title: Tracking metal ions through a Cu/Ag efflux pump assigns the functional roles of the periplasmic proteins.
abstract: Copper is an essential nutrient for all aerobic organisms but is toxic in excess. At the host-pathogen interface, macrophages respond to bacterial infection by copper-dependent killing mechanisms, whereas the invading bacteria are thought to counter with an up-regulation of copper transporters and efflux pumps. The tripartite efflux pump CusCBA and its metallochaperone CusF are vital to the detoxification of copper and silver ions in the periplasm of Escherichia coli. However, the mechanism of efflux by this complex, which requires the activation of the inner membrane pump CusA, is poorly understood. Here, we use selenomethionine (SeM) active site labels in a series of biological X-ray absorption studies at the selenium, copper, and silver edges to establish a "switch" role for the membrane fusion protein CusB. We determine that metal-bound CusB is required for activation of cuprous ion transfer from CusF directly to a site in the CusA antiporter, showing for the first time (to our knowledge) the in vitro activation of the Cus efflux pump. This metal-binding site of CusA is unlike that observed in the crystal structures of the CusA protein and is composed of one oxygen and two sulfur ligands. Our results suggest that metal transfer occurs between CusF and apo-CusB, and that, when metal-loaded, CusB plays a role in the regulation of metal ion transfer from CusF to CusA in the periplasm.
authors.affiliation: Institute of Environmental Health, Oregon Health and Science University, Portland, OR 97239; and.
authors.forename: Kelly N
authors.identifier: http://orcid.org/0000-0003-1332-7283
authors.initials: KN
authors.lastname: Chacón
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721.
authors.forename: Tiffany D
authors.initials: TD
authors.lastname: Mealman
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721 blackbni@ohsu.edu mcevoy@email.arizona.edu.
authors.forename: Megan M
authors.initials: MM
authors.lastname: McEvoy
authors.affiliation: Institute of Environmental Health, Oregon Health and Science University, Portland, OR 97239; and blackbni@ohsu.edu mcevoy@email.arizona.edu.
authors.forename: Ninian J
authors.initials: NJ
authors.lastname: Blackburn
mesh_terms: D001059:Apoproteins / Q000378:metabolism; D003300:Copper / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D007477:Ions; D008954:Models, Biological; D033903:Periplasmic Proteins / Q000378:metabolism*; D012645:Selenomethionine / Q000378:metabolism; D012834:Silver / Q000378:metabolism*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D001059:Apoproteins; D007477:Ions; D033903:Periplasmic Proteins; D012834:Silver; D003300:Copper; D012645:Selenomethionine
keywords: X-ray absorption spectroscopy; copper; host–pathogen interaction; metal ion transport; periplasmic efflux