Structural Basis of Heterochromatin Formation by Human HP1.

Heterochromatin plays important roles in transcriptional silencing and genome maintenance by the formation of condensed chromatin structures, which determine the epigenetic status of eukaryotic cells. The trimethylation of histone H3 lysine 9 (H3K9me3), a target of heterochromatin protein 1 (HP1), is a hallmark of heterochromatin formation. However, the mechanism by which HP1 folds chromatin-containing H3K9me3 into a higher-order structure has not been elucidated. Here we report the three-dimensional structure of the H3K9me3-containing dinucleosomes complexed with human HP1α, HP1ß, and HP1γ, determined by cryogenic electron microscopy with a Volta phase plate. In the structures, two H3K9me3 nucleosomes are bridged by a symmetric HP1 dimer. Surprisingly, the linker DNA between the nucleosomes does not directly interact with HP1, thus allowing nucleosome remodeling by the ATP-utilizing chromatin assembly and remodeling factor (ACF). The structure depicts the fundamental architecture of heterochromatin.

Researchers support preprints and open access publishing, but with reservations: A questionnaire survey of MBSJ members.

Since the 1990s, journals have become increasingly online and open access. In fact, about 50% of articles published in 2021 were open access. The use of preprints (i.e., non-peer-reviewed articles) has also increased. However, there is limited awareness of these concepts among academics. Therefore, we conducted a questionnaire-based survey among members of the Molecular Biology Society of Japan. The survey was conducted between September 2022 and October 2022, with 633 respondents, 500 of whom (79.0%) were faculty members. In total, 478 (76.6%) respondents had published articles as open access, and 571 (91.5%) wanted to publish their articles in open access. Although 540 (86.5%) respondents knew about preprints, only 183 (33.9%) had posted preprints before. In the open-ended section of the questionnaire survey, several comments were made about the cost burdens associated with open access and the difficulty of how academic preprints are handled. Although open access is widespread, and recognition of preprints is increasing, some issues remain that need to be addressed. Academic and institutional support, and transformative agreement may help reduce the cost burden. Guidelines for handling preprints in academia are also important for responding to changes in the research environment.

H3K14 ubiquitylation promotes H3K9 methylation for heterochromatin assembly.

The methylation of histone H3 at lysine 9 (H3K9me), performed by the methyltransferase Clr4/SUV39H, is a key event in heterochromatin assembly. In fission yeast, Clr4, together with the ubiquitin E3 ligase Cul4, forms the Clr4 methyltransferase complex (CLRC), whose physiological targets and biological role are currently unclear. Here, we show that CLRC-dependent H3 ubiquitylation regulates Clr4's methyltransferase activity. Affinity-purified CLRC ubiquitylates histone H3, and mass spectrometric and mutation analyses reveal that H3 lysine 14 (H3K14) is the preferred target of the complex. Chromatin immunoprecipitation analysis shows that H3K14 ubiquitylation (H3K14ub) is closely associated with H3K9me-enriched chromatin. Notably, the CLRC-mediated H3 ubiquitylation promotes H3K9me by Clr4, suggesting that H3 ubiquitylation is intimately linked to the establishment and/or maintenance of H3K9me. These findings demonstrate a cross-talk mechanism between histone ubiquitylation and methylation that is involved in heterochromatin assembly.

Meiosis-specific cohesin component, Rec8, promotes the localization of Mps3 SUN domain protein on the nuclear envelope.

