The proteasome is a multicatalytic proteinase complex with a highly ordered ring-shaped 20S core structure.The core structure 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.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 member of the peptidase T1A family, that is a 20S core alpha subunit.[provided by RefSeq] Synonyms: HC3, MU, PMSA2, PSC2