Fig. 3.

Relationship between the inclined membrane wedge in the monomeric membrane domain and the association of two monomers into a dimer. The dimer in state [s2:s2] is shown. The c₈ ring is gray, and transmembrane α-helices bH2 and bH3 of subunit b (pink) define the wedge angle (2). The rest of the structure is shown in silhouette. (A) Side view showing the relationship between the angles of rotatory axes and the wedge angles when measured from the same reference point, with both rotatory axes aligned in plane. The angle between bH2 and bH3 in this view is 33°, approximately half of the dimer angle. The approximate boundary of the membrane is indicated; in B, the rotatory axis is aligned vertically and the structure is rotated so that bH2 and bH3 in the wedge of the right-hand monomer are in the plane of the paper. The 45° wedge angle is a measure of the inclination of the transmembrane α-helix bH2 relative to bH3 and the rotatory axis of the enzyme, as required for the wedge to provide the membrane curvature in the membrane domain of each monomer. The approximate boundary of the lower leaflet is indicated to provide perspective. (C and D) Top views showing the differences between the angle measurement planes to explain why a simple doubling of the wedge angle does not produce the rotatory axis angle. In the context of the angle variations observed in the consensus reconstructions, the wedge angle and dimer, or rotatory axis, angle are separate considerations. The wedge angle does not change, rather, the way in which the membrane domains are arranged relative to one another does, as described in Fig. 2 and Movie S3.