Proteins in the nuclear envelope (NE) play a role in the dynamics and functions of the nucleus and of chromosomes during mitosis and meiosis. Mps3, a yeast NE protein with a conserved SUN domain, predominantly localizes on a yeast centrosome equivalent, spindle pole body (SPB), in mitotic cells. During meiosis, Mps3, together with SPB, forms a distinct multiple ensemble on NE. How meiosis-specific NE localization of Mps3 is regulated remains largely unknown. In this study, we found that a meiosis-specific component of the protein complex essential for sister chromatid cohesion, Rec8, binds to Mps3 during meiosis and controls Mps3 localization and proper dynamics on NE. Ectopic expression of Rec8 in mitotic yeast cells induced the formation of Mps3 patches/foci on NE. This required the cohesin regulator, WAPL ortholog, Rad61/Wpl1, suggesting that a meiosis-specific cohesin complex with Rec8 controls NE localization of Mps3. We also observed that two domains of the nucleoplasmic region of Mps3 are essential for NE localization of Mps3 in mitotic as well as meiotic cells. We speculate that the interaction of Mps3 with the meiosis-specific cohesin in the nucleoplasm is a key determinant for NE localization/function of Mps3.

Intrinsic nucleic acid-binding activity of Chp1 chromodomain is required for heterochromatic gene silencing.

Centromeric heterochromatin assembly in fission yeast requires the RNAi pathway. Chp1, a chromodomain (CD) protein, forms the Ago1-containing RNA-induced transcriptional silencing (RITS) complex and recruits siRNA-bound RITS to methylated histone H3 lysine 9 (H3K9me) via its CD. Here, we show that the CD of Chp1 (Chp1-CD) possesses unique nucleic acid-binding activities that are essential for heterochromatic gene silencing. Detailed electrophoretic-mobility shift analyses demonstrated that Chp1 binds to RNA via the CD in addition to its central RNA-recognition motif. Interestingly, robust RNA- and DNA-binding activity of Chp1-CD was strongly enhanced when it was bound to H3K9me, which was revealed to involve a positively charged domain within the Chp1-CD by structural analyses. These results demonstrate a role for the CD that provides a link between RNA, DNA, and methylated histone tails to ensure heterochromatic gene silencing.

The intron in centromeric noncoding RNA facilitates RNAi-mediated formation of heterochromatin.

In fission yeast, the formation of centromeric heterochromatin is induced through the RNA interference (RNAi)-mediated pathway. Some pre-mRNA splicing mutants (prp) exhibit defective formation of centromeric heterochromatin, suggesting that splicing factors play roles in the formation of heterochromatin, or alternatively that the defect is caused by impaired splicing of pre-mRNAs encoding RNAi factors. Herein, we demonstrate that the splicing factor spPrp16p is enriched at the centromere, and associates with Cid12p (a factor in the RNAi pathway) and the intron-containing dg ncRNA. Interestingly, removal of the dg intron, mutations of its splice sites, or replacement of the dg intron with an euchromatic intron significantly decreased H3K9 dimethylation. We also revealed that splicing of dg ncRNA is repressed in cells and its repression depends on the distance from the transcription start site to the intron. Inefficient splicing was also observed in other intron-containing centromeric ncRNAs, dh and antisense dg, and splicing of antisense dg ncRNA was repressed in the presence of the RNAi factors. Our results suggest that the introns retained in centromeric ncRNAs work as facilitators, co-operating with splicing factors assembled on the intron and serving as a platform for the recruitment of RNAi factors, in the formation of centromeric heterochromatin.

A novel RNAi protein, Dsh1, assembles RNAi machinery on chromatin to amplify heterochromatic siRNA.

In fission yeast, siRNA is generated from pericentromeric noncoding RNA by the RNAi machinery. siRNA synthesis and heterochromatin formation are interdependent, forming a self-reinforcing loop on chromatin. In this system, siRNA is amplified by the RNA-dependent RNA polymerase complex (RDRC) and the endoribonuclease Dcr1, which synthesizes dsRNA and processes the dsRNA, respectively. The amplification is essential for stable heterochromatin formation. Here, a novel gene, dsh1(+) (defect of the gene silencing at centromeric heterochromatin), is identified as an essential component of RNAi-directed heterochromatin assembly. Loss of dsh1(+) abolishes normal RNAi function and heterochromatic gene silencing at pericentromeres. Dsh1 interacts with Dcr1 and RDRC and couples the reactions of both proteins to the effective production of siRNA in vivo. Dsh1 binds to heterochromatin in the absence of RDRC, while RDRC requires Dsh1 for its chromatin-binding activity, suggesting that Dsh1 recruits RDRC to chromatin. Immunofluorescence analysis shows that Dsh1 forms foci at the nuclear periphery, and some Dsh1 foci colocalize with Dcr1 and RDRC. Dsh1 is required for the colocalization of Dcr1 and RDRC. Moreover, loss of the nuclear periphery localization of Dsh1 abolishes Dsh1 function. Taken together, these results suggest that Dsh1 assembles the RNAi machinery on heterochromatin and forms a perinuclear compartment for amplification of heterochromatic siRNA.

