Functional characterization of kinetochore protein,Ctf19 in meiosis I: an implication of differential impact of Ctf19 on the assembly of mitotic and meiotic kinetochores in Saccharomyces cerevisiae

Meiosis is a specialized cell division process through which chromosome numbers are reduced by half for the generation of gametes. Kinetochore, a multiprotein complex that connects centromeres to microtubules, plays essential role in chromosome segregation. Ctf19 is the key central kinetochore protein that recruits all the other non‐essential proteins of the Ctf19 complex in budding yeast. Earlier studies have shown the role of Ctf19 complex in enrichment of cohesin around the centromeres both during mitosis and meiosis, leading to sister chromatid cohesion and meiosis II disjunction. Here we show that Ctf19 is also essential for the proper execution of the meiosis I specific unique events, such as non‐homologous centromere coupling, homologue pairing, chiasmata resolution and proper orientation of homologues and sister chromatids with respect to the spindle poles. Additionally, this investigation reveals that proper kinetochore function is required for faithful chromosome condensation in meiosis. Finally, this study suggests that absence of Ctf19 affects the integrity of meiotic kinetochore differently than that of the mitotic kinetochore. Consequently, absence of Ctf19 leads to gross chromosome missegregation during meiosis as compared with mitosis. Hence, this study reports for the first time the differential impact of a non‐essential kinetochore protein on the mitotic and meiotic kinetochore ensembles and hence chromosome segregation.     

Dissociation of the Nuf2-Ndc80 complex releases centromeres from the spindle-pole body during meiotic prophase in fission yeast

In the fission yeast Schizosaccharomyces pombe, centromeres remain clustered at the spindle-pole body (SPB) during mitotic interphase. In contrast, during meiotic prophase centromeres dissociate from the SPB. Here we examined the behavior of centromere proteins in living meiotic cells of S. pombe. We show that the Nuf2-Ndc80 complex proteins (Nuf2, Ndc80, Spc24, and Spc25) disappear from the centromere in meiotic prophase when the centromeres are separated from the SPB. The centromere protein Mis12 also dissociates during meiotic prophase; however, Mis6 remains throughout meiosis. When cells are induced to meiosis by inactivation of Pat1 kinase (a key negative regulator of meiosis), centromeres remain associated with the SPB during meiotic prophase. However, inactivation of Nuf2 by a mutation causes the release of centromeres from the SPB in pat1 mutant cells, suggesting that the Nuf2-Ndc80 complex connects centromeres to the SPB. We further found that removal of the Nuf2-Ndc80 complex from the centromere and centromere-SPB dissociation are caused by mating pheromone signaling. Because pat1 mutant cells also show aberrant chromosome segregation in the first meiotic division and this aberration is compensated by mating pheromone signaling, dissociation of the Nuf2-Ndc80 complex may be associated with remodeling of the kinetochore for meiotic chromosome segregation.     

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.     

Separase Is Required for Homolog and Sister Disjunction during Drosophila melanogaster Male Meiosis,but Not for Biorientation of Sister Centromeres

Spatially controlled release of sister chromatid cohesion during progression through the meiotic divisions is of paramount importance for error-free chromosome segregation during meiosis. Cohesion is mediated by the cohesin protein complex and cleavage of one of its subunits by the endoprotease separase removes cohesin first from chromosome arms during exit from meiosis I and later from the pericentromeric region during exit from meiosis II. At the onset of the meiotic divisions, cohesin has also been proposed to be present within the centromeric region for the unification of sister centromeres into a single functional entity, allowing bipolar orientation of paired homologs within the meiosis I spindle. Separase-mediated removal of centromeric cohesin during exit from meiosis I might explain sister centromere individualization which is essential for subsequent biorientation of sister centromeres during meiosis II. To characterize a potential involvement of separase in sister centromere individualization before meiosis II, we have studied meiosis in Drosophila melanogaster males where homologs are not paired in the canonical manner. Meiosis does not include meiotic recombination and synaptonemal complex formation in these males. Instead, an alternative homolog conjunction system keeps homologous chromosomes in pairs. Using independent strategies for spermatocyte-specific depletion of separase complex subunits in combination with time-lapse imaging, we demonstrate that separase is required for the inactivation of this alternative conjunction at anaphase I onset. Mutations that abolish alternative homolog conjunction therefore result in random segregation of univalents during meiosis I also after separase depletion. Interestingly, these univalents become bioriented during meiosis II, suggesting that sister centromere individualization before meiosis II does not require separase.     

