Mechanism of the action of active oxygen species and proteases in the degradation of photosystem II complexes

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When chloroplasts are illuminated with strong light, the reaction-center binding D1 protein of photosystem II (PS II) Is damaged and degraded. In the protein degradation, active oxygen molecules produced at the acceptor-side of PS II and the endogenous cationic radicals generated at the donor-side of PS II are involved. It was also shown that the extrinsic proteins of PS II (OEC subunits) play s regulatory role in the degradation of the D1 protein. In the present study, we analyzed the action of the oxygen- and cationic radicals on the degradation of the D1 protein. We also studied the secondary structure of the OEC subunits. The results are summarized below.(1) Photodamaged D1 protein easily cross-links with the surrounding polypeptides in PS II. We found that the cross-linked products are not the dead-end products, but are digested by a stromal protease(s). The protease(s) is SDS-resistant. A 15-kDa factor in the stroma was found to have the protease activity.(2) Among the cross-linked products, generation of those between the D1 protein and CP43 were shown to be regulated by the OEC33. These results suggest that the OEC subunit plays a molecular chaperone-like role.(3) The secondary structures of OEC24 and OEC18 were analyzed by Fourier transform infrared spectroscopy. The results obtained show that these proteins have a large amount of β-sheet structure and they are relatively stable to heat treatment.