Epitope insertion favors a six transmembrane domain model for the carboxy-terminal portion of the multidrug resistance-associated protein

Multidrug resistance proteins (MRPs): Structure, function and the overcoming of cancer multidrug resistance

ATP-binding cassette (ABC) transporters mediate the ATP-driven translocation of structurally and mechanistically distinct substrates against steep concentration gradients. Among the seven human ABC subfamilies namely ABCA-ABCG, ABCC is the largest subfamily with 13 members. In this respect, 9 of the ABCC members are termed "multidrug resistance proteins" (MRPs1-9) due to their ability to mediate cancer multidrug resistance (MDR) by extruding various chemotherapeutic agents or their metabolites from tumor cells. Furthermore, MRPs are also responsible for the ATP-driven efflux of physiologically important organic anions such as leukotriene C₄, folic acid, bile acids and cAMP. Thus, MRPs are involved in important regulatory pathways. Blocking the anticancer drug efflux function of MRPs has shown promising results in overcoming cancer MDR. As a result, many novel MRP modulators have been developed in the past decade. In the current review, we summarize the structure, tissue distribution, biological and pharmacological functions as well as clinical insights of MRPs. Furthermore, recent updates in MRP modulators and their therapeutic applications in clinical trials are also discussed.

CCTOP: a Consensus Constrained TOPology prediction web server

The Consensus Constrained TOPology prediction (CCTOP; http://cctop.enzim.ttk.mta.hu) server is a web-based application providing transmembrane topology prediction. In addition to utilizing 10 different state-of-the-art topology prediction methods, the CCTOP server incorporates topology information from existing experimental and computational sources available in the PDBTM, TOPDB and TOPDOM databases using the probabilistic framework of hidden Markov model. The server provides the option to precede the topology prediction with signal peptide prediction and transmembrane-globular protein discrimination. The initial result can be recalculated by (de)selecting any of the prediction methods or mapped experiments or by adding user specified constraints. CCTOP showed superior performance to existing approaches. The reliability of each prediction is also calculated, which correlates with the accuracy of the per protein topology prediction. The prediction results and the collected experimental information are visualized on the CCTOP home page and can be downloaded in XML format. Programmable access of the CCTOP server is also available, and an example of client-side script is provided.

Expediting topology data gathering for the TOPDB database

The Topology Data Bank of Transmembrane Proteins (TOPDB, http://topdb.enzim.ttk.mta.hu) contains experimentally determined topology data of transmembrane proteins. Recently, we have updated TOPDB from several sources and utilized a newly developed topology prediction algorithm to determine the most reliable topology using the results of experiments as constraints. In addition to collecting the experimentally determined topology data published in the last couple of years, we gathered topographies defined by the TMDET algorithm using 3D structures from the PDBTM. Results of global topology analysis of various organisms as well as topology data generated by high throughput techniques, like the sequential positions of N- or O-glycosylations were incorporated into the TOPDB database. Moreover, a new algorithm was developed to integrate scattered topology data from various publicly available databases and a new method was introduced to measure the reliability of predicted topologies. We show that reliability values highly correlate with the per protein topology accuracy of the utilized prediction method. Altogether, more than 52 000 new topology data and more than 2600 new transmembrane proteins have been collected since the last public release of the TOPDB database.

Bindings of hMRP1 transmembrane peptides with dodecylphosphocholine and dodecyl-β-d-maltoside micelles: a molecular dynamics simulation study

In this paper, we describe molecular dynamics simulation results of the interactions between four peptides (mTM10, mTM16, TM17 and KTM17) with micelles of dodecylphosphocholine (DPC) and dodecyl-β-d-maltoside (DDM). These peptides represent three transmembrane fragments (TM10, 16 and 17) from the MSD1 and MSD2 membrane-spanning domains of an ABC membrane protein (hMRP1), which play roles in the protein functions. The peptide-micelle complex structures, including the tryptophan accessibility and dynamics were compared to circular dichroism and fluorescence studies obtained in water, trifluoroethanol and with micelles. Our work provides additional results not directly accessible by experiments that give further support to the fact that these peptides adopt an interfacial conformation within the micelles. We also show that the peptides are more buried in DDM than in DPC, and consequently, that they have a larger surface exposure to water in DPC than in DDM. As noted previously by simulations and experiments we have also observed formation of cation-π bonds between the phosphocholine DPC headgroup and Trp peptide residue. Concerning the peptide secondary structures (SS), we find that in TFE their initial helical conformations are maintained during the simulation, whereas in water their initial SS are lost after few nanoseconds of simulation. An intermediate situation is observed with micelles, where the peptides remain partially folded and more structured in DDM than in DPC. Finally, our results show no sign of β-strand structure formation as invoked by far-UV CD experiments even when three identical peptides are simulated either in water or with micelles.

Topology determination of untagged membrane proteins

The topology of integral membrane proteins with a weak topological tendency can be influenced when fused to tags, such as these used for topological determination or protein purification. Here, we describe a technique for topology determination of an untagged membrane protein. This technique, optimized for bacterial cells, allows the visualization of the protein in the native environment and incorporates the substituted-cysteine accessibility method.

Molecular and cellular mechanisms underlying neural tube defects in the loop-tail mutant mouse

Loop-tail (Lp) mice show a very severe neural tube defect (craniorachischisis) caused by mutations in the Vangl2 gene (D255E, S464N). Mammalian Vangl1 and Vangl2 are membrane proteins that play critical roles in development such as establishment of planar cell polarity (PCP) in epithelial layers and convergent extension movements during neurogenesis and cardiogenesis. Vangl proteins are thought to assemble with other PCP proteins (Dvl, Pk) to form membrane-bound PCP signaling complexes that provide polarity information to the cell. In the present study, we show that Vangl1 is expressed exclusively at the plasma membrane of transfected MDCK cells, where it is targeted to the basolateral membrane. Experiments with an inserted exofacial HA epitope indicate that the segment delimited by the predicted transmembrane domains 1 and 2 is exposed to the extracellular milieu. Comparative studies of the Lp-associated pathogenic mutation D255E indicate that the targeting of the mutant variant at the plasma membrane is greatly reduced; the mutant variant is predominantly retained intracellularly in endoplasmic reticulum (ER) vesicles colocalizing with the ER marker calreticulin. In addition, the D255E variant shows drastically reduced stability with a half-life of approximately 2 h, compared to >9 h for its wild type counterpart and is rapidly degraded in a proteasome-dependent and MG132 sensitive pathway. These findings highlight a critical role for D255 for normal folding and processing of Vangl proteins, with highly conservative substitutions not tolerated at that site. Our study provide an experimental framework for the analysis of human VANGL mutations recently identified in familial and sporadic cases of spina bifida.

