Histone proteins are known to stabilize and regulate the nuclear DNA. Core histones, H2A, H2B, H3 and H4 aid the packaging of eukaryotic nuclear DNA into repeating nucleosome units while the linker histone, H1, binds to DNA between the nucleosomes. A diverse and elaborate array of post-translational modifications, including acetylation, phosphorylation and methylation, ubiquitination and ADP ribosylation occur in histones, particularly on the N-terminal tail domains of histones, particularly of H3 and H4.These modifications dictate chromatin states and the gene expression.
Phosphorylation of H3, mainly on Ser10 and Ser28, is localized to a small fraction of highly acetylated H3 and occurs primarily in response to mitogenic and stress stimuli. Phosphorylation of Ser10 is considered a crucial event for the onset of mitosis. It is tightly correlated with chromosome condensation during both mitosis and meiosis. Histone methylation, like acetylation, is a complex, dynamic process involved in a number of processes, including transcriptional regulation, chromatin condensation, mitosis ad heterochromatin assembly. Conserved lysine residues in the N-terminal tail domains of histone H3, Lys4, Lys9, and Lys27 are the preferred sites of methylation. Histone lysine residues can be mono-, di-, and tri-methylated at different heterochromatic subdomains, adding further complexity to the regulation of chromatin structure.
