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Query: FACT and other histone chaperone(s) compensate for Histone 2A (H2A)-histone 2B (H2B) dimer eviction during the histone exchange process.
FACT, a heterodimer of Spt16 and Pob3, is an essential histone chaperone. We show that the H2A-H2B binding activity that is central to FACT function resides in short acidic regions near the C termini of each subunit. Mutations throughout these regions affect binding and cause correlated phenotypes that range from mild to lethal, with the largest individual contributions unexpectedly coming from an aromatic residue and a nearby carboxylate residue within each domain. Spt16 and Pob3 bind overlapping sites on H2A-H2B, and Spt16-Pob3 heterodimers simultaneously bind two H2A-H2B dimers, the same stoichiometry as the components of a nucleosome. An Spt16:H2A-H2B crystal structure explains the biochemical and genetic data, provides a model for Pob3 binding, and implies a mechanism for FACT reorganization that we confirm biochemically. Moreover, unexpected similarity to binding of ANP32E and Swr1 with H2A.Z-H2B reveals that diverse H2A-H2B chaperones use common mechanisms of histone binding and regulating nucleosome functions.
Activation of transcription within chromatin has been correlated with the incorporation of the essential histone variant H2A.Z into nucleosomes. H2A.Z and other histone variants may establish structurally distinct chromosomal domains; however, the molecular mechanism by which they function is largely unknown. Here we report the 2.6 Å crystal structure of a nucleosome core particle containing the histone variant H2A.Z. The overall structure is similar to that of the previously reported 2.8 Å nucleosome structure containing major histone proteins. However, distinct localized changes result in the subtle destabilization of the interaction between the (H2A.Z–H2B) dimer and the (H3–H4)2 tetramer. Moreover, H2A.Z nucleosomes have an altered surface that includes a metal ion. This altered surface may lead to changes in higher order structure, and/or could result in the association of specific nuclear proteins with H2A.Z. Finally, incorporation of H2A.Z and H2A within the same nucleosome is unlikely, due to significant changes in the interface between the two H2A.Z–H2B dimers.
Histone levels are tightly regulated to prevent harmful effects such as genomic instability and hypersensitivity to DNA-damaging agents due to the accumulation of these highly basic proteins when DNA replication slows down or stops. Although chromosomal histones are stable, excess (non-chromatin bound) histones are rapidly degraded in a Rad53 (radiation sensitive 53) kinase-dependent manner in Saccharomyces cerevisiae. Here we demonstrate that excess histones associate with Rad53 in vivo and seem to undergo modifications such as tyrosine phosphorylation and polyubiquitylation, before their proteolysis by the proteasome. We have identified the Tyr 99 residue of histone H3 as being critical for the efficient ubiquitylation and degradation of this histone. We have also identified the ubiquitin conjugating enzymes (E2) Ubc4 and Ubc5, as well as the ubiquitin ligase (E3) Tom1 (temperature dependent organization in mitotic nucleus 1), as enzymes involved in the ubiquitylation of excess histones. Regulated histone proteolysis has major implications for the maintenance of epigenetic marks on chromatin, genomic stability and the packaging of sperm DNA.
In S. cerevisiae, histone variant H2A.Z is deposited in euchromatin at the flanks of silent heterochromatin to prevent its ectopic spread. We show that H2A.Z nucleosomes are found at promoter regions of nearly all genes in euchromatin. They generally occur as two positioned nucleosomes that flank a nucleosome-free region (NFR) that contains the transcription start site. Astonishingly, enrichment at 5' ends is observed not only at actively transcribed genes but also at inactive loci. Mutagenesis of a typical promoter revealed a 22 bp segment of DNA sufficient to program formation of a NFR flanked by two H2A.Z nucleosomes. This segment contains a binding site of the Myb-related protein Reb1 and an adjacent dT:dA tract. Efficient deposition of H2A.Z is further promoted by a specific pattern of histone H3 and H4 tail acetylation and the bromodomain protein Bdf1, a component of the Swr1 remodeling complex that deposits H2A.Z.
