Functional reconstitution of human ABCC3 into proteoliposomes reveals a transport mechanism with positive cooperativity

Placing steroid hormones within the human ABCC3 transporter reveals a compatible amphiphilic substrate-binding pocket

The human ABC transporter ABCC3 (also known as MRP3) transports a wide spectrum of substrates, including endogenous metabolites and exogenous drugs. Accordingly, it participates in multiple physiological processes and is involved in diverse human diseases such as intrahepatic cholestasis of pregnancy, which is caused by the intracellular accumulation of bile acids and estrogens. Here, we report three cryogenic electron microscopy structures of ABCC3: in the apo-form and in complexed forms bound to either the conjugated sex hormones β-estradiol 17-(β-D-glucuronide) and dehydroepiandrosterone sulfate. For both hormones, the steroid nuclei that superimpose against each other occupy the hydrophobic center of the transport cavity, whereas the two conjugation groups are separated and fixed by the hydrophilic patches in two transmembrane domains. Structural analysis combined with site-directed mutagenesis and ATPase activity assays revealed that ABCC3 possesses an amphiphilic substrate-binding pocket able to hold either conjugated hormone in an asymmetric pattern. These data build on consensus features of the substrate-binding pocket of MRPs and provide a structural platform for the rational design of inhibitors.

A single power stroke by ATP binding drives substrate translocation in a heterodimeric ABC transporter

ATP-binding cassette (ABC) transporters constitute the largest family of primary active transporters, responsible for many physiological processes and human maladies. However, the mechanism how chemical energy of ATP facilitates translocation of chemically diverse compounds across membranes is poorly understood. Here, we advance the quantitative mechanistic understanding of the heterodimeric ABC transporter TmrAB, a functional homolog of the transporter associated with antigen processing (TAP) by single-turnover analyses at single-liposome resolution. We reveal that a single conformational switch by ATP binding drives unidirectional substrate translocation. After this power stroke, ATP hydrolysis and phosphate release launch the return to the resting state, which facilitates nucleotide exchange and a new round of substrate binding and translocation. In contrast to hitherto existing steady-state assays, our single-turnover approach uncovers the power stroke in substrate translocation and the tight chemomechanical coupling in these molecular machines.

MicroRNA-448 overexpression inhibits fibroblast proliferation and collagen synthesis and promotes cell apoptosis via targeting ABCC3 through the JNK signaling pathway

Idiopathic pulmonary fibrosis (IPF) is a condition that results in the progressive deterioration of lung function with poor prognosis. The current study is aimed at exploring how microRNA-448 (miR-448) targeting ABCC3 affects fibroblast proliferation, apoptosis, and collagen synthesis of mice with IPF via the Jun N-terminal kinase (JNK) signaling pathway. Bioinformatics and dual-luciferase polymerase chain reaction were used to predict the relationship of miR-448 and ABCC3. The expression of miR-448 and ABCC3 was detected in IPF tissues. Using IPF mouse models, lung fibroblasts for the experiments were treated with miR-448 mimic, miR-448 inhibitor, si-ABCC3, or SP600125 (inhibitor of JNK) to evaluate the cell proliferation and apoptosis in response to miR-448. Reverse transcription quantitative polymerase chain reaction and western blot analysis were used to identify the expression of miR-448, ABCC3, and the activation of the JNK signaling pathway. ABCC3 was targeted and downregulated by miR-448 based on bioinformatics prediction and dual-luciferase reporter gene assay. Additionally, miR-448 was found to be highly expressed in IPF lung tissues with low expression levels of ABCC3. In response to the treatment of miR-448 mimic or si-ABCC3, lung fibroblasts exhibited decreased cell proliferation and increased apoptotic rates, whereas the miR-448 inhibitor reversed the conditions. Notably, we also found that miR-448 mimic inhibited the JNK signaling pathway. In conclusion, by using miR-448 to target and downregulate ABCC3 to block the JNK signaling pathway in mice with IPF, we found an increase in fibroblast apoptosis, inhibited cell proliferation, and decreased collagen synthesis of fibroblasts.

