THE ROLE OF CHAPERONES AND BAMA IN THE OUTER MEMBRANE PROTEIN BIOGENESIS PATHWAY OF ESCHERICHIA COLI

Outer membrane protein (OMP) biogenesis in Gram-negative bacteria is a multi-step, complex process that spans several cellular compartments. OMPs are post-translationally secreted across the bacterial inner membrane and subsequently encounter the aqueous periplasm. In this milieu, nascent OMPs interact with chaperone proteins that prevent the formation of off-folding-pathway species and deleterious aggregates. Here we investigate how two Escherichia coli periplasmic chaperones, FkpA and SurA, bind to unfolded OMP clients to facilitate trafficking. We find that FkpA populates both monomeric and dimeric species and these oligomers have differential affinities for unfolded OMP clients. We present the first structural model for a SurA-OMP complex, in which the unfolded OMP is expanded and makes delocalized contacts with the SurA chaperone. Upon reaching the outer membrane, OMPs are assembled via interactions with the E. coli β-Barrel Assembly Machinery (BAM) multi-protein complex. We consider the functional mechanism for the OMP component of the BAM complex, BamA, and determine that this enzyme works via a catalytic cycle to facilitate the folding of OMPs with an activity similar to the entire BAM complex. Lastly, we interrogate this pathway in a holistic manner by constructing a computational model to simulate OMP flux through this pathway. Our modeling suggests that together the concentrations of periplasmic chaperones and the kinetic and thermodynamic parameters for chaperone-OMP binding are poised for this system to act as a reservoir for OMP flux towards the OM. Our studies highlight the importance of both chaperone-OMP and BAM-OMP interactions in the accurate and efficient process of OMP biogenesis