Proteasomes are large multicatalytic proteinases located in the nuclei and cytoplasm of all eukaryotic cells. Proteasomes are composed of four heptameric rings stacked to form a hollow cylinder (length 16-20 nm, diameter 11-12 nm). The outer two rings are composed of α-subunits, while β-subunits, which contain the active sites, comprise the inner two rings. Proteasomes from archaebacteria contain only one type each of α- and β-subunits. Eukaryotic proteasomes are more divergent; yeast proteasomes have seven different α- and seven different β-subunits, each occupying a unique position in the ring. Only three of the seven yeast β-subunits contain the N-terminal threonine necessary for activity (1-2).