Identification of Specific Functional Subdomains within the Linker Histone H10 C-terminal Domain

177

106

H1 histones bind to DNA as they enter and exit the nucleosome. H1 histones have a tripartite structure consisting of a short N-terminal domain, a highly conserved central globular domain, and a lysine-and arginine-rich C-terminal domain. The C-terminal domain comprises approximately half of the total amino acid content of the protein, is essential for the formation of compact chromatin structures, and contains the majority of the amino acid variations that define the individual histone H1 family members. This region contains several cell cycle-regulated phosphorylation sites and is thought to function through a charge-neutralization process, neutralizing the DNA phosphate backbone to allow chromatin compaction. In this study, we use fluorescence microscopy and fluorescence recovery after photobleaching to define the behavior of the individual histone H1 subtypes in vivo. We find that there are dramatic differences in the binding affinity of the individual histone H1 subtypes in vivo and differences in their preference for euchromatin and heterochromatin. Further, we show that subtype-specific properties originate with the C terminus and that the differences in histone H1 binding are not consistent with the relatively small changes in the net charge of the C-terminal domains.The H1 or "linker" histones are a family of very lysine-rich proteins that associate with the stretch of DNA that enters and exits the nucleosome. In the absence of histone H1, the nucleosome comprises ϳ146 bp of DNA that wraps 1.75 turns around the outer surface of the histone octamer (1-3). In the presence of histone H1, the resulting chromatosome contains two complete turns of 168-bp DNA. Histone H1, which is in an ϳ1:1 ratio with the number of nucleosomes in the cell, bends and alters the path of the DNA entering and exiting the nucleosome such that the nucleosome adopts a lollipop-like conformation (4, 5). This change in the trajectory of the linker DNA defines the first step in folding the polynucleosome chain into interphase chromosomes.The discovery of the H1 histones and the identification of its seven variants in the mid 1970s suggested possible roles in development (6). An additional variant, referred to as H1oo, was recently isolated from the oocytes of mice (7). These studies revealed associations between histone H1 subtypes in cell growth and differentiation (8 -12) and in the development of higher eukaryotes (9 -15). In simplistic terms, variants that have been associated with cellular differentiation are the histone H5 and histone H1.0 subtypes, and they are closer in amino acid sequences and most divergent from the other somatic histone H1 variants. Histone H5 is restricted to amphibian and reptile species, where it is found in high abundance in the nucleated but transcriptionally inert erythrocytes (16,17). The nuclei of these mature erythrocytes are almost entirely heterochromatic, allowing them to maintain a very small total volume. In this case, histone H5 expression correlates with the cessation of the RNA polymerase . I...

supporting

confidence: 71%

mentioning

confidence: 99%

H1 Family Histones in the Nucleus

Th'ng

Sung

et al. 2005

177

106

“…MeCP2 and H1-Wild type human MeCP2 and selected RTT-causing mutations, and human H1 0 were expressed and purified as described (5,19).…”

Section: Methodsmentioning

confidence: 99%

MeCP2-Chromatin Interactions Include the Formation of Chromatosome-like Structures and Are Altered in Mutations Causing Rett Syndrome

