Cryoelectron Microscopy Structures of AdeB Illuminate Mechanisms of Simultaneous Binding and Exporting of Substrates

mBio. 2021 Feb 23;12(1):e03690-20. doi: 10.1128/mBio.03690-20.

Abstract

Acinetobacter baumannii is a Gram-negative pathogen that has emerged as one of the most highly antibiotic-resistant bacteria worldwide. Multidrug efflux within these highly drug-resistant strains and other opportunistic pathogens is a major cause of failure of drug-based treatments of infectious diseases. The best-characterized multidrug efflux system in A. baumannii is the prevalent Acinetobacter drug efflux B (AdeB) pump, which is a member of the resistance-nodulation-cell division (RND) superfamily. Here, we report six structures of the trimeric AdeB multidrug efflux pump in the presence of ethidium bromide using single-particle cryoelectron microscopy (cryo-EM). These structures allow us to directly observe various novel conformational states of the AdeB trimer, including the transmembrane region of trimeric AdeB can be associated with form a trimer assembly or dissociated into "dimer plus monomer" and "monomer plus monomer plus monomer" configurations. We also discover that a single AdeB protomer can simultaneously anchor a number of ethidium ligands and that different AdeB protomers can bind ethidium molecules simultaneously. Combined with molecular dynamics (MD) simulations, we reveal a drug transport mechanism that involves multiple multidrug-binding sites and various transient states of the AdeB membrane protein. Our data suggest that each AdeB protomer within the trimer binds and exports drugs independently.IMPORTANCE Acinetobacter baumannii has emerged as one of the most highly antibiotic-resistant Gram-negative pathogens. The prevalent AdeB multidrug efflux pump mediates resistance to a broad spectrum of clinically relevant antimicrobial agents. Here, we report six cryo-EM structures of the trimeric AdeB pump in the presence of ethidium bromide. We discover that a single AdeB protomer can simultaneously anchor a number of ligands, and different AdeB protomers can bind ethidium molecules simultaneously. The results indicate that each AdeB protomer within the trimer recognizes and extrudes drugs independently.

Keywords: Acinetobacter baumannii; AdeB multidrug efflux pump; cryo-EM; multidrug resistance; structural biology.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Acinetobacter baumannii / drug effects
  • Acinetobacter baumannii / genetics*
  • Acinetobacter baumannii / ultrastructure
  • Anti-Bacterial Agents / pharmacology
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / ultrastructure
  • Cell Division / drug effects
  • Cryoelectron Microscopy*
  • Drug Resistance, Multiple, Bacterial / genetics*
  • Ethidium / pharmacology
  • Membrane Transport Proteins / chemistry*
  • Membrane Transport Proteins / ultrastructure

Substances

  • AdeB protein, Acinetobacter baumannii
  • Anti-Bacterial Agents
  • Bacterial Proteins
  • Membrane Transport Proteins
  • Ethidium