Nucleosome composition regulates the histone H3 tail conformational ensemble and accessibility

Hexasomal particles: consequence or also consequential?

It is long known that an RNA polymerase transcribing through a nucleosome can generate subnucleosomal particles called hexasomes. These particles lack an H2A-H2B dimer, breaking the symmetry of a nucleosome and revealing new interfaces. Whether hexasomes are simply a consequence of RNA polymerase action or they also have a regulatory impact remains an open question. Recent biochemical and structural studies of RNA polymerases and chromatin remodelers with hexasomes motivated us to revisit this question. Here, we build on previous models to discuss how formation of hexasomes can allow sophisticated regulation of transcription and also significantly impact chromatin folding. We anticipate that further cellular and biochemical analysis of these subnucleosomal particles will uncover additional regulatory roles.

Nucleosome conformation dictates the histone code

Histone post-translational modifications (PTMs) play a critical role in chromatin regulation. It has been proposed that these PTMs form localized 'codes' that are read by specialized regions (reader domains) in chromatin-associated proteins (CAPs) to regulate downstream function. Substantial effort has been made to define [CAP: histone PTM] specificities, and thus decipher the histone code and guide epigenetic therapies. However, this has largely been done using the reductive approach of isolated reader domains and histone peptides, which cannot account for any higher-order factors. Here, we show that the [BPTF PHD finger and bromodomain: histone PTM] interaction is dependent on nucleosome context. The tandem reader selectively associates with nucleosomal H3K4me3 and H3K14ac or H3K18ac, a combinatorial engagement that despite being in cis is not predicted by peptides. This in vitro specificity of the BPTF tandem reader for PTM-defined nucleosomes is recapitulated in a cellular context. We propose that regulatable histone tail accessibility and its impact on the binding potential of reader domains necessitates we refine the 'histone code' concept and interrogate it at the nucleosome level.

Conformational and Interaction Landscape of Histone H4 Tails in Nucleosomes Probed by Paramagnetic NMR Spectroscopy

The fundamental repeat unit of chromatin, the nucleosome, consists of approximately 147 base pairs of double-stranded DNA and a histone protein octamer containing two copies each of histones H2A, H2B, H3, and H4. Each histone possesses a dynamically disordered N-terminal tail domain, and it is well-established that the tails of histones H3 and H4 play key roles in chromatin compaction and regulation. Here we investigate the conformational ensemble and interactions of the H4 tail in nucleosomes by means of solution NMR measurements of paramagnetic relaxation enhancements (PREs) in recombinant samples reconstituted with 15N-enriched H4 and nitroxide spin-label tagged H3. The experimental PREs, which report on the proximities of individual H4 tail residues to the different H3 spin-label sites, are interpreted by using microsecond time-scale molecular dynamics simulations of the nucleosome core particle. Collectively, these data enable improved localization of histone H4 tails in nucleosomes and support the notion that H4 tails engage in a fuzzy complex interaction with nucleosomal DNA.

Construction of Histone-Protein Complex Structures by Peptide Growing

The structures of histone complexes are master keys to epigenetics. Linear histone peptide tails often bind to shallow pockets of reader proteins via weak interactions, rendering their structure determination challenging. In the present study, a new protocol, PepGrow, is introduced. PepGrow uses docked histone fragments as seeds and grows the full peptide tails in the reader-binding pocket, producing atomic-resolution structures of histone-reader complexes. PepGrow is able to handle the flexibility of histone peptides, and it is demonstrated to be more efficient than linking pre-docked peptide fragments. The new protocol combines the advantages of popular program packages and allows fast generation of solution structures. AutoDock, a force-field-based program, is used to supply the docked peptide fragments used as structural seeds, and the building algorithm of Modeller is adopted and tested as a peptide growing engine. The performance of PepGrow is compared to ten other docking methods, and it is concluded that in situ growing of a ligand from a seed is a viable strategy for the production of complex structures of histone peptides at atomic resolution.

