Functions as component of the transcription regulatory histone acetylation (HAT) complex ADA. ADA preferentially acetylates nucleosomal histones H3 (at 'Lys-14' and 'Lys-18') and H2B. AHC1 is required for the overall structural integrity of the ADA complex. Component of the 0.8 MDa ADA complex, which at least consists of ADA2, ADA3, AHC1 and GCN5. Present with 339 molecules/cell in log phase SD medium.