Boundary elements hinder the spread of heterochromatin, yet these sites do not fully account for the preservation of adjacent euchromatin. Histone variant H2A.Z (Htz1 in yeast) replaces conventional H2A in many nucleosomes. Microarray analysis revealed that HTZ1-activated genes cluster near telomeres. The reduced expression of most of these genes in htz1Delta cells was reversed by the deletion of SIR2 (sir2Delta) suggesting that H2A.Z antagonizes telomeric silencing. Other Htz1-activated genes flank the silent HMR mating-type locus. Their requirement for Htz1 can be bypassed by sir2Delta or by a deletion encompassing the silencing nucleation sites in HMR. In htz1Delta cells, Sir2 and Sir3 spread into flanking euchromatic regions, producing changes in histone H4 acetylation and H3 4-methylation indicative of ectopic heterochromatin formation. Htz1 is enriched in these euchromatic regions and acts synergistically with a boundary element to prevent the spread of heterochromatin. Thus, euchromatin and heterochromatin each contains components that antagonize switching to the opposite chromatin state.
Histone variants within the H2A family show high divergences in their C-terminal regions. In this work, we have studied how these divergences and in particular, how a part of the H2A COOH-terminus, the docking domain, is implicated in both structural and functional properties of the nucleosome. Using biochemical methods in combination with Atomic Force Microscopy and Electron Cryo-Microscopy, we show that the H2A-docking domain is a key structural feature within the nucleosome. Deletion of this domain or replacement with the incomplete docking domain from the variant H2A.Bbd results in significant structural alterations in the nucleosome, including an increase in overall accessibility to nucleases, un-wrapping of ∼10 bp of DNA from each end of the nucleosome and associated changes in the entry/exit angle of DNA ends. These structural alterations are associated with a reduced ability of the chromatin remodeler RSC to both remodel and mobilize the nucleosomes. Linker histone H1 binding is also abrogated in nucleosomes containing the incomplete docking domain of H2A.Bbd. Our data illustrate the unique role of the H2A-docking domain in coordinating the structural-functional aspects of the nucleosome properties. Moreover, our data suggest that incorporation of a 'defective' docking domain may be a primary structural role of H2A.Bbd in chromatin.
Histone variant H2A.Z-containing nucleosomes are incorporated at most eukaryotic promoters. This incorporation is mediated by the conserved SWR1 complex, which replaces histone H2A in canonical nucleosomes with H2A.Z in an ATP-dependent manner. Here, we show that promoter-proximal nucleosomes are highly heterogeneous for H2A.Z in Saccharomyces cerevisiae, with substantial representation of nucleosomes containing one, two, or zero H2A.Z molecules. SWR1-catalyzed H2A.Z replacement in vitro occurs in a stepwise and unidirectional fashion, one H2A.Z-H2B dimer at a time, producing heterotypic nucleosomes as intermediates and homotypic H2A.Z nucleosomes as end products. The ATPase activity of SWR1 is specifically stimulated by H2A-containing nucleosomes without ensuing histone H2A eviction. Remarkably, further addition of free H2A.Z-H2B dimer leads to hyperstimulation of ATPase activity, eviction of nucleosomal H2A-H2B, and deposition of H2A.Z-H2B. These results suggest that the combination of H2A-containing nucleosome and free H2A.Z-H2B dimer acting as both effector and substrate for SWR1 governs the specificity and outcome of the replacement reaction.
Histone posttranslational modifications are key components of diverse processes that modulate chromatin structure. These marks function as signals during various chromatin-based events, and act as platforms for recruitment, assembly or retention of chromatin-associated factors. The best-known function of histone phosphorylation takes place during cellular response to DNA damage, when phosphorylated histone H2A(X) demarcates large chromatin domains around the site of DNA breakage. However, multiple studies have also shown that histone phosphorylation plays crucial roles in chromatin remodeling linked to other nuclear processes. In this review, we summarize the current knowledge of histone phosphorylation and describe the many kinases and phosphatases that regulate it. We discuss the key roles played by this histone mark in DNA repair, transcription and chromatin compaction during cell division and apoptosis. Additionally, we describe the intricate crosstalk that occurs between phosphorylation and other histone modifications and allows for sophisticated control over the chromatin remodeling processes.
