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 PSMB5 by PSMB8 increases the capacity of the immunoproteasome to cleave model peptides after hydrophobic and basic residues. Involved in the generation of spliced peptides resulting from the ligation of two separate proteasomal cleavage products that are not contiguous in the parental protein (PubMed:27049119). Acts as a major component of interferon gamma-induced sensitivity. Plays a key role in apoptosis via the degradation of the apoptotic inhibitor MCL1. May be involved in the inflammatory response pathway. In cancer cells, substitution of isoform 1 (E2) by isoform 2 (E1) results in immunoproteasome deficiency. Required for the differentiation of preadipocytes into adipocytes. 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 PSMB5. Component of the spermatoproteasome, a form of the proteasome specifically found in testis. Directly interacts with POMP. Interacts with TAP1. (Microbial infection) Interacts with HIV-1 TAT protein. Additional isoforms seem to exist. Highly expressed in immature dendritic cells (at protein level). Up-regulated by IFNG/IFN-gamma and IRF1 (at protein level). Up-regulated by TNF (at protein level). Up-regulated by tetrodotoxin (TTX) in glial cells. Up-regulated in Crohn's bowel disease (CD). Down-regulated by the selective inhibitor PR-957. Down-regulated in mature dendritic cells by HSV-1 infection. Up-regulated by heat shock treatment. Autocleaved. The resulting N-terminal Thr residue of the mature subunit is responsible for the nucleophile proteolytic activity. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase T1B family.