Intrinsic catalytic properties of histone H3 lysine-9 methyltransferases preserve monomethylation levels under low S-adenosylmethionine

S-adenosylmethionine (SAM) is the methyl donor for site-specific methylation reactions on histone proteins, imparting key epigenetic information. During SAM-depleted conditions that can arise from dietary methionine restriction, lysine di- and tri-methylation are reduced while sites such as Histone-3 lysine-9 (H3K9) are actively maintained, allowing cells to restore higher-state methylation upon metabolic recovery. Here, we investigated if the intrinsic catalytic properties of H3K9 histone methyltransferases (HMTs) contribute to this epigenetic persistence. We employed systematic kinetic analyses and substrate binding assays using four recombinant H3K9 HMTs (i.e., EHMT1, EHMT2, SUV39H1, and SUV39H2). At both high and low (i.e., sub-saturating) SAM, all HMTs displayed the highest catalytic efficiency (kcat/KM) for monomethylation compared to di- and trimethylation on H3 peptide substrates. The favored monomethylation reaction was also reflected in kcat values, apart from SUV39H2 which displayed a similar kcat regardless of substrate methylation state. Using differentially methylated nucleosomes as substrates, kinetic analyses of EHMT1 and EHMT2 revealed similar catalytic preferences. Orthogonal binding assays revealed only small differences in substrate affinity across methylation states, suggesting that catalytic steps dictate the monomethylation preferences of EHMT1, EHMT2, and SUV39H1. To link in vitro catalytic rates with nuclear methylation dynamics, we built a mathematical model incorporating measured kinetic parameters and a time course of mass spectrometry-based H3K9 methylation measurements following cellular SAM depletion. The model revealed that the intrinsic kinetic constants of the catalytic domains could recapitulate in vivo observations. Together, these results suggest catalytic discrimination by H3K9 HMTs maintains nuclear H3K9me1, ensuring epigenetic persistence after metabolic stress.

Structural and dynamical investigation of histone H2B in well-hydrated nucleosome core particles by solid-state NMR

H2A-H2B dimer is a key component of nucleosomes and an important player in chromatin biology. Here, we characterized the structure and dynamics of H2B in precipitated nucleosome core particles (NCPs) with a physiologically relevant concentration using solid-state NMR. Our recent investigation of H3-H4 tetramer determined its unique dynamic properties and the present work provides a deeper understanding of the previously observed dynamic networks in NCP that is potentially functionally significant. Nearly complete 13C, 15N assignments were obtained for H2B R30-A121, which permit extracting unprecedented detailed structural and amino-acid site-specific dynamics. The derived structure of H2B in the well-hydrated NCP sample agrees well with that of X-ray crystals. Dynamics at different timescales were determined semi-quantitatively for H2B in a site-specific manner. Particularly, higher millisecond-microsecond dynamics are observed for H2B core regions including partial α1, L1, partial α2, and partial L3. The analysis of these regions in the context of the tertiary structure reveals the clustering of dynamical residues. Overall, this work fills a gap to a complete resonance assignment of all four histones in nucleosomes and delineates that the dynamic networks in NCP extend to H2B, which suggests a potential mechanism to couple histone core with distant DNA to modulate the DNA activities.

The MMP-2 histone H3 N-terminal tail protease is selectively targeted to the transcription start sites of active genes

BACKGROUND

Proteolysis of the histone H3 N-terminal tail (H3NT) is an evolutionarily conserved epigenomic feature of nearly all eukaryotes, generating a cleaved H3 product that is retained in ~ 5-10% of the genome. Although H3NT proteolysis within chromatin was first reported over 60 years ago, the genomic sites targeted for H3NT proteolysis and the impact of this histone modification on chromatin structure and function remain largely unknown. The goal of this study was to identify the specific regions targeted for H3NT proteolysis and investigate the consequence of H3NT "clipping" on local histone post-translational modification (PTM) dynamics.

RESULTS

Leveraging recent findings that matrix metalloproteinase 2 (MMP-2) functions as the principal nuclear H3NT protease in the human U2OS osteosarcoma cell line, a ChIP-Seq approach was used to map MMP-2 localization genome wide. The results indicate that MMP-2 is selectively targeted to the transcription start sites (TSSs) of protein coding genes, primarily at the + 1 nucleosome. MMP-2 localization was exclusive to highly expressed genes, further supporting a functional role for H3NT proteolysis in transcriptional regulation. MMP-2 dependent H3NT proteolysis at the TSSs of these genes resulted in a > twofold reduction of activation-associated histone H3 PTMs, including H3K4me3, H3K9ac and H3K18ac. One of genes requiring MMP-2 mediated H3NT proteolysis for proficient expression was the lysosomal cathepsin B protease (CTSB), which we discovered functions as a secondary nuclear H3NT protease in U2OS cells.

