Allosteric Conformational and Topology Changes in the E.coli Membrane Fusion CusB Dimer

Bacterial cells have developed sophisticated systems to deal with the toxicity of metal ions. The E.coli CusCFBA is a complex efflux system, located in the periplasm, involving four proteins: CusA, CusB, CusC, and CusF which is responsible for transferring Cu(I) and Ag(I) ions. The CusA, CusB and CusC are connected together in an oligomerization ratio of 3:6:3 CusA:CusB:CusC to form the CusCBA periplasm membrane transporter. CusB is an adaptor protein which connects the two membrane proteins CusA (inner membrane) and CusC (outer membrane). The CusF is a metallochaperone who transfers Cu(I) and Ag(I) to the CusCBA transporter from the periplasm. The crystal structures of CusB, CusC, CusF, and CusBA complex have been resolved, shedding some light on the efflux mechanism of this intriguing system. However, since CusB is an adaptor protein, its role in operating this system is significant, and should be understood in detail. Here, we utilize EPR spectroscopy to target the conformational changes of the full CusB protein upon binding Cu(I). We reveal that CusB is a dimer in solution, where the orientation of one molecule with respect to the other molecule is varies up on Cu(I) coordination, resulting in a more compact CusB structure. These structural and topology changes upon Cu(I) binding probably play the role of a switch for opening the channel and transferring metal ions from CusB to CusC and out of the cell.