Component of the 26S proteasome, a multiprotein complex involved in the ATP-dependent degradation of ubiquitinated proteins. This complex plays a key role in the maintenance of protein homeostasis by removing misfolded or damaged proteins, which could impair cellular functions, and by removing proteins whose functions are no longer required. Therefore, the proteasome participates in numerous cellular processes, including cell cycle progression, apoptosis, or DNA damage repair. Component of the 19S proteasome regulatory particle complex (PubMed:16857966). The 26S proteasome consists of a 20S core particle (CP) and two 19S regulatory subunits (RP) (By similarity). The regulatory particle is made of a lid composed of 9 subunits including PSMD12, a base containing 6 ATPases and few additional components (By similarity). Interacts with ERCC6 (By similarity). Belongs to the proteasome subunit p55 family.