Protonation-dependent stepped rotation of the F-type ATP synthase c-ring observed by single-molecule measurements

The two opposed rotary molecular motors of the F₀F₁-ATP synthase work together to provide the majority of ATP in biological organisms. Rotation occurs in 120° power strokes separated by dwells when F₁ synthesizes or hydrolyzes ATP. F₀ and F₁ complexes connect via a central rotor stalk and a peripheral stator stalk. A major unresolved question is the mechanism in which the interaction between subunit-a and rotating subunit-c-ring in the F₀ motor uses the flux of H⁺ across the membrane to induce clockwise rotation against the force of counterclockwise rotation driven by the F₁-ATPase. In single-molecule measurements of F₀F₁ embedded in lipid bilayer nanodiscs, we observed that the ability of the F₀ motor to form transient dwells increases with decreasing pH. Transient dwells can halt counterclockwise rotation powered by the F₁-ATPase in steps equivalent to the rotation of single c-subunits in the c-ring of F₀, and can push the common axle shared by the two motors clockwise by as much as one c-subunit. Because the F₀ proton half-channels that access the periplasm and the cytoplasm are exposed to the same pH, these data are consistent with the conclusion that the periplasmic half-channel is more easily protonated in a manner that halts ATPase-driven rotation by blocking ATPase-dependent proton pumping. The fit of transient dwell occurrence to the sum of three Gaussian curves suggests that the asymmetry of the three ATPase-dependent 120° power strokes imposed by the relative positions of the central and peripheral stalks affects c-subunit stepping efficiency.