MRP4/ABCC4 As a New Therapeutic Target: Meta-Analysis to Determine cAMP Binding Sites as a Tool for Drug Design

MRP4 transports multiple endogenous and exogenous substances and is critical not only for detoxification but also in the homeostasis of several signaling molecules. Its dysregulation has been reported in numerous pathological disorders, thus MRP4 appears as an attractive therapeutic target. However, the efficacy of MRP4 inhibitors is still controversial. The design of specific pharmacological agents with the ability to selectively modulate the activity of this transporter or modify its affinity to certain substrates represents a challenge in current medicine and chemical biology. The first step in the long process of drug rational design is to identify the therapeutic target and characterize the mechanism by which it affects the given pathology. In order to develop a pharmacological agent with high specific activity, the second step is to systematically study the structure of the target and identify all the possible binding sites. Using available homology models and mutagenesis assays, in this review we recapitulate the up-to-date knowledge about MRP structure and aligned amino acid sequences to identify the candidate MRP4 residues where cyclic nucleotides bind. We have also listed the most relevant MRP inhibitors studied to date, considering drug safety and specificity for MRP4 in particular. This meta-analysis platform may serve as a basis for the future development of inhibitors of MRP4 cAMP specific transport.

Proteoliposome-based model for screening inhibitors targeting histidine kinase AgrC

AgrC, as an integral membrane receptor protein with histidine kinase activity, is an important component of the agr quorum-sensing system of Staphylococcus aureus. AgrC acts as a sensor for the recognition of environmental signals and transduction of the signals into the cytoplasm. Therefore, AgrC is considered to be a compelling target for the development of novel quorum-sensing inhibitors. Here, we constructed a proteoliposome-based model for screening inhibitors targeting AgrC by incorporating AgrC into liposomes. We demonstrated that the dissolution state of the liposome was a critical factor in the reconstruction of the AgrC proteoliposome, in which AgrC maintained similar orientation and function as those in natural biological membranes. Two monomers, namely, rhein and aloeemodin, were successfully screened out as inhibitors targeting AgrC by the proteoliposome-based model from 14 traditional Chinese medicine monomers. The inhibitory effects of these compounds on the growth of suspended bacteria was dose dependent, and subinhibitory concentrations of these compounds significantly reduced the expression of three virulence factors (hla, clfA, and clpP), that are regulated by the agr system. The results preliminarily indicated that rhein and aloeemodin can inhibit the agr signaling pathway and also indirectly confirmed the feasibility and effectiveness of the AgrC proteoliposome as a drug screening model.

Moving the Cellular Peptidome by Transporters

Living matter is defined by metastability, implying a tightly balanced synthesis and turnover of cellular components. The first step of eukaryotic protein degradation via the ubiquitin-proteasome system (UPS) leads to peptides, which are subsequently degraded to single amino acids by an armada of proteases. A small fraction of peptides, however, escapes further cytosolic destruction and is transported by ATP-binding cassette (ABC) transporters into the endoplasmic reticulum (ER) and lysosomes. The ER-resident heterodimeric transporter associated with antigen processing (TAP) is a crucial component in adaptive immunity for the transport and loading of peptides onto major histocompatibility complex class I (MHC I) molecules. Although the function of the lysosomal resident homodimeric TAPL-like (TAPL) remains, until today, only loosely defined, an involvement in immune defense is anticipated since it is highly expressed in dendritic cells and macrophages. Here, we compare the gene organization and the function of single domains of both peptide transporters. We highlight the structural organization, the modes of substrate binding and translocation as well as physiological functions of both organellar transporters.

Identification of novel MRP3 inhibitors based on computational models and validation using an in vitro membrane vesicle assay

INTRODUCTION

Multidrug resistance-associated protein 3 (MRP3), an efflux transporter on the hepatic basolateral membrane, may function as a compensatory mechanism to prevent the accumulation of anionic substrates (e.g., bile acids) in hepatocytes. Inhibition of MRP3 may disrupt bile acid homeostasis and is one hypothesized risk factor for the development of drug-induced liver injury (DILI). Therefore, identifying potential MRP3 inhibitors could help mitigate the occurrence of DILI.

