Multiple roles of charged amino acids in cytoplasmic loop 7 for expression and function of the multidrug and organic anion transporter MRP1 (ABCC1)

How Cryo-EM Has Expanded Our Understanding of Membrane Transporters

Over the past two decades, technological advances in membrane protein structural biology have provided insight into the molecular mechanisms that transporters use to move diverse substrates across the membrane. However, the plasticity of these proteins' ligand binding pockets, which allows them to bind a range of substrates, also poses a challenge for drug development. Here we highlight the structure, function, and transport mechanism of ATP-binding cassette/solute carrier transporters that are related to several diseases and multidrug resistance: ABCB1, ABCC1, ABCG2, SLC19A1, and SLC29A1. SIGNIFICANCE STATEMENT: ATP-binding cassette transporters and solute carriers play vital roles in clinical chemotherapeutic outcomes. This paper describes the current understanding of the structure of five pharmacologically relevant transporters and how they interact with their ligands.

The First Cytoplasmic Loop in the Core Structure of the ABCC1 (Multidrug Resistance Protein 1; MRP1) Transporter Contains Multiple Amino Acids Essential for Its Expression

ABCC1 (human multidrug resistance protein 1 (hMRP1)) is an ATP-binding cassette transporter which effluxes xeno- and endobiotic organic anions and confers multidrug resistance through active drug efflux. The 17 transmembrane α-helices of hMRP1 are distributed among three membrane spanning domains (MSD0, 1, 2) with MSD1,2 each followed by a nucleotide binding domain to form the 4-domain core structure. Eight conserved residues in the first cytoplasmic loop (CL4) of MSD1 in the descending α-helix (Gly³⁹², Tyr⁴⁰⁴, Arg⁴⁰⁵), the perpendicular coupling helix (Asn⁴¹², Arg⁴¹⁵, Lys⁴¹⁶), and the ascending α-helix (Glu⁴²², Phe⁴³⁴) were targeted for mutagenesis. Mutants with both alanine and same charge substitutions of the coupling helix residues were expressed in HEK cells at wild-type hMRP1 levels and their transport activity was only moderately compromised. In contrast, mutants of the flanking amino acids (G392I, Y404A, R405A/K, E422A/D, and F434Y) were very poorly expressed although Y404F, E422D, and F434A were readily expressed and transport competent. Modeling analyses indicated that Glu⁴²² and Arg⁶¹⁵ could form an ion pair that might stabilize transporter expression. However, this was not supported by exchange mutations E422R/R615E which failed to improve hMRP1 levels. Additional structures accompanied by rigorous biochemical validations are needed to better understand the bonding interactions crucial for stable hMRP1 expression.

Conserved amino acids in the region connecting membrane spanning domain 1 to nucleotide binding domain 1 are essential for expression of the MRP1 (ABCC1) transporter

Multidrug resistance protein 1 (MRP1) (gene symbol ABCC1) is an ATP-binding cassette (ABC) transporter which effluxes xeno- and endobiotic organic anions including estradiol glucuronide and the pro-inflammatory leukotriene C₄. MRP1 also confers multidrug resistance by reducing intracellular drug accumulation through active efflux. MRP1 has three membrane spanning domains (MSD), and two nucleotide binding domains (NBD). MSD1 and MSD2 are linked to NBD1 and NBD2 by connecting regions (CR) 1 and CR2, respectively. Here we targeted four residues in CR1 (Ser⁶¹², Arg⁶¹⁵, His⁶²², Glu⁶²⁴) for alanine substitution and unexpectedly, found that cellular levels of three mutants (S612A, R615A, E624A) in transfected HEK cells were substantially lower than wild-type MRP1. Whereas CR1-H622A properly trafficked to the plasma membrane and exhibited organic anion transport activity comparable to wild-type MRP1, the poorly expressing R615A and E624A (and to a lesser extent S612A) mutant proteins were retained intracellularly. Analyses of cryogenic electron microscopic and atomic homology models of MRP1 indicated that Arg⁶¹⁵ and Glu⁶²⁴ might participate in bonding interactions with nearby residues to stabilize expression of the transporter. However, this was not supported by double exchange mutations E624K/K406E, R615D/D430R and R615F/F619R which failed to improve MRP1 levels. Nevertheless, these experiments revealed that the highly conserved CR1-Phe⁶¹⁹ and distal Lys⁴⁰⁶ in the first cytoplasmic loop of MSD1 are also essential for expression of MRP1 protein. This study is the first to demonstrate that CR1 contains several highly conserved residues critical for plasma membrane expression of MRP1 but thus far, currently available structures and models do not provide any insights into the underlying mechanism(s). Additional structures with rigorous biochemical validation data are needed to fully understand the bonding interactions critical to stable expression of this clinically important ABC transporter.

