Tripartite CBA efflux complexes driven by RND antiporters export heavy metal cations across both the inner and outer membranes of Gram-negative bacteria.
"The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance-nodulation-cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors."