Activation of the multicatalytic protease. The 11 S regulator and 20 S ATPase complexes contain distinct 30-kilodalton subunits

The multicatalytic protease (MCP) associates with a 20 S ATPase complex in the presence of ATP to form the 26 S ubiquitin/ATP-dependent protease. This association results in a uniform 3-fold activation of peptide hydrolysis by MCP. In the absence of nucleotides, an 11 S regulator binds MCP and differentially stimulates its peptidase activities from 3-fold to 25-fold depending upon the peptide. When incubated separately with ATPase complex or regulator, all MCP molecules are converted to 26 S protease or to an activated MCP, respectively. Competition between ATPase complex and regulator for limiting amounts of MCP results in the 26 S protease as the only assembled species. Rabbit reticulocyte regulator is composed of a single 30-kDa protein. Among the 15 subunits in the ATPase complex, there is also a 30-kDa protein. Three findings demonstrate that the 30-kDa subunits in each complex are distinct proteins. First, two-dimensional polyacrylamide gel revealed different isoelectric points for each 30-kDa protein. Second, anti-regulator antibodies did not cross-react with proteins in the ATPase complex or in the 26 S protease. Third, directly sequenced peptides from the 30-kDa subunit of the ATPase complex are not present in the deduced amino acid sequence of the regulator. Thus, the regulator and ATPase complex are independent activators of MCP.