Figure 5.

Organization of V_o

(A) Section through the V_o region of the map. The red circle shows the approximate boundary of the c ring and lines delineate a four-helix bundle of one c subunit.

(B) Segmentation of the map showing the extent of subunit a around the c ring (represented by the NtpK 10-mer structure (PDB ID 2BL2; green). Arrow donates direction of rotation.

(C and D) Region of low-density “cavities,” enclosed within the gray surface, at the c ring (green)/subunit a interface consistent with a proton-accessible “half channel” (arrow). The conserved glutamate in helix 4 of subunit c playing a key role in proton transfer is shown in stick format. This occurs at the same depth in the membrane as the low-density feature, which is at a position appropriate for the previously hypothetical “proton half channel,” discussed in the text. Views are parallel to the plane of the membrane (C) and from the luminal side (D).