PSME3, also known as PA28-gamma, is a subunit of proteasome. The 26S proteasome multicatalytic proteinase complex has a highly ordered structure composed of 2 complexes, a 2S core and a 19S regulator. The 2S 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 1 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. The immunoproteasome contains an alternate regulator, referred to as the 11S regulator or PA28, that replaces the 19S regulator. Three subunits (alpha, beta and gamma) of the 11S regulator have been identified. PSME3 gene encodes the gamma subunit of the 11S regulator. Six gamma subunits combine to form a homohexameric ring.