Regulation of mitotic recombination between DNA repeats in centromeres.

Centromeres that are essential for faithful segregation of chromosomes consist of unique DNA repeats in many eukaryotes. Although recombination is under-represented around centromeres during meiosis, little is known about recombination between centromere repeats in mitotic cells. Here, we compared spontaneous recombination that occurs between ade6B/ade6X inverted repeats integrated at centromere 1 (cen1) or at a non-centromeric ura4 locus in fission yeast. Remarkably, distinct mechanisms of homologous recombination (HR) were observed in centromere and non-centromere regions. Rad51-dependent HR that requires Rad51, Rad54 and Rad52 was predominant in the centromere, whereas Rad51-independent HR that requires Rad52 also occurred in the arm region. Crossovers between inverted repeats (i.e. inversions) were under-represented in the centromere as compared to the arm region. While heterochromatin was dispensable, Mhf1/CENP-S, Mhf2/CENP-X histone-fold proteins and Fml1/FANCM helicase were required to suppress crossovers. Furthermore, Mhf1 and Fml1 were found to prevent gross chromosomal rearrangements mediated by centromere repeats. These data for the first time uncovered the regulation of mitotic recombination between DNA repeats in centromeres and its physiological role in maintaining genome integrity.

Four domains of Ada1 form a heterochromatin boundary through different mechanisms.

In eukaryotic cells, there are two chromatin states, silenced and active, and the formation of a so-called boundary plays a critical role in demarcating these regions; however, the mechanisms underlying boundary formation are not well understood. In this study, we focused on S. cerevisiae ADA1, a gene previously shown to encode a protein with a robust boundary function. Ada1 is a component of the histone modification complex Spt-Ada-Gcn5-acetyltransferase (SAGA) and the SAGA-like (SLIK) complex, and it helps to maintain the integrity of these complexes. Domain analysis showed that four relatively small regions of Ada1 (Region I; 66-75 aa, II; 232-282 aa, III; 416-436 aa and IV; 476-488 aa) have a boundary function. Among these, Region II could form an intact SAGA complex, whereas the other regions could not. Investigation of cellular factors that interact with these small regions identified a number of proteasome-associated proteins. Interestingly, the boundary functions of Region II and Region III were affected by depletion of Ump1, a maturation and assembly factor of the 20S proteasome. These results suggest that the boundary function of Ada1 is functionally linked to proteasome processes and that the four relatively small regions in ADA1 form a boundary via different mechanisms.

Phosphorylation of Swi6/HP1 regulates transcriptional gene silencing at heterochromatin.

Heterochromatin protein 1 (HP1) recruits various effectors to heterochromatin for multiple functions, but its regulation is unclear. In fission yeast, a HP1 homolog Swi6 recruits SHREC, Epe1, and cohesin, which are involved in transcriptional gene silencing (TGS), transcriptional activation, and sister chromatid cohesion, respectively. We found that casein kinase II (CK2) phosphorylated Swi6. Loss of CK2-dependent Swi6 phosphorylation alleviated heterochromatic TGS without affecting heterochromatin structure. This was due to the inhibited recruitment of SHREC to heterochromatin, accompanied by an increase in Epe1. Interestingly, loss of phosphorylation did not affect cohesion. These results indicate that CK2-dependent Swi6 phosphorylation specifically controls TGS in heterochromatin.

