Assembly and regulation of the chlorhexidine-specific efflux pump AceI

Few antibiotics are effective against Acinetobacter baumannii, one of the most successful pathogens responsible for hospital-acquired infections. Resistance to chlorhexidine, an antiseptic widely used to combat A. baumannii, is effected through the proteobacterial antimicrobial compound efflux (PACE) family. The prototype mem-brane protein of this family, AceI (Acinetobacter chlorhexidine ef-flux protein I), is encoded for by the aceI gene and is under the transcriptional control of AceR (Acinetobacter chlorhexidine efflux protein regulator), a LysR-type transcriptional regulator (LTTR) protein. Here we use native mass spectrometry to probe the re-sponse of AceI and AceR to chlorhexidine assault. Specifically, we show that AceI forms dimers at high pH, and that binding to chlor-hexidine facilitates the functional form of the protein. Changes in the oligomerization of AceR to enable interaction between RNA polymerase and promoter DNA were also observed following chlor-hexidine assault. Taken together, these results provide insight into the assembly of PACE family transporters and their regulation via LTTR proteins on drug recognition and suggest potential routes for intervention.