Role of histone H3 lysine 9 methylation in epigenetic control of heterochromatin assembly

Steps in assembly of silent chromatin in yeast: Sir3-independent binding of a Sir2/Sir4 complex to silencers and role for Sir2-dependent deacetylation

Transcriptional silencing at the budding yeast silent mating type (HM) loci and telomeric DNA regions requires Sir2, a conserved NAD-dependent histone deacetylase, Sir3, Sir4, histones H3 and H4, and several DNA-binding proteins. Silencing at the yeast ribosomal DNA (rDNA) repeats requires a complex containing Sir2, Net1, and Cdc14. Here we show that the native Sir2/Sir4 complex is composed solely of Sir2 and Sir4 and that native Sir3 is not associated with other proteins. We further show that the initial binding of the Sir2/Sir4 complex to DNA sites that nucleate silencing, accompanied by partial Sir2-dependent histone deacetylation, occurs independently of Sir3 and is likely to be the first step in assembly of silent chromatin at the HM loci and telomeres. The enzymatic activity of Sir2 is not required for this initial binding, but is required for the association of silencing proteins with regions distal from nucleation sites. At the rDNA repeats, we show that histone H3 and H4 tails are required for silencing and rDNA-associated H4 is hypoacetylated in a Sir2-dependent manner. However, the binding of Sir2 to rDNA is independent of its histone deacetylase activity. Together, these results support a stepwise model for the assembly of silent chromatin domains in Saccharomyces cerevisiae.

PR-Set7 is a nucleosome-specific methyltransferase that modifies lysine 20 of histone H4 and is associated with silent chromatin

We have purified a human histone H4 lysine 20 methyltransferase and cloned the encoding gene, PR/SET07. A mutation in Drosophila pr-set7 is lethal: second instar larval death coincides with the loss of H4 lysine 20 methylation, indicating a fundamental role for PR-Set7 in development. Transcriptionally competent regions lack H4 lysine 20 methylation, but the modification coincided with condensed chromosomal regions on polytene chromosomes, including chromocenter and euchromatic arms. The Drosophila male X chromosome, which is hyperacetylated at H4 lysine 16, has significantly decreased levels of lysine 20 methylation compared to that of females. In vitro, methylation of lysine 20 and acetylation of lysine 16 on the H4 tail are competitive. Taken together, these results support the hypothesis that methylation of H4 lysine 20 maintains silent chromatin, in part, by precluding neighboring acetylation on the H4 tail.

Chromatin as a eukaryotic template of genetic information

In eukaryotes, chromatin is essential for heredity. Chromatin architecture is sometimes "epistatic" over the DNA and imparts a different heritable state to the same DNA sequence or the same functional state to unrelated DNA sequences. This has been documented recently in a wide variety of studies focused on regulation of the yeast mating type, the function of Polycomb and trithorax group proteins, the specification of eukaryotic centromeres and neocentromeres, and genomic imprinting.

The identification of protein-protein interactions of the nuclear pore complex of Saccharomyces cerevisiae using high throughput matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry

Mass spectrometry has become the technology of choice for detailed identification of proteins in complex mixtures. Although electrophoretic separation, proteolytic digestion, mass spectrometric analysis of unseparated digests, and database searching have become standard methods in widespread use, peptide sequence information obtained by collision-induced dissociation and tandem mass spectrometry is required to establish the most comprehensive and reliable results. Most tandem mass spectrometers in current use employ electrospray ionization. In this work a novel tandem mass spectrometer employing matrix-assisted laser desorption ionization-time-of-flight/time-of-flight operating at 200 Hz has been used to identify proteins interacting with known nucleoporins in the nuclear pore complex of Saccharomyces cerevisiae. Proteins interacting with recombinant proteins as bait were purified from yeast extracts and then separated by one-dimensional SDS-PAGE. Although peptide mass fingerprinting is sometimes sufficient to identify proteins, this study shows the importance of employing tandem mass spectrometry for identifying proteins in mixtures or as covalently modified forms. The rules for incorporating these features into MS-Tag are presented. In addition to providing an evaluation of the sensitivity and overall quality of collision-induced dissociation spectra obtained, standard conditions for ionization and fragmentation have been selected that would allow automatic data collection and analysis, without the need to adjust parameters in a sample-specific fashion. Other considerations essential for successful high throughput protein analysis are discussed.

The fundamental role of epigenetic events in cancer

Patterns of DNA methylation and chromatin structure are profoundly altered in neoplasia and include genome-wide losses of, and regional gains in, DNA methylation. The recent explosion in our knowledge of how chromatin organization modulates gene transcription has further highlighted the importance of epigenetic mechanisms in the initiation and progression of human cancer. These epigenetic changes -- in particular, aberrant promoter hypermethylation that is associated with inappropriate gene silencing -- affect virtually every step in tumour progression. In this review, we discuss these epigenetic events and the molecular alterations that might cause them and/or underlie altered gene expression in cancer.

The many faces of histone lysine methylation

Diverse post-translational modifications of histone amino termini represent an important epigenetic mechanism for the organisation of chromatin structure and the regulation of gene activity. Within the past two years, great progress has been made in understanding the functional implications of histone methylation; in particular through the characterisation of histone methyltransferases that direct the site-specific methylation of, for example, lysine 9 and lysine 4 positions in the histone H3 amino terminus. All known histone methyltransferases of this type contain the evolutionarily conserved SET domain and appear to be able to stimulate either gene repression or gene activation. Methylation of H3 Lys9 and Lys4 has been visualised in native chromatin, indicating opposite roles in structuring repressive or accessible chromatin domains. For example, at the mating-type loci in Schizosaccharomyces pombe, at pericentric heterochromatin and at the inactive X chromosome in mammals, striking differences between these distinct marks have been observed. H3 Lys9 methylation is also important to direct additional epigenetic signals such as DNA methylation--for example, in Neurospora crassa and in Arabidopsis thaliana. Together, the available data strongly establish histone lysine methylation as a central modification for the epigenetic organisation of eukaryotic genomes.

