Adenosine triphosphate (ATP) synthase contains a rotary motor involved in biological energy conversion. Its membrane-embedded FO sector has a rotation generator fueled by the proton-motive force, which provides the energy required for the synthesis of ATP by the F$_1$ domain. An electron density map obtained from crystals Of a subcomplex of yeast mitochondrial ATP synthase shows a ring of 10 c subunits. Each c subunit forms an α-helical hairpin. The interhelical loops of six to seven of the c subunits are in dose contact with the γ and δ subunits of the central stalk. The extensive contact between the c ring and the stalk suggests that they may rotate as an ensemble during catalysis.