The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structure composed of 2 complexes, a 20S core and a 19S regulator. This multi-subunit complex selectively degrades intracellular proteins. The 20S catalytic 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. This activity degrades a variety of peptide substrates and poly-ubiquitinated proteins with broad specificity in non-ATP dependent process. The 20S proteasome can be activated chemically by the addition of detergent or by the proteinaceous activator PA28. This product has been purified from rat erythrocytes.