The dMi-2 chromodomains are DNA binding modules important for ATP-dependent nucleosome mobilization

Drosophila Mi-2 (dMi-2) is the ATPase subunit of a complex combining ATP-dependent nucleosome remodelling and histone deacetylase activities. dMi-2 contains an HMG box-like region, two PHD fingers, two chromodomains and a SNF2-type ATPase domain. It is not known which of these domains contribute to nucleosome remodelling. We have tested a panel of dMi-2 deletion mutants in ATPase, nucleosome mobilization and nucleosome binding assays. Deletion of the chromodomains impairs all three activities. A dMi-2 mutant lacking the chromodomains is incorporated into a functional histone deacetylase complex in vivo but has lost nucleosome-stimulated ATPase activity. In contrast to dHP1, dMi-2 does not bind methylated histone H3 tails and does not require histone tails for nucleosome binding. Instead, the dMi-2 chromodomains display DNA binding activity that is not shared by other chromodomains. Our results suggest that the chromodomains act at an early step of the remodelling process to bind the nucleosome substrate predominantly via protein-DNA interactions. Furthermore, we identify DNA binding as a novel chromodomain-associated activity.

Beyond the ABCs of CKC and SCC. Do centromeres orchestrate sister chromatid cohesion or vice versa?

The centromere-kinetochore complex is a highly specialized chromatin domain that both mediates and monitors chromosome-spindle interactions responsible for accurate partitioning of sister chromatids to daughter cells. Centromeres are distinguished from adjacent chromatin by specific patterns of histone modification and the presence of a centromere-specific histone H3 variant (e.g. CENP-A). Centromere-proximal regions usually correspond to sites of avid and persistent sister chromatid cohesion mediated by the conserved cohesin complex. In budding yeast, there is a substantial body of evidence indicating centromeres direct formation and/or stabilization of centromere-proximal cohesion. In other organisms, the dependency of cohesion on centromere function is not as clear. Indeed, it appears that pericentromeric heterochromatin recruits cohesion proteins independent of centromere function. Nonetheless, aspects of centromere function are impaired in the absence of sister chromatid cohesion, suggesting the two are interdependent. Here we review the nature of centromeric chromatin, the dynamics and regulation of sister chromatid cohesion, and the relationship between the two.

Bop encodes a muscle-restricted protein containing MYND and SET domains and is essential for cardiac differentiation and morphogenesis

Many transcription factors regulate specific temporal-spatial events during cardiac differentiation; however, the mechanisms that regulate such events are largely unknown. Using a modified subtractive hybridization method to identify specific genes that influence early cardiac development, we found that Bop is expressed specifically in cardiac and skeletal muscle precursors before differentiation of these lineages. Bop encodes a protein containing MYND and SET domains, which have been shown to regulate transcription by mediating distinct chromatin modifications. We show that m-Bop is a histone deacetylase-dependent transcriptional repressor. Targeted deletion of Bop in mice disrupted maturation of ventricular cardiomyocytes and interfered with formation of the right ventricle. Normal expression of Hand2, a transcription factor essential for right ventricular development, in cardiomyocyte precursors is dependent upon m-Bop. These results indicate that m-Bop is essential for cardiomyocyte differentiation and cardiac morphogenesis.

Germline X chromosomes exhibit contrasting patterns of histone H3 methylation in Caenorhabditis elegans

In mammals, one of the two somatic X chromosomes in the female is inactivated, thereby equalizing X chromosome-derived transcription in the two sexes, a process known as dosage compensation. In the germline, however, the situation is quite different. Both X chromosomes are transcriptionally active during female oogenesis, whereas the X and Y chromosomes are transcriptionally silent during male spermatogenesis. Previous studies suggest that Caenorhabditis elegans germline X chromosomes might have different transcriptional activity in the two sexes in a manner similar to that in mammals. Using antibodies specific to H3 methylated at either lysine 4 or lysine 9, we show that the pattern of site-specific H3 methylation is different between X chromosomes and autosomes as well as between germline X chromosomes from the two sexes in C. elegans. We show that the pachytene germline X chromosomes in both sexes lack Me(K4)H3 when compared with autosomes, consistent with their being transcriptionally inactive. This transcriptional inactivity of germline X chromosomes is apparently transient in hermaphrodites because both X chromosomes stain brightly for Me(K4)H3 after germ nuclei exit pachytene. The male single X chromosome, on the other hand, remains devoid of Me(K4)H3 staining throughout the germline. Instead, the male germline X chromosome exhibits a high level of Me(K9)H3 that is not detected on any other chromosomes in either sex, consistent with stable silencing of this chromosome. Using mutants defective in the sex determination pathway, we show that X-chromosomal Me(K9)H3 staining is determined by the sexual phenotype, and not karyotype, of the animal. We detect a similar high level of Me(K9)H3 in male mouse XY bodies, suggesting an evolutionarily conserved mechanism for silencing the X chromosome specifically in the male germline.

