The Photosystem II Repair Cycle Requires FtsH Turnover through the EngA GTPase

Specific degradation of photodamaged D1, the photosystem II (PSII) reaction center protein, is a crucial step in the PSII repair cycle to maintain photosynthesis activity. Processive proteolysis by the FtsH protease is fundamental to cooperative D1 degradation. Here, we attempted to purify the FtsH complex to elucidate its regulation mechanisms and substrate recognition in Arabidopsis (Arabidopsis thaliana). Unlike previously reported prokaryotic and mitochondrial FtsHs, the Arabidopsis chloroplastic FtsH does not appear to form a megacomplex with prohibition-like proteins but instead accumulates as smaller complexes. The copurified fraction was enriched with a partial PSII intermediate presumably undergoing repair, although its precise properties were not fully clarified. In addition, we copurified a bacteria-type GTPase localized in chloroplasts, EngA, and confirmed its interaction with FtsH by subsequent pull-down and bimolecular fluorescence complementation assays. While the engA mutation is embryo lethal, the transgenic lines overexpressing EngA (EngA-OX) showed leaf variegation reminiscent of the variegated mutant lacking FtsH2. EngA-OX was revealed to accumulate more cleaved D1 fragments and reactive oxygen species than the wild type, indicative of compromised PSII repair. Based on these results and the fact that FtsH becomes more stable in EngA-OX, we propose that EngA negatively regulates FtsH stability. We demonstrate that proper FtsH turnover is crucial for PSII repair in the chloroplasts of Arabidopsis. Consistent with the increased turnover of FtsH under high-light conditions in Chlamydomonas reinhardtii, our findings underline the rapid turnover of not only D1 but also FtsH proteases in the PSII repair cycle.