FIGURE 2.

Model of PSII repair and regulation of FtsH activity in chloroplasts. In chloroplasts, photodamaged D1 migrates from the grana stacks to the grana margins, which is the location of D1 degradation in the PSII repair cycle. Dephosphorylation of PSII core proteins by chloroplast PP2C phosphatase and partial disassembly of the PSII complex occur prior to the degradation process. D1 degradation is mainly conducted by FtsH (fundamental degradation), and endopeptidic cleavages by Deg proteases facilitate the effective degradation by FtsH in photoinhibitory conditions. Newly synthesized D1 is processed at its C-terminus by CtpA peptidase. Repaired PSII migrates to the grana core to form functional PSII. Phosphorylation of PSII core proteins is carried out by STN8 kinase. On the other hand, the PSII repair cycle might require proper FtsH turnover in chloroplasts. When FtsH has access to damaged PSII, FtsH concomitantly suffers from oxidative damage induced by ROS. The damaged FtsH should be repaired by proper turnover. Newly synthesized FtsH proteins are imported into chloroplasts and subsequently integrated into the thylakoid membrane. FtsH forms functional complexes in the grana margin. The post-translational modification in FtsH might regulate protease activity and/or complex formation.