The proteasome is a multicatalytic proteinase complex witha highly ordered ring-shaped 20S core structure. The core structureis composed of 4 rings of 28 non-identical subunits, 2 rings arecomposed of 7 alpha subunits and 2 rings are composed of 7 betasubunits. Proteasomes are distributed throughout eukaryotic cellsat a high concentration and cleave peptides in anATP/ubiquitin-dependent process in a non-lysosomal pathway. Anessential function of a modified proteasome, the immunoproteasome,is the processing of class I MHC peptides. This gene encodes amember of the peptidase T1A family, that is a 20S core alphasubunit. A pseudogene has been identified on the Y chromosome.