Histone proteins play essential structural and functional roles in the transition between active and inactive chromatin states. Although histones have a high degree of conservation due to constraints to maintain the overall structure of the nucleosomal octameric core, variants have evolved to assume diverse roles in gene regulation and epigenetic silencing. Histone variants, post-translational modifications and interactions with chromatin remodeling complexes influence DNA replication, transcription, repair and recombination. The authors review recent findings on the structure of chromatin that confirm previous interparticle interactions observed in crystal structures.
In yeast, remodeling of PHO5 promoter chromatin upon activation is accompanied by transient hyperacetylation and subsequent eviction of histones from the promoter in trans. In the course of rerepression, nucleosomes have to be reassembled on the promoter. We have analyzed where the histones for reassembly of the inactive promoter chromatin come from. The use of a strain with two differently tagged and differently regulated versions of histone H3 allowed us to discriminate between histones originating from the chromatin fraction and histones arising from the soluble histone pool. In this way, we show that the incorporated histones originate from a source in trans. Promoter closure occurs very rapidly, and the histone chaperones Asf1 and Hir1 as well as the SWI/SNF nucleosome remodeling complex appear to be important for rapid reassembly of nucleosomes at the PHO5 promoter.
H2A.Z is an evolutionary conserved histone variant involved in transcriptional regulation, antisilencing, silencing, and genome stability. The mechanism(s) by which H2A.Z regulates these various biological functions remains poorly defined, in part due to the lack of knowledge regarding its physical location along chromosomes and the bearing it has in regulating chromatin structure. Here we mapped H2A.Z across the yeast genome at an approximately 300-bp resolution, using chromatin immunoprecipitation combined with tiling microarrays. We have identified 4,862 small regions—typically one or two nucleosomes wide—decorated with H2A.Z. Those “Z loci” are predominantly found within specific nucleosomes in the promoter of inactive genes all across the genome. Furthermore, we have shown that H2A.Z can regulate nucleosome positioning at the GAL1 promoter. Within HZAD domains, the regions where H2A.Z shows an antisilencing function, H2A.Z is localized in a wider pattern, suggesting that the variant histone regulates a silencing and transcriptional activation via different mechanisms. Our data suggest that the incorporation of H2A.Z into specific promoter-bound nucleosomes configures chromatin structure to poise genes for transcriptional activation. The relevance of these findings to higher eukaryotes is discussed.
Poly(ADP-ribosyl)ation plays a major role in DNA repair, where it regulates chromatin relaxation as one of the critical events in the repair process. However, the molecular mechanism by which poly(ADP-ribose) modulates chromatin remains poorly understood. Here we identify the poly(ADP-ribose)-regulated protein APLF as a DNA-damage-specific histone chaperone. APLF preferentially binds to the histone H3/H4 tetramer via its C-terminal acidic motif, which is homologous to the motif conserved in the histone chaperones of the NAP1L family (NAP1L motif). We further demonstrate that APLF exhibits histone chaperone activities in a manner that is dependent on its acidic domain and that the NAP1L motif is critical for the repair capacity of APLF in vivo. Finally, we identify structural analogs of APLF in lower eukaryotes with the ability to bind histones and localize to the sites of DNA-damage-induced poly(ADP-ribosyl)ation. Collectively, these findings define the involvement of histone chaperones in poly(ADP-ribose)-regulated DNA repair reactions.
Histone variants help specialize chromatin regions; however, their impact on transcriptional regulation is largely unknown. Here, we determined the genome-wide localization and dynamics of Htz1, the yeast histone H2A variant. Htz1 localizes to hundreds of repressed/basal Pol II promoters and prefers TATA-less promoters. Specific Htz1 deposition requires the SWR1 complex, which largely colocalizes with Htz1. Htz1 occupancy correlates with particular histone modifications, and Htz1 deposition is partially reliant on Gcn5 (a histone acetyltransferase) and Bdf1, an SWR1 complex member that binds acetylated histones. Changes in growth conditions cause a striking redistribution of Htz1 from activated to repressed/basal promoters. Furthermore, Htz1 promotes full gene activation but does not generally impact repression. Importantly, Htz1 releases from purified chromatin in vitro under conditions where H2A and H3 remain associated. We suggest that Htz1-bearing nucleosomes are deposited at repressed/basal promoters but facilitate activation through their susceptibility to loss, thereby helping to expose promoter DNA.
