The resistance nodulation division (RND)-type efflux systems are utilized in Gram-negative bacteria to export a variety of substrates. The CusCFBA system is the $Cu^+$ and $Ag^+$ efflux system in Escherichia coli, conferring resistance to lethal concentrations of $Cu^+$ and $Ag^+$. The periplasmic component, CusB, which is essential for the assembly of the protein complex, has $Cu^+$ or $Ag^+$ binding sites. The twelve-span membrane protein CusA is a homotrimeric transporter, and has a relatively large periplasmic domain. Here, we constructed the periplasmic domain of CusA by joining two DNA segments and then successfully expressed and purified the protein. Isothermal titration calorimetry experiments revealed $Ag^+$ binding sites with $K_ds$ of $10^{-6}-10^{-5}$ M. Our findings suggest that the metal binding in the periplasmic domain of CusA might play an important role in the function of the efflux pump.