The proteasome is a multicatalytic proteinase complex with a highly ordered 20S core structure is composed of four axially stacked rings of 28 non-identical subunits: the two end rings are each formed by 14 alpha subunits, and the two central rings are each formed by seven beta subunits.
This barrel-shaped core structure is composed of 4 axially stacked rings of 28 non-identical subunits: The two end rings are each formed by 7 alpha subunits, and the two central rings are each formed by 7 beta subunits. 
