The Q-loop Disengages from the First Intracellular Loop during the Catalytic Cycle of the Multidrug ABC Transporter BmrA

The transport activity of the multidrug ABC transporter BmrA does not require a wide separation of the nucleotide-binding domains

ATP-binding cassette (ABC) transporters are ubiquitous membrane proteins responsible for the translocation of a wide diversity of substrates across biological membranes. Some of them confer multidrug or antimicrobial resistance to cancer cells and pathogenic microorganisms, respectively. Despite a wealth of structural data gained in the last two decades, the molecular mechanism of these multidrug efflux pumps remains elusive, including the extent of separation between the two nucleotide-binding domains (NBDs) during the transport cycle. Based on recent outward-facing structures of BmrA, a homodimeric multidrug ABC transporter from Bacillus subtilis, we introduced a cysteine mutation near the C-terminal end of the NBDs to analyze the impact of disulfide-bond formation on BmrA function. Interestingly, the presence of the disulfide bond between the NBDs did not prevent the ATPase, nor did it affect the transport of Hoechst 33342 and doxorubicin. Yet, the 7-amino-actinomycin D was less efficiently transported, suggesting that a further opening of the transporter might improve its ability to translocate this larger compound. We solved by cryo-EM the apo structures of the cross-linked mutant and the WT protein. Both structures are highly similar, showing an intermediate opening between their NBDs while their C-terminal extremities remain in close proximity. Distance measurements obtained by electron paramagnetic resonance spectroscopy support the intermediate opening found in these 3D structures. Overall, our data suggest that the NBDs of BmrA function with a tweezers-like mechanism distinct from the related lipid A exporter MsbA.

Effects of Two Natural Bisbenzylisoquinolines, Curine and Guattegaumerine, Extracted from Isolona hexaloba on Rhodamine Efflux by Abcb1b from Rat Glycocholic-Acid-Resistant Hepatocarcinoma Cells

To develop new therapeutic molecules, it is essential to understand the biological effects and targets of clinically relevant compounds. In this article, we describe the extraction and characterization of two alkaloids from the roots of Isolona hexaloba—curine and guattegaumerine. The effect of these alkaloids on the multidrug efflux pump ABCB1 (MDR1/P-Glycoprotein) and their antiproliferative properties were studied. Compared to verapamil, a widely used inhibitor of P-gp, curine and guattegaumerine were found to be weak inhibitors of MDR1/P-Glycoprotein. The highest inhibition of efflux produced by verapamil disappeared in the presence of curine or guattegaumerine as competitors, and the most pronounced effect was achieved with curine. Altogether, this work has provided new insights into the biological effects of these alkaloids on the rat Mdr1b P-gp efflux mechanism and would be beneficial in the design of potent P-gp inhibitors.

Backbone NMR assignment of the nucleotide binding domain of the Bacillus subtilis ABC multidrug transporter BmrA in the post-hydrolysis state

ATP binding cassette (ABC) proteins are present in all phyla of life and form one of the largest protein families. The Bacillus subtilis ABC transporter BmrA is a functional homodimer that can extrude many different harmful compounds out of the cell. Each BmrA monomer is composed of a transmembrane domain (TMD) and a nucleotide binding domain (NBD). While the TMDs of ABC transporters are sequentially diverse, the highly conserved NBDs harbor distinctive conserved motifs that enable nucleotide binding and hydrolysis, interdomain communication and that mark a protein as a member of the ABC superfamily. In the catalytic cycle of an ABC transporter, the NBDs function as the molecular motor that fuels substrate translocation across the membrane via the TMDs and are thus pivotal for the entire transport process. For a better understanding of the structural and dynamic consequences of nucleotide interactions within the NBD at atomic resolution, we determined the ¹H, ¹³C and ¹⁵N backbone chemical shift assignments of the 259 amino acid wildtype BmrA-NBD in its post-hydrolytic, ADP-bound state.

A Conserved Motif in Intracellular Loop 1 Stabilizes the Outward-Facing Conformation of TmrAB

The ATP binding cassette (ABC) family of transporters moves small molecules (lipids, sugars, peptides, drugs, nutrients) across membranes in nearly all organisms. Transport activity requires conformational switching between inward-facing and outward-facing states driven by ATP-dependent dimerization of two nucleotide binding domains (NBDs). The mechanism that connects ATP binding and hydrolysis in the NBDs to conformational changes in a substrate binding site in the transmembrane domains (TMDs) is currently an outstanding question. Here we use sequence coevolution analyses together with biochemical characterization to investigate the role of a highly conserved region in intracellular loop 1 we define as the GRD motif in coordinating domain rearrangements in the heterodimeric peptide exporter from Thermus thermophilus, TmrAB. Mutations in the GRD motif alter ATPase activity as well as transport. Disulfide crosslinking, evolutionary trace, and evolutionary coupling analysis reveal that these effects are likely due to the destabilization of a network in which the GRD motif in TmrA bridges residues of the Q-loop, X-loop, and ABC motif in the NBDs to residues in the TmrAB peptide substrate binding site, thus providing an avenue for conformational coupling. We further find that disruption of this network in TmrA versus TmrB has different functional consequences, hinting at an intrinsic asymmetry in heterodimeric ABC transporters extending beyond that of the NBDs. These results support a mechanism in which the GRD motifs help coordinate a transition to an outward open conformation, and each half of the transporter likely plays a different role in the conformational cycle of TmrAB.

Identification of novel inhibitors of the ABC transporter BmrA

The resistance of microbes to commonly used antibiotics has become a worldwide health problem. A major underlying mechanism of microbial antibiotic resistance is the export of drugs from bacterial cells. Drug efflux is mediated through the action of multidrug resistance efflux pumps located in the bacterial cell membranes. The critical role of bacterial efflux pumps in antibiotic resistance has directed research efforts to the identification of novel efflux pump inhibitors that can be used alongside antibiotics in clinical settings. Here, we aimed to find potential inhibitors of the archetypical ATP-binding cassette (ABC) efflux pump BmrA of Bacillus subtilis via virtual screening of the Mu.Ta.Lig. Chemotheca small molecule library. Molecular docking calculations targeting the nucleotide-binding domain of BmrA were performed using AutoDock Vina. Following a further drug-likeness filtering step based on Lipinski's Rule of Five, top 25 scorers were identified. These ligands were then clustered into separate groups based on their contact patterns with the BmrA nucleotide-binding domain. Six ligands with distinct contact patterns were used for further in vitro inhibition assays based on intracellular ethidium bromide accumulation. Using this methodology, we identified two novel inhibitors of BmrA from the Chemotheca small molecule library.

