Drug-stimulated nucleotide trapping in the human multidrug transporter MDR1. Cooperation of the nucleotide binding domains

Correlation between steady-state ATP hydrolysis and vanadate-induced ADP trapping in Human P-glycoprotein. Evidence for ADP release as the rate-limiting step in the catalytic cycle and its modulation by substrates

P-glycoprotein (Pgp) is a transmembrane protein conferring multidrug resistance to cells by extruding a variety of amphipathic cytotoxic agents using energy from ATP hydrolysis. The objective of this study was to understand how substrates affect the catalytic cycle of ATP hydrolysis by Pgp. The ATPase activity of purified and reconstituted recombinant human Pgp was measured using a continuous cycling assay. Pgp hydrolyzes ATP in the absence of drug at a basal rate of 0.5 micromol x min x mg(-1) with a K(m) for ATP of 0.33 mm. This basal rate can be either increased or decreased depending on the Pgp substrate used, without an effect on the K(m) for ATP or 8-azidoATP and K(i) for ADP, suggesting that substrates do not affect nucleotide binding to Pgp. Although inhibitors of Pgp activity, cyclosporin A, its analog PSC833, and rapamycin decrease the rate of ATP hydrolysis with respect to the basal rate, they do not completely inhibit the activity. Therefore, these drugs can be classified as substrates. Vanadate (Vi)-induced trapping of [alpha-(32)P]8-azidoADP was used to probe the effect of substrates on the transition state of the ATP hydrolysis reaction. The K(m) for [alpha-(32)P]8-azidoATP (20 microm) is decreased in the presence of Vi; however, it is not changed by drugs such as verapamil or cyclosporin A. Strikingly, the extent of Vi-induced [alpha-(32)P]8-azidoADP trapping correlates directly with the fold stimulation of ATPase activity at steady state. Furthermore, P(i) exhibits very low affinity for Pgp (K(i) approximately 30 mm for Vi-induced 8-azidoADP trapping). In aggregate, these data demonstrate that the release of Vi trapped [alpha-(32)P]8-azidoADP from Pgp is the rate-limiting step in the steady-state reaction. We suggest that substrates modulate the rate of ATPase activity of Pgp by controlling the rate of dissociation of ADP following ATP hydrolysis and that ADP release is the rate-limiting step in the normal catalytic cycle of Pgp.

ABC drug transporters: hereditary polymorphisms and pharmacological impact in MDR1, MRP1 and MRP2

Transport by ATP-dependent efflux pumps, such as P-glycoprotein (PGP) and multi-drug resistance related proteins (MRPs), influences bioavailability and disposition of drugs. These efflux pumps serve as defence mechanisms and determine bioavailability and CNS concentrations of many drugs. However, despite the fact that substantial data have been accumulated on the structure, function and pharmacological role of ABC transporters and even though modification of PGP function is an important mechanism of drug interactions and adverse effects in humans, there is a striking lack of data on variability of the underlying genes. This review focuses on the human drug transporter proteins PGP (MDR1) and the multi-drug resistance proteins MRP1 and MRP2. An overview is provided of pharmacologically relevant genetic, structural and functional data as well as on hereditary polymorphisms, their phenotypical consequences and pharmacological implications.

Characterization of the amino-terminal regions in the human multidrug resistance protein (MRP1)

