PRIMARY STRUCTURE OF THE THERMOPLASMA PROTEASOME AND ITS IMPLICATIONS FOR THE STRUCTURE, FUNCTION, AND EVOLUTION OF THE MULTICATALYTIC PROTEINASE

The proteasome or multicatalytic proteinase is a high molecular mass multisubunit complex ubiquitous in eukaryotes but also found in the archaebacterium Thermoplasma acidophilum. While eukaryotic proteasomes contain 15-20 different subunits, 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.