An A666G mutation in transmembrane helix 5 of the yeast multidrug transporter Pdr5 increases drug efflux by enhancing cooperativity between transport sites

Resistance to antimicrobial and chemotherapeutic agents is a significant clinical problem. Overexpression of multidrug efflux pumps often creates broad-spectrum resistance in cancers and pathogens. We describe a mutation, A666G, in the yeast ABC transporter Pdr5 that shows greater resistance to most of the tested compounds than does an isogenic wild-type strain. This mutant exhibited enhanced resistance without increasing either the amount of protein in the plasma membrane or the ATPase activity. In fluorescence quenching transport assays with rhodamine 6G in purified plasma membrane vesicles, the initial rates of rhodamine 6G fluorescence quenching of both the wild type and mutant showed a strong dependence on the ATP concentration, but were about twice as high in the latter. Plots of the initial rate of fluorescence quenching versus ATP concentration exhibited strong cooperativity that was further enhanced in the A666G mutant. Resistance to imazalil sulfate was about 3-4x as great in the A666G mutant strain as in the wild type. When this transport substrate was used to inhibit the rhodamine 6G transport, the A666G mutant inhibition curves also showed greater cooperativity than the wild-type strain. Our results suggest a novel and important mechanism: under selection, Pdr5 mutants can increase drug resistance by improving cooperative interactions between drug transport sites.

Flexible-to-rigid transition is central for substrate transport in the ABC transporter BmrA from Bacillus subtilis

ATP-binding-cassette (ABC) transporters are molecular pumps that translocate molecules across the cell membrane by switching between inward-facing and outward-facing states. To obtain a detailed understanding of their mechanism remains a challenge to structural biology, as these proteins are notoriously difficult to study at the molecular level in their active, membrane-inserted form. Here we use solid-state NMR to investigate the multidrug ABC transporter BmrA reconstituted in lipids. We identify the chemical-shift differences between the inward-facing, and outward-facing state induced by ATP:Mg2+:Vi addition. Analysis of an X-loop mutant, for which we show that ATPase and transport activities are uncoupled, reveals an incomplete transition to the outward-facing state upon ATP:Mg2+:Vi addition, notably lacking the decrease in dynamics of a defined set of residues observed in wild-type BmrA. This suggests that this stiffening is required for an efficient transmission of the conformational changes to allow proper transport of substrate by the pump.

ATP-Dependent Signaling in Simulations of a Revised Model of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily that has uniquely evolved to function as a chloride channel. It binds and hydrolyzes ATP at its nucleotide binding domains to form a pore providing a diffusive pathway within its transmembrane domains. CFTR is the only known protein from the ABC superfamily with channel activity, and its dysfunction causes the disease cystic fibrosis. While much is known about the functional aspects of CFTR, significant gaps remain, such as the structure-function relationship underlying signaling of ATP binding. In the present work, we refined an existing homology model using an intermediate-resolution (9 Å) published cryo-electron microscopy map. The newly derived models have been simulated in equilibrium molecular dynamics simulations for a total of 2.5 μs in multiple ATP-occupancy states. Putative conformational movements connecting ATP binding with pore formation are elucidated and quantified. Additionally, new interdomain interactions between E543, K968, and K1292 have been identified and confirmed experimentally; these interactions may be relevant for signaling ATP binding and hydrolysis to the transmembrane domains and induction of pore opening.

Channel Gating Regulation by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) First Cytosolic Loop

In this study, we present data indicating a robust and specific domain interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) first cytosolic loop (CL1) and nucleotide binding domain 1 (NBD1) that allows ion transport to proceed in a regulated fashion. We used co-precipitation and ELISA to establish the molecular contact and showed that binding kinetics were not altered by the common clinical mutation F508del. Both intrinsic ATPase activity and CFTR channel gating were inhibited severely by CL1 peptide, suggesting that NBD1/CL1 binding is a crucial requirement for ATP hydrolysis and channel function. In addition to cystic fibrosis, CFTR dysregulation has been implicated in the pathogenesis of prevalent diseases such as chronic obstructive pulmonary disease, acquired rhinosinusitis, pancreatitis, and lethal secretory diarrhea (e.g. cholera). On the basis of clinical relevance of the CFTR as a therapeutic target, a cell-free drug screen was established to identify modulators of NBD1/CL1 channel activity independent of F508del CFTR and pharmacologic rescue. Our findings support a targetable mechanism of CFTR regulation in which conformational changes in the NBDs cause reorientation of transmembrane domains via interactions with CL1 and result in channel gating.

Assembly and function of the major histocompatibility complex (MHC) I peptide-loading complex are conserved across higher vertebrates

Antigen presentation to cytotoxic T lymphocytes via major histocompatibility complex class I (MHC I) molecules depends on the heterodimeric transporter associated with antigen processing (TAP). For efficient antigen supply to MHC I molecules in the ER, TAP assembles a macromolecular peptide-loading complex (PLC) by recruiting tapasin. In evolution, TAP appeared together with effector cells of adaptive immunity at the transition from jawless to jawed vertebrates and diversified further within the jawed vertebrates. Here, we compared TAP function and interaction with tapasin of a range of species within two classes of jawed vertebrates. We found that avian and mammalian TAP1 and TAP2 form heterodimeric complexes across taxa. Moreover, the extra N-terminal domain TMD0 of mammalian TAP1 and TAP2 as well as avian TAP2 recruits tapasin. Strikingly, however, only TAP1 and TAP2 from the same taxon can form a functional heterodimeric translocation complex. These data demonstrate that the dimerization interface between TAP1 and TAP2 and the tapasin docking sites for PLC assembly are conserved in evolution, whereas elements of antigen translocation diverged later in evolution and are thus taxon specific.

