Subunit stoichiometry three-dimensional arrangement in proteasomes from Thermoplasma acidophilum

The proteasome or multicatalytic proteinase from the archaebacterium Thennoplasma acidophilum is a 700 kDa multisubunit protein complex. Unlike proteasomes from eukaryotic cells which are composed of 10-20 different subunits, the Thermoplasma proteasome is made of only two types of subunit, α. and β, which have molecular weights of 25.8 and 22.3 kDa, respectively. In this communication we present a three-dimensional stoichiometric model of the archaebacterial proteasome deduced from electron microscopic investigations. The techniques which we have used include image analysis of negatively stained single particles, image analysis of metal decorated small three-dimensional crystals after freeze-etching and STEM mass measurements of freeze-dried particles. The archaebacterial and eukaryotic proteasomes are almost identical in size and shape; the subunits are arranged in four rings which are stacked together such that they collectively form a barrel-shaped complex. According to a previous immunoelectron microscopic investigation, the α-subunits form the two outer rings of the stack, while the two rings composed of β-subunits, which are supposed to carry the active sites, are sandwiched between them. Each of the α- and β-rings contains seven subunits; hence the stoichiometry of the whole proteasome is α₁₄ and β₁₄ the symmetry is 7-fold. Image simulation experiments indicate that the α- and β-subunits are not in register along the cylinder axis; rather it appears that the β-rings are rotated with respect to the α-rings by ∼ 25°. In contrast to some previous reports we have not been able to find stoichiometric amounts of RNA associated with highly purified proteolytically active proteasome preparations.