Genomes are packaged as chromatin in nucleosomes composed of core histones (H2A/H2B/H3/H4). Epigenetic processes, mediated by histone post-translational modifications, allow continued gene expression/repression in cell lineages. Understanding how such ‘marks’ establish and propagate chromatin states is critical for specifying and maintaining distinct cell types. Moreover, epigenetic states provide a source of phenotypic variation, independent of DNA sequence.
Eic1 links Mis18 with the CCAN/Mis6/Ctf19 complex to promote CENP-A assembly.
CENP-A chromatin forms the foundation for kinetochore assembly. Replication-independent incorporation of CENP-A at centromeres depends on its chaperone HJURP(Scm3), and Mis18 in vertebrates and fission yeast. The recruitment of Mis18 and HJURP(Scm3) to centromeres is cell cycle regulated. Vertebrate Mis18 associates with Mis18BP1(KNL2), which is critical for the recruitment of Mis18 and HJURP(Scm3). We identify two novel fission yeast Mis18-interacting proteins (Eic1 and Eic2), components of the Mis18 complex. Eic1 is essential to maintain Cnp1(CENP-A) at centromeres and is crucial for kinetochore integrity; Eic2 is dispensable. Eic1 also associates with Fta7(CENP-Q/Okp1), Cnl2(Nkp2) and Mal2(CENP-O/Mcm21), components of the constitutive CCAN/Mis6/Ctf19 complex. No Mis18BP1(KNL2) orthologue has been identified in fission yeast, consequently it remains unknown how the key Cnp1(CENP-A) loading factor Mis18 is recruited. Our findings suggest that Eic1 serves a function analogous to that of Mis18BP1(KNL2), thus representing the functional counterpart of Mis18BP1(KNL2) in fission yeast that connects with a module within the CCAN/Mis6/Ctf19 complex to allow the temporally regulated recruitment of the Mis18/Scm3(HJURP) Cnp1(CENP-A) loading factors. The novel interactions identified between CENP-A loading factors and the CCAN/Mis6/Ctf19 complex are likely to also contribute to CENP-A maintenance in other organisms. See Subramanian et al (2014) Open Biology.
Telomeric repeats facilitate CENP-A(Cnp1) incorporation via telomere binding proteins.
The histone H3 variant, CENP-A, is normally assembled upon canonical centromeric sequences, but there is no apparent obligate coupling of sequence and assembly, suggesting that centromere location can be epigenetically determined. To explore the tolerances and constraints on CENP-A deposition we investigated whether certain locations are favoured when additional CENP-A(Cnp1) is present in fission yeast cells. Our analyses show that additional CENP-A(Cnp1) accumulates within and close to heterochromatic centromeric outer repeats, and over regions adjacent to rDNA and telomeres. The use of minichromosome derivatives with unique DNA sequences internal to chromosome ends shows that telomeres are sufficient to direct CENP-A(Cnp1) deposition. However, chromosome ends are not required as CENP-A(Cnp1) deposition also occurs at telomere repeats inserted at an internal locus and correlates with the presence of H3K9 methylation near these repeats. The Ccq1 protein, which is known to bind telomere repeats and recruit telomerase, was found to be required to induce H3K9 methylation and thus promote the incorporation of CENP-A(Cnp1) near telomere repeats. These analyses demonstrate that at non-centromeric chromosomal locations the presence of heterochromatin influences the sites at which CENP-A is incorporated into chromatin and, thus, potentially the location of centromeres. See Castillo et al (2013) PlosOne.
CENP-A confers a reduction in height on octameric nucleosomes
Nucleosomes with histone H3 replaced by CENP-A direct kinetochore assembly. CENP-A nucleosomes from human and Drosophila have been reported to have reduced heights as compared to canonical octameric H3 nucleosomes, thus suggesting a unique tetrameric hemisomal composition. We demonstrate that octameric CENP-A nucleosomes assembled in vitro exhibit reduced heights, indicating that they are physically distinct from H3 nucleosomes and negating the need to invoke the presence of hemisomes.
See Miell et al (2013) Nat Struct Mol Biol.
Distinct Roles for Sir2 and RNAi in Centromeric Heterochromatin Nucleation, Spreading and Maintenance
Epigenetically regulated heterochromatin domains govern essential cellular activities. A key feature of heterochromatin domains is the presence of hypoacetylated nucleosomes, which are methylated on lysine 9 of histone H3 (H3K9me). Here, we investigate the requirements for establishment, spreading and maintenance of heterochromatin using fission yeast centromeres as a paradigm. We show that establishment of heterochromatin on centromeric repeats is initiated at modular 'nucleation sites' by RNA interference (RNAi), ensuring the mitotic stability of centromere-bearing minichromosomes. We demonstrate that the histone deacetylases Sir2 and Clr3 and the chromodomain protein Swi6HP1 are required for H3K9me spreading from nucleation sites, thus allowing formation of extended heterochromatin domains. We discovered that RNAi and Sir2 along with Swi6HP1 operate in two independent pathways to maintain heterochromatin. Finally, we demonstrate that tethering of Sir2 is pivotal to the maintenance of heterochromatin at an ectopic locus in the absence of RNAi. These analyses reveal that Sir2, together with RNAi, are sufficient to ensure heterochromatin integrity and provide evidence for sequential establishment, spreading and maintenance steps in the assembly of centromeric heterochromatin.
