Investigating the molecular evolution of small multidrug resistant transporters

The small multidrug resistant (SMR) proteins are a family of membrane proteins that are divided into two major functional clades. One of these is the Qacs, or quaternary ammonium cation transporters which are promiscuous in their substrate specificity and can transport a wide variety of antimicrobials. The other clade of SMRs only export a very specific set of guanidinium ions (Gdxs). Despite very similar structural homology between Qacs and Gdxs, there is no overlap in substrate selectivity. By utilizing evolutionary techniques including directed evolution, we have identified a Gdx-Clo variant with six mutations that can transport Qac substrates. We are working towards the structural and biochemical characterization of this mutant to see exactly how these residues allow Gdx to behave like a Qac.