Deep mutational scan of a drug efflux pump reveals its structure-function landscape

Drug efflux is a common resistance mechanism found in bacteria and cancer cells. Although several structures of drug efflux pumps are available, they provide only limited functional information on the phenomenon of drug efflux. Here, we performed deep mutational scanning (DMS) on the bacterial ATP binding cassette (ABC) transporter EfrCD to determine the drug efflux activity profile of more than 1500 single variants. These systematic measurements revealed that the introduction of negative charges at different locations within the large substrate binding pocket results in strongly increased efflux activity towards positively charged ethidium, while additional aromatic residues did not display the same effect. Data analysis in the context of an inward-facing cryo-EM structure of EfrCD uncovered a high affinity binding site, which releases bound drugs through a peristaltic transport mechanism as the transporter transits to its outward-facing conformation. Finally, we identified substitutions resulting in rapid Hoechst influx without affecting the efflux activity for ethidium and daunorubicin. Hence, single mutations can convert the ABC exporter EfrCD into a drug-specific ABC importer. ### Competing Interest Statement The authors have declared no competing interest.