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Structural basis of RND-type multidrug exporters.

Yamaguchi A, Nakashima R, Sakurai K - Front Microbiol (2015)

Bottom Line: Multidrug recognition is based on a multisite drug-binding mechanism, in which two voluminous multidrug-binding pockets in cell membrane exporters recognize a wide range of substrates as a result of permutations at numerous binding sites that are specific for the partial structures of substrate molecules.Substrates are transported through dual multidrug-binding pockets via the peristaltic motion of the substrate translocation channel.Although there are no clinically available inhibitors of bacterial multidrug exporters, efforts to develop inhibitors based on structural information are underway.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Cell Membrane Structural Biology, Institute of Scientific and Industrial Research, Osaka University Ibaraki, Japan.

ABSTRACT
Bacterial multidrug exporters are intrinsic membrane transporters that act as cellular self-defense mechanism. The most notable characteristics of multidrug exporters is that they export a wide range of drugs and toxic compounds. The overexpression of these exporters causes multidrug resistance. Multidrug-resistant pathogens have become a serious problem in modern chemotherapy. Over the past decade, investigations into the structure of bacterial multidrug exporters have revealed the multidrug recognition and export mechanisms. In this review, we primarily discuss RND-type multidrug exporters particularly AcrAB-TolC, major drug exporter in Gram-negative bacteria. RND-type drug exporters are tripartite complexes comprising a cell membrane transporter, an outer membrane channel and an adaptor protein. Cell membrane transporters and outer membrane channels are homo-trimers; however, there is no consensus on the number of adaptor proteins in these tripartite complexes. The three monomers of a cell membrane transporter have varying conformations (access, binding, and extrusion) during transport. Drugs are exported following an ordered conformational change in these three monomers, through a functional rotation mechanism coupled with the proton relay cycle in ion pairs, which is driven by proton translocation. Multidrug recognition is based on a multisite drug-binding mechanism, in which two voluminous multidrug-binding pockets in cell membrane exporters recognize a wide range of substrates as a result of permutations at numerous binding sites that are specific for the partial structures of substrate molecules. The voluminous multidrug-binding pocket may have numerous binding sites even for a single substrate, suggesting that substrates may move between binding sites during transport, an idea named as multisite-drug-oscillation hypothesis. This hypothesis is consistent with the apparently broad substrate specificity of cell membrane exporters and their highly efficient ejection of drugs from the cell. Substrates are transported through dual multidrug-binding pockets via the peristaltic motion of the substrate translocation channel. Although there are no clinically available inhibitors of bacterial multidrug exporters, efforts to develop inhibitors based on structural information are underway.

No MeSH data available.


Related in: MedlinePlus

Functional-rotation mechanism of drug export mediated through the AcrAB-TolC tripartite complex. The upper and lower panels show side and horizontal views, respectively. The green, blue and red colors indicate the access, binding and extrusion stages of AcrB, respectively. The yellow and pale blue colors indicate AcrA and TolC, respectively. The jagged circles indicate substrates.
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Figure 8: Functional-rotation mechanism of drug export mediated through the AcrAB-TolC tripartite complex. The upper and lower panels show side and horizontal views, respectively. The green, blue and red colors indicate the access, binding and extrusion stages of AcrB, respectively. The yellow and pale blue colors indicate AcrA and TolC, respectively. The jagged circles indicate substrates.

Mentions: The double drug-binding AcrB trimer structure (in which rifampicin binds in the proximal pocket of the access monomer and minocycline binds in the distal pocket of the binding monomer) was determined. No drugs were detected in the distal pocket of the access monomer or the proximal pocket of the binding monomer, indicating that the proximal pocket and distal pocket are only activated during the access stage and the binding stage, respectively (Nakashima et al., 2011). Now the story of drug export is as follows: drugs initially enter the proximal pocket at the access stage. At this stage, HMMDs are bound and recognized in the expanded proximal pocket, but LMMDs are hardly or only weakly bound. Subsequently, the drugs are transferred to the distal pocket through the swinging of a switch loop and the relative motion of the subdomains, PC2, PC1, and PN2, resulting in a reduction in the volume of the proximal pocket and the expansion of the distal pocket during the transition from the access stage to the binding stage. LMMDs are bound and recognized in the distal pocket: HMMDs are not tightly bound but are instead occluded in the distal pocket because the path under the switch loop is too narrow to permit the return of HMMDs to the proximal pocket. Ultimately, the drugs are squeezed out through the TolC channel via a funnel-like opening as a result of conformational changes at the extrusion stage. In other words, drugs are moved through the intramolecular channels by a peristaltic motion of the two tandem drug-binding pockets (Figure 8). Multi-pockets having different substrate-binding specificity contribute to expand substrate spectrum.


