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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

Examples of the substrates and non-substrates of AcrAB-TolC system in E. coli. Black and red frames indicate the substrates and non-substrates of this exporter.
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Figure 2: Examples of the substrates and non-substrates of AcrAB-TolC system in E. coli. Black and red frames indicate the substrates and non-substrates of this exporter.

Mentions: Most notably, these proteins demonstrate an extraordinarily wide substrate specificity (Elkins and Nikaido, 2002). The compounds exported through a typical RND-type exporter includes antibiotics, detergents, antiseptics and toxic dyes as well as anionic, cationic, zwitter ionic, and neutral compounds (Figure 2). These compounds also include both aromatic and aliphatic compounds. Moreover, there is no common chemical characteristic of these molecules, with the exception of amphiphilic, a characteristics of drugs and cellular toxins that assists them in moving through fluid to the target and in invading cells through the lipid bilayer of the cell membrane. However, multidrug exporters are not non-specific transporters. These proteins do not export nutrients or non-toxic metabolites such as glucose or amino acids. Additionally, these proteins transport a defined spectrum of drugs: e.g., AcrB in E. coli and MexB in P. aeruginosa do not export aminoglycoside antibiotics such as kanamycin and streptomycin, whereas AcrD in E. coli and MexY in P. aeruginosa do (Masuda et al., 2000; Elkins and Nikaido, 2002). Various inhibitors are also specific for certain multidrug exporters: e.g., pyridopyrimidine derivatives are potent inhibitors of AcrB and MexB but not MexY (Yoshida et al., 2007). Therefore, multidrug exporters recognize their substrates and are inhibited through specific mechanisms. At the start of the twenty-first century, crystal structure determinations of multidrug exporters first in RND-type (Murakami et al., 2002) followed by ABC-type (Dawson and Locher, 2006) revealed mechanisms of multidrug recognition and the active export of multidrug efflux transporters. In this review, we summarize the structural basis of multidrug recognition and active export, primarily focusing on AcrB and MexB, the most-studied RND-type multidrug exporters. We also discuss future avenues of research into the structural and mechanical aspects of multidrug exporters.


Structural basis of RND-type multidrug exporters.

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

Examples of the substrates and non-substrates of AcrAB-TolC system in E. coli. Black and red frames indicate the substrates and non-substrates of this exporter.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Examples of the substrates and non-substrates of AcrAB-TolC system in E. coli. Black and red frames indicate the substrates and non-substrates of this exporter.
Mentions: Most notably, these proteins demonstrate an extraordinarily wide substrate specificity (Elkins and Nikaido, 2002). The compounds exported through a typical RND-type exporter includes antibiotics, detergents, antiseptics and toxic dyes as well as anionic, cationic, zwitter ionic, and neutral compounds (Figure 2). These compounds also include both aromatic and aliphatic compounds. Moreover, there is no common chemical characteristic of these molecules, with the exception of amphiphilic, a characteristics of drugs and cellular toxins that assists them in moving through fluid to the target and in invading cells through the lipid bilayer of the cell membrane. However, multidrug exporters are not non-specific transporters. These proteins do not export nutrients or non-toxic metabolites such as glucose or amino acids. Additionally, these proteins transport a defined spectrum of drugs: e.g., AcrB in E. coli and MexB in P. aeruginosa do not export aminoglycoside antibiotics such as kanamycin and streptomycin, whereas AcrD in E. coli and MexY in P. aeruginosa do (Masuda et al., 2000; Elkins and Nikaido, 2002). Various inhibitors are also specific for certain multidrug exporters: e.g., pyridopyrimidine derivatives are potent inhibitors of AcrB and MexB but not MexY (Yoshida et al., 2007). Therefore, multidrug exporters recognize their substrates and are inhibited through specific mechanisms. At the start of the twenty-first century, crystal structure determinations of multidrug exporters first in RND-type (Murakami et al., 2002) followed by ABC-type (Dawson and Locher, 2006) revealed mechanisms of multidrug recognition and the active export of multidrug efflux transporters. In this review, we summarize the structural basis of multidrug recognition and active export, primarily focusing on AcrB and MexB, the most-studied RND-type multidrug exporters. We also discuss future avenues of research into the structural and mechanical aspects of multidrug exporters.

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