Securin and separase phosphorylation act redundantly to maintain sister chromatid cohesion in mammalian cells

An autonomous mathematical model for the mammalian cell cycle

A mathematical model for the mammalian cell cycle is developed as a system of 13 coupled nonlinear ordinary differential equations. The variables and interactions included in the model are based on detailed consideration of available experimental data. A novel feature of the model is inclusion of cycle tasks such as origin licensing and initiation, nuclear envelope breakdown and kinetochore attachment, and their interactions with controllers (molecular complexes involved in cycle control). Other key features are that the model is autonomous, except for a dependence on external growth factors; the variables are continuous in time, without instantaneous resets at phase boundaries; mechanisms to prevent rereplication are included; and cycle progression is independent of cell size. Eight variables represent cell cycle controllers: the Cyclin D1-Cdk4/6 complex, APC^Cdh1, SCF^βTrCP, Cdc25A, MPF, NuMA, the securin-separase complex, and separase. Five variables represent task completion, with four for the status of origins and one for kinetochore attachment. The model predicts distinct behaviors corresponding to the main phases of the cell cycle, showing that the principal features of the mammalian cell cycle, including restriction point behavior, can be accounted for in a quantitative mechanistic way based on known interactions among cycle controllers and their coupling to tasks. The model is robust to parameter changes, in that cycling is maintained over at least a five-fold range of each parameter when varied individually. The model is suitable for exploring how extracellular factors affect cell cycle progression, including responses to metabolic conditions and to anti-cancer therapies.

Autocleavage of separase suppresses its premature activation by promoting binding to cyclin B1

Accurate chromosome segregation requires timely activation of separase, a protease that cleaves cohesin during the metaphase-to-anaphase transition. However, the mechanism that maintains the inactivity of separase prior to this event remains unclear. We provide evidence that separase autocleavage plays an essential role in this process. We show that the inhibition of separase autocleavage results in premature activity before the onset of anaphase, accompanied by the formation of chromosomal bridges and spindle rocking. This deregulation is attributed to the reduced binding of cyclin B1 to separase that occurs during the metaphase-to-anaphase transition. Furthermore, when separase is mutated to render the regulation by cyclin B1 irrelevant, which keeps separase in securin-binding form, the deregulation induced by autocleavage inhibition is rescued. Our results reveal a physiological role of separase autocleavage in regulating separase, which ensures faithful chromosome segregation.

Separase Control and Cohesin Cleavage in Oocytes: Should I Stay or Should I Go?

The key to gametogenesis is the proper execution of a specialized form of cell division named meiosis. Prior to the meiotic divisions, the recombination of maternal and paternal chromosomes creates new genetic combinations necessary for fitness and adaptation to an ever-changing environment. Two rounds of chromosome segregation -meiosis I and II- have to take place without intermediate S-phase and lead to the creation of haploid gametes harboring only half of the genetic material. Importantly, the segregation patterns of the two divisions are fundamentally different and require adaptation of the mitotic cell cycle machinery to the specificities of meiosis. Separase, the enzyme that cleaves Rec8, a subunit of the cohesin complex constituting the physical connection between sister chromatids, has to be activated twice: once in meiosis I and immediately afterwards, in meiosis II. Rec8 is cleaved on chromosome arms in meiosis I and in the centromere region in meiosis II. This step-wise cohesin removal is essential to generate gametes of the correct ploidy and thus, embryo viability. Hence, separase control and Rec8 cleavage must be perfectly controlled in time and space. Focusing on mammalian oocytes, this review lays out what we know and what we still ignore about this fascinating mechanism.

CCHCR1-astrin interaction promotes centriole duplication through recruitment of CEP72

BACKGROUND

The centrosome is one of the most important non-membranous organelles regulating microtubule organization and progression of cell mitosis. The coiled-coil alpha-helical rod protein 1 (CCHCR1, also known as HCR) gene is considered to be a psoriasis susceptibility gene, and the protein is suggested to be localized to the P-bodies and centrosomes in mammalian cells. However, the exact cellular function of HCR and its potential regulatory role in the centrosomes remain unexplored.

RESULTS

We found that HCR interacts directly with astrin, a key factor in centrosome maturation and mitosis. Immunoprecipitation assays showed that the coiled-coil region present in the C-terminus of HCR and astrin respectively mediated the interaction between them. Astrin not only recruits HCR to the centrosome, but also protects HCR from ubiquitin-proteasome-mediated degradation. In addition, depletion of either HCR or astrin significantly reduced centrosome localization of CEP72 and subsequent MCPH proteins, including CEP152, CDK5RAP2, and CEP63. The absence of HCR also caused centriole duplication defects and mitotic errors, resulting in multipolar spindle formation, genomic instability, and DNA damage.

CONCLUSION

We conclude that HCR is localized and stabilized at the centrosome by directly binding to astrin. HCR are required for the centrosomal recruitment of MCPH proteins and centriolar duplication. Both HCR and astrin play key roles in keeping normal microtubule assembly and maintaining genomic stability.

Ordered dephosphorylation initiated by the selective proteolysis of cyclin B drives mitotic exit

APC/C-mediated proteolysis of cyclin B and securin promotes anaphase entry, inactivating CDK1 and permitting chromosome segregation, respectively. Reduction of CDK1 activity relieves inhibition of the CDK1-counteracting phosphatases PP1 and PP2A-B55, allowing wide-spread dephosphorylation of substrates. Meanwhile, continued APC/C activity promotes proteolysis of other mitotic regulators. Together, these activities orchestrate a complex series of events during mitotic exit. However, the relative importance of regulated proteolysis and dephosphorylation in dictating the order and timing of these events remains unclear. Using high temporal-resolution proteomics, we compare the relative extent of proteolysis and protein dephosphorylation. This reveals highly-selective rapid proteolysis of cyclin B, securin and geminin at the metaphase-anaphase transition, followed by slow proteolysis of other substrates. Dephosphorylation requires APC/C-dependent destruction of cyclin B and was resolved into PP1-dependent categories with unique sequence motifs. We conclude that dephosphorylation initiated by selective proteolysis of cyclin B drives the bulk of changes observed during mitotic exit.