Identifying assembly-inhibiting and assembly-tolerant sites in the SbsB S-layer protein from Geobacillus stearothermophilus

Surface layer (S-layer) proteins self-assemble into two-dimensional crystalline lattices that cover the cell wall of all archaea and many bacteria. We have generated assembly-negative protein variants of high solubility that will facilitate high-resolution structure determination. Assembly-negative versions of the S-layer protein SbsB from Geobacillus stearothermophilus PV72/p2 were obtained using an insertion mutagenesis screen. The haemagglutinin epitope tag was inserted at 23 amino acid positions known to be located on the monomer protein surface from a previous cysteine accessibility screen. Limited proteolysis, circular dichroism, and fluorescence were used to probe whether the epitope insertion affected the secondary and tertiary structures of the monomer, while electron microscopy and size-exclusion chromatography were employed to examine proteins' ability to self-assemble. The screen not only identified assembly-compromised mutants with native fold but also yielded correctly folded, self-assembling mutants suitable for displaying epitopes for biomedical and biophysical applications, as well as cryo-electron microscopy imaging. Our study marks an important step in the analysis of the S-layer structure. In addition, the approach of concerted insertion and cysteine mutagenesis can likely be applied for other supramolecular assemblies.

The high turnover Drosophila multidrug resistance-associated protein shares the biochemical features of its human orthologues

DMRP, an ABC transporter encoded by the dMRP/CG6214 gene, is the Drosophila melanogaster orthologue of the "long" human multidrug resistance-associated proteins (MRP1/ABCC1, MRP2/ABCC2, MRP3/ABCC3, MRP6/ABCC6, and MRP7/ABCC10). In order to provide a detailed biochemical characterisation we expressed DMRP in Sf9 insect cell membranes. We demonstrated DMRP as a functional orthologue of its human counterparts capable of transporting several human MRP substrates like beta-estradiol 17-beta-D-glucuronide, leukotriene C4, calcein, fluo3 and carboxydichlorofluorescein. Unexpectedly, we found DMRP to exhibit an extremely high turnover rate for the substrate transport as compared to its human orthologues. Furthermore, DMRP showed remarkably high basal ATPase activity (68-75 nmol Pi/mg membrane protein/min), which could be further stimulated by probenecid and the glutathione conjugate of N-ethylmaleimide. Surprisingly, this high level basal ATPase activity was inhibited by the transported substrates. We discussed this phenomenon in the light of a potential endogenous substrate (or activator) present in the Sf9 membrane.

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

Multidrug resistance protein MRP1 mediates the ATP-dependent efflux of many chemotherapeutic agents and organic anions. MRP1 has two nucleotide binding sites (NBSs) and three membrane spanning domains (MSDs) containing 17 transmembrane helices linked by extracellular and cytoplasmic loops (CL). Homology models suggest that CL7 (amino acids 1141-1195) is in a position where it could participate in signaling between the MSDs and NBSs during the transport process. We have individually replaced eight charged residues in CL7 with Ala, and in some cases, an amino acid with the same charge, and then investigated the effects on MRP1 expression, transport activity, and nucleotide and substrate interactions. A triple mutant in which Glu(1169), Glu(1170), and Glu(1172) were all replaced with Ala was also examined. The properties of R1173A and E1184A were comparable with those of wild-type MRP1, whereas the remaining mutants were either poorly expressed (R1166A, D1183A) or exhibited reduced transport of one or more organic anions (E1144A, D1179A, K1181A, (1169)AAQA). Same charge mutant D1183E was also not expressed, whereas expression and activity of R1166K were similar to wild-type MRP1. The moderate substrate-selective changes in transport activity displayed by mutants E1144A, D1179A, K1181A, and (1169)AAQA were accompanied by changes in orthovanadate-induced trapping of [alpha-(32)P]azidoADP by NBS2 indicating changes in ATP hydrolysis or release of ADP. In the case of E1144A, estradiol glucuronide no longer inhibited trapping of azidoADP. Together, our results demonstrate the extreme sensitivity of CL7 to mutation, consistent with its critical and complex dual role in both the proper folding and transport activity of MRP1.

The ocular albinism type 1 gene product, OA1, spans intracellular membranes 7 times

OA1 (GPR143) is a pigment cell-specific intracellular glycoprotein consisting of 404 amino acid residues that is mutated in patients with ocular albinism type 1, the most common form of ocular albinism. While its cellular localization is suggested to be endolysosomal and melanosomal, the physiological function of OA1 is currently unclear. Recent reports predicted that OA1 functions as a G protein coupled receptor (GPCR) based on its weak amino acid sequence similarity to known GPCRs, and on demonstration of GPCR activity in OA1 mislocalized to the plasma membrane. Because mislocalization of proteins is often caused by or induces defects in their proper folding/assembly, the significance of these studies remains unclear. A characteristic feature of GPCRs is a seven transmembrane domain structure. We analyzed the membrane topology of OA1 properly localized to intracellular lysosomal organelles in COS-1 cells. To accomplish this analysis, we established experimental conditions that allowed selective permeabilization of the plasma membrane while leaving endolysosomal membranes intact. Domains were mapped by the insertion of a hemagglutinin (HA) tag into the predicted cytosolic/luminal regions of OA1 molecule and the accessibility of tag to HA antibody was determined by immunofluorescence. HA-tagged lysosome associated membrane protein 1 (LAMP1), a type I membrane protein, was employed as a reporter for selective permeabilization of the plasma membrane. Our results show experimentally that the C-terminus of OA1 is directed to the cytoplasm and that the protein spans the intracellular membrane 7 times. Thus, OA1, properly localized intracellularly, is a 7 transmembrane domain integral membrane protein consistent with its putative role as an intracellular GPCR.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

The predicted transmembrane fragment 17 of the human multidrug resistance protein 1 (MRP1) behaves as an interfacial helix in membrane mimics

