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Gram-negative bacteria frequently expel toxic chemicals through tripartite efflux pumps that span both the inner and outer membranes. The three parts are the inner membrane, substrate-binding transporter (or pump); a periplasmic membrane fusion protein (MFP, or adaptor); and an outer membrane-anchored channel. The fusion protein connects the transporter to the channel within the periplasmic space. One such efflux system CusCBA is responsible for extruding biocidal Cu(I) and Ag(I) ions. We previously described the crystal structures of both the inner membrane transporter CusA and the MFP CusB of Escherichia coli. We also determined the co-crystal structure of the CusBA adaptor–transporter efflux complex, showing that the transporter CusA, which is present as a trimer, interacts with six CusB protomers and that the periplasmic domain of CusA is involved in these interactions. Here, we summarize the structural information of these efflux proteins, and present the accumulated evidence that this efflux system uses methionine residues to bind and export Cu(I) and Ag(I). Genetic and structural analyses suggest that the CusA pump is capable of picking up the metal ions from both the periplasm and the cytoplasm. We propose a stepwise shuttle mechanism for this pump to export metal ions from the cell. [ABSTRACT FROM AUTHOR]

Abstract: Gram-negative bacteria expel various toxic chemicals via tripartite efflux pumps belonging to the resistance–nodulation–cell division superfamily. These pumps span both the inner and outer membranes of the cell. The three components of these tripartite systems are an inner‐membrane, substrate-binding transporter (or pump); a periplasmic membrane fusion protein (or adaptor); and an outer‐membrane-anchored channel. These three efflux proteins interact in the periplasmic space to form the three-part complexes. We previously presented the crystal structures of both the inner‐membrane transporter CusA and membrane fusion protein CusB of the CusCBA tripartite efflux system from Escherichia coli. We also described the co-crystal structure of the CusBA adaptor–transporter, revealing that the trimeric CusA efflux pump assembles with six CusB protein molecules to form the complex CusB6–CusA3. We here report three different conformers of the crystal structures of CusBA–Cu(I), suggesting a mechanism on how Cu(I) binding initiates a sequence of conformational transitions in the transport cycle. Genetic analysis and transport assays indicate that charged residues, in addition to the methionine pairs and clusters, are essential for extruding metal ions out of the cell. [Copyright &y& Elsevier]