What makes a histone variant a variant: Changing H2A to become H2A.Z

Replacement of Arabidopsis H2A.Z with human H2A.Z orthologs reveals extensive functional conservation and limited importance of the N-terminal tail sequence for Arabidopsis development

The incorporation of histone variants, distinct paralogs of core histones, into chromatin affects all DNA-templated processes in the cell, including the regulation of transcription. In recent years, much research has been focused on H2A.Z, an evolutionarily conserved H2A variant found in all eukaryotes. In order to investigate the functional conservation of H2A.Z histones during eukaryotic evolution we transformed h2a.z deficient plants with each of the three human H2A.Z variants to assess their ability to rescue the mutant defects. We discovered that human H2A.Z.1 and H2A.Z.2.1 fully complement the phenotypic abnormalities of h2a.z plants despite significant divergence in the N-terminal tail sequences of Arabidopsis and human H2A.Zs. In contrast, the brain-specific splice variant H2A.Z.2.2 has a dominant-negative effect in wild-type plants, mimicking an H2A.Z deficiency phenotype. Furthermore, H2A.Z.1 almost completely re-establishes normal H2A.Z chromatin occupancy in h2a.z plants and restores the expression of more than 84% of misexpressed genes. Finally, we used a series of N-terminal tail truncations of Arabidopsis HTA11 to reveal that the N-terminal tail of Arabidopsis H2A.Z is not necessary for normal plant development but does play an important role in mounting proper environmental stress responses.

Beyond the Usual Suspects: Examining the Role of Understudied Histone Variants in Breast Cancer

The incorporation of histone variants has structural ramifications on nucleosome dynamics and stability. Due to their unique sequences, histone variants can alter histone-histone or histone-DNA interactions, impacting the folding of DNA around the histone octamer and the overall higher-order structure of chromatin fibers. These structural modifications alter chromatin compaction and accessibility of DNA by transcription factors and other regulatory proteins to influence gene regulatory processes such as DNA damage and repair, as well as transcriptional activation or repression. Histone variants can also generate a unique interactome composed of histone chaperones and chromatin remodeling complexes. Any of these perturbations can contribute to cellular plasticity and the progression of human diseases. Here, we focus on a frequently overlooked group of histone variants lying within the four human histone gene clusters and their contribution to breast cancer.

Truncating the spliceosomal 'rope protein' Prp45 results in Htz1 dependent phenotypes

Spliceosome assembly contributes an important but incompletely understood aspect of splicing regulation. Prp45 is a yeast splicing factor which runs as an extended fold through the spliceosome, and which may be important for bringing its components together. We performed a whole genome analysis of the genetic interaction network of the truncated allele of PRP45 (prp45(1-169)) using synthetic genetic array technology and found chromatin remodellers and modifiers as an enriched category. In agreement with related studies, H2A.Z-encoding HTZ1, and the components of SWR1, INO80, and SAGA complexes represented prominent interactors, with htz1 conferring the strongest growth defect. Because the truncation of Prp45 disproportionately affected low copy number transcripts of intron-containing genes, we prepared strains carrying intronless versions of SRB2, VPS75, or HRB1, the most affected cases with transcription-related function. Intron removal from SRB2, but not from the other genes, partly repaired some but not all the growth phenotypes identified in the genetic screen. The interaction of prp45(1-169) and htz1Δ was detectable even in cells with SRB2 intron deleted (srb2Δi). The less truncated variant, prp45(1-330), had a synthetic growth defect with htz1Δ at 16°C, which also persisted in the srb2Δi background. Moreover, htz1Δ enhanced prp45(1-330) dependent pre-mRNA hyper-accumulation of both high and low efficiency splicers, genes ECM33 and COF1, respectively. We conclude that while the expression defects of low expression intron-containing genes contribute to the genetic interactome of prp45(1-169), the genetic interactions between prp45 and htz1 alleles demonstrate the sensitivity of spliceosome assembly, delayed in prp45(1-169), to the chromatin environment.

Competitive Chemical Reaction Kinetic Model of Nucleosome Assembly Using the Histone Variant H2A.Z and H2A In Vitro

Nucleosomes not only serve as the basic building blocks for eukaryotic chromatin but also regulate many biological processes, such as DNA replication, repair, and recombination. To modulate gene expression in vivo, the histone variant H2A.Z can be dynamically incorporated into the nucleosome. However, the assembly dynamics of H2A.Z-containing nucleosomes remain elusive. Here, we demonstrate that our previous chemical kinetic model for nucleosome assembly can be extended to H2A.Z-containing nucleosome assembly processes. The efficiency of H2A.Z-containing nucleosome assembly, like that of canonical nucleosome assembly, was also positively correlated with the total histone octamer concentration, reaction rate constant, and reaction time. We expanded the kinetic model to represent the competitive dynamics of H2A and H2A.Z in nucleosome assembly, thus providing a novel method through which to assess the competitive ability of histones to assemble nucleosomes. Based on this model, we confirmed that histone H2A has a higher competitive ability to assemble nucleosomes in vitro than histone H2A.Z. Our competitive kinetic model and experimental results also confirmed that in vitro H2A.Z-containing nucleosome assembly is governed by chemical kinetic principles.

