title: Redundancy in the periplasmic adaptor proteins AcrA and AcrE provides resilience and an ability to export substrates of multidrug efflux.
abstract: OBJECTIVE
  The components of the AcrAB-TolC efflux pump function as a tripartite efflux system conferring resistance to multiple antibiotics and the individual components can also function in conjunction with other efflux pumps. This study aimed to establish whether redundancy exists between the homologous periplasmic adaptor proteins (PAPs) AcrA and AcrE and to measure the impact of this redundancy on antimicrobial resistance and the potential efficacy of inhibitor molecules.
  
  
  METHODS
  The acrE gene was inactivated in Salmonella enterica serovar Typhimurium SL1344 and a ΔacrA mutant by insertion of the aph gene. The mutants were complemented with plasmids carrying acrA or acrE. The antimicrobial susceptibility of the mutants to various antimicrobials was determined and the accumulation or efflux of various substrates was measured.
  
  
  RESULTS
  Inactivation of acrE alone had no phenotypic effect. However, the effect of inactivation of PAPs was additive; the acrA acrE mutant was more susceptible to certain antimicrobials and accumulated more Hoechst dye than single acrA, acrE or acrB mutants. In addition, the double mutant invaded human intestinal epithelial cells poorly. The phenotypic defects of the acrA acrE mutant were ameliorated by expression of either acrA or acrE, but the proteins exhibited some substrate specificity.
  
  
  CONCLUSIONS
  These data show for the first time the level of redundancy between the PAPs AcrA and AcrE, and highlight the PAPs as excellent targets for inhibitor molecules that could be used to potentiate the action of clinical antimicrobials. However, the redundancy that exists between AcrA and AcrE means potential inhibitors must act on both targets to be effective.
authors.affiliation: Antimicrobials Research Group, School of Immunity and Infection, College of Medical and Dental Sciences, and Institute of Microbiology and Infection, The University of Birmingham, Birmingham B15 2TT, UK.
authors.forename: Helen E
authors.initials: HE
authors.lastname: Smith
authors.forename: Jessica M A
authors.initials: JM
authors.lastname: Blair
mesh_terms: D000900:Anti-Bacterial Agents / Q000378:metabolism* / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D017353:Gene Deletion; D005816:Genetic Complementation Test; D006801:Humans; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D008826:Microbial Sensitivity Tests; D012486:Salmonella typhimurium / Q000235:genetics* / Q000378:metabolism*
chemical_list: C493251:AcrAB-TolC protein, Salmonella enterica; C534525:AcrE protein, Salmonella enterica; D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D002352:Carrier Proteins; D026901:Membrane Transport Proteins
keywords: RND; Salmonella spp.; antibiotic resistance; drug targets; efflux pumps; multidrug resistance