The delivery complex in outer membrane protein biogenesis: How the SurA chaperone interacts with the BAM enzyme

SurA is the major chaperone of unfolded outer membrane proteins (OMPs) in the periplasm of E. coli. Its function has not been fully resolved but current literature suggests roles in binding OMPs as they emerge from the inner membrane, preventing their aggregation, and maintaining them in a folding competent state. Less well studied is how OMPs can be delivered to the BAM complex via SurA diffusing in the periplasm. We use AlphaFold models as a starting point to explore the ‘delivery complex’ between SurA and BAM. Through the solution of cryoEM-derived structures, in vitro biophysics, and in vivo functional assays we can build a mechanistic model that incorporates chaperone delivery into the current BAM-assisted OMP folding paradigm.