Centromere domain organization and histone modifications

Centromere function requires the proper coordination of several subfunctions, such as kinetochore assembly, sister chromatid cohesion, binding of kinetochore microtubules, orientation of sister kinetochores to opposite spindle poles, and their movement towards the spindle poles. Centromere structure appears to be organized in different, separable domains in order to accomplish these functions. Despite the conserved nature of centromere functions, the molecular genetic definition of the DNA sequences that form a centromere in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, in the fruit fly Drosophila melanogaster, and in humans has revealed little conservation at the level of centromere DNA sequences. Also at the protein level few centromere proteins are conserved in all of these four organisms and many are unique to the different organisms. The recent analysis of the centromere structure in the yeast S. pombe by electron microscopy and detailed immunofluorescence microscopy of Drosophila centromeres have brought to light striking similarities at the overall structural level between these centromeres and the human centromere. The structural organization of the centromere is generally multilayered with a heterochromatin domain and a central core/inner plate region, which harbors the outer plate structures of the kinetochore. It is becoming increasingly clear that the key factors for assembly and function of the centromere structure are the specialized histones and modified histones which are present in the centromeric heterochromatin and in the chromatin of the central core. Thus, despite the differences in the DNA sequences and the proteins that define a centromere, there is an overall structural similarity between centromeres in evolutionarily diverse eukaryotes.

Regulation of transcription by H1 phosphorylation in Tetrahymena is position independent and requires clustered sites

In Tetrahymena cells, constitutive phosphorylation of histone H1 phenocopies the loss of H1 from chromatin. Regulation of transcription by H1 phosphorylation is achieved by altering the overall charges of a small domain. Here, we further explore the electrostatic properties of this domain and the mechanism by which it regulates transcription. We demonstrate that the regulatory effect of the clustered charges does not require any long-range interaction and is position independent. However, when the same number of charges was dispersed throughout the H1 molecule, the effect became undetectable. The results are explained by a nucleation-propagation model and provide in vivo evidence that the synergy of the clustered positive charges plays a role in histone function and gene regulation.

Isoform-specific interaction of HP1 with human TAFII130

The general transcription factor TFIID facilitates recruitment of the transcription machinery to gene promoters and regulates initiation of transcription by RNA polymerase II. hTAF(II)130, a component of TFIID, interacts with and serves as a coactivator for multiple transcriptional regulatory proteins, including Sp1 and CREB. A yeast two-hybrid screen has identified an interaction between hTAF(II)130 and heterochromatin protein 1 (HP1), a chromatin-associated protein whose function has been implicated in gene silencing. We find that hTAF(II)130 associates with HP1 in an isoform-specific manner: HP1alpha and HP1gamma bind to hTAF(II)130, but not HP1beta. In addition, we show that endogenous hTAF(II)130 and components of TFIID in HeLa nuclear extracts associate with glutathione S-transferase-HP1alpha and -HP1gamma. hTAF(II)130 possesses a pentapeptide HP1-binding motif, and mutation of the hTAF(II)130 HP1 box compromises the interaction of hTAF(II)130 with HP1. We demonstrate that Gal4-HP1 proteins interfere with hTAF(II)130-mediated activation of transcription. Our results suggest that HP1alpha and HP1gamma associate with hTAF(II)130 to mediate repression of transcription, supporting a new model of transcriptional repression involving a specific interaction between a component of TFIID and chromatin.

SETDB1: a novel KAP-1-associated histone H3, lysine 9-specific methyltransferase that contributes to HP1-mediated silencing of euchromatic genes by KRAB zinc-finger proteins

Posttranslational modification of histones has emerged as a key regulatory signal in eukaryotic gene expression. Recent genetic and biochemical studies link H3-lysine 9 (H3-K9) methylation to HP1-mediated heterochromatin formation and gene silencing. However, the mechanisms that target and coordinate these activities to specific genes is poorly understood. Here we report that the KAP-1 corepressor for the KRAB-ZFP superfamily of transcriptional silencers binds to SETDB1, a novel SET domain protein with histone H3-K9-specific methyltransferase activity. Although acetylation and phosphorylation of the H3 N-terminal tail profoundly affect the efficiency of H3-K9 methylation by SETDB1, we found that methylation of H3-K4 does not affect SETDB1-mediated methylation of H3-K9. In vitro methylation of the N-terminal tail of histone H3 by SETDB1 is sufficient to enhance the binding of HP1 proteins, which requires both an intact chromodomain and chromoshadow domain. Indirect immunofluoresence staining of interphase nuclei localized SETDB1 predominantly in euchromatic regions that overlap with HP1 staining in nonpericentromeric regions of chromatin. Moreover, KAP-1, SETDB1, H3-MeK9, and HP1 are enriched at promoter sequences of a euchromatic gene silenced by the KRAB-KAP-1 repression system. Thus, KAP-1 is a molecular scaffold that is targeted by KRAB-ZFPs to specific loci and coordinates both histone methylation and the deposition of HP1 proteins to silence gene expression.

Histone H3 lysine 4 methylation disrupts binding of nucleosome remodeling and deacetylase (NuRD) repressor complex

Histone N-terminal tails are post-translationally modified in many ways. At lysine residues, histones can be either acetylated or methylated. Both modifications lead to the binding of specific proteins; bromodomain proteins, such as GCN5, bind acetyl lysines and the chromodomain protein, HP1, binds methyl lysine 9 of histone H3. Here we show that the previously characterized transcriptional repressor complex NuRD (nucleosome remodeling and deacetylase) binds to the histone H3 N-terminal tail and that methylation at lysine 4, but not lysine 9, prevents binding. Given that lysine 4 methylation is found at sites of active transcription, these results suggest that a function of lysine 4 methylation is to disrupt the association of histones with a repressor complex.