Securin-independent regulation of separase by checkpoint-induced shugoshin-MAD2

Separation of eukaryotic sister chromatids during the cell cycle is timed by the spindle assembly checkpoint (SAC) and ultimately triggered when separase cleaves cohesion-mediating cohesin1-3. Silencing of the SAC during metaphase activates the ubiquitin ligase APC/C (anaphase-promoting complex, also known as the cyclosome) and results in the proteasomal destruction of the separase inhibitor securin1. In the absence of securin, mammalian chromosomes still segregate on schedule, but it is unclear how separase is regulated under these conditions4,5. Here we show that human shugoshin 2 (SGO2), an essential protector of meiotic cohesin with unknown functions in the soma6,7, is turned into a separase inhibitor upon association with SAC-activated MAD2. SGO2-MAD2 can functionally replace securin and sequesters most separase in securin-knockout cells. Acute loss of securin and SGO2, but not of either protein individually, resulted in separase deregulation associated with premature cohesin cleavage and cytotoxicity. Similar to securin8,9, SGO2 is a competitive inhibitor that uses a pseudo-substrate sequence to block the active site of separase. APC/C-dependent ubiquitylation and action of the AAA-ATPase TRIP13 in conjunction with the MAD2-specific adaptor p31comet liberate separase from SGO2-MAD2 in vitro. The latter mechanism facilitates a considerable degree of sister chromatid separation in securin-knockout cells that lack APC/C activity. Thus, our results identify an unexpected function of SGO2 in mitotically dividing cells and a mechanism of separase regulation that is independent of securin but still supervised by the SAC.

The cyclin B2/CDK1 complex inhibits separase activity in mouse oocyte meiosis I

Chromosome segregation is driven by separase, activity of which is inhibited by binding to securin and cyclin B1/CDK1. In meiosis, premature separase activity will induce aneuploidy or abolish chromosome segregation owing to the untimely destruction of cohesin. Recently, we have proved that cyclin B2 can compensate for cyclin B1 in CDK1 activation for the oocyte meiosis G2/M transition. In the present study, we identify an interaction between cyclin B2/CDK1 and separase in mouse oocytes. We find that cyclin B2 degradation is required for separase activation during the metaphase I-anaphase I transition because the presence of stable cyclin B2 leads to failure of homologous chromosome separation and to metaphase I arrest, especially in the simultaneous absence of securin and cyclin B1. Moreover, non-phosphorylatable separase rescues the separation of homologous chromosomes in stable cyclin B2-arrested cyclin B1-null oocytes. Our results indicate that cyclin B2/CDK1 is also responsible for separase inhibition via inhibitory phosphorylation to regulate chromosome separation in oocyte meiosis, which may not occur in other cell types.

Cdk1 phosphorylation of Esp1/Separase functions with PP2A and Slk19 to regulate pericentric Cohesin and anaphase onset

Anaphase onset is an irreversible cell cycle transition that is triggered by the activation of the protease Separase. Separase cleaves the Mcd1 (also known as Scc1) subunit of Cohesin, a complex of proteins that physically links sister chromatids, triggering sister chromatid separation. Separase is regulated by the degradation of the anaphase inhibitor Securin which liberates Separase from inhibitory Securin/Separase complexes. In many organisms, Securin is not essential suggesting that Separase is regulated by additional mechanisms. In this work, we show that in budding yeast Cdk1 activates Separase (Esp1 in yeast) through phosphorylation to trigger anaphase onset. Esp1 activation is opposed by protein phosphatase 2A associated with its regulatory subunit Cdc55 (PP2ACdc55) and the spindle protein Slk19. Premature anaphase spindle elongation occurs when Securin (Pds1 in yeast) is inducibly degraded in cells that also contain phospho-mimetic mutations in ESP1, or deletion of CDC55 or SLK19. This striking phenotype is accompanied by advanced degradation of Mcd1, disruption of pericentric Cohesin organization and chromosome mis-segregation. Our findings suggest that PP2ACdc55 and Slk19 function redundantly with Pds1 to inhibit Esp1 within pericentric chromatin, and both Pds1 degradation and Cdk1-dependent phosphorylation of Esp1 act together to trigger anaphase onset.

Zap70 and downstream RanBP2 are required for the exact timing of the meiotic cell cycle in oocytes

In previous studies, we observed that Zeta-chain-associated protein kinase 70 (Zap70) regulates spindle assembly and chromosome alignment in mouse oocyte and that Ran binding protein 2 (RanBP2) is a highly associated gene with Zap70 based on a microarray analysis. Because RanBP2 is related to nuclear envelope breakdown (NEBD) during mitosis, the aim of the present study was to elucidate the molecular mechanism of Zap70 with respect to RanBP2 in the germinal vesicle breakdown (GVBD) of oocytes. Results indicated that RanBP2 expression was regulated by Zap70 and that depletion of RanBP2 using RanBP2 RNAi manifested comparable phenotypes to those observed in Zap70 RNAi-treated oocytes, which presented faster processing of GVBD. Additionally, Zap70 RNAi-treated oocytes showed faster meiotic resumption with premature activation of maturation-promoting factor (MPF), premature division of chromosomes at approximately 6-8 h and more rapid degradation of securin. In conclusion, we report that Zap70 is a crucial factor for controlling the exact timing of meiotic progression in mouse oocytes.

Targeted TPX2 increases chromosome missegregation and suppresses tumor cell growth in human prostate cancer

Prostate cancer is a complex disease that can be relatively harmless or extremely aggressive. Although androgen-deprivation therapy is a commonly used treatment for men with prostate cancer, the adverse effects can be detrimental to patient health and quality of life. Therefore, identifying new target genes for tumor growth will enable the development of novel therapeutic intervention. TPX2 plays a critical role in chromosome segregation machinery during mitosis. Low rates of chromosome missegregation can promote tumor development, whereas higher levels might promote cell death and suppress tumorigenesis. Hence, the strategy of promoting cell death by inducing massive chromosome missegregation has been a therapeutic application for selectively eliminating highly proliferating tumor cells. RNAi was used for TPX2 protein expression knockdown, and a clonogenic assay, immunostaining, double thymidine block, image-cytometry analysis, and tumor spheroid assay were used to analyze the role of TPX2 in tumor cell growth, cell cycle progression, multinuclearity, ploidy, and tumorigenicity, respectively; finally, Western blotting was used to analyze anticancer mechanisms in TPX2 targeting. We demonstrated that targeting TPX2 reduced cell cycle regulators and chromosome segregation genes, resulting in increased cell micronucleation. Moreover, TPX2 depletion led to prostate cancer cell growth inhibition, increased apoptosis, and reduced tumorigenesis. These results confirmed the therapeutic potential of targeting TPX2 in prostate cancer treatment. Moreover, we found that TPX2 silencing led to deregulation of CDK1, cyclin B, securin, separase, and aurora A proteins; by contrast, p21 mRNA was upregulated. We also determined the molecular mechanisms for TPX2 targeting in prostate cancer cells. In conclusion, our study illustrates the power of TPX2 as a potential novel target gene for prostate cancer treatment.