CONCLUSIONS

This study revealed that the MMP-2 H3NT protease is selectively targeted to the TSSs of active protein coding genes in U2OS cells. The resulting H3NT proteolysis directly alters local histone H3 PTM patterns at TSSs, which likely functions to regulate transcription. MMP-2 mediated H3NT proteolysis directly activates CTSB, a secondary H3NT protease that generates additional cleaved H3 products within chromatin.

Arginine anchor points govern H3 tail dynamics

Chromatin is dynamically reorganized spatially and temporally, and the post-translational modification of histones is a key component of this regulation. The basic subunit of chromatin is the nucleosome core particle, consisting of two copies each of the histones H2A, H2B, H3, and H4 around which ∼147 base pairs of DNA wrap. The intrinsically disordered histone termini, or tails, protrude from the core and are heavily post-translationally modified. Previous studies have shown that the histone tails exist in dynamic ensembles of DNA-bound states within the nucleosome. Histone tail interactions with DNA are involved in nucleosome conformation and chromatin organization. Charge-modulating histone post-translational modifications (PTMs) are poised to perturb the dynamic interactions between histone tails and DNA. Arginine side chains form favorable interactions with DNA and are sites of charge-modulating PTMs such as citrullination. Our current focus is on the H3 tail, the longest histone tail. Four arginine residues are relatively evenly spaced along the H3 tail sequence, suggesting multivalent interactions with DNA poised for regulation by PTMs. In this study, we use NMR nuclear spin relaxation experiments to investigate the contribution of arginine residues to H3 tail dynamics within the nucleosome core particle. By neutralizing arginine via mutation to glutamine, we begin to work towards a comprehensive understanding of the contribution of individual residues to H3 tail dynamics. We find that neutralization of arginine residues results in increased regional mobility of the H3 tails, with implications for understanding the direct effects of arginine citrullination. Altogether, these studies support a role for dynamics within the histone language and emphasize the importance of charge-modulating histone PTMs in regulating chromatin dynamics, starting at the level of the basic subunit of chromatin.

Basic helix-loop-helix pioneer factors interact with the histone octamer to invade nucleosomes and generate nucleosome-depleted regions

Nucleosomes drastically limit transcription factor (TF) occupancy, while pioneer transcription factors (PFs) somehow circumvent this nucleosome barrier. In this study, we compare nucleosome binding of two conserved S. cerevisiae basic helix-loop-helix (bHLH) TFs, Cbf1 and Pho4. A cryo-EM structure of Cbf1 in complex with the nucleosome reveals that the Cbf1 HLH region can electrostatically interact with exposed histone residues within a partially unwrapped nucleosome. Single-molecule fluorescence studies show that the Cbf1 HLH region facilitates efficient nucleosome invasion by slowing its dissociation rate relative to DNA through interactions with histones, whereas the Pho4 HLH region does not. In vivo studies show that this enhanced binding provided by the Cbf1 HLH region enables nucleosome invasion and ensuing repositioning. These structural, single-molecule, and in vivo studies reveal the mechanistic basis of dissociation rate compensation by PFs and how this translates to facilitating chromatin opening inside cells.

The effects of RNA.DNA-DNA triple helices on nucleosome structures and dynamics

Noncoding RNAs (ncRNAs) are an emerging epigenetic factor and have been recognized as playing a key role in many gene expression pathways. Structurally, binding of ncRNAs to isolated DNA is strongly dependent on sequence complementary and results in the formation of an RNA.DNA-DNA (RDD) triple helix. However, in vivo DNA is not isolated but is rather packed in chromatin fibers, the fundamental unit of which is the nucleosome. Biochemical experiments have shown that ncRNA binding to nucleosomal DNA is elevated at DNA entry and exit sites and is dependent on the presence of the H3 N-terminal tails. However, the structural and dynamical bases for these mechanisms remain unknown. Here, we have examined the mechanisms and effects of RDD formation in the context of the nucleosome using a series of all-atom molecular dynamics simulations. Results highlight the importance of DNA sequence on complex stability, elucidate the effects of the H3 tails on RDD structures, show how RDD formation impacts the structure and dynamics of the H3 tails, and show how RNA alters the local and global DNA double-helical structure. Together, our results suggest ncRNAs can modify nucleosome, and potentially higher-order chromatin, structures and dynamics as a means of exerting epigenetic control.

