H2A histones

Kim, Y.-J., Hwang, I., Tres, L.L., Kierszenbaum, A.L., and Chae, C.-B. (1987) Molecular cloning and differential expression of somatic and testis-specific H2A histone genes during rat spermatogenesis. Dev. Biol. 124, 23-34. 

Grosschedl, R., and Birnstiel, M. L., 1980a, Identification of regulatory sequences in the prelude sequences of an H2A histone gene by the study of specific deletion mutants in vivo, Proc. Natl. Acad. Sci. USA 77 1432-1436. 

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). 

Toyopearl HW-50S resin has been used to help isolate the ubiquitin-histone conjugate mH2A from the unicellular ciliated protozoan Tetrahymena pyriformis. Figure 4.49 shows the separation of mH2A from the histone, H2A. The sole difference between these two components is a small polypeptide, ubiquitin (approximately 8500 Da). The mH2A fraction was then further purified by HPLC on a Tosoh ODS-silica column (52). One of the many benefits... 

FIGURE 4.49 Isolation of a complex protein conjugate on Toyopearl HW-50S. Column 22 mm X 83 cm. Sample Fraction from crude Tetrahymena H2A containing the ubiquitin-histone conjugate uH2A. Elution 10 nM HCI. Flow rate 0.1 ml/min. Detection UV at 230 nm. 

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. 

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. 

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. 

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.

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 ... 

The H2a-H2b histone dimer also has strong salt-dependent conformational properties, with a transition near 0.5 M NaCl.(93) Above 0.5 M NaCl, the tyrosine fluorescence emission becomes less quenchable by Cs+, and the dimer structure becomes more compact. 

The purpose of this article is to correlate the rather unique structural aspects of the five histone molecules—the differences among them as well as their similarities—with their biological function. Such an analysis is best approached, we believe, via a study of the many protein-protein (Section II) and protein-DNA (Section III) interactions in which the histones participate. Emphasis will be placed on the four core histones H2A, H2B, H3, and H4 HI will be discussed briefly, mainly in relation to its interaction with DNA. In no sense is the bibliography meant to be exhaustive. 

The nucleosome is composed of 200 base pairs of DNA and an octamer of the histones H2A, H2B, H3, and H4 as well as histone HI (Komberg, 1974, 1977). Nucleosomes can be obtained by mild digestion of chromatin with micrococcal nuclease (Noll, 1974a Axel, 1975), followed by fractionation on a sucrose gradient. Further digestion of the nucleosomes results in the formation of nucleosome core particles composed of 145 base pairs of DNA and an octamer of the histones H2A, H2B, H3, and H4 (Rill and Van Holde, 1973 Sollner-Webb and Felsenfeld, 1975 Axel, 1975 Bakayev et al., 1975 Whitlock and Simpson, 1976 Noll and Komberg, 1977). The DNA piece thus excised is called linker DNA which serves as a link... 

Fic. 1. The amino acid sequences of calf histones H2A (Yeoman et al., 1972 Sau-tiere et al., 1974) and H2B (Iwai et al., 1972). A one-letter code is used A, alanine R, arginine N, asparagine D, aspartic acid C, cysteine E, glutamic acid Q, glutamine ... 

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