The ubiquitin ligase CRL2ZYG11 targets cyclin B1 for degradation in a conserved pathway that facilitates mitotic slippage

Cells arrested in mitosis by inactivation of the APC/C complex sometimes manage to exit mitosis in a process called mitotic slippage, which helps cancer cells circumvent chemotherapy drugs. Balachandran et al. show that mitotic slippage occurs as a result of targeting of cyclin B1 for degradation by the ligase CRL2^ZYG11.

The anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase is known to target the degradation of cyclin B1, which is crucial for mitotic progression in animal cells. In this study, we show that the ubiquitin ligase CRL2^ZYG-11 redundantly targets the degradation of cyclin B1 in Caenorhabditis elegans and human cells. In C. elegans, both CRL2^ZYG-11 and APC/C are required for proper progression through meiotic divisions. In human cells, inactivation of CRL2^ZYG11A/B has minimal effects on mitotic progression when APC/C is active. However, when APC/C is inactivated or cyclin B1 is overexpressed, CRL2^ZYG11A/B-mediated degradation of cyclin B1 is required for normal progression through metaphase. Mitotic cells arrested by the spindle assembly checkpoint, which inactivates APC/C, often exit mitosis in a process termed “mitotic slippage,” which generates tetraploid cells and limits the effectiveness of antimitotic chemotherapy drugs. We show that ZYG11A/B subunit knockdown, or broad cullin–RING ubiquitin ligase inactivation with the small molecule MLN4924, inhibits mitotic slippage in human cells, suggesting the potential for antimitotic combination therapy.

Robust Ordering of Anaphase Events by Adaptive Thresholds and Competing Degradation Pathways

The splitting of chromosomes in anaphase and their delivery into the daughter cells needs to be accurately executed to maintain genome stability. Chromosome splitting requires the degradation of securin, whereas the distribution of the chromosomes into the daughter cells requires the degradation of cyclin B. We show that cells encounter and tolerate variations in the abundance of securin or cyclin B. This makes the concurrent onset of securin and cyclin B degradation insufficient to guarantee that early anaphase events occur in the correct order. We uncover that the timing of chromosome splitting is not determined by reaching a fixed securin level, but that this level adapts to the securin degradation kinetics. In conjunction with securin and cyclin B competing for degradation during anaphase, this provides robustness to the temporal order of anaphase events. Our work reveals how parallel cell-cycle pathways can be temporally coordinated despite variability in protein concentrations.

Positive and negative regulation of vertebrate separase by Cdk1-cyclin B1 may explain why securin is dispensable

Sister chromatid cohesion is established during replication by entrapment of both dsDNAs within the cohesin ring complex. It is dissolved in anaphase when separase, a giant cysteine endopeptidase, cleaves the Scc1/Rad21 subunit of cohesin, thereby triggering chromosome segregation. Separase is held inactive by association with securin until this anaphase inhibitor is destroyed at the metaphase-to-anaphase transition by ubiquitin-dependent degradation. The relevant ubiquitin ligase, the anaphase-promoting complex/cyclosome, also targets cyclin B1, thereby causing inactivation of Cdk1 and mitotic exit. Although separase is essential, securin knock-out mice are surprisingly viable and fertile. Capitalizing on our previous finding that Cdk1-cyclin B1 can also bind and inhibit separase, we investigated whether this kinase might be suitable to maintain faithful timing and execution of anaphase in the absence of securin. We found that, similar to securin, Cdk1-cyclin B1 regulates separase in both a positive and negative manner. Although securin associates with nascent separase to co-translationally assist proper folding, Cdk1-cyclin B1 acts on native state separase. Upon entry into mitosis, Cdk1-cyclin B1-dependent phosphorylation of Ser-1126 renders separase prone to inactivation by aggregation/precipitation. Stable association of Cdk1-cyclin B1 with phosphorylated separase counteracts this tendency and stabilizes separase in an inhibited yet activatable state. These opposing effects are suited to prevent premature cleavage of cohesin in early mitosis while ensuring timely activation of separase by anaphase-promoting complex/cyclosome-dependent degradation of cyclin B1. Coupling sister chromatid separation with subsequent exit from mitosis by this simplified mode might have been the common scheme of mitotic control prior to the evolution of securin.

PP2A delays APC/C-dependent degradation of separase-associated but not free securin

The universal triggering event of eukaryotic chromosome segregation is cleavage of centromeric cohesin by separase. Prior to anaphase, most separase is kept inactive by association with securin. Protein phosphatase 2A (PP2A) constitutes another binding partner of human separase, but the functional relevance of this interaction has remained enigmatic. We demonstrate that PP2A stabilizes separase-associated securin by dephosphorylation, while phosphorylation of free securin enhances its polyubiquitylation by the ubiquitin ligase APC/C and proteasomal degradation. Changing PP2A substrate phosphorylation sites to alanines slows degradation of free securin, delays separase activation, lengthens early anaphase, and results in anaphase bridges and DNA damage. In contrast, separase-associated securin is destabilized by introduction of phosphorylation-mimetic aspartates or extinction of separase-associated PP2A activity. G2- or prometaphase-arrested cells suffer from unscheduled activation of separase when endogenous securin is replaced by aspartate-mutant securin. Thus, PP2A-dependent stabilization of separase-associated securin prevents precocious activation of separase during checkpoint-mediated arrests with basal APC/C activity and increases the abruptness and fidelity of sister chromatid separation in anaphase.

Pttg1/securin is required for the branching morphogenesis of the mammary gland and suppresses mammary tumorigenesis

Pituitary tumor transforming gene 1 (Pttg1) encodes the mammalian securin, which is an inhibitor of separase (a protease required for the separation of sister chromatids in mitosis and meiosis). PTTG1 is overexpressed in a number of human cancers and has been suggested to be an oncogene. However, we found that, in Pttg1-mutant females, the mammary epithelial cells showed increased proliferation and precocious branching morphogenesis. In accord with these phenotypic changes, progesterone receptor, cyclin D1, and Mmp2 were up-regulated whereas p21 (Cdkn1a) was down-regulated. These molecular changes provide explanation for the observed developmental defects, and suggest that Pttg1 is a tumor suppressor. Indeed, mice lacking Pttg1 developed spontaneous mammary tumors. Furthermore, in human breast tumors, PTTG1 protein levels were down-regulated and the reduction was significantly correlated with the tumor grade.