The human multidrug resistance protein (MRP1) contributes to drug resistance in cancer cells. In addition to an MDR1-like core, MRP1 contains an N-terminal membrane-bound (TMD(0)) region and a cytoplasmic linker (L(0)), both characteristic of several members of the MRP family. In order to study the role of the TMD(0) and L(0) regions, we constructed various truncated and mutated MRP1, and chimeric MRP1-MDR1 molecules, which were expressed in insect (Sf9) and polarized mammalian (MDCKII) cells. The function of the various proteins was examined in isolated membrane vesicles by measuring the transport of leukotriene C(4) and other glutathione conjugates, and by vanadate-dependent nucleotide occlusion. Cellular localization, and glutathione-conjugate and drug transport, were also studied in MDCKII cells. We found that chimeric proteins consisting of N-terminal fragments of MRP1 fused to the N terminus of MDR1 preserved the transport, nucleotide occlusion and apical membrane routing of wild-type MDR1. As shown before, MRP1 without TMD(0)L(0) (Delta MRP1), was non-functional and localized intracellularly, so we investigated the coexpression of Delta MRP1 with the isolated L(0) region. Coexpression yielded a functional MRP1 molecule in Sf9 cells and routing to the lateral membrane in MDCKII cells. Interestingly, the L(0) peptide was found to be associated with membranes in Sf9 cells and could only be solubilized by urea or detergent. A 10-amino-acid deletion in a predicted amphipathic region of L(0) abolished its attachment to the membrane and eliminated MRP1 transport function, but did not affect membrane routing. Taken together, these experiments suggest that the L(0) region forms a distinct domain within MRP1, which interacts with hydrophobic membrane regions and with the core region of MRP1.

Mutational analysis of conserved carboxylate residues in the nucleotide binding sites of P-glycoprotein

Mutagenesis was used to investigate the functional role of six pairs of aspartate and glutamate residues (D450/D1093, E482/E1125, E552/E1197, D558/D1203, D592/D1237, and E604/E1249) that are highly conserved in the nucleotide binding sites of P-glycoprotein (Mdr3) and of other ABC transporters. Removal of the charge in E552Q/E1197Q and D558N/D1203N produced proteins with severely impaired biological activity when the proteins were analyzed in yeast cells for cellular resistance to FK506 and restoration of mating in a ste6Delta mutant. Mutations at other acidic residues had no apparent effect in the same assays. These four mutants were expressed in Pichia pastoris, purified to homogeneity, and biochemically characterized with respect to ATPase activity. Studies with purified proteins showed that mutants D558N and D1203N retained 14 and 30% of the drug-stimulated ATPase activity of wild-type (WT) Mdr3, respectively, and vanadate trapping of 8-azido[alpha-(32)P]nucleotide confirmed slower basal and drug-stimulated 8-azido-ATP hydrolysis compared to that for WT Mdr3. The E552Q and E1197Q mutants showed no drug-stimulated ATPase activity. Surprisingly, drugs did stimulate vanadate trapping of 8-azido[alpha-(32)P]nucleotide in E552Q and E1197Q at a level similar to that of WT Mdr3. This suggests that formation of the catalytic transition state can occur in these mutants, and that the bond between the beta- and gamma-phosphates is hydrolyzed. In addition, photolabeling by 8-azido[alpha-(32)P]nucleotide in the presence or absence of drug was also detected in the absence of vanadate in these mutants. These results suggest that steps after the transition state, possibly involved in release of MgADP, are severely impaired in these mutant enzymes.

Cooperativity in the inhibition of P-glycoprotein-mediated daunorubicin transport: evidence for half-of-the-sites reactivity

P-Glycoprotein (Pgp) is an important transport enzyme composed of two homologous domains and transports a wide range of structurally diverse xenobiotics from the cell. Recent studies have indicated that allosteric interactions occur between the nucleotide binding domains and between the substrate binding domains of the two halves, but the extent of this interaction as well as the means by which the enzyme can transport such a wide variety of substrates has not been elucidated. Herein, the Pgp-mediated transport of a marker substrate, daunorubicin (DNR), out of viable cells was examined in the presence of a variety of other known substrates of Pgp. For most of the typical Pgp substrates examined, the relationship between inhibition of DNR efflux and competing substrate concentration was sigmoidal and therefore not a simple mutually exclusive competitive inhibition of transport. The Hill coefficient ranged from about 3 to 5 for the inhibition of transport of DNR. This negative cooperativity in combination with recent evidence, including several examples of noncompetitive inhibition between the homologous halves of Pgp, indicates a "half-of-the-sites" reactivity. Our data support the mechanistic proposal that substrate binding at one putative transport binding site precludes activity at another unequal site; many of the substrates examined exert a negative allosteric effect on the other transport site (and vice versa). A half-of-the-sites reactivity model would account for many of these observations and may be critical to the efficiency of Pgp substrate transport of a broad spectrum of compounds.

