CusB is a component of the copper efflux complex CusCFBA of Escherichia coli. The Cus system, unlike its multi-drug exporter homologs, such as AcrAB-TolC, consists of an additional periplasmic component called CusF. CusF is unique to putative copper/silver CBA transport systems and bears no significant sequence similarity to other proteins outside of its orthologs in other organisms. CusC, CusB and CusA, in accordance with CBA-type multi-drug exporters, function as the Outer Membrane Factor (OMF), Membrane Fusion Protein (MFP) and Resistance, Nodulation, Cell Division (RND) type antiporter, respectively. Membrane Fusion Proteins are shown to interact with inner and outer membrane components in other CBA type systems. However, their direct involvement in the uptake of periplasmic solutes is not indicated. The evidence for the periplasmic uptake in Mex and Acr multi-drug resistance systems pointed at the inner membrane component as being the major player. In some organisms, the MFP CusB is a single polypeptide with CusF. This suggests an interaction between these two components in the Cus system and a possible periplasmic copper uptake by CusB. CusF, which has been shown to bind Cu (I)/Ag (I) through in vivo and in vitro studies might act as a chaperone or a regulator of CusB. In order to understand the molecular details of copper regulation demonstrated by Cus system and more specifically, define the role of MFPs in CBA type metal transporters, we aim to characterize the MFP CusB and probe its interactions with CusF using various structural and biochemical tools. The study might enhance our general understanding of multi-drug transporters.