Histone cell cycle regulation defective homolog A, also known as TUP1-like enhancer of split protein 1, is the mammalian homolog of the yeast HIR1 and HIR2 transcriptional repressor proteins. HIRA interacts with UBN1, CABIN, and ASF1A in the cell nucleus to form the evolutionarily conserved HUCA histone chaperone complex that deposits the variant histone H3.3 into chromatin in a DNA-replication independent manner. HIRA is required for deposition of histone H3.3 at the transcription start sites of genes, where incorporation of histone H3.3 facilitates nucleosome destabilization and contributes to transcriptional activation. Histone H3.3 is also linked to gene silencing and is incorporated into regions of the genome thought to be transcriptionally inactive. While some incorporation of H3.3 into heterochromatin is facilitated by a different histone chaperone complex that contains ATRX and DAXX, HIRA is required for incorporation of histone H3.3 and formation of senescence-associated heterochromatin foci during cellular senescence. HIRA is ubiquitously expressed during mouse embryonic development. In the adult mouse, HIRA is expressed at high levels in the kidney, skeletal muscle, and pancreas, but it is expressed at lower levels in the heart, lung, placenta, brain, and liver. A missing copy of the HIRA gene on human chromosome region 22q11.2 is a common characteristic of DiGeorge syndrome patients and insufficient production of the HIRA protein may disrupt normal embryonic development.
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