Transition-state formation in ATPase-negative mutants of human MDR1 protein

In this work we have studied the partial catalytic reactions in MDR1 variants carrying mutations in the conserved Walker A region (K433M and K1076M) of either the N-terminal or C-terminal ABC domain. Both mutations have been demonstrated to cause a loss of drug transport, drug-stimulated ATPase, and vanadate-dependent nucleotide trapping activity. Here we show that these mutants still allow transition state formation (nucleotide trapping) when fluoro-aluminate or beryllium fluoride is used as a complex-stabilizing anion. Drug stimulation of nucleotide trapping was found to be preserved in both mutants. Limited trypsin digestion revealed that whenever MDR1-nucleotide trapping occurred, both ABC domains were involved in the formation of the catalytic intermediates. Our results show that details of the MDR1-ATPase cycle can be studied even in ATPase-negative mutants. These data also demonstrate that the conformational alteration caused by a mutation in one of the ABC domains is propagated to the other, nonmutated domain, indicating a tight coupling between the functioning of the two ABC domains.

Investigation of the role of glutamine-471 and glutamine-1114 in the two catalytic sites of P-glycoprotein

P-glycoprotein, also known as multidrug resistance protein, pumps drugs out of cells using ATP hydrolysis as the energy source. Glutamine-471 and the corresponding glutamine-1114 in the two catalytic sites of P-glycoprotein are conserved in ABC transporters. X-ray structures show that they lie close to the bound nucleotide. Proposed functional roles are (1) activation of the attacking water for ATP hydrolysis, (2) coordination of the essential Mg(2+) cofactor in Mg nucleotide, and (3) signal communication between catalytic site reaction chemistry and drug-binding sites. We made mutations Q471A, Q471E, Q1114A, and Q1114E in mouse MDR3 P-glycoprotein. Pure mutant and wild-type proteins were prepared and subjected to enzymatic and biochemical characterization. We conclude from the results that the primary role of this glutamine residue is in interdomain signal communication. Coordination of the Mg(2+) cofactor is not a critical functional role, neither is activation of the attacking water molecule, although an auxiliary role in positioning the water cannot be ruled out. We found that equivalent mutations (Ala or Glu) in either of the two P-glycoprotein catalytic sites produced the same effects, implying functional symmetry of the two sites.

Conserved walker A Ser residues in the catalytic sites of P-glycoprotein are critical for catalysis and involved primarily at the transition state step

P-glycoprotein mutants S430A/T and S1073A/T, affecting conserved Walker A Ser residues, were characterized to elucidate molecular roles of the Ser and functioning of the two P-glycoprotein catalytic sites. Results showed the Ser-OH is critical for MgATPase activity and formation of the normal transition state, although not for initial MgATP binding. Mutation to Ala in either catalytic site abolished MgATPase and transition state formation in both sites, whereas Thr mutants had similar MgATPase to wild-type. Trapping of 1 mol of MgADP/mol of P-glycoprotein by vanadate, shown here with pure protein, yielded full inhibition of ATPase. Thus, congruent with previous work, both sites must be intact and must interact for catalysis. Equivalent mutations (Ala or Thr) in the two catalytic sites had identical effects on a wide range of activities, emphasizing that the two catalytic sites function symmetrically. The role of the Ser-OH is to coordinate Mg(2+) in MgATP, but only at the stage of the transition state are its effects tangible. Initial substrate binding is apparently to an "open" catalytic site conformation, where the Ser-OH is dispensable. This changes to a "closed" conformation required to attain the transition state, in which the Ser-OH is a critical ligand. Formation of the latter conformation requires both sites; both sites may provide direct ligands to the transition state.

