Ccq1p and the condensin proteins Cut3p and Cut14p prevent telomere entanglements in the fission yeast Schizosaccharomyces pombe

Condensin II subunit NCAPH2 associates with shelterin protein TRF1 and is required for telomere stability

Condensin II subunits are known to be expressed and localized to interphase nuclei of eukaryotic cells. Although some studies have shown that condensin II likely exerts axial compaction forces, organizes chromosome territories, and has possible transcriptional modulatory functions, the full range of condensin II interphase activities are not known. In particular, it is not known if condensin II interphase activities are generally genome‐wide or if they have additional local activities unique to specific chromosomal structures such as telomeres. Here, we find that NCAPH2 interacts with TRF1 and these two proteins co‐localize at telomeres. Depletion of NCAPH2 leads to ATR‐dependent accumulation of 53BP1 and γH2AX DNA damage foci, including damage specific to telomeres. Furthermore, depletion of NCAPH2 results in a fragile telomere phenotype and apparent sister‐telomere fusions only days after NCAPH2 depletion. Taken together these observations suggest that NCAPH2 promotes telomere stability, possibly through a direct interaction with the TRF1 shelterin component, and prevents telomere dysfunction resulting from impaired DNA replication. Because proper telomere function is essential for chromosome integrity these observations reveal a previously unappreciated function for NCAPH2 in ensuring genome and telomere stability.

Shelterin and subtelomeric DNA sequences control nucleosome maintenance and genome stability

Telomeres and the shelterin complex cap and protect the ends of chromosomes. Telomeres are flanked by the subtelomeric sequences that have also been implicated in telomere regulation, although their role is not well defined. Here, we show that, in Schizosaccharomyces pombe, the telomere-associated sequences (TAS) present on most subtelomeres are hyper-recombinogenic, have metastable nucleosomes, and unusual low levels of H3K9 methylation. Ccq1, a subunit of shelterin, protects TAS from nucleosome loss by recruiting the heterochromatic repressor complexes CLRC and SHREC, thereby linking nucleosome stability to gene silencing. Nucleosome instability at TAS is independent of telomeric repeats and can be transmitted to an intrachromosomal locus containing an ectopic TAS fragment, indicating that this is an intrinsic property of the underlying DNA sequence. When telomerase recruitment is compromised in cells lacking Ccq1, DNA sequences present in the TAS promote recombination between chromosomal ends, independent of nucleosome abundance, implying an active function of these sequences in telomere maintenance. We propose that Ccq1 and fragile subtelomeres co-evolved to regulate telomere plasticity by controlling nucleosome occupancy and genome stability.

Fundamental mechanisms of telomerase action in yeasts and mammals: understanding telomeres and telomerase in cancer cells

Aberrant activation of telomerase occurs in 85–90% of all cancers and underpins the ability of cancer cells to bypass their proliferative limit, rendering them immortal. The activity of telomerase is tightly controlled at multiple levels, from transcriptional regulation of the telomerase components to holoenzyme biogenesis and recruitment to the telomere, and finally activation and processivity. However, studies using cancer cell lines and other model systems have begun to reveal features of telomeres and telomerase that are unique to cancer. This review summarizes our current knowledge on the mechanisms of telomerase recruitment and activation using insights from studies in mammals and budding and fission yeasts. Finally, we discuss the differences in telomere homeostasis between normal cells and cancer cells, which may provide a foundation for telomere/telomerase targeted cancer treatments.

Ccq1-Tpz1TPP1 interaction facilitates telomerase and SHREC association with telomeres in fission yeast

Through characterization of ccq1 mutants that disrupt Ccq1-Tpz1^TPP1 interaction, the authors establish that Ccq1-Tpz1^TPP1 interaction contributes to optimal binding of the Ccq1-SHREC complex and is required for Ccq1 Thr93 phosphorylation and telomerase recruitment.