H2A Ubiquitination Alters H3-tail Dynamics on Linker-DNA to Enhance H3K27 Methylation

Polycomb repressive complex 1 (PRC1) and PRC2 are responsible for epigenetic gene regulation. PRC1 ubiquitinates histone H2A (H2Aub), which subsequently promotes PRC2 to introduce the H3 lysine 27 tri-methyl (H3K27me3) repressive chromatin mark. Although this mechanism provides a link between the two key transcriptional repressors, PRC1 and PRC2, it is unknown how histone-tail dynamics contribute to this process. Here, we have examined the effect of H2A ubiquitination and linker-DNA on H3-tail dynamics and H3K27 methylation by PRC2. In naïve nucleosomes, the H3-tail dynamically contacts linker DNA in addition to core DNA, and the linker-DNA is as important for H3K27 methylation as H2A ubiquitination. H2A ubiquitination alters contacts between the H3-tail and DNA to improve the methyltransferase activity of the PRC2-AEBP2-JARID2 complex. Collectively, our data support a model in which H2A ubiquitination by PRC1 synergizes with linker-DNA to hold H3 histone tails poised for their methylation by PRC2-AEBP2-JARID2.

Nucleosome Core Particle Reconstitution with Recombinant Histones and Widom 601 DNA

Reconstitution of nucleosomes from recombinant histones and DNA is a widely used tool for studying nucleosome structure, dynamics, and interactions. Preparation of reconstituted nucleosomes allows for the study of nucleosomes with defined compositions. Here, we describe methods for refolding recombinant human histones, reconstituting nucleosome core particles with 147 bp Widom 601 DNA, and purification via sucrose gradient.

Molecular dynamics simulations for the study of chromatin biology

The organization of Eukaryotic DNA into chromatin has profound implications for the processing of genetic information. In the past years, molecular dynamics (MD) simulations proved to be a powerful tool to investigate the mechanistic basis of chromatin biology. We review recent all-atom and coarse-grained MD studies revealing how the structure and dynamics of chromatin underlie its biological functions. We describe the latest method developments; the structural fluctuations of nucleosomes and the various factors affecting them; the organization of chromatin fibers, with particular emphasis on its liquid-like character; the interactions and dynamics of transcription factors on chromatin; and how chromatin organization is modulated by molecular motors acting on DNA.

OCT4 interprets and enhances nucleosome flexibility

Pioneer transcription factors are proteins that induce cellular identity transitions by binding to inaccessible regions of DNA in nuclear chromatin. They contribute to chromatin opening and recruit other factors to regulatory DNA elements. The structural features and dynamics modulating their interaction with nucleosomes are still unresolved. From a combination of experiments and molecular simulations, we reveal here how the pioneer factor and master regulator of pluripotency, Oct4, interprets and enhances nucleosome structural flexibility. The magnitude of Oct4’s impact on nucleosome dynamics depends on the binding site position and the mobility of the unstructured tails of nucleosomal histone proteins. Oct4 uses both its DNA binding domains to propagate and stabilize open nucleosome conformations, one for specific sequence recognition and the other for nonspecific interactions with nearby regions of DNA. Our findings provide a structural basis for the versatility of transcription factors in engaging with nucleosomes and have implications for understanding how pioneer factors induce chromatin dynamics.

Differential enrichment of H3K9me3 in intrahepatic cholangiocarcinoma

Background

Intrahepatic cholangiocarcinoma (ICC) is a malignant tumor, which poses a serious threat to human health. Histone 3 lysine 9 trimethylation (H3K9me3) is a post-translational modification involved in regulating a broad range of biological processes and has been considered as potential therapeutic target in types of cancer. However, there is limited research on investigating profiles of histone modification H3K9me3 in ICC patients.

Methods

In this study, we applied the ChIP-seq technique to investigate the effect of H3K9me3 on ICC. Anti-H3K9me3 antibody was used for ChIP-seq in ICC (RBE cell lines) and HIBEpic (normal cell lines). MACS2 (peak-calling tools) was then used to identify the peaks recorded in RBE and HIBEpic cell lines. Gene expression, mutation and clinical data were downloaded from TCGA and cBioPortal databases.

Results

H3K9me3 exhibited abnormal methylation and influenced the process of abnormal gene expression in patients suffering from ICC. The Wnt/β-Catenin signaling pathway (also known as simply the WNT signaling pathway) was enriched in H3K9me3-regulated genes.

Conclusions

We are the first to report that H3K9me3 may play an important role in the progression of ICC. It promotes the understanding of epigenetic molecular mechanisms for ICC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01338-1.

