Mg-dependent mechanism of environmental versatility in a multidrug efflux pump.

Tripartite resistance nodulation and cell division multidrug efflux pumps span the periplasm and are major drivers of multidrug resistance among gram-negative bacteria. Cations, such as Mg, become concentrated within the periplasm and, in contrast to the cytoplasm, its pH is sensitive to conditions outside the cell. Here, we reveal an interplay between Mg and pH in modulating the structural dynamics of the periplasmic adapter protein, AcrA, and its function within the prototypical AcrAB-TolC multidrug pump from Escherichia coli. In the absence of Mg, AcrA becomes increasingly plastic within acidic conditions, but when Mg is bound this is ameliorated, resulting instead in domain specific organization. We establish a unique histidine residue directs these dynamics and is essential for sustaining pump activity across acidic, neutral, and basic regimes. Overall, we propose Mg conserves AcrA structural mobility to ensure optimal AcrAB-TolC function within rapidly changing environments commonly faced during bacterial infection and colonization.