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

Functionality of membrane proteins overexpressed and purified from E. coli is highly dependent upon the strain

Overexpression of correctly folded membrane proteins is a fundamental prerequisite for functional and structural studies. One of the most commonly used expression systems for the production of membrane proteins is Escherichia coli. While misfolded proteins typically aggregate and form inclusions bodies, membrane proteins that are addressed to the membrane and extractable by detergents are generally assumed to be properly folded. Accordingly, GFP fusion strategy is often used as a fluorescent proxy to monitor their expression and folding quality. Here we investigated the functionality of two different multidrug ABC transporters, the homodimer BmrA from Bacillus subtilis and the heterodimer PatA/PatB from Streptococcus pneumoniae, when produced in several E. coli strains with T7 expression system. Strikingly, while strong expression in the membrane of several strains could be achieved, we observed drastic differences in the functionality of these proteins. Moreover, we observed a general trend in which mild detergents mainly extract the population of active transporters, whereas a harsher detergent like Fos-choline 12 could solubilize transporters irrespective of their functionality. Our results suggest that the amount of T7 RNA polymerase transcripts may indirectly but notably impact the structure and activity of overexpressed membrane proteins, and advise caution when using GFP fusion strategy.

Function and Role of ATP-Binding Cassette Transporters as Receptors for 3D-Cry Toxins

When ABC transporter family C2 (ABCC2) and ABC transporter family B1 (ABCB1) were heterologously expressed in non-susceptible cultured cells, the cells swelled in response to Cry1A and Cry3 toxins, respectively. Consistent with the notion that 3D-Cry toxins form cation-permeable pores, Bombyx mori ABCC2 (BmABCC2) facilitated cation-permeable pore formation by Cry1A when expressed in Xenopus oocytes. Furthermore, BmABCC2 had a high binding affinity (KD) to Cry1Aa of 3.1 × 10-10 M. These findings suggest that ABC transporters, including ABCC2 and ABCB1, are functional receptors for 3D-Cry toxins. In addition, the Cry2 toxins most distant from Cry1A toxins on the phylogenetic tree used ABC transporter A2 as a receptor. These data suggest that 3D-Cry toxins use ABC transporters as receptors. In terms of inducing cell swelling, ABCC2 has greater activity than cadherin-like receptor. The pore opening of ABC transporters was hypothesized to be linked to their receptor function, but this was repudiated by experiments using mutants deficient in export activity. The synergistic relationship between ABCC2 and cadherin-like receptor explains their ability to cause resistance in one species of insect.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid–protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid–protein interactions, and to link lipid–protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid–protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid–protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid–protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid–protein interactions.

Activity of the purified plant ABC transporter NtPDR1 is stimulated by diterpenes and sesquiterpenes involved in constitutive and induced defenses

Within the plant ATP-binding cassette transporter family, pleiotropic drug resistance (PDR) transporters play essential functions, such as in hormone transport or defense against biotic and abiotic stresses. NtPDR1 from Nicotiana tabacum has been shown to be involved in the constitutive defense against pathogens through the secretion of toxic cyclic diterpenes, such as the antimicrobial substrates cembrene and sclareol from the leaf hairs (trichomes). However, direct evidence of an interaction between NtPDR1 and terpenes is lacking. Here, we stably expressed NtPDR1 in N. tabacum BY-2 suspension cells. NtPDR1 was purified as an active monomer glycosylated at a single site in the third external loop. NtPDR1 reconstitution in proteoliposomes stimulated its basal ATPase activity from 21 to 38 nmol of Pi·mg-1·min-1, and ATPase activity was further stimulated by the NtPDR1 substrates cembrene and sclareol, providing direct evidence of an interaction between NtPDR1 and its two substrates. Interestingly, NtPDR1 was also stimulated by capsidiol, a sesquiterpene produced by N. tabacum upon pathogen attack. We also monitored the transcriptional activity from the NtPDR1 promoter in situ with a reporter gene and found that, although NtPDR1 expression was limited to trichomes under normal conditions, addition of methyl jasmonate, a biotic stress hormone, induced expression in all leaf tissues. This finding indicated that NtPDR1 is involved not only in constitutive but also in induced plant defenses. In conclusion, we provide direct evidence of an interaction between the NtPDR1 transporter and its substrates and that NtPDR1 transports compounds involved in both constitutive (diterpenes) and induced (sesquiterpenes) plant defenses.