Structural basis of RND-type multidrug exporters.

Yamaguchi A, Nakashima R, Sakurai K - Front Microbiol (2015)

Functional-rotation mechanism of drug export mediated through the AcrAB-TolC tripartite complex. The upper and lower panels show side and horizontal views, respectively. The green, blue and red colors indicate the access, binding and extrusion stages of AcrB, respectively. The yellow and pale blue colors indicate AcrA and TolC, respectively. The jagged circles indicate substrates.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4403515&req=5

Figure 8: Functional-rotation mechanism of drug export mediated through the AcrAB-TolC tripartite complex. The upper and lower panels show side and horizontal views, respectively. The green, blue and red colors indicate the access, binding and extrusion stages of AcrB, respectively. The yellow and pale blue colors indicate AcrA and TolC, respectively. The jagged circles indicate substrates.
Mentions: The double drug-binding AcrB trimer structure (in which rifampicin binds in the proximal pocket of the access monomer and minocycline binds in the distal pocket of the binding monomer) was determined. No drugs were detected in the distal pocket of the access monomer or the proximal pocket of the binding monomer, indicating that the proximal pocket and distal pocket are only activated during the access stage and the binding stage, respectively (Nakashima et al., 2011). Now the story of drug export is as follows: drugs initially enter the proximal pocket at the access stage. At this stage, HMMDs are bound and recognized in the expanded proximal pocket, but LMMDs are hardly or only weakly bound. Subsequently, the drugs are transferred to the distal pocket through the swinging of a switch loop and the relative motion of the subdomains, PC2, PC1, and PN2, resulting in a reduction in the volume of the proximal pocket and the expansion of the distal pocket during the transition from the access stage to the binding stage. LMMDs are bound and recognized in the distal pocket: HMMDs are not tightly bound but are instead occluded in the distal pocket because the path under the switch loop is too narrow to permit the return of HMMDs to the proximal pocket. Ultimately, the drugs are squeezed out through the TolC channel via a funnel-like opening as a result of conformational changes at the extrusion stage. In other words, drugs are moved through the intramolecular channels by a peristaltic motion of the two tandem drug-binding pockets (Figure 8). Multi-pockets having different substrate-binding specificity contribute to expand substrate spectrum.

Bottom Line: Multidrug recognition is based on a multisite drug-binding mechanism, in which two voluminous multidrug-binding pockets in cell membrane exporters recognize a wide range of substrates as a result of permutations at numerous binding sites that are specific for the partial structures of substrate molecules.Substrates are transported through dual multidrug-binding pockets via the peristaltic motion of the substrate translocation channel.Although there are no clinically available inhibitors of bacterial multidrug exporters, efforts to develop inhibitors based on structural information are underway.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Cell Membrane Structural Biology, Institute of Scientific and Industrial Research, Osaka University Ibaraki, Japan.

ABSTRACT
Bacterial multidrug exporters are intrinsic membrane transporters that act as cellular self-defense mechanism. The most notable characteristics of multidrug exporters is that they export a wide range of drugs and toxic compounds. The overexpression of these exporters causes multidrug resistance. Multidrug-resistant pathogens have become a serious problem in modern chemotherapy. Over the past decade, investigations into the structure of bacterial multidrug exporters have revealed the multidrug recognition and export mechanisms. In this review, we primarily discuss RND-type multidrug exporters particularly AcrAB-TolC, major drug exporter in Gram-negative bacteria. RND-type drug exporters are tripartite complexes comprising a cell membrane transporter, an outer membrane channel and an adaptor protein. Cell membrane transporters and outer membrane channels are homo-trimers; however, there is no consensus on the number of adaptor proteins in these tripartite complexes. The three monomers of a cell membrane transporter have varying conformations (access, binding, and extrusion) during transport. Drugs are exported following an ordered conformational change in these three monomers, through a functional rotation mechanism coupled with the proton relay cycle in ion pairs, which is driven by proton translocation. Multidrug recognition is based on a multisite drug-binding mechanism, in which two voluminous multidrug-binding pockets in cell membrane exporters recognize a wide range of substrates as a result of permutations at numerous binding sites that are specific for the partial structures of substrate molecules. The voluminous multidrug-binding pocket may have numerous binding sites even for a single substrate, suggesting that substrates may move between binding sites during transport, an idea named as multisite-drug-oscillation hypothesis. This hypothesis is consistent with the apparently broad substrate specificity of cell membrane exporters and their highly efficient ejection of drugs from the cell. Substrates are transported through dual multidrug-binding pockets via the peristaltic motion of the substrate translocation channel. Although there are no clinically available inhibitors of bacterial multidrug exporters, efforts to develop inhibitors based on structural information are underway.

No MeSH data available.


Related in: MedlinePlus