Immunochemical Studies on the Clp-protease in Chloroplasts: Evidence for the Formation of a CIpC/P Complex*

Chloroplasts contain a proteolytic system whose activity is ATP-dependent. The presence of genes encoding homologues of the ATP-dependent E. coli CIpA/P protease on the plastome and nuclear genome suggests that a similar protease is located in chloroplasts. Antibodies raised against a recombinant chloroplast-encoded proteolytic ClpP subunit detect this polypeptide in chloroplasts prepared from barley leaves or the eukaryotic algae Chlamydomonas reinhardtii and Euglena gracilis. Co-immunoprecipitation experiments using the anti-ClpP antibody and an antibody against the nuclear encoded regulatory CIpC component (a ClpA homologue) provide direct evidence for the existence of a CIpC/P complex in the chloroplast stroma. These results suggest that at least a part of the ATP-dependent proteolytic reactions in the chloroplast is catalyzed by an enzyme complex similar to the E. coli CIpA/P protease.