Threonine-978 in the Transmembrane Segment of the Multidrug Efflux Pump AcrB of Escherichia coli Is Crucial for Drug Transport as a Probable Component of the Proton Relay Network

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Abstract
Escherichia coli AcrB is a multidrug efflux transporter that recognizes multiple toxic chemicals and expels them from cells. It is a proton antiporter belonging to the resistance-nodulation-division (RND) superfamily. Asp407, Asp408, Lys940, and Arg971 in transmembrane (TM) helices of this transporter have been identified as essential amino acid residues that probably function as components of the proton relay system. In this study, we identified a novel residue in TM helix 11, Thr978, as an essential residue by alanine scanning mutagenesis. Its location close to Asp407 suggests that it is also a component of the proton translocation pathway, a prediction confirmed by the similar conformations adopted by T978A, D407A, D408A, and K940A mutant proteins (see the accompanying paper). Sequence alignment of 566 RND transporters showed that this threonine residue is conserved in about 96% of cases. Our results suggest the hypotheses that Thr978 functions through hydrogen bonding with Asp407 and that protonation of the latter alters the salt bridging and hydrogen bonding pattern in the proton relay network, thus initiating a series of conformational changes that ultimately result in drug extrusion.