The human multidrug resistance protein MRP1 (or ABCC1) is one of the most important members of the large ABC transporter family, in terms of both its biological (tissue defense) and pharmacological functions. Many studies have investigated the function of MRP1, but structural data remain scarce for this protein. We investigated the structure and dynamics of predicted transmembrane fragment 17 (TM17, from Ala(1227) to Ser(1251)), which contains a single Trp residue (W(1246)) involved in MRP1 substrate specificity and transport function. We synthesized TM17 and a modified peptide in which Ala(1227) was replaced by a charged Lys residue. Both peptides were readily solubilized in dodecylmaltoside (DM) or dodecylphosphocholine (DPC) micelles, as membrane mimics. The interaction of these peptides with DM or DPC micelles was studied by steady-state and time-resolved Trp fluorescence spectroscopy, including experiments in which Trp was quenched by acrylamide or by two brominated analogs of DM. The secondary structure of these peptides was determined by circular dichroism. Overall, the results obtained indicated significant structuring ( approximately 50% alpha-helix) of TM17 in the presence of either DM or DPC micelles as compared to buffer. A main interfacial location of TM17 is proposed, based on significant accessibility of Trp(1246) to brominated alkyl chains of DM and/or acrylamide. The comparison of various fluorescence parameters including lambda(max), lifetime distributions and Trp rotational mobility with those determined for model fluorescent transmembrane helices in the same detergents is also consistent with the interfacial location of TM17. We therefore suggest that TM17 intrinsic properties may be insufficient for its transmembrane insertion as proposed by the MRP1 consensus topological model. This insertion may also be controlled by additional constraints such as interactions with other TM domains and its position in the protein sequence. The particular pattern of behavior of this predicted transmembrane peptide may be the hallmark of a fragment involved in substrate transport.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Membrane topology of human ABC proteins

In this review, we summarize the currently available information on the membrane topology of some key members of the human ABC protein subfamilies, and present the predicted domain arrangements. In the lack of high-resolution structures for eukaryotic ABC transporters this topology is based only on prediction algorithms and biochemical data for the location of various segments of the polypeptide chain, relative to the membrane. We suggest that topology models generated by the available prediction methods should only be used as guidelines to provide a basis of experimental strategies for the elucidation of the membrane topology.

Multidrug resistance-associated protein MRP-1 regulates dauer diapause by its export activity in Caenorhabditis elegans

Multidrug resistance-associated proteins (MRPs), when overexpressed, confer drug resistance to cancer cells by exporting anti-cancer agents through the cell membrane, but their role in animal development has not been elucidated. Here we show that an MRP homolog regulates larval development in the nematode Caenorhabditis elegans. C. elegans forms a special third-stage larva called a dauer larva under conditions inappropriate for growth. By contrast, we found that mutants in mrp-1, an MRP homolog gene, form dauer larvae even under conditions appropriate for growth, in the background of certain mutations that partially block the insulin signaling pathway. A functional mrp-1::GFP gene was shown to be expressed in many tissues, and the wild-type mrp-1 gene must be expressed in multiple tissues for a wild-type phenotype. Human MRP1 could substitute for C. elegans MRP-1 in dauer larva regulation, and an inhibitor of the human MRP1 transport activity impaired this function, showing that export activity is required for normal dauer larva regulation. Epistasis studies revealed that MRP-1 acts in neither the TGF-beta nor the cGMP signaling pathway. mrp-1 mutations enhanced the dauer-constitutive phenotype of mutants in the insulin signaling pathway more strongly than that in other pathways. Thus, MRP-1, through its export activity, supports the induction of the normal (non-dauer) life cycle by the insulin signaling pathway.

Molecular mechanisms of reduced glutathione transport: role of the MRP/CFTR/ABCC and OATP/SLC21A families of membrane proteins

The initial step in reduced glutathione (GSH) turnover in all mammalian cells is its transport across the plasma membrane into the extracellular space; however, the mechanisms of GSH transport are not clearly defined. GSH export is required for the delivery of its constituent amino acids to other tissues, detoxification of drugs, metals, and other reactive compounds of both endogenous and exogenous origin, protection against oxidant stress, and secretion of hepatic bile. Recent studies indicate that some members of the multidrug resistance-associated protein (MRP/CFTR or ABCC) family of ATP-binding cassette (ABC) proteins, as well as some members of the organic anion transporting polypeptide (OATP or SLC21A) family of transporters contribute to this process. In particular, five of the 12 members of the MRP/CFTR family appear to mediate GSH export from cells namely, MRP1, MRP2, MRP4, MRP5, and CFTR. Additionally, two members of the OATP family, rat Oatp1 and Oatp2, have been identified as GSH transporters. For the Oatp1 transporter, efflux of GSH may provide the driving force for the uptake of extracellular substrates. In humans, OATP-B and OATP8 do not appear to transport GSH; however, other members of this family have yet to be characterized in regards to GSH transport. In yeast, the ABC proteins Ycf1p and Bpt1p transport GSH from the cytosol into the vacuole, whereas Hgt1p mediates GSH uptake across the plasma membrane. Because transport is a key step in GSH homeostasis and is intimately linked to its biological functions, GSH export proteins are likely to modulate essential cellular functions.

Thermotolerance induced at a mild temperature of 40°C protects cells against heat shock-induced apoptosis

Apoptosis constitutes a response of organisms to various physiological or pathological stimuli, and to different stresses. The ability of thermotolerance induced at a mild temperature of 40 degrees C to protect against activation of the apoptotic cascade by heat shock was investigated. When Chinese hamster ovary and human adenocarcinoma cervical cells were pretreated at 40 degrees C for 3 h, they were resistant to subsequent lethal heat shock at 43 degrees C. Induction of thermotolerance at 40 degrees C led to increased expression of heat shock proteins 27, 32, 72, and 90. Heat shock induced apoptotic events at the mitochondrial level, involving a decrease in membrane potential, translocation of Bax to mitochondria, and liberation of cytochrome c into the cytosol. These events were diminished in thermotolerant cells. Heat shock (42-45 degrees C) caused activation of initiator caspase-9 and effector caspases-3, -6, and -7, relative to controls at 37 degrees C. Activation of caspases was decreased in thermotolerant cells. Heat shock caused fragmentation of the caspase substrate, inhibitor of caspase-activated DNase. Fragmentation was diminished in thermotolerant cells. Thermotolerance afforded protection against heat shock-induced nuclear chromatin condensation, but not against necrosis.

Mechanisms of drug resistance in cancer chemotherapy

The management of cancer involves procedures, which include surgery, radiotherapy and chemotherapy. Development of chemoresistance is a persistent problem during the treatment of local and disseminated disease. A plethora of cytotoxic drugs that selectively, but not exclusively, target actively proliferating cells include such diverse groups as DNA alkylating agents, antimetabolites, intercalating agents and mitotic inhibitors. Resistance constitutes a lack of response to drug-induced tumour growth inhibition; it may be inherent in a subpopulation of heterogeneous cancer cells or be acquired as a cellular response to drug exposure. Resistance varies. Although regulatory approval may require efficacy in as few as 20% of trial cohorts, a drug may subsequently be used in unselected patients displaying resistance to the treatment. Principal mechanisms may include altered membrane transport involving the P-glycoprotein product of the multidrug resistance (MDR) gene as well as other associated proteins, altered target enzyme (e.g. mutated topoisomerase II), decreased drug activation, increased drug degradation due to altered expression of drug-metabolising enzymes, drug inactivation due to conjugation with increased glutathione, subcellular redistribution, drug interaction, enhanced DNA repair and failure to apoptose as a result of mutated cell cycle proteins such as p53. Attempts to overcome resistance mainly involve the use of combination drug therapy using different classes of drugs with minimally overlapping toxicities to allow maximal dosages and with narrowest cycle intervals, necessary for bone marrow recovery. Adjuvant therapy with P-glycoprotein inhibitors and, in specific instances, the use of growth factor and protein kinase C inhibitors are newer experimental approaches that may also prove effective in abrogating or delaying onset of resistance. Gene knockout using antisense molecules may be another effective way of blocking drug resistance genes. Conversely, drug resistance may also be used to good purpose by transplanting retrovirally transformed CD34 cells expressing the MDR gene to protect the bone marrow during high-dose chemotherapy.