Study on functional sites in human multidrug resistance protein 1 (hMRP1)

Human multidrug resistance protein 1 (hMRP1) is an important member of the ATP-binding cassette (ABC) transporter superfamily. It can extrude a variety of anticancer drugs and physiological organic anions across the plasma membrane, which is activated by substrate binding, and is accompanied by large-scale cooperative movements between different domains. Currently, it remains unclear completely about how the specific interactions between hMRP1 and its substrate are and which critical residues are responsible for allosteric signal transduction. To the end, we first construct an inward-facing state of hMRP1 using homology modeling method, and then dock substrate proinflammatory agent leukotriene C4 (LTC4) to hMRP1 pocket. The result manifests LTC4 interacts with two parts of hMRP1 pocket, namely the positively charged pocket (P pocket) and hydrophobic pocket (H pocket), similar to its binding mode with bMRP1 (bovine MRP1). Additionally, we use the Gaussian network model (GNM)-based thermodynamic method proposed by us to identify the key residues whose perturbations markedly alter their binding free energy. Here the conventional GNM is improved with covalent/non-covalent interactions and secondary structure information considered (denoted as sscGNM). In the result, sscGNM improves the flexibility prediction, especially for the nucleotide binding domains with rich kinds of secondary structures. The 46 key residue clusters located in different subdomains are identified which are highly consistent with experimental observations. Furtherly, we explore the long-range cooperation within the transporter. This study is helpful for strengthening the understanding of the work mechanism in ABC exporters and can provide important information to scientists in drug design studies.

Molecular analysis of the massive GSH transport mechanism mediated by the human Multidrug Resistant Protein 1/ABCC1

The transporter Multidrug Resistance Protein 1 (MRP1, ABCC1) is implicated in multidrug resistant (MDR) phenotype of cancer cells. Glutathione (GSH) plays a key role in MRP1 transport activities. In addition, a ligand-stimulated GSH transport which triggers the death of cells overexpressing MRP1, by collateral sensitivity (CS), has been described. This CS could be a way to overcome the poor prognosis for patients suffering from a chemoresistant cancer. The molecular mechanism of such massive GSH transport and its connection to the other transport activities of MRP1 are unknown. In this context, we generated MRP1/MRP2 chimeras covering different regions, MRP2 being a close homolog that does not trigger CS. The one encompassing helices 16 and 17 led to the loss of CS and MDR phenotype without altering basal GSH transport. Within this region, the sole restoration of the original G1228 (D1236 in MRP2) close to the extracellular loop between the two helices fully rescued the CS (massive GSH efflux and cell death) but not the MDR phenotype. The flexibility of that loop and the binding of a CS agent like verapamil could favor a particular conformation for the massive transport of GSH, not related to other transport activities of MRP1.

Structure-guided probing of the leukotriene C4 binding site in human multidrug resistance protein 1 (MRP1; ABCC1)

The human multidrug resistance protein 1 (hMRP1) transporter is implicated in cancer multidrug resistance as well as immune responses involving its physiologic substrate, glutathione (GSH)-conjugated leukotriene C₄ (LTC₄). LTC₄ binds a bipartite site on hMRP1, which a recent cryoelectron microscopy structure of LTC₄-bound bovine Mrp1 depicts as composed of a positively charged pocket and a hydrophobic (H) pocket that binds the GSH moiety and surrounds the fatty acid moiety, respectively, of LTC₄. Here, we show that single Ala and Leu substitutions of H-pocket hMRP1-Met¹⁰⁹³ have no effect on LTC₄ binding or transport. Estrone 3-sulfate transport is also unaffected, but both hMRP1-Met¹⁰⁹³ mutations eliminate estradiol glucuronide transport, demonstrating that these steroid conjugates have binding sites distinct from each other and from LTC₄. To eliminate LTC₄ transport by hMRP1, mutation of 3 H-pocket residues was required (W553/M1093/W1246A), indicating that H-pocket amino acids are key to the vastly different affinities of hMRP1 for LTC₄ vs. GSH alone. Unlike organic anion transport, hMRP1-mediated drug resistance was more diminished by Ala than Leu substitution of Met¹⁰⁹³. Although our findings generally support a structure in which H-pocket residues bind the lipid tail of LTC₄, their critical and differential role in the transport of conjugated estrogens and anticancer drugs remains unexplained.-Conseil, G., Arama-Chayoth, M., Tsfadia, Y., Cole, S. P. C. Structure-guided probing of the leukotriene C₄ binding site in human multidrug resistance protein 1 (MRP1; ABCC1).

