Histone proteins

Histones are proteins found in the nucleus of eucaryotic cells that associate with DNA and help condense it into chromatin. Histones are abundant in lysine and arginine residues which makes them positively charged and very basic. Their positive charge allows them to associate with DNA, which is negatively charged. By doing so, histones protect DNA from being damaged and prevent DNA from being tangled. Histones also play a vital role in gene regulation and DNA replication. Without histones, unwound DNA would be exceptionally long and impossible to fit in the nucleus or in the cell. For instance, each human cell has about 1.8 meters of DNA if completely stretched out, however when wound about histones, this length is reduced to about 90 micrometers (0.09 mm). Nuclear DNA does not appear in free linear strands; it is highly condensed and wrapped around histones in order to fit inside of the nucleus and take part in the formation of chromosomes.

Histones act as spools around which DNA winds to create the structural unit also known as nucleosome. Each nucleosome is made of DNA wrapped around eight histone proteins and is called a histone octamer. Each histone octamer is composed of two copies of each of the histone proteins H2A, H2B, H3, and H4. The chain of nucleosomes is then wrapped into a 30 nm spiral called a solenoid, where additional H1 histone proteins are associated with each nucleosome to maintain the chromosome structure. Nucleosomes are then assembled into a compact structure also known as chromatin and eventually into very large, high-order structures known as chromosomes.

Histones may be chemically modified through the action of enzymes to regulate gene transcription. There are two types of main modification that histone proteins undergo:

• Methylation of arginine or lysine residues. Methylation can affect how other proteins such as transcription factors interact with the nucleosomes.
• Acetylation of lysine residues. Lysine acetylation eliminates a positive charge on lysine thereby weakening the electrostatic attraction between histone and DNA resulting in partial unwinding of the DNA making it more accessible for gene expression.

As histone variants have such ubiquitous expression and pleiotropic roles, it is not surprising that they are involved in normal physiology as well as many pathological processes. For example, histone variants may be linked to brain-related diseases such as neurodevelopmental and neurodegenerative diseases. Moreover, many cancers and developmental syndromes are known to carry recurrent mutations in genes encoding chromatin-associated proteins, including histone variants and their deposition machineries.

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