Histone acetyltransferase
Histone acetyltransferases (HATs) are enzymes that acetylate conserved lysine amino acids on histone proteins by transferring an acetyl group from acetyl-CoA to form ε-N-acetyllysine. DNA is wrapped around histones, and, by transferring an acetyl group to the histones, genes can be turned on and off. In general, histone acetylation increases gene expression. In general, histone acetylation is linked to transcriptional activation and associated with euchromatin. Euchromatin, which is less densely compact, allows transcription factors to bind more easily to regulatory sites on DNA, causing transcriptional activation. When it was first discovered, it was thought that acetylation of lysine neutralizes the positive charge normally present, thus reducing affinity between histone and (negatively charged) DNA, which renders DNA more accessible to transcription factors. Research has emerged, since, to show that lysine acetylation and other posttranslational modifications of histones generate binding sites for specific protein–protein interaction domains, such as the acetyllysine-binding bromodomain. Histone acetyltransferases can also acetylate non-histone proteins, such as nuclear receptors and other transcription factors to facilitate gene expression. HATs are traditionally divided into two different classes based on their subcellular localization. Type A HATs are located in the nucleus and are involved in the regulation of gene expression through acetylation of nucleosomal histones in the context of chromatin. They contain a bromodomain, which helps them recognize and bind to acetylated lysine residues on histone substrates. Gcn5, p300/CBP, and TAFII250 are some examples of type A HATs that cooperate with activators to enhance transcription. Type B HATs are located in the cytoplasm and are responsible for acetylating newly synthesized histones prior to their assembly into nucleosomes. These HATs lack a bromodomain, as their targets are unacetylated. The acetyl groups added by type B HATs to the histones are removed by HDACs once they enter the nucleus and are incorporated into chromatin.