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.

An Outward-Facing Aromatic Amino Acid Is Crucial for Signaling between the Membrane-Spanning and Nucleotide-Binding Domains of Multidrug Resistance Protein 1 (MRP1; ABCC1)

The 190-kDa human MRP1 is an ATP-binding cassette multidrug and multiorganic anion efflux transporter. The 17 transmembrane helices of its three membrane-spanning domains, together with its two nucleotide binding domains (NBDs), form a stabilizing network of domain-domain interactions that ensure substrate binding in the cytoplasm is efficiently coupled to ATP binding and hydrolysis to effect solute efflux into the extracellular milieu. Here we show that Ala substitution of Phe583 in an outward-facing loop between the two halves of the transporter essentially eliminates the binding of multiple organic anions by MRP1. Conservative substitutions with Trp and Tyr had little or no effect. The F583A mutation also caused a substantial increase in orthovanadate-induced trapping of azidoADP by the cytoplasmic NBDs of MRP1, although the binding of ATP was unaffected. These observations indicate that the loss of the aromatic side chain at position 583 impairs the release of ADP and thus effectively locks the transporter in a low-affinity solute binding state. Phe583 is the first outward-facing amino acid in MRP1 found to be critical for its transport function. Our data provide evidence for long-range coupling, presumably via allosteric interaction, between this outward-facing region of MRP1 and both the solute binding and nucleotide binding regions of the transporter. Cryoelectron microscopy structural and homology models of MRP1 indicate that the orientation of the Phe583 side chain is altered by ATP binding but are currently unable to provide insights into the molecular mechanism by which this long-range signaling is propagated.

Structural Studies of Multidrug Resistance Protein 1 Using "Almost" Cysless Template

Multidrug resistance protein, MRP1 (ABCC1) is a broad-spectrum ATP-binding cassette transporter that plays a major role in defense against dietary and environmental toxicants, in addition to contributing toward multidrug resistance of certain types of malignancy. Elucidating the molecular structure of hMRP1 is key to determining its mechanism of substrate recognition and transport. Here, we report the first successful attempt using cysteine-scanning mutagenesis coupled with cross-linking studies to probe the structure of hMRP1 in its native environment of the cell membrane or in membrane vesicles. We have established that an active 3Cys ΔMRP1 (MRP204-1531) mutant, described in previous studies from our laboratory, is a suitable template with which to generate single- and double-cysteine mutants for performing cysteine mutagenesis studies. We have now used 3Cys ΔMRP1 to probe the arrangement of several TM segments, as well as the location of individual amino acids in these regions. Cysteine residues were introduced into TMs 8, 14, 15, and 16 of 3Cys ΔMRP1. The mutants were then subjected to chemical cross-linking analyses, and cross-linking was detected between the following cysteine pairs: Cys388 (TM7) and I1193C (TM16); Cys388 (TM7) and E1144C (TM15); R433C (TM8) and E1144C (TM15); and R433C (TM8) and T1082C (TM14). The aqueous accessibility of these residues and the possible implications of the differences between the open and closed states of the protein are also discussed. Moreover, using competition experiments involving a well characterized substrate and a cross-linking reagent for probing the Cys388/ I1193C mutant, we have defined these amino acid positions as a component of the potential site for estrone sulfate binding.

