The membrane-bound CzcA protein, a member of the resistance-nodulation-cell division (RND) permease superfamily, is part of the CzcCB(2)A complex that mediates heavy metal resistance in Ralstonia sp. CH34 by an active cation efflux mechanism driven by cation/proton antiport. CzcA was purified to homogeneity after expression in Escherichia coli, reconstituted into proteoliposomes, and the kinetics of heavy metal transport by CzcA was determined. CzcA is composed of 12 transmembrane alpha-helices and two large periplasmic domains. Two conserved aspartate and a glutamate residue in one of these transmembrane spans are essential for heavy metal resistance and proton/cation antiport but not for facilitated diffusion of cations. Generalization of the resulting model for the function of CzcA as a two-channel pump might help to explain the functions of other RND proteins in bacteria and eukaryotes.