Biased Brownian stepping rotation of FoF1-ATP synthase driven by proton motive force.

F_oF₁-ATP synthase (F_oF₁) produces most of the ATP in cells, uniquely, by converting the proton motive force (pmf) into ATP production via mechanical rotation of the inner rotor complex. Technical difficulties have hampered direct investigation of pmf-driven rotation, which are crucial to elucidating the chemomechanical coupling mechanism of F_oF₁. Here we develop a novel supported membrane system for direct observation of the rotation of F_oF₁ driven by pmf that was formed by photolysis of caged protons. Upon photolysis, F_oF₁ initiated rotation in the opposite direction to that of the ATP-driven rotation. The step size of pmf-driven rotation was 120°, suggesting that the kinetic bottleneck is a catalytic event on F₁ with threefold symmetry. The reaction equilibrium was slightly biased to ATP synthesis like under physiological conditions, and F_oF₁ showed highly stochastic behaviour, frequently making a 120° backward step. This new experimental system would be applicable to single-molecule study of other membrane proteins. [ABSTRACT FROM AUTHOR]