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An RND-type efflux system in Borrelia burgdorferi is involved in virulence and resistance to antimicrobial compounds.

Bunikis I, Denker K, Ostberg Y, Andersen C, Benz R, Bergström S - PLoS Pathog. (2008)

Bottom Line: The biophysical properties of BesC could be explained by a model based on the channel-tunnel structure.We have also generated a structural model of the efflux apparatus showing the putative spatial orientation of BesC with respect to the AcrAB homologs BesAB.We believe that our findings will be helpful in unraveling the pathogenic mechanisms of borreliae as well as in developing novel therapeutic agents aiming to block the function of this secretion apparatus.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology, Umeå University, Umeå, Sweden.

ABSTRACT
Borrelia burgdorferi is remarkable for its ability to thrive in widely different environments due to its ability to infect various organisms. In comparison to enteric Gram-negative bacteria, these spirochetes have only a few transmembrane proteins some of which are thought to play a role in solute and nutrient uptake and excretion of toxic substances. Here, we have identified an outer membrane protein, BesC, which is part of a putative export system comprising the components BesA, BesB and BesC. We show that BesC, a TolC homolog, forms channels in planar lipid bilayers and is involved in antibiotic resistance. A besC knockout was unable to establish infection in mice, signifying the importance of this outer membrane channel in the mammalian host. The biophysical properties of BesC could be explained by a model based on the channel-tunnel structure. We have also generated a structural model of the efflux apparatus showing the putative spatial orientation of BesC with respect to the AcrAB homologs BesAB. We believe that our findings will be helpful in unraveling the pathogenic mechanisms of borreliae as well as in developing novel therapeutic agents aiming to block the function of this secretion apparatus.

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Comparison of the periplasmic tunnel entrance of E. coli TolC with the modeled structure of Borrelia BesC and a model of the Borrelia efflux pump.The backbone of the peptide chains of the three monomers are colored differently. Side chains are omitted except those lining the tunnel entrance. These are D371 and D374 in TolC (A) and D363 and K366 in BesC (B) or those involved in forming the circular network in TolC - D153, Y362, and R367. (C) The channel-tunnel BesC is colored red, the adaptor protein BesA is shown in yellow and the RND transporter in green. Structures of all three components are modeled and formed according to existing models [28],[82],[83]. The model of the adaptor protein BesA shows just residues 38–219 of the mature chain. The remaining parts of the protein are not modeled because of the missing template. In this model the complex is comprised of three adaptor protein protomers. It should be mentioned that there are other models proposed suggesting six adaptor protein protomers per efflux apparatus [31],[59]. Note that the adaptor proteins do not have the alpha-helical domain, which is thought to interact with the helices of the tunnel region of the outer membrane component in other bacterial efflux pumps.
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ppat-1000009-g004: Comparison of the periplasmic tunnel entrance of E. coli TolC with the modeled structure of Borrelia BesC and a model of the Borrelia efflux pump.The backbone of the peptide chains of the three monomers are colored differently. Side chains are omitted except those lining the tunnel entrance. These are D371 and D374 in TolC (A) and D363 and K366 in BesC (B) or those involved in forming the circular network in TolC - D153, Y362, and R367. (C) The channel-tunnel BesC is colored red, the adaptor protein BesA is shown in yellow and the RND transporter in green. Structures of all three components are modeled and formed according to existing models [28],[82],[83]. The model of the adaptor protein BesA shows just residues 38–219 of the mature chain. The remaining parts of the protein are not modeled because of the missing template. In this model the complex is comprised of three adaptor protein protomers. It should be mentioned that there are other models proposed suggesting six adaptor protein protomers per efflux apparatus [31],[59]. Note that the adaptor proteins do not have the alpha-helical domain, which is thought to interact with the helices of the tunnel region of the outer membrane component in other bacterial efflux pumps.

Mentions: To explain our biophysical data, a special focus was put on residues lining the periplasmic entrance, which are known to have a major influence on the electrophysiological properties of channel-tunnels [30]. In comparison to TolC of E. coli, which forms pores of 80 pS in 1 M KCl with a high preference for cations due to six aspartate residues (Asp371 and Asp374) lining the tunnel entrance (Figure 4A), the pores formed by BesC are almost non-selective. Looking at the modeled BesC tunnel entrance, it becomes apparent that the opening is slightly wider than that of TolC, explaining the higher single channel conductance. Furthermore, the charges lining the channel entrance are balanced. There are two oppositely charged residues per monomer, Asp363 and Lys366, which could explain the low ion selectivity of BesC (Figure 4B).


