Transport of leukotriene C4 by a cysteine-less multidrug resistance protein 1 (MRP1)

Directed evolution of P-glycoprotein cysteines reveals site-specific, non-conservative substitutions that preserve multidrug resistance

Pgp (P-glycoprotein) is a prototype ABC (ATP-binding-cassette) transporter involved in multidrug resistance of cancer. We used directed evolution to replace six cytoplasmic Cys (cysteine) residues in Pgp with all 20 standard amino acids and selected for active mutants. From a pool of 75000 transformants for each block of three Cys, we identified multiple mutants that preserved drug resistance and yeast mating activity. The most frequent substitutions were glycine and serine for Cys427 (24 and 20%, respectively) and Cys1070 (37 and 25%) of the Walker A motifs in the NBDs (nucleotide-binding domains), Cys1223 in NBD2 (25 and 8%) and Cys638 in the linker region (24 and 16%), whereas close-by Cys669 tolerated glycine (16%) and alanine (14%), but not serine (absent). Cys1121 in NBD2 showed a clear preference for positively charged arginine (38%) suggesting a salt bridge with Glu269 in the ICL2 (intracellular loop 2) may stabilize domain interactions. In contrast, three Cys residues in transmembrane α-helices could be successfully replaced by alanine. The resulting CL (Cys-less) Pgp was fully active in yeast cells, and purified proteins displayed drug-stimulated ATPase activities indistinguishable from WT (wild-type) Pgp. Overall, directed evolution identified site-specific, non-conservative Cys substitutions that allowed building of a robust CL Pgp, an invaluable new tool for future functional and structural studies, and that may guide the construction of other CL proteins where alanine and serine have proven unsuccessful.

Molecular mechanism of ATP-dependent solute transport by multidrug resistance-associated protein 1

Millions of new cancer patients are diagnosed each year and over half of these patients die from this devastating disease. Thus, cancer causes a major public health problem worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Over-expression of ATP-binding cassette transporters, such as P-glycoprotein, breast cancer resistance protein and/or multidrug resistance-associated protein 1 (MRP1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs across the cell membrane barrier. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

Effect of cysteine mutagenesis on the function and disulfide bond formation of human ABCG2

ABCG2 is a member of the ATP-binding cassette (ABC) transporter superfamily. Its overexpression causes multidrug resistance in cancer chemotherapy. Based on its apparent half size in sequence when compared with other traditional ABC transporters, ABCG2 has been thought to exist and function as a homodimer linked by intermolecular disulfide bonds. However, recent evidence suggests that ABCG2 may exist as a higher form of oligomers due to noncovalent interactions. In this study, we attempted to create a cysless mutant ABCG2 as a tool for further characterization of this molecule. However, we found that the cysless mutant ABCG2 is well expressed but not functional. Mapping of the cysteine residues showed that three cysteine residues (Cys284, Cys374, and Cys438) are required concurrently for the function of ABCG2 and potentially for intramolecular disulfide bond formation. We also found that the cysteine residues (Cys592, Cys603, and Cys608) in the third extracellular loop are involved in forming intermolecular disulfide bonds and that mutation of these residues does not affect the expression or drug transport activity of human ABCG2. Thus, we conclude that Cys284, Cys374, and Cys438, which may be involved in intramolecular disulfide bond formation, are concurrently required for ABCG2 function, whereas Cys592, Cys603, and Cys608, potentially involved in intermolecular disulfide bond formation, are not required.

