Core component of nucleosome. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. The nucleosome is a histone octamer containing two molecules each of H2A, H2B, H3 and H4 assembled in one H3-H4 heterotetramer and two H2A-H2B heterodimers. The octamer wraps approximately 147 bp of DNA. Contains 2 SPKK motifs which may interact with the minor groove of A/T-rich DNA sites. Phosphorylation of this motif may regulate DNA binding. This motif is reiterated in both termini of histone H1 and in the N-terminus of sea urchin histones H2B, but its presence in the C-terminus seems to be unique to plant H2A. Phosphorylated within its C-terminal part, probably at the SPKK motifs. Belongs to the histone H2A family.