An archaebacterial ATPase, homologous to ATPases in the eukaryotic 26 S proteasome, activates protein breakdown by 20 S proteasomes

In eukaryotes, the 20 S proteasome is the proteolytic core of the 26 S prot easome, which degrades ubiquitinated proteins in an ATP-dependent process. Archaebacteria lack ubiquitin and 26 S proteasomes but do contain 20 S prot easomes. Many archaebacteria, such as Methanococcus jannaschii, also contai n a gene (S4) that is highly homologous to the six ATPases in the 19 S (PA7 00) component of the eukaryotic 26 S proteasome, To test if this putative A TPase may regulate proteasome function, we expressed it in Escherichia colt and purified the 50-kDa product as a 650-kDa complex with ATPase activity. When mixed with the well characterized 20 S proteasomes from Thermoplasma acidophilum and ATP, this complex stimulated degradation of several unfolde d proteins 8-25-fold, It also stimulated proteolysis by 20 S proteasomes fr om another archaebacterium and mammals, This effect required ATP hydrolysis since ADP and the nonhydrolyzable analog, 5'-adenylyl beta,gamma-imidophos phate, were ineffective. CTP and to a lesser extent GTP and UTP were also h ydrolyzed and also stimulated proteolysis. We therefore named this complex PAN for proteasome-activating nucleotidase. However, PAN did not promote th e degradation of small peptides, which, unlike proteins, should readily dif fuse into the proteasome. This ATPase complex appears to have been the evol utionary precursor of the eukaryotic 19 S complex, before the coupling of p roteasome function to ubiquitination.