METHODS

Bayesian models were developed using MRP3 transporter inhibition data for 86 structurally diverse drugs. The compounds were split into training and test sets of 57 and 29 compounds, respectively, and six models were generated based on distinct inhibition thresholds and molecular fingerprint methods. The six Bayesian models were validated against the test set and the model with the highest accuracy was utilized for a virtual screen of 1470 FDA-approved drugs from DrugBank. Compounds that were predicted to be inhibitors were selected for in vitro validation. The ability of these compounds to inhibit MRP3 transport at a concentration of 100μM was measured in membrane vesicles derived from stably transfected MRP3-over-expressing HEK-293 cells with [³H]-estradiol-17β-d-glucuronide (E₂17G; 10μM; 5min uptake) as the probe substrate.

RESULTS

A predictive Bayesian model was developed with a sensitivity of 73% and specificity of 71% against the test set used to evaluate the six models. The area under the Receiver Operating Characteristic (ROC) curve was 0.710 against the test set. The final selected model was based on compounds that inhibited substrate transport by at least 50% compared to the negative control, and functional-class fingerprints (FCFP) with a circular diameter of six atoms, in addition to one-dimensional physicochemical properties. The in vitro screening of predicted inhibitors and non-inhibitors resulted in similar model performance with a sensitivity of 64% and specificity of 70%. The strongest inhibitors of MRP3-mediated E₂17G transport were fidaxomicin, suramin, and dronedarone. Kinetic assessment revealed that fidaxomicin was the most potent of these inhibitors (IC₅₀=1.83±0.46μM). Suramin and dronedarone exhibited IC₅₀ values of 3.33±0.41 and 47.44±4.41μM, respectively.

CONCLUSION

Bayesian models are a useful screening approach to identify potential inhibitors of transport proteins. Novel MRP3 inhibitors were identified by virtual screening using the selected Bayesian model, and MRP3 inhibition was confirmed by an in vitro transporter inhibition assay. Information generated using this modeling approach may be valuable in predicting the potential for DILI and/or MRP3-mediated drug-drug interactions.

The synthesis of recombinant membrane proteins in yeast for structural studies

Historically, recombinant membrane protein production has been a major challenge meaning that many fewer membrane protein structures have been published than those of soluble proteins. However, there has been a recent, almost exponential increase in the number of membrane protein structures being deposited in the Protein Data Bank. This suggests that empirical methods are now available that can ensure the required protein supply for these difficult targets. This review focuses on methods that are available for protein production in yeast, which is an important source of recombinant eukaryotic membrane proteins. We provide an overview of approaches to optimize the expression plasmid, host cell and culture conditions, as well as the extraction and purification of functional protein for crystallization trials in preparation for structural studies.

Interaction of α-Lipoic Acid with the Human Na+/Multivitamin Transporter (hSMVT)

The human Na(+)/multivitamin transporter (hSMVT) has been suggested to transport α-lipoic acid (LA), a potent antioxidant and anti-inflammatory agent used in therapeutic applications, e.g. in the treatment of diabetic neuropathy and Alzheimer disease. However, the molecular basis of the cellular delivery of LA and in particular the stereospecificity of the transport process are not well understood. Here, we expressed recombinant hSMVT in Pichia pastoris and used affinity chromatography to purify the detergent-solubilized protein followed by reconstitution of hSMVT in lipid bilayers. Using a combined approach encompassing radiolabeled LA transport and equilibrium binding studies in conjunction with the stabilized R-(+)- and S-(-)-enantiomers and the R,S-(+/-) racemic mixture of LA or lipoamide, we identified the biologically active form of LA, R-LA, to be the physiological substrate of hSMVT. Interaction of R-LA with hSMVT is strictly dependent on Na(+). Under equilibrium conditions, hSMVT can simultaneously bind ~2 molecules of R-LA in a biphasic binding isotherm with dissociation constants (Kd) of 0.9 and 7.4 μm. Transport of R-LA in the oocyte and reconstituted system is exclusively dependent on Na(+) and exhibits an affinity of ~3 μm. Measuring transport with known amounts of protein in proteoliposomes containing hSMVT in outside-out orientation yielded a catalytic turnover number (kcat) of about 1 s(-1), a value that is well in agreement with other Na(+)-coupled transporters. Our data suggest that hSMVT-mediated transport is highly specific for R-LA at our tested concentration range, a finding with wide ramifications for the use of LA in therapeutic applications.

