The molecular basis of sister-chromatid cohesion

Distinct developmental function of two Caenorhabditis elegans homologs of the cohesin subunit Scc1/Rad21

Cohesin, which mediates sister chromatid cohesion, is composed of four subunits, named Scc1/Rad21, Scc3, Smc1, and Smc3 in yeast. Caenorhabditis elegans has a single homolog for each of Scc3, Smc1, and Smc3, but as many as four for Scc1/Rad21 (COH-1, SCC-1/COH-2, COH-3, and REC-8). Except for REC-8 required for meiosis, function of these C. elegans proteins remains largely unknown. Herein, we examined their possible involvement in mitosis and development. Embryos depleted of the homolog of either Scc3, or Smc1, or Smc3 by RNA interference revealed a defect in mitotic chromosome segregation but not in chromosome condensation and cytokinesis. Depletion of SCC-1/COH-2 caused similar phenotypes. SCC-1/COH-2 was present in cells destined to divide. It localized to chromosomes in a cell cycle-dependent manner. Worms depleted of COH-1 arrested at either the late embryonic or the larval stage, with no indication of mitotic dysfunction. COH-1 associated chromosomes throughout the cell cycle in all somatic cells undergoing late embryogenesis or larval development. Thus, SCC-1/COH-2 and the homologs of Scc3, Smc1, and Smc3 facilitate mitotic chromosome segregation during the development, presumably by forming a cohesin complex, whereas COH-1 seems to play a role important for development but unrelated to mitosis.

Cohesins determine the attachment manner of kinetochores to spindle microtubules at meiosis I in fission yeast

During mitosis, sister kinetochores attach to microtubules that extend to opposite spindle poles (bipolar attachment) and pull the chromatids apart at anaphase (equational segregation). A multisubunit complex called cohesin, including Rad21/Scc1, plays a crucial role in sister chromatid cohesion and equational segregation at mitosis. Meiosis I differs from mitosis in having a reductional pattern of chromosome segregation, in which sister kinetochores are attached to the same spindle (monopolar attachment). During meiosis, Rad21/Scc1 is largely replaced by its meiotic counterpart, Rec8. If Rec8 is inactivated in fission yeast, meiosis I is shifted from reductional to equational division. However, the reason rec8Delta cells undergo equational rather than random division has not been clarified; therefore, it has been unclear whether equational segregation is due to a loss of cohesin in general or to a loss of a specific requirement for Rec8. We report here that the equational segregation at meiosis I depends on substitutive Rad21, which relocates to the centromeres if Rec8 is absent. Moreover, we demonstrate that even if sufficient amounts of Rad21 are transferred to the centromeres at meiosis I, thereby establishing cohesion at the centromeres, rec8Delta cells never recover monopolar attachment but instead secure bipolar attachment. Thus, Rec8 and Rad21 define monopolar and bipolar attachment, respectively, at meiosis I. We conclude that cohesin is a crucial determinant of the attachment manner of kinetochores to the spindle microtubules at meiosis I in fission yeast.

Chromosome-microtubule interactions during mitosis

Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.

The cohesin SMC3 is a target the for beta-catenin/TCF4 transactivation pathway

The structural maintenance of chromosome protein SMC3 is a component of the cohesin complex that mediates sister chromatid cohesion and segregation in prokaryotes and eukaryotes. It is also present extracellularly in the form of a chondroitin sulfate proteoglycan known as bamacan. We have found previously that SMC3 expression is elevated in a large fraction of human colon carcinomas. The additional finding that the protein is significantly increased in the intestinal polyps of ApcMin/+ mice has led us to hypothesize that SMC3 expression is linked to activation of the APC/beta-catenin/TCF4 pathway. The immunohistochemical analysis of colon adenocarcinomas from clinical specimens revealed that beta-catenin and SMC3 antigens co-localize with maximal stain intensity within the transformed areas. Cloning and sequencing of 1578 bp of the human SMC3 promoter unveiled the presence of seven putative consensus sequences for beta-catenin/TCF4 binding, two of which are conserved in the mouse Smc3 promoter. Transient transfection experiments in HCT116 and SW480 human colon carcinoma cells using deletion and mutated promoter constructs in luciferase reporter vectors confirmed that the putative sites, the first located at -48 bp and the second located at -701 bp, are susceptible to beta-catenin/TCF4 transactivation. Co-transfection with a beta-catenin expression vector enhanced the promoter activity whereas E-cadherin had the opposite effect. Binding of beta-catenin/TCF4 complexes from SW480 nuclear extracts to these sequences was confirmed by electrophoretic shift and supershift mobility assays. Altogether these results are consistent with the idea that the beta-catenin/TCF4 transactivation pathway contributes to SMC3 overexpression in intestinal tumorigenesis.