Control of CpNpG DNA methylation by the KRYPTONITE histone H3 methyltransferase

Gene silencing in eukaryotes is associated with the formation of heterochromatin, a complex of proteins and DNA that block transcription. Heterochromatin is characterized by the methylation of cytosine nucleotides of the DNA, the methylation of histone H3 at lysine 9 (H3 Lys 9), and the specific binding of heterochromatin protein 1 (HP1) to methylated H3 Lys 9 (refs 1-7). Although the relationship between these chromatin modifications is generally unknown, in the fungus Neurospora crassa, DNA methylation acts genetically downstream of H3 Lys 9 methylation. Here we report the isolation of KRYPTONITE, a methyltransferase gene specific to H3 Lys 9, identified in a mutant screen for suppressors of gene silencing at the Arabidopsis thaliana SUPERMAN (SUP) locus. Loss-of-function kryptonite alleles resemble mutants in the DNA methyltransferase gene CHROMOMETHYLASE3 (CMT3), showing loss of cytosine methylation at sites of CpNpG trinucleotides (where N is A, C, G or T) and reactivation of endogenous retrotransposon sequences. We show that CMT3 interacts with an Arabidopsis homologue of HP1, which in turn interacts with methylated histones. These data suggest that CpNpG DNA methylation is controlled by histone H3 Lys 9 methylation, through interaction of CMT3 with methylated chromatin.

Transcriptional silencing in Saccharomyces cerevisiae and Schizosaccharomyces pombe

Transcriptional silencing is a heritable form of gene inactivation that involves the assembly of large regions of DNA into a specialized chromatin structure that inhibits transcription. This phenomenon is responsible for inhibiting transcription at silent mating-type loci, telomeres and rDNA repeats in both budding yeast Saccharomyces cerevisiae and fission yeast Schizosaccharomyces pombe, as well as at centromeres in fission yeast. Although transcriptional silencing in both S.cerevisiae and S.pombe involves modification of chromatin, no apparent amino acid sequence similarities have been reported between the proteins involved in establishment and maintenance of silent chromatin in these two distantly related yeasts. Silencing in S.cerevisiae is mediated by Sir2p-containing complexes, whereas silencing in S.pombe is mediated primarily by Swi6-containing complexes. The Swi6 complexes of S.pombe contain proteins closely related to their counterparts in higher eukaryotes, but have no apparent orthologs in S.cerevisiae. Silencing proteins from both yeasts are also actively involved in other chromosome-related nuclear functions, including DNA repair and the regulation of chromatin structure.

Functional divergence between histone deacetylases in fission yeast by distinct cellular localization and in vivo specificity

Histone deacetylases (HDACs) are important for gene regulation and the maintenance of heterochromatin in eukaryotes. Schizosaccharomyces pombe was used as a model system to investigate the functional divergence within this conserved enzyme family. S. pombe has three HDACs encoded by the hda1(+), clr3(+), and clr6(+) genes. Strains mutated in these genes have previously been shown to display strikingly different phenotypes when assayed for viability, chromosome loss, and silencing. Here, conserved differences in the substrate binding pocket identify Clr6 and Hda1 as class I HDACs, while Clr3 belongs in the class II family. Furthermore, these HDACs were shown to have strikingly different subcellular localization patterns. Hda1 was localized to the cytoplasm, while most of Clr3 resided throughout the nucleus. Finally, Clr6 was localized exclusively on the chromosomes in a spotted pattern. Interestingly, Clr3, the only HDAC present in the nucleolus, was required for ribosomal DNA (rDNA) silencing. Clr3 presumably acts directly on heterochromatin, since it colocalized with the centromere, mating-type region, and rDNA as visualized by in situ hybridization. In addition, Clr3 could be cross-linked to mat3 in chromatin immunoprecipitation experiments. Western analysis of bulk histone preparations indicated that Hda1 (class I) had a generally low level of activity in vivo and Clr6 (class I) had a high level of activity and broad in vivo substrate specificity, whereas Clr3 (class II) displayed its main activity on acetylated lysine 14 of histone H3. Thus, the distinct functions of the S. pombe HDACs are likely explained by their distinct cellular localization and their different in vivo specificities.

Braking the silence: how heterochromatic gene repression is stopped in its tracks

Eukaryotic DNA is assembled into nucleosomes, which are further packaged into higher order chromatin structures containing many non-histone chromosomal proteins. The details of this packaging have profound effects on gene expression and other cellular processes involving the genetic material. Heterochromatic domains of the genome are usually transcriptionally repressed, while euchromatic regions are transcriptionally competent. Current models of gene activation postulate the existence of boundary elements that either prevent inappropriate activation of genes by distal enhancers (enhancer blockers), or sequences that block the propagation of heterochromatin into euchromatic regions (barriers). While numerous boundary sequences have been identified, little is known with regard to the molecular mechanisms used to punctuate the genome. This review will focus on recent data that provide insight into the mode of action of barrier elements.

Sequence relationships, conserved domains, and expression patterns for maize homologs of the polycomb group genes E(z), esc, and E(Pc)

Polycomb group (PcG) proteins play an important role in developmental and epigenetic regulation of gene expression in fruit fly (Drosophila melanogaster) and mammals. Recent evidence has shown that Arabidopsis homologs of PcG proteins are also important for the regulation of plant development. The objective of this study was to characterize the PcG homologs in maize (Zea mays). The 11 cloned PcG proteins from fruit fly and the Enhancer of zeste [E(z)], extra sex combs (esc), and Enhancer of Polycomb [E(Pc)] homologs from Arabidopsis were used as queries to perform TBLASTN searches against the public maize expressed sequence tag database and the Pioneer Hi-Bred database. Maize homologs were found for E(z), esc, and E(Pc), but not for Polycomb, pleiohomeotic, Posterior sex combs, Polycomblike, Additional sex combs, Sex combs on midleg, polyhometoic, or multi sex combs. Transcripts of the three maize Enhancer of zeste-like genes, Mez1, Mez2, and Mez3, were detected in all tissues tested, and the Mez2 transcript is alternatively spliced in a tissue-dependent pattern. Zea mays fertilization independent endosperm1 (ZmFie1) expression was limited to developing embryos and endosperms, whereas ZmFie2 expression was found throughout plant development. The conservation of E(z) and esc homologs across kingdoms indicates that these genes likely play a conserved role in repressing gene expression.

