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

SUMMARYTransmembrane β-barrel proteins are folded into the outer membrane (OM) of Gram-negative bacteria by the β-barrel assembly machine (BAM) via an unexplained 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 remarkable “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 towards 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.

Local Bilayer Hydrophobicity Modulates Membrane Protein Stability

ABSTRACTThrough the insertion of nonpolar side chains into the bilayer, the hydrophobic effect has long been accepted as a driving force for membrane protein folding. However, how the changing chemical composition of the bilayer affects the magnitude side chain transfer free energies has historically not been well understood. A particularly challenging region for experimental interrogation is the bilayer interfacial region that is characterized by a steep polarity gradient. In this study we have determined the for nonpolar side chains as a function of bilayer position using a combination of experiment and simulation. We discovered an empirical correlation between the surface area of nonpolar side chain, the transfer free energies, and the local water concentration in the membrane that allows for to be accurately estimated at any location in the bilayer. Using these water-to-bilayer values, we calculated the interface-to-bilayer transfer free energy . We find that the are similar to the “biological”, translocon-based transfer free energies, indicating that the translocon energetically mimics the bilayer interface. Together these findings can be applied to increase the accuracy of computational workflows used to identify and design membrane proteins, as well as bring greater insight into our understanding of how disease-causing mutations affect membrane protein folding and function.