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Domain 4:

Synthesis and Processing of Macromolecules

Outer Membrane Protein Insertion by the β-barrel Assembly Machine

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  • Authors: Dante P. Ricci1, and Thomas J. Silhavy2
  • Editors: Maria Sandkvist3, Eric Cascales4, Peter J. Christie5
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Department of Early Research, Achaogen, Inc., South San Francisco, CA 94080; 2: Department of Molecular Biology, Princeton University, Princeton, NJ 08544; 3: Department of Microbiology and Immunology, University of Michigan, Ann Arbor, Michigan; 4: CNRS Aix-Marseille Université, Mediterranean Institute of Microbiology, Marseille, France; 5: Department of Microbiology and Molecular Genetics, McGovern Medical School, Houston, Texas
  • Received 25 September 2018 Accepted 11 January 2019 Published 13 March 2019
  • Address correspondence to Thomas J. Silhavy, [email protected]
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  • Abstract:

    Like all outer membrane (OM) constituents, integral OM β-barrel proteins in Gram-negative bacteria are synthesized in the cytoplasm and trafficked to the OM, where they are locally assembled into the growing OM by the ubiquitous β-barrel assembly machine (Bam). While the identities and structures of all essential and accessory Bam components have been determined, the basic mechanism of Bam-assisted OM protein integration remains elusive. Here we review mechanistic analyses of OM β-barrel protein folding and Bam dynamics and summarize recent insights that inform a general model for OM protein recognition and assembly by the Bam complex.

  • Citation: Ricci D, Silhavy T. 2019. Outer Membrane Protein Insertion by the β-barrel Assembly Machine, EcoSal Plus 2019; doi:10.1128/ecosalplus.ESP-0035-2018

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/content/journal/ecosalplus/10.1128/ecosalplus.ESP-0035-2018
2019-03-13
2019-03-23

Abstract:

Like all outer membrane (OM) constituents, integral OM β-barrel proteins in Gram-negative bacteria are synthesized in the cytoplasm and trafficked to the OM, where they are locally assembled into the growing OM by the ubiquitous β-barrel assembly machine (Bam). While the identities and structures of all essential and accessory Bam components have been determined, the basic mechanism of Bam-assisted OM protein integration remains elusive. Here we review mechanistic analyses of OM β-barrel protein folding and Bam dynamics and summarize recent insights that inform a general model for OM protein recognition and assembly by the Bam complex.

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Figures

Image of Figure 1
Figure 1

(a) The BamA β-barrel (pink) is asymmetric, with one face forming a narrow protein-lipid interface (approximated by the dashed line) that is thought to physically alter the local properties of the bilayer (green). (b) The activated BamA β-barrel undergoes a dramatic conformational rearrangement that disrupts the continuous β-barrel structure, separates the β-strands comprising the lateral gate (β1 and β16), and exposes an aqueous channel that spans the membrane. Additionally, a highly conserved extracellular loop (L6, blue) internally braces and globally stabilizes the β-barrel domain and compensates for the instability introduced by the conformational dynamics. This image was generated using PDB structures 4K3B (left) and 5EKQ (right).

Citation: Ricci D, Silhavy T. 2019. Outer Membrane Protein Insertion by the β-barrel Assembly Machine, EcoSal Plus 2019; doi:10.1128/ecosalplus.ESP-0035-2018
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Image of Figure 2
Figure 2

(a) A nascent eight-stranded OMP rapidly adsorbs to the surface (green) of the lipid bilayer, where hydrophobic lipid-facing side chains begin to penetrate into the membrane and β-strands assume a cloverleaf-like circular arrangement according to their relative position in the folded protein. (b) β-hairpins begin to form as the ends of the TM β-strands, oriented toward the center of the cloverleaf, plunge into the lipid phase. (c) Hydrogen bonds form between neighboring β-hairpins as they enter the membrane, stabilizing the native fold in concert with membrane insertion. The highlighted dashed line indicates the proposed path of the leading () ends of the β-strands.

Citation: Ricci D, Silhavy T. 2019. Outer Membrane Protein Insertion by the β-barrel Assembly Machine, EcoSal Plus 2019; doi:10.1128/ecosalplus.ESP-0035-2018
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Image of Figure 3
Figure 3

(a) Nascent OMPs (red), maintained in a folding-competent state by periplasmic chaperones (green), are transferred to the Bam complex (blue). (b) Client proteins associate with multiple epitopes on Bam, potentially stimulating formation of early β-structure and orienting circularly-arranged β-strands/hairpins toward the presumptive substrate exit pore. Recognition of conserved OMP motifs triggers a conformational change in BamA that exposes the barrel lumen and destabilizes the lateral gate, further perturbing the local membrane environment and generating an OM integration path for OMP substrates. (c) The Bam complex prevents aggregation, protects substrates from proteolysis, and lowers the kinetic barrier to OM integration to enable rapid OMP folding along the native pathway. (d) OMPs spontaneously fold into the locally destabilized membrane, with the exposed BamA lumen potentially accommodating the folding barrel and/or secreted extracellular domains of client proteins. (e) Release of substrate from the complex prompts restoration of the closed, inert state of the complex to enable an ensuing round of assembly.

Citation: Ricci D, Silhavy T. 2019. Outer Membrane Protein Insertion by the β-barrel Assembly Machine, EcoSal Plus 2019; doi:10.1128/ecosalplus.ESP-0035-2018
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