Histone methyltransferase

Histone methyltransferases (HMT) are histone-modifying enzymes (e.g., histone-lysine N-methyltransferases and histone-arginine N-methyltransferases), 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 (which can be SET (Su(var)3-9, Enhancer of Zeste, Trithorax) domain containing or non-SET domain containing) and arginine-specific. In both types of histone methyltransferases, cofactor S-Adenosyl methionine (SAM) serves as a cofactor and methyl donor group. In eukaryotic cells, the genome is tightly condensed into chromatin (composed of DNA and histone proteins), so enzymes, such as histone methyltransferases, must overcome this inaccessibility. Histone methyltransferase does so by modifying histones at certain sites through methylation. Methylation of histones is important biologically because it is the principal epigenetic modification of chromatin that determines gene expression, genomic stability, stem cell maturation, cell lineage development, genetic imprinting, DNA methylation, and cell mitosis.