Single liposome analysis of peptide translocation by the ABC transporter TAPL

ATP-binding cassette (ABC) transporters use ATP to drive solute transport across biological membranes. Members of this superfamily have crucial roles in cell physiology, and some of the transporters are linked to severe diseases. However, understanding of the transport mechanism, especially of human ABC exporters, is scarce. We reconstituted the human lysosomal polypeptide ABC transporter TAPL, expressed in Pichia pastoris, into lipid vesicles (liposomes) and performed explicit transport measurements. We analyzed solute transport at the single liposome level by monitoring the coincident fluorescence of solutes and proteoliposomes in the focal volume of a confocal microscope. We determined a turnover number of eight peptides per minute, which is two orders of magnitude higher than previously estimated from macroscopic measurements. Moreover, we show that TAPL translocates peptides against a large concentration gradient. Maximal filling is not limited by an electrochemical gradient but by trans-inhibition. Countertransport and reversibility studies demonstrate that peptide translocation is a strictly unidirectional process. Altogether, these data are included in a refined model of solute transport by ABC exporters.

Clinical and molecular description of a 17q21.33 microduplication in a girl with severe kyphoscoliosis and developmental delay

High proportion of disease-associated copy number variant maps to chromosome 17. Genomic studies have provided an insight into its complex genomic structure such as relative abundance of segmental duplication and intercepted repetitive elements. 17q21.31, 17q11.2 and 17q12 loci are well known on this chromosome and are associated with microdeletion and microduplication syndrome. No syndrome associated with 17q21.33 locus have been described. We report clinical, cytogenetic and molecular investigations of a 13 years-old girl admitted for evaluation of microcephaly, scoliosis, skeletal defects and learning difficulties. We carried out detailed analysis of the clinical phenotype of this patient and investigated the genetic basis using Agilent 180K Array Comparative Genomic Hybridization. We identified a ∼0.9 Mb de novo microduplication on chromosome 17q21.33. Four genes, COL1A1, SGCA, PPP1R9B and CHAD located within the duplicated region are possible candidates for clinical features present in our patients. Gene expression studies by real-time RT-PCR assay only showed an overexpression of SGCA (P < 0.01), a component of the dystrophin glycoprotein complex. Defect of SGCA was previously shown to lead to severe childhood autosomal recessive muscular dystrophy (LGMD2D) which result in progressive muscle weakness and can also be associated with hyperlordosis or scoliosis. Further cases with similar duplications are expected to be diagnosed. This will contribute to the delineation of this potential new microduplication syndrome and to improve genetic counseling.

Real-time monitoring of membrane-protein reconstitution by isothermal titration calorimetry

Phase diagrams offer a wealth of thermodynamic information on aqueous mixtures of bilayer-forming lipids and micelle-forming detergents, providing a straightforward means of monitoring and adjusting the supramolecular state of such systems. However, equilibrium phase diagrams are of very limited use for the reconstitution of membrane proteins because of the occurrence of irreversible, unproductive processes such as aggregation and precipitation that compete with productive reconstitution. Here, we exemplify this by dissecting the effects of the K⁺ channel KcsA on the process of bilayer self-assembly in a mixture of Escherichia coli polar lipid extract and the nonionic detergent octyl-β-d-glucopyranoside. Even at starting concentrations in the low micromolar range, KcsA has a tremendous impact on the supramolecular organization of the system, shifting the critical lipid/detergent ratios at the onset and completion of vesicle formation by more than 2-fold. Thus, equilibrium phase diagrams obtained for protein-free lipid/detergent mixtures would be misleading when used to guide the reconstitution process. To address this issue, we demonstrate that, even under such nonequilibrium conditions, high-sensitivity isothermal titration calorimetry can be exploited to monitor the progress of membrane-protein reconstitution in real time, in a noninvasive manner, and at high resolution to yield functional proteoliposomes with a narrow size distribution for further downstream applications.