Structure of centromere chromatin: from nucleosome to chromosomal architecture

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

The Birth of the Centromere

The centromere is the region of the eukaryotic chromosome that determines kinetochore formation and sister chromatid cohesion. Centromeres interact with spindle microtubules to ensure chromatid segregation during mitosis and homologous chromosome segregation during meiosis I. In recent years, the overall organization of centromeres in several eukaryotic species has been described, yet the mechanisms of centromere definition remain elusive. Understanding the evolutionary origin of the centromere may well elucidate aspects of its function. With such intention, we hypothesize that centromeres were derived from telomeres during the evolution of the eukaryotic chromosome. We propose that the proto-eukaryotic cell could not have evolved a nucleus without concurrently evolving a new tubulin-based cytoskeleton, the microtubules, and a specific chromosomal region that enabled the chromosome-microtubule interaction, the centromere. The repetitive nature of the subtelomeric regions that gave rise to the centromeres forced the concerted evolution of the centromeres. Although this implies the absence of a conserved primary sequence, a conserved centromere-specific structural motif could still exist and determine where in the chromosome the centromere is to be formed.To support the “centromeres-from-telomeres” hypothesis, we discuss several situations, in meiosis and mitosis, where telomeric regions took over centromeric roles. The recently discovered phenomenon of centromere repositioning is also discussed because it has revealed new insights into how neocentromeres evolve.     

The Synaptonemal Complex Protein Zip1 Promotes Bi-Orientation of Centromeres at Meiosis I

In meiosis I, homologous chromosomes become paired and then separate from one another to opposite poles of the spindle. In humans, errors in this process are a leading cause of birth defects, mental retardation, and infertility. In most organisms, crossing-over, or exchange, between the homologous partners provides a link that promotes their proper, bipolar, attachment to the spindle. Attachment of both partners to the same pole can sometimes be corrected during a delay that is triggered by the spindle checkpoint. Studies of non-exchange chromosomes have shown that centromere pairing serves as an alternative to exchange by orienting the centromeres for proper microtubule attachment. Here, we demonstrate a new role for the synaptonemal complex protein Zip1. Zip1 localizes to the centromeres of non-exchange chromosomes in pachytene and mediates centromere pairing and segregation of the partners at meiosis I. Exchange chromosomes were also found to experience Zip1-dependent pairing at their centromeres. Zip1 was found to persist at centromeres, after synaptonemal complex disassembly, remaining there until microtubule attachment. Disruption of this centromere pairing, in spindle checkpoint mutants, randomized the segregation of exchange chromosomes. These results demonstrate that Zip1-mediated pairing of exchange chromosome centromeres promotes an initial, bipolar attachment of microtubules. This activity of Zip1 lessens the load on the spindle checkpoint, greatly reducing the chance that the cell will exit the checkpoint delay with an improperly oriented chromosome pair. Thus exchange, the spindle checkpoint, and centromere pairing are complementary mechanisms that ensure the proper segregation of homologous partners at meiosis I.     

A new role for the transcriptional corepressor SIN3; regulation of centromeres

Centromeres play a vital role in maintaining the genomic stability of eukaryotes by coordinating the equal distribution of chromosomes to daughter cells during mitosis and meiosis. Fission yeast (S. pombe) centromeres consist of a 4-9 kb central core region and 30-100 kb of flanking inner (imr/B) and outer (otr/K) repeats. These sequences direct a laminar kinetochore structure similar to that of human centromeres. Centromeric heterochromatin is generally underacetylated. We have previously shown that inhibition of histone deacetylases (HDACs) caused hyperacetylation of centromeres and defective chromosome segregation. SIN3 is a HDAC corepressor that has the ability to mediate HDAC targeting in the repression of promoters. In this study, we have characterized S. pombe sin three corepressors (Pst1p and Pst2p) to investigate whether SIN3-HDAC is required in the regulation of centromeres. We show that only pst1-1 and not pst2Delta cells displayed anaphase defects and thiabendazole sensitivity. pst1-1 cells showed reduced centromeric silencing, increased histone acetylation in centromeric chromatin, and defective centromeric sister chromatid cohesion. The HDAC Clr6p and Pst1p coimmunoprecipitated, and Pst1p colocalized with centromeres, particularly in binucleate cells. These data are consistent with a model in which Pst1p-Clr6p temporally associate with centromeres to carry out the initial deacetylation necessary for subsequent steps in heterochromatin formation.     