Single Nanoparticle Plasmonic Spectroscopy for Study of Charge-Dependent Efflux Function of Multidrug ABC Transporters of Single Live Bacillus subtilis Cells

Multidrug membrane transporters can selectively extrude a wide variety of structurally and functionally unrelated substrates, and they are responsible for ineffective treatment of a wide range of diseases (e.g., infection and cancer). Their underlying molecular mechanisms remain elusive. In this study, we functionalized Ag NPs (11 nm in diameter) with two biocompatible peptides (CALNNK, CALNNE) to prepare positively and negatively charged Ag-peptide NPs (Ag-CALNNK NPs+ζ, Ag-CALNNE NPs-4ζ), respectively. We used them as photostable plasmonic imaging probes to study charge-dependent efflux kinetics of BmrA (ABC) membrane transporter of single live Bacillus (B.) subtilis cells. Two strains of the cells, normal expression of BmrA (WT) or devoid of BmrA (ΔBmrA), were used to study the charge-dependent efflux kinetics of single NPs upon the expression of BmrA. The NPs (1.4 nM) were stable (non-aggregated) in a PBS buffer and biocompatible to the cells. We found the high dependent accumulation of the intracellular NPs in both WT and ΔBmrA upon the charge and concentration of NPs. Notably, the accumulation rates of the positively charged NPs in single live WT cells are nearly identical to those in ΔBmrA cells, showing independence upon the expression of BmrA. In contrast, the accumulation rates of the negatively charged NPs in WT are much lower than in ΔBmrA, showing high dependence upon the expression of BmrA and suggesting that BmrA extrude the negatively charged NPs, but not positively charged NPs, out of the WT. The accumulation of positively charged NPs in both WT and ΔBmrA increases nearly proportionally to the NP concentration. The accumulation of negatively charged NPs in ΔBmrA, but not in WT, also increases nearly proportionally to the NP concentration. These results suggest that both negatively and positively charged NPs enter the cells via passive diffusion driven by concentration gradients across the cellular membrane, and BmrA can only extrude the negatively charged NPs out of the WT. This study shows that single NP plasmon spectroscopy can serve as a powerful tool to identify single plasmonic NPs and to probe the charge-dependent efflux kinetics and function of single membrane transporters in single live cells in real time.

Polymorphic variants of MRP4/ABCC4 differentially modulate the transport of methylated arsenic metabolites and physiological organic anions

Broad inter-individual variation exists in susceptibility to arsenic-induced tumours, likely involving differences in the ability of individuals to eliminate this metalloid. We recently identified human multidrug resistance protein 4 (MRP4/ABCC4) as a novel pathway for the cellular export of dimethylarsinic acid (DMA^V), the major urinary arsenic metabolite in humans, and the diglutathione conjugate of the highly toxic monomethylarsonous acid [MMA(GS)₂]. These findings, together with the basolateral and apical membrane localization of MRP4 in hepatocytes and renal proximal tubule cells, respectively, suggest a role for MRP4 in the urinary elimination of hepatic arsenic metabolites. Accordingly, we have now investigated the influence of non-synonymous single nucleotide polymorphisms (SNPs) on MRP4 levels, cellular localization, and arsenical transport. Of eight MRP4 variants (C171G-, G187W-, K304N-, G487E-, Y556C-, E757K-, V776I- and C956S-MRP4) characterized, two (V776I- and C956S-MRP4) did not localize appropriately to the plasma membrane of HEK293T and LLC-PK1 cells. Characterization of the six correctly localized mutants revealed that MMA(GS)₂ transport by C171G-, G187W-, and K304N-MRP4 was 180%, 73%, and 30% of WT-MRP4 activity, respectively, whereas DMA^V transport by K304N- and Y556C-MRP4 was 30% and 184% of WT-MRP4, respectively. Transport of the prototypical physiological MRP4 substrates prostaglandin E₂ and 17β-estradiol 17-(β-d-glucuronide) by the six variants was also differentially affected. Thus, MRP4 variants have differing abilities to transport arsenic and endogenous metabolites through both altered function and membrane localization. Further investigation is warranted to determine if genetic variations in ABCC4 contribute to inter-individual differences in susceptibility to arsenic-induced (and potentially other) diseases.