Direct Spectroscopic Detection of ATP Turnover Reveals Mechanistic Divergence of ABC Exporters

We have applied high-field (W-band) pulse electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) to characterize the coordination sphere of the Mn²⁺ co-factor in the nucleotide binding sites (NBSs) of ABC transporters. MsbA and BmrCD are two efflux transporters hypothesized to represent divergent catalytic mechanisms. Our results reveal distinct coordination of Mn²⁺ to ATP and transporter residues in the consensus and degenerate NBSs of BmrCD. In contrast, the coordination of Mn²⁺ at the two NBSs of MsbA is similar, which provides a mechanistic rationale for its higher rate constant of ATP hydrolysis relative to BmrCD. Direct detection of vanadate ion, trapped in a high-energy post-hydrolysis intermediate, further supports the notion of asymmetric hydrolysis by the two NBSs of BmrCD. The integrated spectroscopic approach presented here, which link energy input to conformational dynamics, can be applied to a variety of systems powered by ATP turnover.

The putative drug efflux systems of the Bacillus cereus group

The Bacillus cereus group of bacteria includes seven closely related species, three of which, B. anthracis, B. cereus and B. thuringiensis, are pathogens of humans, animals and/or insects. Preliminary investigations into the transport capabilities of different bacterial lineages suggested that genes encoding putative efflux systems were unusually abundant in the B. cereus group compared to other bacteria. To explore the drug efflux potential of the B. cereus group all putative efflux systems were identified in the genomes of prototypical strains of B. cereus, B. anthracis and B. thuringiensis using our Transporter Automated Annotation Pipeline. More than 90 putative drug efflux systems were found within each of these strains, accounting for up to 2.7% of their protein coding potential. Comparative analyses demonstrated that the efflux systems are highly conserved between these species; 70-80% of the putative efflux pumps were shared between all three strains studied. Furthermore, 82% of the putative efflux system proteins encoded by the prototypical B. cereus strain ATCC 14579 (type strain) were found to be conserved in at least 80% of 169 B. cereus group strains that have high quality genome sequences available. However, only a handful of these efflux pumps have been functionally characterized. Deletion of individual efflux pump genes from B. cereus typically had little impact to drug resistance phenotypes or the general fitness of the strains, possibly because of the large numbers of alternative efflux systems that may have overlapping substrate specificities. Therefore, to gain insight into the possible transport functions of efflux systems in B. cereus, we undertook large-scale qRT-PCR analyses of efflux pump gene expression following drug shocks and other stress treatments. Clustering of gene expression changes identified several groups of similarly regulated systems that may have overlapping drug resistance functions. In this article we review current knowledge of the small molecule efflux pumps encoded by the B. cereus group and suggest the likely functions of numerous uncharacterised pumps.

Rae1/YacP, a new endoribonuclease involved in ribosome-dependent mRNA decay in Bacillus subtilis

The PIN domain plays a central role in cellular RNA biology and is involved in processes as diverse as rRNA maturation, mRNA decay and telomerase function. Here, we solve the crystal structure of the Rae1 (YacP) protein of Bacillus subtilis, a founding member of the NYN (Nedd4-BP1/YacP nuclease) subfamily of PIN domain proteins, and identify potential substrates in vivo Unexpectedly, degradation of a characterised target mRNA was completely dependent on both its translation and reading frame. We provide evidence that Rae1 associates with the B. subtilis ribosome and cleaves between specific codons of this mRNA in vivo Critically, we also demonstrate translation-dependent Rae1 cleavage of this substrate in a purified translation assay in vitro Multiple lines of evidence converge to suggest that Rae1 is an A-site endoribonuclease. We present a docking model of Rae1 bound to the B. subtilis ribosomal A-site that is consistent with this hypothesis and show that Rae1 cleaves optimally immediately upstream of a lysine codon (AAA or AAG) in vivo.