Maintenance of cohesin at centromeres after meiosis I in budding yeast requires a kinetochore-associated protein related to MEI-S332

BACKGROUND: The halving of chromosome number that occurs during meiosis depends on three factors. First, homologs must pair and recombine. Second, sister centromeres must attach to microtubules that emanate from the same spindle pole, which ensures that homologous maternal and paternal pairs can be pulled in opposite directions (called homolog biorientation). Third, cohesion between sister centromeres must persist after the first meiotic division to enable their biorientation at the second. RESULTS: A screen performed in fission yeast to identify meiotic chromosome missegregation mutants has identified a conserved protein called Sgo1 that is required to maintain sister chromatid cohesion after the first meiotic division. We describe here an orthologous protein in the budding yeast S. cerevisiae (Sc), which has not only meiotic but also mitotic chromosome segregation functions. Deletion of Sc SGO1 not only causes frequent homolog nondisjunction at meiosis I but also random segregation of sister centromeres at meiosis II. Meiotic cohesion fails to persist at centromeres after the first meiotic division, and sister centromeres frequently separate precociously. Sgo1 is a kinetochore-associated protein whose abundance declines at anaphase I but, nevertheless, persists on chromatin until anaphase II. CONCLUSIONS: The finding that Sgo1 is localized to the centromere at the time of the first division suggests that it may play a direct role in preventing the removal of centromeric cohesin. The similarity in sequence composition, chromosomal location, and mutant phenotypes of sgo1 mutants in two distant yeasts with that of MEI-S332 in Drosophila suggests that these proteins define an orthologous family conserved in most eukaryotic lineages.     

Slk19p is necessary to prevent separation of sister chromatids in meiosis I

BACKGROUND: A fundamental difference between meiotic and mitotic chromosome segregation is that in meiosis I, sister chromatids remain joined, moving as a unit to one pole of the spindle rather than separating as they do in mitosis. It has long been known that the sustained linkage of sister chromatids through meiotic anaphase I is accomplished by association of the chromatids at the centromere region. The localization of the cohesin Rec8p to the centromeres is essential for maintenance of sister chromatid cohesion through meiosis I, but the molecular basis for the regulation of Rec8p and sister kinetochores in meiosis remains a mystery. RESULTS: We show that the SLK19 gene product from Saccharomyces cerevisiae is essential for proper chromosome segregation during meiosis I. When slk19 mutants were induced to sporulate they completed events characteristic of meiotic prophase I, but at the first meiotic division they segregated their sister chromatids to opposite poles at high frequencies. The vast majority of these cells did not perform a second meiotic division and proceeded to form dyads (asci containing two spores). Slk19p was found to localize to centromere regions of chromosomes during meiotic prophase where it remained until anaphase I. In the absence of Slk19p, Rec8p was not maintained at the centromere region through anaphase I as it is in wild-type cells. Finally, we demonstrate that Slk19p appears to function downstream of the meiosis-specific protein Spo13p in control of sister chromatid behavior during meiosis I. CONCLUSIONS: Our results suggest that Slk19p is essential at the centromere of meiotic chromosomes to prevent the premature separation of sister chromatids at meiosis I.     

The kinetochore proteins Pcs1 and Mde4 and heterochromatin are required to prevent merotelic orientation

BACKGROUND: Accurate chromosome segregation depends on the establishment of correct-amphitelic-kinetochore orientation. Merotelic kinetochore orientation is an error that occurs when a single kinetochore attaches to microtubules emanating from opposite spindle poles, a condition that hinders segregation of the kinetochore to a spindle pole in anaphase. To avoid chromosome missegregation resulting from merotelic kinetochore orientation, cells have developed mechanisms to prevent or correct merotelic attachment. A protein called Pcs1 has been implicated in preventing merotelic attachment in mitosis and meiosis II in the fission yeast S. pombe. RESULTS: We report that Pcs1 forms a complex with a protein called Mde4. Both Pcs1 and Mde4 localize to the central core of centromeres. Deletion of mde4(+), like that of pcs1(+), causes the appearance of lagging chromosomes during the anaphases of mitotic and meiosis II cells. We provide evidence that the kinetochores of lagging chromosomes in both pcs1 and mde4 mutant cells are merotelically attached. In addition, we find that lagging chromosomes in cells with defective centromeric heterochromatin also display features consistent with merotelic attachment. CONCLUSIONS: We suggest that the Pcs1/Mde4 complex is the fission yeast counterpart of the budding yeast monopolin subcomplex Csm1/Lrs4, which promotes the segregation of sister kinetochores to the same pole during meiosis I. We propose that the Pcs1/Mde4 complex acts in the central kinetochore domain to clamp microtubule binding sites together, the centromeric heterochromatin coating the flanking domains provides rigidity, and both systems contribute to the prevention of merotelic attachment.     