A molecular understanding of ATP-dependent solute transport by multidrug resistance-associated protein MRP1

Over a million new cases of cancers are diagnosed each year in the United States and over half of these patients die from these devastating diseases. Thus, cancers cause a major public health problem in the United States and worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Numerous mechanisms of MDR exist in cancer cells, such as intrinsic or acquired MDR. Overexpression of ATP-binding cassette (ABC) drug transporters, such as P-glycoprotein (P-gp or ABCB1), breast cancer resistance protein (BCRP or ABCG2) and/or multidrug resistance-associated protein (MRP1 or ABCC1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs. In addition to their roles in MDR, there is substantial evidence suggesting that these drug transporters have functions in tissue defense. Basically, these drug transporters are expressed in tissues important for absorption, such as in lung and gut, and for metabolism and elimination, such as in liver and kidney. In addition, these drug transporters play an important role in maintaining the barrier function of many tissues including blood-brain barrier, blood-cerebral spinal fluid barrier, blood-testis barrier and the maternal-fetal barrier. Thus, these ATP-dependent drug transporters play an important role in the absorption, disposition and elimination of the structurally diverse array of the endobiotics and xenobiotics. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

The amino terminus of the human multidrug resistance transporter ABCC1 has a U-shaped folding with a gating function

Multidrug resistance is a serious problem in successful cancer chemotherapy. Studies using model cell lines have demonstrated that overexpression of some members of the ATP-binding cassette (ABC) transporter superfamily, such as ABCC1, causes enhanced efflux and, thus, decreased accumulation of multiple anticancer drugs, which leads to increased cell survival. Unlike most other ABC transporters, ABCC1 has an additional membrane-spanning domain (MSD0) with a putative extracellular amino terminus of 32 amino acids. However, the function of MSD0 and the role of the extracellular amino terminus are largely unknown. In this study, we examined the structural folding and the function of the amino terminus. We found that it has a U-shaped folding with the bottom of the U-structure facing cytoplasm and both ends in extracellular space. We also found that this U-shaped amino terminus probably functions as a gate to regulate the drug transport activity of human ABCC1.

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.

The remarkable transport mechanism of P-glycoprotein: a multidrug transporter

Human P-glycoprotein (ABCB1) is a primary multidrug transporter located in plasma membranes, that utilizes the energy of ATP hydrolysis to pump toxic xenobiotics out of cells. P-glycoprotein employs a most unusual molecular mechanism to perform this drug transport function. Here we review our work to elucidate the molecular mechanism of drug transport by P-glycoprotein. High level heterologous expression of human P-glycoprotein, in the yeast Saccharomyces cerevisiae, has facilitated biophysical studies in purified proteoliposome preparations. Development of novel spin-labeled transport substrates has allowed for quantitative and rigorous measurements of drug transport in real time by EPR spectroscopy. We have developed a new drug transport model of P-glycoprotein from the results of mutagenic, quantitative thermodynamic and kinetic studies. This model satisfactorily accounts for most of the unusual kinetic, coupling, and physiological features of P-glycoprotein. Additionally, an atomic detail structural model of P-glycoprotein has been devised to place our results within a proper structural context.

Analysis of human multidrug resistance protein 1 (ABCC1) by matrix-assisted laser desorption ionization/time of flight mass spectrometry: toward identification of leukotriene C4 binding sites

Multidrug resistance in tumor cells may be caused by reduced drug accumulation resulting from expression of one or more proteins belonging to the ATP-binding cassette (ABC) transporter superfamily. In addition to their drug efflux properties, certain ABC proteins such as multidrug resistance protein 1 (MRP1) (ABCC1) mediate the ATP-dependent transport of a broad array of organic anions. The intrinsically photoreactive glutathione-conjugated cysteinyl leukotriene C4 (LTC4) is a high-affinity physiological substrate of MRP1 and is widely regarded as a model compound for evaluating the substrate binding and transport properties of wild-type and mutant forms of the transporter. In the present study, we have optimized high-level expression of recombinant human MRP1 in Pichia pastoris and developed a two-step purification scheme that results in purification of the transporter to >90% homogeneity. Peptide mapping by matrix-assisted laser desorption ionization/time of flight mass spectrometry of the peptides generated by in-gel protease digestions of purified underglycosylated MRP1 identified 96.7% of the MRP1 sequence with >98% coverage of its 17 transmembrane helices. Subsequent comparisons with mass spectra of MRP1 photolabeled with LTC4 identified six candidate LTC4-modified peptide fragments that are consistent with the conclusion that the intracellular juxtamembrane positions of transmembrane helices 6, 7, 10, 17, and a COOH-proximal portion of the cytoplasmic loop that links the first and second membrane spanning domains are part of the LTC4 binding site of the transporter. Our studies confirm the usefulness of mass spectrometry for analysis of mammalian polytopic membrane proteins and for identification of substrate binding sites of human MRP1.