High-efficient production and biophysical characterisation of nicastrin and its interaction with APPC100

Nicastrin, the largest member among the four components of the γ-secretase complex, has been identified to be the substrate recognizer for the proteolytic activity of the complex. Here we report that full-length human nicastrin (hNCT) can be obtained by heterologous expression in E. coli. Milligram quantities of the target protein are purified in a two-step purification protocol using affinity chromatography followed by SEC. The FOS-choline 14 purified tetrameric hNCT exhibits a proper folding with 31% α-helix and 23% β-sheet content. Thermal stability studies reveal stable secondary and tertiary structure of the detergent purified hNCT. A physical interaction between nicastrin and the γ-secretase substrate APPC100 confirmed the functionality of hNCT as a substrate recognizer.

Probing allocrite preferences of 2 naturally occurring variants of the wheat LR34 ABC transporter

For almost a century, the wheat Lr34 gene has conferred durable resistance against fungal rust diseases. While sequence homology predicts a putative ATP binding cassette transporter, the molecules that are transported (allocrites) by the encoded LR34 variants, and any associated mechanism of resistance, remain enigmatic. Here, the in vitro transport characteristics of 2 naturally occurring Lr34 variants (that differ in their ability to mediate disease resistance; Lr34sus and Lr34res) are investigated. Initially, a method to express and purify recombinant LR34Sus and LR34Res pseudo half-molecules from Saccharomyces cerevisiae, is described. Subsequently, a semi-targeted chlorophyll catabolite (CC) extraction from Lr34res-expressing wheat plants was performed based on previous reports highlighting increased accumulation of CCs in Lr34res-expressing flag leaves. Following partial biochemical characterization, this extract was applied to an LR34 in vitro proteoliposome transport assay. Mass spectroscopic analyses of transported metabolites revealed that LR34Sus imported a wheat metabolite of 618 Da and that the LR34Res transporter did not. While the identity of the LR34Sus transported metabolite remains to be confirmed and any allocrites of LR34Res remain to be detected, this work demonstrates that these variants have different allocrite preferences, a finding that may be relevant to the mechanism of disease resistance.

The Heterodimeric ABC Transporter EfrCD Mediates Multidrug Efflux in Enterococcus faecalis

Nosocomial infections with Enterococcus faecalis are an emerging health problem. However, drug efflux pumps contributing to intrinsic drug resistance are poorly studied in this Gram-positive pathogen. In this study, we functionally investigated seven heterodimeric ABC transporters of E. faecalis that are annotated as drug efflux pumps. Deletion of ef0789-ef0790 on the chromosome of E. faecalis resulted in increased susceptibility to daunorubicin, doxorubicin, ethidium, and Hoechst 33342, and the corresponding transporter was named EfrCD. Unexpectedly, the previously described heterodimeric multidrug ABC transporter EfrAB contributes marginally to drug efflux in the endogenous context of E. faecalis In contrast, heterologous expression in Lactococcus lactis revealed that EfrAB, EfrCD, and the product of ef2226-ef2227 (EfrEF) mediate the efflux of fluorescent substrates and confer resistance to multiple dyes and drugs, including fluoroquinolones. Four of seven transporters failed to exhibit drug efflux activity for the set of drugs and dyes tested, even upon overexpression in L. lactis Since all seven transporters were purified as heterodimers after overexpression in L. lactis, a lack of drug efflux activity is not attributed to poor expression or protein aggregation. Reconstitution of the purified multidrug transporters EfrAB, EfrCD, and EfrEF in proteoliposomes revealed functional coupling between ATP hydrolysis and drug binding. Our analysis creates an experimental basis for the accurate prediction of drug efflux transporters and indicates that many annotated multidrug efflux pumps might be incapable of drug transport and thus might fulfill other physiological functions in the cell.