FACT modulates the conformations of histone H2A and H2B N-terminal tails within nucleosomes

Gene expression is regulated by the modification and accessibility of histone tails within nucleosomes. The histone chaperone FACT (facilitate chromatin transcription), comprising SPT16 and SSRP1, interacts with nucleosomes through partial replacement of DNA with the phosphorylated acidic intrinsically disordered (pAID) segment of SPT16; pAID induces an accessible conformation of the proximal histone H3 N-terminal tail (N-tail) in the unwrapped nucleosome with FACT. Here, we use NMR to probe the histone H2A and H2B tails in the unwrapped nucleosome. Consequently, both the H2A and H2B N-tails on the pAID-proximal side bind to pAID with robust interactions, which are important for nucleosome assembly with FACT. Furthermore, the conformations of these N-tails on the distal DNA-contact site are altered from those in the canonical nucleosome. Our findings highlight that FACT both proximally and distally regulates the conformations of the H2A and H2B N-tails in the asymmetrically unwrapped nucleosome.

Histone tail network and modulation in a nucleosome

The structural unit of eukaryotic chromatin is a nucleosome, comprising two histone H2A/H2B heterodimers and one histone (H3/H4)₂ tetramer, wrapped around by ∼146-bp core DNA and linker DNA. Flexible histone tails sticking out from the core undergo posttranslational modifications that are responsible for various epigenetic functions. Recently, the functional dynamics of histone tails and their modulation within the nucleosome and nucleosomal complexes have been investigated by integrating NMR, molecular dynamics simulations, and cryo-electron microscopy approaches. In particular, recent NMR studies have revealed correlations in the structures of histone N-terminal tails between H2A and H2B, as well as between H3 and H4 depending on linker DNA, suggesting that histone tail networks exist even within the nucleosome.

Histone variants H3.3 and H2A.Z/H3.3 facilitate excision of uracil from nucleosome core particles

At the most fundamental level of chromatin organization, DNA is packaged as nucleosome core particles (NCPs) where DNA is wound around a core of histone proteins. This ubiquitous sequestration of DNA within NCPs presents a significant barrier to many biological processes, including DNA repair. We previously demonstrated that histone variants from the H2A family facilitate excision of uracil (U) lesions by DNA base excision repair (BER) glycosylases. Here, we consider how the histone variant H3.3 and double-variant H2A.Z/H3.3 modulate the BER enzymes uracil DNA glycosylase (UDG) and single-strand selective monofunctional uracil DNA glycosylase (SMUG1). Using an NCP model system with U:G base pairs at a wide variety of geometric positions we generate the global repair profile for both glycosylases. Enhanced excision of U by UDG and SMUG1 is observed with the H3.3 variant. We demonstrate that these H3.3-containing NCPs form two species: (1) octasomes, which contain the full complement of eight histone proteins and (2) hexasomes which are sub-nucleosomal particles that contain six histones. Both the octasome and hexasome species facilitate excision activity of UDG and SMUG1, with the largest impacts observed at sterically-occluded lesion sites and in terminal regions of DNA of the hexasome that do not closely interact with histones. For the double-variant H2A.Z/H3.3 NCPs, which exist as octasomes, the global repair profile reveals that UDG but not SMUG1 has increased U excision activity. The enhanced glycosylase activity reveals potential functions for these histone variants to facilitate BER in packaged DNA and contributes to our understanding of DNA repair in chromatin and its significance regarding mutagenesis and genomic integrity.

Recent Advances in Investigating Functional Dynamics of Chromatin

Dynamics spanning the picosecond-minute time domain and the atomic-subcellular spatial window have been observed for chromatin in vitro and in vivo. The condensed organization of chromatin in eukaryotic cells prevents regulatory factors from accessing genomic DNA, which requires dynamic stabilization and destabilization of structure to initiate downstream DNA activities. Those processes are achieved through altering conformational and dynamic properties of nucleosomes and nucleosome–protein complexes, of which delineating the atomistic pictures is essential to understand the mechanisms of chromatin regulation. In this review, we summarize recent progress in determining chromatin dynamics and their modulations by a number of factors including post-translational modifications (PTMs), incorporation of histone variants, and binding of effector proteins. We focus on experimental observations obtained using high-resolution techniques, primarily including nuclear magnetic resonance (NMR) spectroscopy, Förster (or fluorescence) resonance energy transfer (FRET) microscopy, and molecular dynamics (MD) simulations, and discuss the elucidated dynamics in the context of functional response and relevance.

