Refined cryo-EM structure of the Polytomella c-ring. (A) The c-ring, viewed along the membrane plane. The ring consists of 10 hairpin-like c subunits, with the N-terminal helix in the interior and the outer helix in the exterior. A conserved glutamate in each of the outer helices (red spheres) marks the location of the proton-binding sites. (B) The c-ring, viewed from the mitochondrial interior. Note that the outer helices are staggered relative to the inner helices, rather than projecting radially (dashed black lines). (C) Close-up of one of the proton-binding sites, at the interface between two adjacent c subunits, featuring the conserved glutamate side chain (Glu58).