ABSTRACT

A lthough the 20S proteasome is responsib le for the b u lk of extralysosom al p ro teo lysis, the purified enzyme has negligible activity toward native proteins and only poorly degrades oligopeptides and unfolded proteins such as casein. This property has been referred to as latency.1 The degradation of some synthetic substrates can be stimulated by low con­ centrations of SDS 2 and the degradation of larger substrates such as casein and the oxidized B chain of insulin can be activated by various nonphysiological manipulations such as heating, dialysis against distilled water, and treatment with a variety of protein side chain modifying reagents.1,3 The physical basis for the property of latency as revealed by the x-ray crystallographic solution of the structure of the Thermoplasma enzyme is the narrow opening at each end of the particle which serves as a barrier against the entrance of native proteins.4 Since the active sites present on β-type subunits are in the interior chamber of the molecule, the cell is protected from uncon­ trolled proteolysis by this major intracellular proteinase.5 Activation of the degradation of large substrates by chemical or physical modification is due to a gross alteration of proteasome conformation, providing a means for substrate access to the active sites. More recently, the X-ray crystallographic structure

of the yeast proteasome solved to 2.4 A resolution surprisingly revealed that this molecule is sealed at both ends and that the only visible access of substrates to the catalytic interior of the molecule appears to be by very narrow side entrances.6