A critical role of a carboxylate in proton conduction by the ATP-binding cassette multidrug transporter LmrA

FASEB J. 2005 Oct;19(12):1698-700. doi: 10.1096/fj.04-3558fje. Epub 2005 Jul 22.


The ATP binding cassette (ABC) transporter LmrA from the bacterium Lactococcus lactis is a homolog of the human multidrug resistance P-glycoprotein (ABCB1), the activity of which impairs the efficacy of chemotherapy. In a previous study, LmrA was shown to mediate ethidium efflux by an ATP-dependent proton-ethidium symport reaction in which the carboxylate E314 is critical. The functional importance of this key residue for ABC proteins was suggested by its conservation in a wider family of related transporters; however, the structural basis of its role was not apparent. Here, we have used homology modeling to define the structural environment of E314. The residue is nested in a hydrophobic environment that probably elevates its pKa, accounting for the pH dependency of drug efflux that we report in this work. Functional analyses of wild-type and mutant proteins in cells and proteoliposomes support our proposal for the mechanistic role of E314 in proton-coupled ethidium transport. As the carboxylate is known to participate in proton translocation by secondary-active transporters, our observations suggest that this substituent can play a similar role in the activity of ABC transporters.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / metabolism
  • ATP-Binding Cassette Transporters / chemistry*
  • Adenosine Triphosphate / chemistry
  • Amino Acid Sequence
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism
  • Biological Transport
  • Carboxylic Acids / chemistry*
  • Cytoplasm / metabolism
  • Databases, Protein
  • Drug Resistance, Microbial
  • Ethidium / chemistry
  • Hydrogen-Ion Concentration
  • Kinetics
  • Lactococcus lactis / metabolism
  • Membrane Potentials
  • Membrane Transport Proteins / chemistry
  • Models, Biological
  • Models, Molecular
  • Molecular Sequence Data
  • Multidrug Resistance-Associated Proteins / chemistry*
  • Multidrug Resistance-Associated Proteins / metabolism
  • Mutation
  • Plasmids / metabolism
  • Protons
  • Sequence Homology, Amino Acid
  • Time Factors


  • ATP Binding Cassette Transporter, Subfamily B, Member 1
  • ATP-Binding Cassette Transporters
  • Bacterial Proteins
  • Carboxylic Acids
  • LmrA protein, Lactococcus lactis
  • Membrane Transport Proteins
  • Multidrug Resistance-Associated Proteins
  • Protons
  • Adenosine Triphosphate
  • Ethidium