Outer membrane biogenesis in Escherichia coli

Abstract

The outer membrane (OM) of Gram-negative bacteria, which serves as a selective permeability barrier, contains lipoproteins and integral β-barrel proteins (OMPs) assembled into an asymmetric lipid bilayer. In E. coli, the Bam complex, which is required for insertion of β-barrel proteins into the OM, is composed of the OMP BamA and four associated lipoproteins (BamBCDE). In addition to its β-barrel domain, BamA contains an N-terminal periplasmic extension that interacts independently and simultaneously with BamB and the BamCDE subcomplex.

The mechanism by which the Bam complex catalyzes the assembly of OMPs is not known. Furthermore, the interaction interfaces between BamA and its accessory lipoproteins have not been precisely defined. Due to the essentiality of BamA, loss-of-function analysis has been limited to a small library of mutants and depletion experiments; therefore, thorough genetic analysis of the organization and function of the Bam complex requires the isolation of additional mutants. This thesis primarily describes a series of Bam mutants that, through extensive characterization and suppressor analysis, have expanded our understanding of the interactions between and functions of the various components of the Bam complex.

We present results suggesting that activation of the Bam machine is dependent on an allosteric interaction between BamA and BamD. We provide the first direct evidence implicating the PPIase domains of the periplasmic OMP chaperone SurA in β-barrel folding and targeting, and we show the existence of two separable SurA-dependent OMP assembly pathways. We demonstrate the functional importance of a conserved extracellular loop of the BamA β-barrel and establish a reciprocal genetic relationship between the periplasmic and β-barrel domains of BamA. Finally, we explore the role of the C-terminal peptide in OMP targeting and identify a periplasmic protease involved in periplasmic quality control. Based on results presented here, we propose a model for the function and evolution of the Bam complex in Gram-negative bacteria.