Functional reconstitution of Staphylococcus aureus truncated AgrC histidine kinase in a model membrane system

The integral membrane protein AgrC is a histidine kinase whose sensor domains interact with an autoinducing peptide, resulting in a series of downstream responses. In this study, truncated AgrCTM5-6C and AgrCTM5-6C-GFP with GFP as a reporter gene were produced using a bacterial system. Purified AgrCTM5-6C and AgrCTM5-6C-GFP were reconstituted into liposomes by a detergent-mediated method. To achieve high-yield protein incorporation, we investigated the effect of different detergents on protein reconstitution efficiency. The highest incorporation was found with N,N-dimethyldode-cylamine N-oxide during complete liposome solubilization, which resulted in a yield of 85±5%. The COOH-terminus of the protein AgrCTM5-6C was almost exclusively oriented towards the inside of the vesicles. AgrCTM5-6C in proteoliposomes exhibited approximately a 6-fold increase in constitutive activity compared with AgrCTM5-6C in detergent micelles. The reconstitution of AgrCTM5-6C or AgrCTM5-6C-GFP was characterized using dynamic light scattering, fluorescence microscopy, and transmission electron microscopy. Based on the results, the optimal conditions for protein incorporation were defined. These findings contribute to the study of membrane protein structure and function in vitro using a reconstitution system.

Improving the stability and function of purified ABCB1 and ABCA4: the influence of membrane lipids

ATP Binding Cassette (ABC) transporters play prominent roles in numerous cellular processes and many have been implicated in human diseases. Unfortunately, detailed mechanistic information on the majority of ABC transporters has not yet been elucidated. The slow rate of progress of molecular and high resolution structural studies may be attributed to the difficulty in the investigation of integral membrane proteins. These difficulties include the expression of functional, non-aggregated protein in heterologous systems. Furthermore, the extraction of membrane proteins from source material remains a major bottle-neck in the process since there are relatively few guidelines for selection of an appropriate detergent to achieve optimal extraction. Whilst affinity tag strategies have simplified the purification of membrane proteins; many challenges remain. For example, the chromatographic process and associated steps can rapidly lead to functional inactivation, random aggregation, or even precipitation of the target protein. Furthermore, optimisation of high yield and purity, does not guarantee successful structure determination. Based on this series of potential issues, any investigation into structure-function of membrane proteins requires a systematic evaluation of preparation quality. In particular, the evaluation should focus on function, homogeneity and mono-dispersity. The present investigation provides a detailed assessment of the quality of purified ATP Binding Cassette (ABC) transporters; namely ABCB1 (P-gp) and ABCA4 (ABCR). A number of suggestions are provided to facilitate the production of functional, homogeneous and mono-disperse preparations using the insect cell expression system. Finally, the ABCA4 samples have been used to provide structural insights into this essential photo-receptor cell protein.

Efficient purification and reconstitution of ATP binding cassette transporter B6 (ABCB6) for functional and structural studies

The mitochondrial ATP binding cassette transporter ABCB6 has been associated with a broad range of physiological functions, including growth and development, therapy-related drug resistance, and the new blood group system Langereis. ABCB6 has been proposed to regulate heme synthesis by shuttling coproporphyrinogen III from the cytoplasm into the mitochondria. However, direct functional information of the transport complex is not known. To understand the role of ABCB6 in mitochondrial transport, we developed an in vitro system with pure and active protein. ABCB6 overexpressed in HEK293 cells was solubilized from mitochondrial membranes and purified to homogeneity. Purified ABCB6 showed a high binding affinity for MgATP (Kd = 0.18 μM) and an ATPase activity with a Km of 0.99 mM. Reconstitution of ABCB6 into liposomes allowed biochemical characterization of the ATPase including (i) substrate-stimulated ATPase activity, (ii) transport kinetics of its proposed endogenous substrate coproporphyrinogen III, and (iii) transport kinetics of substrates identified using a high throughput screening assay. Mutagenesis of the conserved lysine to alanine (K629A) in the Walker A motif abolished ATP hydrolysis and substrate transport. These results suggest a direct interaction between mitochondrial ABCB6 and its transport substrates that is critical for the activity of the transporter. Furthermore, the simple immunoaffinity purification of ABCB6 to near homogeneity and efficient reconstitution of ABCB6 into liposomes might provide the basis for future studies on the structure/function of ABCB6.