Distinct cohesin complexes organize meiotic chromosome domains

Meiotic cohesin complexes at centromeres behave differently from those along chromosome arms, but the basis for these differences has remained elusive. The fission yeast cohesin molecule Rec8 largely replaces its mitotic counterpart, Rad21/Scc1, along the entire chromosome during meiosis. Here we show that Rec8 complexes along chromosome arms contain Rec11, whereas those in the vicinity of centromeres have a different partner subunit, Psc3. The arm associated Rec8-Rec11 complexes are critical for meiotic recombination. The Rec8-Psc3 complexes comprise two different types of assemblies. First, pericentromeric Rec8-Psc3 complexes depend on histone methylation-directed heterochromatin for their localization and are required for cohesion during meiosis II. Second, central core Rec8-Psc3 complexes form independently of heterochromatin and are presumably required for establishing monopolar attachment at meiosis I. These findings define distinct modes of assembly and functions for cohesin complexes at different regions along chromosomes.

Division of the nucleolus and its release of CDC14 during anaphase of meiosis I depends on separase, SPO12, and SLK19

Disjunction of maternal and paternal centromeres during meiosis I requires crossing over between homologous chromatids, which creates chiasmata that hold homologs together. It also depends on a mechanism ensuring that maternal and paternal sister kinetochore pairs attach to oppositely oriented microtubules. Proteolytic cleavage of cohesin's Rec8 subunit by separase destroys cohesion between sister chromatid arms at anaphase I and thereby resolves chiasmata. The Spo12 and Slk19 proteins have been implicated in regulating meiosis I kinetochore orientation and/or in preventing cleavage of Rec8 at centromeres. We show here that the role of these proteins is instead to promote nucleolar segregation, including release of the Cdc14 phosphatase required for Cdk1 inactivation and disassembly of the anaphase I spindle. Separase is also required but surprisingly not its protease activity. It has two mechanistically different roles during meiosis I. Loss of the protease-independent function alone results in a second meiotic division occurring on anaphase I spindles in spo12delta and slk19delta mutants.

Requirement of Cks2 for the first metaphase/anaphase transition of mammalian meiosis

We generated mice lacking Cks2, one of two mammalian homologs of the yeast Cdk1-binding proteins, Suc1 and Cks1, and found them to be viable but sterile in both sexes. Sterility is due to failure of both male and female germ cells to progress past the first meiotic metaphase. The chromosomal events up through the end of prophase I are normal in both CKS2-/- males and females, suggesting that the phenotype is due directly to failure to enter anaphase and not a consequence of a checkpoint-mediated metaphase I arrest.

Role of Polo-like kinase CDC5 in programming meiosis I chromosome segregation

Meiosis is a specialized cell division in which two chromosome segregation phases follow a single DNA replication phase. The budding yeast Polo-like kinase Cdc5 was found to be instrumental in establishing the meiosis I chromosome segregation program. Cdc5 was required to phosphorylate and remove meiotic cohesin from chromosomes. Furthermore, in the absence of CDC5 kinetochores were bioriented during meiosis I, and Mam1, a protein essential for coorientation, failed to associate with kinetochores. Thus, sister-kinetochore coorientation and chromosome segregation during meiosis I are coupled through their dependence on CDC5.

