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EcoSal Plus

Domain 4:

Synthesis and Processing of Macromolecules

Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

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  • Authors: Juliana C. Malinverni1, and Thomas J. Silhavy2
  • Editors: James M. Slauch3, Harris Bernstein4
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Department of Molecular Biology, Princeton University, Princeton NJ 08544; 2: Department of Molecular Biology, Princeton University, Princeton NJ 08544; 3: The Schoold of Molecular and Cellular Biology, University of Illinois at Urbana-Champaign, Urbana, IL; 4: National Institutes of Health, Bethesda, MD
  • Received 06 January 2010 Accepted 22 March 2010 Published 26 March 2011
  • Address correspondence to Thomas J. Silhavy tsilhavy@princeton.edu
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  • Abstract:

    The major class of integral proteins found in the outer membrane (OM) of and adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In , the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.

  • Citation: Malinverni J, Silhavy T. 2011. Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex, EcoSal Plus 2011; doi:10.1128/ecosalplus.4.3.8

Key Concept Ranking

Outer Membrane Proteins
0.45503902
BAM Complex Proteins
0.41547915
Periplasmic Chaperone DegP
0.40562898
0.45503902

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/content/journal/ecosalplus/10.1128/ecosalplus.4.3.8
2011-03-26
2017-03-24

Abstract:

The major class of integral proteins found in the outer membrane (OM) of and adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In , the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.

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Figures

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Figure 1

In , most OM lipoproteins and OMPs are synthesized and transported to the IM secretion machinery posttranslationally. SecB chaperone molecules bind to hydrophobic segments of OM proteins to prevent their aggregation in the cytoplasm. OM proteins are delivered to the SecA ATPase complexed with the IM translocation machinery, SecYEG, as well as other associated proteins of unknown function that are not depicted here (see reference  1 ). OM lipoproteins and OMPs are distinguished by the composition of their signal sequences and sorted for processing by separate pathways ( 2 , 3 ). OM lipoprotein biogenesis pathway (depicted to the left of the IM secretion machinery): the asterisk indicates a lipoprotein without an Asp amino acid at the +2 position, the main signal for sorting by the LolABCDE pathway to the OM in ( 4 ). OMP biogenesis pathway (depicted to the right of the IM secretion machinery): after signal sequence processing, the SurA chaperone is thought to complex with the bulk of OMPs in the periplasmic space and deliver them to the OM assembly site, the Bam complex. In the absence of SurA, the Skp/DegP pathway becomes an indispensable substitute. Details concerning the mechanisms behind Bam-mediated OMP assembly into the OM are unknown. See the text for references and further details.

Citation: Malinverni J, Silhavy T. 2011. Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex, EcoSal Plus 2011; doi:10.1128/ecosalplus.4.3.8
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Figure 2

The Bam complex is composed of the integral OMP, BamA, and four OM lipoproteins, BamB, -C, -D, and -E ( 41 , 42 ). BamA consists of an essential N-terminal periplasmic region with five POTRA domains and a C-terminal β-barrel domain ( 104 , 108 , 109 ). The arrangement of the periplasmic domain may be dynamic, with a putative hinge region between P2 and P3 that ranges from a comparatively straight (130°) ( 111 ) to a bent (100°) ( 108 ) conformation. The relative orientation of P5 (shaded in white) with respect to P1 through P4 is not represented in the existing crystal structures but is portrayed in a sharply kinked conformation in agreement with SAX data (see the text for details) ( 112 ). The C-terminal portion of BamA is predicted to be a β-barrel with several extracellular loops. Extracellular loop 6 (L6) is particularly interesting in that portions of L6 are highly conserved and are predicted to interact with other conserved features of the barrel ( 34 , 105 ). Biochemical and structural data from a BamA homolog, FhaC, imply that L6 may be a dynamic loop capable of extending into the barrel and reaching the periplasmic space ( 34 , 110 , 114 ).

Citation: Malinverni J, Silhavy T. 2011. Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex, EcoSal Plus 2011; doi:10.1128/ecosalplus.4.3.8
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Tables

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Table 1

Gram-negative phyla containing Bam complex and periplasmic chaperone proteins

Citation: Malinverni J, Silhavy T. 2011. Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex, EcoSal Plus 2011; doi:10.1128/ecosalplus.4.3.8

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