An RND-type efflux system in Borrelia burgdorferi is involved in virulence and resistance to antimicrobial compounds.

Bunikis I, Denker K, Ostberg Y, Andersen C, Benz R, Bergström S - PLoS Pathog. (2008)

Comparison of the periplasmic tunnel entrance of E. coli TolC with the modeled structure of Borrelia BesC and a model of the Borrelia efflux pump.The backbone of the peptide chains of the three monomers are colored differently. Side chains are omitted except those lining the tunnel entrance. These are D371 and D374 in TolC (A) and D363 and K366 in BesC (B) or those involved in forming the circular network in TolC - D153, Y362, and R367. (C) The channel-tunnel BesC is colored red, the adaptor protein BesA is shown in yellow and the RND transporter in green. Structures of all three components are modeled and formed according to existing models [28],[82],[83]. The model of the adaptor protein BesA shows just residues 38–219 of the mature chain. The remaining parts of the protein are not modeled because of the missing template. In this model the complex is comprised of three adaptor protein protomers. It should be mentioned that there are other models proposed suggesting six adaptor protein protomers per efflux apparatus [31],[59]. Note that the adaptor proteins do not have the alpha-helical domain, which is thought to interact with the helices of the tunnel region of the outer membrane component in other bacterial efflux pumps.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2279261&req=5

ppat-1000009-g004: Comparison of the periplasmic tunnel entrance of E. coli TolC with the modeled structure of Borrelia BesC and a model of the Borrelia efflux pump.The backbone of the peptide chains of the three monomers are colored differently. Side chains are omitted except those lining the tunnel entrance. These are D371 and D374 in TolC (A) and D363 and K366 in BesC (B) or those involved in forming the circular network in TolC - D153, Y362, and R367. (C) The channel-tunnel BesC is colored red, the adaptor protein BesA is shown in yellow and the RND transporter in green. Structures of all three components are modeled and formed according to existing models [28],[82],[83]. The model of the adaptor protein BesA shows just residues 38–219 of the mature chain. The remaining parts of the protein are not modeled because of the missing template. In this model the complex is comprised of three adaptor protein protomers. It should be mentioned that there are other models proposed suggesting six adaptor protein protomers per efflux apparatus [31],[59]. Note that the adaptor proteins do not have the alpha-helical domain, which is thought to interact with the helices of the tunnel region of the outer membrane component in other bacterial efflux pumps.
Mentions: To explain our biophysical data, a special focus was put on residues lining the periplasmic entrance, which are known to have a major influence on the electrophysiological properties of channel-tunnels [30]. In comparison to TolC of E. coli, which forms pores of 80 pS in 1 M KCl with a high preference for cations due to six aspartate residues (Asp371 and Asp374) lining the tunnel entrance (Figure 4A), the pores formed by BesC are almost non-selective. Looking at the modeled BesC tunnel entrance, it becomes apparent that the opening is slightly wider than that of TolC, explaining the higher single channel conductance. Furthermore, the charges lining the channel entrance are balanced. There are two oppositely charged residues per monomer, Asp363 and Lys366, which could explain the low ion selectivity of BesC (Figure 4B).

Bottom Line: The biophysical properties of BesC could be explained by a model based on the channel-tunnel structure.We have also generated a structural model of the efflux apparatus showing the putative spatial orientation of BesC with respect to the AcrAB homologs BesAB.We believe that our findings will be helpful in unraveling the pathogenic mechanisms of borreliae as well as in developing novel therapeutic agents aiming to block the function of this secretion apparatus.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Biology, Umeå University, Umeå, Sweden.

ABSTRACT
Borrelia burgdorferi is remarkable for its ability to thrive in widely different environments due to its ability to infect various organisms. In comparison to enteric Gram-negative bacteria, these spirochetes have only a few transmembrane proteins some of which are thought to play a role in solute and nutrient uptake and excretion of toxic substances. Here, we have identified an outer membrane protein, BesC, which is part of a putative export system comprising the components BesA, BesB and BesC. We show that BesC, a TolC homolog, forms channels in planar lipid bilayers and is involved in antibiotic resistance. A besC knockout was unable to establish infection in mice, signifying the importance of this outer membrane channel in the mammalian host. The biophysical properties of BesC could be explained by a model based on the channel-tunnel structure. We have also generated a structural model of the efflux apparatus showing the putative spatial orientation of BesC with respect to the AcrAB homologs BesAB. We believe that our findings will be helpful in unraveling the pathogenic mechanisms of borreliae as well as in developing novel therapeutic agents aiming to block the function of this secretion apparatus.

Show MeSH
Related in: MedlinePlus