordinates Cohesin Cleavage, Gametogenesis, and Exit from M Phase in Meiosis II

Meiosis consists of DNA replication followed by two consecutive nuclear divisions and gametogenesis or spore formation. While meiosis I has been studied extensively, less is known about the regulation of meiosis II. Here we show that Hrr25, the conserved casein kinase 1δ of budding yeast, links three mutually independent key processes of meiosis II. First, Hrr25 induces nuclear division by priming centromeric cohesin for cleavage by separase. Hrr25 simultaneously phosphorylates Rec8, the cleavable subunit of cohesin, and removes from centromeres the cohesin protector composed of shugoshin and the phosphatase PP2A. Second, Hrr25 initiates the sporulation program by inducing the synthesis of membranes that engulf the emerging nuclei at anaphase II. Third, Hrr25 mediates exit from meiosis II by activating pathways that trigger the destruction of M-phase-promoting kinases. Thus, Hrr25 synchronizes formation of the single-copy genome with gamete differentiation and termination of meiosis.

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.

Fin1-PP1 Helps Clear Spindle Assembly Checkpoint Protein Bub1 from Kinetochores in Anaphase

The spindle assembly checkpoint (SAC) monitors chromosome attachment defects, and the assembly of SAC proteins at kinetochores is essential for its activation, but the SAC disassembly process remains unknown. We found that deletion of a 14-3-3 protein, Bmh1, or hyperactivation of Cdc14 early anaphase release (FEAR) allows premature SAC silencing in budding yeast, which depends on a kinetochore protein Fin1 that forms a complex with protein phosphatase PP1. Previous works suggest that FEAR-dependent Fin1 dephosphorylation promotes Bmh1-Fin1 dissociation, which enables kinetochore recruitment of Fin1-PP1. We found persistent kinetochore association of SAC protein Bub1 in fin1Δ mutants after anaphase entry. Therefore, we revealed a mechanism that clears SAC proteins from kinetochores. After anaphase entry, FEAR activation promotes kinetochore enrichment of Fin1-PP1, resulting in SAC disassembly at kinetochores. This mechanism is required for efficient SAC silencing after SAC is challenged, and untimely Fin1-kinetochore association causes premature SAC silencing and chromosome missegregation.

Mitotic Transcription Installs Sgo1 at Centromeres to Coordinate Chromosome Segregation

Human sister chromatids at metaphase are primarily linked by centromeric cohesion, forming the iconic X shape. Premature loss of centromeric cohesion disrupts orderly mitotic progression. Shugoshin (Sgo1) binds to and protects cohesin at inner centromeres. The kinetochore kinase Bub1 phosphorylates histone H2A at T120 (H2A-pT120) and recruits Sgo1 to kinetochores, 0.5 μm from inner centromeres. Here, we show that Sgo1 is a direct reader of the H2A-pT120 mark. Bub1 also recruits RNA polymerase II (Pol II) to unattached kinetochores and promotes active transcription at mitotic kinetochores. Mitosis-specific inactivation of Pol II traps Sgo1 at kinetochores and weakens centromeric cohesion. Sgo1 interacts with Pol II in human cells and with RNA in vitro. We propose that Pol II-dependent transcription enables kinetochore-bound Sgo1 initially recruited by H2A-pT120 to reach cohesin embedded in centromeric chromatin. Our study implicates mitotic transcription in targeting regulatory factors to highly compacted mitotic chromatin.

Genetic instability in budding and fission yeast-sources and mechanisms

Cells are constantly confronted with endogenous and exogenous factors that affect their genomes. Eons of evolution have allowed the cellular mechanisms responsible for preserving the genome to adjust for achieving contradictory objectives: to maintain the genome unchanged and to acquire mutations that allow adaptation to environmental changes. One evolutionary mechanism that has been refined for survival is genetic variation. In this review, we describe the mechanisms responsible for two biological processes: genome maintenance and mutation tolerance involved in generations of genetic variations in mitotic cells of both Saccharomyces cerevisiae and Schizosaccharomyces pombe. These processes encompass mechanisms that ensure the fidelity of replication, DNA lesion sensing and DNA damage response pathways, as well as mechanisms that ensure precision in chromosome segregation during cell division. We discuss various factors that may influence genome stability, such as cellular ploidy, the phase of the cell cycle, transcriptional activity of a particular region of DNA, the proficiency of DNA quality control systems, the metabolic stage of the cell and its respiratory potential, and finally potential exposure to endogenous or environmental stress.

