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Combining Mutations That Inhibit Two Distinct Steps of the ATP Hydrolysis Cycle Restores Wild-Type Function in the Lipopolysaccharide Transporter and Shows that ATP Binding Triggers Transport.

ABSTRACT: ATP-binding cassette (ABC) transporters constitute a large family of proteins present in all domains of life. They are powered by dynamic ATPases that harness energy from binding and hydrolyzing ATP through a cycle that involves the closing and reopening of their two ATP-binding domains. The LptB2FGC exporter is an essential ABC transporter that assembles lipopolysaccharides (LPS) on the surface of Gram-negative bacteria to form a permeability barrier against many antibiotics. LptB2FGC extracts newly synthesized LPS molecules from the inner membrane and powers their transport across the periplasm and through the outer membrane. How LptB2FGC functions remains poorly understood. Here, we show that the C-terminal domain of the dimeric LptB ATPase is essential for LPS transport in Escherichia coli Specific changes in the C-terminal domain of LptB cause LPS transport defects that can be repaired by intragenic suppressors altering the ATP-binding domains. Surprisingly, we found that each of two lethal changes in the ATP-binding and C-terminal domains of LptB, when present in combined form, suppressed the defects associated with the other to restore LPS transport to wild-type levels both in vivo and in vitro We present biochemical evidence explaining the effect that each of these mutations has on LptB function and how the observed cosuppression results from the opposing lethal effects these changes have on the dimerization state of the LptB ATPase. We therefore propose that these sites modulate the closing and reopening of the LptB dimer, providing insight into how the LptB2FGC transporter cycles to export LPS to the cell surface and how to inhibit this essential envelope biogenesis process.IMPORTANCE Gram-negative bacteria are naturally resistant to many antibiotics because their surface is covered by the glycolipid LPS. Newly synthesized LPS is transported across the cell envelope by the multiprotein Lpt machinery, which includes LptB2FGC, an unusual ABC transporter that extracts LPS from the inner membrane. Like in other ABC transporters, the LptB2FGC transport cycle is driven by the cyclical conformational changes that a cytoplasmic, dimeric ATPase, LptB, undergoes when binding and hydrolyzing ATP. How these conformational changes are controlled in ABC transporters is poorly understood. Here, we identified two lethal changes in LptB that, when combined, remarkably restore wild-type transport function. Biochemical studies revealed that the two changes affect different steps in the transport cycle, having opposing, lethal effects on LptB's dimerization cycle. Our work provides mechanistic details about the LptB2FGC extractor that could be used to develop Lpt inhibitors that would overcome the innate antibiotic resistance of Gram-negative bacteria.

SUBMITTER: Simpson BW

PROVIDER: S-EPMC6703430 | biostudies-literature | 2019 Aug

REPOSITORIES: biostudies-literature

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Combining Mutations That Inhibit Two Distinct Steps of the ATP Hydrolysis Cycle Restores Wild-Type Function in the Lipopolysaccharide Transporter and Shows that ATP Binding Triggers Transport.

Simpson Brent W BW Pahil Karanbir S KS Owens Tristan W TW Lundstedt Emily A EA Davis Rebecca M RM Kahne Daniel D Ruiz Natividad N

mBio 20190820 4

ATP-binding cassette (ABC) transporters constitute a large family of proteins present in all domains of life. They are powered by dynamic ATPases that harness energy from binding and hydrolyzing ATP through a cycle that involves the closing and reopening of their two ATP-binding domains. The LptB<sub>2</sub>FGC exporter is an essential ABC transporter that assembles lipopolysaccharides (LPS) on the surface of Gram-negative bacteria to form a permeability barrier against many antibiotics. LptB<su ...[more]