MDR3 P-glycoprotein, a phosphatidylcholine translocase, transports several cytotoxic drugs and directly interacts with drugs as judged by interference with nucleotide trapping

The human MDR3 gene is a member of the multidrug resistance (MDR) gene family. The MDR3 P-glycoprotein is a transmembrane protein that translocates phosphatidylcholine. The MDR1 P-glycoprotein related transports cytotoxic drugs. Its overexpression can make cells resistant to a variety of drugs. Attempts to show that MDR3 P-glycoprotein can cause MDR have been unsuccessful thus far. Here, we report an increased directional transport of several MDR1 P-glycoprotein substrates, such as digoxin, paclitaxel, and vinblastine, through polarized monolayers of MDR3-transfected cells. Transport of other good MDR1 P-glycoprotein substrates, including cyclosporin A and dexamethasone, was not detectably increased. MDR3 P-glycoprotein-dependent transport of a short-chain phosphatidylcholine analog and drugs was inhibited by several MDR reversal agents and other drugs, indicating an interaction between these compounds and MDR3 P-gp. Insect cell membranes from Sf9 cells overexpressing MDR3 showed specific MgATP binding and a vanadate-dependent, N-ethylmaleimide-sensitive nucleotide trapping activity, visualized by covalent binding with [alpha-(32)P]8-azido-ATP. Nucleotide trapping was (nearly) abolished by paclitaxel, vinblastine, and the MDR reversal agents verapamil, cyclosporin A, and PSC 833. We conclude that MDR3 P-glycoprotein can bind and transport a subset of MDR1 P-glycoprotein substrates. The rate of MDR3 P-glycoprotein-mediated transport is low for most drugs, explaining why this protein is not detectably involved in multidrug resistance. It remains possible, however, that drug binding to MDR3 P-glycoprotein could adversely affect phospholipid or toxin secretion under conditions of stress (e.g. in pregnant heterozygotes with one MDR3 null allele).

Expression and characterization of the N- and C-terminal ATP-binding domains of MRP1

The His(6)-tagged N- and C-terminal nucleotide binding (ATP Binding Cassette, ABC) domains of the human multidrug resistance associated protein, MRP1, were expressed in bacteria in fusion to the bacterial maltose binding protein and a two-step affinity purification was utilized. Binding of a fluorescent ATP-analogue occurred with micromolar dissociation constants, MgATP was able to inhibit the ATP-analogue binding with 70 and 200 micromolar apparent inhibition constants, while AMP was nearly ineffective. Both MRP1 nucleotide binding domains showed ATPase activities (V(max) values between 5-10 nmoles/mg protein/min), which is fifty to hundred times lower than that of parent transporter. The K(M) value of the ATP hydrolysis by the nucleotide binding domains were 1.5 mM and 1.8 mM, which is similar to the K(M) value of the native or the purified and reconstituted transporter, N-ethylmaleinimide and A1F(4) inhibited the ATPase activity of both nucleotide binding domains.

Inhibitory effect of the reversal agents V-104, GF120918 and Pluronic L61 on MDR1 Pgp-, MRP1- and MRP2-mediated transport

The human multidrug transporter MDR1 P-glycoprotein and the multidrug resistance proteins MRP1 and MRP2 transport a range of cytotoxic drugs, resulting in multidrug resistance in tumour cells. To overcome this form of drug resistance in patients, several inhibitors (reversal agents) of these transporters have been isolated. Using polarized cell lines stably expressing human MDR1, MRP1 or MRP2cDNA, and 2008 ovarian carcinoma cells stably expressing MRP1 cDNA, we have investigated in this study the specificity of the reversal agents V-104 (a pipecolinate derivative), GF120918 (an acridone carboxamide derivative also known as GG918), and Pluronic L61 (a (poly)oxypropethylene and (poly)oxypropylene block copolymer). Transport experiments with cytotoxic drugs with polarized cell lines indicate that all three compounds efficiently inhibit MDR1 Pgp. Furthermore, V-104 partially inhibits daunorubicin transport by MRP1 but not vinblastine transport by MRP2. V-104 reverses etoposide resistance of 2008/MRP1 cells, whereas GF120918 does not reverse resistance due to MRP1. V-104 partially inhibits the export of the organic anion dinitrophenyl S-glutathione by MDCKII-MRP1 but not by MDCKII-MRP2 cells. Unexpectedly, export of the organic anion calcein by MDCKII-MRP1 and MDCKII-MRP2 cells is stimulated by Pluronic L61, probably because it relieves the block on entry of calcein AM into the cell by endogenous MDR1 Pgp.

