Copper Binding and Oligomerization Studies of the Metal Resistance Determinant CrdA from .

Within this research, the CrdA protein from (CrdA), a putative copper-binding protein important for the survival of bacterium, was biophysically characterized in a solution, and its binding affinity toward copper was experimentally determined. Incubation of CrdA with Cu(II) ions favors the formation of the monomeric species in the solution. The modeled CrdA structure shows a conserved methionine-rich region, a potential binding site for Cu(I), as in the structures of similar copper-binding proteins, CopC and PcoC, from and from , respectively. Within the conserved amino acid motif, CrdA contains two additional methionines and two glutamic acid residues (MXMPGMXXXM) in comparison to CopC and PcoC but lacks the canonical Cu(II) binding site (two His) since the sequence has no His residues. The methionine-rich site is in a flexible loop and can adopt different geometries for the two copper oxidation states. It could bind copper in both oxidation states (I and II), but with different binding affinities, micromolar was found for Cu(II), and less than nanomolar is proposed for Cu(I). Considering that CrdA is a periplasmic protein involved in chaperoning copper export and delivery in the cell and that the affinity of the interaction corresponds to a middle or strong metal-protein interaction depending on the copper oxidation state, we conclude that the interaction also occurs in vivo and is physiologically relevant for .