Proton gradient across the chloroplast thylakoid membrane governs the redox regulatory function of ATP synthase.

Chloroplast ATP synthase (CFCF) synthesizes ATP by using a proton electrochemical gradient across the thylakoid membrane, termed ΔμH, as an energy source. This gradient is necessary not only for ATP synthesis but also for reductive activation of CFCF by thioredoxin, using reducing equivalents produced by the photosynthetic electron transport chain. ΔμH comprises two thermodynamic components: pH differences across the membrane (ΔpH) and the transmembrane electrical potential (ΔΨ). In chloroplasts, the ratio of these two components in ΔμH is crucial for efficient solar energy utilization. However, the specific contribution of each component to the reductive activation of CFCF remains unclear. In this study, an in vitro assay system for evaluating thioredoxin-mediated CFCF reduction is established, allowing manipulation of ΔμH components in isolated thylakoid membranes using specific chemicals. Our biochemical analyses revealed that ΔpH formation is essential for thioredoxin-mediated CFCF reduction on the thylakoid membrane, whereas ΔΨ formation is nonessential.