Multidrug-resistant hela cells overexpressing MRP1 exhibit sensitivity to cell killing by hyperthermia: Interactions with etoposide

PURPOSE

Multidrug resistance (MDR) remains one of the primary obstacles in cancer chemotherapy and often involves overexpression of drug efflux transporters such as P-glycoprotein and multidrug resistance protein 1 (MRP1). Regional hyperthermia is undergoing clinical investigation in combination with chemotherapy or radiotherapy. This study evaluates whether hyperthermia can reverse MDR mediated by MRP1 in human cervical adenocarcinoma (HeLa) cells.

METHODS AND MATERIALS

Cytotoxicity of hyperthermia and/or etoposide was evaluated using sulforhodamine-B in HeLa cells overexpressing MRP1 and their drug-sensitive counterparts. Glutathione, glutathione peroxidase (GPx), and glutathione S-transferase (GST) were quantified by spectrophotometry. GST isoenzymes were quantified by immunodetection. Caspase activation was evaluated by fluorometry and chromatin condensation by fluorescence microscopy using Hoechst 33258. Necrosis was determined using propidium iodide.

RESULTS

The major finding is that HeLa and HeLaMRP cells are both sensitive to cytotoxicity of hyperthermia (41-45 degrees C). Hyperthermia induced activation of caspase 3 and chromatin condensation. Although total levels of cell killing were similar, there was a switch from apoptotic to necrotic cell death in MDR cells. This could be explained by decreased glutathione and GPx in MDR cells. MDR cells also contained very low levels of GST and were resistant to etoposide-induced apoptosis. Hyperthermia caused a modest increase in etoposide-induced apoptosis in HeLa and HeLaMRP cells, which required appropriate heat-drug scheduling.

CONCLUSIONS

Hyperthermia could be useful in eliminating MDR cells that overexpress MRP1.

The dMRP/CG6214 gene of Drosophila is evolutionarily and functionally related to the human multidrug resistance-associated protein family

ATP-binding cassette (ABC) transporters are involved in the transport of substrates across biological membranes and are essential for many cellular processes. Of the fifty-six Drosophila ABC transporter genes only white, brown, scarlet, E23 and Atet have been studied in detail. Phylogenetic analyses identify the Drosophila gene dMRP/CG6214 as an orthologue to the human multidrug-resistance associated proteins MRP1, MRP2, MRP3 and MRP6. To study evolutionarily conserved roles of MRPs we have initiated a characterization of dMRP. In situ hybridization and Northern analysis indicate that dMRP is expressed throughout development and appears to be head enriched in adults. Functional studies indicate that DMRP is capable of transporting a known MRP1 substrate and establishes DMRP as a high capacity ATP-dependent, vanadate-sensitive organic anion transporter.

Independent mutations in mouse Vangl2 that cause neural tube defects in looptail mice impair interaction with members of the Dishevelled family

Mammalian Vangl1 and Vangl2 are highly conserved membrane proteins that have evolved from a single ancestral protein Strabismus/Van Gogh found in Drosophila. Mutations in the Vangl2 gene cause a neural tube defect (craniorachischisis) characteristic of the looptail (Lp) mouse. Studies in model organisms indicate that Vangl proteins play a key developmental role in establishing planar cell polarity (PCP) and in regulating convergent extension (CE) movements during embryogenesis. The role of Vangl1 in these processes is virtually unknown, and the molecular function of Vangl1 and Vangl2 in PCP and CE is poorly understood. Using a yeast two-hybrid system, glutathione S-transferase pull-down and co-immunoprecipitation assays, we show that both mouse Vangl1 and Vangl2 physically interact with the three members of the cytoplasmic Dishevelled (Dvl) protein family. This interaction is shown to require both the predicted cytoplasmic C-terminal half of Vangl1/2 and a portion of the Dvl protein containing PDZ and DIX domains. In addition, we show that the two known Vangl2 loss-of-function mutations identified in two independent Lp alleles associated with neural tube defects impair binding to Dvl1, Dvl2, and Dvl3. These findings suggest a molecular mechanism for the neural tube defect seen in Lp mice. Our observations indicate that Vangl1 biochemical properties parallel those of Vangl2 and that Vangl1 might, therefore, participate in PCP and CE either in concert with Vangl2 or independently of Vangl2 in discrete cell types.

Subcellular localization and N-glycosylation of human ABCC6, expressed in MDCKII cells

Mutations in the gene coding for a human ABC transporter protein, ABCC6 (MRP6), are responsible for the development of pseudoxanthoma elasticum. Here, we demonstrate that human ABCC6, when expressed by retroviral transduction in polarized mammalian (MDCKII) cells, is exclusively localized to the basolateral membrane. The human ABCC6 in MDCKII cells was found to be glycosylated, in contrast to the underglycosylated form of the protein, as expressed in Sf9 cells. In order to localize the major glycosylation site(s) in ABCC6, we applied limited proteolysis on the fully glycosylated and underglycosylated forms, followed by immunodetection with region-specific antibodies for ABCC6. Our results indicate that Asn15, which is located in the extracellular N-terminal region of human ABCC6, is the only N-glycosylation site in this protein. The polarized mammalian expression system characterized here provides a useful tool for further examination of routing, glycosylation, and function of the normal and pathological variants of human ABCC6.