HP1 complexes and heterochromatin assembly

Since its discovery almost two decades ago, heterochromatin protein 1 (HP1) has emerged as a major player in the transcriptional regulation of both heterochromatic and euchromatic genes as well as the mechanics of chromosome segregation and the functional and structural organization of the interphase nucleus. Recent years have brought the identification of a myriad of HP1-interacting proteins. Each of these is discussed in relationship to its role in heterochromatin assembly and HP1 function. The breadth of functions represented by HP1-interacting proteins testifies to its pivotal role in the daily operations of the nucleus.

Kinetochore recruitment of two nucleolar proteins is required for homolog segregation in meiosis I

Halving of the chromosome number during meiosis I depends on the segregation of maternal and paternal centromeres. This process relies on the attachment of sister centromeres to microtubules emanating from the same spindle pole. We describe here the identification of a protein complex, Csm1/Lrs4, that is essential for monoorientation of sister kinetochores in Saccharomyces cerevisiae. Both proteins are present in vegetative cells, where they reside in the nucleolus. Only shortly before meiosis I do they leave the nucleolus and form a "monopolin" complex with the meiosis-specific Mam1 protein, which binds to kinetochores. Surprisingly, Csm1's homolog in Schizosaccharomyces pombe, Pcs1, is essential for accurate chromosome segregation during mitosis and meiosis II. Csm1 and Pcs1 might clamp together microtubule binding sites on the same (Pcs1) or sister (Csm1) kinetochores.

The Synaptonemal complex component C(2)M regulates meiotic crossing over in Drosophila

BACKGROUND

The synaptonemal complex (SC) is a proteinaceous structure that forms between homologously paired meiotic chromosomes. Previous studies have suggested that the SC is required for meiotic crossing over in Drosophila. However, only one component of this structure, C(3)G, has been identified in Drosophila.

RESULTS

Mutations in c(2)M cause a reduced frequency of meiotic crossing over due, in part, to how recombination events are resolved. Cytological evidence suggests that C(2)M is a component of the SC and is required for the assembly of C(3)G (a putative transverse filament of the SC) along the chromosomes. Additionally, C(2)M localizes along the chromosomes in the absence of C(3)G. Despite having a defect in C(3)G localization, c(2)M mutants unexpectedly affect crossing over less severely than a c(3)G mutant. There is virtually no crossing over in a c(3)G mutant, but c(2)M or c(2)M; c(3)G double mutants produce a substantial number of crossovers. The appearance of C(3)G-independent crossovers in c(2)M mutants suggests that C(2)M prevents recombination in the absence of complete SC formation.

CONCLUSIONS

We have identified a new Drosophila SC component, C(2)M, that promotes the formation of crossovers. Furthermore, the appearance of C(3)G-independent crossovers in c(2)M mutants suggests a novel role in preventing recombination in the absence of complete SC.

Depletion of Drad21/Scc1 in Drosophila cells leads to instability of the cohesin complex and disruption of mitotic progression

BACKGROUND

The coordination of cell cycle events is necessary to ensure the proper duplication and dissemination of the genome. In this study, we examine the consequences of depleting Drad21 and SA, two non-SMC subunits of the cohesin complex, by dsRNA-mediated interference in Drosophila cultured cells.

RESULTS

We have shown that a bona fide cohesin complex exists in Drosophila embryos. Strikingly, the Drad21/Scc1 and SA/Scc3 non-SMC subunits associate more intimately with one another than they do with the SMCs. We have observed defects in mitotic progression in cells from which Drad21 has been depleted: cells delay in prometaphase with normally condensed, but prematurely separated, sister chromatids and with abnormal spindle morphology. Much milder defects are observed when SA is depleted from cells. The dynamics of the chromosome passenger protein, INCENP, are affected after Drad21 depletion. We have also made the surprising observation that SA is unstable in the absence of Drad21; however, we have shown that the converse is not true. Interference with Drad21 in living Drosophila embryos also has deleterious effects on mitotic progression.