Efficient and stable reconstitution of the ABC transporter BmrA for solid-state NMR studies

We present solid-state NMR sample preparation and first 2D spectra of the Bacillus subtilis ATP-binding cassette (ABC) transporter BmrA, a membrane protein involved in multidrug resistance. The homodimeric 130-kDa protein is a challenge for structural characterization due to its membrane-bound nature, size, inherent flexibility and insolubility. We show that reconstitution of this protein in lipids from Bacillus subtilis at a lipid-protein ratio of 0.5 w/w allows for optimal protein insertion in lipid membranes with respect to two central NMR requirements, high signal-to-noise in the spectra and sample stability over a time period of months. The obtained spectra point to a well-folded protein and a highly homogenous preparation, as witnessed by the narrow resonance lines and the signal dispersion typical for the expected secondary structure distribution of BmrA. This opens the way for studies of the different conformational states of the transporter in the export cycle, as well as on interactions with substrates, via chemical-shift fingerprints and sequential resonance assignments.

A Computational Approach towards the Understanding of Plasmodium falciparum Multidrug Resistance Protein 1

The emergence of drug resistance in Plasmodium falciparum tremendously affected the chemotherapy worldwide while the intense distribution of chloroquine-resistant strains in most of the endemic areas added more complications in the treatment of malaria. The situation has even worsened by the lack of molecular mechanism to understand the resistance conferred by Plasmodia species. Recent studies have suggested the association of antimalarial resistance with P. falciparum multidrug resistance protein 1 (PfMDR1), an ATP-binding cassette (ABC) transporter and a homologue of human P-glycoprotein 1 (P-gp1). The present study deals about the development of PfMDR1 computational model and the model of substrate transport across PfMDR1 with insights derived from conformations relative to inward- and outward-facing topologies that switch on/off the transportation system. Comparison of ATP docked positions and its structural motif binding properties were found to be similar among other ATPases, and thereby contributes to NBD domains dimerization, a unique structural agreement noticed in Mus musculus Pgp and Escherichia coli MDR transporter homolog (MsbA). The interaction of leading antimalarials and phytochemicals within the active pocket of both wild-type and mutant-type PfMDR1 demonstrated the mode of binding and provided insights of less binding affinity thereby contributing to parasite's resistance mechanism.

Mechanism of the ABC transporter ATPase domains: catalytic models and the biochemical and biophysical record

ABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane domains that bind substrate and form the channel, and two ATP-binding cassettes, which bind and hydrolyze ATP to energize the translocase function. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the nucleotide binding domains are proposed to dimerise upon binding of two ATP molecules, and thence dissociate upon sequential hydrolysis of the ATP. This idea appears consistent with crystal structures of both isolated subunits and whole transporters, as well as with a significant body of biochemical data. Nonetheless, an alternative Constant Contact Model has been proposed, in which the nucleotide binding domains do not fully dissociate, and ATP hydrolysis occurs alternately at each of the two active sites. Here, we review the biochemical and biophysical data relating to the ABC catalytic mechanism, to show how they may be construed as consistent with a Constant Contact Model, and to assess to what extent they support the Switch Model.

The role of ATP-binding cassette transporters in bacterial pathogenicity

The ATP-binding cassette transporter superfamily is present in all three domains of life. This ubiquitous class of integral membrane proteins have diverse biological functions, but their fundamental role involves the unidirectional translocation of compounds across cellular membranes in an ATP coupled process. The importance of this class of proteins in eukaryotic systems is well established as typified by their association with genetic diseases and roles in the multi-drug resistance of cancer. In stark contrast, the ABC transporters of prokaryotes have not been exhaustively investigated due to the sheer number of different roles and organisms in which they function. In this review, we examine the breadth of functions associated with microbial ABC transporters in the context of their contribution to bacterial pathogenicity and virulence.

Dynamics of a bacterial multidrug ABC transporter in the inward- and outward-facing conformations

The study of membrane proteins remains a challenging task, and approaches to unravel their dynamics are scarce. Here, we applied hydrogen/deuterium exchange (HDX) coupled to mass spectrometry to probe the motions of a bacterial multidrug ATP-binding cassette (ABC) transporter, BmrA, in the inward-facing (resting state) and outward-facing (ATP-bound) conformations. Trypsin digestion and global or local HDX support the transition between inward- and outward-facing conformations during the catalytic cycle of BmrA. However, in the resting state, peptides from the two intracellular domains, especially ICD2, show a much faster HDX than in the closed state. This shows that these two subdomains are very flexible in this conformation. Additionally, molecular dynamics simulations suggest a large fluctuation of the Cα positions from ICD2 residues in the inward-facing conformation of a related transporter, MsbA. These results highlight the unexpected flexibility of ABC exporters in the resting state and underline the power of HDX coupled to mass spectrometry to explore conformational changes and dynamics of large membrane proteins.

ABC transporters and immunity: mechanism of self-defense

The transporter associated with antigen processing (TAP) is a prototype of an asymmetric ATP-binding cassette (ABC) transporter, which uses ATP binding and hydrolysis to translocate peptides from the cytosol to the lumen of the endoplasmic reticulum (ER). Here, we review molecular details of peptide binding and ATP binding and hydrolysis as well as the resulting allosteric cross-talk between the nucleotide-binding domains and the transmembrane domains that drive translocation of the solute across the ER membrane. We also discuss the general molecular architecture of ABC transporters and demonstrate the importance of structural and functional studies for a better understanding of the role of the noncanonical site of asymmetric ABC transporters. Several aspects of peptide binding and specificity illustrate details of peptide translocation by TAP. Furthermore, this ABC transporter forms the central part of the major histocompatibility complex class I (MHC I) peptide-loading machinery. Hence, TAP is confronted with a number of viral factors, which prevent antigen translocation and MHC I loading in virally infected cells. We review how these viral factors have been used as molecular tools to decipher mechanistic aspects of solute translocation and discuss how they can help in the structural analysis of TAP.

Molecular-dynamics simulations of the ATP/apo state of a multidrug ATP-binding cassette transporter provide a structural and mechanistic basis for the asymmetric occluded state

ATP-binding cassette transporters use the energy of ATP hydrolysis to transport substrates across cellular membranes. They have two transmembrane domains and two cytosolic nucleotide-binding domains. Biochemical studies have characterized an occluded state of the transporter in which nucleotide is tenaciously bound in one active site, whereas the opposite active site is empty or binds nucleotide loosely. Here, we report molecular-dynamics simulations of the bacterial multidrug ATP-binding cassette transporter Sav1866. In two simulations of the ATP/apo state, the empty site opened substantially by way of rotation of the nucleotide-binding domain (NBD) core subdomain, whereas the ATP-bound site remained occluded and intact. We correlate our findings with elastic network and molecular-dynamics simulation analyses of the Sav1866 NBD monomer, and with existing experimental data, to argue that the observed transition is physiological, and that the final structure observed in the ATP/apo simulations corresponds to the tight/loose state of the NBD dimer characterized experimentally.

