36° step size of proton-driven c-ring rotation in FoF1-ATP synthase

Synthesis of adenosine triphosphate ATP, the ‘biological energy currency', is accomplished by FoF1-ATP synthase. In the plasma membrane of Escherichia coli, proton-driven rotation of a ring of 10 c subunits in the Fo motor powers catalysis in the F1 motor. Although F1 uses 120° stepping during ATP synthesis, models of Fo predict either an incremental rotation of c subunits in 36° steps or larger step sizes comprising several fast substeps. Using single-molecule fluorescence resonance energy transfer, we provide the first experimental determination of a 36° sequential stepping mode of the c-ring during ATP synthesis.