CONCLUSIONS

We conclude that Drad21, as a member of a cohesin complex, is required in Drosophila cultured cells and embryos for proper mitotic progression. The protein is required in cultured cells for chromosome cohesion, spindle morphology, dynamics of a chromosome passenger protein, and stability of the cohesin complex, but apparently not for normal chromosome condensation. The observation of SA instability in the absence of Drad21 implies that the expression of cohesin subunits and assembly of the cohesin complex will be tightly regulated.

Dissecting plant meiosis using Arabidopsis thaliana mutants

Meiosis is a key stage in the life cycle of all sexually reproducing eukaryotes. In plants, specialized reproductive cells differentiate from somatic tissue. These cells then undergo a single round of DNA replication followed by two rounds of chromosome division to produce haploid cells that then undergo further rounds of mitotic division to produce the pollen grain and embryo sac. A detailed cytological description of meiosis has been built up over many years, based on studies in a wide range of plants. Until recently, comparable molecular studies have proved too challenging, however, a number of groups are beginning to use Arabidopsis thaliana to overcome this problem. A range of meiotic mutants affecting key stages in meiosis have been identified using a combination of screening for plants exhibiting reduced fertility and, more recently, using a reverse genetics approach. These are now providing the means to identify and characterize the activity of key meiotic genes in flowering plants.

SMC protein complexes and the maintenance of chromosome integrity

Structural maintenance of chromosomes (SMC) family proteins have attracted much attention for their unique protein structure and critical roles in mitotic chromosome organization. Elegant genetic and biochemical studies in yeast and Xenopus identified two different SMC heterodimers in two conserved multiprotein complexes termed 'condensin' and 'cohesin'. These complexes are required for mitotic chromosome condensation and sister chromatid cohesion, respectively, both of which are prerequisite to accurate segregation of chromosomes. Although structurally similar, the SMC proteins in condensin and cohesin appear to have distinct functions, whose specificity and cell cycle regulation are critically determined by their interactions with unique sets of associated proteins. Recent studies of subcellular localization of SMC proteins and SMC-containing complexes, identification of their interactions with other cellular factors, and discovery of new SMC family members have uncovered unexpected roles for SMC proteins and SMC-containing complexes in different aspects of genome functions and chromosome organization beyond mitosis, all of which are critical for the maintenance of chromosome integrity.

Cohesin release is required for sister chromatid resolution, but not for condensin-mediated compaction, at the onset of mitosis

The establishment of metaphase chromosomes is an essential prerequisite of sister chromatid separation in anaphase. It involves the coordinated action of cohesin and condensin, protein complexes that mediate cohesion and condensation, respectively. In metazoans, most cohesin dissociates from chromatin at prophase, coincident with association of condensin. Whether loosening of cohesion at the onset of mitosis facilitates the compaction process, resolution of the sister chromatids, or both, remains unknown. We have found that the prophase release of cohesin is completely blocked when two mitotic kinases, aurora B and polo-like kinase (Plx1), are simultaneously depleted from Xenopus egg extracts. Condensin loading onto chromatin is not affected under this condition, and rod-shaped chromosomes are produced that show an apparently normal level of compaction. However, the resolution of sister chromatids within these chromosomes is severely compromised. This is not because of inhibition of topoisomerase II activity that is also required for the resolution process. We propose that aurora B and Plx1 cooperate to destabilize the sister chromatid linkage through distinct mechanisms that may involve phosphorylation of histone H3 and cohesin, respectively. More importantly, our results strongly suggest that cohesin release at the onset of mitosis is essential for sister chromatid resolution but not for condensin-mediated compaction.

