Outer membrane β-barrel protein folding is physically controlled by periplasmic lipid head groups and BamA

Abstract

Outer membrane β-barrel proteins (OMPs) are crucial for numerous cellular processes in prokaryotes and eukaryotes. Despite extensive studies on OMP biogenesis, it is unclear why OMPs require assembly machineries to fold into their native outer membranes, as they are capable of folding quickly and efficiently through an intrinsic folding pathway in vitro. By investigating the folding of several bacterial OMPs using membranes with naturally occurring Escherichia coli lipids, we show that phosphoethanolamine and phosphoglycerol head groups impose a kinetic barrier to OMP folding. The kinetic retardation of OMP folding places a strong negative pressure against spontaneous incorporation of OMPs into inner bacterial membranes, which would dissipate the proton motive force and undoubtedly kill bacteria. We further show that prefolded β-barrel assembly machinery subunit A (BamA), the evolutionarily conserved, central subunit of the BAM complex, accelerates OMP folding by lowering the kinetic barrier imposed by phosphoethanolamine head groups. Our results suggest that OMP assembly machineries are required in vivo to enable physical control over the spontaneously occurring OMP folding reaction in the periplasm. Mechanistic studies further allowed us to derive a model for BamA function, which explains how OMP assembly can be conserved between prokaryotes and eukaryotes.membrane protein folding | beta-barrel transmembrane protein

The Bam complex catalyzes efficient insertion of bacterial outer membrane proteins into membrane vesicles of variable lipid composition

“…Alternatively, by distorting the lipid bilayer, the Bam complex might promote the integration of OMPs based on their intrinsic thermodynamic properties. Consistent with the possibility that the Bam complex directly overcomes inhibitory effects exerted by lipids, BamA alone accelerates the folding of at least some OMPs into liposomes containing small amounts of native head groups (34,36) as well as relatively thick bilayers that often inhibit spontaneous assembly (19). More significantly, in the presence of SurA, the Bam complex has been shown to catalyze the assembly of OMPs into proteoliposomes that contain native E. coli lipids near neutral pH and within roughly physiological timescales (37)(38)(39)(40).…”

Multiple Driving Forces Required for Efficient Secretion of Autotransporter Virulence Proteins

“…Consistent with this model, it has been shown that the passenger C terminus crosses the OM first (12). However, several periplasmic chaperones, including the BAM complex (13)(14)(15), have been shown to interact with the passenger before and during OM translocation (16 -19). Additionally, although there are limits to the size and bulkiness of prefolded passenger structural elements that can cross the OM without blocking translocation (20,21), some of these structures are surprisingly large when compared with the 1-2-nm diameter of the translocator pore (22,23).…”

A combined kinetic push and thermodynamic pull as driving forces for outer membrane protein sorting and folding in bacteria

“…No configuration of natural or mimicked E. coli lipids support fast folding in vitro: neither of the forms of E. coli lipids available for purchase from Avanti support high efficiency folding; nor do the lipids purified from E. coli; nor do bilayers prepared with synthetic lipids mixed at biological mole ratios; and the addition of lipopolysaccharide has no dramatic effect on folding [14,15]. Upon discovery, this was surprising and counterintuitive.…”