Mitochondria have a crucial role in replenishing ATP, the molecule used to provide cellular energy. This is achieved by tightly coupling the movement of protons (H+) across the inner mitochondrial membrane to power the rotary action of ATP synthase in the final step of oxidative phosphorylation. Folds of the inner mitochondrial membrane called cristae provide a large, ion-impermeable surface area studded with respiratory enzymes including ATP synthase dimers. Structures of isolated ATP synthases have previously been determined, and by reconstituting into liposomes were observed to spontaneously form dimer rows that curve the liposomal membrane; however, the structure of ATP synthase has not been determined in its natural environment, which preserves the electrochemical proton gradient.

Now, using cryo-electron tomography (cryo-ET), Dietrich et al. have determined the structure of the mitochondrial ATP synthase within the unicellular alga Polytomella, which was flash-frozen under active growth conditions. The team observed an arrangement of parallel rows of ATP synthase dimers that twist around the cristae ridges with a left-handed helical geometry. The ATP synthase dimer rows define the shape of the cristae.