PfMDR1: mechanisms of transport modulation by functional polymorphisms

ATP-Binding Cassette (ABC) transporters are efflux pumps frequently associated with multidrug resistance in many biological systems, including malaria. Antimalarial drug-resistance involves an ABC transporter, PfMDR1, a homologue of P-glycoprotein in humans. Twenty years of research have shown that several single nucleotide polymorphisms in pfmdr1 modulate in vivo and/or in vitro drug susceptibility. The underlying physiological mechanism of the effect of these mutations remains unclear. Here we develop structural models for PfMDR1 in different predicted conformations, enabling the study of transporter motion. Such analysis of functional polymorphisms allows determination of their potential role in transport and resistance. The bacterial MsbA ABC pump is a PfMDR1 homologue. MsbA crystals in different conformations were used to create PfMDR1 models with Modeller software. Sequences were aligned with ClustalW and analysed by Ali2D revealing a high level of secondary structure conservation. To validate a potential drug binding pocket we performed antimalarial docking simulations. Using aminoquinoline as probe drugs in PfMDR1 mutated parasites we evaluated the physiology underlying the mechanisms of resistance mediated by PfMDR1 polymorphisms. We focused on the analysis of well known functional polymorphisms in PfMDR1 amino acid residues 86, 184, 1034, 1042 and 1246. Our structural analysis suggested the existence of two different biophysical mechanisms of PfMDR1 drug resistance modulation. Polymorphisms in residues 86/184/1246 act by internal allosteric modulation and residues 1034 and 1042 interact directly in a drug pocket. Parasites containing mutated PfMDR1 variants had a significant altered aminoquinoline susceptibility that appears to be dependent on the aminoquinoline lipophobicity characteristics as well as vacuolar efflux by PfCRT. We previously described the in vivo selection of PfMDR1 polymorphisms under antimalarial drug pressure. Now, together with recent PfMDR1 functional reports, we contribute to the understanding of the specific structural role of these polymorphisms in parasite antimalarial drug response.

Glutamine residues in Q-loops of multidrug resistance protein MRP1 contribute to ATP binding via interaction with metal cofactor

Structural analyses of bacterial ATP-binding-cassette transporters revealed that the glutamine residue in Q-loop plays roles in interacting with: 1) a metal cofactor to participate in ATP binding; 2) a putative catalytic water molecule to participate in ATP hydrolysis; 3) other residues to transmit the conformational changes between nucleotide-binding-domains and transmembrane-domains, in ATP-dependent solute transport. We have mutated the glutamines at 713 and 1375 to asparagine, methionine or leucine to determine the functional roles of these residues in Q-loops of MRP1. All these single mutants significantly decreased Mg·ATP binding and increased the K(m) (Mg·ATP) and V(max) values in Mg·ATP-dependent leukotriene-C4 transport. However, the V(max) values of the double mutants Q713N/Q1375N, Q713M/Q1375M and Q713L/Q1375L were lower than that of wtMRP1, implying that the double mutants cannot efficiently bind Mg·ATP. Interestingly, MRP1 has higher affinity for Mn·ATP than for Mg·ATP and the Mn·ATP-dependent leukotriene-C4 transport activities of Q713N/Q1375N and Q713M/Q1375M are significantly higher than that of wtMRP1. All these results suggest that: 1) the glutamine residues in Q-loops contribute to ATP-binding via interaction with a metal cofactor; 2) it is most unlikely that these glutamine residues would play crucial roles in ATP hydrolysis and in transmitting the conformational changes between nucleotide-binding-domains and transmembrane-domains.

Functional hot spots in human ATP-binding cassette transporter nucleotide binding domains

The human ATP-binding cassette (ABC) transporter superfamily consists of 48 integral membrane proteins that couple the action of ATP binding and hydrolysis to the transport of diverse substrates across cellular membranes. Defects in 18 transporters have been implicated in human disease. In hundreds of cases, disease phenotypes and defects in function can be traced to nonsynonymous single nucleotide polymorphisms (nsSNPs). The functional impact of the majority of ABC transporter nsSNPs has yet to be experimentally characterized. Here, we combine experimental mutational studies with sequence and structural analysis to describe the impact of nsSNPs in human ABC transporters. First, the disease associations of 39 nsSNPs in 10 transporters were rationalized by identifying two conserved loops and a small α-helical region that may be involved in interdomain communication necessary for transport of substrates. Second, an approach to discriminate between disease-associated and neutral nsSNPs was developed and tailored to this superfamily. Finally, the functional impact of 40 unannotated nsSNPs in seven ABC transporters identified in 247 ethnically diverse individuals studied by the Pharmacogenetics of Membrane Transporters consortium was predicted. Three predictions were experimentally tested using human embryonic kidney epithelial (HEK) 293 cells stably transfected with the reference multidrug resistance transporter 4 and its variants to examine functional differences in transport of the antiviral drug, tenofovir. The experimental results confirmed two predictions. Our analysis provides a structural and evolutionary framework for rationalizing and predicting the functional effects of nsSNPs in this clinically important membrane transporter superfamily.

ABC proteins in antigen translocation and viral inhibition

How ABC transporters work is a key issue because of their important roles in multidrug resistance of pathogenic bacteria, reduced efficacy of antitumor drugs, cholesterol metabolism, cell homeostasis and immune response. In the past few years, significant progress has been made in crystallization and structure determination of (mostly) bacterial ABC transporters, as well as in functional studies on ABC systems involved in human pathology. In this review, we use the transporter associated with antigen processing (TAP) to illustrate what is known regarding the mechanism of substrate transport. We also discuss the chemical basis of substrate recognition by TAP and the allosteric cross-talk between the binding of substrate, the release of chemical energy by ATP hydrolysis and cross-membrane translocation. Finally, we detail the role of TAP in a large macromolecular assembly, which optimally loads MHC class I molecules, and the interference with this machinery by TAP-targeted viral factors. Because of structural and probable mechanistic similarities, the understanding of the detailed structure and mechanism of TAP will be applicable to all ABC systems, including those of medical relevance.

