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EcoSal Plus

Domain 3:

Metabolism

Solute and Ion Transport: Outer Membrane Pores and Receptors

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  • Authors: Satoshi Yamashita1, and Susan K. Buchanan2
  • Editor: Valley Stewart3
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Disease, National Institutes of Health, Bethesda, MD 20892; 2: Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Disease, National Institutes of Health, Bethesda, MD 20892; 3: University of California, Davis, Davis, CA
  • Received 04 June 2009 Accepted 10 August 2009 Published 31 March 2010
  • Address correspondence to Susan K. Buchanan skbuchan@helix.nih.gov
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  • Abstract:

    Two membranes enclose Gram-negative bacteria-an inner membrane consisting of phospholipid and an outer membrane having an asymmetric structure in which the inner leaflet contains phospholipid and the outer leaflet consists primarily of lipopolysaccharide. The impermeable nature of the outer membrane imposes a need for numerous outer membrane pores and transporters to ferry substances in and out of the cell. These outer membrane proteins have structures distinct from their inner membrane counterparts and most often function without any discernable energy source. In this chapter, we review the structures and functions of four classes of outer membrane protein: general and specific porins, specific transporters, TonB-dependent transporters, and export channels. While not an exhaustive list, these classes exemplify small-molecule transport across the outer membrane and illustrate the diversity of structures and functions found in Gram-negative bacteria.

  • Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1

Key Concept Ranking

Outer Membrane Proteins
0.4563372
Integral Membrane Proteins
0.440876
ABC Transporters
0.3382937
0.4563372

References

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2010-03-31
2017-11-22

Abstract:

Two membranes enclose Gram-negative bacteria-an inner membrane consisting of phospholipid and an outer membrane having an asymmetric structure in which the inner leaflet contains phospholipid and the outer leaflet consists primarily of lipopolysaccharide. The impermeable nature of the outer membrane imposes a need for numerous outer membrane pores and transporters to ferry substances in and out of the cell. These outer membrane proteins have structures distinct from their inner membrane counterparts and most often function without any discernable energy source. In this chapter, we review the structures and functions of four classes of outer membrane protein: general and specific porins, specific transporters, TonB-dependent transporters, and export channels. While not an exhaustive list, these classes exemplify small-molecule transport across the outer membrane and illustrate the diversity of structures and functions found in Gram-negative bacteria.

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Figures

Image of Figure 1
Figure 1

(a) Monomer structure as viewed from the side. (b) Trimer structure as viewed from extracellular surface. Extracellular loop 3 (red) folds into the barrel to constrict the pore.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 2
Figure 2

(a) Side view of the structure in closed conformation. (b) The structures in closed (left) and open (right) conformations as viewed from the extracellular surface.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 3
Figure 3

(a) The structure with a bound lauryldimethylamine--oxide (LDAO) molecule (space filling representation) as viewed from the side. The position of gap in the middle of barrel wall is in red. The putative position of the OM is indicated by horizontal lines. (b) Surface representation as viewed from the same direction and in the same colors as (a). The gap is connected to the binding pocket of the bound LDAO molecule and is thought to form a tunnel for passage of the putative substrate.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 4
Figure 4

(a) Side view of the structure bound with two thymidine molecules (space filling representation). (b) Surface representation viewed from the extracellular surface without bound substrates.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 5
Figure 5

The bound detergent molecules are indicated in space filling representation. The positions of detergent molecules indicate a putative movement of the substrate from the extracellular side to the middle of barrel. The periplasmic side of the barrel is completely closed by the N-terminal hatch domain (blue). CE, tetraethylene glycol monooctyl ether; LDAO, lauryldimethylamine--oxide.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 6
Figure 6

The transporter (green) is shown to interact with a TonB complex (yellow) at the TonB box motif (red). Transport of a ferric siderophore across the IM requires a PBP and an ABC transporter (blue). Once the ferric siderophore enters the cytoplasm, the ferric ion (Fe) is reduced to the ferrous ion (Fe).

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 7
Figure 7

The C-terminal domain of TonB (green) is attached to the N-terminal end of BtuB (red) at the periplasmic side. The BtuB plug domain is in cyan. Bound cobalamine is shown in space filling representation in orange.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 8
Figure 8

Left, the complex between colicin E2 R-domain and its receptor, BtuB. Right, the complex between colicin Ia R-domain and its receptor, Cir. The receptors are green, and the colicin molecules are purple.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 9
Figure 9

TolC (red) and AcrB (orange) form a complex to export substrates from the cytoplasm to the extracellular space. AcrA molecules (green) are predicted to bind the side walls of AcrB and TolC in the periplasm, and are thought to behave as molecular clamps. The export of substrate (pink) is driven by a proton/substrate antiport mechanism.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 10
Figure 10

The β-barrel channel (OM) and the α-helical tunnel (periplasmic) are formed by three protomers that are colored by monomer.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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Image of Figure 11
Figure 11

(a) The octameric structure of Wza as viewed from the side. Each monomer is a different color. The α-barrel spans the OM, and three large domains extend to the periplasm. (b) View of the continuous pore of Wza octamer from the extracellular surface.

Citation: Yamashita S, Buchanan S. 2010. Solute and Ion Transport: Outer Membrane Pores and Receptors, EcoSal Plus 2010; doi:10.1128/ecosalplus.3.3.1
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