Short-chain diamines are the physiological substrates of PACE family efflux pumps

Significance Multidrug efflux pumps are highly promiscuous determinants of antimicrobial resistance in bacterial pathogens. Since efflux pumps evolved long before the widespread use of antimicrobials, drug transport is likely to be a side reaction in many pumps, fortuitously beneficial to bacteria in hospitals. The AceI efflux protein from Acinetobacter baumannii is the prototype for the proteobacterial antimicrobial compound efflux (PACE) family. AceI was only known to transport the synthetic biocide chlorhexidine, which was incongruous with its ancient origin. Here we demonstrate that short-chain diamines are the physiological substrates of AceI and other PACE members, and that transport is energized by an electrochemical gradient of protons. These observations are important, because diamines play vital roles in bacterial physiology and virulence and have significant commercial uses.