Durable proteo-hybrid vesicles for the extended functional lifetime of membrane proteins in bionanotechnology

Significant enhancement of membrane protein functional durability is demonstrated when reconstituted in hybrid lipid–block copolymer vesicles compared to conventional proteoliposomes.

The full capabilities of membrane proteins in bionanotechnology can only be realised through improvements in their reconstitution environments that combine biocompatibility to support function and durability for long term stability. We demonstrate that hybrid vesicles composed of natural phospholipids and synthetic diblock copolymers have the potential to achieve these criteria.

Stubborn contaminants: influence of detergents on the purity of the multidrug ABC transporter BmrA

Despite the growing interest in membrane proteins, their crystallization remains a major challenge. In the course of a crystallographic study on the multidrug ATP-binding cassette transporter BmrA, mass spectral analyses on samples purified with six selected detergents revealed unexpected protein contamination visible for the most part on overloaded SDS-PAGE. A major contamination from the outer membrane protein OmpF was detected in purifications with Foscholine 12 (FC12) but not with Lauryldimethylamine-N-oxide (LDAO) or any of the maltose-based detergents. Consequently, in the FC12 purified BmrA, OmpF easily crystallized over BmrA in a new space group, and whose structure is reported here. We therefore devised an optimized protocol to eliminate OmpF during the FC12 purification of BmrA. On the other hand, an additional band visible at ∼110 kDa was detected in all samples purified with the maltose-based detergents. It contained AcrB that crystallized over BmrA despite its trace amounts. Highly pure BmrA preparations could be obtained using either a ΔacrAB E. coli strain and n-dodecyl-β-D-maltopyranoside, or a classical E. coli strain and lauryl maltose neopentyl glycol for the overexpression and purification, respectively. Overall our results urge to incorporate a proteomics-based purity analysis into quality control checks prior to commencing crystallization assays of membrane proteins that are notoriously arduous to crystallize. Moreover, the strategies developed here to selectively eliminate obstinate contaminants should be applicable to the purification of other membrane proteins overexpressed in E. coli.

The multidrug ABC transporter BmrC/BmrD of Bacillus subtilis is regulated via a ribosome-mediated transcriptional attenuation mechanism

Expression of particular drug transporters in response to antibiotic pressure is a critical element in the development of bacterial multidrug resistance, and represents a serious concern for human health. To obtain a better understanding of underlying regulatory mechanisms, we have dissected the transcriptional activation of the ATP-binding cassette (ABC) transporter BmrC/BmrD of the Gram-positive model bacterium Bacillus subtilis. By using promoter-GFP fusions and live cell array technology, we demonstrate a temporally controlled transcriptional activation of the bmrCD genes in response to antibiotics that target protein synthesis. Intriguingly, bmrCD expression only occurs during the late-exponential and stationary growth stages, irrespective of the timing of the antibiotic challenge. We show that this is due to tight transcriptional control by the transition state regulator AbrB. Moreover, our results show that the bmrCD genes are co-transcribed with bmrB (yheJ), a small open reading frame immediately upstream of bmrC that harbors three alternative stem-loop structures. These stem-loops are apparently crucial for antibiotic-induced bmrCD transcription. Importantly, the antibiotic-induced bmrCD expression requires translation of bmrB, which implies that BmrB serves as a regulatory leader peptide. Altogether, we demonstrate for the first time that a ribosome-mediated transcriptional attenuation mechanism can control the expression of a multidrug ABC transporter.

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.