ABC transporters in lipid transport

Since it was found that the P-glycoproteins encoded by the MDR3 (MDR2) gene in humans and the Mdr2 gene in mice are primarily phosphatidylcholine translocators, there has been increasing interest in the possibility that other ATP binding cassette (ABC) transporters are involved in lipid transport. The evidence reviewed here shows that the MDR1 P-glycoprotein and the multidrug resistance (-associated) transporter 1 (MRP1) are able to transport lipid analogues, but probably not major natural membrane lipids. Both transporters can transport a wide range of hydrophobic drugs and may see lipid analogues as just another drug. The MDR3 gene probably arose in evolution from a drug-transporting P-glycoprotein gene. Recent work has shown that the phosphatidylcholine translocator has retained significant drug transport activity and that this transport is inhibited by inhibitors of drug-transporting P-glycoproteins. Whether the phosphatidylcholine translocator also functions as a transporter of some drugs in vivo remains to be seen. Three other ABC transporters were recently shown to be involved in lipid transport: ABCR, also called Rim protein, was shown to be defective in Stargardt's macular dystrophy; this protein probably transports a complex of retinaldehyde and phosphatidylethanolamine in the retina of the eye. ABC1 was shown to be essential for the exit of cholesterol from cells and is probably a cholesterol transporter. A third example, the ABC transporter involved in the import of long-chain fatty acids into peroxisomes, is discussed in the chapter by Hettema and Tabak in this volume.

Nonequivalent nucleotide trapping in the two nucleotide binding folds of the human multidrug resistance protein MRP1

Multidrug resistance protein 1 (MRP1) and P-glycoprotein, which are ATP-dependent multidrug efflux pumps and involved in multidrug resistance of tumor cells, are members of the ATP binding cassette proteins and contain two nucleotide-binding folds (NBFs). P-glycoprotein hydrolyzes ATP at both NBFs, and vanadate-induced nucleotide trapping occurs at both NBFs. We examined vanadate-induced nucleotide trapping in MRP1 stably expressed in KB cell membrane by using 8-azido-[alpha-(32)P]ATP. Vanadate-induced nucleotide trapping in MRP1 was found to be stimulated by reduced glutathione, glutathione disulfide, and etoposide and to be synergistically stimulated by the presence of etoposide and either glutathione. These results suggest that glutathione and etoposide interact with MRP1 at different sites and that those bindings cooperatively stimulate the nucleotide trapping. Mild trypsin digestion of MRP1 revealed that vanadate-induced nucleotide trapping mainly occurs at NBF2. Our results suggest that the two NBFs of MRP1 might be functionally nonequivalent.

Comparison of the functional characteristics of the nucleotide binding domains of multidrug resistance protein 1

Multidrug Resistance Protein 1 (MRP1) transports diverse organic anionic conjugates and confers resistance to cytotoxic xenobiotics. The protein contains two nucleotide binding domains (NBDs) with features characteristic of members of the ATP-binding cassette superfamily and exhibits basal ATPase activity that can be stimulated by certain substrates. It is not known whether the two NBDs of MRP1 are functionally equivalent. To investigate this question, we have used a baculovirus dual expression vector encoding both halves of MRP1 to reconstitute an active transporter and have compared the ability of each NBD to be photoaffinity-labeled with 8-azido-[(32)P]ATP and to trap 8-azido-[(32)P]ADP in the presence of orthovanadate. We found that NBD1 was preferentially labeled with 8-azido-[(32)P]ATP, while trapping of 8-azido-[(32)P]ADP occurred predominantly at NBD2. Although trapping at NBD2 was dependent on co-expression of both halves of MRP1, binding of 8-azido-ATP by NBD1 remained detectable when the NH(2)-proximal half of MRP1 was expressed alone and when NBD1 was expressed as a soluble polypeptide. Mutation of the conserved Walker A lysine 684 or creation of an insertion mutation between Walker A and B motifs eliminated binding by NBD1 and all detectable trapping of 8-azido-ADP at NBD2. Both mutations decreased leukotriene C(4) (LTC(4)) transport by approximately 70%. Mutation of the NBD2 Walker A lysine 1333 eliminated trapping of 8-azido-ADP by NBD2 but, in contrast to the mutations in NBD1, essentially eliminated LTC(4) transport activity without affecting labeling of NBD1 with 8-azido-[(32)P]ATP.