ABC proteins in antigen translocation and viral inhibition

How ABC transporters work is a key issue because of their important roles in multidrug resistance of pathogenic bacteria, reduced efficacy of antitumor drugs, cholesterol metabolism, cell homeostasis and immune response. In the past few years, significant progress has been made in crystallization and structure determination of (mostly) bacterial ABC transporters, as well as in functional studies on ABC systems involved in human pathology. In this review, we use the transporter associated with antigen processing (TAP) to illustrate what is known regarding the mechanism of substrate transport. We also discuss the chemical basis of substrate recognition by TAP and the allosteric cross-talk between the binding of substrate, the release of chemical energy by ATP hydrolysis and cross-membrane translocation. Finally, we detail the role of TAP in a large macromolecular assembly, which optimally loads MHC class I molecules, and the interference with this machinery by TAP-targeted viral factors. Because of structural and probable mechanistic similarities, the understanding of the detailed structure and mechanism of TAP will be applicable to all ABC systems, including those of medical relevance.

The signaling interface of the yeast multidrug transporter Pdr5 adopts a cis conformation, and there are functional overlap and equivalence of the deviant and canonical Q-loop residues

ABC transporters are polytopic proteins. ATP hydrolysis and substrate transport take place in separate domains, and these activities must be coordinated through a signal interface. We previously characterized a mutation (S558Y) in the yeast multidrug transporter Pdr5 that uncouples ATP hydrolysis and drug transport. To characterize the transmission interface, we used a genetic screen to isolate second-site mutations of S558Y that restore drug transport. We recovered suppressors that restore drug resistance; their locations provide functional evidence for an interface in the cis rather than the trans configuration indicated by structural and cross-linking studies of bacterial and eukaryotic efflux transporters. One mutation, E244G, defines the Q-loop of the deviant portion of NBD1, which is the hallmark of this group of fungal transporters. When moved to an otherwise wild-type background, this mutation and its counterpart in the canonical ATP-binding site Q951G show a similar reduction in drug resistance and in the very high basal-level ATP hydrolysis characteristic of Pdr5. A double E244G, Q951G mutant is considerably more drug sensitive than either of the single mutations. Surprisingly, then, the deviant and canonical Q-loop residues are functionally overlapping and equivalent in a strikingly asymmetric ABC transporter.

Multidrug efflux pumps: the structures of prokaryotic ATP-binding cassette transporter efflux pumps and implications for our understanding of eukaryotic P-glycoproteins and homologues

One of the Holy Grails of ATP-binding cassette transporter research is a structural understanding of drug binding and transport in a eukaryotic multidrug resistance pump. These transporters are front-line mediators of drug resistance in cancers and represent an important therapeutic target in future chemotherapy. Although there has been intensive biochemical research into the human multidrug pumps, their 3D structure at atomic resolution remains unknown. The recent determination of the structure of a mouse P-glycoprotein at subatomic resolution is complemented by structures for a number of prokaryotic homologues. These structures have provided advances into our knowledge of the ATP-binding cassette exporter structure and mechanism, and have provided the template data for a number of homology modelling studies designed to reconcile biochemical data on these clinically important proteins.

Probing the conformation of the resting state of a bacterial multidrug ABC transporter, BmrA, by a site-directed spin labeling approach

Previously published 3-D structures of a prototypic ATP-binding cassette (ABC) transporter, MsbA, have been recently corrected revealing large rigid-body motions possibly linked to its catalytic cycle. Here, a closely related multidrug bacterial ABC transporter, BmrA, was studied using site-directed spin labeling by focusing on a region connecting the transmembrane domain and the nucleotide-binding domain (NBD). Electron paramagnetic resonance (EPR) spectra of single spin-labeled cysteine mutants suggests that, in the resting state, this sub-domain essentially adopts a partially extended conformation, which is consistent with the crystal structures of MsbA and Sav1866. Interestingly, one of the single point mutants (Q333C) yielded an immobilized EPR spectrum that could arise from a direct interaction with a vicinal tyrosine residue. Inspection of different BmrA models pointed to Y408, within the NBD, as the putative interacting partner, and its mutation to a Phe residue indeed dramatically modified the EPR spectra of the spin labeled Q333C. Moreover, unlike the Y408F mutation, the Y408A mutation abolished both ATPase activity and drug transport of BmrA, suggesting that a nonpolar bulky residue is required at this position. The spatial proximity of Q333 and Y408 was also confirmed by formation of a disulfide bond when both Q333 and T407 (or S409) were replaced jointly by a cysteine residue. Overall, these results indicate that the two regions surrounding Q333 and Y408 are close together in the 3-D structure of BmrA and that residues within these two sub-domains are essential for proper functioning of this transporter.

An intramolecular signaling element that modulates dynamin function in vitro and in vivo

Dynamin exhibits a high basal rate of GTP hydrolysis that is enhanced by self-assembly on a lipid template. Dynamin's GTPase effector domain (GED) is required for this stimulation, though its mechanism of action is poorly understood. Recent structural work has suggested that GED may physically dock with the GTPase domain to exert its stimulatory effects. To examine how these interactions activate dynamin, we engineered a minimal GTPase-GED fusion protein (GG) that reconstitutes dynamin's basal GTPase activity and utilized it to define the structural framework that mediates GED's association with the GTPase domain. Chemical cross-linking of GG and mutagenesis of full-length dynamin establishes that the GTPase-GED interface is comprised of the N- and C-terminal helices of the GTPase domain and the C-terminus of GED. We further show that this interface is essential for structural stability in full-length dynamin. Finally, we identify mutations in this interface that disrupt assembly-stimulated GTP hydrolysis and dynamin-catalyzed membrane fission in vitro and impair the late stages of clathrin-mediated endocytosis in vivo. These data suggest that the components of the GTPase-GED interface act as an intramolecular signaling module, which we term the bundle signaling element, that can modulate dynamin function in vitro and in vivo.

