F1F0 ATP synthases use a transmembrane proton gradient to drive the synthesis of cellular ATP. The structure of the cytosolic F1 portion of the enzyme and the basic mechanism of ATP hydrolysis by F1 are now well established, but how proton translocation through the transmembrane F0 portion drives these catalytic changes is less clear. Here we describe the structural changes in the proton-translocating F0 subunit c that are induced by deprotonating the specific aspartic acid involved in proton transport. Conformational changes between the protonated and deprotonated forms of subunit c provide the structural basis for an explicit mechanism to explain coupling of proton translocation by F0 to the rotation of subunits within the core of F1. Rotation of these subunits within F1 causes the catalytic conformational changes in the active sites of F1 that result in ATP synthesis.