The Assembly of Beta-Barrel Proteins into Bacterial Outer Membranes

Abstract

In order to cause disease, bacterial pathogens must (i) avoid immune detection, (ii) inhibit immune responses, and (iii) induce pathology favoring bacterial replication. In gram-negative bacteria, pathogenesis depend on structures assembled in the bacterial outer membrane. We have been working to understand the mechanism by which beta-barrel and other integral membrane proteins are assembled into bacterial outer membranes.The BAM complex is the core machinery for the assembly of membrane proteins, and inhibition of BAM complex activity is lethal to bacteria. In Escherichia coli, the BAM coplex is formed from four lipoproteins (BamB, BamC, BamD and BamE) attached to the integral membrane protein BamA to modulate its function, with the BamCDE module spanning the outer membrane. An additional, enigmatic module of the beta-barrel membrane protein assembly is the translocation and assembly module (the TAM). Composed of an outer membrane protein (TamA) and an integral inner membrane protein (TamB), the TAM is not essential for cell viability under laboratory conditions. However, the tamA and tamB genes are often hits in genetic screens of virulence, and have been well-conserved through evolution suggesting selection for an important function. Recent work demonstrating the reconstitution of active forms of the BAM complex and the TAM, and new assays for the assembly of a diverse array of outer membrane protein substrates will be discussed. Comparative genome analysis will be presented that demonstrates evolutionary tinkering in the interplay between the various modules (BamAB, BamCDE and TamAB) of the beta-barrel membrane protein assembly machinery. Finally, new data from neutron imaging and super-resolution microscopy will be presented which details the molecular organization of assembly precincts in the outer membrane: specialized clusters of BAM complexes that suggest current models for the mechanism of beta-barrel assembly need revision.