The stability of budding and fission yeast genomes is influenced by two contradictory factors: (1) the need to be fully functional, which is ensured through the replication fidelity pathways of nuclear and mitochondrial genomes through sensing and repairing DNA damage, through precise chromosome segregation during cell division; and (2) the need to acquire changes for adaptation to environmental challenges.

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.

Sgo1 recruits PP2A to chromosomes to ensure sister chromatid bi-orientation during mitosis

Sister chromatid bi-orientation on the mitotic spindle is essential for proper chromosome segregation. Defects in bi-orientation are sensed and corrected to prevent chromosome mis-segregation and aneuploidy. This response depends on the adaptor protein Sgo1, which associates with pericentromeric chromatin in mitosis. The mechanisms underlying Sgo1 function and regulation are unclear. Here, we show that Sgo1 is an anaphase-promoting complex/cyclosome (APC/C) substrate in budding yeast (Saccharomyces cerevisiae), and that its mitotic destruction depends on an unusual D-box-related sequence motif near its C-terminus. We find that the removal of Sgo1 from chromosomes before anaphase is not dependent on its destruction, but rather on other mechanisms responsive to tension between sister chromatids. Additionally, we find that Sgo1 recruits the protein phosphatase 2A (PP2A) isoform containing Rts1 to the pericentromeric region prior to bi-orientation, and that artificial recruitment of Rts1 to this region of a single chromosome is sufficient to perform the function of Sgo1 on that chromosome. We conclude that in early mitosis, Sgo1 associates transiently with pericentromeric chromatin to promote bi-orientation, in large part by recruiting the Rts1 isoform of PP2A.

Tethering sister centromeres to each other suggests the spindle checkpoint detects stretch within the kinetochore

The spindle checkpoint ensures that newly born cells receive one copy of each chromosome by preventing chromosomes from segregating until they are all correctly attached to the spindle. The checkpoint monitors tension to distinguish between correctly aligned chromosomes and those with both sisters attached to the same spindle pole. Tension arises when sister kinetochores attach to and are pulled toward opposite poles, stretching the chromatin around centromeres and elongating kinetochores. We distinguished between two hypotheses for where the checkpoint monitors tension: between the kinetochores, by detecting alterations in the distance between them, or by responding to changes in the structure of the kinetochore itself. To distinguish these models, we inhibited chromatin stretch by tethering sister chromatids together by binding a tetrameric form of the Lac repressor to arrays of the Lac operator located on either side of a centromere. Inhibiting chromatin stretch did not activate the spindle checkpoint; these cells entered anaphase at the same time as control cells that express a dimeric version of the Lac repressor, which cannot cross link chromatids, and cells whose checkpoint has been inactivated. There is no dominant checkpoint inhibition when sister kinetochores are held together: cells expressing the tetrameric Lac repressor still arrest in response to microtubule-depolymerizing drugs. Tethering chromatids together does not disrupt kinetochore function; chromosomes are successfully segregated to opposite poles of the spindle. Our results indicate that the spindle checkpoint does not monitor inter-kinetochore separation, thus supporting the hypothesis that tension is measured within the kinetochore.

The spindle checkpoint monitors tension on chromosomes to distinguish between chromosomes that are correctly and incorrectly attached to the spindle. Tension is generated across a correctly attached chromosome as microtubules from opposite poles attach to and pull kinetochores apart, but are resisted by the cohesin that holds sister chromatids together. This tension generates separation between kinetochores as pericentric chromatin stretches and it also elongates the kinetochores. To monitor tension, the checkpoint could measure the separation between kinetochores or the stretch within them. We inhibited the ability of pericentric chromatin to stretch by tethering sister centromeres to each other, and we asked whether the resulting reduction in inter-kinetochore separation artificially activated the spindle checkpoint. Inhibiting inter-kinetochore separation does not delay anaphase, and the timing of mitosis was the same in cells with or without the spindle checkpoint, showing that the checkpoint is not activated. Inhibiting chromatin stretch does not alter the function of kinetochores as chromosomes are still segregated correctly, nor does it hinder the checkpoint. Cells whose sister kinetochores are held together can still activate the checkpoint in response to microtubule depolymerization. Our results indicate the spindle checkpoint does not monitor inter-kinetochore separation and likely monitors tension within kinetochores.

