Dissipation of the proton electrochemical gradient in chloroplasts promotes the oxidation of ATP synthase by thioredoxin-like proteins.

Chloroplast FF-ATP synthase (CFCF) uses an electrochemical gradient of protons across the thylakoid membrane (ΔμH) as an energy source in the ATP synthesis reaction. CFCF activity is regulated by the redox state of a Cys pair on its central axis, that is, the γ subunit (CF-γ). When the ΔμH is formed by the photosynthetic electron transfer chain under light conditions, CF-γ is reduced by thioredoxin (Trx), and the entire CFCF enzyme is activated. The redox regulation of CFCF is a key mechanism underlying the control of ATP synthesis under light conditions. In contrast, the oxidative deactivation process involving CFCF has not been clarified. In the present study, we analyzed the oxidation of CF-γ by two physiological oxidants in the chloroplast, namely the proteins Trx-like 2 and atypical Cys-His-rich Trx. Using the thylakoid membrane containing the reduced form of CFCF, we were able to assess the CF-γ oxidation ability of these Trx-like proteins. Our kinetic analysis indicated that these proteins oxidized CF-γ with a higher efficiency than that achieved by a chemical oxidant and typical chloroplast Trxs. Additionally, the CF-γ oxidation rate due to Trx-like proteins and the affinity between them were changed markedly when ΔμH formation across the thylakoid membrane was manipulated artificially. Collectively, these results indicate that the formation status of the ΔμH controls the redox regulation of CFCF to prevent energetic disadvantages in plants.