Transport of metformin metabolites by guanidinium exporters of the small multidrug resistance family

Peripheral positions encode transport specificity in the small multidrug resistance exporters

In secondary active transporters, a relatively limited set of protein folds have evolved diverse solute transport functions. Because of the conformational changes inherent to transport, altering substrate specificity typically involves remodeling the entire structural landscape, limiting our understanding of how novel substrate specificities evolve. In the current work, we examine a structurally minimalist family of model transport proteins, the small multidrug resistance (SMR) transporters, to understand the molecular basis for the emergence of a novel substrate specificity. We engineer a selective SMR protein to promiscuously export quaternary ammonium antiseptics, similar to the activity of a clade of multidrug exporters in this family. Using combinatorial mutagenesis and deep sequencing, we identify the necessary and sufficient molecular determinants of this engineered activity. Using X-ray crystallography, solid-supported membrane electrophysiology, binding assays, and a proteoliposome-based quaternary ammonium antiseptic transport assay that we developed, we dissect the mechanistic contributions of these residues to substrate polyspecificity. We find that substrate preference changes not through modification of the residues that directly interact with the substrate but through mutations peripheral to the binding pocket. Our work provides molecular insight into substrate promiscuity among the SMRs and can be applied to understand multidrug export and the evolution of novel transport functions more generally.

On the link between antibiotic resistance, diabetes, and wastewater

The study by Lucero et al. (https://doi.org/10.1085/jgp.202313464) sheds light on the remarkable capabilities of bacterial transporters to adapt to new selective pressures. Their findings provide insight into the mechanism of a subtype of SMR transporters.

SMR transporters meet the challenge of metformin metabolites

JGP study (Lucero et al. http://www.doi.org/10.1085/jgp.202313464) shows that members of the SMRGdx subtype can export the degradation products of metformin, helping bacteria adapt to high environmental levels of the commonly prescribed diabetes medication.