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

• Published in: The Journal of biological chemistry Vol. 293; no. 8; pp. 2959 - 2973
• Main Authors: Hussain, Sunyia, Bernstein, Harris D.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 23.02.2018; American Society for Biochemistry and Molecular Biology
• Subjects: Bacterial Outer Membrane Proteins - chemistry; Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - metabolism; Bam complex; beta-barrel proteins; Escherichia coli - chemistry; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Escherichia coli Proteins - metabolism; Gram-negative bacteria; Lipid Bilayers - chemistry; Lipid Bilayers - metabolism; Lipid-Linked Proteins - chemistry; Lipid-Linked Proteins - genetics; Lipid-Linked Proteins - metabolism; Liposomes; Membrane Biology; membrane protein; molecular chaperone; outer membrane; Peptide Fragments - chemistry; Peptide Fragments - genetics; Peptide Fragments - metabolism; Porins - chemistry; Porins - genetics; Porins - metabolism; Protein Conformation, alpha-Helical; Protein Conformation, beta-Strand; Protein Folding; Protein Interaction Domains and Motifs; Protein Multimerization; Protein Transport; protein-lipid interaction; Recombinant Fusion Proteins - chemistry; Recombinant Fusion Proteins - metabolism; Serine Endopeptidases - chemistry; Serine Endopeptidases - genetics; Serine Endopeptidases - metabolism; outer membrane; molecular chaperone; beta-barrel proteins; Bam complex; protein-lipid interaction; membrane protein; protein folding; Gram-negative bacteria
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.RA117.000349

Most proteins that reside in the bacterial outer membrane (OM) have a distinctive “β-barrel” architecture, but the assembly of these proteins is poorly understood. The spontaneous assembly of OM proteins (OMPs) into pure lipid vesicles has been studied extensively but often requires non-physiological conditions and time scales and is strongly influenced by properties of the lipid bilayer, including surface charge, thickness, and fluidity. Furthermore, the membrane insertion of OMPs in vivo is catalyzed by a heterooligomer called the β-barrel assembly machinery (Bam) complex. To determine the role of lipids in the assembly of OMPs under more physiological conditions, we exploited an assay in which the Bam complex mediates their insertion into membrane vesicles. After reconstituting the Bam complex into vesicles that contain a variety of different synthetic lipids, we found that two model OMPs, EspP and OmpA, folded efficiently regardless of the lipid composition. Most notably, both proteins folded into membranes composed of a gel-phase lipid that mimics the rigid bacterial OM. Interestingly, we found that EspP, OmpA, and another model protein (OmpG) folded at significantly different rates and that an α-helix embedded inside the EspP β-barrel accelerates folding. Our results show that the Bam complex largely overcomes effects that lipids exert on OMP assembly and suggest that specific interactions between the Bam complex and an OMP influence its rate of folding.