The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structure composed of 2 complexes, a 20S core and a 19S regulator.The 20S core is composed of 4 rings of 28 non-identical subunits, 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits.The 19S regulator is composed of a base, which contains 6 ATPase subunits and 2 non-ATPase subunits, and a lid, which contains up to 10 non-ATPase subunits.Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway.An essential function of a modified proteasome, the immunoproteasome, is the processing of class I MHC peptides.This gene encodes a protein that inhibits the activation of the proteasome by the 11S and 19S regulators.Alternative transcript variants have been identified for this gene.[provided by RefSeq]