The Role of Flexible Loops in Folding, Trafficking and Activity of Equilibrative Nucleoside Transporters

Equilibrative nucleoside transporters (ENTs) are integral membrane proteins, which reside in plasma membranes of all eukaryotic cells and mediate thermodynamically downhill transport of nucleosides. This process is essential for nucleoside recycling, and also plays a key role in terminating adenosine-mediated cellular signaling. Furthermore, ENTs mediate the uptake of many drugs, including anticancer and antiviral nucleoside analogues. The structure and mechanism, by which ENTs catalyze trans-membrane transport of their substrates, remain unknown. To identify the core of the transporter needed for stability, activity, and for its correct trafficking to the plasma membrane, we have expressed human ENT deletion mutants in Xenopus laevis oocytes and determined their localization, transport properties and susceptibility to inhibition. We found that the carboxyl terminal trans-membrane segments are essential for correct protein folding and trafficking. In contrast, the soluble extracellular and intracellular loops appear to be dispensable, and must be involved in the fine-tuning of transport regulation.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): CLOSED AND OPEN STATE CHANNEL MODELS

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily. CFTR controls the flow of anions through the apical membrane of epithelia. Dysfunctional CFTR causes the common lethal genetic disease cystic fibrosis. Transitions between open and closed states of CFTR are regulated by ATP binding and hydrolysis on the cytosolic nucleotide binding domains, which are coupled with the transmembrane (TM) domains forming the pathway for anion permeation. Lack of structural data hampers a global understanding of CFTR and thus the development of "rational" approaches directly targeting defective CFTR. In this work, we explored possible conformational states of the CFTR gating cycle by means of homology modeling. As templates, we used structures of homologous ABC transporters, namely TM(287-288), ABC-B10, McjD, and Sav1866. In the light of published experimental results, structural analysis of the transmembrane cavity suggests that the TM(287-288)-based CFTR model could correspond to a commonly occupied closed state, whereas the McjD-based model could represent an open state. The models capture the important role played by Phe-337 as a filter/gating residue and provide structural information on the conformational transition from closed to open channel.

Multidrug resistance protein 1 (MRP1, ABCC1), a "multitasking" ATP-binding cassette (ABC) transporter

The multidrug resistance protein 1 (MRP1) encoded by ABCC1 was originally discovered as a cause of multidrug resistance in tumor cells. However, it is now clear that MRP1 serves a broader role than simply mediating the ATP-dependent efflux of drugs from cells. The antioxidant GSH and the pro-inflammatory cysteinyl leukotriene C4 have been identified as key physiological organic anions effluxed by MRP1, and an ever growing body of evidence indicates that additional lipid-derived mediators are also substrates of this transporter. As such, MRP1 is a multitasking transporter that likely influences the etiology and progression of a host of human diseases.

Two polymorphic variants of ABCC1 selectively alter drug resistance and inhibitor sensitivity of the multidrug and organic anion transporter multidrug resistance protein 1

In this study we compared the in silico predictions of the effect of ABCC1 nonsynonymous single nucleotide polymorphisms (nsSNPs) with experimental data on MRP1 transport function and response to chemotherapeutics and multidrug resistance protein 1 (MRP1) inhibitors. Vectors encoding seven ABCC1 nsSNPs were stably expressed in human embryonic kidney (HEK) cells, and levels and localization of the mutant MRP1 proteins were determined by confocal microscopy and immunoblotting. The function of five of the mutant proteins was determined using cell-based drug and inhibitor sensitivity and efflux assays, and membrane-based organic anion transport assays. Predicted consequences of the mutations were determined by multiple bioinformatic methods. Mutants C43S and S92F were correctly routed to the HEK cell plasma membrane, but the levels were too low to permit functional characterization. In contrast, levels and membrane trafficking of R633Q, G671V, R723Q, A989T, and C1047S were similar to wild-type MRP1. In cell-based assays, all five mutants were equally effective at effluxing calcein, but only two exhibited reduced resistance to etoposide (C1047S) and vincristine (A989T; C1047S). The GSH-dependent inhibitor LY465803 (LY465803 [N-[3-(9-chloro-3-methyl-4-oxo-4H-isoxazolo-[4,3-c]quinolin-5-yl)-cyclohexylmethyl]-benzamide)] was less effective at blocking calcein efflux by A989T, but in a membrane-based assay, organic anion transport by A989T and C1047S was inhibited by MRP1 modulators as well as wild-type MRP1. GSH accumulation assays suggest cellular GSH efflux by A989T and C1047S may be impaired. In conclusion, although six in silico analyses consistently predict deleterious consequences of ABCC1 nsSNPs G671V, changes in drug resistance and inhibitor sensitivity were only observed for A989T and C1047S, which may relate to GSH transport differences.

