A small-molecule inhibitor of BamA impervious to efflux and the outer membrane permeability barrier

Proc Natl Acad Sci U S A. 2019 Oct 22;116(43):21748-21757. doi: 10.1073/pnas.1912345116. Epub 2019 Oct 7.

Abstract

The development of new antimicrobial drugs is a priority to combat the increasing spread of multidrug-resistant bacteria. This development is especially problematic in gram-negative bacteria due to the outer membrane (OM) permeability barrier and multidrug efflux pumps. Therefore, we screened for compounds that target essential, nonredundant, surface-exposed processes in gram-negative bacteria. We identified a compound, MRL-494, that inhibits assembly of OM proteins (OMPs) by the β-barrel assembly machine (BAM complex). The BAM complex contains one essential surface-exposed protein, BamA. We constructed a bamA mutagenesis library, screened for resistance to MRL-494, and identified the mutation bamAE470K BamAE470K restores OMP biogenesis in the presence of MRL-494. The mutant protein has both altered conformation and activity, suggesting it could either inhibit MRL-494 binding or allow BamA to function in the presence of MRL-494. By cellular thermal shift assay (CETSA), we determined that MRL-494 stabilizes BamA and BamAE470K from thermally induced aggregation, indicating direct or proximal binding to both BamA and BamAE470K Thus, it is the altered activity of BamAE470K responsible for resistance to MRL-494. Strikingly, MRL-494 possesses a second mechanism of action that kills gram-positive organisms. In microbes lacking an OM, MRL-494 lethally disrupts the cytoplasmic membrane. We suggest that the compound cannot disrupt the cytoplasmic membrane of gram-negative bacteria because it cannot penetrate the OM. Instead, MRL-494 inhibits OMP biogenesis from outside the OM by targeting BamA. The identification of a small molecule that inhibits OMP biogenesis at the cell surface represents a distinct class of antibacterial agents.

Keywords: BAM complex; Escherichia coli; antibiotic development; gram-negative bacteria; outer membrane biogenesis.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Bacterial Outer Membrane Proteins / antagonists & inhibitors
  • Bacterial Outer Membrane Proteins / genetics
  • Bacterial Outer Membrane Proteins / metabolism*
  • Biological Transport / physiology
  • Cell Membrane / drug effects
  • Cell Membrane Permeability / physiology
  • Drug Evaluation, Preclinical
  • Drug Resistance, Bacterial / genetics
  • Escherichia coli / drug effects*
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / antagonists & inhibitors*
  • Escherichia coli Proteins / genetics
  • Microbial Sensitivity Tests
  • Protein Multimerization / drug effects*
  • Triazines / pharmacology*

Substances

  • Anti-Bacterial Agents
  • Bacterial Outer Membrane Proteins
  • BamA protein, E coli
  • Escherichia coli Proteins
  • Triazines