New functions for the three subunits of the CzcCBA cation-proton antiporter

The membrane-bound CzcCBA protein complex mediates heavy metal resistance in Alcaligenes eutrophus by an active cation efflux mechanism driven by cation-proton antiport. The CzcA protein alone is able to mediate weak resistance to zinc and cobalt and is thus the central antiporter subunit. The two histidine-rich motifs in the CzcB subunit are not essential for zinc resistance; however, deletion of both motifs led to a small but significant loss of resistance to this cation. Translation of the czcC gene encoding the third subunit of the CzcCBA complex starts earlier than predicted, and CzcC is probably a periplasmic protein, as judged by the appearance of two bands after expression of czcC in Escherichia coli under control of the phage T7 promoter. Fusions of CzcC and CzcB with alkaline phosphatase and beta-galactosidase are in agreement with a periplasmic location of most parts of both proteins. Both CzcC and CzcB are bound to a membrane, probably the outer membrane, by themselves and do not need either CzcA or each other as an anchoring protein. Based on these data, a new working model for the function of the Czc system is discussed.