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+).