A novel degradation pathway of the D1 protein of photosystem II via covalent cross-linked adducts under illumination in vivo

Abstract

Damage to the D1 protein of the photosystem II reaction center (PSII) under photoinhibitory illumination was compared among three materials, namely, thylakoids, intact chloroplasts and leaf discs. In all the materials, illumination at 25° C led to cleavage of the protein in the loop that connects membrane-spanning helixes D and E (DE loop) and cross-linking of the protein with other PSII reaction center protein, the D2 protein or cytochrome b559 although resultant fragments and cross-linked adducts barely accumulated in chloroplasts and leaf discs. Under illumination at 2° C, fragments and adducts were accumulated in leaf discs, but they gradually disappeared after transfer to 25° C, an indication that fragments and adducts were digested by proteases at 25° C. It was found that a novel protease(s) that selectively digests cross-linked adducts of the D1 protein but not the protein in a free form was present in the stroma. These results suggest that cross-linking reactions are involved in complete degradation of the D1 protein in vivo. Proteolytic mapping of the adduct of the D1 and D2 proteins indicated that cross-linking occurred in residues 226-224 on the N-terminal side of the DE loop of the D1 protein, while the cleavage occurred on the C-terminal side.