Figure 2.

Configuration of ion-binding sites in the A. woodii c-ring. (a) One of the eight Na⁺-binding sites at the interfaces between c_2/3 subunits, viewed from the membrane. The protein residues (gray sticks) and water molecule (red sphere) coordinating the ion (yellow sphere) are highlighted. (b The Na⁺-binding site at the c_2/3/c₁ interface. (c) The water-binding site within the c₁ subunit. (d) The Na⁺-binding site at the c₁/c_2/3 interface. Dashed lines indicate inferred hydrogen-bonds in (a) and (c), and ion-protein contacts in (b) and (d). (e) F_o−F_c difference map (green mesh) for all bound Na⁺ and water molecules, contoured at +5.0σ. The ring is viewed from the cytoplasmic side. (f) F_o−F_c maps for either Na⁺ or water molecules (yellow and green mesh, respectively) bound within c₁ and the c₁/c_2/3 interface, contoured at +4.0σ. The 2F_o−F_c map for the protein is also shown, contoured at 2.4σ (light blue mesh). Supplementary Fig. 6 shows a stereo-view of this figure.