The interface between thec-subunit oligomer and the asubunit in the F0sector of the ATP synthase is believed to formthe core of the rotatingmotor powered by the protonic ¯ow. Besides the essential cAsp61 and aArg210 residues (Escherichia colinumbering), a few other residues at this interface, although nonessential, show a high degree of conservation, among these aGlu219.