ChIPs of the beta-globin locus: unraveling gene regulation within an active domain

Recent studies of beta-globin gene expression have concentrated on the analysis of factor binding and chromatin structure within the endogenous locus. These studies have more precisely defined the extent and nature of the active chromosomal domain and the elements that organize it. Surprisingly, the beta-globin locus control region (LCR), although critical for high-level gene expression, plays little role in the overall architecture of the active locus. Analysis of the effects of targeted deletion of the beta-globin LCR, along with emerging knowledge of the behavior of the erythroid transcription factor NF-E2, leads to a new perspective on factor binding and LCR function.

Histone methylation in transcriptional control

Over the past year or so, methylation of histones has come to be recognised as a major player in the regulation of gene activity. This notion follows the discovery of lysine and arginine methyltransferases and proteins that recognise the methyl-lysine 'mark' on histones. Methylated histones have been implicated in heterochromatic repression, promoter regulation and the propagation of a repressed state via DNA methylation.

Breaking through to the other side: silencers and barriers

The establishment and restriction of transcriptionally inactive regions in the nucleus is mediated by silencer and barrier elements. Silencer-bound proteins recruit additional factors to establish the silenced domain during the S-phase of the cell cycle but, contrary to previous models, DNA replication is not a pre-requisite for the establishment. Characteristically, silenced domains contain hypoacetylated histones and recent data have identified residue-specific methylation of histone H3 as an additional signature that distinguishes active regions from inactive ones. Peaks of acetylated histones demarcate the boundaries between these regions and recruitment of HAT activities provides a mechanism to restrict the spread of heterochromatin.

Heterochromatin: new possibilities for the inheritance of structure

Significant portions of the eukaryotic genome are heterochromatic, made up largely of repetitious sequences and possessing a distinctive chromatin structure associated with gene silencing. New insights into the form of packaging, the associated histone modifications, and the associated nonhistone chromosomal proteins of heterochromatin have suggested a mechanism for providing an epigenetic mark that allows this distinctive chromatin structure to be maintained following replication and to spread within a given domain.

Histone modifications in transcriptional regulation

Covalent modifications of the amino termini of the core histones in nucleosomes have important roles in gene regulation. Research in the past two years reveals these modifications to consist of phosphorylation, methylation and ubiquitination, in addition to the better-characterized acetylation. This multiplicity of modifications, and their occurrence in patterns and dependent sequences, argues persuasively for the existence of a histone code.

Structure of HP1 chromodomain bound to a lysine 9-methylated histone H3 tail

The chromodomain of the HP1 family of proteins recognizes histone tails with specifically methylated lysines. Here, we present structural, energetic, and mutational analyses of the complex between the Drosophila HP1 chromodomain and the histone H3 tail with a methyllysine at residue 9, a modification associated with epigenetic silencing. The histone tail inserts as a beta strand, completing the beta-sandwich architecture of the chromodomain. The methylammonium group is caged by three aromatic side chains, whereas adjacent residues form discerning contacts with one face of the chromodomain. Comparison of dimethyl- and trimethyllysine-containing complexes suggests a role for cation-pi and van der Waals interactions, with trimethylation slightly improving the binding affinity.

Structure of the HP1 chromodomain bound to histone H3 methylated at lysine 9

Specific modifications to histones are essential epigenetic markers---heritable changes in gene expression that do not affect the DNA sequence. Methylation of lysine 9 in histone H3 is recognized by heterochromatin protein 1 (HP1), which directs the binding of other proteins to control chromatin structure and gene expression. Here we show that HP1 uses an induced-fit mechanism for recognition of this modification, as revealed by the structure of its chromodomain bound to a histone H3 peptide dimethylated at Nzeta of lysine 9. The binding pocket for the N-methyl groups is provided by three aromatic side chains, Tyr21, Trp42 and Phe45, which reside in two regions that become ordered on binding of the peptide. The side chain of Lys9 is almost fully extended and surrounded by residues that are conserved in many other chromodomains. The QTAR peptide sequence preceding Lys9 makes most of the additional interactions with the chromodomain, with HP1 residues Val23, Leu40, Trp42, Leu58 and Cys60 appearing to be a major determinant of specificity by binding the key buried Ala7. These findings predict which other chromodomains will bind methylated proteins and suggest a motif that they recognize.

Higher-order structure in pericentric heterochromatin involves a distinct pattern of histone modification and an RNA component

Post-translational modification of histone tails is thought to modulate higher-order chromatin structure. Combinations of modifications including acetylation, phosphorylation and methylation have been proposed to provide marks recognized by specific proteins. This is exemplified, in both mammalian cells and fission yeast, by transcriptionally silent constitutive pericentric heterochromatin. Such heterochromatin contains histones that are generally hypoacetylated and methylated by Suv39h methyltransferases at lysine 9 of histone H3 (H3-K9). Each of these modification states has been implicated in the maintenance of HP1 protein-binding at pericentric heterochromatin, in transcriptional silencing and in centromere function. In particular, H3-K9 methylation is thought to provide a marking system for the establishment and maintenance of stably repressed regions and heterochromatin subdomains. To address the question of how these two types of modifications, as well as other unidentified parameters, function to maintain pericentric heterochromatin, we used a combination of histone deacetylase inhibitors, RNAse treatments and an antibody raised against methylated branched H3-K9 peptides. Our results show that both H3-K9 acetylation and methylation can occur on independent sets of H3 molecules in pericentric heterochromatin. In addition, we identify an RNA- and histone modification-dependent structure that brings methylated H3-K9 tails together in a specific configuration required for the accumulation of HP1 proteins in these domains.

