Mrp-type antiporters comprise the cation/proton antiporter family 3 (CPA3), commonly referred to as Mrp. They are the products of operons that carry either six or seven genes (mrpA-G), and form complexes containing all subunits [, ]. They have Na(+)/H(+) antiporter activity []. Two of the Mrp proteins, MrpA and MrpD, resemble NuoL/ND5, NuoM/ND4, NuoN/ND2, the homologous subunits that constitute the membrane-embedded, proton-translocating core of complex I [, , , ]. They also resemble subunits of membrane-bound hydrogenases, such as HyfB, HyfD and HyfF from E.coli []and F420H2 dehydrogenasa (FPO complex) subunits L, N and M [].This domain represents an N-terminal extension of . It is found in complex I subunit NuoL/ND5, CPA3 antiporters subunit MrpA, and in F(420)H(2) dehydrogenase subunit L. Only NADH-Ubiquinone chain 5 and eubacterial chain L are in this family. This sub-family is part of complex I which catalyses the transfer of two electrons from NADH to ubiquinone in a reaction that is associated with proton translocation across the membrane.
