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

摘要

The D1 protein of the PSII reaction center is selectively damaged under strong illumination that causes photoinhibition of photosynthesis. The molecular mechanisms of damage to the D1 protein have been intensively studied in vitro while those in vivo have not been fully understood yet. In PSII preparations under photoinhibitory illumination, the D1 protein is either cleaved mainly at the loop connecting membrane helixes D and E (DE loop) or covalently cross-linked with other reaction center protein, the D2 protein or the alpha-subunit of cytochrome b559, to generate the heterodimer or the 41-kDa adduct (Aro et al. 1993). By contrast, studies in vivo have suggested that the D1 protein is cleaved in a site distinct from that in vitro and that cross-linking rarely occurs (Kettunen et al. 1996). To bridge a gap between observations in vivo and in vitro, we compared damage to the D1 protein under photoinhibitory illumination in three materials, namely, leaf discs, intact chloroplasts, and thylakoids.We found that the initial damage to the D1 protein occurs in almost the same way in vivo and in vitro and that cross-linking is a process involved in complete degradation of the D1 protein in vivo.