Localization of the GSH-dependent photolabelling site of an agosterol A analog on human MRP1

1. Human multidrug resistance protein 1 (MRP1) is a 190 kDa membrane glycoprotein that confers multidrug resistance (MDR) to tumor cells. We recently demonstrated that glutathione (GSH) is required for the labelling of the C-terminal half of MRP1 with a photoanalog of agosterol A (azido AG-A). In this study, we further characterized the GSH-dependent photolabelling site of azido AG-A on MRP1. 2. An epitope-inserted MRP1, MRP1 1222HA, which has two hemagglutinin A (HA) epitopes in the extracellular loop between transmembrane segment (TM) 16 and TM17 of the transporter, could bind azido AG-A in a GSH-dependent manner. 3. Protease digestion of the photolabelled MRP1 1222HA, followed by immunoprecipitation with an anti-HA antibody suggested that the GSH-dependent azido AG-A photolabelling site on MRP1 resides in the region within TM14-17 and the cytoplasmic region proximate to the C-terminus of TM17. 4. Arg(1210) in human MRP2 that corresponds to Arg(1202) in human MRP1 has an important role in the transporting activity of MRP2. Therefore, we replaced the Arg residue at position 1202 of MRP1 with Gly. Whereas photolabelling of the mutant MRP1 R1202G was greatly reduced, it retained leukotriene C(4) (LTC(4)) transport activity and conferred Vincristine resistance in LLC-PK1 cells. 5. In summary, this study demonstrated that the GSH-dependent azido AG-A photolabelling site on MRP1 resides in the region within TM14-17 and the cytoplasmic region proximate to the C-terminus of TM17. The charged amino acid Arg(1202) proximate to TM helix 16 is of critical importance for the GSH-dependent photolabelling of MRP1 with azido AG-A. Arg(1202) itself or the region nearby Arg(1202) may be involved in azido AG-A photolabelling.

The Drosophila melanogaster multidrug-resistance protein 1 (MRP1) homolog has a novel gene structure containing two variable internal exons

Drosophila melanogaster has a gene very similar to human MRP1 that encodes a full ABC-transporter containing three membrane-spanning domains and two nucleotide-binding domains. This 19 exon insect gene, dMRP (FBgn0032456), spans slightly more than 22 kb. The cDNA SD07655 representing this gene was sequenced and found to contain sequences from 12 exons including single copies of two exons having multiple genomic copies. The gene contains two variant copies of exon 4 and seven of exon 8. While a cDNA contains only one version of each variable exon, all forms of these variable exons were detected in adult fly mRNA. These results predict that Drosophila could make 14 different MRP isoforms from a single gene by substituting different variable exons. This is the first report of any organism using differential splicing of alternative, internal exons, to produce such a large array of MRP isoforms having the same size, but with limited and defined internal variations. Defining the functional differences in the dMRP isoforms should provide clues to the structure/function relationships of the amino acids in these MRP domains, both for the insect enzyme and for those of other species.

Overexpression, purification, and functional characterization of ATP-binding cassette transporters in the yeast, Pichia pastoris

The ATP-binding cassette (ABC) transporter superfamily is a large gene family that has been highly conserved throughout evolution. The physiological importance of these membrane transporters is highlighted by the large variety of substrates they transport, and by the observation that mutations in many of them cause heritable diseases in human. Likewise, overexpression of certain ABC transporters, such as P-glycoprotein and members of the multidrug resistance associated protein (MRP) family, is associated with multidrug resistance in various cells and organisms. Understanding the structure and molecular mechanisms of transport of the ABC transporters in normal tissues and their possibly altered function in human diseases requires large amounts of purified and active proteins. For this, efficient expression systems are needed. The methylotrophic yeast Pichia pastoris has proven to be an efficient and inexpensive experimental model for high-level expression of many proteins, including ABC transporters. In the present review, we will summarize recent advances on the use of this system for the expression, purification, and functional characterization of P-glycoprotein and two members of the MRP subfamily.

Substitution of Trp1242 of TM17 alters substrate specificity of human multidrug resistance protein 3

Multidrug resistance protein 3 (MRP3) is an ATP-dependent transporter of 17beta-estradiol 17beta(d-glucuronide) (E(2)17betaG), leukotriene C(4) (LTC(4)), methotrexate, and the bile salts taurocholate and glycocholate. In the present study, the role of a highly conserved Trp residue at position 1242 on MRP3 transport function was examined by expressing wild-type MRP3 and Ala-, Cys-, Phe-, Tyr-, and Pro-substituted mutants in human embryonic kidney 293T cells. Four MRP3-Trp(1242) mutants showed significantly increased E(2)17betaG uptake, whereas transport by the Pro mutant was undetectable. Similarly, the Pro mutant did not transport LTC(4). By comparison, LTC(4) transport by the Ala, Cys, Phe, and Tyr mutants was reduced by approximately 35%. The Ala, Cys, Phe, and Tyr mutants all showed greatly reduced methotrexate and leucovorin transport, except the Tyr mutant, which transported leucovorin at levels comparable with wild-type MRP3. In contrast, the MRP3-Trp(1242) substitutions did not significantly affect taurocholate transport or taurocholate and glycocholate inhibition of E(2)17betaG uptake. Thus Trp(1242) substitutions markedly alter the substrate specificity of MRP3 but leave bile salt binding and transport intact.

Intermediate structural states involved in MRP1-mediated drug transport. Role of glutathione

Human multidrug resistance protein 1 (MRP1) is a member of the ATP-binding cassette transporter family and transports chemotherapeutic drugs as well as diverse organic anions such as leukotriene LTC(4). The transport of chemotherapeutic drugs requires the presence of reduced GSH. By using hydrogen/deuterium exchange kinetics and limited trypsin digestion, the structural changes associated with each step of the drug transport process are analyzed. Purified MRP1 is reconstituted into lipid vesicles with an inside-out orientation, exposing its cytoplasmic region to the external medium. The resulting proteoliposomes have been shown previously to exhibit both ATP-dependent drug transport and drug-stimulated ATPase activity. Our results show that during GSH-dependent drug transport, MRP1 does not undergo secondary structure changes but only modifications in its accessibility toward the external environment. Drug binding induces a restructuring of MRP1 membrane-embedded domains that does not affect the cytosolic domains, including the nucleotide binding domains, responsible for ATP hydrolysis. This demonstrates that drug binding to MRP1 is not sufficient to propagate an allosteric signal between the membrane and the cytosolic domains. On the other hand, GSH binding induces a conformational change that affects the structural organization of the cytosolic domains and enhances ATP binding and/or hydrolysis suggesting that GSH-mediated conformational changes are required for the coupling between drug transport and ATP hydrolysis. Following ATP binding, the protein adopts a conformation characterized by a decreased stability and/or an increased accessibility toward the aqueous medium. No additional change in the accessibility toward the solvent and/or the stability of this specific conformational state and no change of the transmembrane helices orientation are observed upon ATP hydrolysis. Binding of a non-transported drug affects the dynamic changes occurring during ATP binding and hydrolysis and restricts the movement of the drug and its release.

