A free‐exchange mathematical model of EmrE

Abstract

EmrE is a multidrug resistance transport protein from E. coli that couples the efflux of toxic, lipophilic cations to the influx of protons down their electrochemical gradient. Active transporters require alternating access between open‐in (binding pocket accessible to the cytoplasm) and open‐out (binding pocket accessible to the periplasm) conformations to move substrate across the membrane. Strict restrictions on alternating access or substrate binding lead to simplified models that achieve stoichiometric symport (coupling movement of both substrates in same direction) or “pure‐exchange” antiport (coupling movement of both substrates in opposite directions). However, recent data suggests that common strictly‐coupled models of transport may be oversimplified. For example, the EmrE point mutant, W63G, confers resistance to erythromycin, indicating H+‐coupled transport of that substrate, while also conferring susceptibility to polyamines, indicating uniport or H+‐coupled symport of those substrates. We recently developed a “free‐exchange” model of EmrE transport that can account for this behavior as well as NMR data showing that EmrE can simultaneously bind drug and proton and can alternate access when bound to both, one, or no substrates in violation of pure‐exchange models. Here, we develop a free‐exchange mathematical model to explore features that tip the balance between different H+‐coupled transport regimes. We find that under physiological conditions, transport is faster in the free‐exchange model and comparable substrate gradients can be produced. Using the available kinetic data for EmrE and our mathematical model, we show that EmrE can concentrate drug as experimentally observed in liposomal‐flux assays and also show how drug identity can switch directionality of transport, as observed for W63G‐EmrE.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.