The proteasome or multicatalytic proteinase is a high molecular mass multisubunit complex ubiquitous in eukaryotes but also found in the archaebacterial proteasome is made of two different subunits only, and yet the complexes are almost identical in size and shape. Cloning and sequencing the gene encoding the small (beta) subunit of the T. acidophilum complex completes the primary structure of the archaebacterial proteasome. The similarity of the derived amino acid sequences of 233 (alpha) and 211 (beta) residues, respectively, indicates that they arose from a common ancestral gene. All the sequences of proteasomal subunits from eukaryotes available to date can be related to either the alpha-subunit or beta-subunit of the T. acidophilum "Urproteasome", and they can be distinguished by means of a highly conserved N-terminal extension, which is characteristic for alpha-type subunits. On the basis of circumstantial evidence we suggest that the alpha-subunits have regulatory and targeting functions, while the beta-subunits carry the active sites.

We have purified proteasomes to apparent homogeneity from the archaebacterium Thermoplasma acidophilum. This proteinase has a molecular mass of about 650 kDa and an isoelectric point of 5.6. The proteasome hydrolyses peptide substrates containing an aromatic residue adjacent to the reporter group, as well as [14C]methylated casein optimally at pH 8.5 and 90°C. The enzyme activity is enhanced severalford by Mg2+ and Ca2+ at 25–500 mM. This increase in activity results primarily from a change in Km. The serine‐proteinase inhibitors diisopropylfluorophosphate and 3,4‐dichlorosiocoumarin irreversibly inhibit the enzyme, obviously by modification of both the α and β subunits in the proteasome. The inhibition of proteasomal activity by the peptidylchloromethanes, Cbz‐Leu‐Leu‐CH2Cl and Cbz‐Ala‐Ala‐Phe‐Ch2Cl (Cbz, benzyloxycarbonyl), is reversible and predominantly of a competitive type. The enzyme is not activated by any of the compounds that typically stimulate the activities of the eukaryotic proteasome.

The subunit topography of the Thermoplasma acidophilum proteasome was determined by immunoelectron microscopy using monospecific antibodies directed against the two constituent subunits (,v~). Anti-at-subunit lgG was found to bind to the outer disks of the cylinder-or barrel.shaped molecule, while the binding sites of the anti-fl-subunit lgG were mapped on the two inner rings. Probably the homologues of the two subunits in the compositionally more complex but isomorphous eukaryotie proteasomes occupy equivalent positions.Proteasome; Multicatalytic proteinase; Archaebacterium: lmmunoelcctron microscopy

The proteasome from the thermoacidophilic archaeon Thermoplasma acidophilum in its native state represents a 20S particle with significant secondary structure (∼ 35%α 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.

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