Comparison of three structures of the multidrug transporter EmrE

The small multidrug resistance proteins constitute a family of bacterial antiporters that confer multidrug resistance by H+-linked drug efflux across the bacterial cytoplasmic membrane. The structure of EmrE, the family archetype, has been determined by electron crystallography and shows that EmrE in the membrane is an asymmetric homodimer composed of a tightly packed bundle of eight α helices, six of which form the substrate-binding site, which has a single molecule of tetraphenylphosphonium at its centre. Two X-ray structures of EmrE have been determined; the first structure was of a non-native conformation of EmrE that formed a crystallographic tetramer, whereas EmrE in the second structure was an asymmetric dimer containing a single molecule of bound tetraphenylphosphonium. This recent EmrE structure bears a superficial resemblance to the electron crystallographic structure and the differences were ascribed to conformational changes. However, the biological relevance of these conformational differences is questionable.