Any of a class of rather simple proteins occurring in cell nuclei and in combination with DNA to form nucleoproteins. They can be obtained from both plants and animals. Unlike most proteins, they dissolve readily in water. They were discovered circa 1884.
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Two each of the class H2A, H2B, H3 and H4, so-called core histones, assemble to form one octameric nucleosome core particle by wrapping 146 base pairs of DNA around the protein spool in 1.65 left-handed super-helical turn. The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA spaced between each nucleosome (also referred to as linker DNA). The assembled histones and DNA is called chromatin. Higher order structures include the 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins.
In all, histones make five types of interactions with DNA:
The highly basic nature of histones, aside from facilitating DNA-histone interactions, contributes to the water solubility of histones.
Histones are subject to posttranslational modification by enzymes primarily on their N-terminal tails, but also in their globular domains. Such modifications include methylation, citrullination, acetylation, phosphorylation, Sumoylation, ubiquitination, and ADP-ribosylation. This affects their function of gene regulation (see functions).
In general, genes that are active have less bound histone, while inactive genes are highly associated with histones during interphase. It also appears that the structure of histones has been evolutionarily conserved, as any deleterious mutations would be severely maladaptive.
The common nomenclature of histone modifications is as follows:
So H3K4me1 denotes the monomethylation of the 4th residue (a lysine) from the start (i.e., the N-terminal) of the H3 protein.
|Type of modification|
Core histones are highly conserved proteins, that is, there are very few differences among the amino acid sequences of the histone proteins of different species. Linker histone usually has more than one form within a species and is also less conserved than the core histones.
There are some variant forms in some of the major classes. They share amino acid sequence homology and core structural similarity to a specific class of major histones but also have their own feature that is distinct from the major histones. These minor histones usually carry out specific functions of the chromatin metabolism. For example, histone H3-like CenpA is a histone only associated with centromere region of the chromosome. Histone H2A variant H2A.Z is associated with the promoters of actively transcribed genes and also involved in the formation of the heterochromatin. Another H2A variant H2A.X binds to the DNA with double strand breaks and marks the region undergoing DNA repair. Histone H3.3 is associated with the body of actively transcribed genes.
Histone gene expression and histone mRNA 3' end structure in Caenorhabditis elegans.(Research article)(ribonucleic acid)(Clinical report)
Jun 14, 2007; Authors: Rebecca Keall (equal contributor) ; Sandra Whitelaw (equal contributor) ; Jonathan Pettitt ; Berndt Müller...