Single cell visualization of yeast gene expression shows correlation of epigenetic switching between multiple heterochromatic regions through multiple generations.

Differences in gene expression between individual cells can be mediated by epigenetic regulation; thus, methods that enable detailed analyses of single cells are crucial to understanding this phenomenon. In this study, genomic silencing regions of Saccharomyces cerevisiae that are subject to epigenetic regulation, including the HMR, HML, and telomere regions, were investigated using a newly developed single cell analysis method. This method uses fluorescently labeled proteins to track changes in gene expression over multiple generations of a single cell. Epigenetic control of gene expression differed depending on the specific silencing region at which the reporter gene was inserted. Correlations between gene expression at the HMR-left and HMR-right regions, as well as the HMR-right and HML-right regions, were observed in the single-cell level; however, no such correlations involving the telomere region were observed. Deletion of the histone acetyltransferase GCN5 gene from a yeast strain carrying a fluorescent reporter gene at the HMR-left region reduced the frequency of changes in gene expression over a generation. The results presented here suggest that epigenetic control within an individual cell is reversible and can be achieved via regulation of histone acetyltransferase activity.

siRNA-mediated heterochromatin establishment requires HP1 and is associated with antisense transcription.

Heterochromatic gene silencing at the pericentromeric DNA repeats in fission yeast requires the RNA interference (RNAi) machinery. The RNA-induced transcriptional silencing (RITS) complex mediates histone H3 lysine 9 (H3K9) methylation and recruits the RNA-dependent RNA polymerase complex (RDRC) to promote double-stranded RNA (dsRNA) synthesis and siRNA generation. Here we show that ectopic expression of a long hairpin RNA bypasses the requirement for chromatin-dependent steps in siRNA generation. The ability of hairpin-produced siRNAs to silence homologous sequences in trans is subject to local chromatin structure, requires HP1, and correlates with antisense transcription at the target locus. Furthermore, although hairpin siRNAs can be produced in the absence of RDRC, trans-silencing of reporter genes by hairpin-produced siRNAs is completely dependent on the dsRNA synthesis activity of RDRC. These results provide insights into the regulation of siRNA action and reveal roles for cis-dsRNA synthesis and HP1 in siRNA-mediated heterochromatin assembly.

N-terminal phosphorylation of HP1α increases its nucleosome-binding specificity.

Heterochromatin protein 1 (HP1) is an evolutionarily conserved chromosomal protein that binds to lysine 9-methylated histone H3 (H3K9me), a hallmark of heterochromatin. Although HP1 phosphorylation has been described in several organisms, the biological implications of this modification remain largely elusive. Here we show that HP1's phosphorylation has a critical effect on its nucleosome binding properties. By in vitro phosphorylation assays and conventional chromatography, we demonstrated that casein kinase II (CK2) is the kinase primarily responsible for phosphorylating the N-terminus of human HP1α. Pull-down assays using in vitro-reconstituted nucleosomes showed that unmodified HP1α bound H3K9-methylated and H3K9-unmethylated nucleosomes with comparable affinity, whereas CK2-phosphorylated HP1α showed a high specificity for H3K9me3-modified nucleosomes. Electrophoretic mobility shift assays showed that CK2-mediated phosphorylation diminished HP1α's intrinsic DNA binding, which contributed to its H3K9me-independent nucleosome binding. CK2-mediated phosphorylation had a similar effect on the nucleosome-binding specificity of fly HP1a and S. pombe Swi6. These results suggested that HP1 phosphorylation has an evolutionarily conserved role in HP1's recognition of H3K9me-marked nucleosomes.

Physical and functional interactions between the histone H3K4 demethylase KDM5A and the nucleosome remodeling and deacetylase (NuRD) complex.