The Coprinus cinereus adherin Rad9 functions in Mre11-dependent DNA repair, meiotic sister-chromatid cohesion, and meiotic homolog pairing

Mitotic sister-chromatid cohesion (SCC) is known to depend in part on conserved proteins called adherins, which although necessary for SCC are not themselves localized between sister chromatids. We have examined mitotic DNA-repair and meiotic chromosome behavior in the Coprinus cinereus adherin mutant rad9-1. Genetic pathway analysis established that Rad9 functions in an Mre11-dependent pathway of DNA repair. Using fluorescence in situ hybridization, we found that the rad9-1 mutant is defective in the establishment of meiotic homolog pairing at both interstitial and subtelomeric sites but in the maintenance of pairing at only interstitial loci. To determine the role of Rad9 in meiotic SCC, we hybridized nuclear spreads simultaneously with a homolog-specific probe and a probe that recognizes both members of a homologous pair. We found that Rad9 is required for wild-type levels of meiotic SCC, and that nuclei showing loss of cohesion were twice as likely also to fail at homolog pairing. To ask whether the contribution of Rad9 to homolog pairing is solely in the establishment of SCC, we examined a rad9-1;msh5-22 double mutant, in which premeiotic DNA replication is inhibited. The msh5-22 mutation partially suppressed the deleterious effects of the rad9-1 mutation on homolog pairing; however, pairing in the double mutant still was significantly lower than in the msh5-22 single mutant control. Because the role of Rad9 in homolog pairing is not obviated by the absence of a sister chromatid, we conclude that adherins have one or more early meiotic functions distinct from the establishment of cohesion.

Meiotic cohesion requires accumulation of ORD on chromosomes before condensation

Cohesion between sister chromatids is a prerequisite for accurate chromosome segregation during mitosis and meiosis. To allow chromosome condensation during prophase, the connections that hold sister chromatids together must be maintained but still permit extensive chromatin compaction. In Drosophila, null mutations in the orientation disruptor (ord) gene lead to meiotic nondisjunction in males and females because cohesion is absent by the time that sister kinetochores make stable microtubule attachments. We provide evidence that ORD is concentrated within the extrachromosomal domains of the nuclei of Drosophila primary spermatocytes during early G2, but accumulates on the meiotic chromosomes by mid to late G2. Moreover, using fluorescence in situ hybridization to monitor cohesion directly, we show that cohesion defects first become detectable in ord(null) spermatocytes shortly after the time when wild-type ORD associates with the chromosomes. After condensation, ORD remains bound at the centromeres of wild-type spermatocytes and persists there until centromeric cohesion is released during anaphase II. Our results suggest that association of ORD with meiotic chromosomes during mid to late G2 is required to maintain sister-chromatid cohesion during prophase condensation and that retention of ORD at the centromeres after condensation ensures the maintenance of centromeric cohesion until anaphase II.

Targeted stimulation of meiotic recombination

Meiotic recombination in Saccharomyces cerevisiae is initiated by programmed DNA double-strand breaks (DSBs), a process that requires the Spo11 protein. DSBs usually occur in intergenic regions that display open chromatin accessibility, but other determinants that control their frequencies and non-random chromosomal distribution remain obscure. We report that a Spo11 construct bearing the Gal4 DNA binding domain not only rescues spo11Delta spore inviability and catalyzes DSB formation at natural sites but also strongly stimulates DSB formation near Gal4 binding sites. At GAL2, a naturally DSB-cold locus, Gal4BD-Spo11 creates a recombinational hotspot that depends on all the other DSB gene functions, showing that the targeting of Spo11 to a specific site is sufficient to stimulate meiotic recombination that is under normal physiological control.

mcl1+, the Schizosaccharomyces pombe homologue of CTF4, is important for chromosome replication, cohesion, and segregation

The fission yeast minichromosome loss mutant mcl1-1 was identified in a screen for mutants defective in chromosome segregation. Missegregation of the chromosomes in mcl1-1 mutant cells results from decreased centromeric cohesion between sister chromatids. mcl1+ encodes a beta-transducin-like protein with similarity to a family of eukaryotic proteins that includes Ctf4p from Saccharomyces cerevisiae, sepB from Aspergillus nidulans, and AND-1 from humans. The previously identified fungal members of this protein family also have chromosome segregation defects, but they primarily affect DNA metabolism. Chromosomes from mcl1-1 cells were heterogeneous in size or structure on pulsed-field electrophoresis gels and had elongated heterogeneous telomeres. mcl1-1 was lethal in combination with the DNA checkpoint mutations rad3delta and rad26delta, demonstrating that loss of Mcl1p function leads to DNA damage. mcl1-1 showed an acute sensitivity to DNA damage that affects S-phase progression. It interacts genetically with replication components and causes an S-phase delay when overexpressed. We propose that Mcl1p, like Ctf4p, has a role in regulating DNA replication complexes.