Evolutionarily conserved shelterin complex is essential for telomere maintenance in the fission yeast Schizosaccharomyces pombe. Elimination of the fission yeast shelterin subunit Ccq1 causes progressive loss of telomeres due to the inability to recruit telomerase, activates the DNA damage checkpoint, and loses heterochromatin at telomere/subtelomere regions due to reduced recruitment of the heterochromatin regulator complex Snf2/histone deacetylase–containing repressor complex (SHREC). The shelterin subunit Tpz1^TPP1 directly interacts with Ccq1 through conserved C-terminal residues in Tpz1^TPP1, and tpz1 mutants that fail to interact with Ccq1 show telomere shortening, checkpoint activation, and loss of heterochromatin. While we have previously concluded that Ccq1-Tpz1^TPP1 interaction contributes to Ccq1 accumulation and telomerase recruitment based on analysis of tpz1 mutants that fail to interact with Ccq1, another study reported that loss of Ccq1-Tpz1^TPP1 interaction does not affect accumulation of Ccq1 or telomerase. Furthermore, it remained unclear whether loss of Ccq1-Tpz1^TPP1 interaction affects SHREC accumulation at telomeres. To resolve these issues, we identified and characterized a series of ccq1 mutations that disrupt Ccq1-Tpz1^TPP1 interaction. Characterization of these ccq1 mutants established that Ccq1-Tpz1^TPP1 interaction contributes to optimal binding of the Ccq1-SHREC complex, and is critical for Rad3^ATR/Tel1^ATM-dependent Ccq1 Thr93 phosphorylation and telomerase recruitment.

Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction

The segregation of centromeres and telomeres at mitosis is coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. Mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during fission yeast mitosis. Telomere dispersion initiates in metaphase, whereas disjunction takes place in anaphase. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres, whereas disjunction occurs at anaphase after the phosphorylation of condensin subunit Cnd2. Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescued cell death after Aurora inhibition by promoting the loading of condensin on chromosome arms. Our findings reveal an essential role for telomeres in chromosome arm segregation.

Condensin I associates with structural and gene regulatory regions in vertebrate chromosomes

The condensin complex is essential for correct packaging and segregation of chromosomes during mitosis and meiosis in all eukaryotes. To date, the genome-wide location and the nature of condensin-binding sites have remained elusive in vertebrates. Here we report the genome-wide map of condensin I in chicken DT40 cells. Unexpectedly, we find that condensin I binds predominantly to promoter sequences in mitotic cells. We also find a striking enrichment at both centromeres and telomeres, highlighting the importance of the complex in chromosome segregation. Taken together, the results show that condensin I is largely absent from heterochromatic regions. This map of the condensin I binding sites on the chicken genome reveals that patterns of condensin distribution on chromosomes are conserved from prokaryotes, through yeasts to vertebrates. Thus in three kingdoms of life, condensin is enriched on promoters of actively transcribed genes and at loci important for chromosome segregation.

The Ctf18RFC clamp loader is essential for telomere stability in telomerase-negative and mre11 mutant alleles

The function of the replication clamp loaders in the semi-conservative telomere replication and their relationship to telomerase- and recombination mechanisms of telomere addition remains ambiguous. We have investigated the variant clamp loader Ctf18 RFC (Replication Factor C). To understand the role of Ctf18 at the telomere, we first investigated genetic interactions after loss of Ctf18 and TLC1 (the yeast telomerase RNA). We find that the tlc1Δ ctf18Δ double mutant confers a rapid >1000-fold decrease in viability. The rate of loss was similar to the kinetics of cell death in rad52Δ tlc1Δ cells. However, the Ctf18 pathway is distinct from Rad52, required for the repair of DSBs, as demonstrated by the synthetic lethality of rad52▵ tlc1Δ ctf18Δ triple mutants. These data suggest that each mutant elicits non-redundant defects acting on the same substrate. Second, interactions of the yeast hyper-recombinational mutant, mre11A470T, with ctf18▵ confer a synergistic cold sensitivity. The phenotype of these double mutants ultimately results in telomere loss and the generation of recombinational survivors. We observed a similar synergism between single mutants that led to hypersensitivity to the DNA alkylating agent, methane methyl sulphonate (MMS), the replication fork inhibitor hydroxyurea (HU), and to a failure to separate telomeres of sister chromatids. Hence, ctf18Δ and mre11A470T act in different pathways on telomere substrates for multiple phenotypes. The mre11A470T cells also displayed a DNA damage response (DDR) at 15°C but not at 30°C while ctf18Δ mutants conferred a constitutive DDR activity. Both the 15°C DDR pattern and growth rate were reversible at 30°C and displayed telomerase activity in vivo. We hypothesize that Ctf18 confers protection against stalling and/or breaks at the replication fork in cells that either lack, or are compromised for, telomerase activity. This Ctf18-based function is likely to contribute another level to telomere size homeostasis.

