ATP-dependent substrate transport by the ABC transporter MsbA is proton-coupled.

ATP-binding cassette transporters mediate the transbilayer movement of a vast number of substrates in or out of cells in organisms ranging from bacteria to humans. Current alternating access models for ABC exporters including the multidrug and Lipid A transporter MsbA from Escherichia coli suggest a role for nucleotide as the fundamental source of free energy. These models involve cycling between conformations with inward- and outward-facing substrate-binding sites in response to engagement and hydrolysis of ATP at the nucleotide-binding domains. Here we report that MsbA also utilizes another major energy currency in the cell by coupling substrate transport to a transmembrane electrochemical proton gradient. The dependence of ATP-dependent transport on proton coupling, and the stimulation of MsbA-ATPase by the chemical proton gradient highlight the functional integration of both forms of metabolic energy. These findings introduce ion coupling as a new parameter in the mechanism of this homodimeric ABC transporter.Himansha Singh is supported by the Cambridge Commonwealth, European and International Trust. Saroj Velamakanni was a recipient of a Cambridge Nehru Scholarship. Shen L. Wei was funded by the Cambridge Overseas Trust. This research in the Van Veen group was supported by Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/I002383/1 and BB/C004663/1, Medical Research Council (MRC) grant G0401165 and by further support from the Human Frontier Science Program (HFSP) and the British Society for Antimicrobial Chemotherapy (BSAC).This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms1238

Engineered MATE multidrug transporters reveal two functionally distinct ion-coupling pathways in NorM from Vibrio cholerae.

Multidrug and toxic compound extrusion (MATE) transport proteins confer multidrug resistance on pathogenic microorganisms and affect pharmacokinetics in mammals. Our understanding of how MATE transporters work, has mostly relied on protein structures and MD simulations. However, the energetics of drug transport has not been studied in detail. Many MATE transporters utilise the electrochemical H+ or Na+ gradient to drive substrate efflux, but NorM-VC from Vibrio cholerae can utilise both forms of metabolic energy. To dissect the localisation and organisation of H+ and Na+ translocation pathways in NorM-VC we engineered chimaeric proteins in which the N-lobe of H+-coupled NorM-PS from Pseudomonas stutzeri is fused to the C-lobe of NorM-VC, and vice versa. Our findings in drug binding and transport experiments with chimaeric, mutant and wildtype transporters highlight the versatile nature of energy coupling in NorM-VC, which enables adaptation to fluctuating salinity levels in the natural habitat of V. cholerae

Complexities of a protonatable substrate in measurements of Hoechst 33342 transport by multidrug transporter LmrP

Funder: AstraZeneca PhD studentshipFunder: China Scholarship Council; doi: http://dx.doi.org/10.13039/501100004543Funder: Cambridge Trust; doi: http://dx.doi.org/10.13039/501100003343Abstract: Multidrug transporters can confer drug resistance on cells by extruding structurally unrelated compounds from the cellular interior. In transport assays, Hoechst 33342 (referred to as Hoechst) is a commonly used substrate, the fluorescence of which changes in the transport process. With three basic nitrogen atoms that can be protonated, Hoechst can exist as cationic and neutral species that have different fluorescence emissions and different abilities to diffuse across cell envelopes and interact with lipids and intracellular nucleic acids. Due to this complexity, the mechanism of Hoechst transport by multidrug transporters is poorly characterised. We investigated Hoechst transport by the bacterial major facilitator superfamily multidrug-proton antiporter LmrP in Lactococcus lactis and developed a novel assay for the direct quantitation of cell-associated Hoechst. We observe that changes in Hoechst fluorescence in cells do not always correlate with changes in the amount of Hoechst. Our data indicate that chemical proton gradient-dependent efflux by LmrP in cells converts populations of highly fluorescent, membrane-intercalated Hoechst in the alkaline interior into populations of less fluorescent, cell surface-bound Hoechst in the acidic exterior. Our methods and findings are directly relevant for the transport of many amphiphilic antibiotics, antineoplastic agents and cytotoxic compounds that are differentially protonated within the physiological pH range

Powering the ABC multidrug exporter LmrA: How nucleotides embrace the ion-motive force.