Transmembrane topology of the sulfonylurea receptor SUR1

Sulfonylurea receptors (SURx) are multi-spanning transmembrane proteins of the ATP-binding cassette (ABC) family, which associate with Kir6.x to form ATP-sensitive potassium channels. Two models, with 13-17 transmembrane segments, have been proposed for SURx topologies. Recently, we demonstrated that the amino-terminal region of SUR1 contains 5 transmembrane segments, supporting the 17-transmembrane model. To investigate the topology of the complete full-length SUR1, two strategies were employed. Topology was probed by accessibility of introduced cysteines to a membrane-impermeable biotinylating reagent, biotin maleimide. Amino acid positions 6/26, 99, 159, 337, 567, 1051, and 1274 were accessible, therefore extracellular, whereas many endogenous and some introduced cysteines were inaccessible, thus likely cytoplasmic or intramembrane. These sites correspond to extracellular loops 1-3, 5-6, and 8 and the NH2 terminus, and intracellular loops 3-8 and COOH terminus in the 17-transmembrane model. Immunofluorescence was used to determine accessibility of epitope-tagged SUR1 in intact and permeabilized cells. Epitopes at positions 337 and 1050 (putative external loops 3 and 6) were labeled in intact cells, therefore external, whereas positions 485 and 1119 (putative internal loops 5 and 7) only were accessible after permeabilization and therefore internal. These results are compatible with the 17-transmembrane model with two pairs of transmembrane segments as possible reentrant loops.

The structure of the multidrug resistance protein 1 (MRP1/ABCC1). crystallization and single-particle analysis

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-binding cassette (ABC) polytopic membrane transporter of considerable clinical importance that confers multidrug resistance on tumor cells by reducing drug accumulation by active efflux. MRP1 is also an efficient transporter of conjugated organic anions. Like other ABC proteins, including the drug resistance conferring 170-kDa P-glycoprotein (ABCB1), the 190-kDa MRP1 has a core structure consisting of two membrane-spanning domains (MSDs), each followed by a nucleotide binding domain (NBD). However, unlike P-glycoprotein and most other ABC superfamily members, MRP1 contains a third MSD with five predicted transmembrane segments with an extracytosolic NH(2) terminus. Moreover, the two nucleotide-binding domains of MRP1 are considerably more divergent than those of P-glycoprotein. In the present study, the first structural details of MRP1 purified from drug-resistant lung cancer cells have been obtained by electron microscopy of negatively stained single particles and two-dimensional crystals formed after reconstitution of purified protein with lipids. The crystals display p2 symmetry with a single dimer of MRP1 in the unit cell. The overall dimensions of the MRP1 monomer are approximately 80 x 100 A. The MRP1 monomer shows some pseudo-2-fold symmetry in projection, and in some orientations of the detergent-solubilized particles, displays a stain filled depression (putative pore) appearing toward the center of the molecule, presumably to enable transport of substrates. These data represent the first structural information of this transporter to approximately 22-A resolution and provide direct structural evidence for a dimeric association of the transporter in a reconstituted lipid bilayer.

Nramp 2 (DCT1/DMT1) expressed at the plasma membrane transports iron and other divalent cations into a calcein-accessible cytoplasmic pool

Nramp2, also known as DMT1 and DCT1, is a 12-transmembrane (TM) domain protein responsible for dietary iron uptake in the duodenum and iron acquisition from transferrin in peripheral tissues. Nramp2/DMT1 produces by alternative splicing two isoforms differing at their C terminus (isoforms I and II). The subcellular localization, mechanism of action, and destination of divalent cations transported by the two Nramp2 isoforms are not completely understood. Stable CHO transfectants expressing Nramp2 isoform II modified by addition of a hemaglutinin epitope in the loop defined by the TM7-TM8 interval were generated. Immunofluorescence with permeabilized and intact cells established that Nramp2 isoform II is expressed at the plasma membrane and demonstrated the predicted extracytoplasmic location of the TM7-TM8 loop. Using the fluorescent, metal-sensitive dye calcein, and a combination of membrane-permeant and -impermeant iron chelators, Nramp2 transport was measured and quantitated with respect to kinetic parameters and at steady state. Iron transport at the plasma membrane was time- and pH-dependent, saturable, and proportional to the amount of Nramp2 expression. Iron uptake by Nramp2 at the plasma membrane was into the nonferritin-bound, calcein-accessible so-called "labile iron pool." Ion selectivity experiments show that Nramp2 isoform II can also transport Co(2+) and Cd(2+) but not Mg(2+) into the calcein-accessible pool. Parallel experiments with transfectants expressing the lysosomal Nramp1 homolog do not show any divalent cation transport activity, establishing major functional differences between Nramp1 and Nramp2. Monitoring the effect of Nramp2 on the calcein-sensisitve labile iron pool allows a simple, rapid, and nonisotopic approach to the functional study of this protein.

Plant ABC transporters

The ATP binding cassette (ABC) superfamily is a large, ubiquitous and diverse group of proteins, most of which mediate transport across biological membranes. ABC transporters have been shown to function not only as ATP-dependent pumps, but also as ion channels and channel regulators. Whilst members of this gene family have been extensively characterised in mammalian and microbial systems, the study of plant ABC transporters is a relatively new field of investigation. Sequences of over 20 plant ABC proteins have been published and include homologues of P-glycoprotein, MRP, PDR5 and organellar transporters. At present, functions have been assigned to a small proportion of these genes and only the MRP subclass has been extensively characterised. This review aims to summarise literature relevant to the study of plant ABC transporters, to review methods of cloning, to discuss the utility of yeast and mammalian systems as models and to speculate on possible roles of uncharacterised ABC transporters in plants.

Molecular basis for K(ATP) assembly: transmembrane interactions mediate association of a K+ channel with an ABC transporter

K(ATP) channels are large heteromultimeric complexes containing four subunits from the inwardly rectifying K+ channel family (Kir6.2) and four regulatory sulphonylurea receptor subunits from the ATP-binding cassette (ABC) transporter family (SUR1 and SUR2A/B). The molecular basis for interactions between these two unrelated protein families is poorly understood. Using novel trafficking-based interaction assays, coimmunoprecipitation, and current measurements, we show that the first transmembrane segment (M1) and the N terminus of Kir6.2 are involved in K(ATP) assembly and gating. Additionally, the transmembrane domains, but not the nucleotide-binding domains, of SUR1 are required for interaction with Kir6.2. The identification of specific transmembrane interactions involved in K(ATP) assembly may provide a clue as to how ABC proteins that transport hydrophobic substrates evolved to regulate other membrane proteins.

Membrane topology and insertion of membrane proteins: search for topogenic signals

Integral membrane proteins are found in all cellular membranes and carry out many of the functions that are essential to life. The membrane-embedded domains of integral membrane proteins are structurally quite simple, allowing the use of various prediction methods and biochemical methods to obtain structural information about membrane proteins. A critical step in the biosynthetic pathway leading to the folded protein in the membrane is its insertion into the lipid bilayer. Understanding of the fundamentals of the insertion and folding processes will significantly improve the methods used to predict the three-dimensional membrane protein structure from the amino acid sequence. In the first part of this review, biochemical approaches to elucidate membrane protein topology are reviewed and evaluated, and in the second part, the use of similar techniques to study membrane protein insertion is discussed. The latter studies search for signals in the polypeptide chain that direct the insertion process. Knowledge of the topogenic signals in the nascent chain of a membrane protein is essential for the evaluation of membrane topology studies.

