X-ray structure of the metal-sensor CnrX in both the apo- and copper-bound forms

Both the X-ray structures of the apo- and the copper-bound forms of the metal-sensor domain (residues 31–148) of CnrX from Cupriavidus metallidurans CH34 were obtained at 1.74 Å resolution from a selenomethionine derivative. This four-helix hooked-hairpin is the first structure of a metal-sensor in an ECF-type signaling pathway. The copper ion is bound in a type 2-like center with a 3N1O coordination in the equatorial plane and shows an unprecedented remote fifth axial ligand with Met93 contributing a weak S–Cu bond. The signal onset cannot be explained by conformational changes associated with CnrX metallation.