LmrA is a bacterial ATP-binding cassette (ABC) multidrug exporter that uses metabolic energy to transport ions, cytotoxic drugs, and lipids. Voltage clamping in a Port-a-Patch was used to monitor electrical currents associated with the transport of monovalent cationic HEPES+ by single-LmrA transporters and ensembles of transporters. In these experiments, one proton and one chloride ion are effluxed together with each HEPES+ ion out of the inner compartment, whereas two sodium ions are transported into this compartment. Consequently, the sodium-motive force (interior negative and low) can drive this electrogenic ion exchange mechanism in cells under physiological conditions. The same mechanism is also relevant for the efflux of monovalent cationic ethidium, a typical multidrug transporter substrate. Studies in the presence of Mg-ATP (adenosine 5'-triphosphate) show that ion-coupled HEPES+ transport is associated with ATP-bound LmrA, whereas ion-coupled ethidium transport requires ATP binding and hydrolysis. HEPES+ is highly soluble in a water-based environment, whereas ethidium has a strong preference for residence in the water-repelling plasma membrane. We conclude that the mechanism of the ABC transporter LmrA is fundamentally related to that of an ion antiporter that uses extra steps (ATP binding and hydrolysis) to retrieve and transport membrane-soluble substrates from the phospholipid bilayer.This research was supported by the Biotechnology and Biological Sciences Research Council grants BB/R00224X/1, BB/I002383/1 and BB/K017713/1, and Medical Research Council grant G0401165 (to H.W.V.V.). We are also grateful for funding by the Human Frontier Science Program (grant RGP0034/2013), Strategic International Cooperative Program (Japan Science and Technology Agency, Japan) and Royal Society (UK) for collaborative research between H.W.V.V. and S.M. C.H.F.L. received a research studentship of Peterhouse, Cambridge. Y.S.K.K. received a Federal Training Award from the Ministry of Health in Malaysia. H.S. and S.R. were supported by the Cambridge Commonwealth, European and International Trust

Energy coupling in ABC exporters.

Multidrug transporters are important and interesting molecular machines that extrude a wide variety of cytotoxic drugs from target cells. This review summarizes novel insights in the energetics and mechanisms of bacterial ATP-binding cassette multidrug transporters as well as recent advances connecting multidrug transport to ion and lipid translocation processes in other membrane proteins

Energetics of lipid transport by the ABC transporter MsbA is lipid dependent.

Funder: China Scholarship Council (CSC); doi: https://doi.org/10.13039/501100004543Funder: Cambridge Commonwealth Trust; doi: https://doi.org/10.13039/501100003342The ABC multidrug exporter MsbA mediates the translocation of lipopolysaccharides and phospholipids across the plasma membrane in Gram-negative bacteria. Although MsbA is structurally well characterised, the energetic requirements of lipid transport remain unknown. Here, we report that, similar to the transport of small-molecule antibiotics and cytotoxic agents, the flopping of physiologically relevant long-acyl-chain 1,2-dioleoyl (C18)-phosphatidylethanolamine in proteoliposomes requires the simultaneous input of ATP binding and hydrolysis and the chemical proton gradient as sources of metabolic energy. In contrast, the flopping of the large hexa-acylated (C12-C14) Lipid-A anchor of lipopolysaccharides is only ATP dependent. This study demonstrates that the energetics of lipid transport by MsbA is lipid dependent. As our mutational analyses indicate lipid and drug transport via the central binding chamber in MsbA, the lipid availability in the membrane can affect the drug transport activity and vice versa.This research was funded by Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/R00224X/1 (to H.W.V.V). D.G. and Y.T. were funded by China Scholarship Council – Cambridge Trust PhD Scholarships. C.G. was funded by a BBSRC Doctoral Training Partnership (DTP) Targeted PhD studentship (project 2114197). T.N. received a student grant from Christ’s College Cambridge

Drug‐dependent inhibition of nucleotide hydrolysis in the heterodimeric ABC multidrug transporter PatAB from Streptococcus pneumoniae

Funder: Croucher Foundation; Id: http://dx.doi.org/10.13039/501100001692The bacterial heterodimeric ATP-binding cassette (ABC) multidrug exporter PatAB has a critical role in conferring antibiotic resistance in multidrug-resistant infections by Streptococcus pneumoniae. As with other heterodimeric ABC exporters, PatAB contains two transmembrane domains that form a drug translocation pathway for efflux and two nucleotide-binding domains that bind ATP, one of which is hydrolysed during transport. The structural and functional elements in heterodimeric ABC multidrug exporters that determine interactions with drugs and couple drug binding to nucleotide hydrolysis are not fully understood. Here, we used mass spectrometry techniques to determine the subunit stoichiometry in PatAB in our lactococcal expression system and investigate locations of drug binding using the fluorescent drug-mimetic azido-ethidium. Surprisingly, our analyses of azido-ethidium-labelled PatAB peptides point to ethidium binding in the PatA nucleotide-binding domain, with the azido moiety crosslinked to residue Q521 in the H-like loop of the degenerate nucleotide-binding site. Investigation into this compound and residue’s role in nucleotide hydrolysis pointed to a reduction in the activity for a Q521A mutant and ethidium-dependent inhibition in both mutant and wild type. Most transported drugs did not stimulate or inhibit nucleotide hydrolysis of PatAB in detergent solution or lipidic nanodiscs. However, further examples for ethidium-like inhibition were found with propidium, novobiocin and coumermycin A1, which all inhibit nucleotide hydrolysis by a non-competitive mechanism. These data cast light on potential mechanisms by which drugs can regulate nucleotide hydrolysis by PatAB, which might involve a novel drug binding site near the nucleotide-binding domains