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.

The current view for the silencing of the spindle assembly checkpoint

Chromosome bipolar attachment is achieved when sister kinetochores are attached by microtubules emanating from opposite spindle poles, and this process is essential for faithful chromosome segregation during anaphase. A fundamental question in cell biology is how cells ensure that chromosome segregation only occurs after bipolar attachment. It is well documented that unattached kinetochores activate the spindle assembly checkpoint (SAC) to delay chromosome segregation. Therefore, the silencing of the SAC is thought to trigger anaphase onset, but how correct chromosome attachment is coupled with SAC silencing and the subsequent anaphase onset is poorly understood. The establishment of chromosome bipolar attachment not only results in the occupancy of kinetochores by microtubules but also applies tension on sister kinetochores. A long-standing debate is whether the kinetochore attachment (occupancy) or the tension silences the SAC. Recent work in budding yeast reveals the SAC silencing network SSN that prevents SAC silencing prior to tension generation at kinetochores. Therefore, this signaling pathway ensures that SAC silencing and the subsequent anaphase onset occur only after chromosome bipolar attachment applies tension on chromosomes. This review will summarize the recent advances in the understanding of the SAC silencing process.

SHUGOSHINs and PATRONUS protect meiotic centromere cohesion in Arabidopsis thaliana

In meiosis, chromosome cohesion is maintained by the cohesin complex, which is released in a two-step manner. At meiosis I, the meiosis-specific cohesin subunit Rec8 is cleaved by the protease Separase along chromosome arms, allowing homologous chromosome segregation. Next, in meiosis II, cleavage of the remaining centromere cohesin results in separation of the sister chromatids. In eukaryotes, protection of centromeric cohesion in meiosis I is mediated by SHUGOSHINs (SGOs). The Arabidopsis genome contains two SGO homologs. Here we demonstrate that Atsgo1 mutants show a premature loss of cohesion of sister chromatid centromeres at anaphase I and that AtSGO2 partially rescues this loss of cohesion. In addition to SGOs, we characterize PATRONUS which is specifically required for the maintenance of cohesion of sister chromatid centromeres in meiosis II. In contrast to the Atsgo1 Atsgo2 double mutant, patronus T-DNA insertion mutants only display loss of sister chromatid cohesion after meiosis I, and additionally show disorganized spindles, resulting in defects in chromosome segregation in meiosis. This leads to reduced fertility and aneuploid offspring. Furthermore, we detect aneuploidy in sporophytic tissue, indicating a role for PATRONUS in chromosome segregation in somatic cells. Thus, ploidy stability is preserved in Arabidopsis by PATRONUS during both meiosis and mitosis.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

Phosphorylation of Sli15 by Ipl1 is important for proper CPC localization and chromosome stability in Saccharomyces cerevisiae

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 have identified multiple sites of CPC autophosphorylation on yeast Sli15 that are located within its central microtubule-binding domain and examined the functional significance of their phosphorylation by Ipl1 through mutation of these sites, either to non-phosphorylatable alanine (sli15-20A) or to acidic residues to mimic constitutive phosphorylation (sli15-20D). Both mutant sli15 alleles confer chromosome instability, but this is mediated neither by changes in the capacity of Sli15 to activate Ipl1 kinase nor by decreased efficiency of chromosome biorientation, a key process in cell division that requires CPC function. Instead, we find that mimicking constitutive phosphorylation of Sli15 on the Ipl1 phosphorylation sites causes delocalization of the CPC in metaphase, whereas blocking phosphorylation of Sli15 on the Ipl1 sites drives excessive localization of Sli15 to the mitotic spindle in pre-anaphase cells. Consistent with these results, direct interaction of Sli15 with microtubules in vitro is greatly reduced either following phosphorylation by Ipl1 or when constitutive phosphorylation at the Ipl1-dependent phosphorylation sites is mimicked by aspartate or glutamate substitutions. Furthermore, we find that mimicking Ipl1 phosphorylation of Sli15 interferes with the ‘tension checkpoint’ – the CPC-dependent mechanism through which cells activate the spindle assembly checkpoint to delay anaphase in the absence of tension on kinetochore-microtubule attachments. Ipl1-dependent phosphorylation of Sli15 therefore inhibits its association with microtubules both in vivo and in vitro and may negatively regulate the tension checkpoint mechanism.

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.