Post-translational modifications of histones
Post-translational modification (PTM) of histones is a crucial step in epigenetic regulation of a gene. N-terminal tails of histones are the most accessible regions of these peptide as they protrude from the nucleosome and possess no specific structure. These tails are subjected to various modifications such as acetylation, methylation, phosphorylation, ubiquitination etc. by the 'writers'. PTMs are believed to function in a combinatorial pattern referred to as the 'histone code'. The major function of PTMs is to either create sites for the recruitment of specific factors or modify existing sites so as to abolish previous interactions. This alters the expression states of associated loci by multiple ways thus enabling gene regulation. PTMs can recruite enzymes that can ‘write’, ‘erase’ or ‘read’ modifications and the repertoire of such modifiers is found to be fairly large in number (~150 different enzymes in humans). Certain modifications such as acetylation, phosphorylation, change the overall charge on basic histone proteins and thereby interfere with the histone-DNA interaction essential for nucleosome stability. In terms of molecular weight, these modifications range from light (acetylation, methylation, phosphorylation) to heavy (ubiquitination, poly ADP ribosylation). Here we include 8 different types of modifications that exist on all histone peptides.
PTMs are often found to be cell cycle dependent. Role of various histone PTMs has been evaluated in many important cellular processes such as demarcating euchromatin and hetrochromatin regions, transcriptional regulation of Hox gene clusters, maintainance of stemness, cell cycle control etc. Presence or absence of certain PTMs is shown to be a hallmark of different cancers.Types of PTMs Arginine citrullination, Arginine methylation, Lysine acetylation, Lysine biotinylation, Lysine methylation, Lysine ribosylation, Lysine ubiquitination, Serine/threonine/tyrosine phosphorylation