Histone H3K4 methylation has been linked to transcriptional activation. KDM5A (also known as RBP2 or JARID1A), a member of the KDM5 protein family, is an H3K4 demethylase, previously implicated in the regulation of transcription and differentiation. Here, we show that KDM5A is physically and functionally associated with two histone deacetylase complexes. Immunoaffinity purification of KDM5A confirmed a previously described association with the SIN3B-containing histone deacetylase complex and revealed an association with the nucleosome remodeling and deacetylase (NuRD) complex. Sucrose density gradient and sequential immunoprecipitation analyses further confirmed the stable association of KDM5A with these two histone deacetylase complexes. KDM5A depletion led to changes in the expression of hundreds of genes, two-thirds of which were also controlled by CHD4, the NuRD catalytic subunit. Gene ontology analysis confirmed that the genes commonly regulated by both KDM5A and CHD4 were categorized as developmentally regulated genes. ChIP analyses suggested that CHD4 modulates H3K4 methylation levels at the promoter and coding regions of target genes. We further demonstrated that the Caenorhabditis elegans homologues of KDM5 and CHD4 function in the same pathway during vulva development. These results suggest that KDM5A and the NuRD complex cooperatively function to control developmentally regulated genes.

Heterochromatin protein 1 homologue Swi6 acts in concert with Ers1 to regulate RNAi-directed heterochromatin assembly.

In fission yeast, the RNAi pathway is required for centromeric heterochromatin assembly. siRNAs derived from centromeric transcripts are incorporated into the RNA-induced transcriptional silencing (RITS) complex and direct it to nascent homologous transcripts. The RNA-induced transcriptional silencing-bound nascent transcripts further recruit the RNA-directed RNA polymerase complex (RDRC) to promote dsRNA synthesis and siRNA production. Heterochromatin coated with Swi6/Heterochromain Protein 1 is then formed following recruitment of chromatin modification machinery. Swi6 is also required for the upstream production of siRNA, although the mechanism for this has remained obscure. Here, we demonstrate that Swi6 recruits RDRC to heterochromatin through Ers1, an RNAi factor intermediate. An ers1(+) mutant allele (ers1-C62) was identified in a genetic screen for mutants that alleviate centromeric silencing, and this phenotype was suppressed by overexpression of either the Hrr1 RDRC subunit or Clr4 histone H3-K9 methyltransferase. Ers1 physically interacts with Hrr1, and loss of Ers1 impairs RDRC centromeric localization. Although Ers1 failed to bind Clr4, a direct interaction with Swi6 was detected, and centromeric localization of Swi6 was enhanced by Clr4 overexpression in ers1-C62 cells. Consistent with this, deletion of swi6(+) reduced centromeric localization of Ers1 and RDRC. Moreover, tethering of Ers1 or Hrr1 to centromeric heterochromatin partially bypassed Swi6 function. These findings demonstrate an alternative mechanism for RDRC recruitment and explain the essential role of Swi6/Heterochromain Protein 1 in RNAi-directed heterochromatin assembly.

C-terminus of the Sgf73 subunit of SAGA and SLIK is important for retention in the larger complex and for heterochromatin boundary function.

The budding yeast Saccharomyces cerevisiae contains active and inactive chromatin separated by boundary domains. Previously, we used genome-wide screening to identify 55 boundary-related genes. Here, we focus on Sgf73, a boundary protein that is a component of the Spt-Ada-Gcn5 acetyltransferase (SAGA) and SLIK (SAGA-like) complexes. These complexes have histone acetyltransferase (HAT) and histone deubiquitinase activity, and Sgf73 is one of the factors necessary to anchor the deubiquitination module. Domain analysis of Sgf73 was carried out, and the minimum region (373-402 aa) essential for boundary function was identified. This minimum region does not include the domain involved in anchoring the deubiquitination module, suggesting that the histone deubiquitinase activity of Sgf73 is not important for its boundary function. Next, Sgf73-mediated boundary function was analyzed in disruption strains in which different protein subunits of the SAGA/SLIK/ADA complexes were deleted. Deletion of ada2, ada3 or gcn5 (a HAT module component) caused complete loss of the boundary function of Sgf73. The importance of SAGA or SLIK complex binding to the boundary function of Sgf73 was also analyzed. Western blot analysis detected both the full-length and truncated forms of Spt7, suggesting that SAGA and SLIK complex formation is important for the boundary function of Sgf73.