See Buscaino et al (2013) EMBO J.
Factors that Promote H3 Chromatin intergrity during transciption prevent promiscuous deposition of CENP-A(Cnp1) in fission yeast
Specialized chromatin containing CENP-A nucleosomes instead of H3 nucleosomes is found at all centromeres. However, the mechanisms that specify the locations at which CENP-A chromatin is assembled remain elusive in organisms with regional, epigenetically regulated centromeres. It is known that normal centromeric DNA is transcribed in several systems including the fission yeast, Schizosaccharomyces pombe. Here, we show that factors which preserve stable histone H3 chromatin during transcription also play a role in preventing promiscuous CENP-A(Cnp1) deposition in fission yeast. Mutations in the histone chaperone FACT impair the maintenance of H3 chromatin on transcribed regions and promote widespread CENP-A(Cnp1) incorporation at non-centromeric sites. FACT has little or no effect on CENP-A(Cnp1) assembly at endogenous centromeres where CENP-A(Cnp1) is normally assembled. In contrast, Clr6 complex II (Clr6-CII; equivalent to Rpd3S) histone deacetylase function has a more subtle impact on the stability of transcribed H3 chromatin and acts to prevent the ectopic accumulation of CENP-A(Cnp1) at specific loci, including subtelomeric regions, where CENP-A(Cnp1) is preferentially assembled. Moreover, defective Clr6-CII function allows the de novo assembly of CENP-A(Cnp1) chromatin on centromeric DNA, bypassing the normal requirement for heterochromatin. Thus, our analyses show that alterations in the process of chromatin assembly during transcription can destabilize H3 nucleosomes and thereby allow CENP-A(Cnp1) to assemble in its place. We propose that normal centromeres provide a specific chromatin context that limits reassembly of H3 chromatin during transcription and thereby promotes the establishment of CENP-A(Cnp1) chromatin and associated kinetochores. These findings have important implications for genetic and epigenetic processes involved in centromere specification.
See Choi et al (2012) PloS Genetics
Quantitative Single Molecule Microscopy Reveals that CENP-A(Cnp1) Deposition Occurs During G2 in Fission Yeast
The inheritance of the histone H3 variant CENP-A in nucleosomes at centromeres following DNA replication is mediated by an epigenetic mechanism. To understand the process of epigenetic inheritance, or propagation of histones and histone variants, as nucleosomes are disassembled and reassembled in living eukaryotic cells, we have explored the feasibility of exploiting photo-activated localization microscopy (PALM). PALM of single molecules in living cells has the potential to reveal new concepts in cell biology, providing insights into stochastic variation in cellular states. However, thus far, its use has been limited to studies in bacteria or to processes occurring near the surface of eukaryotic cells. With PALM, one literally observes and 'counts' individual molecules in cells one-by-one and this allows the recording of images with a resolution higher than that determined by the diffraction of light (the so-called super-resolution microscopy). Here, we investigate the use of different fluorophores and develop procedures to count the centromere-specific histone H3 variant CENP-A(Cnp1) with single-molecule sensitivity in fission yeast (Schizosaccharomyces pombe). The results obtained are validated by and compared with ChIP-seq analyses. Using this approach, CENP-A(Cnp1) levels at fission yeast (S. pombe) centromeres were followed as they change during the cell cycle. Our measurements show that CENP-A(Cnp1) is deposited solely during the G2 phase of the cell cycle.
See Lando et al (2012) Open Biol
Raf1 is a DCAF for the Rik1 DDB1-Like Protein and has Separable Roles in siRNA Generation and Chromatin Modification
Non-coding transcription can trigger histone post-translational modifications forming specialized chromatin. In fission yeast, heterochromatin formation requires RNAi and the histone H3K9 methyltransferase complex CLRC, composed of Clr4, Raf1, Raf2, Cul4, and Rik1. CLRC mediates H3K9 methylation and siRNA production; it also displays E3-ubiquitin ligase activity in vitro. DCAFs act as substrate receptors for E3 ligases and may couple ubiquitination with histone methylation. Here, structural alignment and mutation of signature WDxR motifs in Raf1 indicate that it is a DCAF for CLRC. We demonstrate that Raf1 promotes H3K9 methylation and siRNA amplification via two distinct, separable functions. The association of the DCAF Raf1 with Cul4-Rik1 is critical for H3K9 methylation, but dispensable for processing of centromeric transcripts into siRNAs. Thus the association of a DCAF, Raf1, with its adaptor, Rik1, is required for histone methylation and to allow RNAi to signal to chromatin.
See Buscaino et al (2012) PLoS Genetics.