Structural Characterization of Histone H2A Variants

Abstract

protein to form chromatin, the fundamental unit of which is the nucleosome core particle (NCP). An NCP consists of two copies each of the four core histones H2A, H2B, H3, and H4. This histone octamer binds 147 base pairs of DNA around its outer surface in 1.65 tight superhelical turns (Fig. 1A) (Luger et al. 1997; Richmond and Davey 2003). Linker histones and other nonhistone proteins promote or stabilize the folding of nucleosomal arrays into superstructures of increasing complexity and largely unknown architecture (Hansen 2002). Covalent modification of the core histones and variations in the fundamental biochemical composition of nucleosomes distinguish transcriptionally active from inactive chromatin regions, by either changing the structure of the nucleosomes, altering their ability to interact with other protein factors, or modifying their propensity to fold into varying degrees of higher-order structures (or by any combination of the above). Studying the mechanism for establishing distinct chromatin domains is essential to understanding differential regulation of gene expression and all other DNA-dependent processes. Much progress has been made in this direction in the past few years. Substitution of one or more of the core histones with the corresponding histone variants has the potential to exert considerable influence on the structure and function of chromatin. Histone variants are distinct nonallelic forms of conventional, major-type histones that form the bulk of nucleosomes during replication and whose synthesis is tightly coupled to S phase. Histone variants are characterized by a completely different expression pattern that is not restricted to S phase. They are found in most eukaryotic organisms and are expressed in all tissue types (unlike some H2B isoforms that are found only in specialized tissues such as testes). Compared to their majortype counterparts, histone variants exhibit moderate to significant degrees of sequence homology (Fig. 1B). H2A.X (82%) and H3.3 (~96%) are the least divergent of all histone variants. H2A.Z (~60%), macroH2A (~65%), H2A.Bbd (40%), and CenpA, which has a 93 amino acid domain that is 62% identical to H3 (Palmer et al. 1991; Sullivan et al. 1994), are increasingly divergent in their Structural Characterization of Histone H2A Variants