The proteasome is a multicatalytic proteinase complex which is characterized by its ability to cleave peptides with Arg, Phe, Tyr, Leu, and Glu adjacent to the leaving group at neutral or slightly basic pH. The proteasome has an ATP-dependent proteolytic activity. This subunit is involved in antigen processing to generate class I binding peptides. Replacement of PSMB6 by PSMB9 increases the capacity of the immunoproteasome to cleave model peptides after hydrophobic and basic residues. Cleavage of peptide bonds with very broad specificity. The 26S proteasome consists of a 20S proteasome core and two 19S regulatory subunits. The 20S proteasome core is composed of 28 subunits that are arranged in four stacked rings, resulting in a barrel-shaped structure. The two end rings are each formed by seven alpha subunits, and the two central rings are each formed by seven beta subunits. The catalytic chamber with the active sites is on the inside of the barrel. Component of the immunoproteasome, where it displaces the equivalent housekeeping subunit PSMB6. Component of the spermatoproteasome, a form of the proteasome specifically found in testis. (Microbial infection) Interacts with HIV-1 TAT protein. Highly expressed in immature dendritic cells (at protein level). Up-regulated by interferon gamma (at protein level). Up-regulated by IRF1. Up-regulated by tumor necrosis factor-alpha (at protein level). Up-regulated by tetrodotoxin (TTX) in glial cells. Up-regulated in Crohn's bowel disease (CD). Up-regulated by heat shock treatment. Up-regulated by CD40L via the NFKB1 pathway in cancer cells. Autocleaved. The resulting N-terminal Thr residue of the mature subunit is responsible for the nucleophile proteolytic activity. The disease may be caused by variants affecting distinct genetic loci, including the gene represented in this entry. Encoded in the MHC class II region. A model for self-activation in which residue Thr-21 serves as nucleophile and Lys-53 as proton donor/acceptor has been proposed. Subunit processing of mammalian beta-subunits proceeds via a novel ordered two-step mechanism involving autocatalysis. Belongs to the peptidase T1B family.