The GreenCut protein TEF6 maintains photosystem II homeostasis under high light by stabilizing FtsH accumulation in Chlamydomonas reinhardtii.

Photosystem II (PSII) is highly susceptible to photodamage under high-light stress, necessitating an efficient repair cycle involving degradation of the damaged D1 protein, primarily mediated by FtsH proteases. Although the role of FtsH in D1 turnover is well established, the regulatory mechanisms that ensure precise and controlled degradation remain unclear. Here, we characterize TEF6, a conserved thylakoid membrane protein containing two transmembrane domains in Chlamydomonas reinhardtii. The tef6 mutant exhibits severe growth inhibition, reduced PSII activity, impaired accumulation of PSII supercomplexes, and disorganized thylakoid membranes specifically under high-light conditions. Physiological, cellular, biochemical, and genetic analyses demonstrate that loss of TEF6 compromises PSII stability and repair. Multiple approaches-including co-immunoprecipitation coupled with mass spectrometry, yeast two-hybrid assays, and bimolecular fluorescence complementation (BiFC)-further reveal that TEF6 directly interacts with both the D1 protein and the FtsH proteases (FtsH1/FtsH2). Loss of TEF6 leads to misregulated and excessive accumulation of FtsH under high light, correlating with accelerated and uncontrolled degradation of D1 and ultimately disrupting PSII repair and homeostasis. Collectively, our findings identify TEF6 as a crucial scaffold-like factor in the PSII repair machinery. TEF6 stabilizes the proper accumulation of the FtsH protease complex in the thylakoid membrane, thereby ensuring accurate and regulated turnover of photodamaged D1. This study reveals a novel regulatory mechanism mediated by a GreenCut protein that maintains PSII quality control and photosynthetic efficiency under light stress.