Identification and characterization of functionally important elements in the multidrug resistance protein 1 COOH-terminal region

Structural Characterization of ABCB1, the Gene Underlying the d2 Dwarf Phenotype in Pearl Millet, Cenchrus Americanus (L.) Morrone

Pearl millet is an important food crop in arid and semi-arid regions of South Asia and sub-Saharan Africa and is grown in Australia and the United States as a summer fodder crop. The d2 dwarf germplasm has been widely used in the last half-century to develop high-performing pearl millet hybrids. We previously mapped the d2 phenotype to a 1.6 cM region in linkage group (LG) 4 and identified the ABCB1 gene as a candidate underlying the trait. Here, we report the sequence, structure and expression of ABCB1 in tall (D2D2) and d2 dwarf (d2d2) germplasm. The ABCB1 allele in d2 dwarfs differs from that in tall inbreds by the presence of two different high copy transposable elements, one in the coding region and the second located 664 bp upstream of the ATG start codon. These transposons were present in all d2 dwarfs tested that were reported to be of independent origin and absent in the analyzed wild-type tall germplasm. We also compared the expression profile of this gene in different organs of multiple tall and d2 dwarf inbreds, including the near-isogenic inbreds at the d2 locus, Tift 23B (D2D2) and Tift 23DB (d2d2). Heterologous transformation of the tall (Ca_ABCB1) and the d2 dwarf (Ca_abcb1) pearl millet alleles in the Arabidopsis double mutant abcb1abcb19 showed that the pearl millet D2 but not the d2 allele complements the Arabidopsis abcb1 mutation. Our studies also show the importance of the COOH-terminal 22 amino acids of the ABCB1 protein in either protein function or stability.

MRP1 and its role in anticancer drug resistance

The phenomenon of multidrug resistance (MDR) in cancer is associated with the overexpression of the ATP-binding cassette (ABC) transporter proteins, including multidrug resistance-associated protein 1 (MRP1) and P-glycoprotein. MRP1 plays an active role in protecting cells by its ability to efflux a vast array of drugs to sub-lethal levels. There has been much effort in elucidating the mechanisms of action, structure and substrates and substrate binding sites of MRP1 in the last decade. In this review, we detail our current understanding of MRP1, its clinical relevance and highlight the current environment in the search for MRP1 inhibitors. We also look at the capacity for the rapid intercellular transfer of MRP1 phenotype from spontaneously shed membrane vesicles known as microparticles and discuss the clinical and therapeutic significance of this in the context of cancer MDR.

Domain location within the cystic fibrosis transmembrane conductance regulator protein investigated by electron microscopy and gold labelling

The domain organisation of the cystic fibrosis transmembrane conductance regulator (CFTR) protein was studied using electron microscopy of detergent-solubilised dimeric complexes. Ni-NTA nanogold labelling data suggest that in the nonphosphorylated, nucleotide-free state, the C-terminus is intimately associated with the cytoplasmic ATP-binding regions, whilst part of the regulatory domain occupies a position close to the cytoplasmic surface of the lipid membrane. Removal of the entire second nucleotide binding domain (NBD2) results in a deficit in the CFTR structure that is consistent with the size and shape of a single NBD. The data suggest that NBD2 lies closer to the C2 symmetry axis than the first nucleotide binding domain (NBD1) and that NBD2 from one CFTR monomer also contacts NBD1 from the opposing one. These data suggest that current homology models for CFTR based on other ATP-binding cassette proteins appear to be reasonable, at least to low resolution. We also find that Ni-NTA nanogold labelling of an internal hexa-Histidine sequence is a valuable approach to locate individual protein domains.

Identification of regions required for apical membrane localization of human multidrug resistance protein 2

Multidrug resistance proteins MRP1 and MRP2 transport a wide range of endo- and xenobiotics. However, with the exception of certain parts of the brain, MRP1 traffics to basolateral membranes of polarized cells, whereas MRP2 is apical in location and thus it is particularly important for systemic elimination of such compounds. Different regions of MRP1 and MRP2 seem to target them to their respective membrane locations. In addition to two "core" membrane spanning domains (MSDs) characteristic of ATP-binding cassette transporters, MRP1 and MRP2 have a third NH2-terminal MSD (MSD0), which is not required for basolateral targeting of MRP1, or for transport of at least some substrates. Here, we demonstrate that all elements necessary for apical targeting of MRP2 reside in MSD0 and the adjacent cytoplasmic loop (CL) 3. Furthermore, we show that this region of MRP2 can target the core of MRP1 to an exclusively apical location. Within MRP2 CL3, we identified a lysine-rich element that is essential for apical targeting. When introduced into MRP1, this element alone is sufficient to result in partial apical localization. However, exclusive targeting to the apical membrane seems to require the integrity of the entire region encompassing MSD0 and CL3 of MRP2. Because CL3 of MRP1 is critical for binding, transport, or both of several compounds, we also examined the function of hybrids containing all, or portions of MRP2 MSD0 and CL3. Our results indicate that CL3 is important for interaction with both the glutathione and glucuronide conjugates tested, but that different regions may be involved.

