Cryo-EM map of the subunit a–c-ring complex from the Polytomella mitochondrial ATP synthase. The map shown is a selection of that previously reported for the complete ATP synthase dimer (Allegretti et al., 2015). (A) The c-ring, comprising 10 c subunits, viewed along the plane of the membrane. The proton-motive force drives protons inwards, from the electropositive or P side of the membrane to the electronegative or N side, i.e., inwards. (B) In the cryo-EM map, subunit a appears as two hairpins of two transmembrane helices each, adjacent to the c-ring, and highly tilted relative to the membrane perpendicular. (C) Same as B, viewed from the mitochondrial interior. To energize the synthesis of ATP, inward proton translocation downhill results in the counterclockwise rotation of the c-ring against subunit a, which is believed to remain static.