An antiport mechanism for a member of the cation diffusion facilitator family: Divalent cations efflux in exchange for K⁺ and H⁺

Members of the cation diffusion facilitator (CDF) family of membrane transport proteins are found in eukaryotes and prokaryotes. The family encompasses transporters of zinc ions, with cobalt, cadmium and lead ions being additional substrates for some prokaryotic examples. No transport mechanism has previously been established for any CDF protein. It is shown here that the CzcD protein of Bacillus subtilis, a CDF protein, uses an antiporter mechanism, catalysing active efflux of Zn²⁺ in exchange for K⁺ and H⁺. The exchange is probably electroneutral, energized by the transmembrane pH gradient and oppositely oriented gradients of the other cation substrates. The data suggest that Co²⁺ and Cd²⁺ are additional cytoplasmic substrates for CzcD. A second product of the same operon that encodes czcD has sequence similarity to oxidoreductases and is here designated CzcO. CzcO modestly enhances the activity of CzcD but is not predicted to be an integral membrane protein and has no antiport activity of its own.