Composition, roles, and regulation of cullin-based ubiquitin e3 ligases

Due to their sessile nature, plants depend on flexible regulatory systems that allow them to adequately regulate developmental and physiological processes in context with environmental cues. The ubiquitin proteasome pathway, which targets a great number of proteins for degradation, is cellular tool that provides the necessary flexibility to accomplish this task. Ubiquitin E3 ligases provide the needed specificity to the pathway by selectively binding to particular substrates and facilitating their ubiquitylation. The largest group of E3 ligases known in plants is represented by CULLIN-REALLY INTERESTING NEW GENE (RING) E3 ligases (CRLs). In recent years, a great amount of knowledge has been generated to reveal the critical roles of these enzymes across all aspects of plant life. This review provides an overview of the different classes of CRLs in plants, their specific complex compositions, the variety of biological processes they control, and the regulatory steps that can affect their activities.

Increased CDK1 activity determines the timing of kinetochore-microtubule attachments in meiosis I

The slow increase in CDK1 activity in meiosis I acts as a timing mechanism to allow stable kinetochore–microtubule attachments only after bipolar spindle formation, thus preventing attachment errors.

Chromosome segregation during cell division depends on stable attachment of kinetochores to spindle microtubules. Mitotic spindle formation and kinetochore–microtubule (K-MT) capture typically occur within minutes of nuclear envelope breakdown. In contrast, during meiosis I in mouse oocytes, formation of the acentrosomal bipolar spindle takes 3–4 h, and stabilization of K-MT attachments is delayed an additional 3–4 h. The mechanism responsible for this delay, which likely prevents stabilization of erroneous attachments during spindle formation, is unknown. Here we show that during meiosis I, attachments are regulated by CDK1 activity, which gradually increases through prometaphase and metaphase I. Partial reduction of CDK1 activity delayed formation of stable attachments, whereas a premature increase in CDK1 activity led to precocious formation of stable attachments and eventually lagging chromosomes at anaphase I. These results indicate that the slow increase in CDK1 activity in meiosis I acts as a timing mechanism to allow stable K-MT attachments only after bipolar spindle formation, thus preventing attachment errors.

Cyclin A2 is required for sister chromatid segregation, but not separase control, in mouse oocyte meiosis

In meiosis, two specialized cell divisions allow the separation of paired chromosomes first, then of sister chromatids. Separase removes the cohesin complex holding sister chromatids together in a stepwise manner from chromosome arms in meiosis I, then from the centromere region in meiosis II. Using mouse oocytes, our study reveals that cyclin A2 promotes entry into meiosis, as well as an additional unexpected role; namely, its requirement for separase-dependent sister chromatid separation in meiosis II. Untimely cyclin A2-associated kinase activity in meiosis I leads to precocious sister separation, whereas inhibition of cyclin A2 in meiosis II prevents it. Accordingly, endogenous cyclin A is localized to kinetochores throughout meiosis II, but not in anaphase I. Additionally, we found that cyclin B1, but not cyclin A2, inhibits separase in meiosis I. These findings indicate that separase-dependent cohesin removal is differentially regulated by cyclin B1 and A2 in mammalian meiosis.

From ubiquitin-proteasomal degradation to CDK1 inactivation: requirements for the first polar body extrusion in mouse oocytes

Completion of the first meiotic division, manifested by extrusion of the first polar body (PBI), depends on proteasomal degradation of cyclin B1 and securin and the subsequent respective CDK1 inactivation and chromosome segregation. We aimed at identifying the polyubiquitin signal that mediates proteasomal action and at a better characterization of the role of CDK1 inactivation at this stage of meiosis. Microinjections of mutated ubiquitin proteins into mouse oocytes revealed that interference with lysine-11 polyubiquitin chains abrogated chromosome segregation and reduced PBI extrusion by 63% as compared to WT ubiquitin-injected controls. Inactivation of CDK1 in oocytes arrested at first metaphase by a proteasome inhibitor fully rescued PBI extrusion. However, removal of CDK1 inhibition failed to allow progression to the second metaphase, rather, inducing PBI reengulfment in 62% of the oocytes. Inhibition of either PLK1 or MEK1/2 during the first anaphase changed spindle dimensions. The PLK1 inhibitor also blocked PBI emission and prevented RhoA translocation. Our results identified lysine-11 rather than the canonic lysine-48 ubiquitin chains as the degradation signal in oocytes resuming meiosis, further disclosing that CDK1 inactivation is necessary and sufficient for PBI emission. This information significantly contributes to our understanding of faulty chromosome segregation that may lead to aneuploidy.

Separase sensor reveals dual roles for separase coordinating cohesin cleavage and cdk1 inhibition

Complete dissociation of sister chromatid cohesion and subsequent induction of poleward movement of disjoined sisters are two essential events underlying chromosome segregation; however, how cells coordinate these two processes is not well understood. Here, we developed a fluorescence-based sensor for the protease separase that mediates cohesin cleavage. We found that separase undergoes an abrupt activation shortly before anaphase onset in the vicinity of chromosomes. This activation profile of separase depends on the abilities of two of its binding proteins, securin and cyclin B1, to inhibit its protease activity and target it to chromosomes. Subsequent to its proteolytic activation, separase then binds to and inhibits a subset of cyclin B1-cdk1, which antagonizes cdk1-mediated phosphorylation on chromosomes and facilitates poleward movement of sisters in anaphase. Therefore, by consecutively acting as a protease and a cdk1 inhibitor, separase coordinates two key processes to achieve simultaneous and abrupt separation of sister chromatids.

Separases: biochemistry and function

Tight regulation of cell cycle is of critical importance for eukaryotic biology and is achieved through a combined action of a large number of highly specialized proteins. Separases are evolutionarily conserved caspase-like proteases playing a crucial role in cell cycle regulation, as they execute sister chromatid separation at metaphase to anaphase transition. In contrast to extensively studied yeast and metazoan separases, very little is known about the role of separases in plant biology. Here we describe the molecular mechanisms of separase-mediated chromatid segregation in yeast and metazoan models, discuss new emerging but less-understood functions of separases and highlight major gaps in our knowledge about plant separases.

Age-dependent susceptibility of chromosome cohesion to premature separase activation in mouse oocytes

A hypothesis to explain the maternal age-dependent increase in formation of aneuploid eggs is deterioration of chromosome cohesion. Although several lines of evidence are consistent with this hypothesis, whether cohesion is actually reduced in naturally aged oocytes has not been directly tested by any experimental perturbation. To directly target cohesion, we increased the activity of separase, the protease that cleaves the meiotic cohesin REC8, in oocytes. We show that cohesion is more susceptible to premature separase activation in old oocytes than in young oocytes, demonstrating that cohesion is significantly reduced. Furthermore, cohesion is protected by two independent mechanisms that inhibit separase, securin and an inhibitory phosphorylation of separase by CDK1; both mechanisms must be disrupted to prematurely activate separase. With the continual loss of cohesins from chromosomes that occurs throughout the natural reproductive lifespan, tight regulation of separase in oocytes may be particularly important to maintain cohesion and prevent aneuploidy.

The sister bonding of duplicated chromosomes

Sister chromatid cohesion and separation are two fundamental chromosome dynamics that are essential to equal chromosome segregation during cell proliferation. In this review, I will discuss the major steps that regulate these dynamics during mitosis, with an emphasis on vertebrate cells. The implications of these machineries outside of sister chromatid cohesion and separation are also discussed.

S-phase cyclin-dependent kinases promote sister chromatid cohesion in budding yeast

Genome stability depends on faithful chromosome segregation, which relies on maintenance of chromatid cohesion during S phase. In eukaryotes, Pds1/securin is the only known inhibitor that can prevent loss of cohesion. However, pds1Δ yeast cells and securin-null mice are viable. We sought to identify redundant mechanisms that promote cohesion within S phase in the absence of Pds1 and found that cells lacking the S-phase cyclins Clb5 and Clb6 have a cohesion defect under conditions of replication stress. Similar to the phenotype of pds1Δ cells, loss of cohesion in cells lacking Clb5 and Clb6 is dependent on Esp1. However, Pds1 phosphorylation by Cdk-cyclin is not required for cohesion. Moreover, cells lacking Clb5, Clb6, and Pds1 are inviable and lose cohesion during an unperturbed S phase, indicating that Pds1 and specific B-type cyclins promote cohesion independently of one another. Consistent with this, we find that Mcd1/Scc1 is less abundant on chromosomes in cells lacking Clb5 and Clb6 during replication stress. However, clb5Δ clb6Δ cells do accumulate Mcd1/Scc1 at centromeres upon mitotic arrest, suggesting that the cyclin-dependent mechanism is S phase specific. These data indicate that Clb5 and Clb6 promote cohesion which is then protected by Pds1 and that both mechanisms are required during replication stress.

Separase phosphosite mutation leads to genome instability and primordial germ cell depletion during oogenesis

To ensure equal chromosome segregation and the stability of the genome during cell division, Separase is strictly regulated primarily by Securin binding and inhibitory phosphorylation. By generating a mouse model that contained a mutation to the inhibitory phosphosite of Separase, we demonstrated that mice of both sexes are infertile. We showed that Separase deregulation leads to chromosome mis-segregation, genome instability, and eventually apoptosis of primordial germ cells (PGCs) during embryonic oogenesis. Although the PGCs of mutant male mice were completely depleted, a population of PGCs from mutant females survived Separase deregulation. The surviving PGCs completed oogenesis but produced deficient initial follicles. These results indicate a sexual dimorphism effect on PGCs from Separase deregulation, which may be correlated with a gender-specific discrepancy of Securin. Our results reveal that Separase phospho-regulation is critical for genome stability in oogenesis. Furthermore, we provided the first evidence of a pre-zygotic mitotic chromosome segregation error resulting from Separase deregulation, whose sex-specific differences may be a reason for the sexual dimorphism of aneuploidy in gametogenesis.

Orthologues of the anaphase-promoting complex/cyclosome coactivators Cdc20p and Cdh1p are important for mitotic progression and morphogenesis in Candida albicans

The conserved anaphase-promoting complex/cyclosome (APC/C) system mediates protein degradation during mitotic progression. Conserved coactivators Cdc20p and Cdh1p regulate the APC/C during early to late mitosis and G(1) phase. Candida albicans is an important fungal pathogen of humans, and it forms highly polarized cells when mitosis is blocked through depletion of the polo-like kinase Cdc5p or other treatments. However, the mechanisms governing mitotic progression and associated polarized growth in the pathogen are poorly understood. In order to gain insights into these processes, we characterized C. albicans orthologues of Cdc20p and Cdh1p. Cdc20p-depleted cells were blocked in early or late mitosis with elevated levels of Cdc5p and the mitotic cyclin Clb2p, suggesting that Cdc20p is essential and has some conserved functions during mitosis. However, the yeast cells formed highly polarized buds in contrast to the large doublets of S. cerevisiae cdc20 mutants, implying a distinct role in morphogenesis. In comparison, cdh1Δ/cdh1Δ cells were viable but showed enrichment of Clb2p and Cdc5p, suggesting that Cdh1p may influence mitotic exit. The cdh1Δ/cdh1Δ phenotype was pleiotropic, consisting of normal or enlarged yeast, pseudohyphae, and some elongated buds, whereas S. cerevisiae cdh1Δ yeast cells were reduced in size. Thus, C. albicans Cdh1p may have some distinct functions. Finally, absence of Cdh1p or Cdc20p had a minor or no effect on hyphal development, respectively. Overall, the results suggest that Cdc20p and Cdh1p may be APC/C activators that are important for mitosis but also morphogenesis in C. albicans. Their novel features imply additional variations in function and underscore rewiring in the emerging mitotic regulatory networks of the pathogen.

Pituitary tumor-transforming gene and its binding factor in endocrine cancer

The pituitary tumor-transforming gene (PTTG1) encodes a multifunctional protein (PTTG) that is overexpressed in numerous tumours, including pituitary, thyroid, breast and ovarian carcinomas. PTTG induces cellular transformation in vitro and tumourigenesis in vivo, and several mechanisms by which PTTG contributes to tumourigenesis have been investigated. Also known as the human securin, PTTG is involved in cell cycle regulation, controlling the segregation of sister chromatids during mitosis. This review outlines current information regarding PTTG structure, expression, regulation and function in the pathogenesis of neoplasia. Recent progress concerning the use of PTTG as a prognostic marker or therapeutic target will be considered. In addition, the PTTG binding factor (PBF), identified through its interaction with PTTG, has also been established as a proto-oncogene that is upregulated in several cancers. Current knowledge regarding PBF is outlined and its role both independently and alongside PTTG in endocrine and related cancers is discussed.