The CHD remodeling factor Hrp1 stimulates CENP-A loading to centromeres

Centromeres of fission yeast are arranged with a central core DNA sequence flanked by repeated sequences. The centromere-associated histone H3 variant Cnp1 (SpCENP-A) binds exclusively to central core DNA, while the heterochromatin proteins and cohesins bind the surrounding outer repeats. CHD (chromo-helicase/ATPase DNA binding) chromatin remodeling factors were recently shown to affect chromatin assembly in vitro. Here, we report that the CHD protein Hrp1 plays a key role at fission yeast centromeres. The hrp1Δ mutant disrupts silencing of the outer repeats and central core regions of the centromere and displays chromosome segregation defects characteristic for dysfunction of both regions. Importantly, Hrp1 is required to maintain high levels of Cnp1 and low levels of histone H3 and H4 acetylation at the central core region. Hrp1 interacts directly with the centromere in early S-phase when centromeres are replicated, suggesting that Hrp1 plays a direct role in chromatin assembly during DNA replication.     

Nucleosome depletion alters the chromatin structure of Saccharomyces cerevisiae centromeres.

Saccharomyces cerevisiae centromeric DNA is packaged into a highly nuclease-resistant chromatin core of approximately 200 base pairs of DNA. The structure of the centromere in chromosome III is somewhat larger than a 160-base-pair nucleosomal core and encompasses the conserved centromere DNA elements (CDE I, II, and III). Extensive mutational analysis has revealed the sequence requirements for centromere function. Mutations affecting the segregation properties of centromeres also exhibit altered chromatin structures in vivo. Thus the structure, as delineated by nuclease digestion, correlated with functional centromeres. We have determined the contribution of histone proteins to this unique structural organization. Nucleosome depletion by repression of either histone H2B or H4 rendered the cell incapable of chromosome segregation. Histone repression resulted in increased nuclease sensitivity of centromere DNA, with up to 40% of CEN3 DNA molecules becoming accessible to nucleolytic attack. Nucleosome depletion also resulted in an alteration in the distribution of nuclease cutting sites in the DNA surrounding CEN3. These data provide the first indication that authentic nucleosomal subunits flank the centromere and suggest that nucleosomes may be the central core of the centromere itself.     

Mutations in the alpha-tubulin 67C gene specifically impair achiasmate segregation in Drosophila melanogaster

Drosophila melanogaster oocytes heterozygous for mutations in the alpha-tubulin 67C gene (alphatub67C) display defects in centromere positioning during prometaphase of meiosis I. The centromeres do not migrate to the poleward edges of the chromatin mass, and the chromatin fails to stretch during spindle lengthening. These results suggest that the poleward forces acting at the kinetochore are compromised in the alphatub67C mutants. Genetic studies demonstrate that these mutations also strongly and specifically decrease the fidelity of achiasmate chromosome segregation. Proper centromere orientation, chromatin elongation, and faithful segregation can all be restored by a decrease in the amount of the Nod chromokinesin. These results suggest that the accurate segregation of achiasmate chromosomes requires the proper balancing of forces acting on the chromosomes during prometaphase.     

Centromere formation: from epigenetics to self-assembly

This review is part of the Chromosome segregation and Aneuploidy series that focuses on the importance of chromosome segregation mechanisms in maintaining genome stability. Centromeres are specialized chromosomal domains that serve as the foundation for the mitotic kinetochore, the interaction site between the chromosome and the mitotic spindle. The chromatin of centromeres is distinguished from other chromosomal loci by the unique incorporation of the centromeric histone H3 variant, centromere protein A. Here, we review the genetic and epigenetic factors that control the formation and maintenance of centromeric chromatin and propose a chromatin self-assembly model for organizing the higher-order structure of the centromere.     

