T he proteasome was first discovered as a cylinder-shaped particle of unknown function on electron micrographs of human erythrocyte cell lysates.1 More than a decade later, a large multisubunit protease with multicatalytic activity was isolated from bovine pituitary cells,2 and has since been found in all eukarytic cells thus far examined. More recently, related complexes, albeit much simpler in subunit composition, have been purified from archaea and some bacteria.3The simpler subunit composition of the Thermo­ plasma acidophilum proteasome has greatly facilitated structural and functional studies, and revealed the long time enigmatic pro­ teolytic mechanism of the proteasome.4