Histone variants
Histone variants are proteins that substitute for the core canonical histones (H3, H4, H2A, H2B) in nucleosomes in eukaryotes and often confer specific structural and functional features. The term might also include a set of linker histone (H1) variants, which lack a distinct canonical isoform. The differences between the core canonical histones and their variants can be summarized as follows: (1) canonical histones are replication-dependent and are expressed during the S-phase of cell cycle whereas histone variants are replication-independent and are expressed during the whole cell cycle; (2) in animals, the genes encoding canonical histones are typically clustered along the chromosome, are present in multiple copies and are among the most conserved proteins known, whereas histone variants are often single-copy genes and show high degree of variation among species; (3) canonical histone genes lack introns and use a stem loop structure at the 3’ end of their mRNA, whereas histone variant genes may have introns and their mRNA tail is usually polyadenylated. Complex multicellular organisms typically have a large number of histone variants providing a variety of different functions. Recent data are accumulating about the roles of diverse histone variants highlighting the functional links between variants and the delicate regulation of organism development.
Histone variants nomenclature
Different names historically assigned to homologous proteins in different species complicate the nomenclature of histone variants. A recently suggested unified nomenclature of histone variants follows phylogeny-based approach to naming the variants.[1] According to this nomenclature, letter suffixes or prefixes are mainly used to denote structurally distinct monophyletic clades of a histone family (e.g. H2A.Z, H2B.W, subH2B). Number suffixes are assumed to be species-specific (e.g. H1.1), but are encouraged to be used consistently between species where unique orthologies are clear. However, due to historical reasons naming of certain variants may still deviate from these rules.
Variants of histone H3
Throughout
Variants of histone H4
Histone H4 is one of the slowest evolving proteins with no functional variants in the majority of species. The reason for a lack of sequence variants remains unclear. Trypanosoma are known to have a variant of H4 named H4.V.[1] In Drosophila there are H4 replacement genes that are constitutively expressed throughout the cell cycle that encode proteins that are identical in sequence to the major H4.[6]
Variants of histone H2A
Variants of histone H2B
H2B histone type is known to have a limited number of variants at least in mammals, apicomplexa and sea urchins.[1][2][7][8] H2B.1 is a testis, oocyte and zygote specific variant that forms subnucleosomal particles, at least, in spermatids. It can dimerize with H2A.L and H2A.1. H2B.W is involved in spermatogenesis, telomere associated functions in sperm and is found in spermatogenic cells. It is characterized by the extension of the N-terminal tail. subH2B participates in regulation of spermiogenesis and is found in non-nucleosomal particle in the subacrosome of spermatozoa. This variant has a bipartite nuclear localization signal. H2B.Z is an apicomplexan specific variant that is known to interact with
Databases and resources
References
- ^ PMID 22650316.
- ^ a b c d "Histone Variants Database 2.0". National Center for Biotechnology Information. Retrieved 13 January 2017.
- PMID 12408966.
- PMID 16267050.
- PMID 26159997.
- PMID 15687254.
- ^ PMID 26989147.
- ^ PMID 25731851.
- ^ "Histome: The Histone Infobase". Retrieved 13 January 2017.
- PMID 28096900.