Histone variants. signaling or structural modules

Histones are the main protein component of chromatin and they can be divided into two major groups, core histones and linker histones. Both core histones and linker histones exhibit inter and intra-specific amino acid sequence variability and each have their own set of variants. This chapter focuses on some of the most recent achievements regarding the information on each individual histone variant. It focuses on structural and signaling aspects that contribute to the epigenetic role recently assigned to these variants. Histone variants can modulate the highly dynamic nature of the nucleosome to accommodate for different nuclear DNA metabolic needs such as transcription, replication, and repair. The existence of heterotypic single and multiple variant nucleosomes add to an additional layer of dynamic complexity. All of the conformational effects of histone variants may be enhanced further by the presence of specific posttranslational modifications (PTMs) to create a code of structures and signals with strong epigenetic implications. Following this, the study explores how histone variants can have both structural and signaling roles that can be directly mediated by the nature of the variability of the protein sequence itself and in combination with PTMs. It provides examples where core histone variants can have structural implications of their own at the levels of the NCP and the chromatin fiber. Finally, it states that discerning the individual contribution of the structural and signaling components of histone variants to the epigenetic changes they impart on chromatin may still be a question that requires more than what is currently known. © 2010 Elsevier Inc. All rights reserved