Structural arrangement of the transmission interface in the antigen ABC transport complex TAP

The transporter associated with antigen processing (TAP) represents a focal point in the immune recognition of virally or malignantly transformed cells by translocating proteasomal degradation products into the endoplasmic reticulum-lumen for loading of MHC class I molecules. Based on a number of experimental data and the homology to the bacterial ABC exporter Sav1866, we constructed a 3D structural model of the core TAP complex and used it to examine the interface between the transmembrane and nucleotide-binding domains (NBD) by cysteine-scanning and cross-linking approaches. Herein, we demonstrate the functional importance of the newly identified X-loop in the NBD in coupling substrate binding to downstream events in the transport cycle. We further verified domain swapping in a heterodimeric ABC half-transporter complex by cysteine cross-linking. Strikingly, either substrate binding or translocation can be blocked by cross-linking the X-loop to coupling helix 2 or 1, respectively. These results resolve the structural arrangement of the transmission interface and point to different functions of the cytosolic loops and coupling helices in substrate binding, signaling, and transport.

Purification and structural analyses of ABCG2

ABCG2 is best known as a multidrug transporter capable of conferring resistance to cancer cells. However, the protein is also inherently expressed in numerous barrier tissues and intriguingly within hematopoietic stem cells. Unlike its partners ABCB1 and ABCC1, there is considerably less information available on the molecular mechanism of ABCG2. The transporter has a distinct topology and is presumed to function as a homodimer. However, a number of biochemical studies have presented data to suggest that the protein adopts higher order oligomers. This review focuses on this controversial issue with particular reference to findings from low resolution structural data. In addition, a number of molecular models of ABCG2 based on high resolution structures of bacterial ABC transporters have recently become available and are critically assessed. ABCG2 is a structurally distinct member of the triumvirate of human multidrug transporters and continues to evade description of a unifying molecular mechanism.

Structure, function, and evolution of bacterial ATP-binding cassette systems

SUMMARY

ATP-binding cassette (ABC) systems are universally distributed among living organisms and function in many different aspects of bacterial physiology. ABC transporters are best known for their role in the import of essential nutrients and the export of toxic molecules, but they can also mediate the transport of many other physiological substrates. In a classical transport reaction, two highly conserved ATP-binding domains or subunits couple the binding/hydrolysis of ATP to the translocation of particular substrates across the membrane, through interactions with membrane-spanning domains of the transporter. Variations on this basic theme involve soluble ABC ATP-binding proteins that couple ATP hydrolysis to nontransport processes, such as DNA repair and gene expression regulation. Insights into the structure, function, and mechanism of action of bacterial ABC proteins are reported, based on phylogenetic comparisons as well as classic biochemical and genetic approaches. The availability of an increasing number of high-resolution structures has provided a valuable framework for interpretation of recent studies, and realistic models have been proposed to explain how these fascinating molecular machines use complex dynamic processes to fulfill their numerous biological functions. These advances are also important for elucidating the mechanism of action of eukaryotic ABC proteins, because functional defects in many of them are responsible for severe human inherited diseases.

Multiple membrane-cytoplasmic domain contacts in the cystic fibrosis transmembrane conductance regulator (CFTR) mediate regulation of channel gating

The cystic fibrosis transmembrane conductance regulator (CFTR) is a unique ATP-binding cassette (ABC) ion channel mutated in patients with cystic fibrosis. The most common mutation, deletion of phenylalanine 508 (DeltaF508) and many other disease-associated mutations occur in the nucleotide binding domains (NBD) and the cytoplasmic loops (CL) of the membrane-spanning domains (MSD). A recently constructed computational model of the CFTR three-dimensional structure, supported by experimental data (Serohijos, A. W., Hegedus, T., Aleksandrov, A. A., He, L., Cui, L., Dokholyan, N. V., and Riordan, J. R. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 3256-3261) revealed that several of these mutations including DeltaF508 disrupted interfaces between these domains. Here we have used cysteine cross-linking experiments to verify all NBD/CL interfaces predicted by the structural model and observed that their cross-linking has a variety of different effects on channel gating. The interdomain contacts comprise aromatic clusters important for stabilization of the interfaces and also involve the Q-loops and X-loops that are in close proximity to the ATP binding sites. Cross-linking of all domain-swapping contacts between NBDs and MSD cytoplasmic loops in opposite halves of the protein rapidly and reversibly arrest single channel gating while those in the same halves have lesser impact. These results reinforce the idea that mediation of regulatory signals between cytoplasmic- and membrane-integrated domains of the CFTR channel apparently relies on an array of precise but highly dynamic interdomain structural joints.

A comparative electron paramagnetic resonance study of the nucleotide-binding domains' catalytic cycle in the assembled maltose ATP-binding cassette importer

We present a quantitative analysis of conformational changes of the nucleotide-binding subunits, MalK(2), of the maltose ATP-binding cassette importer MalFGK(2) during the transport cycle. Distance changes occurring between selected residues were monitored in the full transporter by site-directed spin-labeling electron paramagnetic resonance spectroscopy and site-directed chemical cross-linking. We considered S83C and A85C from the conserved Q-loop and V117C located on the outer surface of MalK. Additionally, two native cysteines (C350, C360) were included in the study. On ATP binding, small rearrangements between the native sites, and no distance changes between positions 117 were detected. In contrast, positions 85 come closer together in the ATP-bound state and in the vanadate-trapped intermediate and move back toward the apo-state after ATP hydrolysis. The distance between positions 83 is shown to slightly decrease on ATP binding, and to further decrease after ATP hydrolysis. Results from cross-linking experiments are in agreement with these findings. The data are compared with in silico spin-labeled x-ray structures from both isolated MalK(2) and the MalFGK(2)-E complex. Our results are consistent with a slightly modified "tweezers-like" model of closure and reopening of MalK(2) during the catalytic cycle, and show an unforeseen potential interaction between MalK and the transmembrane subunit MalG.

Investigating the role of the invariant carboxylate residues E552 and E1197 in the catalytic activity of Abcb1a (mouse Mdr3)

The invariant carboxylate residue which follows the Walker B motif (hyd(4)DE/D) in the nucleotide-binding domains (NBDs) of ATP-binding cassette transporters is thought to be involved in the hydrolysis of the gamma-phosphate of MgATP, either by activating the attacking water molecule or by promoting substrate-assisted catalysis. In Abcb1a, this invariant carboxylate residue corresponds to E552 in NBD1 and E1197 in NBD2. To further characterize the role of these residues in catalysis, we created in Abcb1a the single-site mutants E552D, N and A in NBD1, and E1197D, N and A in NBD2, as well as the double-mutant E552Q/E1197Q. In addition, we created mutants in which the Walker A K --> R mutation known to abolish ATPase activity was introduced in the non-mutant NBD of E552Q and E1197Q. ATPase activity, binding affinity and trapping properties were tested for each Abcb1a variant. The results suggest that the length of the invariant carboxylate residue is important for the catalytic activity, whereas the charge of the side chain is critical for full turnover to occur. Moreover, in the double-mutants where the K --> R mutation is introduced in the 'wild-type' NBD of the E --> Q mutants, single-site turnover is observed, especially when NBD2 can undergo gamma-P(i) cleavage. The results further support the idea that the NBDs are not symmetric and suggest that the invariant carboxylates are involved both in NBD-NBD communication and transition-state formation through orientation of the linchpin residue.

