The proteasome is a multicatalytic proteinase complex with an extremely organized ring-shaped 20S core structure. The core structure consists of 4 rings of 28 non-identical subunits； 2 rings consist of 7 alpha subunits and 2 rings consist of 7 beta subunits. Proteasomes are spread all over the eukaryotic cells in large quantities and cleave peptides in an ATP/ubiquitin-dependent procedure in a non-lysosomal pathway. A vital role of a modified proteasome， the immunoproteasome， is the handling of class I MHC peptides.
