title: Distinct characteristics of Ag+ and Cd2+ binding to CopZ from Bacillus subtilis.
abstract: The chaperone CopZ together with the P-type ATPase transporter CopA constitute a copper-detoxification system in Bacillus subtilis that is commonly found in bacteria and higher cells. Previous studies of the regulation of the copZA operon showed that expression is significantly upregulated in response to elevated concentrations of environmental silver and cadmium, as well as copper. Here, we have used spectroscopic and bioanalytical methods to investigate in detail the capacity of CopZ to bind these metal ions (as Ag(+) and Cd(2+)). We demonstrate that Ag(+) binding mimics closely that of Cu(+): Ag(+)-mediated dimerisation of the protein occurs, and distinct Ag(+)-bound species are formed at higher Ag(+) loadings. Cd(2+) also binds to CopZ, but exhibits significantly different behaviour. Cd(2+)-mediated dimerisation is only observed at low loadings, such that at 0.5 and one Cd(2+) per CopZ the protein is present mainly in a monomeric form; and multinuclear higher-order forms of Cd(2+)-CopZ are not observed. Competition binding studies reveal that Ag(+) binds with an affinity very similar to that of Cu(+), while Cd(2+) binding is significantly weaker. These data provide support for the proposal that CopZ may be involved in the detoxification of silver and cadmium, in addition to copper.
authors.affiliation: Centre for Molecular and Structural Biochemistry, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK.
authors.forename: Margaret A
authors.initials: MA
authors.lastname: Kihlken
authors.forename: Chloe
authors.initials: C
authors.lastname: Singleton
authors.forename: Nick E
authors.initials: NE
authors.lastname: Le Brun
mesh_terms: D001412:Bacillus subtilis / Q000737:chemistry*; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry*; D002151:Calorimetry / Q000379:methods; D002850:Chromatography, Gel / Q000379:methods; D002942:Circular Dichroism / Q000379:methods; D003300:Copper / Q000737:chemistry; D018832:Molecular Chaperones / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*; D013050:Spectrometry, Fluorescence / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014461:Ultracentrifugation / Q000379:methods
chemical_list: D001426:Bacterial Proteins; D018832:Molecular Chaperones; D002104:Cadmium; D012834:Silver; D003300:Copper