Histone H2A

Histone H2A is approximately 14 kDa protein consisting of ~130 amino acid residues. Interestingly, atypical variants such as MacroH2As, H2A.X, H2A-Bbd, differ considerably in size. H2A-H2B heterodimer forms a core component of the 'nucleosome'. The Histone H2A family contains 265 different members reported from diverse species. H2A undergoes many post translational modifications like acetylation, methylation, citrullination, phosphorylation etc. Histone H2A has by far the maximum number of variants (total 19 in number) among other core histones. These are coded by 26 genes with majority of them present in cluster 1 and 2 and a single gene in cluster 3. Out of these, 9 genes are not part of any cluster and code for atypical histone H2A such as macro-H2A, H2A-Bbd etc.

Macro-H2A is a bulky variant with 327 amino acid and ~40 kDa compared to the normal 14 kDa weight. It has specific functions in X chromosome inactivation and transcriptional regulation. On the other hand, H2A-Bbd variant is exclusively present on the 'active' X chromosome. Other variants such as H2A.X, H2A.Z play important roles wherein they are differentially modified (e.g. phosphorylated) during various events e.g. DNA damage or heat shock response. H2A.Z and macro-H2A have substitutions even in the histone fold.

Cytogenetic map of histone H2A coding genes
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Variants of histone H2A
Variant (UniprotKB recommended name) UniprotKB Accession No. of coding gene(s)
Core histone macro-H2A.1O753671 (H2AFY)
Core histone macro-H2A.2Q9P0M61 (H2AFY2)
Histone H2A type 1-JQ998781 (HIST1H2AJ)
Histone H2A type 1-CQ930771 (HIST1H2AC)
Histone H2A.xP161041 (H2AFX)
Histone H2A type 2-AQ6FI132 (HIST2H2AA4, HIST2H2AA3)
Histone H2A type 2-CQ167771 (HIST2H2AC)
Histone H2A type 1-DP206711 (HIST1H2AD)
Histone H2A type 1P0C0S85 (HIST1H2AI, HIST1H2AK, HIST1H2AL, HIST1H2AM, HIST1H2AG)
Histone H2A type 1-B/EP049082 (HIST1H2AE, HIST1H2AB)
Histone H2A type 2-BQ8IUE61 (HIST2H2AB)
Histone H2A.ZP0C0S51 (H2AFZ)
Histone H2A type 1-HQ96KK51 (HIST1H2AH)
Histone H2A type 1-AQ96QV61 (HIST1H2AA)
Histone H2A type 3Q7L7L01 (HIST3H2A)
Histone H2A.VQ71UI91 (H2AFV)
Histone H2A-Bbd type 2/3P0C5Z02 (H2AFB2, H2AFB3)
Histone H2A-Bbd type 1P0C5Y91 (H2AFB1)
Histone H2A.JQ9BTM11 (H2AFJ)