Confined Mobility of TonB and FepA in Escherichia coli Membranes

Role of the lipid bilayer in outer membrane protein folding in Gram-negative bacteria

β-Barrel outer membrane proteins (OMPs) represent the major proteinaceous component of the outer membrane (OM) of Gram-negative bacteria. These proteins perform key roles in cell structure and morphology, nutrient acquisition, colonization and invasion, and protection against external toxic threats such as antibiotics. To become functional, OMPs must fold and insert into a crowded and asymmetric OM that lacks much freely accessible lipid. This feat is accomplished in the absence of an external energy source and is thought to be driven by the high thermodynamic stability of folded OMPs in the OM. With such a stable fold, the challenge that bacteria face in assembling OMPs into the OM is how to overcome the initial energy barrier of membrane insertion. In this review, we highlight the roles of the lipid environment and the OM in modulating the OMP-folding landscape and discuss the factors that guide folding in vitro and in vivo We particularly focus on the composition, architecture, and physical properties of the OM and how an understanding of the folding properties of OMPs in vitro can help explain the challenges they encounter during folding in vivo Current models of OMP biogenesis in the cellular environment are still in flux, but the stakes for improving the accuracy of these models are high. OMP folding is an essential process in all Gram-negative bacteria, and considering the looming crisis of widespread microbial drug resistance it is an attractive target. To bring down this vital OMP-supported barrier to antibiotics, we must first understand how bacterial cells build it.

Establishment of a Protein Concentration Gradient in the Outer Membrane Requires Two Diffusion-Limiting Mechanisms

OmpA-like proteins are involved in the stabilization of the outer membrane, resistance to osmotic stress, and pathogenesis. In Caulobacter crescentus, OmpA2 forms a physiologically relevant concentration gradient that forms by an uncharacterized mechanism, in which the gradient orientation depends on the position of the gene locus. This suggests that OmpA2 is synthesized and translocated to the periplasm close to the position of the gene and that the gradient forms by diffusion of the protein from this point. To further understand how the OmpA2 gradient is established, we determined the localization and mobility of the full protein and of its two structural domains. We show that OmpA2 does not diffuse and that both domains are required for gradient formation. The C-terminal domain binds tightly to the cell wall and the immobility of the full protein depends on the binding of this domain to the peptidoglycan; in contrast, the N-terminal membrane β-barrel diffuses slowly. Our results support a model in which once OmpA2 is translocated to the periplasm, the N-terminal membrane β-barrel is required for an initial fast restriction of diffusion until the position of the protein is stabilized by the binding of the C-terminal domain to the cell wall. The implications of these results on outer membrane protein diffusion and organization are discussed.IMPORTANCE Protein concentration gradients play a relevant role in the organization of the bacterial cell. The Caulobacter crescentus protein OmpA2 forms an outer membrane polar concentration gradient. To understand the molecular mechanism that determines the formation of this gradient, we characterized the mobility and localization of the full protein and of its two structural domains an integral outer membrane β-barrel and a periplasmic peptidoglycan binding domain. Each domain has a different role in the formation of the OmpA2 gradient, which occurs in two steps. We also show that the OmpA2 outer membrane β-barrel can diffuse, which is in contrast to what has been reported previously for several integral outer membrane proteins in Escherichia coli, suggesting a different organization of the outer membrane proteins.

The Starch Utilization System Assembles around Stationary Starch-Binding Proteins

Bacteroides thetaiotaomicron (Bt) is a prominent member of the human gut microbiota with an extensive capacity for glycan harvest. This bacterium expresses a five-protein complex in the outer membrane, called the starch utilization system (Sus), which binds, degrades, and imports starch into the cell. Sus is a model system for the many glycan-targeting polysaccharide utilization loci found in Bt and other members of the Bacteroidetes phylum. Our previous work has shown that SusG, a lipidated amylase in the outer membrane, explores the entire cell surface but diffuses more slowly as it interacts with starch. Here, we use a combination of single-molecule tracking, super-resolution imaging, reverse genetics, and proteomics to show that SusE and SusF, two proteins that bind starch, are immobile on the cell surface even when other members of the system are knocked out and under multiple different growth conditions. This observation suggests a new paradigm for protein complex formation: binding proteins form immobile complexes that transiently associate with a mobile enzyme partner.