Nikitina

Ghosh

Horowitz-Scherer

et al. 2007

Self Cite

103

112

Sporadic mutations in hMeCP2 (human methylated DNAbinding protein 2) cause the majority of cases of the X-linked neurodevelopmental disease known as Rett syndrome (RTT), 2 a severe autism spectrum disorder (reviewed in Refs. 1-3). The disease results in a diverse range of debilitating physical and neurological symptoms that typically make their initial appearance in the first 6 -18 months of life in affected girls. Although most RTT cases result from a loss of function effect, mutations that increase the MeCP2 dose may also give rise to similar symptoms, a finding seen also in mice engineered to synthesize additional MeCP2 (4). Extensive research has suggested that MeCP2 acts as a transmitter of epigenetic information by binding to symmetrically methylated CpG dinucleotides, recruiting complexes that include histone deacetylase and methyltransferase, and leading to local transcriptional repression. However, there is also considerable evidence that MeCP2 governs additional processes such as chromatin condensation (5, 6) and that its function is not restricted to transcriptional repression (3).To better understand MeCP2 function, we are studying the basic interactions between MeCP2 and its DNA and chromatin substrates. In addition to a methylation-dependent interaction with DNA that is mediated by the methylated DNA-binding domain (MBD), MeCP2 has a C-terminal chromatin-binding region as well as additional DNA-binding regions (5, 6). MeCP2 binding contributes to the formation in nucleosomal arrays of a distinctive structural motif in which the entering and exiting linker DNA segments are brought into close juxtaposition forming a "stem." The stem motif appears very similar to structures induced by histone H1 on mono-nucleosomes and polynucleosomes (7,8) and is thought to initiate the zigzag conformation and compaction of H1-containing chromatin. MeCP2 shares with H1 the ability to induce chromatin compaction, but the multiple chromatin-binding regions of MeCP2 lead to a higher level of condensation (9).A distinctive property of H1-containing chromatin is the protection from micrococcal nuclease (Mnase) digestion of ϳ20 bp of DNA beyond the ϳ146 bp of the nucleosome core particle (NCP). There is considerable evidence that the globular domain of H1 binds near the linker entry-exit site of the nucleosome (reviewed in Ref. 10). However, the location of the ϳ20 bp of protected DNA with respect to the parent nucleosome is influenced by the underlying DNA sequence (11), and the detailed molecular structure of the H1-containing unit, termed the chromatosome (12), remains controversial (10).In the present study we show that hMeCP2, like H1, provides specific protection of linker DNA. However, the two proteins differ significantly in the length of protected DNA. With the

“…32). For example, a set of studies on the role of the murine H1(0) subtype in chromatin folding in vitro suggests that this function resides in the unique amino acid composition of two subdomains within the CTD (17,18). The H1(0) CTD was also reported to bind and stimulate the activity of the apoptotic nuclease, DNA fragmentation factor DFF40/caspase-activated DNase (33).…”

Section: Discussionmentioning

confidence: 99%

Independent Biological and Biochemical Functions for Individual Structural Domains of Drosophila Linker Histone H1

Kavi¹,

Emelyanov

Fyodorov

et al. 2016

Linker histone H1 is among the most abundant components of chromatin. H1 has profound effects on chromosome architecture. H1 also helps to tether DNA-and histone-modifying enzymes to chromatin. Metazoan linker histones have a conserved tripartite structure comprising N-terminal, globular, and long, unstructured C-terminal domains. Here we utilize truncated Drosophila H1 polypeptides in vitro and H1 mutant transgenes in vivo to interrogate the roles of these domains in multiple biochemical and biological activities of H1. We demonstrate that the globular domain and the proximal part of the C-terminal domain are essential for H1 deposition into chromosomes and for the stability of H1-chromatin binding. The two domains are also essential for fly viability and the establishment of a normal polytene chromosome structure. Additionally, through interaction with the heterochromatin-specific histone H3 Lys-9 methyltransferase Su(var)3-9, the H1 C-terminal domain makes important contributions to formation and H3K9 methylation of heterochromatin as well as silencing of transposons in heterochromatin. Surprisingly, the N-terminal domain does not appear to be required for any of these functions. However, it is involved in the formation of a single chromocenter in polytene chromosomes. In summary, we have discovered that linker histone H1, similar to core histones, exerts its multiple biological functions through independent, biochemically separable activities of its individual structural domains.DNA in the eukaryotic nucleus is packaged into a compact nucleoprotein complex called chromatin (1, 2). The histones constitute a family of basic proteins that play a vital role in organizing chromatin. There are five major classes of histones: the core histones H2A, H2B, H3, and H4 and the linker histone H1. The nucleosome core particle, the fundamental unit of chromatin, consists of an octamer of two copies of each of the core histones around which about 146 bp of DNA is wrapped. H1 binds to the nucleosome core particle and the linker DNA between adjacent core particles. The stoichiometry of H1 to nucleosome core particles in cells of complex eukaryotes ranges from about 0.5 to approximately equimolar (3). Given this abundance, the linker histone H1 is expected to play important roles in chromatin structure. Indeed, numerous in vitro studies, as well as experiments in vivo, have shown that H1 has a profound effect on chromatin structure (reviewed in Refs. 4 and 5). For example, incorporation of H1 into chromatin leads to an increase in the spacing between nucleosome core particles. H1 also facilitates the folding of chromatin into more compact structures. In addition to these architectural roles, there are now an increasing number of reports describing interactions between H1 linker histones and a variety of other nuclear proteins (6), including functional interactions between H1, histone-modifying enzymes, and DNA methyltransferases that lead to modulation of epigenetic marking of chromatin (7,8). In this context, it is notable th...