Meiosis: how male flies do meiosis

In meiosis in male fruitflies, chromosome pairing events do not facilitate genetic exchange, but rather create bivalents that can be sequestered to discrete pockets of the prophase nucleus. This assignment of homologs to a common pocket likely facilitates the orientation of homologous centromeres to opposite poles.

Xkid chromokinesin is required for the meiosis I to meiosis II transition in Xenopus laevis oocytes

Xkid chromokinesin is required for chromosome alignment on the metaphase plate of spindles formed in Xenopus laevis egg extracts. We have investigated the role of Xkid in Xenopus oocyte meiotic maturation, a progesterone-triggered process that reinitiates the meiotic cell cycle in oocytes arrested at the G2/M border of meiosis I. Here we show that Xkid starts to accumulate at the time of germinal vesicle breakdown and reaches its largest quantities at metaphase II in oocytes treated with progesterone. Both germinal vesicle breakdown and spindle assembly at meiosis I can occur normally in the absence of Xkid. But Xkid-depleted oocytes cannot reactivate Cdc2/cyclin B after meiosis I and, instead of proceeding to meiosis II, they enter an interphase-like state and undergo DNA replication. Expression of a Xkid mutant that lacks the DNA-binding domain allows Xkid-depleted oocytes to complete meiotic maturation. Our results show that Xkid has a role in the meiotic cell cycle that is independent from its role in metaphase chromosome alignment.

The dynamics of homologous chromosome pairing during male Drosophila meiosis

BACKGROUND

Meiotic pairing is essential for the proper orientation of chromosomes at the metaphase plate and their subsequent disjunction during anaphase I. In male Drosophila melanogaster, meiosis occurs in the absence of recombination or a recognizable synaptonemal complex (SC). Due to limitations in available cytological techniques, the early stages of homologous chromosome pairing in male Drosophila have not been observed, and the mechanisms involved are poorly understood.

RESULTS

Chromosome tagging with GFP-Lac repressor protein allowed us to track, for the first time, the behavior of meiotic chromosomes at high resolution, live, at all stages of male Drosophila meiosis. Homologous chromosomes pair throughout the euchromatic regions in spermatogonia and during the early phases of spermatocyte development. Extensive separation of homologs and sister chromatids along the chromosome arms occurs in mid-G2, several hours before the first meiotic division, and before the G2/M transition. Centromeres, on the other hand, show complex association patterns, with specific homolog pairing taking place in mid-G2. These changes in chromosome pairing parallel changes in large-scale chromosome organization.

CONCLUSIONS

Our results suggest that widespread interactions along the euchromatin are required for the initiation, but not the maintenance, of meiotic pairing of autosomes in male Drosophila. We propose that heterochromatic associations, or chromatid entanglement, may be responsible for the maintenance of homolog association during late G2. Our data also suggest that the formation of chromosome territories in the spermatocyte nucleus may play an active role in ensuring the specificity of meiotic pairing in late prophase by disrupting interactions between nonhomologous chromosomes.

Manipulation of the oocyte: possible damage to the spindle apparatus

Oocytes are structured, polarized cells. For high developmental potential, it is essential that the distribution of organelles and molecules, and the function of meiotic spindles remain intact during handling of oocytes in assisted reproduction. Spindles are dynamic cell organelles. Spindle formation depends on activity of motor proteins, energy supply and temperature. Disturbances in spindle function may predispose oocytes to aneuploidy or maturation arrest. Thus, perturbation of the cytoskeletal integrity of oocytes may critically influence the fate of the embryo. Recently, enhanced polarizing microscopy has been developed for non-invasive analysis of spindle morphology in living mammalian oocytes. Chemically induced dynamic alterations have been characterized in the spindle in individual mouse oocytes and it has been shown that spindle aberrations are predictive of risks for non-disjunction. Spindle imaging identified adverse, irreversible effects of handling in living human oocytes (for instance, the extreme susceptibility of human oocytes to cooling). Also, oocyte immaturity may be detected. Selection of metaphase II oocytes and an injection site for intracytoplasmic sperm injection (ICSI) that avoids spindle damage may increase the yield of euploid embryos. The detection of genetic, environmentally induced, or treatment-related defects in oocyte maturation by non-invasive spindle imaging can improve quality control and assist in the selection of morphologically normal oocytes for assisted reproduction.