Visualizing Conformational Ensembles of the Nucleosome by NMR

The formation of chromatin not only compacts the eukaryotic genome into the nucleus but also provides a mechanism for the regulation of all DNA templated processes. Spatial and temporal modulation of the chromatin structure is critical in such regulation and involves fine-tuned functioning of the basic subunit of chromatin, the nucleosome. It has become apparent that the nucleosome is an inherently dynamic system, but characterization of these dynamics at the atomic level has remained challenging. NMR spectroscopy is a powerful tool for investigating the conformational ensemble and dynamics of proteins and protein complexes, and recent advances have made the study of large systems possible. Here, we review recent studies which utilize NMR spectroscopy to uncover the atomic level conformation and dynamics of the nucleosome and provide a better understanding of the importance of these dynamics in key regulatory events.

Characteristic H3 N-tail dynamics in the nucleosome core particle, nucleosome, and chromatosome

The nucleosome core particle (NCP) comprises a histone octamer, wrapped around by ∼146-bp DNA, while the nucleosome additionally contains linker DNA. We previously showed that, in the nucleosome, H4 N-tail acetylation enhances H3 N-tail acetylation by altering their mutual dynamics. Here, we have evaluated the roles of linker DNA and/or linker histone on H3 N-tail dynamics and acetylation by using the NCP and the chromatosome (i.e., linker histone H1.4-bound nucleosome). In contrast to the nucleosome, H3 N-tail acetylation and dynamics are greatly suppressed in the NCP regardless of H4 N-tail acetylation because the H3 N-tail is strongly bound between two DNA gyres. In the chromatosome, the asymmetric H3 N-tail adopts two conformations: one contacts two DNA gyres, as in the NCP; and one contacts linker DNA, as in the nucleosome. However, the rate of H3 N-tail acetylation is similar in the chromatosome and nucleosome. Thus, linker DNA and linker histone both regulate H3-tail dynamics and acetylation.

Methylation of recombinant mononucleosomes by DNMT3A demonstrates efficient linker DNA methylation and a role of H3K36me3

Recently, the structure of the DNMT3A2/3B3 heterotetramer complex bound to a mononucleosome was reported. Here, we investigate DNA methylation of recombinant unmodified, H3K_C4me3 and H3K_C36me3 containing mononucleosomes by DNMT3A2, DNMT3A catalytic domain (DNMT3AC) and the DNMT3AC/3B3C complex. We show strong protection of the nucleosomal bound DNA against methylation, but efficient linker-DNA methylation next to the nucleosome core. High and low methylation levels of two specific CpG sites next to the nucleosome core agree well with details of the DNMT3A2/3B3-nucleosome structure. Linker DNA methylation next to the nucleosome is increased in the absence of H3K4me3, likely caused by binding of the H3-tail to the ADD domain leading to relief of autoinhibition. Our data demonstrate a strong stimulatory effect of H3K36me3 on linker DNA methylation, which is independent of the DNMT3A-PWWP domain. This observation reveals a direct functional role of H3K36me3 on the stimulation of DNA methylation, which could be explained by hindering the interaction of the H3-tail and the linker DNA. We propose an evolutionary model in which the direct stimulatory effect of H3K36me3 on DNA methylation preceded its signaling function, which could explain the evolutionary origin of the widely distributed "active gene body-H3K36me3-DNA methylation" connection.

