Cryo-EM structures of the SurA-BAM complex reveal conformational changes in outer membrane protein assembly.

The outer membrane (OM) of Gram-negative bacteria acts as a permeability barrier against toxic compounds. Its integrity is maintained by various outer membrane proteins (OMPs), which are inserted into the OM by the β-barrel assembly machinery (BAM) complex. The periplasmic chaperone SurA delivers unfolded OMPs to BAM; however, the mechanism of substrate transfer remains unclear. Here, we show that the flexible P1 and P2 domains of SurA regulate the function of its Core domain and interact with BAM components, including BamE, whose interaction with the P2 domain is crucial for efficient OMP assembly. Moreover, cryo-electron microscopy reveals four distinct Escherichia coli SurA-BAM structures, suggesting dynamic domain rearrangements of SurA. Based on these findings, we propose a dynamic model in which SurA transfers substrates to BAM through multiple conformational changes, providing a unified framework for chaperone-assisted OMP biogenesis.