Spo13 protects meiotic cohesin at centromeres in meiosis I

In the absence of Spo13, budding yeast cells complete a single meiotic division during which sister chromatids often separate. We investigated the function of Spo13 by following chromosomes tagged with green fluorescent protein. The occurrence of a single division in spo13Delta homozygous diploids depends on the spindle checkpoint. Eliminating the checkpoint accelerates meiosis I in spo13Delta cells and allows them to undergo two divisions in which sister chromatids often separate in meiosis I and segregate randomly in meiosis II. Overexpression of Spo13 and the meiosis-specific cohesin Rec8 in mitotic cells prevents separation of sister chromatids despite destruction of Pds1 and activation of Esp1. This phenotype depends on the combined overexpression of both proteins and mimics one aspect of meiosis I chromosome behavior. Overexpressing the mitotic cohesin, Scc1/Mcd1, does not substitute for Rec8, suggesting that the combined actions of Spo13 and Rec8 are important for preventing sister centromere separation in meiosis I.

Spo13 regulates cohesin cleavage

A key aspect of meiotic chromosome segregation is that cohesin, the protein complex that holds sister chromatids together, dissociates from chromosome arms during meiosis I and from centromeric regions during meiosis II. The budding yeast protein Spo13 plays a key role in preventing centromeric cohesin from being lost during meiosis I. We have determined the molecular basis for the metaphase arrest obtained when SPO13 is overexpressed during the mitotic cell cycle. Overexpression of SPO13 inhibits anaphase onset by at least two mechanisms. First, Spo13 causes a transient delay in degradation of the anaphase inhibitor Pds1. Second, Spo13 inhibits cleavage of the cohesin subunit Scc1/Mcd1 or its meiosis-specific homolog, Rec8, by the separase Esp1. The finding that Spo13 did not prevent cleavage of another Esp1 substrate, Slk19, suggests that overexpression of SPO13 is sufficient to prevent cohesin cleavage by protecting specific substrates from separase activity.

STAG2 and Rad21 mammalian mitotic cohesins are implicated in meiosis

STAG/SA proteins are specific cohesin complex subunits that maintain sister chromatid cohesion in mitosis and meiosis. Two members of this family, STAG1/SA1 and STAG2/SA2,double dagger are classified as mitotic cohesins, as they are found in human somatic cells and in Xenopus laevis as components of the cohesin(SA1) and cohesin(SA2) complexes, in which the shared subunits are Rad21/SCC1, SMC1 and SMC3 proteins. A recently reported third family member, STAG3, is germinal cell-specific and is a subunit of the meiotic cohesin complex. To date, the meiosis-specific cohesin complex has been considered to be responsible for sister chromatid cohesion during meiosis. We studied replacement of the mitotic by the meiotic cohesin complex during mouse germinal cell maturation, and we show that mammalian STAG2 and Rad21 are also involved in several meiosis stages. Immunofluorescence results suggest that a cohesin complex containing Rad21 and STAG2 cooperates with a STAG3-specific complex to maintain sister chromatid cohesion during the diplotene stage of meiosis.

Only connect: linking meiotic DNA replication to chromosome dynamics

The process of meiosis reduces a diploid cell to four haploid gametes and is accompanied by extensive recombination. Thus, chromosome dynamics in meiosis are significantly different than in mitotic cells. This review analyzes unique features of meiotic DNA replication and describes how it affects subsequent recombination and chromosome segregation.