The stable contact of ISW2 with nucleosomal DNA approximately 20 bp from the dyad was shown by DNA footprinting and photoaffinity labeling using recombinant histone octamers to require the histone H4 N-terminal tail. Efficient ISW2 remodeling also required the H4 N-terminal tail, although the lack of the H4 tail can be mostly compensated for by increasing the incubation time or concentration of ISW2. Similarly, the length of extranucleosomal DNA affected the stable contact of ISW2 with this same internal nucleosomal site, with the optimal length being 70 to 85 bp. These data indicate the histone H4 tail, in concert with a favorable length of extranucleosomal DNA, recruits and properly orients ISW2 onto the nucleosome for efficient nucleosome remodeling. One consequence of this property of ISW2 is likely its previously observed nucleosome spacing activity.
Variant histone H2AZ-containing nucleosomes are involved in the regulation of gene expression. In Saccharomyces cerevisiae, chromatin deposition of histone H2AZ is mediated by the fourteen-subunit SWR1 complex, which catalyzes ATP-dependent exchange of nucleosomal histone H2A for H2AZ. Previous work defined the role of seven SWR1 subunits (Swr1 ATPase, Swc2, Swc3, Arp6, Swc5, Yaf9, and Swc6) in maintaining complex integrity and H2AZ histone replacement activity. Here we examined the function of three additional SWR1 subunits, bromodomain containing Bdf1, actin-related protein Arp4 and Swc7, by analyzing affinity-purified mutant SWR1 complexes. We observed that depletion of Arp4 (arp4-td) substantially impaired the association of Bdf1, Yaf9, and Swc4. In contrast, loss of either Bdf1 or Swc7 had minimal effects on overall complex integrity. Furthermore, the basic H2AZ histone replacement activity of SWR1 in vitro required Arp4, but not Bdf1 or Swc7. Thus, three out of fourteen SWR1 subunits, Bdf1, Swc7, and previously noted Swc3, appear to have roles auxiliary to the basic histone replacement activity. The N-terminal region of the Swr1 ATPase subunit is necessary and sufficient to direct association of Bdf1 and Swc7, as well as Arp4, Act1, Yaf9 and Swc4. This same region contains an additional H2AZ-H2B specific binding site, distinct from the previously identified Swc2 subunit. These findings suggest that one SWR1 enzyme might be capable of binding two H2AZ-H2B dimers, and provide further insight on the hierarchy and interdependency of molecular interactions within the SWR1 complex.
Chromatin assembly factor 1 (CAF-1) and Rtt106 participate in the deposition of newly synthesized histones onto replicating DNA to form nucleosomes. This process is critical for the maintenance of genome stability and inheritance of functionally specialized chromatin structures in proliferating cells. However, the molecular functions of the acetylation of newly synthesized histones in this DNA replication-coupled nucleosome assembly pathway remain enigmatic. Here we show that histone H3 acetylated at lysine 56 (H3K56Ac) is incorporated onto replicating DNA and, by increasing the binding affinity of CAF-1 and Rtt106 for histone H3, H3K56Ac enhances the ability of these histone chaperones to assemble DNA into nucleosomes. Genetic analysis indicates that H3K56Ac acts in a nonredundant manner with the acetylation of the N-terminal residues of H3 and H4 in nucleosome assembly. These results reveal a mechanism by which H3K56Ac regulates replication-coupled nucleosome assembly mediated by CAF-1 and Rtt106.