PMID: 31431556

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Project description:BackgroundTherapeutic blood plasma concentrations of anti-malarial drugs are essential for successful treatment. Pharmacokinetics of pharmaceutical compounds are dependent of adsorption, distribution, metabolism, and excretion. ATP binding cassette (ABC) transport proteins are particularly involved in drug deposition, as they are located at membranes of many uptake and excretory organs and at protective barriers, where they export endogenous and xenobiotic compounds, including pharmaceuticals. In this study, a panel of well-established anti-malarial drugs which may affect drug plasma concentrations was tested for interactions with human ABC transport proteins.MethodsThe interaction of chloroquine, quinine, artemisinin, mefloquine, lumefantrine, atovaquone, dihydroartemisinin and proguanil, with transport activity of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), bile salt export pump (BSEP) and multidrug resistance-associated proteins (MRP) 1-4 were analysed. The effect of the anti-malarials on the ATP-dependent uptake of radio-labelled substrates was measured in membrane vesicles isolated from HEK293 cells overexpressing the ABC transport proteins.ResultsA strong and previously undescribed inhibition of BCRP-mediated transport by atovaquone with a 50% inhibitory concentration (IC50) of 0.23 μM (95% CI 0.17-0.29 μM) and inhibition of P-gp-mediated transport by quinine with an IC50 of 6.8 μM (95% CI 5.9-7.8 μM) was observed. Furthermore, chloroquine and mefloquine were found to significantly inhibit P-gp-mediated transport. BCRP transport activity was significantly inhibited by all anti-malarials tested, whereas BSEP-mediated transport was not inhibited by any of the compounds. Both MRP1- and MRP3-mediated transport were significantly inhibited by mefloquine.ConclusionsAtovaquone and quinine significantly inhibit BCRP- and P-gp- mediated transport at concentrations within the clinically relevant prophylactic and therapeutic range. Co-administration of these established anti-malarials with drugs that are BCRP or P-gp substrates may potentially lead to drug-drug interactions.

Project description:P-Glycoprotein, a member of the ATP-binding cassette (ABC) superfamily, is a multidrug transporter responsible for cellular efflux of hundreds of structurally unrelated compounds, including natural products, many clinically used drugs and anti-cancer agents. Expression of P-glycoprotein has been linked to multidrug resistance in human cancers. ABC transporters are driven by ATP hydrolysis at their two cytoplasmic nucleotide-binding domains, which interact to form a closed ATP-bound sandwich dimer. Intimate knowledge of the catalytic cycle of these proteins is clearly essential for understanding their mechanism of action. P-Glycoprotein has been proposed to hydrolyse ATP by an alternating mechanism, for which there is substantial experimental evidence, including inhibition of catalytic activity by trapping of ortho-vanadate at one nucleotide-binding domain, and the observation of an asymmetric occluded state. Despite many studies of P-glycoprotein ATPase activity over the past 20 years, no comprehensive kinetic analysis has yet been carried out, and some puzzling features of its behaviour remain unexplained. In this work, we have built several progressively more complex kinetic models, and then carried out simulations and detailed analysis, to test the validity of the proposed reaction pathway employed by P-glycoprotein for ATP hydrolysis. To establish kinetic parameters for the catalytic cycle, we made use of the large amount of published data on ATP hydrolysis by hamster P-glycoprotein, both purified and in membrane vesicles. The proposed kinetic scheme(s) include a high affinity priming reaction for binding of the first ATP molecule, and an independent pathway for ADP binding outside the main catalytic cycle. They can reproduce to varying degrees the observed behavior of the protein's ATPase activity and its inhibition by ortho-vanadate. The results provide new insights into the mode of action of P-glycoprotein, and some hypotheses about the nature of the occluded nucleotide-bound state.

Dataset Information

Combining Mutations That Inhibit Two Distinct Steps of the ATP Hydrolysis Cycle Restores Wild-Type Function in the Lipopolysaccharide Transporter and Shows that ATP Binding Triggers Transport.

Publications

Combining Mutations That Inhibit Two Distinct Steps of the ATP Hydrolysis Cycle Restores Wild-Type Function in the Lipopolysaccharide Transporter and Shows that ATP Binding Triggers Transport.

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