Microcalorimetry of EmrE, a Multi‐Drug Transporter.

The efflux of structurally diverse toxic substrates by multi‐drug transporters is the cause of significant resistance of pathogenic bacteria to a number of therapeutic compounds. EmrE is a small multi‐drug transporter from Escherichia coli that utilises the proton gradient to extrude a number of hydrophobic cations from the cell, thus resulting in resistance to toxic compounds. Interactions between EmrE and drug molecules can be determined by microcalorimetry, which also allows non‐invasive determination of absolute thermodynamic quantities. These interactions can also be measured and quantified in lipid environments that are not amenable to other biophysical techniques. We have investigated the thermodynamic parameters of binding of the substrate tetraphenylphosphonium (TPP+) to EmrE, reconstituted in various detergent and lipid environments, by Isothermal Titration Calorimetry. The Stoiciometry and binding affinity is in agreement with previous work, with a stoichiometry of 1 molecule of ligand to each EmrE dimer.