H3 histones

Histone H3 Histones are DNA-binding proteins found in chromosomes 135 amino acid residues. Note die very basic nature of this protein dne to its abmidance of Arg and Lys residues. It also lacks tryptophan. 

Witt O, Albig W, Doenecke D (1996) Testis-specific expression of a novel human H3 histone gene. Exp Cell Res 229 301-306... 

Found in the chromatin of all eukaryotic cells, histones have molecular weights between 11,000 and 21,000 and are very rich in the basic amino acids arginine and lysine (together these make up about one-fourth of the amino acid residues). All eukaryotic cells have five major classes of histones, differing in molecular weight and amino acid composition (Table 24-3). The H3 histones are nearly identical in amino acid sequence in all eukaryotes, as are the H4 histones, suggesting strict conservation of their functions. For example, only 2 of 102 amino acid residues differ between the H4 histone molecules of peas and cows, and only 8 differ between the H4 histones of humans and yeast. Histones HI, H2A, and H2B show less sequence similarity among eukaryotic species. 

DADS has also reported to induce the differentiation of DS19 mouse erythroleukemic cells (Lea et al. 1999). DADS caused increase in acetylation ofH4 and H3 histones in DS19 cells and K562 human leukemic cells. DADS was more effective than DAS and diallyl sulfone. Histone acetylation was also induced in rat hepatoma and human breast cancer cells by DADS or its metabolite, allyl mercaptan. The allyl mercaptan is a more potent inhibitor of histone deacetylase than DADS. Differentiation in erytiiroleuke-... 

Therefore, various histone fractions have different specializations in chromatin structure. The phosphorylation of HI histone determines the molecular level of chromatin organization the phosphorylation of H2a histone determines the heterochromatization pf chromatin and the superphosphorylation of H1 histone, and normal phosphorylation of H3 histone affects the microscopic level of organization (Curley et al., 1978). 

Chromatin is composed of nucleosomes, where each comprise 147 base pairs of DNA wrapped around an octamer oftwo copies of each histone H2A, H2B, H3, and H4. Nucleosomes are folded into higher-order structures that are stabilized by linker histones. Chromatin structure can be altered by enzymes that posttranslationally modify histones (e.g., through phosphorylation, acetylation, methylation, or ubiquitination) or by ATP-driven chromatin-remodeling complexes that alter nucleosome position and/or composition. 

Histones are small, basic proteins required to condense DNA into chromatin. They have been first described and named in 1884 by Albrecht Kossel. There are five main histones HI, H2A, H2B, H3 andH4. An octamer of core histones H2A, H2B, H3 andH4 is located inside a nucleosome, the central building block of chromatin, with about 150 base pairs of DNA wrapped around. The basic nature of histones, mediated by the high content of lysine and arginine residues, allows a direct interaction with the acidic phosphate back bone of DNA. The fifth histone HI is located outside at the junction between nucleosomes and is referred to as the linker histone. Besides the main histones, so-called histone variants are known, which replace core histones in certain locations like centromers. 

Macdonald N, Welburn JP, Noble ME et al (2005) Molecular basis for the recognition of phosphorylated and phosphoacetylated histone H3 by 14-3-3. Mol Cell 20 199-211... 

The core unit of the chromatin, the nucleosome, consists of histones arranged as an octamer consisting of a (H3/ H4)2-tetramer complexed with two histone H2A/H2B dimers. Accessibility to DNA-binding proteins (for replication, repair, or transcription) is achieved by posttranslational modifications of the amino-termini of the histones, the histone tails phosphorylation, acetylation, methylation, ubiquitination, and sumoyla-tion. Especially acetylation of histone tails has been linked to transcriptional activation, leading to weakened interaction of the core complexes with DNA and subsequently to decondensation of chromatin. In contrast, deacetylation leads to transcriptional repression. As mentioned above, transcriptional coactivators either possess HAT activity or recruit HATs. HDACs in turn act as corepressors. 

The histones interact with each other in very specific ways. H3 and H4 form a tetramer containing two mol-... 

When the histone octamer is mixed with purified, double-stranded DNA, the same x-ray diffraction pattern is formed as that observed in freshly isolated chromatin. Electron microscopic studies confirm the existence of reconstituted nucleosomes. Furthermore, the reconsti-mtion of nucleosomes from DNA and histones H2A, H2B, H3, and H4 is independent of the organismal or cellular origin of the various components. The histone HI and the nonhistone proteins are not necessary for the reconstitution of the nucleosome core. 

Acetylation of histones H3 and H4 is associated with the activation or inactivation of gene transcription (Chapter 37). 

Figure 36-2. Model for the structure of the nucleosome, in which DNA is wrapped around the surface of a flat protein cylinder consisting of two each of histones H2A, H2B, H3, and H4 that form the histone octamer. The 146 base pairs of DNA, consisting of 1.75 superhelical turns, are in contact with the histone octamer. This protects the DNA from digestion by a nuclease. The position of histone HI, when it is present, is indicated by the dashed outline at the bottom of the figure.

Interestingly, post integration latency in microglial cells seems to be the result of the concerted action of both HDACs as well as HMTs on core histone H3 in nuc-1 (Marban et al. 2007). COUP-TF interacting protein 2 (CTIP2), a transcriptional repressor interacts with HDACl and HDAC2 via its N-terminus to repress... 

Bannister AJ, Zegerman P, Partridge JF, Miska EA, Thomas JO, AUshire RC, Kouzarides T (2001) Selective recognition of methylated lysine 9 on histone H3 by the HPl chromo domain. Nature 410(6824) 120-124... 

In the nuclei of all eukaryotic cells, DNA is tightly wrapped around an octamer of histone proteins and is compacted into a dense structure known as chromatin. In order to access the genetic information which is required in numerous essential cellular processes including DNA replication, gene expression and DNA repair, chromatin needs to be partially unwound. One important mechanism to regulate chromatin structure and thus to control the access of the genomic DNA is through histone modifications [1-6]. The histone octamer is composed of two copies of H2A, H2B, H3 and H4 core histone proteins. Their tails, that protrude out of the surface of the... 

The four histone groups that are composed of ho-mogeneous proteins, H2A, H2B, H3, and H4, make up the nucleosome core. Each core consists of two copies of the four histones. The double-stranded DNA is wrapped twice around each core in a left-handed superhelix. A superhelix is the name given to the additional helix made by the double-stranded, helical DNA as it is wrapped around the nucleosome core. A familiar superhelix in everyday life is a twisted spiral telephone cord. The nucleosome core of histones do not recognize specific DNA structures rather, they can bind to any stretch of DNA as long as it is not too close to a neighboring nucleosome. The order of contact of histones to the DNA is as follows ... 

---