Mechanistic studies of a multidrug efflux protein from <i>Escherichia coli</i>

Chloramphenicol and tetraphenylphosphonium appear to compete with protons for binding to MdtM at pH values < 6.7. This apparent competition may be as a result of direct competition for the binding site but is more likely as a result of proton mediated conformational changes. These changes could lead to MdtM no longer being in the high affinity inward open conformation.

Charged residues play a critical role in linking proton and substrate transport. Using in silico analysis of homology models, critical interactions between charged residues have been identified and functionally investigated. The conformational switch and charge relay triad have been identified.

MdtM tolerates mutation of the conserved charged residue inside the binding site; the transport activity is reduced but not abolished and binding affinity is not impacted to the extent of removal of binding residues. Mutation to the conserved residues of loop regions has a detrimental impact on the stability of the transporter when extracted from the membrane.