Atypical histone H2A which replaces conventional H2A during late spermatogenesis and is involved in the replacement of histones to protamine in male germ cells (PubMed:28366643). Core component of nucleosome: nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template (PubMed:19506029). Nucleosomes containing H2AB1 only wrap 130 bp of DNA, compared to 147 bp for classical nucleosomes (PubMed:19506029). In condensing spermatids, the heterodimer between H2AB1 and H2BC1/TH2B is loaded onto the nucleosomes and promotes loading of transition proteins (TNP1 and TNP2) onto the nucleosomes (PubMed:28366643). Inclusion of the H2AB1-H2BC1/TH2B dimer into chromatin opens the nucleosomes, releasing the nucleosomal DNA ends and allowing the invasion of nucleosomes by transition proteins (TNP1 and TNP2) (PubMed:28366643). Then, transition proteins drive the recruitment and processing of protamines, which are responsible for histone eviction (PubMed:28366643). 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 (PubMed:19506029). Incorporated into nucleosomes during late spermatogenesis (PubMed:19506029). Interacts with H2BC1/TH2B; preferentially dimerizes with H2BC1/TH2B to form nucleosomes (PubMed:17261847, PubMed:28366643). Specifically localizes to the pericentric regions in condensing spermatids (PubMed:17261847). Highly expressed in adult testis, mainly in spermatocytes (PubMed:20008104, PubMed:20188161, PubMed:17261847). Expressed since postnatal day (P) 21, peaks at P30, and gradually decreases in the testis of aging mouse (PubMed:20008104, PubMed:20188161). Coexpressed with transition proteins during late spermiogenesis (PubMed:28366643). Strongly enriched in step 12-16 spermatids and accumulate during late spermiogenesis, in condensing spermatids (PubMed:17261847). Remains present in mature spermatozoa isolated from epididymis (PubMed:17261847). Rapidly disappears from the paternal pericentric heterochromatin regions after sperm-egg fusion (PubMed:18703863). Male mice are completely sterile due to defects in spermatogenesis. Chromatin in mature spermatozoa shows defects in density, due to impaired histone replacement by protamines. A significant proportion of Prm2 remains unprocessed. In contrast to other H2A histones, it does not contain the conserved residues that are the target of post-translational modifications. Belongs to the histone H2A family. Although related to variant histone H2AB1 in human (AC P0C5Y9), it is unclear whether human and mouse H2AB1 proteins are involved in similar processes. In mouse, variant histone H2AB1 is specifically required to direct the transformation of dissociating nucleosomes to protamine in male germ cells during spermatogenesis (PubMed:28366643). It is however unclear whether human protein, which participates in mRNA processing and is associated with active transcription, is also involved in nucleosomes to protamine replacement.