Implicated in immunoproteasome assembly and required for efficient antigen processing. The PA28 activator complex enhances the generation of class I binding peptides by altering the cleavage pattern of the proteasome (By similarity). Heterodimer of PSME1 and PSME2, which forms a hexameric ring. PSME1 can form homoheptamers (By similarity). Belongs to the PA28 family.