CryoEM structure of QacA, an antibacterial efflux transporter from Staphylococcus aureus

Efflux of antibacterial compounds is a major mechanism for developing antimicrobial resistance. In the Gram-positive pathogen Staphylococcus aureus, QacA, a 14 transmembrane (TM) helix containing major facilitator superfamily antiporter, mediates proton-coupled efflux of mono and divalent cationic antibacterial compounds. In this study, we report the cryoEM structure of QacA, with a single mutation D411N that improves homogeneity and retains efflux activity against divalent cationic compounds like dequalinium and chlorhexidine. The structure of substrate-free QacA, complexed to two single-domain camelid antibodies, was elucidated to a resolution of 3.6 [A]. The structure displays an outward-open conformation with an extracellular hairpin loop, which is conserved in a subset of DHA2 transporters and its deletion causes a loss of function in the transporter. Modeling and simulations of QacAs cytosol-facing and occluded conformations reveal asymmetry in the rocker-switch mode of QacAs conformational shifts, providing new insights into the organization and structural dynamics of DHA2 members.