(A) Lumenal channel seen from the crista lumen. (B) Matrix channel seen from the matrix. (C) Side view of both channels seen from the c-ring, with outer c-ring helices in transparent yellow. Lumenal channel, left; matrix channel, right. The strictly conserved aArg239 in H5 separates the lumenal and matrix channels. (D) The lumenal channel passes through the H5/H6 hairpin at the small sidechains aAla246, aGly247 (H5) and aAla292 (H6) (green). (E) H4, the N-terminal half of H5 and the connecting H4/H5 loop at the matrix channel. Subunit a, blue; c10-ring, yellow; ASA 6, brick. Channels are shown as potential surfaces (red, negative; blue, positive; grey, neutral). (A) and (B) display a 5 Å slice of the c10-ring at the level of the protonated cGlu111.