Protonated Structure of EmrE Reveals C-terminal Tail Gating Mechanism

The multidrug efflux pump EmrE is one of the smallest known active transporters and has become a model system for studying multidrug recognition and transport. While recent high-resolution structures have illuminated its dynamic substrate binding pocket, the conformations of its interhelical loops and C-terminal tail, regions critical for controlling proton coupling and gating, remain poorly characterized. Here, we report the high-resolution structure of protonated S64V EmrE determined using solution and solid-state NMR data. This new structural model shows the C-terminal tail occluding the open face of the transport pore, providing a structural basis for how EmrE minimizes proton leak in the absence of substrate. These findings support growing evidence that relatively simple model transporters must leverage an occluded state during alternating access to avoid physiologically unfavorable proton leak.