Conformational dynamics of the nucleotide binding domains and the power stroke of a heterodimeric ABC transporter

Multidrug ATP binding cassette (ABC) exporters are ubiquitous ABC transporters that extrude cytotoxic molecules across cell membranes. Despite recent progress in structure determination of these transporters, the conformational motion that transduces the energy of ATP hydrolysis to the work of substrate translocation remains undefined. Here, we have investigated the conformational cycle of BmrCD, a representative of the heterodimer family of ABC exporters that have an intrinsically impaired nucleotide binding site. We measured distances between pairs of spin labels monitoring the movement of the nucleotide binding (NBD) and transmembrane domains (TMD). The results expose previously unobserved structural intermediates of the NBDs arising from asymmetric configuration of catalytically inequivalent nucleotide binding sites. The two-state transition of the TMD, from an inward- to an outward-facing conformation, is driven exclusively by ATP hydrolysis. These findings provide direct evidence of divergence in the mechanism of ABC exporters.DOI: http://dx.doi.org/10.7554/eLife.02740.001.

Characterization of hydrophobic peptides in the presence of detergent by photoionization mass spectrometry

The characterization of membrane proteins is still challenging. The major issue is the high hydrophobicity of membrane proteins that necessitates the use of detergents for their extraction and solubilization. The very poor compatibility of mass spectrometry with detergents remains a tremendous obstacle in studies of membrane proteins. Here, we investigated the potential of atmospheric pressure photoionization (APPI) for mass spectrometry study of membrane proteins. This work was focused on the tetraspanin CD9 and the multidrug transporter BmrA. A set of peptides from CD9, exhibiting a broad range of hydropathicity, was investigated using APPI as compared to electrospray ionization (ESI). Mass spectrometry experiments revealed that the most hydrophobic peptides were hardly ionized by ESI whereas all peptides, including the highly hydrophobic one that corresponds to the full sequence of the first transmembrane domain of CD9, were easily ionized by APPI. The native protein BmrA purified in the presence of the non-ionic detergent beta-D-dodecyl maltoside (DDM) was digested in-solution using trypsin. The resulting peptides were investigated by flow injection analysis of the mixture followed by mass spectrometry. Upon ESI, only detergent ions were detected and the ionic signals from the peptides were totally suppressed. In contrast, APPI allowed many peptides distributed along the sequence of the protein to be detected. Furthermore, the parent ion corresponding to the first transmembrane domain of the protein BmrA was detected under APPI conditions. Careful examination of the APPI mass spectrum revealed a-, b-, c- and y- fragment ions generated by in-source fragmentation. Those fragment ions allowed unambiguous structural characterization of the transmembrane domain. In conclusion, APPI-MS appears as a versatile method allowing the ionization and fragmentation of hydrophobic peptides in the presence of detergent.

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.

Recombinant expression, detergent solubilisation and purification of insect odorant receptor subunits

Insect odorant receptors (ORs) are seven transmembrane domain proteins that comprise a novel family of ligand-gated non-selective cation channels. The functional channel is made up of an odour activated ligand-binding OR and the OR co-receptor, Orco. However, the structure, stoichiometry and mechanism of activation of the receptor complex are not well understood. Here we demonstrate that baculovirus-mediated Sf9 cell expression and wheat germ cell-free expression, but not Escherichia coli cell-based or cell-free expression, can be used successfully to over-express a selection of insect ORs. From a panel of 19 detergents, 1%w/v Zwittergent 3-16 was able to solubilise five Drosophila melanogaster ORs produced from both eukaryotic expression systems. A large-scale purification protocol was then developed for DmOrco and the ligand-binding receptor, DmOr22a. The proteins were nickel-affinity purified using a deca-histidine tag in a buffer containing 0.2 mM Zwittergent 3-16, followed by size exclusion chromatography. These purified ORs appear to form similarly sized protein-detergent complexes when isolated from both expression systems. Circular dichroism analysis of both purified proteins suggests they are folded correctly. We also provide evidence that when DmOrco is expressed in Sf9 cells it undergoes post translational modification, probably glycosylation. Finally we show that the recombinant ORs can be incorporated into pre-formed liposomes. The ability to recombinantly express and purify insect ORs to homogeneity on a preparative scale, as well as insert them into liposomes, is a major step forward in enabling future structural and functional studies, as well as their use in OR based biosensors.