Differential functional rescue of Lys(513) and Lys(516) processing mutants of MRP1 (ABCC1) by chemical chaperones reveals different domain-domain interactions of the transporter

Multidrug resistance protein 1 (MRP1) extrudes drugs as well as pharmacologically and physiologically important organic anions across the plasma membrane in an ATP-dependent manner. We previously showed that Ala substitutions of Lys(513) and Lys(516) in the cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 cause misfolding of MRP1, abrogating its expression at the plasma membrane in transfected human embryonic kidney (HEK) cells. Exposure of HEK cells to the chemical chaperones glycerol, DMSO, polyethylene glycol (PEG) and 4-aminobutyric acid (4-PBA) improved levels of K513A to wild-type MRP1 levels but transport activity was only fully restored by 4-PBA or DMSO treatments. Tryptic fragmentation patterns and conformation-dependent antibody immunoreactivity of the transport-deficient PEG- and glycerol-rescued K513A proteins indicated that the second nucleotide binding domain (NBD2) had adopted a more open conformation than in wild-type MRP1. This structural change was accompanied by differences in ATP binding and hydrolysis but no changes in substrate Km. In contrast to K513A, K516A levels in HEK cells were not significantly enhanced by chemical chaperones. In more permissive insect cells, however, K516A levels were comparable to wild-type MRP1. Nevertheless, organic anion transport by K516A in insect cell membranes was reduced by >80% due to reduced substrate Km. Tryptic fragmentation patterns indicated a more open conformation of the third membrane spanning domain of MRP1. Thus, despite their close proximity to one another in CL5, Lys(513) and Lys(516) participate in different interdomain interactions crucial for the proper folding and assembly of MRP1.

Mutation of Glu521 or Glu535 in cytoplasmic loop 5 causes differential misfolding in multiple domains of multidrug and organic anion transporter MRP1 (ABCC1)

The polytopic 5-domain multidrug resistance protein 1 (MRP1/ABCC1) extrudes a variety of drugs and organic anions across the plasma membrane. Four charged residues in the fifth cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 are critical for stable expression of MRP1 at the plasma membrane. Thus Ala substitution of Lys(513), Lys(516), Glu(521), and Glu(535) all cause misfolding of MRP1 and target the protein for proteasome-mediated degradation. Of four chemical chaperones tested, 4-phenylbutyric acid (4-PBA) was the most effective at restoring expression of MRP1 mutants K513A, K516A, E521A, and E535A. However, although 4-PBA treatment of K513A resulted in wild-type protein levels (and activity), the same treatment had little or no effect on the expression of K516A. On the other hand, 4-PBA treatment allowed both E521A and E535A to exit the endoplasmic reticulum and be stably expressed at the plasma membrane. However, the 4-PBA-rescued E535A mutant exhibited decreased transport activity associated with reduced substrate affinity and conformational changes in both halves of the transporter. By contrast, E521A exhibited reduced transport activity associated with alterations in the mutant interactions with ATP as well as a distinct conformational change in the COOH-proximal half of MRP1. These findings illustrate the critical and complex role of CL5 for stable expression of MRP1 at the plasma membrane and more specifically show the differential importance of Glu(521) and Glu(535) in interdomain interactions required for proper folding and assembly of MRP1 into a fully transport competent native structure.

Analysis of recombinant tagged equilibrative nucleoside transporter 1 (ENT1) expressed in E. coli

Nucleoside transporters (NTs) are integral membrane proteins necessary for the cellular entry of nucleoside analog drugs used in chemotherapeutic treatment of conditions such as cancer and viral or parasitic infections. NTs are also the targets of certain drugs used in the treatment of various cardiovascular conditions. Because of the importance of NTs in drug uptake, determination of the three-dimensional structure of these proteins, particularly hENT1, has the potential to improve these treatments through structure-based design of more specifically targeted and transported drugs. In this paper, we use NMR spectroscopy to investigate the structure of the large intracellular loop between transmembrane domains 6 and 7 and we also describe a method for the successful overexpression of full-length hENT1 in a bacterial system. Recombinant tandem histidine-affinity (HAT) and 3×FLAG tagged hENT1 was overexpressed in E. coli, affinity purified, and functionally characterized by nitrobenzylthioinosine (NBTI) binding. Anti-3×FLAG immunodetection confirmed the expression of N-HAT-3×FLAG-hENT1, while increased NBTI binding (3.2-fold compared with controls) confirmed the conformational integrity of the recombinant hENT1 within the bacterial inner membrane. Yields of recombinant hENT1 using this approach were ~15 µg/L of bacterial culture and this approach provides a basis for large-scale production of protein for a variety of purposes.

