(a) The Mrp monomer is shown in the centre in side view. Alignments to MBH are in the left column and to complex I in the right column. The top row shows view from the cytosol, with the conserved domains underlined. The arrow indicates ‘swinging out’ of the MrpE TMH1-2 in comparison to MBH. The second row shows the side view, with Mrp subunits coloured as in the centre and complex I/MBH in grey. In the bottom row only MBH and complex I are shown in the same orientation as above, with subunits homologous to Mrp coloured as in Mrp and the rest grey. Quinone-binding subunit in complex I (Nqo8/NuoH) is not present in Mrp and is highlighted in slate. Additional MrpD-like subunit in complex I (Nqo13/NuoM) is in orange. MBH has its redox module attached to the Mrp-like domain on the opposite side compared to complex I. MbhF is homologous to MrpB, MbhG to MrpC, MbhH to MrpD, MbhA to MrpE, MbhB to MrpF, MbhC to MrpG and MbhD together with MbhE to MrpAC. Nqo12/NuoL is homologous to MrpAN, Nqo13/NuoM and Nqo14/NuoN to MrpD, Nqo11/NuoK to MrpC and Nqo10/NuoJ to MrpAC. (b) Overlay of Mrp with the membrane domain of complex I. Key residues, important for H+ translocation and Na+ binding are shown in stick representation, with the residue number indicated in black for Mrp and the corresponding homologous residue of complex I indicated in red. Except for MrpAN and MrpD, suffixes indicate subunit (in E. coli nomenclature for complex I). (c) Overlay of Mrp with the homologous subunits of MBH. Key residues, important for H+ translocation and Na+ binding are shown in stick representation, with the residue number indicated in black for Mrp and the corresponding homologous residue of MBH shown in red. Except for MrpD, suffixes indicate subunit. The positions of two bound K+ ions in Mrp are indicated by purple spheres, with one of them being less visible behind H37c.