Architecture and nomenclature of the intact T. thermophilus A-ATPase/ synthase. (a) Side view of the A-ATPase. The stator subunits (A, B, E, G and I) are labeled in white and the rotor subunits (D, F, C and L) are labeled in black. (b) View from above. The black arrow indicates the rotational direction of the central stalk during ATP synthesis and the white arrows indicate the torque in the A3B3 nucleotide-binding domain that subunit E counteracts.