Monopolin recruits condensin to organize centromere DNA and repetitive DNA sequences

Higher-order structure of chromatin is essential for chromosome segregation and repetitive DNA stability. Monopolin recruits condensin to organize centromere DNA irrespective of the number of kinetochore–microtubule attachments. In addition, the role of monopolin in stabilizing repeat tracts observed in budding yeast is conserved in Candida albicans.

The establishment and maintenance of higher-order structure at centromeres is essential for accurate chromosome segregation. The monopolin complex is thought to cross-link multiple kinetochore complexes to prevent merotelic attachments that result in chromosome missegregation. This model is based on structural analysis and the requirement that monopolin execute mitotic and meiotic chromosome segregation in Schizosaccharomyces pombe, which has more than one kinetochore–microtubule attachment/centromere, and co-orient sister chromatids in meiosis I in Saccharomyces cerevisiae. Recent data from S. pombe suggest an alternative possibility: that the recruitment of condensin is the primary function of monopolin. Here we test these models using the yeast Candida albicans. C. albicans cells lacking monopolin exhibit defects in chromosome segregation, increased distance between centromeres, and decreased stability of several types of repeat DNA. Of note, changing kinetochore–microtubule copy number from one to more than one kinetochore–microtubule/centromere does not alter the requirement for monopolin. Furthermore, monopolin recruits condensin to C. albicans centromeres, and overexpression of condensin suppresses chromosome segregation defects in strains lacking monopolin. We propose that the key function of monopolin is to recruit condensin in order to promote the assembly of higher-order structure at centromere and repetitive DNA.

Condensin phosphorylated by the Aurora-B-like kinase Ark1 is continuously required until telophase in a mode distinct from Top2

Condensin is a conserved protein complex that functions in chromosome condensation and segregation. It has not been previously unequivocally determined whether condensin is required throughout mitosis. Here, we examined whether Schizosaccharomyces pombe condensin continuously acts on chromosomes during mitosis and compared its role with that of DNA topoisomerase II (Top2). Using double mutants containing a temperature-sensitive allele of the condensin SMC2 subunit cut14 (cut14-208) or of top2, together with the cold-sensitive nda3-KM311 mutation (in β-tubulin), temperature-shift experiments were performed. These experiments allowed inactivation of condensin or Top2 at various stages throughout mitosis, even after late anaphase. The results established that mitotic chromosomes require condensin and Top2 throughout mitosis, even in telophase. We then showed that the Cnd2 subunit of condensin (also known as Barren) is the target subunit of Aurora-B-like kinase Ark1 and that Ark1-mediated phosphorylation of Cnd2 occurred throughout mitosis. The phosphorylation sites in Cnd2 were determined by mass spectrometry, and alanine and glutamate residue replacement mutant constructs for these sites were constructed. Alanine substitution mutants of Cnd2, which mimic the unphosphorylated protein, exhibited broad mitotic defects, including at telophase, and overexpression of these constructs caused a severe dominant-negative effect. By contrast, glutamate substitution mutants, which mimic the phosphorylated protein, alleviated the segregation defect in Ark1-inhibited cells. In telophase, the condensin subunits in cut14-208 mutant accumulated in lumps that contained telomeric DNA and proteins that failed to segregate. Condensin might thus serve to keep the segregated chromosomes apart during telophase.