Trp521 oxidation affects FtsH2 stability and its role in PSII repair.

Photosynthesis generates reactive oxygen species that damage photosynthetic machinery. Singlet oxygen (1O2) produced at photosystem II (PSII) damages PSII proteins, with D1 as a primary target due to its proximity to the site of 1O2 generation. During PSII repair, D1 is degraded by the FtsH metalloprotease complex, followed by resynthesis and PSII reassembly. Oxidation of a conserved tryptophan residue (Trp14) in N-terminal D1 enhances its recognition and degradation by FtsH, serving as a regulatory signal under photooxidative stress. The FtsH complex also undergoes turnover, but the mechanism remains unclear. In Arabidopsis thaliana, the FtsH complex comprises four isomers (FtsH1/2/5/8), grouped into types A and B. We identified a conserved, oxidation-prone Trp residue in type B isomers (FtsH2/8). Using transgenic plants expressing FtsH2 variants with Trp substitutions, we found that Trp oxidation affects FtsH2 stability and its ability to degrade D1. In ftsh2 ftsh8 double mutants, expressing FtsH2 with a Trp-to-phenylalanine substitution (oxidation-mimicking) failed to sustain growth and did not survive to reproduction, whereas a Trp-to-leucine substitution (oxidation-insensitive) enhanced PSII repair and improved growth under conditions tested. These findings demonstrate that oxidative modification influences FtsH2 function and provide mechanistic insight into how redox-sensitive residues affect PSII repair and chloroplast protein homeostasis.