The proteasome from the thermoacidophilic archaeon Thermoplasma acidophilum in its native state represents a 20S particle with significant secondary structure (approximately 35% alpha helix) of its subunits. Electron microscopy, ultracentrifugal and spectral analysis demonstrate that at pH of less than 3 dissociation to partially denatured subunits occurs. Upon dialysis against near neutral pH buffers, at low protein concentration, reconstitution occurs, leading to the restoration of up to 90% of the native fluorescence signal. The recovery of activity depends on several parameters, including the buffer system, the pH used to dissociate the complex, and the duration of exposure to low pH. High concentrations of Ca2+ and Mg2+ cause partial dissociation of the Thermoplasma proteasome, yielding distinct subcomplexes. Neither the completely nor the partially dissociated complexes have proteolytic activity, indicating that function is linked to fully assembled proteasomes.