Identification of DNA regions required for mitotic and meiotic functions within the centromere of Schizosaccharomyces pombe chromosome I.

We have determined the structural organization and functional roles of centromere-specific DNA sequence repeats in cen1, the centromere region from chromosome I of the fission yeast Schizosaccharomyces pombe. cen1 is composed of various classes of repeated sequences designated K', K"(dgl), L, and B', arranged in a 34-kb inverted repeat surrounding a 4- to 5-kb nonhomologous central core. Artificial chromosomes containing various portions of the cen1 region were constructed and assayed for mitotic and meiotic centromere function in S. pombe. Deleting K' and L from the distal portion of one arm of the inverted repeat had no effect on mitotic centromere function but resulted in greatly increased precocious sister chromatid separation in the first meiotic division. A centromere completely lacking K' and L, but containing the central core, one copy of B' and K" in one arm, and approximately 2.5 kb of the core-proximal portion of B' in the other arm, was also fully functional mitotically but again did not maintain sister chromatid attachment in meiosis I. However, deletion of K" from this minichromosome resulted in complete loss of centromere function. Thus, one copy of at least a portion of the K" (dgl) repeat is absolutely required but is not sufficient for S. pombe centromere function. The long centromeric inverted-repeat region must be relatively intact to maintain sister chromatid attachment in meiosis I.     

The Cohesion Protein SOLO Associates with SMC1 and Is Required for Synapsis,Recombination, Homolog Bias and Cohesion and Pairing of Centromeres in Drosophila Meiosis

Cohesion between sister chromatids is mediated by cohesin and is essential for proper meiotic segregation of both sister chromatids and homologs. solo encodes a Drosophila meiosis-specific cohesion protein with no apparent sequence homology to cohesins that is required in male meiosis for centromere cohesion, proper orientation of sister centromeres and centromere enrichment of the cohesin subunit SMC1. In this study, we show that solo is involved in multiple aspects of meiosis in female Drosophila. Null mutations in solo caused the following phenotypes: 1) high frequencies of homolog and sister chromatid nondisjunction (NDJ) and sharply reduced frequencies of homolog exchange; 2) reduced transmission of a ring-X chromosome, an indicator of elevated frequencies of sister chromatid exchange (SCE); 3) premature loss of centromere pairing and cohesion during prophase I, as indicated by elevated foci counts of the centromere protein CID; 4) instability of the lateral elements (LE)s and central regions of synaptonemal complexes (SCs), as indicated by fragmented and spotty staining of the chromosome core/LE component SMC1 and the transverse filament protein C(3)G, respectively, at all stages of pachytene. SOLO and SMC1 are both enriched on centromeres throughout prophase I, co-align along the lateral elements of SCs and reciprocally co-immunoprecipitate from ovarian protein extracts. Our studies demonstrate that SOLO is closely associated with meiotic cohesin and required both for enrichment of cohesin on centromeres and stable assembly of cohesin into chromosome cores. These events underlie and are required for stable cohesion of centromeres, synapsis of homologous chromosomes, and a recombination mechanism that suppresses SCE to preferentially generate homolog crossovers (homolog bias). We propose that SOLO is a subunit of a specialized meiotic cohesin complex that mediates both centromeric and axial arm cohesion and promotes homolog bias as a component of chromosome cores.     

INCENP and Aurora B promote meiotic sister chromatid cohesion through localization of the Shugoshin MEI-S332 in Drosophila

The chromosomal passenger complex protein INCENP is required in mitosis for chromosome condensation, spindle attachment and function, and cytokinesis. Here, we show that INCENP has an essential function in the specialized behavior of centromeres in meiosis. Mutations affecting Drosophila incenp profoundly affect chromosome segregation in both meiosis I and II, due, at least in part, to premature sister chromatid separation in meiosis I. INCENP binds to the cohesion protector protein MEI-S332, which is also an excellent in vitro substrate for Aurora B kinase. A MEI-S332 mutant that is only poorly phosphorylated by Aurora B is defective in localization to centromeres. These results implicate the chromosomal passenger complex in directly regulating MEI-S332 localization and, therefore, the control of sister chromatid cohesion in meiosis.