Detergent screening and purification of the human liver ABC transporters BSEP (ABCB11) and MDR3 (ABCB4) expressed in the yeast Pichia pastoris

The human liver ATP-binding cassette (ABC) transporters bile salt export pump (BSEP/ABCB11) and the multidrug resistance protein 3 (MDR3/ABCB4) fulfill the translocation of bile salts and phosphatidylcholine across the apical membrane of hepatocytes. In concert with ABCG5/G8, these two transporters are responsible for the formation of bile and mutations within these transporters can lead to severe hereditary diseases. In this study, we report the heterologous overexpression and purification of human BSEP and MDR3 as well as the expression of the corresponding C-terminal GFP-fusion proteins in the yeast Pichia pastoris. Confocal laser scanning microscopy revealed that BSEP-GFP and MDR3-GFP are localized in the plasma membrane of P. pastoris. Furthermore, we demonstrate the first purification of human BSEP and MDR3 yielding ∼1 mg and ∼6 mg per 100 g of wet cell weight, respectively. By screening over 100 detergents using a dot blot technique, we found that only zwitterionic, lipid-like detergents such as Fos-cholines or Cyclofos were able to extract both transporters in sufficient amounts for subsequent functional analysis. For MDR3, fluorescence-detection size exclusion chromatography (FSEC) screens revealed that increasing the acyl chain length of Fos-Cholines improved monodispersity. BSEP purified in n-dodecyl-β-D-maltoside or Cymal-5 after solubilization with Fos-choline 16 from P. pastoris membranes showed binding to ATP-agarose. Furthermore, detergent-solubilized and purified MDR3 showed a substrate-inducible ATPase activity upon addition of phosphatidylcholine lipids. These results form the basis for further biochemical analysis of human BSEP and MDR3 to elucidate the function of these clinically relevant ABC transporters.

Effects of the lipid environment, cholesterol and bile acids on the function of the purified and reconstituted human ABCG2 protein

The human ABCG2 multidrug transporter actively extrudes a wide range of hydrophobic drugs and xenobiotics recognized by the transporter in the membrane phase. In order to examine the molecular nature of the transporter and its effects on the lipid environment, we have established an efficient protocol for the purification and reconstitution of the functional protein. We found that the drug-stimulated ATPase and the transport activity of ABCG2 are fully preserved by applying excess lipids and mild detergents during solubilization, whereas a detergent-induced dissociation of the ABCG2 dimer causes an irreversible inactivation. By using the purified and reconstituted protein we demonstrate that cholesterol is an essential activator, whereas bile acids are important modulators of ABCG2 activity. Both wild-type ABCG2 and its R482G mutant variant require cholesterol for full activity, although they exhibit different cholesterol sensitivities. Bile acids strongly decrease the basal ABCG2-ATPase activity both in the wild-type ABCG2 and in the mutant variant. These data reinforce the results for the modulatory effects of cholesterol and bile acids of ABCG2 investigated in a complex cell membrane environment. Moreover, these experiments open the possibility to perform functional and structural studies with a purified, reconstituted and highly active ABCG2 multidrug transporter.

ATPase activity of human ABCG1 is stimulated by cholesterol and sphingomyelin

ATP-binding cassette protein G1 (ABCG1) is important for the formation of HDL. However, the biochemical properties of ABCG1 have not been reported, and the mechanism of how ABCG1 is involved in HDL formation remains unclear. We established a procedure to express and purify human ABCG1 using the suspension-adapted human cell FreeStyle293-F. ABCG1, fused at the C terminus with green fluorescent protein and Flag-peptide, was solubilized with n-dodecyl-β-D-maltoside and purified via a single round of Flag-M2 antibody affinity chromatography. The purified ABCG1 was reconstituted in liposome of various lipid compositions, and the ATPase activity was analyzed. ABCG1 reconstituted in egg lecithin showed ATPase activity (150 nmol/min/mg), which was inhibited by beryllium fluoride. The ATPase activity of ABCG1, reconstituted in phosphatidylserine liposome, was stimulated by cholesterol and choline phospholipids (especially sphingomyelin), and the affinity for cholesterol was increased by the addition of sphingomyelin. These results suggest that ABCG1 is an active lipid transporter and possesses different binding sites for cholesterol and sphingomyelin, which may be synergistically coupled.