Centromere DNA decatenation depends on cohesin removal and is required for mammalian cell division

Sister chromatid cohesion is mediated by DNA catenation and proteinaceous cohesin complexes. The recent visualization of PICH (Plk1-interacting checkpoint helicase)-coated DNA threads in anaphase cells raises new questions as to the role of DNA catenation and its regulation in time and space. In the present study we show that persistent DNA catenation induced by inhibition of Topoisomerase-IIalpha can contribute to sister chromatid cohesion in the absence of cohesin complexes and that resolution of catenation is essential for abscission. Furthermore, we use an in vitro chromatid separation assay to investigate the temporal and functional relationship between cohesin removal and Topoisomerase-IIalpha-mediated decatenation. Our data suggest that centromere decatenation can occur only after separase activation and cohesin removal, providing a plausible explanation for the persistence of centromere threads after anaphase onset.

DNA-dependent cohesin cleavage by separase

Eukaryotic genomes are organized into chromosomes. In order to maintain genomic stability during cell proliferation, a series of elaborate processes is employed to ensure that chromosomes are duplicated and segregated equally into daughter cells. Sister chromatid cohesion, a tight association of duplicated sister chromatids, allows their attachment to the opposite centrosomes. Sister chromatid cohesion depends on the cohesin complex, a proteinaceous ring that entraps the chromatids together. At the metaphase-to-anaphase transition, a protease called separase is activated and completely dissolves the cohesion by cleaving SCC1, a subunit of the cohesin complex. As one of the key executors of anaphase, separase is regulated temporally and spatially by often redundant mechanisms. A recent study revealed that chromosomal DNA is required as a cofactor for the cleavage of cohesin to occur. This DNA dependence is the underlying biochemical mechanism that allows separase to selectively cleave only the chromosome-associated cohesin. We propose that the chromosomal DNA dependent cohesin cleavage by separase is a component of a regulatory pathway that cells utilize to protect the bulk of cohesin. This intact cohesin becomes immediately available in G(1) to resume its other function-regulation of gene transcription by means of chromatin insulation.

Loss of spindle assembly checkpoint-mediated inhibition of Cdc20 promotes tumorigenesis in mice

Genomic instability is a hallmark of human cancers. Spindle assembly checkpoint (SAC) is a critical cellular mechanism that prevents chromosome missegregation and therefore aneuploidy by blocking premature separation of sister chromatids. Thus, SAC, much like the DNA damage checkpoint, is essential for genome stability. In this study, we report the generation and analysis of mice carrying a Cdc20 allele in which three residues critical for the interaction with Mad2 were mutated to alanine. The mutant Cdc20 protein (AAA-Cdc20) is no longer inhibited by Mad2 in response to SAC activation, leading to the dysfunction of SAC and aneuploidy. The dysfunction could not be rescued by the additional expression of another Cdc20 inhibitor, BubR1. Furthermore, we found that Cdc20^AAA/AAA mice died at late gestation, but Cdc20^+/AAA mice were viable. Importantly, Cdc20^+/AAA mice developed spontaneous tumors at highly accelerated rates, indicating that the SAC-mediated inhibition of Cdc20 is an important tumor-suppressing mechanism.

Separase is recruited to mitotic chromosomes to dissolve sister chromatid cohesion in a DNA-dependent manner

Sister chromatid separation is triggered by the separase-catalyzed cleavage of cohesin. This process is temporally controlled by cell-cycle-dependent factors, but its biochemical mechanism and spatial regulation remain poorly understood. We report that cohesin cleavage by human separase requires DNA in a sequence-nonspecific manner. Separase binds to DNA in vitro, but its proteolytic activity, measured by its autocleavage, is not stimulated by DNA. Instead, biochemical characterizations suggest that DNA mediates cohesin cleavage by bridging the interaction between separase and cohesin. In human cells, a fraction of separase localizes to the mitotic chromosome. The importance of the chromosomal DNA in cohesin cleavage is further demonstrated by the observation that the cleavage of the chromosome-associated cohesins is sensitive to nuclease treatment. Our observations explain why chromosome-associated cohesins are specifically cleaved by separase and the soluble cohesins are left intact in anaphase.

Preimplantation mouse embryos depend on inhibitory phosphorylation of separase to prevent chromosome missegregation

Separase is a critical protease that catalyzes the cleavage of sister chromatid cohesins to allow the separation of sister chromatids in the anaphase. Its activity must be inhibited prior to the onset of the anaphase. Two inhibitory mechanisms exist in vertebrates that block the protease activity. One mechanism is through binding and inhibition by securin, and another is phosphorylation on Ser1126 (in humans [Ser1121 in mice]). These two mechanisms are largely redundant. However, phosphorylation on Ser1121 is critical for the prevention of premature sister separation in embryonic germ cells. As a result, Ser1121-to-Ala mutation leads to depletion of germ cells in development and subsequently to infertility in mice. Here, we report that the same mutation also causes embryogenesis failure between the 8- and 16-cell stages in mice. Our results indicate a critical role of separase phosphorylation in germ cell development as well as in early embryogenesis. Thus, deregulation of separase may be a significant contributor to infertility in humans.

The cohesin complex and its roles in chromosome biology

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

Inhibitory phosphorylation of separase is essential for genome stability and viability of murine embryonic germ cells

Activity of separase, a cysteine protease that cleaves sister chromatid cohesin at the onset of anaphase, is tightly regulated to ensure faithful chromosome segregation and genome stability. Two mechanisms negatively regulate separase: inhibition by securin and phosphorylation on serine 1121. To gauge the physiological significance of the inhibitory phosphorylation, we created a mouse strain in which Ser1121 was mutated to Ala (S1121A). Here we report that this S1121A point mutation causes infertility in mice. We show that germ cells in the mutants are depleted during development. We further demonstrate that S1121A causes chromosome misalignment during proliferation of the postmigratory primordial germ cells, resulting in mitotic arrest, aneuploidy, and eventual cell death. Our results indicate that inhibitory phosphorylation of separase plays a critical role in the maintenance of sister chromatid cohesion and genome stability in proliferating postmigratory primordial germ cells.

