The thylakoid FtsH protease plays a role in the light-induced turnover of the photosystem II D1 protein

The photosystem II reaction center D1 protein is known to turn over frequen tly. This protein is prone to irreversible damage caused by reactive oxygen species that are formed in the light; the damaged, nonfunctional D1 protei n is degraded and replaced by a new copy. However, the proteases responsibl e for D1 protein degradation remain unknown. In this study, we investigate the possible role of the FtsH protease, an ATP-dependent zinc metalloprotea se, during this process. The primary light-induced cleavage product of the D1 protein, a 23-kD fragment, was found to be degraded in isolated thylakoi ds in the dark during a process dependent on ATP hydrolysis and divalent me tal ions, suggesting the involvement of FtsH. Purified FtsH degraded the 23 -kD D1 fragment present in isolated photosystem II core complexes, as well as that in thylakoid membranes depleted of endogenous FtsH. In this study, we definitively identify the chloroplast protease acting on the D1 protein during its light-induced turnover. Unlike previously identified membrane-bo und substrates for FtsH in bacteria and mitochondria, the 23-kD D1 fragment represents a novel class of FtsH substrate-functionally assembled proteins that have undergone irreversible photooxidative damage and cleavage.