Targeted degradation of ABC transporters in health and disease

ATP binding cassette (ABC) transporters comprise an extended protein family involved in the transport of a broad spectrum of solutes across membranes. They consist of a common architecture including two ATP-binding domains converting chemical energy into conformational changes and two transmembrane domains facilitating transport via alternating access. This review focuses on the biogenesis, and more precisely, on the degradation of mammalian ABC transporters in the endoplasmic reticulum (ER). We enlighten the ER-associated degradation pathway in the context of misfolded, misassembled or tightly regulated ABC transporters with a closer view on the cystic fibrosis transmembrane conductance regulator (CFTR) and the transporter associated with antigen processing (TAP), which plays an essential role in the adaptive immunity. Three rather different scenarios affecting the stability and degradation of ABC transporters are discussed: (1) misfolded domains caused by a lack of proper intra- and intermolecular contacts within the ABC transporters, (2) deficient assembly with auxiliary factors, and (3) arrest and accumulation of an intermediate or 'dead-end' state in the transport cycle, which is prone to be recognized by the ER-associated degradation machinery.

The Q-loop of DrrA is involved in producing the closed conformation of the nucleotide binding domains and in transduction of conformational changes between DrrA and DrrB

DrrA and DrrB proteins form an ATP-dependent efflux pump for doxorubicin and daunorubicin in Streptomyces peucetius. DrrA, the catalytic subunit, forms a complex with the integral membrane protein DrrB. Previous studies have provided evidence for strong interaction between these two proteins, which was found to be critical for binding of ATP to DrrA and for stability of DrrB. Chemical cross-linking experiments carried out previously showed that in the resting state of the complex DrrA and DrrB are in contact with each other. Use of a cysteine-to-amine cross-linker then allowed identification of the N-terminal cytoplasmic tail of DrrB (residues 1-53) as the primary region of contact with DrrA. In this study, single-cysteine substitutions were introduced into different domains of DrrA in a strain already containing the S23C substitution in the N-terminal tail of DrrB. By using different arm-length disulfide cross-linkers, we found that a cysteine placed in the Q-loop region of DrrA traps DrrA in the dimeric state, thus indicating that in the closed conformation the Q-loops from opposing subunits are in the proximity of each other. Furthermore, the same region of DrrA was also found to interact with the N-terminus of DrrB, although the A-A interaction was much more prominent than the A-B interaction under these conditions. On the basis of additional data shown here, we propose that the interaction of the Q-loop with the N-terminal cytoplasmic tail of DrrB identifies an important step in the communication of conformational changes between DrrA and DrrB. The significance of these findings in the mechanism of the DrrAB complex is discussed, and a model based on analyses of different conformations of DrrA and DrrB is presented.

Conformational change induced by ATP binding in the multidrug ATP-binding cassette transporter BmrA

ATP-binding cassette (ABC) transporters are involved in the transport of a wide variety of substrates, and ATP-driven dimerization of their nucleotide binding domains (NBDs) has been suggested to be one of the most energetic steps of their catalytic cycle. Taking advantage of the propensity of BmrA, a bacterial multidrug resistance ABC transporter, to form stable, highly ordered ring-shaped structures [Chami et al. (2002) J. Mol. Biol. 315, 1075-1085], we show here that addition of ATP in the presence of Mg2+ prevented ring formation or destroyed the previously formed rings. To pinpoint the catalytic step responsible for such an effect, two classes of hydrolysis-deficient mutants were further studied. In contrast to hydrolytically inactive glutamate mutants that behaved essentially as the wild-type, lysine Walker A mutants formed ring-shaped structures even in the presence of ATP-Mg. Although the latter mutants still bound ATP-Mg, and even slowly hydrolyzed it for the K380R mutant, they were most likely unable to undergo a proper NBD dimerization upon ATP-Mg addition. The ATP-driven dimerization step, which was still permitted in glutamate mutants and led to a stable conformation suitable to monitor the growth of 2D crystals, appeared therefore responsible for destabilization of the BmrA ring structures. Our results provide direct visual evidence that the ATP-induced NBD dimerization triggers a conformational change large enough in BmrA to destabilize the rings, which is consistent with the assumption that this step might constitute the "power stroke" for ABC transporters.

Crystal structure of Bacillus stearothermophilus UvrA provides insight into ATP-modulated dimerization, UvrB interaction, and DNA binding

The nucleotide excision repair pathway corrects many structurally unrelated DNA lesions. Damage recognition in bacteria is performed by UvrA, a member of the ABC ATPase superfamily whose functional form is a dimer with four nucleotide-binding domains (NBDs), two per protomer. In the 3.2 A structure of UvrA from Bacillus stearothermophilus, we observe that the nucleotide-binding sites are formed in an intramolecular fashion and are not at the dimer interface as is typically found in other ABC ATPases. UvrA also harbors two unique domains; we show that one of these is required for interaction with UvrB, its partner in lesion recognition. In addition, UvrA contains three zinc modules, the number and ligand sphere of which differ from previously published models. Structural analysis, biochemical experiments, surface electrostatics, and sequence conservation form the basis for models of ATP-modulated dimerization, UvrA-UvrB interaction, and DNA binding during the search for lesions.

