Interacts with the core proteasome alpha-subunit (PrcA) through its C-terminal hydrophobic-tyrosine-X motif (HbYX motif) (PubMed:25469515). Interaction of Bpa with the proteasome stimulates proteosomal peptidase and casein degradation activity, which suggests Bpa could play a role in the removal of non-native or damaged proteins by influencing the conformation of the proteasome complex upon interaction (By similarity). Forms a homooligomeric, either hexameric or heptameric, ring-like structure which stacks co-axially with the proteasomal alpha-rings. Belongs to the Bpa family.