Crystal structure of the Staphylococcus aureus pI258 CadC Cd(II)/Pb(II)/Zn(II)-responsive repressor

The Staphylococcus aureus plasmid pI258 cadCA operon encodes a P-type ATPase, CadA, that confers resistance to the heavy metals Cd(II), Zn(II), and Pb(II). Expression of this heavy-metal efflux pump is regulated by CadC, a homodimeric repressor that dissociates from the cad operator/promoter upon binding of Cd(II), Pb(H), or Zn(H). CadC is a member of the ArsR/SmtB family of metalloregulatory proteins. Here we report the X-ray crystal structure of CadC at 1.9 angstrom resolution. The dimensions of the protein dimer are approximately 30 angstrom by 40 angstrom by 70 angstrom. Each monomer contains six alpha-helices and a three-stranded beta-sheet. Helices 4 and 5 form a classic helix-turn-helix motif that is the putative DNA binding region. The alpha 1 helix of one monomer crosses the dimer to approach the alpha 4 helix of the other monomer, consistent with the previous proposal that these two regulatory metal binding sites for the inducer cadmium or lead are each formed by Cys-7 and Cys-11 from the N terminus of one monomer and Cys-58 and Cys-60 of the other monomer. Two nonregulatory metal binding sites containing zinc are formed between the two antiparallel alpha 6 helices at the dimerization interface. This is the first reported three-dimensional structure of a member of the ArsR/SmtB family with regulatory metal binding sites at the DNA binding domain and the first structure of a transcription repressor that responds to the heavy metals Cd(II) and Pb(II).