Structures of ABCB10, a human ATP-binding cassette transporter in apo- and nucleotide-bound states

ABCB10 is one of the three ATP-binding cassette (ABC) transporters found in the inner membrane of mitochondria. In mammals ABCB10 is essential for erythropoiesis, and for protection of mitochondria against oxidative stress. ABCB10 is therefore a potential therapeutic target for diseases in which increased mitochondrial reactive oxygen species production and oxidative stress play a major role. The crystal structure of apo-ABCB10 shows a classic exporter fold ABC transporter structure, in an open-inwards conformation, ready to bind the substrate or nucleotide from the inner mitochondrial matrix or membrane. Unexpectedly, however, ABCB10 adopts an open-inwards conformation when complexed with nonhydrolysable ATP analogs, in contrast to other transporter structures which adopt an open-outwards conformation in complex with ATP. The three complexes of ABCB10/ATP analogs reported here showed varying degrees of opening of the transport substrate binding site, indicating that in this conformation there is some flexibility between the two halves of the protein. These structures suggest that the observed plasticity, together with a portal between two helices in the transmembrane region of ABCB10, assist transport substrate entry into the substrate binding cavity. These structures indicate that ABC transporters may exist in an open-inwards conformation when nucleotide is bound. We discuss ways in which this observation can be aligned with the current views on mechanisms of ABC transporters.

Tethered proteoliposomes containing human ABC transporter MRP3: new perspectives for biosensor application based on transmembrane proteins

While transmembrane proteins and transporters comprise one of the largest protein families, their use in biosensors like biochips or lab-on-a-chip devices has so far been limited by their demanding requirements of a stable and compartmentalized lipid environment. A possible remedy lies in the tethering of proteoliposomes containing the reconstituted transmembrane protein to the biosensoric surface. As a proof of concept, we reconstituted the human ABC transporter MRP3 into biotinylated proteoliposomes and tethered those to a gold surface coated with streptavidin on a biotinylated self-assembled thiol monolayer. The tethering process was investigated by quartz crystal microbalance with dissipation monitoring. The final assembly of tethered proteoliposomes exhibited biological activity in terms of drug-stimulated ATP hydrolysis and substrate translocation. The presented facile immobilization approach can be easily extended to other transmembrane proteins as it does not require any modification of the protein and will open up transmembrane proteins for future application in biosensors.

Substrate translocation and stimulated ATP hydrolysis of human ABC transporter MRP3 show positive cooperativity and are half-coupled

ABC transporters are involved in countless processes from lipid excretion over cellular detoxification to multidrug resistance of cancer cells. The latter is especially conferred by the ABCC subfamily also called multidrug resistance-associated proteins (MRPs) that excrete a variety of amphipathics including anticancer drugs by ATP-dependent transport. As the mechanisms of substrate translocation and ATP hydrolysis are still unclear for MRPs, we investigated the kinetics of both processes with focus on cooperativity and coupling between ATPase activity and substrate transport using purified MRP3 in proteoliposomes. Although the ATP-dependent uptake of amphipathics and the hydrophilic 5(6)-carboxy-2'-7'-dichlorofluorescein (CDCF) into the lumen of proteoliposomes showed affinity constants similar to those reported for cell-based assays, the maximal uptake rates were up to 250 times higher. Moreover, all substrates showed cooperative interactions of two subunits. Upon stimulation with amphipathics, ATPase activity of MRP3 increased from 80nmol/(mgmin) to 180nmol/(mgmin) showing positive cooperativity with a Hill coefficient of 2. While Hill coefficient and maximal ATPase activity were found to be substrate independent, the affinity constants are characteristic for a given substrate and correspond to the value for transport. Therefore, cooperative interactions of the two nucleotide binding domains (NBDs) in MRP3 are mediated by substrate binding to the transmembrane domains (TMDs). In contrast to amphipathic substrates, CDCF did not stimulate ATPase activity despite being transported in an ATP-dependent manner. This indicates that ATP hydrolysis and substrate translocation are half-coupled in MRP3 as CDCF shuttles on a basal TMD activity resulting from the basal ATPase activity.

Influence of detergents on the activity of the ABC transporter LmrA

The ABC transporter LmrA from Lactococcus lactis has been intensively studied and a role in multidrug resistance was proposed. Here, we performed a comprehensive detergent screen to analyze the impact of detergents for a successful solubilization, purification and retention of functional properties of this ABC transporter. Our screen revealed the preference of LmrA for zwitterionic detergents. In detergent solution, LmrA purified with FC-16 was highly active with respect to ATPase activity, which could be stimulated by a substrate (rhodamine 123) of LmrA. Both, high ATPase activity and substrate stimulation were not detected for LmrA solubilized in DDM. Interestingly, reconstituted LmrA showed an opposite behavior, with a high basal ATPase activity and stimulation by rhodamine 123 for a DDM-reconstituted, but only low ATPase activity and no substrate stimulation for a FC-16 reconstituted sample.