Set2 is a nucleosomal histone H3-selective methyltransferase that mediates transcriptional repression

Recent studies of histone methylation have yielded fundamental new insights pertaining to the role of this modification in gene activation as well as in gene silencing. While a number of methylation sites are known to occur on histones, only limited information exists regarding the relevant enzymes that mediate these methylation events. We thus sought to identify native histone methyltransferase (HMT) activities from Saccharomyces cerevisiae. Here, we describe the biochemical purification and characterization of Set2, a novel HMT that is site-specific for lysine 36 (Lys36) of the H3 tail. Using an antiserum directed against Lys36 methylation in H3, we show that Set2, via its SET domain, is responsible for methylation at this site in vivo. Tethering of Set2 to a heterologous promoter reveals that Set2 represses transcription, and part of this repression is mediated through the HMT activity of the SET domain. These results suggest that Set2 and methylation at H3 Lys36 play a role in the repression of gene transcription.

The PWWP domain of mammalian DNA methyltransferase Dnmt3b defines a new family of DNA-binding folds

The PWWP domain is a weakly conserved sequence motif found in > 60 eukaryotic proteins, including the mammalian DNA methyltransferases Dnmt3a and Dnmt3b. These proteins often contain other chromatin-association domains. A 135-residue PWWP domain from mouse Dnmt3b (amino acids 223--357) has been structurally characterized at 1.8 A resolution. The N-terminal half of this domain resembles a barrel-like five-stranded structure, whereas the C-terminal half contains a five-helix bundle. The two halves are packed against each other to form a single structural module that exhibits a prominent positive electrostatic potential. The PWWP domain alone binds DNA in vitro, probably through its basic surface. We also show that recombinant Dnmt3b2 protein (a splice variant of Dnmt3b) and two N-terminal deletion mutants (Delta218 and Delta369) have approximately equal methyl transfer activity on unmethylated and hemimethylated CpG-containing oligonucleotides. The Delta218 protein, which includes the PWWP domain, binds DNA more strongly than Delta369, which lacks the PWWP domain.

The roX genes encode redundant male-specific lethal transcripts required for targeting of the MSL complex

The roX1 and roX2 genes of Drosophila produce male-specific non-coding RNAs that co-localize with the Male-Specific Lethal (MSL) protein complex. This complex mediates up-regulation of the male X chromosome by increasing histone H4 acetylation, thus contributing to the equalization of X-linked gene expression between the sexes. Both roX genes overlap two of approximately 35 chromatin entry sites, DNA sequences proposed to act in cis to direct the MSL complex to the X chromosome. Although dosage compensation is essential in males, an intact roX1 gene is not required by either sex. We have generated flies lacking roX2 and find that this gene is also non-essential. However, simultaneous removal of both roX RNAs causes a striking male-specific reduction in viability accompanied by relocation of the MSL proteins and acetylated histone H4 from the X chromosome to autosomal sites and heterochromatin. Males can be rescued by roX cDNAs from autosomal transgenes, demonstrating the genetic separation of the chromatin entry and RNA-encoding functions. Therefore, the roX1 and roX2 genes produce redundant, male-specific lethal transcripts required for targeting the MSL complex.

Conserved organization of centromeric chromatin in flies and humans

Recent studies have highlighted the importance of centromere-specific histone H3-like (CENP-A) proteins in centromere function. We show that Drosophila CID and human CENP-A appear at metaphase as a three-dimensional structure that lacks histone H3. However, blocks of CID/CENP-A and H3 nucleosomes are linearly interspersed on extended chromatin fibers, and CID is close to H3 nucleosomes in polynucleosomal preparations. When CID is depleted by RNAi, it is replaced by H3, demonstrating flexibility of centromeric chromatin organization. Finally, contrary to models proposing that H3 and CID/CENP-A nucleosomes are replicated at different times in S phase, we show that interspersed H3 and CID/CENP-A chromatin are replicated concurrently during S phase in humans and flies. We propose that the unique structural arrangement of CID/CENP-A and H3 nucleosomes presents centromeric chromatin to the poleward face of the condensing mitotic chromosome.

A second catalytic domain in the Elp3 histone acetyltransferases: a candidate for histone demethylase activity?

A new subfamily of two-domain histone acetyltransferases (HATs) related to Elp3 has been identified. In addition to a HAT domain in the C terminus, these proteins have an N-terminal domain similar to the catalytic domain of S-adenosylmethionine radical enzymes. Two-domain organization is preserved in evolution, suggesting that both enzymatic activities are functionally or mechanistically coupled and directed towards highly conserved substrates. The functional implications of this similarity and a possible role for Elp3-related proteins as histone demethylases are discussed.

Epigenetic codes for heterochromatin formation and silencing: rounding up the usual suspects

Recent results from diverse organisms point to a self-reinforcing network of interactions among the three best-characterized covalent modifications that mark heterochromatin: histone hypoacetylation, histone H3-Lys9 methylation, and cytosine methylation. These modification systems suggest a mechanistic basis for spreading of heterochromatin over large domains and for stable epigenetic inheritance of the silent state. All three modifications used in packaging heterochromatin are also used in stable silencing of euchromatic genes.

SET-domain proteins of the Su(var)3-9, E(z) and trithorax families

SET-domain (SET: Su(var)3-9, E(z) and Trithorax)-containing proteins were collected through sequence searches of the available databases. After removing redundancies, the proteins belonging to three families, SU(VAR)3-9, E(Z) and Trithorax, were selected. Analysis of the relationship between the different members is based on pairwise alignment, compilation, and comparison of their SET-domains. The level of homology of the SET-domains defined the distribution of the proteins into families and into clades within the families. The architecture of the entire protein supported the distribution pattern built upon SET-domain similarity. Parallel cladistic and protein-architecture analyses outlined two plausible criteria for predicting function.