Higher eukaryotes rely on a separate cell lineage, the germline, to pass genetic information from generation to generation. To ensure faithful transmission of genetic information, cell cycle checkpoint mechanisms are engaged during mitotic and meiotic divisions of germ cells. The identity and function of these checkpoints is not well understood. In mammals, the germline is specified early in embryogenesis as primordial germ cells (PGCs) at the epiblast stage (around embryonic day 5.0 in mice). PGCs then migrate out from their birthplace and arrive at the genital ridge several days later. In the genital ridge, PGCs undergo a great expansion in number through mitosis. During this expansion, PGCs critically depend on the inhibitory phosphorylation of separase to prevent premature separation of sister chromatids and hence progeny with abnormal chromosome number. Separase is a protease which cleaves the Scc1 subunit of sister chromatid cohesin complex. Its activity must be suppressed before all sisters are aligned at the metaphase plate. Two mechanisms are known that can inhibit separase: phosphorylation and binding by securin, both of which are activated at the spindle assembly checkpoint. Although these two mechanisms are redundant in somatic cells, our results indicate that the inhibitory phosphorylation of separase is uniquely required in the germline.

A single point mutation of separase that blocks its phosphorylation has a profound and dominant effect on germ cell biology.

Phosphorylation-dependent binding of cyclin B1 to a Cdc6-like domain of human separase

Sister chromatids are held together by the ring-shaped cohesin complex, which likely entraps both DNA-double strands in its middle. This tie is resolved in anaphase when separase, a giant protease, becomes active and cleaves the kleisin subunit of cohesin. Premature activation of separase and, hence, chromosome missegregation are prevented by at least two inhibitory mechanisms. Although securin has long been appreciated as a direct inhibitor of separase, surprisingly its loss has basically no phenotype in mammals. Phosphorylation-dependent binding of Cdk1 constitutes an alternative way to inhibit vertebrate separase. Its importance is illustrated by the premature loss of cohesion when Cdk1-resistant separase is expressed in mammalian cells without or with limiting amounts of securin. Here, we demonstrate that crucial inhibitory phosphorylations occur within a region of human separase that is also shown to make direct contact with the cyclin B1 subunit of Cdk1. This region exhibits a weak homology to Saccharomyces cerevisiae Cdc6 of similar Cdk1 binding behavior, thereby establishing phosphoserine/threonine-mediated binding of partners as a conserved characteristic of B-type cyclins. In contrast to the Cdc6-like domain, the previously identified serine 1126 phosphorylation is fully dispensable for Cdk1 binding to separase fragments. This suggests that despite its in vivo relevance, it promotes complex formation indirectly, possibly by inducing a conformational change in full-length separase.

Separase, securin and Rad21 in neural cell growth

The key mitotic regulator securin is expressed at low levels in fetal brain compared with adult, and modulates the proliferation of human embryonic neuronal N-Tera2 (NT2) cells. We now examine the function and expression of securin's interacting partner separase, along with Rad21, the functional component of cohesin, which is cleaved by separase following interaction with securin. In contrast to securin, the cleaved forms of separase and Rad21 were highly expressed in human fetal cerebral cortex compared with adult. In a murine model of absent securin expression - the PTTG knock-out mouse - separase and Rad21 were over-expressed in multiple brain regions. In addition, cDNA array analysis of other key mitotic regulators additionally identified cyclin C and sestrin 2 to be induced in the brains of securin-null mice compared with wild type. Further, Rad21 mRNA expression was highly correlated with that of securin, separase, cyclin C and sestrin 2 in fetal brains. In embryonic neuronal NT2 cells, siRNA repression of separase failed to significantly alter cell turnover, whereas repression of securin expression resulted in increased levels of the activated forms of Rad21 and separase, and promoted cell proliferation. Our data suggest that the co-ordinated expression of separase, securin and Rad21 is fundamental for the developing brain.

Discordant proliferation and differentiation in pituitary tumor-transforming gene-null bone marrow stem cells

The mammalian securin, pituitary tumor-transforming gene ( Pttg), regulates sister chromatid separation during mitosis. Mice deficient in Pttg expression exhibit organ-specific hypoplasia of the testis, spleen, pituitary, and postmaturity pancreatic β-cells, pointing to a possible adult stem cell defect. Bone marrow stem cells (BMSCs) contribute to bone, cartilage, and fat tissue repair and regeneration, and multipotent adult progenitor cells (MAPCs) have broader differentiation ability. Bone marrow cells derived under MAPC conditions are involved in a spectrum of tissue repair. We therefore tested whether Pttg deletion affects stem cell proliferation and differentiation. BMSCs were isolated under MAPC conditions, although unlike MAPCs, wild-type (WT) and Pttg−/− BMSCs do not express octamer-binding transcription factor 4 and are stem cell antigen-I positive. WT and Pttg−/− cells did not differ in their ability to differentiate into adipogenic, osteogenic, or hepatocyte-like cells or in phenotypic markers. Cells underwent >100 population doublings, with no observed transforming events. Pttg-null BMSCs replicated 27% slower than WT BMSCs, and under hypoxic conditions, this difference widened. Although apoptosis was not enhanced in Pttg−/− cells, Pttg−/− BMSC senescence-associated β-galactosidase activity was elevated, consistent with enhanced p21 protein levels. Using gene array assays, DNA repair genes were shown to be upregulated in Pttg−/− BMSCs, whereas genes involved in cell cycle progression, including cyclin D1, were decreased. Separase, the protease regulated by Pttg, has been implicated in DNA damage repair and was downregulated in Pttg−/− BMSCs. Separase was constitutively phosphorylated in Pttg−/− cells, a modification likely serving as a compensatory mechanism for Pttg deletion. The results indicate that Pttg deletion reduces BMSC proliferation, renders cells more sensitive to hypoxia, and enhances senescent features, thus pointing to a role for Pttg in the maintenance and proliferation of BMSCs.

