Structure and in Situ Organization of ATP Sythase and Respiratory Chain Complexes

Mitochondria are the powerhouses of eukaryotic cells and the main site of ATP synthesis in cells performing aerobic respiration. Located on the cristae of the inner mitochondrial membrane, the ATP synthase uses the energy stored in a transmembrane gradient of protons to power the synthesis of ATP while the respiratory chain complexes replenish the gradient. Using the technique of electron cryo-tomography and subtomogram averaging, we are characterizing how the structure and spatial distribution of membrane proteins influence mitochondrial function. So far we have discovered: 1) the structure of the ATP synthase varies dramatically between different eukaryotic lineages, 2) that in all species studied so far, the ATP synthase form rows of dimers along the highly curved edges of the cristae membranes, 3) that the structure of the rows varies between protozoan and multicellular organisms, and 4) the purpose of the rows appears to be the maintenance of organized inner membrane invaginations. In parallel, we have shown that the proteins of the respiratory chain are located in the flat membrane regions of cristae and that they form species specific supercomplexes. Alongside the cryo-ET studies, we have determined a 6.2A structure of the mitochondrial ATP synthase of Polytomella sp. using single particle cryo-EM. Our structure revealed four highly tilted membrane-intrinsic helices (∼20° to the membrane plane) adjacent to the rotor ring which we assign to the elusive a-subunit. Based on this structure, we present a model describing how protons move through the ATP synthase.