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In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn are wrapped into 30-nanometer fibers that form tightly packed chromatin. Histones prevent DNA from becoming tangled and protect it from DNA damage. In addition, histones play important roles in gene regulation and DNA replication. Without histones, unwound DNA in chromosomes would be very long. For example, 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) of 30 nm diameter chromatin fibers.
Pyrrolysine is an α-amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacteria; it is not present in humans. It contains an α-amino group and a carboxylic acid group. Its pyrroline side-chain is similar to that of lysine in being basic and positively charged at neutral pH.
Methylation, in the chemical sciences, is the addition of a methyl group on a substrate, or the substitution of an atom by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These terms are commonly used in chemistry, biochemistry, soil science, and biology.
Histone methyltransferases (HMT) are histone-modifying enzymes, that catalyze the transfer of one, two, or three methyl groups to lysine and arginine residues of histone proteins. The attachment of methyl groups occurs predominantly at specific lysine or arginine residues on histones H3 and H4. Two major types of histone methyltranferases exist, lysine-specific and arginine-specific. In both types of histone methyltransferases, S-Adenosyl methionine (SAM) serves as a cofactor and methyl donor group.
The genomic DNA of eukaryotes associates with histones to form chromatin. The level of chromatin compaction depends heavily on histone methylation and other post-translational modifications of histones. Histone methylation is a principal epigenetic modification of chromatin that determines gene expression, genomic stability, stem cell maturation, cell lineage development, genetic imprinting, DNA methylation, and cell mitosis.
Histone H4 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H4 is involved with the structure of the nucleosome of the 'beads on a string' organization. Histone proteins are highly post-translationally modified. Covalently bonded modifications include acetylation and methylation of the N-terminal tails. These modifications may alter expression of genes located on DNA associated with its parent histone octamer. Histone H4 is an important protein in the structure and function of chromatin, where its sequence variants and variable modification states are thought to play a role in the dynamic and long term regulation of genes.
Histone methylation is a process by which methyl groups are transferred to amino acids of histone proteins that make up nucleosomes, which the DNA double helix wraps around to form chromosomes. Methylation of histones can either increase or decrease transcription of genes, depending on which amino acids in the histones are methylated, and how many methyl groups are attached. Methylation events that weaken chemical attractions between histone tails and DNA increase transcription because they enable the DNA to uncoil from nucleosomes so that transcription factor proteins and RNA polymerase can access the DNA. This process is critical for the regulation of gene expression that allows different cells to express different genes.
Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossmann fold for binding S-Adenosyl methionine (SAM). Class II methyltransferases contain a SET domain, which are exemplified by SET domain histone methyltransferases, and class III methyltransferases, which are membrane associated. Methyltransferases can also be grouped as different types utilizing different substrates in methyl transfer reactions. These types include protein methyltransferases, DNA/RNA methyltransferases, natural product methyltransferases, and non-SAM dependent methyltransferases. SAM is the classical methyl donor for methyltransferases, however, examples of other methyl donors are seen in nature. The general mechanism for methyl transfer is a SN2-like nucleophilic attack where the methionine sulfur serves as the leaving group and the methyl group attached to it acts as the electrophile that transfers the methyl group to the enzyme substrate. SAM is converted to S-Adenosyl homocysteine (SAH) during this process. The breaking of the SAM-methyl bond and the formation of the substrate-methyl bond happen nearly simultaneously. These enzymatic reactions are found in many pathways and are implicated in genetic diseases, cancer, and metabolic diseases. Another type of methyl transfer is the radical S-Adenosyl methionine (SAM) which is the methylation of unactivated carbon atoms in primary metabolites, proteins, lipids, and RNA.
rRNA 2'-O-methyltransferase fibrillarin is an enzyme that in humans is encoded by the FBL gene.
Protein arginine N-methyltransferase 1 is an enzyme that in humans is encoded by the PRMT1 gene. The HRMT1L2 gene encodes a protein arginine methyltransferase that functions as a histone methyltransferase specific for histone H4.