Histone H3 and H4 tails play an important role in nucleosome phase separation

Chromatin organization and its dynamic regulation are crucial in governing the temporal and spatial accessibility of DNA for proper gene expression. Disordered chains of nucleosomes comprise the basis of eukaryotic chromatin, forming higher-level organization across a range of length scales. Models of chromatin organization involving phase separation driven by chromatin-associating proteins have been proposed. More recently, evidence has emerged that nucleosome arrays can phase separate in the absence of other protein factors, yet questions remain regarding the molecular basis of chromatin phase separation that governs this dynamic nuclear organization. Here, we break chromatin down into its most basic subunit, the nucleosome core particle, and investigate phase separation using turbidity assays in conjunction with differential interference contrast microscopy. We show that, at physiologically-relevant concentrations, this fundamental subunit of chromatin undergoes phase separation. Individually removing the H3 and H4 tails abrogates phase separation under the same conditions. Taking a reductionist approach to investigate H3 and H4 tail peptide interactions in-trans with DNA and nucleosome core particles supports the direct involvement of these tails in chromatin phase separation. These results provide insight into fundamental mechanisms underlying phase separation of chromatin, which starts at the level of the nucleosome core particle, and support that long-range inter-nucleosomal interactions are sufficient to drive phase separation at nuclear concentrations. Additionally, our data have implications for understanding crosstalk between histone tails and provide a lens through which to interpret the effect of histone post-translational modifications and sequence variants. STATEMENT OF SIGNIFICANCE: Emerging models propose that chromatin organization is based in phase separation, however, mechanisms that drive this dynamic nuclear organization are only beginning to be understood. Previous focus has been on phase separation driven by chromatin-associating proteins, but this has recently shifted to recognize a direct role of chromatin in phase separation. Here, we take a fundamental approach in understanding chromatin phase separation and present new findings that the basic subunit of chromatin, the nucleosome core particle, undergoes phase separation under physiological concentrations of nucleosome and monovalent salt. Furthermore, the histone H3 and H4 tails are involved in phase separation in a manner independent of histone-associating proteins. These data suggest that H3 and H4 tail epigenetic factors may modulate chromatin phase separation.

Emerging Contributions of Solid-State NMR Spectroscopy to Chromatin Structural Biology

The eukaryotic genome is packaged into chromatin, a polymer of DNA and histone proteins that regulates gene expression and the spatial organization of nuclear content. The repetitive character of chromatin is diversified into rich layers of complexity that encompass DNA sequence, histone variants and post-translational modifications. Subtle molecular changes in these variables can often lead to global chromatin rearrangements that dictate entire gene programs with far reaching implications for development and disease. Decades of structural biology advances have revealed the complex relationship between chromatin structure, dynamics, interactions, and gene expression. Here, we focus on the emerging contributions of magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (MAS NMR), a relative newcomer on the chromatin structural biology stage. Unique among structural biology techniques, MAS NMR is ideally suited to provide atomic level information regarding both the rigid and dynamic components of this complex and heterogenous biological polymer. In this review, we highlight the advantages MAS NMR can offer to chromatin structural biologists, discuss sample preparation strategies for structural analysis, summarize recent MAS NMR studies of chromatin structure and dynamics, and close by discussing how MAS NMR can be combined with state-of-the-art chemical biology tools to reconstitute and dissect complex chromatin environments.

Binding of regulatory proteins to nucleosomes is modulated by dynamic histone tails

Little is known about the roles of histone tails in modulating nucleosomal DNA accessibility and its recognition by other macromolecules. Here we generate extensive atomic level conformational ensembles of histone tails in the context of the full nucleosome, totaling 65 microseconds of molecular dynamics simulations. We observe rapid conformational transitions between tail bound and unbound states, and characterize kinetic and thermodynamic properties of histone tail-DNA interactions. Different histone types exhibit distinct binding modes to specific DNA regions. Using a comprehensive set of experimental nucleosome complexes, we find that the majority of them target mutually exclusive regions with histone tails on nucleosomal/linker DNA around the super-helical locations ± 1, ± 2, and ± 7, and histone tails H3 and H4 contribute most to this process. These findings are explained within competitive binding and tail displacement models. Finally, we demonstrate the crosstalk between different histone tail post-translational modifications and mutations; those which change charge, suppress tail-DNA interactions and enhance histone tail dynamics and DNA accessibility.

The intrinsic disorder of histone tails poses challenges in their characterization. Here the authors apply extensive molecular dynamics simulations of the full nucleosome to show reversible binding to DNA with specific binding modes of different types of histone tails, where charge-altering modifications suppress tail-DNA interactions and may boost interactions between nucleosomes and nucleosome-binding proteins.

Histone Tail Conformations: A Fuzzy Affair with DNA

The core histone tails are critical in chromatin structure and signaling. Studies over the past several decades have provided a wealth of information on the histone tails and their interaction with chromatin factors. However, the conformation of the histone tails in a chromatin relevant context has remained elusive. Only recently has enough evidence emerged to start to build a structural model of the tails in the context of nucleosomes and nucleosome arrays. Here, we review these studies and propose that the histone tails adopt a high-affinity fuzzy complex with DNA, characterized by robust but dynamic association. Furthermore, we discuss how these DNA-bound conformational ensembles promote distinct chromatin structure and signaling, and that their fuzzy nature is important in transitioning between functional states.