Nucleotide-induced conformational changes in P-glycoprotein and in nucleotide binding site mutants monitored by trypsin sensitivity

Limited trypsin digestion was used to monitor nucleotide-induced conformational changes in wild-type P-glycoprotein (Pgp) as well as in nucleotide binding domain (NBD) Pgp mutants. Purified and reconstituted wild-type or mutant mouse Mdr3 Pgps were preincubated with different hydrolyzable or nonhydrolyzable nucleotides, followed by limited proteolytic cleavage at different trypsin:protein ratios. The Pgp tryptic digestion products were separated by SDS-PAGE followed by immunodetection with the mouse monoclonal anti-Pgp antibody C219, which recognizes a conserved epitope (VVQE/AALD) in each half of the protein. Different trypsin digestion patterns were observed for wild-type Pgp incubated with MgCl(2) alone, MgADP, MgAMP.PNP, MgATP, and MgATP + vanadate. A unique trypsin digestion profile suggestive of enhanced resistance to trypsin was observed under conditions of vanadate-induced trapping of nucleotides (MgATP + vanadate). The trypsin sensitivity profiles of Pgp mutants bearing either single or double mutations in Walker A (K429R, K1072R) and Walker B (D551N, D1196N) sequence signatures of NBD1 and NBD2 were analyzed under conditions of vanadate-induced trapping of nucleotides. The proteolytic cleavage pattern observed for the double mutants K429R/K1072R and D551N/D1196N, and for the single mutants K429R, K1072R, and D1196N were similar and clearly distinct from wild-type Pgp under the same conditions. This is consistent with the absence of ATP hydrolysis and of vanadate-induced trapping of 8-azido-ADP previously reported for these mutants [Urbatsch et al. (1998) Biochemistry 37, 4592-4602]. Interestingly, the trypsin digestion profiles observed under vanadate-induced trapping for the D551N and D1196N mutants were quite different, with the D551N mutant showing a profile resembling that seen for wild-type Pgp. The different sensitivity profiles of Pgp mutants bearing mutations at the homologous residue in NBD1 (D551N) and NBD2 (D1196N) suggest possible structural and functional differences between the two sites.

Activation of the human P-glycoprotein ATPase by trypsin

The human MDR1 gene product, P-glycoprotein (Pgp), a tandemly duplicated molecule containing two putative ATP- and perhaps two drug-binding sites, is responsible for multidrug resistance in tumors. In this report, we characterized the effects of trypsinization of Pgp on its ATPase function. Incubation of Pgp-containing membranes with trypsin at a ratio of 1000:1 (w/w) resulted in a gradual increase in the basal- and the drug-stimulated ATPase activities of Pgp in a time-dependent manner. The maximal basal-, verapamil-, and vinblastine-stimulated ATPase activities of the trypsinized Pgp were approximately 1.8-, 1.5-, and 1.75-fold higher than the activities of the native Pgp, respectively. Increased basal- and drug-stimulated ATPase activities of the Pgp were also observed when the ratio of membrane protein to trypsin in the incubation mixtures was raised to 10:1 (w/w). Immunoblotting analysis of Pgp tryptic digests using Pgp-specific NH(2)11, C219, and C494 antibodies together revealed the degradation of full-length Pgp and formation of at least eight peptides migrating in the 36-60 kDa range. Immunoprecipitation reactions using NH(2)11 and C494 antibodies have suggested that the peptides originating from the NH(2) half of Pgp are in strong association with the COOH half of the peptide. These findings suggest that while Pgp fragments together exhibit the ATPase functional characteristics, Pgp possesses a cleavage activation site or region, and its cleavage leads to the activation of basal ATPase function of Pgp.

