Figure 2.

(a) Schematic of the rotor–stator assembly in P. modestum. During ATP synthesis, the rotor turns to the left (clockwise viewed from the periplasm). The rotor section below the level of the membrane contains the 12 ion-binding sites. Each site consists of the triad Q32/E65/S66, which coordinates a sodium ion. The stator contains an aqueous channel that conducts ions from the periplasmic (positive) reservoir to the level of the horizontal hydrophilic strip. The positive stator charge, R227, blocks leakage of ions along this strip to the cytoplasm. (b) Face-on view of the rotor–stator assembly. Rotation during synthesis is to the left. There are four rotor sites located near the stator, two within the rotor–stator interface and two adjacent laterally. The stator is penetrated by an aqueous channel that admits ions from the periplasm, but ions can exit to the cytoplasm only by boarding a rotor site and passing through the dielectric barrier forming the left wall of the channel. If the occupied site moves to the right, it quickly loses its ion back to the channel when it approaches the positive stator charge, R227.