Structure and function of the C-terminal domain of MrpA in the Bacillus subtilis Mrp-antiporter complex--the evolutionary progenitor of the long horizontal helix in complex I.

•Transmembrane topology of the C-terminal domain of MrpA was determined.•The first part of the C-terminal domain corresponds to the horizontal helix in NuoL.•The second part of the domain is homologous to the NuoJ subunit in complex I.•The function of the domain was investigated in a B. subtilis model system.