On the mechanisms of transport and energy coupling in ABC exporters

The rapid emergence of multidrug resistant bacterial strains represents a major global healthcare issue. Amongst five known classes of membrane transporters, which play a huge role in multidrug efflux, primary-active ATP-binding cassette (ABC) transporters are ATP powered whilst secondary-active transporters utilize electrochemical ion gradients to drive substrate transport. Mechanistic insights into transport by these proteins can help with the design and development of novel therapeutic agents against multidrug resistance, and can increase our understanding of the physiological functions of these transporters. Although available crystal structures illustrate a common alternate access model for transport by ABC transporters, the mechanisms by which metabolic energy is coupled to the transport cycle is still elusive. This thesis presents a series of functional studies using whole cells as well as artificial phospholipid membranes to study the energetics of transport, and the influence of membrane phospholipids on substrate transport by the homodimeric Escherichia coli lipid A/multidrug ABC exporter MsbA. Current alternating access models for ABC exporters involve cycling between conformations with inward- and outward-facing substrate-binding sites in membrane domains (MDs) in response to engagement and hydrolysis of ATP at the nucleotide-binding domains (NBDs). Here we report that MsbA also utilizes another major energy currency in the cell by coupling substrate transport to a transmembrane electrochemical proton gradient. In this thesis, analogous substrate transport reactions are also studied for two other ABC exporters, the MsbA homologue LmrA and the human multidrug transporter ABCG2. The dependence of ATP-dependent transport on proton coupling, and the stimulation of MsbA-ATPase by the chemical proton gradient highlight the functional integration of both forms of metabolic energy. It also raises questions about the role of NBDs in the transport process. Comparisons of drug transport and resistance in cells expressing MsbA-MD (truncated MsbA lacking the NBD) and full length MsbA (MsbA-WT) demonstrate increased transport efficiency of MsbA-WT compared to MsbA-MD. In addition, growth studies using E. coli WD2 cells, which are conditionally defective in MsbA’s essential activity in lipid A transport, show that lipid A transport can be restored by the expression of MsbA-WT but not MsbA-MD or ATP-hydrolysis impaired Walker A mutant (MsbA- ΔK382). Lastly, we also present biochemical experiments with proteoliposomes with a defined phospholipid composition, which suggest that cardiolipin is essential for the transport activity of MsbA. These techniques open the way to further explore lipid-proteins interactions and examine the physiological role(s) of MsbA. In conclusion, this thesis produces new insights in the mechanisms of transport and energy coupling in ABC exporters1) Cambridge International Scholarship Scheme 2) Rajiv Gandhi Foundatio

Research data supporting 'Complexities of a protonatable substrate in measurements of Hoechst 33342 transport by multidrug transporter LmrP'

Multidrug transporters can confer drug resistance on cells by extruding structurally unrelated compounds from the cellular interior. In transport assays, Hoechst 33342 (referred to as Hoechst) is a commonly used substrate, the fluorescence of which changes in the transport process. With three basic nitrogen atoms that can be protonated, Hoechst can exist as cationic and neutral species that have different fluorescence emissions and abilities to diffuse across cell envelopes and bind to lipids and intracellular nucleic acids. Due to this complexity, the mechanism of Hoechst transport by multidrug transporters is poorly characterised. We investigated Hoechst transport by the bacterial major facilitator superfamily multidrug-proton antiporter LmrP in Lactococcus lactis, and developed a novel assay for the direct quantitation of cell-associated Hoechst. We observe that changes in Hoechst fluorescence in cells do not always correlate with changes in the amount of Hoechst. Our data indicate that chemical proton gradient-dependent efflux by LmrP in cells converts populations of highly fluorescent, membrane-intercalated Hoechst in the alkaline interior into populations of less fluorescent, cell surface-bound Hoechst in the acidic exterior. Our methods and findings are directly relevant for the transport of many amphiphilic antibiotics, antineoplastic agents and cytotoxic compounds that are differentially protonated within the physiological pH range. The research data in this dataset record support the publication by Swain et al. in Scientific Reports and refer to the figures that are incorporated in the paper. Descriptions of the experimental details and statistical analyses are included in the Materials and Methods and figure legends of the paper

Research Data Supporting "ATP-dependent substrate transport by the ABC transporter MsbA is proton-coupled"

ATP-binding cassette transporters mediate the transbilayer movement of a vast number of substrates in or out of cells in organisms ranging from bacteria to humans. Current alternating access models for ABC exporters including the multidrug and Lipid A transporter MsbA from Escherichia coli suggest a role for nucleotide as the fundamental source of free energy. These models involve cycling between conformations with inward- and outward-facing substrate binding sites in response to engagement and hydrolysis of ATP at the nucleotide-binding domains. Here, we report that MsbA also utilizes another major energy currency in the cell by coupling substrate transport to a transmembrane electrochemical proton gradient. The dependence of ATP-dependent transport on proton coupling, and the stimulation of MsbA-ATPase by the chemical proton gradient highlight the functional integration of both forms of metabolic energy in MsbA-mediated transport. These findings introduce ion coupling as a new parameter in the mechanism of this homodimeric ABC transporter. The data for this publication have been uploaded in the Data Repository, and are numbered in accordance with the numbering of the figures in the Nature Communications publication.BBSRC (BB/I002383/1) BBSRC (BB/C004663/1) MRC (G0401165