Membrane Transport Proteins with Complete Replacement of Transmembrane Helices with Polyalanine Sequences Remain Functional

Approximately 25% of all genome coding sequences correspond to membrane proteins, which perform varied and essential functions in cells. Eukaryotic integral membrane proteins are predominantly alpha-helical proteins that span the membrane several times. The most frequent approach to identifying transmembrane-helix amino acids essential for function is to substitute native residues, one at a time, with Cys or Ala (Cys- and Ala-scanning mutagenesis). Here, we present a new approach, in which complete transmembrane-helix native sequences are substituted with poly-Ala sequences. We show that the basic functional features of two dissimilar membrane proteins, which function as a channel and a pump, respectively, are maintained when certain individual alpha-helices are replaced with poly-Ala sequences. This approach ("helix-scanning mutagenesis") allows for rapid identification of helices containing residues essential for function and can be used as a primary helix-screening tool, followed by individual amino acid substitutions when specific helix poly-Ala replacements cause functional changes in the protein.

Identification of proline residues in the core cytoplasmic and transmembrane regions of multidrug resistance protein 1 (MRP1/ABCC1) important for transport function, substrate specificity, and nucleotide interactions

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-binding cassette transporter that confers resistance to drugs and mediates the transport of organic anions. MRP1 has a core structure of two membrane spanning domains (MSDs) each followed by a nucleotide binding domain. This core structure is preceded by a third MSD with five transmembrane (TM) helices, whereas MSD2 and MSD3 each contain six TM helices. We investigated the consequences of Ala substitution of 18 Pro residues in both the non-membrane and TM regions of MSD2 and MSD3 on MRP1 expression and organic anion transport function. All MRP1-Pro mutants except P1113A were expressed in human embryonic kidney cells at levels comparable with wild-type MRP1. In addition, five mutants containing substitutions of Pro residues in or proximal to the TM helices of MSD2 (TM6-Pro(343), TM8-Pro(448), TM10-Pro(557), and TM11-Pro(595)) and MSD3 (TM14-Pro(1088)) exhibited significantly reduced transport of five organic anion substrates. In contrast, mutation of Pro(1150) in the cytoplasmic loop (CL7) linking TM15 to TM16 caused a substantial increase in 17beta-estradiol-17-beta-(D-glucuronide) and methotrexate transport, whereas transport of other organic anions was reduced or unchanged. Significant substrate-specific changes in the ATP dependence of transport and binding by the P1150A mutant were also observed. Our findings demonstrate the importance of TM6, TM8, TM10, TM11, and TM14 in MRP1 transport function and suggest that CL7 may play a differential role in coupling the activity of the nucleotide binding domains to the translocation of different substrates across the membrane.

Expression of functional multidrug-resistance protein 1 in Saccharomyces cerevisiae: effects of N- and C-terminal affinity tags

Studies of the multidrug-resistance protein 1 (MRP1) have been hampered by the lack of a simple expression system allowing for rapid generation of mutants and yielding milligram amounts of protein. Here, we describe a Saccharomyces cerevisiae expression system that meets those conditions. MRP1 was expressed under the control of the constitutive PMA1 (yeast proton pump) promoter. The best conditions for expression were determined, including the use of the chemical chaperone glycerol, which increased MRP1 expression. N-terminal poly-histidine or FLAG affinity tags reduce MRP1 expression, whereas the same tags fused to the C-terminus had no effect. All the fusion proteins were functional. We conclude that because of its low cost and simplicity, the S. cerevisiae-based MRP1-expression system will be useful for studies where a large number of mutants or milligram amounts of purified MRP1 are needed.