Set9, a novel histone H3 methyltransferase that facilitates transcription by precluding histone tail modifications required for heterochromatin formation

A novel histone methyltransferase, termed Set9, was isolated from human cells. Set9 contains a SET domain, but lacks the pre- and post-SET domains. Set9 methylates specifically lysine 4 (K4) of histone H3 (H3-K4) and potentiates transcription activation. The histone H3 tail interacts specifically with the histone deacetylase NuRD complex. Methylation of histone H3-K4 by Set9 precludes the association of NuRD with the H3 tail. Moreover, methylation of H3-K4 impairs Suv39h1-mediated methylation at K9 of H3 (H3-K9). The interplay between the Set9 and Suv39h1 histone methyltransferases is specific, as the methylation of H3-K9 by the histone methyltransferase G9a was not affected by Set9 methylation of H3-K4. Our studies suggest that Set9-mediated methylation of H3-K4 functions in transcription activation by competing with histone deacetylases and by precluding H3-K9 methylation by Suv39h1. Our results suggest that the methylation of histone tails can have distinct effects on transcription, depending on its chromosomal location, the combination of posttranslational modifications, and the enzyme (or protein complex) involved in the particular modification.

Histone-dependent association of Tup1-Ssn6 with repressed genes in vivo

The Tup1-Ssn6 complex regulates diverse classes of genes in Saccharomyces cerevisiae and serves as a model for corepressor functions in many organisms. Tup1-Ssn6 does not directly bind DNA but is brought to target genes through interactions with sequence-specific DNA binding factors. Full repression by Tup1-Ssn6 appears to require interactions with both the histone tails and components of the general transcription machinery, although the relative contribution of these two pathways is not clear. Here, we map Tup1 locations on two classes of Tup1-Ssn6-regulated genes in vivo via chromatin immunoprecipitations. Distinct profiles of Tup1 are observed on a cell-specific genes and DNA damage-inducible genes, suggesting that alternate repressive architectures may be created on different classes of repressed genes. In both cases, decreases in acetylation of histone H3 colocalize with Tup1. Strikingly, although loss of the Srb10 mediator protein had no effect on Tup1 localization, both histone tail mutations and histone deacetylase mutations crippled the association of Tup1 with target loci. Together with previous findings that Tup1-Ssn6 physically associates with histone deacetylase activities, these results indicate that the repressor complex alters histone modification states to facilitate interactions with histones and that these interactions are required to maintain a stable repressive state.

A long terminal repeat retrotransposon of fission yeast has strong preferences for specific sites of insertion

The successful dispersal of transposons depends on the critical balance between the fitness of the host and the ability of the transposon to insert into the host genome. One method transposons may use to avoid the disruption of coding sequences is to target integration into safe havens. We explored the interaction between the long terminal repeat retrotransposon Tf1 and the genome of the yeast Schizosaccharomyces pombe. Using techniques that were specifically designed to detect integration of Tf1 throughout the genome and to avoid bias in this detection, we generated 51 insertion events. Although 60.2% of the genome of S. pombe is coding sequence, all but one of the insertions occurred in intergenic regions. We also found that Tf1 was significantly more likely to insert into intergenic regions that included polymerase II promoters than into regions between convergent gene pairs. Interestingly, 8 of the 51 insertion sites were isolated multiple times from genetically independent cultures. This result suggests that specific sites in intergenic regions are targeted by Tf1. Perhaps the most surprising observation was that per kilobase of nonrepetitive sequence, Tf1 was significantly more likely to insert into chromosome 3 than into one of the other two chromosomes. This preference was found not to be due to differences in the distribution or composition of intergenic sequences within the three chromosomes.

Global regulation of post-translational modifications on core histones

Full-length masses of histones were analyzed by mass spectrometry to characterize post-translational modifications of bulk histones and their changes induced by cell stimulation. By matching masses of unique peptides with full-length masses, H4 and the variants H2A.1, H2B.1, and H3.1 were identified as the main histone forms in K562 cells. Mass changes caused by covalent modifications were measured in a dose- and time-dependent manner following inhibition of phosphatases by okadaic acid. Histones H2A, H3, and H4 underwent changes in mass consistent with altered acetylation and phosphorylation, whereas H2B mass was largely unchanged. Unexpectedly, histone H4 became almost completely deacetylated in a dose-dependent manner that occurred independently of phosphorylation. Okadaic acid also partially blocked H4 hyperacetylation induced by trichostatin-A, suggesting that the mechanism of deacetylation involves inhibition of H4 acetyltransferase activity, following perturbation of cellular phosphatases. In addition, mass changes in H3 in response to okadaic acid were consistent with phosphorylation of methylated, acetylated, and phosphorylated forms. Finally, kinetic differences were observed with respect to the rate of phosphorylation of H2A versus H4, suggesting differential regulation of phosphorylation at sites on these proteins, which are highly related by sequence. These results provide novel evidence that global covalent modifications of chromatin-bound histones are regulated through phosphorylation-dependent mechanisms.