Optimized purification of a heterodimeric ABC transporter in a highly stable form amenable to 2-D crystallization

Optimized protocols for achieving high-yield expression, purification and reconstitution of membrane proteins are required to study their structure and function. We previously reported high-level expression in Escherichia coli of active BmrC and BmrD proteins from Bacillus subtilis, previously named YheI and YheH. These proteins are half-transporters which belong to the ABC (ATP-Binding Cassette) superfamily and associate in vivo to form a functional transporter able to efflux drugs. In this report, high-yield purification and functional reconstitution were achieved for the heterodimer BmrC/BmrD. In contrast to other detergents more efficient for solubilizing the transporter, dodecyl-ß-D-maltoside (DDM) maintained it in a drug-sensitive and vanadate-sensitive ATPase-competent state after purification by affinity chromatography. High amounts of pure proteins were obtained which were shown either by analytical ultracentrifugation or gel filtration to form a monodisperse heterodimer in solution, which was notably stable for more than one month at 4°C. Functional reconstitution using different lipid compositions induced an 8-fold increase of the ATPase activity (k_cat∼5 s⁻¹). We further validated that the quality of the purified BmrC/BmrD heterodimer is suitable for structural analyses, as its reconstitution at high protein densities led to the formation of 2-D crystals. Electron microscopy of negatively stained crystals allowed the calculation of a projection map at 20 Å resolution revealing that BmrC/BmrD might assemble into oligomers in a lipidic environment.

Nucleotide binding to the human multidrug resistance protein 3, MRP3

We used a spin-labeled ATP analog, SL-ATP, to study nucleotide binding to highly purified human multidrug resistance protein 3, MRP3, which had been expressed in the yeast Pichia pastoris. SL-ATP was shown to be a good substrate analog and is hydrolyzed by MRP3 at about 10% of the Vmax for normal ATP. ESR titrations showed that 2 mol of SL-ATP readily bound per mole of MRP3 with a dissociation constant of about 100 microM in the presence of Mg(2+) ions. The binding curve was easily fitted for a hyperbolic binding relationship. SL-ATP also bound readily to MRP3 in the absence of divalent ions and presence of EDTA. The resulting binding curve, however, could not be satisfactorily fitted using the equation for hyperbola. Analysis showed that a good fit was only obtained with the Hill equation using a Hill coefficient of 4 or close to 4. Lower Hill coefficients resulted in lower goodness of the fit. Such cooperative binding may be explained by a dimerization event triggered in the absence of divalent ions and a close communication of nucleotide binding sites of the interacting dimers. These findings may be of great importance for the overall mechanism and regulation of multidrug resistance proteins.

Reconstitution in liposome bilayers enhances nucleotide binding affinity and ATP-specificity of TrwB conjugative coupling protein

Bacterial conjugative systems code for an essential membrane protein that couples the relaxosome to the DNA transport apparatus, called type IV coupling protein (T4CP). TrwB is the T4CP of the conjugative plasmid R388. In earlier work we found that this protein, purified in the presence of detergents, binds preferentially purine nucleotides trisphosphate. In contrast a soluble truncated mutant TrwBΔN70 binds uniformly all nucleotides tested. In this work, TrwB has been successfully reconstituted into liposomes. The non-membranous portion of the protein is almost exclusively oriented towards the outside of the vesicles. Functional analysis of TrwB proteoliposomes demonstrates that when the protein is inserted into the lipid bilayer the affinity for adenine and guanine nucleotides is enhanced as compared to that of the protein purified in detergent or to the soluble deletion mutant, TrwBΔN70. The protein specificity for adenine nucleotides is also increased. No ATPase activity has been found in TrwB reconstituted in proteoliposomes. This result suggests that the N-terminal transmembrane segment of this T4CP interferes with its ATPase activity and can be taken to imply that the TrwB transmembrane domain plays a regulatory role in its biological activity.