Apical membrane targeting and trafficking of the human proton-coupled transporter in polarized epithelia

The human proton-coupled folate transporter (hPCFT) is a recently discovered intestinal transporter involved in folate uptake in epithelia (and possibly other cells). Little is currently known about the structure-function relationship of the different domains of this transporter, particularly which regions are important for substrate transport as well as targeting of the transporter to the apical cell surface of polarized cells. Here we have investigated the role of the COOH-terminal domain and a well-conserved sequence separating transmembrane (TM) domains TM2 and TM3 (DXXGRR; amino acids 109-114) speculated by others to be important for transport function. Using live cell imaging approaches, we show that 1) an hPCFT-yellow fluorescent protein construct is functionally expressed at the apical membrane domain and is localized differentially to the human reduced folate carrier; 2) the predicted cytoplasmic COOH-terminal region of hPCFT is not essential for apical targeting or transporter functionality; 3) mutations that ablate a consensus beta-turn sequence separating predicted TM2 and TM3 abolished apical [(3)H]folic acid uptake as a consequence of endoplasmic reticulum retention of mutant, likely misfolded, transporters; and 4) cell surface delivery of hPCFT is disrupted by microtubule depolymerization or by overexpression of the dynactin complex dynamitin (p50). For the first time, our data present information regarding structure-function and membrane targeting of the hPCFT polypeptide, as well as the mechanisms that control its steady-state expression in polarized cells.

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.

Insight in eukaryotic ABC transporter function by mutation analysis

With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

Substrate recognition and transport by multidrug resistance protein 1 (ABCC1)

Multidrug resistance protein (MRP) 1 belongs to the 'C' branch of the ABC transporter superfamily. MRP1 is a high-affinity transporter of the cysteinyl leukotriene C(4) and is responsible for the systemic release of this cytokine in response to an inflammatory stimulus. However, the substrate specificity of MRP1 is extremely broad and includes many organic anion conjugates of structurally unrelated endo- and xenobiotics. In addition, MRP1 transports unmodified hydrophobic compounds, such as natural product type chemotherapeutic agents and mutagens, such as aflatoxin B(1). Transport of several of these compounds has been shown to be dependent on the presence of reduced glutathione (GSH). More recently, GSH has also been shown to stimulate the transport of some conjugated compounds, including sulfates and glucuronides. Here, we summarize current knowledge of the substrate specificity and modes of transport of MRP1 and discuss how the protein may recognize its structurally diverse substrates.

Role of the NH2-terminal membrane spanning domain of multidrug resistance protein 1/ABCC1 in protein processing and trafficking

Multidrug resistance protein (MRP)1/ABCC1 transports organic anionic conjugates and confers resistance to cytotoxic xenobiotics. In addition to two membrane spanning domains (MSDs) typical of most ATP-binding cassette (ABC) transporters, MRP1 has a third MSD (MSD0) of unknown function. Unlike some topologically similar ABCC proteins, removal of MSD0 has minimal effect on function, nor does it prevent MRP1 from trafficking to basolateral membranes in polarized cells. However, we find that independent of cell type, the truncated protein accumulates in early/recycling endosomes. Using a real-time internalization assay, we demonstrate that MSD0 is important for MRP1 retention in, or recycling to, the plasma membrane. We also show that MSD0 traffics independently to the cell surface and promotes membrane localization of the core-region of MRP1 when the two protein fragments are coexpressed. Finally, we demonstrate that MSD0 becomes essential for trafficking of MRP1 when the COOH-terminal region of the protein is mutated. These studies demonstrate that MSD0 and the COOH-terminal region contain redundant trafficking signals, which only become essential when one or the other region is missing or is mutated. These data explain apparent differences in the trafficking requirement for MSD0 and the COOH-terminal region of MRP1 compared with other ABCC proteins.

The Dileucine Motif at the COOH Terminus of Human Multidrug Resistance P-glycoprotein Is Important for Folding but Not Activity

P-glycoprotein (P-gp, ABCB1) actively transports a broad range of cytotoxic compounds out of the cell. The COOH terminus of P-gp contains a dileucine motif (Leu(1260)-Leu(1261)) and a conserved phenylalanine (Phe(1268)). Similar residues in SUR1 (ABCC8) were reported to be important plasma membrane-targeting signals (Sharma, N., Crane, A., Clement, J. P. t., Gonzalez, G., Babenko, A. P., Bryan, J., and Aguilar-Bryan, L. (1999) J. Biol. Chem. 274, 20628-20632). Here, we used alanine-scanning mutagenesis to test whether these residues were essential for trafficking of P-gp to the cell surface. Mutant L1260A expressed a 150-kDa immature protein that did not reach the cell surface and was sensitive to digestion by Endo H(f). By contrast, mutants L1261A, F1268A, and wild-type P-gps expressed the 170-kDa mature proteins at the cell surface. Mutation of Leu(1260) to Gly, Ile, Trp, Lys, or Glu also resulted in the expression of the 150-kDa immature protein. All of the mutants, however, expressed the 170-kDa protein in the presence of the drug substrate/specific chemical chaperone cyclosporin A. Mutant L1260A P-gp exhibited drug-stimulated ATPase activities similar to that of wild-type enzyme after rescue with cyclosporin A. Deletion of the last 22 amino acids (Q(1259)-Q(1280)) also caused misprocessing. The mutant, however, was rescued by expression in the presence of cyclosporin A and conferred resistance to colchicine in transfected cells. These results show that the dileucine motif is not a plasma membrane targeting signal. The COOH terminus is required for proper folding of P-gp but not for activity.