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 comple x in the presence of ATP to form the 26 S ubiquitin/ATP dependent prot ease. This association results in a uniform 3-fold activation of pepti de 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 separate ly with ATPase complex or regulator, ah MCP molecules are converted to 26 S protease or to an activated MCP, respectively. Competition betwe en 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 subunit s in the ATPase complex, there is also a 30-kDa protein. Three finding s demonstrate that the 30-kDa subunits in each complex are distinct pr oteins. First, two-dimensional polyacrylamide gel revealed different i soelectric points for each 30-kDa protein. Second, anti-regulator anti bodies did not cross-react with proteins in the ATPase complex or in t he 26 S protease. Third, directly sequenced peptides from the 30-kDa s ubunit 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.