Protein Phosphatase 2A and Separase Form a Complex Regulated by Separase Autocleavage

The onset of anaphase is triggered by the activation of a site-specific protease called separase. Separase cleaves the chromosomal cohesins holding the duplicated sister chromatids together, allowing sisters to simultaneously separate and segregate to opposite ends of the cell before division. Activated separase cleaves not only cohesin, but also itself; however, the biological significance of separase self-cleavage has remained elusive. Before anaphase, separase is inhibited by at least two mechanisms. The first involves the binding of securin, whereas the second requires the phosphorylation-dependent binding of cyclin-dependent kinase 1 (Cdk1)/cyclin B1. Because securin and Cdk1/cyclin B1 interact with separase in a mutually exclusive manner, the degradation of both these inhibitors plays an important role in activating separase at anaphase. Here we identify a new separase interacting partner, a specific subtype of the heterotrimeric protein phosphatase 2A (PP2A). PP2A associates with separase through the B' (B56) regulatory subunit and does so independently of securin and cyclin B1 binding. The association of PP2A with separase requires a 55-amino acid domain closely juxtaposed to separase autocleavage sites. Strikingly, mutation of these cleavage sites increases PP2A binding, suggesting that separase cleavage disrupts the interaction of PP2A with separase. Furthermore, expression of a non-cleavable separase, but not a non-cleavable mutant that cannot bind PP2A, causes a premature loss of centromeric cohesion. Together these observations provide a new mechanistic insight into a physiological function for separase self-cleavage.

Loss of Cdc20 causes a securin-dependent metaphase arrest in two-cell mouse embryos

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase mediating targeted proteolysis through ubiquitination of protein substrates to control the progression of mitosis. The APC/C recognizes its substrates through two adapter proteins, Cdc20 and Cdh1, which contain similar C-terminal domains composed of seven WD-40 repeats believed to be involved in interacting with their substrates. During the transition from metaphase to anaphase, APC/C-Cdc20 mediates the ubiquitination of securin and cyclin B1, allowing the activation of separase and the onset of anaphase and mitotic exit. APC/C-Cdc20 and APC/C-Cdh1 have overlapping substrates. It is unclear whether they are redundant for mitosis. Using a gene-trapping approach, we have obtained mice which lack Cdc20 function. These mice show failed embryogenesis. The embryos were arrested in metaphase at the two-cell stage with high levels of cyclin B1, indicating an essential role of Cdc20 in mitosis that is not redundant with that of Cdh1. Interestingly, Cdc20 and securin double mutant embryos could not maintain the metaphase arrest, suggesting a role of securin in preventing mitotic exit.

Cyclin-B1-mediated inhibition of excess separase is required for timely chromosome disjunction

Separase, the cysteine protease that cleaves cohesin and thereby triggers chromosome disjunction, is inhibited by both securin- and phosphorylation-dependent cyclin B1 binding. Using a novel phosphorylation-specific antibody, we show that mitotic-specific phosphorylation of human separase on S1126 is required to establish, but not maintain, cyclin B1 binding. Cells expressing a non-phosphorylatable S1126A mutant maintain cohesion early in mitosis, aligning their chromosomes. Cohesion is then synchronously lost 5 minutes ahead of schedule, without degrading securin or cyclin B1. This premature chromatid disjunction requires the catalytic activity of separase, indicating that it is dependent on cohesin cleavage. Single chromatids then attempt to realign but the lack of tension results in unstable kinetochore-microtubule interactions and Aurora-B-dependent spindle checkpoint activation. Separase mutants that cannot bind cyclin B1 but are phosphorylated on S1126 phenocopy separase S1126A, indicating that cyclin B1 binding, rather than phosphorylation, is the key inhibitory event. Significantly, by overexpressing separase S1126A, we have simultaneously overridden the two known inhibitory mechanisms. First, by elevating separase levels above securin, securin-mediated inhibition is alleviated. Second, by preventing phosphorylation, cyclin-B1-mediated inhibition is also alleviated. Surprisingly, however, cohesion is maintained during the early stages of mitosis, indicating the existence of another mechanism that either inhibits separase or protects its substrate during early mitosis.

Essential CDK1-inhibitory role for separase during meiosis I in vertebrate oocytes

Separase not only triggers anaphase of meiosis I by proteolytic cleavage of cohesin on chromosome arms, but in vitro vertebrate separase also acts as a direct inhibitor of cyclin-dependent kinase 1 (Cdk1) on liberation from the inhibitory protein, securin. Blocking separase-Cdk1 complex formation by microinjection of anti-separase antibodies prevents polar-body extrusion in vertebrate oocytes. Importantly, proper meiotic maturation is rescued by chemical inhibition of Cdk1 or expression of Cdk1-binding separase fragments lacking cohesin-cleaving activity.

Mitosis: a matter of getting rid of the right protein at the right time

There are two major problems for the cell to solve in mitosis: how to ensure that each daughter cell receives an equal and identical complement of the genome, and how to prevent cell separation before chromosome segregation. Both these problems are solved by controlling when two specific proteins are destroyed: securin, an inhibitor of chromosome segregation, and cyclin B, which inhibits cell separation (cytokinesis). It has recently become clear that several other proteins are degraded at specific points in mitosis. This review (which is part of the Chromosome Segregation and Aneuploidy series) focuses on how specific proteins are selected for proteolysis at defined points in mitosis and how this contributes to the proper coordination of chromosome segregation and cytokinesis.

Securin is not required for chromosomal stability in human cells

Abnormalities of chromosome number are frequently observed in cancers. The mechanisms regulating chromosome segregation in human cells are therefore of great interest. Recently it has been reported that human cells without an hSecurin gene lose chromosomes at a high frequency. Here we show that, after hSecurin knockout through homologous recombination, chromosome losses are only a short, transient effect. After a few passages hSecurin(-/-) cells became chromosomally stable and executed mitoses normally. This was unexpected, as the securin loss resulted in a persisting reduction of the sister-separating protease separase and inefficient cleavage of the cohesin subunit Scc1. Our data demonstrate that securin is dispensable for chromosomal stability in human cells. We propose that human cells possess efficient mechanisms to compensate for the loss of genes involved in chromosome segregation.

The many phases of anaphase

Anaphase is the stage of the cell cycle in which duplicated chromosomes separate and move towards opposite poles of the cell. Although its chromosome movements have always been viewed as majestic, until recently anaphase lacked obvious landmarks of regulation. The picture has changed with numerous recent studies that have highlighted the raison d'être of anaphase. It is now known to be associated with a series of regulatory pathways that promote a switch from high to low cyclin-dependent kinase activity--an essential feature for proper mitotic exit. The balance between protein phosphorylation and protein dephosphorylation drives and coordinates diverse processes such as chromosome movement, spindle dynamics and cleavage furrow formation. This well-ordered sequence of events is central to successful mitosis.