New structure model for the ATP-binding cassette multidrug transporter LmrA

Multidrug resistance of pathogenic microorganisms and mammalian tumors can be associated with the overexpression of multidrug transporters. These integral membrane proteins are capable of extruding a wide range of structurally unrelated compounds from the cell. Among the different classes of multidrug transporters are the ATP binding cassette (ABC) transporters, which are dependent on the binding and hydrolysis of ATP. In the past five years, many researchers have built homology models of ABC extrusion systems using the atomic coordinates of crystallized MsbA, a lipopolysaccharide transporter in Gram-negative bacteria. Likewise, we have previously used the Vibrio cholera MsbA structure as a template in the modeling of the multidrug transporter LmrA from Lactococcus lactis. In view of the recently discovered inaccuracies in the MsbA structure, we have remodelled LmrA using the atomic coordinates of the MsbA homologue Sav1866 from Staphylococcus aureus. To compare and test our MsbA-based and Sav1866-based LmrA models we performed cysteine cross-linking at three key positions in LmrA. The pattern of cross-linking at these positions was consistent with the overall topology of transmembrane helices in Sav1866, suggesting that its crystal structure might be physiologically relevant. We recently identified E314 as a residue important in proton conduction by LmrA. The predicted location of this residue at the interface between the two half-transporters in the Sav1866-based homodimer, within the inner leaflet of the phospholipid bilayer, provides a new structural basis for the role of E314 in LmrA-mediated transport.

ATP-driven MalK dimer closure and reopening and conformational changes of the "EAA" motifs are crucial for function of the maltose ATP-binding cassette transporter (MalFGK2)

We have investigated conformational changes of the purified maltose ATP-binding cassette transporter (MalFGK(2)) upon binding of ATP. The transport complex is composed of a heterodimer of the hydrophobic subunits MalF and MalG constituting the translocation pore and of a homodimer of MalK, representing the ATP-hydrolyzing subunit. Substrate is delivered to the transporter in complex with periplasmic maltose-binding protein (MalE). Cross-linking experiments with a variant containing an A85C mutation within the Q-loop of each MalK monomer indicated an ATP-induced shortening of the distance between both monomers. Cross-linking caused a substantial inhibition of MalE-maltose-stimulated ATPase activity. We further demonstrated that a mutation affecting the "catalytic carboxylate" (E159Q) locks the MalK dimer in the closed state, whereas a transporter containing the "ABC signature" mutation Q140K permanently resides in the resting state. Cross-linking experiments with variants containing the A85C mutation combined with cysteine substitutions in the conserved EAA motifs of MalF and MalG, respectively, revealed close proximity of these residues in the resting state. The formation of a MalK-MalG heterodimer remained unchanged upon the addition of ATP, indicating that MalG-EAA moves along with MalK during dimer closure. In contrast, the yield of MalK-MalF dimers was substantially reduced. This might be taken as further evidence for asymmetric functions of both EAA motifs. Cross-linking also caused inhibition of ATPase activity, suggesting that transporter function requires conformational changes of both EAA motifs. Together, our data support ATP-driven MalK dimer closure and reopening as crucial steps in the translocation cycle of the intact maltose transporter and are discussed with respect to a current model.

Mechanism of Substrate Sensing and Signal Transmission within an ABC Transporter

By translocating proteasomal degradation products into the endoplasmic reticulum for loading of major histocompatibility complex I molecules, the ABC transporter TAP plays a focal role in the adaptive immunity against infected or malignantly transformed cells. A key question regarding the transport mechanism is how the quality of the incoming peptide is detected and how this information is transmitted to the ATPase domains. To identify residues involved in this process, we evolved a Trojan horse strategy in which a small artificial protease is inserted into antigenic epitopes. After binding, the TAP backbone in contact is cleaved, allowing the peptide sensor site to be mapped by mass spectrometry. Within this sensor site, we identified residues that are essential for tight coupling of peptide binding and transport. This sensor and transmission interface is restructured during the ATP hydrolysis cycle, emphasizing its important function in the cross-talk between the transmembrane and the nucleotide-binding domains. This allocrite sensor may be similarly positioned in other members of the ABC exporter family.

Intracellular peptide transporters in human--compartmentalization of the "peptidome"

In the human genome, the five adenosine triphosphate (ATP)-binding cassette (ABC) half transporters ABCB2 (TAP1), ABCB3 (TAP2), ABCB9 (TAP-like), and in part, also ABCB8 and ABCB10 are closely related with regard to their structural and functional properties. Although targeted to different cellular compartments such as the endoplasmic reticulum (ER), lysosomes, and mitochondria, they are involved in intracellular peptide trafficking across membranes. The transporter associated with antigen processing (TAP1 and TAP2) constitute a key machinery in the major histocompatibility complex (MHC) class I-mediated cellular immune defense against infected or malignantly transformed cells. TAP translocates the cellular "peptidome" derived primarily from cytosolic proteasomal degradation into the ER lumen for presentation by MHC class I molecules. The homodimeric ABCB9 (TAP-like) complex located in lysosomal compartments shares structural and functional similarities to TAP; however, its biological role seems to be different from the MHC I antigen processing. ABCB8 and ABCB10 are targeted to the inner mitochondrial membrane. MDL1, the yeast homologue of ABCB10, is involved in the export of peptides derived from proteolysis of inner-membrane proteins into the intermembrane space. As such peptides are presented as minor histocompatibility antigens on the surface of mammalian cells, a physiological role of ABCB10 in the antigen processing can be accounted.

The ABC transporter BmrA from Bacillus subtilis is a functional dimer when in a detergent-solubilized state

BmrA from Bacillus subtilis is a half-size ABC (ATP-binding cassette) transporter involved in multidrug resistance. Although its supramolecular organization has been investigated after reconstitution in a lipid bilayer environment, and shows a dimeric and possibly a tetrameric form, the precise quaternary structure in a detergent-solubilized state has never been addressed. In the present study, BmrA was purified from Escherichia coli membranes using an optimized purification protocol and different detergents. Furthermore, the ATPase activity of BmrA and the quantity of bound lipids and detergent were determined, and the oligomeric state was analysed using SEC (size-exclusion chromatography) and analytical ultracentrifugation. The activity and the quaternary structure of BmrA appeared to be strongly influenced by the type and concentration of the detergent used. SEC data showed that BmrA could be purified in a functional form in 0.05 and 0.01% DDM (n-dodecyl-beta-D-maltoside) and was homogeneous and monodisperse with an R(s) (Stokes radius) of 5.6 nm that is compatible with a dimer structure. Sedimentation-velocity and equilibrium experiments unequivocally supported that BmrA purified in DDM is a dimer and excluded the presence of other oligomeric states. These observations, which are discussed in relation to results obtained in proteoliposomes, also constitute an important first step towards crystallographic studies of BmrA structure.