The mechanism of activation of the alternative lengthening of telomeres (ALT) pathway of mammalian chromosome-end maintenance has been unclear. We have now discovered that co-depletion of the histone chaperones ASF1a and ASF1b in human cells induced all hallmarks of ALT in both primary and cancer cells. These included the formation of ALT-associated PML (promyelocytic leukemia) bodies (APBs), the presence of extrachromosomal telomeric DNA species, an elevated frequency of telomeric sister chromatid exchanges (t-SCE) events and intertelomeric exchange of an integrated tag. The induction of ALT characteristics in this setting led to the simultaneous suppression of telomerase. We determined that ALT induction is positively regulated by the proteins RAD17 and BLM and negatively regulated by EXO1 and DNA2. The induction of ALT phenotypes as a consequence of ASF1 depletion strongly supports the hypothesis that ALT is a consequence of histone management dysfunction.
INO80 is an evolutionarily conserved, ATP-dependent chromatin-remodeling enzyme that plays roles in transcription, DNA repair, and replication. Here, we show that yeast INO80 facilitates these diverse processes at least in part by controlling genome-wide distribution of the histone variant H2A.Z. In the absence of INO80, H2A.Z nucleosomes are mislocalized, and H2A.Z levels at promoters show reduced responsiveness to transcriptional changes, suggesting that INO80 controls H2A.Z dynamics. Additionally, we demonstrate that INO80 has a histone-exchange activity in which the enzyme can replace nucleosomal H2A.Z/H2B with free H2A/H2B dimers. Genetic interactions between ino80 and htz1 support a model in which INO80 catalyzes the removal of unacetylated H2A.Z from chromatin as a mechanism to promote genome stability.
Histone acetyltransferases (HATs) and ATP-dependent chromatin remodeling factors (ADCRs) are involved in selective gene regulation via modulation of local chromatin configuration. Activation of the recombination hotspot ade6-M26 of Schizosaccharomyces pombe is mediated by a cAMP responsive element (CRE)-like sequence, M26, and a heterodimeric ATF/CREB transcription factor, Atf1.Pcr1. Chromatin remodeling occurs meiotically around M26. We examined the roles of HATs and ADCRs in chromatin remodeling around M26. Histones H3 and H4 around M26 were hyperacetylated in an M26- and Atf1-dependent manner early in meiosis. SpGcn5, the S. pombe homolog of Gcn5p, was required for the majority of histone H3 acetylation around M26 in vivo. Deletion of gcn5+ caused a significant delay in chromatin remodeling but only partial reduction of M26 meiotic recombination frequency. The snf22+ (a Swi2/Snf2-ADCR homologue) deletion and snf22+ gcn5+ double deletion abolished chromatin remodeling and significant reduction of meiotic recombination around M26. These results suggest that HATs and ADCRs cooperatively alter local chromatin structure, as in selective transcription activation, to activate meiotic recombination at M26 in a site-specific manner.
Nuclear receptors undergo ligand-dependent conformational changes that are required for corepressor-coactivator exchange, but whether there is an actual requirement for specific epigenetic landmarks to impose ligand dependency for gene activation remains unknown. Here we report an unexpected and general strategy that is based on the requirement for specific cohorts of inhibitory histone methyltransferases (HMTs) to impose gene-specific gatekeeper functions that prevent unliganded nuclear receptors and other classes of regulated transcription factors from binding to their target gene promoters and causing constitutive gene activation in the absence of stimulating signals. This strategy, based at least in part on an HMT-dependent inhibitory histone code, imposes a requirement for specific histone demethylases, including LSD1, to permit ligand- and signal-dependent activation of regulated gene expression. These events link an inhibitory methylation component of the histone code to a broadly used strategy that circumvents pathological constitutive gene induction by physiologically regulated transcription factors.
