Fig. 1: EmrE binds and transports F₄-TPP⁺ in a similar fashion to TPP⁺.

a Schematic model of the alternating-access mechanism of the asymmetric EmrE homodimer to export polyaromatic substrates out of bacterial cells. The F₄-TPP⁺ structure is shown on the right. b Amide H^N and ¹⁵N chemical shift difference between F₄-TPP⁺ and TPP⁺ bound S64V-EmrE in lipid bicelles. Red: subunit A; Blue, subunit B. Small chemical shift differences are observed, indicating that TPP⁺ fluorination has little effect on the protein structure. Error bars are ±0.025 ppm based on spectral resolution. c Amide H^N and ¹⁵N chemical shift difference between subunits A and B of bicelle-bound S64V-EmrE. Red: F₄-TPP⁺ bound protein data; Black: TPP⁺-bound protein data. The structural asymmetry between the two subunits is similar for the two substrates. Error bars are ±0.025 ppm based on spectral resolution. d Solid-supported membrane electrophysiology data of F₄-TPP⁺ transport by wild-type EmrE, driven by an inward pH gradient. When the F₄-TPP⁺ gradient is in the opposite direction from the pH gradient, net current increases compared with when the drug gradient is absent. When the substrate gradient is in the same direction as the pH gradient, net current decreases. Thus, F₄-TPP⁺ is a canonical antiported substrate of EmrE. The E14Q mutant data serve as controls. Raw current traces and additional details are shown in Supplementary Figure 3. Error bars represent the standard error of three replicates using independently prepared sensors and the three individual data points are shown as circles.