Interleukin enhancer-binding factor 3 is a protein that in humans is encoded by the ILF3 gene.
DNA (cytosine-5)-methyltransferase 3A (DNMT3A) is an enzyme that catalyzes the transfer of methyl groups to specific CpG structures in DNA, a process called DNA methylation. The enzyme is encoded in humans by the DNMT3A gene.
Protein arginine N-methyltransferase 5 is an enzyme that in humans is encoded by the PRMT5 gene. PRMT5 symmetrically dimethylates H2AR3, H4R3, H3R2, and H3R8 in vivo, all of which are linked to a range of transcriptional regulatory events.
Serine/arginine-rich splicing factor 7 (SRSF7) also known as splicing factor, arginine/serine-rich 7 (SFRS7) or splicing factor 9G8 is a protein that in humans is encoded by the SRSF7 gene.
Protein arginine N-methyltransferase 3 is an enzyme that in humans is encoded by the PRMT3 gene.
In molecular biology, a Tudor domain is a conserved protein structural domain originally identified in the Tudor protein encoded in Drosophila. The Tudor gene was found in a Drosophila screen for maternal factors that regulate embryonic development or fertility. Mutations here are lethal for offspring, inspiring the name Tudor, as a reference to the Tudor King Henry VIII and the several miscarriages experienced by his wives.
Protein arginine methyltransferase 8 is a protein that in humans is encoded by the PRMT8 gene. Arginine methylation is a posttranslational modification involved in a number of cellular processes, including DNA repair, RNA transcription, signal transduction and protein compartmentalization. PRMT8 binds and dimethylates Ewing sarcoma breakpoint region 1 (EWS) protein.
Euchromatic histone-lysine N-methyltransferase 1, also known as G9a-like protein (GLP), is a protein that in humans is encoded by the EHMT1 gene.
Radical SAM enzymes belong to a superfamily of enzymes that use an iron-sulfur cluster (4Fe-4S) to reductively cleave S-adenosyl-L-methionine (SAM) to generate a radical, usually a 5′-deoxyadenosyl radical (5'-dAdo), as a critical intermediate. These enzymes utilize this radical intermediate to perform diverse transformations, often to functionalize unactivated C-H bonds. Radical SAM enzymes are involved in cofactor biosynthesis, enzyme activation, peptide modification, post-transcriptional and post-translational modifications, metalloprotein cluster formation, tRNA modification, lipid metabolism, biosynthesis of antibiotics and natural products etc. The vast majority of known radical SAM enzymes belong to the radical SAM superfamily, and have a cysteine-rich motif that matches or resembles CxxxCxxC. Radical SAM enzymes comprise the largest superfamily of metal-containing enzymes.
SET domain containing 6 is a protein in humans that is encoded by the SETD6 gene.
Protein methylation is a type of post-translational modification featuring the addition of methyl groups to proteins. It can occur on the nitrogen-containing side-chains of arginine and lysine, but also at the amino- and carboxy-termini of a number of different proteins. In biology, methyltransferases catalyze the methylation process, activated primarily by S-adenosylmethionine. Protein methylation has been most studied in histones, where the transfer of methyl groups from S-adenosyl methionine is catalyzed by histone methyltransferases. Histones that are methylated on certain residues can act epigenetically to repress or activate gene expression.
References
- 1 2 "Protein arginine methyltransferase 7" . Retrieved 2011-12-06.
- ↑ Miranda, T. B.; Miranda, M.; Frankel, A.; Clarke, S. (2004). "PRMT7 is a Member of the Protein Arginine Methyltransferase Family with a Distinct Substrate Specificity". Journal of Biological Chemistry. 279 (22): 22902–22907. doi: 10.1074/jbc.M312904200 . PMID 15044439.
- ↑ Tollefsbol, Trygve O., ed. (2023). Handbook of epigenetics: the new molecular and medical genetics (Third ed.). London, United Kingdom ; San Diego, CA: Academic Press, an imprint of Elseiver. p. 36. ISBN 978-0-323-91909-8.