Expression and function of human MRP1 (ABCC1) is dependent on amino acids in cytoplasmic loop 5 and its interface with nucleotide binding domain 2

Multidrug resistance protein 1 (MRP1) is an ATP-binding cassette transporter that effluxes drugs and organic anions across the plasma membrane. The 17 transmembrane helices of MRP1 are linked by extracellular and cytoplasmic loops (CLs), but their role in coupling the ATPase activity of MRP1 to the translocation of its substrates is poorly understood. Here we have examined the importance of CL5 by mutating eight conserved charged residues and the helix-disrupting Gly(511) in this region. Ala substitution of Lys(513), Lys(516), Glu(521), and Glu(535) markedly reduced MRP1 levels. Because three of these residues are predicted to lie at the interface of CL5 and the second nucleotide binding domain (NBD2), a critical role is indicated for this region in the plasma membrane expression of MRP1. Further support for this idea was obtained by mutating NBD2 amino acids His(1364) and Arg(1367) at the CL5 interface, which also resulted in reduced MRP1 levels. In contrast, mutation of Arg(501), Lys(503), Glu(507), Arg(532), and Gly(511) had no effect on MRP1 levels. Except for K503A, however, transport by these mutants was reduced by 50 to 75%, an effect largely attributable to reduced substrate binding and affinity. Studies with (32)P-labeled azido-ATP also indicated that whereas ATP binding by the G511I mutant was unchanged, vanadate-induced trapping of azido-ADP was reduced, indicating changes in the catalytic activity of MRP1. Together, these data demonstrate the multiple roles for CL5 in the membrane expression and function of MRP1.

Molecular basis for reduced estrone sulfate transport and altered modulator sensitivity of transmembrane helix (TM) 6 and TM17 mutants of multidrug resistance protein 1 (ABCC1)

Multidrug resistance protein 1 (MRP1) confers drug resistance and also mediates cellular efflux of many organic anions. MRP1 also transports glutathione (GSH); furthermore, this tripeptide stimulates transport of several substrates, including estrone 3-sulfate. We have previously shown that mutations of Lys(332) in transmembrane helix (TM) 6 and Trp(1246) in TM17 cause different substrate-selective losses in MRP1 transport activity. Here we have extended our characterization of mutants K332L and W1246C to further define the different roles these two residues play in determining the substrate and inhibitor specificity of MRP1. Thus, we have shown that TM17-Trp(1246) is crucial for conferring drug resistance and for binding and transport of methotrexate, estradiol glucuronide, and estrone 3-sulfate, as well as for binding of the tricyclic isoxazole inhibitor N-[3-(9-chloro-3-methyl-4-oxo-4H-isoxazolo-[4,3-c]quinolin-5-yl)-cyclohexylmethyl]-benzamide (LY465803). In contrast, TM6-Lys(332) is important for enabling GSH and GSH-containing compounds to serve as substrates (e.g., leukotriene C(4)) or modulators (e.g., S-decyl-GSH, GSH disulfide) of MRP1 and, further, for enabling GSH (or S-methyl-GSH) to enhance the transport of estrone 3-sulfate and increase the inhibitory potency of LY465803. On the other hand, both mutants are as sensitive as wild-type MRP1 to the non-GSH-containing inhibitors (E)-3-[[[3-[2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid (MK571), 1-[2-hydroxy-3-propyl-4-[4-(1H-tetrazol-5-yl)butoxy]phenyl]-ethanone (LY171883), and highly potent 6-[4'-carboxyphenylthio]-5[S]-hydroxy-7[E], 11[Z]14[Z]-eicosatetrenoic acid (BAY u9773). Finally, the differing abilities of the cysteinyl leukotriene derivatives leukotriene C(4), D(4), and F(4) to inhibit estradiol glucuronide transport by wild-type and K332L mutant MRP1 provide further evidence that TM6-Lys(332) is involved in the recognition of the gamma-Glu portion of substrates and modulators containing GSH or GSH-like moieties.