Clp P represents a unique family of serine proteases

The amino acid sequence of Clp P, the proteolytic subunit of the ATP-dependent Clp protease of Escherichia coli, closely resembles a protein encoded by chloroplast DNA, which is well conserved between chloroplasts of different plant species. The homology extends over almost the full length of the sequences of both proteins and consists of approximately 46% identical and approximately 70% similar amino acids. Antibodies against E. coli Clp P cross-reacted with proteins with Mr of 20,000-30,000 in bacteria, lower eukaryotes, plants, and animal cells. Since the regulatory subunit of Clp protease, Clp A, also has a homolog in plants, as well as in other bacteria and in lower eukaryotes, it is likely that ATP-dependent proteolysis in chloroplasts is catalyzed in part by a Clp-like protease and that both components of Clp-like proteases are widespread in living cells. We have identified Ser-111 as the active site serine in E. coli Clp P modified by diisopropyl fluorophosphate. Mutational alteration of Ser-111 or His-136 eliminates proteolytic activity of Clp P. Both residues are found in highly conserved regions of the protein. The sequences around the active site residues suggest that Clp P represents a unique class of serine protease. Amino-terminal processing of cloned Clp P mutated at either Ser-111 or His-136 occurs efficiently when wild-type clpP is present in the chromosome but is blocked in clpP- hosts. Processing of Clp P appears, therefore, to involve an intermolecular autocatalytic cleavage reaction. Since processing of Clp P occurs in clpA- cells, the autoprocessing activity of Clp P is independent of Clp A.