Periplasmic Metal-Resistance Protein CusF Exhibits High Affinity and Specificity for Both CuI and AgI

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Abstract
The periplasmic protein CusF, as a part of the CusCFBA efflux complex, plays a role in resistance to elevated levels of copper and silver in Escherichia coli. Although homologues have been identified in other Gram-negative bacteria, the substrate of CusF and its precise role in metal resistance have not been described. Here, isothermal titration calorimetry (ITC) was used to demonstrate that CusF binds with high affinity to both CuI and AgI but not CuII. The affinity of CusF for AgI was higher than that for CuI, which could reflect more efficient detoxification of AgI given the lack of a cellular need for AgI. The chemical shifts in the nuclear magnetic resonance (NMR) spectra of CusF−AgI as compared to apo-CusF show that the region of CusF most affected by AgI binding encompasses three absolutely conserved residues: H36, M47, and M49. This suggests that these residues may play a role in AgI coordination. The NMR spectra of CusF in the presence of CuII do not indicate specific binding, which is in agreement with the ITC data. We conclude that CuI and AgI are the likely physiological substrates.