Evidence for a requirement for ATP hydrolysis at two distinct steps during a single turnover of the catalytic cycle of human P-glycoprotein

P-glycoprotein (Pgp) is an ATP-dependent hydrophobic natural product anticancer drug efflux pump whose overexpression confers multidrug resistance to tumor cells. The work reported here deals with the elucidation of the energy requirement for substrate interaction with Pgp during the catalytic cycle. We show that the K(d) (412 nM) of the substrate analogue [(125)I]iodoarylazidoprazoin for Pgp is not altered by the presence of the nonhydrolyzable nucleotide 5'-adenylylimididiphosphate and vanadate (K(d) = 403 nM). Though binding of nucleotide per se does not affect interactions with the substrate, ATP hydrolysis results in a dramatic conformational change where the affinity of [(125)I]iodoarylazidoprazoin for Pgp trapped in transition-state conformation (Pgp x ADP x vanadate) is reduced >30-fold. To transform Pgp from this intermediate state of low affinity for substrate to the next catalytic cycle, i.e., a conformation that binds substrate with high affinity, requires conditions that permit ATP hydrolysis. Additionally, there is an inverse correlation (R(2) = 0.96) between 8AzidoADP (or ADP) release and the recovery of substrate binding. These results suggest that the release of nucleotide is necessary for reactivation but not sufficient. The hydrolysis of additional molecule(s) of ATP (or 8AzidoATP) is obligatory for the catalytic cycle to advance to completion. These data are consistent with the observed stoichiometry of two ATP molecules hydrolyzed for the transport of every substrate molecule. Our data demonstrate two distinct roles for ATP hydrolysis in a single turnover of the catalytic cycle of Pgp, one in the transport of substrate and the other in effecting conformational changes to reset the pump for the next catalytic cycle.

Large scale purification of detergent-soluble P-glycoprotein from Pichia pastoris cells and characterization of nucleotide binding properties of wild-type, Walker A, and Walker B mutant proteins

P-glycoprotein (Pgp; mouse MDR3) was expressed in Pichia pastoris, grown in fermentor culture, and purified. The final pure product is of high specific ATPase activity and is soluble at low detergent concentration. 120 g of cells yielded 6 mg of pure Pgp; >4 kg of cells were obtained from a single fermentor run. Properties of the pure protein were similar to those of previous preparations, except there was significant ATPase activity in absence of added lipid. Mutant mouse MDR3 P-glycoproteins were purified by the same procedure after growth of cells in flask culture, with similar yields and purity. This procedure should open up new avenues of structural, biophysical, and biochemical studies of Pgp. Equilibrium nucleotide-binding parameters of wild-type mouse MDR3 Pgp were studied using 2'-(3')-O-(2,4,6-trinitrophenyl)adenosine tri- and diphosphate. Both analogs were found to bind with K(d) in the low micromolar range, to a single class of site, with no evidence of cooperativity. ATP displacement of the analogs was seen. Similar binding was seen with K429R/K1072R and D551N/D1196N mutant mouse MDR3 Pgp, showing that these Walker A and B mutations had no significant effect on affinity or stoichiometry of nucleotide binding. These residues, known to be critical for catalysis, are concluded to be involved primarily in stabilization of the catalytic transition state in Pgp.

