Cryo-EM structures reveal multiple stages of bacterial outer membrane protein folding

Cell. 2022 Mar 31;185(7):1143-1156.e13. doi: 10.1016/j.cell.2022.02.016. Epub 2022 Mar 15.


Transmembrane β barrel proteins are folded into the outer membrane (OM) of Gram-negative bacteria by the β barrel assembly machinery (BAM) via a poorly understood process that occurs without known external energy sources. Here, we used single-particle cryo-EM to visualize the folding dynamics of a model β barrel protein (EspP) by BAM. We found that BAM binds the highly conserved "β signal" motif of EspP to correctly orient β strands in the OM during folding. We also found that the folding of EspP proceeds via "hybrid-barrel" intermediates in which membrane integrated β sheets are attached to the essential BAM subunit, BamA. The structures show an unprecedented deflection of the membrane surrounding the EspP intermediates and suggest that β sheets progressively fold toward BamA to form a β barrel. Along with in vivo experiments that tracked β barrel folding while the OM tension was modified, our results support a model in which BAM harnesses OM elasticity to accelerate β barrel folding.

Keywords: BAM; membrane dynamics; membrane protein folding; outer membrane protein; β barrel.

Publication types

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

MeSH terms

  • Bacterial Outer Membrane Proteins / metabolism
  • Bacterial Outer Membrane Proteins / ultrastructure*
  • Cryoelectron Microscopy
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / metabolism
  • Protein Folding*


  • Bacterial Outer Membrane Proteins
  • BamA protein, E coli
  • Escherichia coli Proteins