Publisher Summary This chapter summarizes the development and characterization of rabbit polyclonal antibodies named histone that are directed against the methylated H3-K9 position. It provides protocols for peptide design, rabbit immunizations, and quality controls of methyl-lysine histone antibodies, followed by their in vivo characterization using indirect IF of inter-and metaphase chromatin in wild-type (wt) and mutant mouse cells that are deficient for the Suv39h histone methyltransferases (HMTases). Histone amino-termini (tails) protrude from the nucleosome core and are subject to a variety of post-translational modifications, including acetylation (on lysine residues), phosphorylation (on serine and threonine residues), methylation (on lysine and arginine residues), ubiquitination (on lysine residues), and ADP-ribosylation (on glutamic acid residues). In addition to their structural roles, histones play important functions in the control of gene expression by regulating access to the underlying nucleosomal template. It is without doubt that the development of high-quality, position-specific methyl-lysine histone antibodies can provide important tools for the further decoding of the epigenetic information, which is in part, indexed by distinct methylation states of selective lysine residues in the histone amino-termini. A comparative analysis indicates significant discrepancies in the specificity and avidity of the available methyl-lysine histone antibodies and highlights the need for extensive quality controls, such that experimental data can be correctly interpreted despite the exquisite complexity of histone lysine methylation.
Histone acetylation and deacetylation in the yeast Saccharomyces cerevisiae occur by targeting acetyltransferase and deacetylase enzymes to gene promoters and, in an untargeted and global manner, by affecting most nucleosomes. Recently, new roles for histone acetylation have been uncovered, not only in transcription but also in DNA replication, repair and heterochromatin formation. Interestingly, specific acetylatable lysines can function as binding sites for regulatory factors. Moreover, histone deacetylation is not only repressive but can be required for gene activity.
Chromosome segregation during mitosis requires assembly of the kinetochore complex at the centromere. Kinetochore assembly depends on specific recognition of the histone variant CENP-A in the centromeric nucleosome by centromere protein C (CENP-C). We have defined the determinants of this recognition mechanism and discovered that CENP-C binds a hydrophobic region in the CENP-A tail and docks onto the acidic patch of histone H2A and H2B. We further found that the more broadly conserved CENP-C motif uses the same mechanism for CENP-A nucleosome recognition. Our findings reveal a conserved mechanism for protein recruitment to centromeres and a histone recognition mode whereby a disordered peptide binds the histone tail through hydrophobic interactions facilitated by nucleosome docking.
Methylation of histones has been regarded as a stable modification defining the epigenetic program of the cell, which regulates chromatin structure and transcription. However, the recent discovery of histone demethylases has challenged the stable nature of histone methylation. Here we demonstrate that the JARID1 proteins RBP2, PLU1, and SMCX are histone demethylases specific for di- and trimethylated histone 3 lysine 4 (H3K4). Consistent with a role for the JARID1 Drosophila homolog Lid in regulating expression of homeotic genes during development, we show that RBP2 is displaced from Hox genes during embryonic stem (ES) cell differentiation correlating with an increase of their H3K4me3 levels and expression. Furthermore, we show that mutation or RNAi depletion of the C. elegans JARID1 homolog rbr-2 leads to increased levels of H3K4me3 during larval development and defects in vulva formation. Taken together, these results suggest that H3K4me3/me2 demethylation regulated by the JARID1 family plays an important role during development.
We have conducted a genomewide investigation into the enzymatic specificity, expression profiles, and binding locations of four histone deacetylases (HDACs), representing the three different phylogenetic classes in fission yeast (Schizosaccharomyces pombe). By directly comparing nucleosome density, histone acetylation patterns and HDAC binding in both intergenic and coding regions with gene expression profiles, we found that Sir2 (class III) and Hos2 (class I) have a role in preventing histone loss; Clr6 (class I) is the principal enzyme in promoter-localized repression. Hos2 has an unexpected role in promoting high expression of growth-related genes by deacetylating H4K16Ac in their open reading frames. Clr3 (class II) acts cooperatively with Sir2 throughout the genome, including the silent regions: rDNA, centromeres, mat2/3 and telomeres. The most significant acetylation sites are H3K14Ac for Clr3 and H3K9Ac for Sir2 at their genomic targets. Clr3 also affects subtelomeric regions which contain clustered stress- and meiosis-induced genes. Thus, this combined genomic approach has uncovered different roles for fission yeast HDACs at the silent regions in repression and activation of gene expression.