Mrp complex is a Na(+)/H(+) antiporter that is considered to be the major Na(+) excretion system in B.subtilis. Has a major role in Na(+) resistance and a minor role in Na(+)- and K(+)-dependent pH homeostasis as compared to TetB. MrpA may be the actual Na(+)/H(+) antiporter, although the six other Mrp proteins are all required for Na(+)/H(+) antiport activity and Na(+) resistance. MrpA is required for initiation of sporulation when external Na(+) concentration increases. Also transports Li(+) but not K(+), Ca(2+) or Mg(2+). Forms a heterooligomeric complex that consists of seven subunits: MrpA, MrpB, MrpC, MrpD, MrpE, MrpF and MrpG. Mrp-dependent antiport apparently occurs by a secondary, proton motive force-dependent mechanism, but the similarity of several Mrp proteins to membrane-embedded subunits of energy-coupled NADH dehydrogenase complexes raises the possibility that there is a capacity for electron transport that could provide a primary energy coupling option for Mrp functions. Belongs to the CPA3 antiporters (TC 2.A.63) subunit A family. Truncated N-terminus.