Conjugate export pumps of the multidrug resistance protein (MRP) family: localization, substrate specificity, and MRP2-mediated drug resistance

The membrane proteins mediating the ATP-dependent transport of lipophilic substances conjugated to glutathione, glucuronate, or sulfate have been identified as members of the multidrug resistance protein (MRP) family. Several isoforms of these conjugate export pumps with different kinetic properties and domain-specific localization in polarized human cells have been cloned and characterized. Orthologs of the human MRP isoforms have been detected in many different organisms. Studies in mutant rats lacking the apical isoform MRP2 (symbol ABCC2) indicate that anionic conjugates of endogenous and exogenous substances cannot exit from cells at a sufficient rate unless an export pump of the MRP family is present in the plasma membrane. Several mutations in the human MRP2 gene have been identified which lead to the absence of the MRP2 protein from the hepatocyte canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. Overexpression of recombinant MRP2 confers resistance to multiple chemotherapeutic agents. Because of its function in the terminal excretion of cytotoxic and carcinogenic substances, MRP2 as well as other members of the MRP family, play an important role in detoxification and chemoprevention.

Structural, mechanistic and clinical aspects of MRP1

The cDNA encoding ATP-binding cassette (ABC) multidrug resistance protein MRP1 was originally cloned from a drug-selected lung cancer cell line resistant to multiple natural product chemotherapeutic agents. MRP1 is the founder of a branch of the ABC superfamily whose members (from species as diverse as plants and yeast to mammals) share several distinguishing structural features that may contribute to functional and mechanistic similarities among this subgroup of transport proteins. In addition to its role in resistance to natural product drugs, MRP1 (and related proteins) functions as a primary active transporter of structurally diverse organic anions, many of which are formed by the biotransformation of various endo- and xenobiotics by Phase II conjugating enzymes, such as the glutathione S-transferases. MRP1 is involved in a number of glutathione-related cellular processes. Glutathione also appears to play a key role in MRP1-mediated drug resistance. This article reviews the discovery of MRP1 and its relationships with other ABC superfamily members, and summarizes current knowledge of the structure, transport functions and relevance of this protein to in vitro and clinical multidrug resistance.

An inventory of the human ABC proteins

Currently 30 human ABC proteins are represented by full sequences in various databases, and this paper provides a brief overview of these proteins. ABC proteins are composed of transmembrane domains (TMDs), and nucleotide binding domains (NBDs, or ATP-binding cassettes, ABSs). The arrangement of these domains, together with available membrane topology models of the family members, are presented. Based on their sequence similarity scores, the members of the human ABC protein family can be grouped into eight subfamilies. At present the MDR/TAP, the ALD, the MRP/CFTR, the ABC1, the White, the RNAseL inhibitor, the ANSA, and the GCN20 subfamilies are identified. Mutations of many human ABC proteins are known to be causative in inherited diseases, and a short description of the molecular pathology of these ABC gene-related genetic diseases is also provided.

Membrane topology of the amino-terminal region of the sulfonylurea receptor

The sulfonylurea receptor (SUR) is a member of the ATP-binding cassette family that is associated with Kir 6.x to form ATP-sensitive potassium channels. SUR is involved in nucleotide regulation of the channel and is the site of pharmacological interaction with sulfonylurea drugs and potassium channel openers. SUR contains three hydrophobic domains, TM(0), TM(1), and TM(2), with nucleotide binding folds following TM(1) and TM(2). Two topological models of SUR have been proposed containing either 13 transmembrane segments (in a 4+5+4 arrangement) or 17 transmembrane segments (in a 5+6+6 arrangement) (Aguilar-Bryan, L., Nichols, C. G., Wechsler, S. W., Clement, J. P. t., Boyd, A. E., III, González, G., Herrera-Sosa, H., Nguy, K., Bryan, J., and Nelson, D. A. (1995) Science 268, 423-426; Tusnády, G. E., Bakos, E., Váradi, A., and Sarkadi, B. (1997) FEBS Lett. 402, 1-3; Aguilar-Bryan, L., Clement, J. P., IV, González, G., Kunjilwar, K., Babenko, A., and Bryan, J. (1998) Physiol. Rev. 78, 227-245). We analyzed the topology of the amino-terminal TM(0) region of SUR1 using glycosylation and protease protection studies. Deglycosylation using peptide-N-glycosidase F and site-directed mutagenesis established that Asn(10), near the amino terminus, and Asn(1050) are the only sites of N-linked glycosylation, thus placing these sites on the extracellular side of the membrane. To study in detail the topology of SUR1, we constructed and expressed in vitro fusion proteins containing 1-5 hydrophobic segments of the TM(0) region fused to the reporter prolactin. The fusion proteins were subjected to a protease protection assay that reported the accessibility of the prolactin epitope. Our results indicate that the TM(0) region is comprised of 5 transmembrane segments. These data support the 5+6+6 model of SUR1 topology.

MRP3, an organic anion transporter able to transport anti-cancer drugs

The human multidrug-resistance protein (MRP) gene family contains at least six members: MRP1, encoding the multidrug-resistance protein; MRP2 or cMOAT, encoding the canalicular multispecific organic anion transporter; and four homologs, called MRP3, MRP4, MRP5, and MRP6. In this report, we characterize MRP3, the closest homolog of MRP1. Cell lines were retrovirally transduced with MRP3 cDNA, and new monoclonal antibodies specific for MRP3 were generated. We show that MRP3 is an organic anion and multidrug transporter, like the GS-X pumps MRP1 and MRP2. In Madin-Darby canine kidney II cells, MRP3 routes to the basolateral membrane and mediates transport of the organic anion S-(2,4-dinitrophenyl-)glutathione toward the basolateral side of the monolayer. In ovarian carcinoma cells (2008), expression of MRP3 results in low-level resistance to the epipodophyllotoxins etoposide and teniposide. In short-term drug exposure experiments, MRP3 also confers high-level resistance to methotrexate. Neither 2008 cells nor Madin-Darby canine kidney II cells overexpressing MRP3 showed an increase in glutathione export or a decrease in the level of intracellular glutathione, in contrast to cells overexpressing MRP1 or MRP2. We discuss the possible function of MRP3 in (hepatic) physiology and its potential contribution to drug resistance of cancer cells.