Trimethylated lysine 9 of histone H3 is a mark for DNA methylation in Neurospora crassa.

Besides serving to package nuclear DNA, histones carry information in the form of a diverse array of post-translational modifications. Methylation of histones H3 and H4 has been implicated in long-term epigenetic 'memory'. Dimethylation or trimethylation of Lys4 of histone H3 (H3 Lys4) has been found in expressible euchromatin of yeasts and mammals. In contrast, methylation of Lys9 of histone H3 (H3 Lys9) has been implicated in establishing and maintaining the largely quiescent heterochromatin of mammals, yeasts, Drosophila melanogaster and plants. We have previously shown that a DNA methylation mutant of Neurospora crassa, dim-5 (defective in methylation), has a nonsense mutation in the SET domain of an H3-specific histone methyltransferase and that substitutions of H3 Lys9 cause gross hypomethylation of DNA. Similarly, the KRYPTONITE histone methyltransferase is required for full DNA methylation in Arabidopsis thaliana. We used biochemical, genetic and immunological methods to investigate the specific mark for DNA methylation in N. crassa. Here we show that trimethylated H3 Lys9, but not dimethylated H3 Lys9, marks chromatin regions for cytosine methylation and that DIM-5 specifically creates this mark.

Regulation of the SUV39H Family Methyltransferases: Insights from Fission Yeast.

Histones, which make up nucleosomes, undergo various post-translational modifications, such as acetylation, methylation, phosphorylation, and ubiquitylation. In particular, histone methylation serves different cellular functions depending on the location of the amino acid residue undergoing modification, and is tightly regulated by the antagonistic action of histone methyltransferases and demethylases. The SUV39H family of histone methyltransferases (HMTases) are evolutionarily conserved from fission yeast to humans and play an important role in the formation of higher-order chromatin structures called heterochromatin. The SUV39H family HMTases catalyzes the methylation of histone H3 lysine 9 (H3K9), and this modification serves as a binding site for heterochromatin protein 1 (HP1) to form a higher-order chromatin structure. While the regulatory mechanism of this family of enzymes has been extensively studied in various model organisms, Clr4, a fission yeast homologue, has made an important contribution. In this review, we focus on the regulatory mechanisms of the SUV39H family of proteins, in particular, the molecular mechanisms revealed by the studies of the fission yeast Clr4, and discuss their generality in comparison to other HMTases.

Cooperative DNA-binding activities of Chp2 are critical for its function in heterochromatin assembly.

Heterochromatin protein 1 (HP1) is an evolutionarily conserved protein that plays a critical role in heterochromatin assembly. HP1 proteins share a basic structure consisting of an N-terminal chromodomain (CD) and a C-terminal chromoshadow domain (CSD) linked by a disordered hinge region. The CD recognizes histone H3 lysine 9 methylation, a hallmark of heterochromatin, while the CSD forms a dimer to recruit other chromosomal proteins. HP1 proteins have been shown to bind DNA or RNA primarily through the hinge region. However, how DNA or RNA binding contributes to their function remains elusive. Here, we focus on Chp2, one of the two HP1 proteins in fission yeast, and investigate how Chp2's DNA-binding ability contributes to its function. Similar to other HP1 proteins, the Chp2 hinge exhibits clear DNA-binding activity. Interestingly, the Chp2 CSD also shows robust DNA-binding activity. Mutational analysis revealed that basic residues in the Chp2 hinge and at the N-terminus of the CSD are essential for DNA binding, and the combined amino acid substitutions of these residues alter Chp2 stability, impair Chp2 heterochromatin localization and lead to a silencing defect. These results demonstrate that the cooperative DNA-binding activities of Chp2 play an important role in heterochromatin assembly in fission yeast.