Active detergent-solubilized H+,K+-ATPase is a monomer

The H(+),K(+)-ATPase pumps protons or hydronium ions and is responsible for the acidification of the gastric fluid. It is made up of an α-catalytic and a β-glycosylated subunit. The relation between cation translocation and the organization of the protein in the membrane are not well understood. We describe here how pure and functionally active pig gastric H(+),K(+)-ATPase with an apparent Stokes radius of 6.3 nm can be obtained after solubilization with the non-ionic detergent C(12)E(8), followed by exchange of C(12)E(8) with Tween 20 on a Superose 6 column. Mass spectroscopy indicates that the β-subunit bears an excess mass of 9 kDa attributable to glycosylation. From chemical analysis, there are 0.25 g of phospholipids and around 0.024 g of cholesterol bound per g of protein. Analytical ultracentrifugation shows one main complex, sedimenting at s(20,)(w) = 7.2 ± 0.1 S, together with minor amounts of irreversibly aggregated material. From these data, a buoyant molecular mass is calculated, corresponding to an H(+),K(+)-ATPase α,β-protomer of 147.3 kDa. Complementary sedimentation velocity with deuterated water gives a picture of an α,β-protomer with 0.9-1.4 g/g of bound detergent and lipids and a reasonable frictional ratio of 1.5, corresponding to a Stokes radius of 7.1 nm. An α(2),β(2) dimer is rejected by the data. Light scattering coupled to gel filtration confirms the monomeric state of solubilized H(+),K(+)-ATPase. Thus, α,β H(+),K(+)-ATPase is active at least in detergent and may plausibly function as a monomer, as has been established for other P-type ATPases, Ca(2+)-ATPase and Na(+),K(+)-ATPase.

Functional expression and characterisation of membrane transport proteins

Membrane transporters set the framework organising the complexity of plant metabolism in cells, tissues and organisms. Their substrate specificity and controlled activity in different cells is a crucial part for plant metabolism to run pathways in concert. Transport proteins catalyse the uptake and exchange of ions, substrates, intermediates, products and cofactors across membranes. Given the large number of metabolites, a wide spectrum of transporters is required. The vast majority of in silico annotated membrane transporters in plant genomes, however, has not yet been functionally characterised. Hence, to understand the metabolic network as a whole, it is important to understand how transporters connect and control the metabolic pathways of plant cells. Heterologous expression and in vitro activity studies of recombinant transport proteins have highly improved their functional analysis in the last two decades. This review provides a comprehensive overview of the recent advances in membrane protein expression and functional characterisation using various host systems and transport assays.

Tuning the drug efflux activity of an ABC transporter in vivo by in vitro selected DARPin binders

ABC transporters use the energy from binding and hydrolysis of ATP to import or extrude substrates across the membrane. Using ribosome display, we raised designed ankyrin repeat proteins (DARPins) against detergent solubilized LmrCD, a heterodimeric multidrug ABC exporter from Lactococcus lactis. Several target-specific DARPin binders were identified that bind to at least three distinct, partially overlapping epitopes on LmrD in detergent solution as well as in native membranes. Remarkably, functional screening of the LmrCD-specific DARPin pools in L. lactis revealed three homologous DARPins which, when generated in LmrCD-expressing cells, strongly activated LmrCD-mediated drug transport. As LmrCD expression in the cell membrane was unaltered upon the co-expression of activator DARPins, the activation is suggested to occur at the level of LmrCD activity. Consistent with this, purified activator DARPins were found to stimulate the ATPase activity of LmrCD in vitro when reconstituted in proteoliposomes. This study suggests that membrane transporters are tunable in vivo by in vitro selected binding proteins. Our approach could be of biopharmaceutical importance and might facilitate studies on molecular mechanisms of ABC transporters.