Topological and functional analysis of the human reduced folate carrier by hemagglutinin epitope insertion

The membrane topology of the human reduced folate carrier protein (591 amino acids) was assessed by single insertions of the hemagglutinin epitope into nine sites of the protein. Reduced folate carrier-deficient Chinese hamster ovary cells expressing each of these constructs were probed with anti-hemagglutinin epitope monoclonal antibodies to assess whether the insertion was exposed to the external environment or to the cytoplasm. The results are consistent with the 12-transmembrane topology predicted for this protein. The hemagglutinin epitope insertion mutants were also tested for their effects on the function of the reduced folate carrier. For these studies, each of the constructs had a carboxyl-terminal fusion of the enhanced green fluorescent protein to monitor and quantitate expression. Insertions into the external loop between transmembrane regions 7 and 8 (Pro-297), the cytoplasmic loop between transmembrane regions 6 and 7 (Ser-225), and near the cytoplasmic amino and carboxyl termini (Pro-20 and Gly-492, respectively) had minor effects on methotrexate binding and uptake. The insertion into the cytoplasmic loop between transmembrane regions 10 and 11 (Gln-385) greatly reduced both binding and uptake of methotrexate, whereas the insertion into the external loop between transmembrane regions 11 and 12 (Pro-427) selectively interfered with uptake but not binding.

MRP3, a new ATP-binding cassette protein localized to the canalicular domain of the hepatocyte

Bile secretion in liver is driven in large part by ATP-binding cassette (ABC)-type proteins that reside in the canalicular membrane and effect ATP-dependent transport of bile acids, phospholipids, and non-bile acid organic anions. Canalicular ABC-type proteins can be classified into two subfamilies based on membrane topology and sequence identity: MDR1, MDR3, and SPGP resemble the multidrug resistance (MDR) P-glycoprotein, whereas MRP2 is similar in structure and sequence to the multidrug resistance protein MRP1 and transports similar substrates. We now report the isolation of the rMRP3 gene from rat liver, which codes for a protein 1522 amino acids in length that exhibits extensive sequence similarity with MRP1 and MRP2. Northern blot analyses indicate that rMRP3 is expressed in lung and intestine of Sprague-Dawley rats as well as in liver of Eisai hyperbilirubinemic rats and TR- mutant rats, which are deficient in MRP2 expression. rMRP3 expression is also transiently induced in liver shortly after birth and during obstructive cholestasis. Antibodies raised against MRP3 recognize a polypeptide of 190-200 kDa, which is reduced in size to 155-165 kDa after treatment with endoglycosidases. Immunoblot analysis and immunoconfocal microscopy indicate that rMRP3 is present in the canalicular membrane, suggesting that it may play a role in bile formation.

CpABC, a Cryptosporidium parvum ATP-binding cassette protein at the host-parasite boundary in intracellular stages

The intracellular parasite Cryptosporidium parvum develops inside a vacuole at the apex of its epithelial host cell. The developing parasite is separated from the host cell cytoplasm by a zone of attachment that consists of an extensively folded membranous structure known as the feeder organelle. It has been proposed that the feeder organelle is the site of regulation of transport of nutrients and drugs into the parasite. In this report, we localize an approximately 200-kDa integral membrane protein, CpABC, from Cryptosporidium parvum to the host-parasite boundary, possibly the feeder organelle. The predicted amino acid sequence of CpABC has significant structural similarity with the cystic fibrosis conductance regulator and the multidrug resistance protein subfamily of ATP-binding cassette proteins. This is an example of a parasite-encoded transport protein localized at the parasite-host interface of an intracellular protozoan.

A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane K(ATP) channels

Proper ion channel function often requires specific combinations of pore-forming alpha and regulatory beta subunits, but little is known about the mechanisms that regulate the surface expression of different channel combinations. Our studies of ATP-sensitive K+ channel (K(ATP)) trafficking reveal an essential quality control function for a trafficking motif present in each of the alpha (Kir6.1/2) and beta (SUR1) subunits of the K(ATP) complex. We show that this novel motif for endoplasmic reticulum (ER) retention/retrieval is required at multiple stages of K(ATP) assembly to restrict surface expression to fully assembled and correctly regulated octameric channels. We conclude that exposure of a three amino acid motif (RKR) can explain how assembly of an ion channel complex is coupled to intracellular trafficking.

The human multidrug resistance protein MRP1 translocates sphingolipid analogs across the plasma membrane

Recently, we have provided evidence that the ABC-transporter MDR1 P-glycoprotein translocates analogs of various lipid classes across the apical plasma membrane of polarized LLC-PK1 cells transfected with MDR1 cDNA. Here, we show that expression of the basolateral ABC-transporter MRP1 (the multidrug resistance protein) induced lipid transport to the exoplasmic leaflet of the basolateral plasma membrane of LLC-PK1 cells at 15 degreesC. C6-NBD-glucosylceramide synthesized on the cytosolic side of the Golgi complex, but not C6-NBD-sphingomyelin synthesized in the Golgi lumen, became accessible to depletion by BSA in the basal culture medium. This suggests the absence of vesicular traffic and direct translocation of C6-NBD-glucosylceramide by MRP1 across the basolateral membrane. In line with this, transport of the lipid to the exoplasmic leaflet depended on the intracellular glutathione concentration and was inhibited by the MRP1-inhibitors sulfinpyrazone and indomethacin, but not by the MDR1 P-glycoprotein inhibitor PSC 833. In contrast to the broad substrate specificity of the MDR1 P-glycoprotein, MRP1 selectively transported C6-NBD-glucosylceramide and C6-NBD-sphingomyelin, the latter only when it was released from the Golgi lumen by brefeldin A. This shows the specific nature of the lipid translocation. We conclude that the transport activity of MDR1 P-glycoprotein and MRP1 must be taken into account in studies on the transport of lipids to the cell surface.

Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin, the oxidative product of heme in mammals, is excreted into the bile after its esterification with glucuronic acid to polar mono- and diconjugated derivatives. The accumulation of unconjugated and conjugated bilirubin in the serum is caused by several types of hereditary disorder. The Crigler-Najjar syndrome is caused by a defect in the gene which encodes bilirubin UDP-glucuronosyltransferase (UGT), whereas the Dubin-Johnson syndrome is characterized by a defect in the gene which encodes the canalicular bilirubin conjugate export pump of hepatocytes. Animal models such as the unconjugated hyperbilirubinemic Gunn rat, the conjugated hyperbilirubinemic GY/TR-, and the Eisai hyperbilirubinemic rat, have contributed to the understanding of the molecular basis of hyperbilirubinemia in humans. Elucidation of both the structure of the UGT1 gene complex, and the Mrp2 (cMoat) gene which encodes the canalicular conjugate export pump, has led to a greater understanding of the genetic basis of hyperbilirubinemia.