Activating alternative transport modes in a multidrug resistance efflux pump to confer chemical susceptibility

Small multidrug resistance (SMR) transporters perform coupled antiport of protons and toxic substrates, contributing to antibiotic resistance through efflux of these compounds from the bacterial cytoplasm. Extensive biophysical studies of the molecular transport mechanism of the E. coli SMR transporter EmrE indicate that it should also be capable of performing proton/drug symport or uniport, either of which will lead to drug susceptibility rather than drug resistance in vivo. Here we show that EmrE does indeed confer susceptibility to some small molecule substrates in the native E. coli in addition to conferring resistance to known polyaromatic cation substrates. In vitro experiments show that substrate binding at a secondary site triggers uncoupled proton uniport that leads to susceptibility. These results suggest that the SMR transporters provide one avenue for bacterial-selective dissipation of the proton-motive force. This has potential for antibiotic development and disruption of antibiotic resistance due to drug efflux more broadly.