Evidence that Set1, a factor required for methylation of histone H3, regulates rDNA silencing in S. cerevisiae by a Sir2-independent mechanism

Several types of histone modifications have been shown to control transcription. Recent evidence suggests that specific combinations of these modifications determine particular transcription patterns. The histone modifications most recently shown to play critical roles in transcription are arginine-specific and lysine-specific methylation. Lysine-specific histone methyltransferases all contain a SET domain, a conserved 130 amino acid motif originally identified in polycomb- and trithorax-group proteins from Drosophila. Members of the SU(VAR)3-9 family of SET-domain proteins methylate K9 of histone H3. Methylation of H3 has also been shown to occur at K4. Several studies have suggested a correlation between K4-methylated H3 and active transcription. In this paper, we provide evidence that K4-methylated H3 is required in a negative role, rDNA silencing in Saccharomyces cerevisiae. In a screen for rDNA silencing mutants, we identified a mutation in SET1, previously shown to regulate silencing at telomeres and HML. Recent work has shown that Set1 is a member of a complex and is required for methylation of K4 of H3 at several genomic locations. In addition, we demonstrate that a K4R change in H3, which prevents K4 methylation, impairs rDNA silencing, indicating that Set1 regulates rDNA silencing, directly or indirectly, via H3 methylation. Furthermore, we present several lines of evidence that the role of Set1 in rDNA silencing is distinct from that of the histone deacetylase Sir2. Together, these results suggest that Set1-dependent H3 methylation is required for rDNA silencing in a Sir2-independent fashion.

Functional and physical interaction between the histone methyl transferase Suv39H1 and histone deacetylases

The histone methyl transferase Suv39H1 is involved in silencing by pericentric heterochromatin. It specifically methylates K9 of histone H3, thereby creating a high affinity binding site for HP1 proteins. We and others have shown recently that it is also involved in transcriptional repression by the retinoblastoma protein Rb. Strikingly, both HP1 localisation and repression by Rb also require, at least in part, histone deacetylases. We found here that repression of a heterologous promoter by Suv39H1 is dependent on histone deacetylase activity. However, the enzymatic activity of Suv39H1 is not required, since the N-terminal part is by itself a transcriptional repression domain. Coimmunoprecipitation experiments indicated that Suv39H1 can physically interact with HDAC1, -2 and -3, therefore suggesting that transcriptional repression by Suv39H1 could be the consequence of histone deacetylases recruitment. Consistent with this interpretation, the N-terminal transcriptional repression domain of Suv39H1 bound the so-called 'core histone deacetylase complex', composed of HDAC1, HDAC2 and the Rb-associated proteins RbAp48 and RbAp46. Taken together, our results suggest that a complex containing both the Suv39H1 histone methyl transferase and histone deacetylases could be involved in heterochromatin silencing or transcriptional repression by Rb.

Histone H3 lysine 9 methylation is an epigenetic imprint of facultative heterochromatin

Post-translational modifications of histone amino termini are an important regulatory mechanism that induce transitions in chromatin structure, thereby contributing to epigenetic gene control and the assembly of specialized chromosomal subdomains. Methylation of histone H3 at lysine 9 (H3-Lys9) by site-specific histone methyltransferases (Suv39h HMTases) marks constitutive heterochromatin. Here, we show that H3-Lys9 methylation also occurs in facultative heterochromatin of the inactive X chromosome (Xi) in female mammals. H3-Lys9 methylation is retained through mitosis, indicating that it might provide an epigenetic imprint for the maintenance of the inactive state. Disruption of the two mouse Suv39h HMTases abolishes H3-Lys9 methylation of constitutive heterochromatin but not that of the Xi. In addition, HP1 proteins, which normally associate with heterochromatin, do not accumulate with the Xi. These observations suggest the existence of an Suv39h-HP1-independent pathway regulating H3-Lys9 methylation of facultative heterochromatin.

Fission yeast F-box protein Pof3 is required for genome integrity and telomere function

The Skp1-Cullin-1/Cdc53-F-box protein (SCF) ubiquitin ligase plays an important role in various biological processes. In this enzyme complex, a variety of F-box proteins act as receptors that recruit substrates. We have identified a fission yeast gene encoding a novel F-box protein Pof3, which contains, in addition to the F-box, a tetratricopeptide repeat motif in its N terminus and a leucine-rich-repeat motif in the C terminus, two ubiquitous protein-protein interaction domains. Pof3 forms a complex with Skp1 and Pcu1 (fission yeast cullin-1), suggesting that Pof3 functions as an adaptor for specific substrates. In the absence of Pof3, cells exhibit a number of phenotypes reminiscent of genome integrity defects. These include G2 cell cycle delay, hypersensitivity to UV, appearance of lagging chromosomes, and a high rate of chromosome loss. pof3 deletion strains are viable because the DNA damage checkpoint is continuously activated in the mutant, and this leads to G2 cell cycle delay, thereby preventing the mutant from committing lethal mitosis. Pof3 localizes to the nucleus during the cell cycle. Molecular analysis reveals that in this mutant the telomere is substantially shortened and furthermore transcriptional silencing at the telomere is alleviated. The results highlight a role of the SCF(Pof3) ubiquitin ligase in genome integrity via maintaining chromatin structures.

Differentially methylated forms of histone H3 show unique association patterns with inactive human X chromosomes

Studies of histone methylation have shown that H3 can be methylated at lysine 4 (Lys4) or lysine 9 (Lys9). Whereas H3-Lys4 methylation has been correlated with active gene expression, H3-Lys9 methylation has been linked to gene silencing and assembly of heterochromatin in mouse and Schizosaccharomyces pombe. The chromodomain of mouse HP1 (and Swi6 in S. pombe) binds H3 methylated at Lys9, and methylation at this site is thought to mark and promote heterochromatin assembly. We have used a well-studied model of mammalian epigenetic silencing, the human inactive X chromosome, to show that enrichment for H3 methylated at Lys9 is also a distinguishing mark of facultative heterochromatin. In contrast, H3 methylated at Lys4 is depleted in the inactive X chromosome, except in three 'hot spots' of enrichment along its length. Chromatin immunoprecipitation analyses further show that Lys9 methylation is associated with promoters of inactive genes, whereas Lys4 methylation is associated with active genes on the X chromosome. These data demonstrate that differential methylation at two distinct sites of the H3 amino terminus correlates with contrasting gene activities and may be part of a 'histone code' involved in establishing and maintaining facultative heterochromatin.