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.

小型多药耐药（SMR）转运蛋白通过协同反转运质子和有毒底物，通过将这些化合物从细菌细胞质中排出而促成抗生素耐药性。对大肠杆菌SMR转运蛋白EmrE的分子转运机制的广泛生物物理研究指出，它还应具备执行质子/药物同向转运或单向转运的能力，无论哪种转运方式，在体内都将导致药物敏感性而非耐药性。本研究揭示，EmrE确实赋予了原核大肠杆菌对某些小分子底物的敏感性，同时赋予了已知的多环芳族阳离子底物的耐药性。体外实验表明，在次级位点上的底物结合触发了非协同质子单向转运，从而导致敏感性。这些结果表明，SMR转运蛋白为细菌选择性消散质子动力的途径之一。这具有开发抗生素和更广泛地破坏由于药物排出导致的抗生素耐药性的潜力。