The Histone Variant H2A.Z C-Terminal Domain Has Locus-Specific Differential Effects on H2A.Z Occupancy and Nucleosome Localization

The incorporation of histone variant H2A.Z into nucleosomes creates specialized chromatin domains that regulate DNA-templated processes, such as gene transcription. In Saccharomyces cerevisiae, the diverging H2A.Z C terminus is thought to provide the H2A.Z exclusive functions. To elucidate the roles of this H2A.Z C terminus genome-wide, we used derivatives in which the C terminus was replaced with the corresponding region of H2A (ZA protein), or the H2A region plus a transcriptional activating peptide (ZA-rII'), with the intent of regenerating the H2A.Z-dependent regulation globally. The distribution of these H2A.Z derivatives indicates that the H2A.Z C-terminal region is crucial for both maintaining the occupation level of H2A.Z and the proper positioning of targeted nucleosomes. Interestingly, the specific contribution on incorporation efficiency versus nucleosome positioning varies enormously depending on the locus analyzed. Specifically, the role of H2A.Z in global transcription regulation relies on its C-terminal region. Remarkably, however, this mostly involves genes without a H2A.Z nucleosome in the promoter. Lastly, we demonstrate that the main chaperone complex which deposits H2A.Z to gene regulatory region (SWR1-C) is necessary to localize all H2A.Z derivatives at their specific loci, indicating that the differential association of these derivatives is not due to impaired interaction with SWR1-C. IMPORTANCE We provide evidence that the Saccharomyces cerevisiae C-terminal region of histone variant H2A.Z can mediate its special function in performing gene regulation by interacting with effector proteins and chaperones. These functional interactions allow H2A.Z not only to incorporate to very specific gene regulatory regions, but also to facilitate the gene expression process. To achieve this, we used a chimeric protein which lacks the native H2A.Z C-terminal region but contains an acidic activating region, a module that is known to interact with components of chromatin-remodeling entities and/or transcription modulators. We reasoned that because this activating region can fulfill the role of the H2A.Z C-terminal region, at least in part, the role of the latter would be to interact with these activating region targets.

Histone variant H2A.Z modulates nucleosome dynamics to promote DNA accessibility

Nucleosomes, containing histone variants H2A.Z, are important for gene transcription initiation and termination, chromosome segregation and DNA double-strand break repair, among other functions. However, the underlying mechanisms of how H2A.Z influences nucleosome stability, dynamics and DNA accessibility are not well understood, as experimental and computational evidence remains inconclusive. Our modeling efforts of human nucleosome stability and dynamics, along with comparisons with experimental data show that the incorporation of H2A.Z results in a substantial decrease of the energy barrier for DNA unwrapping. This leads to the spontaneous DNA unwrapping of about forty base pairs from both ends, nucleosome gapping and increased histone plasticity, which otherwise is not observed for canonical nucleosomes. We demonstrate that both N- and C-terminal tails of H2A.Z play major roles in these events, whereas the H3.3 variant exerts a negligible impact in modulating the DNA end unwrapping. In summary, our results indicate that H2A.Z deposition makes nucleosomes more mobile and DNA more accessible to transcriptional machinery and other chromatin components.

Direct assessment of histone function using histone replacement

Histones serve many purposes in eukaryotic cells in the regulation of diverse genomic processes, including transcription, replication, DNA repair, and chromatin organization. As such, experimental systems to assess histone function are fundamental resources toward elucidating the regulation of activities occurring on chromatin. One set of important tools for investigating histone function are histone replacement systems, in which endogenous histone expression can be partially or completely replaced with a mutant histone. Histone replacement systems allow systematic screens of histone regulatory functions and the direct assessment of functions for histone residues. In this review, we describe existing histone replacement systems in model organisms, the benefits and limitations of these systems, and opportunities for future research with histone replacement strategies.

Epigenetic stress memory: A new approach to study cold and heat stress responses in plants

Understanding plant stress memory under extreme temperatures such as cold and heat could contribute to plant development. Plants employ different types of stress memories, such as somatic, intergenerational and transgenerational, regulated by epigenetic changes such as DNA and histone modifications and microRNAs (miRNA), playing a key role in gene regulation from early development to maturity. In most cases, cold and heat stresses result in short-term epigenetic modifications that can return to baseline modification levels after stress cessation. Nevertheless, some of the modifications may be stable and passed on as stress memory, potentially allowing them to be inherited across generations, whereas some of the modifications are reactivated during sexual reproduction or embryogenesis. Several stress-related genes are involved in stress memory inheritance by turning on and off transcription profiles and epigenetic changes. Vernalization is the best example of somatic stress memory. Changes in the chromatin structure of the Flowering Locus C (FLC) gene, a MADS-box transcription factor (TF), maintain cold stress memory during mitosis. FLC expression suppresses flowering at high levels during winter; and during vernalization, B3 TFs, cold memory cis-acting element and polycomb repressive complex 1 and 2 (PRC1 and 2) silence FLC activation. In contrast, the repression of SQUAMOSA promoter-binding protein-like (SPL) TF and the activation of Heat Shock TF (HSFA2) are required for heat stress memory. However, it is still unclear how stress memory is inherited by offspring, and the integrated view of the regulatory mechanisms of stress memory and mitotic and meiotic heritable changes in plants is still scarce. Thus, in this review, we focus on the epigenetic regulation of stress memory and discuss the application of new technologies in developing epigenetic modifications to improve stress memory.