Subunit interactions in ABC transporters: towards a functional architecture

The ABC superfamily is a diverse group of integral membrane proteins involved in the ATP-dependent transport of solutes across biological membranes in both prokaryotes and eukaryotes. Although ABC transporters have been studied for over 30 years, very little is known about the mechanism by which the energy of ATP hydrolysis is used to transport substrate across the membrane. The recent report of the high resolution crystal structure of HisP, the nucleotide-binding subunit of the histidine permease complex of Salmonella typhimurium, represents a significant breakthrough toward the elucidation of the mechanism of solute translocation by ABC transporters. In this review, we use data from the crystallographic structures of HisP and other nucleotide-binding proteins, combined with sequence analysis of a subset of atypical ABC transporters, to argue a new model for the dimerisation of the nucleotide-binding domains that embraces the notion that the C motif from one subunit forms part of the ATP-binding site in the opposite subunit. We incorporate this dimerisation of the ATP-binding domains into our recently reported beta-barrel model for P-glycoprotein and present a general model for the cooperative interaction of the two nucleotide-binding domains and the translocation of mechanical energy to the transmembrane domains in ABC transporters.

Purification of the human apical conjugate export pump MRP2 reconstitution and functional characterization as substrate-stimulated ATPase

The multidrug resistance protein MRP2 (ABCC2) acts as an ATP-dependent conjugate export pump in apical membranes of polarized cells and confers multidrug resistance. Purified MRP2 is essential for the detailed functional characterization of this member of the family of ATP-binding cassette (ABC) transporter proteins. In human embryonic kidney cells (HEK293), we have permanently expressed MRP2 containing an additional C-terminal (His)6-tag. Immunoblot and immunofluorescence analyses detected the MRP2-(His)6 overexpressing clones. Isolated membrane vesicles from the MRP2-(His)6-expressing cells were active in ATP-dependent transport of the glutathione S-conjugate leukotriene C4 and were photoaffinity-labelled with 8-azido-[alpha-32P]ATP. MRP2-(His)6 was solubilized from membranes of MRP2-(His)6-cells and purified to homogeneity in a three-step procedure using immobilized metal affinity chromatography, desalting, and immunoaffinity chromatography. The identity of the pure MRP2-(His)6 was verified by MS analysis of tryptic peptides. The purified MRP2-(His)6 glycoprotein was reconstituted into proteoliposomes and showed functional activity as ATPase in a protein-dependent manner with a Km for ATP of 2.1 mM and a Vmax of 25 nmol ADP x mg MRP2-1 x min-1. This ATPase activity was substrate-stimulated by oxidized and reduced glutathione and by S-decyl-glutathione. Future studies using pure MRP2 reconstituted in proteoliposomes should allow further insight into the molecular parameters contributing to MRP2 transport function and to define its intracellular partners for transport and multidrug resistance.

Nucleotide occlusion in the human cystic fibrosis transmembrane conductance regulator. Different patterns in the two nucleotide binding domains

The function of the human cystic fibrosis transmembrane conductance regulator (CFTR) protein as a chloride channel or transport regulator involves cellular ATP binding and cleavage. Here we describe that human CFTR expressed in insect (Sf9) cell membranes shows specific, Mg2+-dependent nucleotide occlusion, detected by covalent labeling with 8-azido-[alpha-32P]ATP. Nucleotide occlusion in CFTR requires incubation at 37 degrees C, and the occluded nucleotide can not be removed by repeated washings of the membranes with cold MgATP-containing medium. By using limited tryptic digestion of the labeled CFTR protein we found that the adenine nucleotide occlusion preferentially occurred in the N-terminal nucleotide binding domain (NBD). Addition of the ATPase inhibitor vanadate, which stabilizes an open state of the CFTR chloride channel, produced an increased nucleotide occlusion and resulted in the labeling of both the N-terminal and C-terminal NBDs. Protein modification with N-ethylmaleimide prevented both vanadate-dependent and -independent nucleotide occlusion in CFTR. The pattern of nucleotide occlusion indicates significant differences in the ATP hydrolyzing activities of the two NBDs, which may explain their different roles in the CFTR channel regulation.

Functional multidrug resistance protein (MRP1) lacking the N-terminal transmembrane domain

The human multidrug resistance protein (MRP1) causes drug resistance by extruding drugs from tumor cells. In addition to an MDR-like core, MRP1 contains an N-terminal membrane-bound region (TMD0) connected to the core by a cytoplasmic linker (L0). We have studied truncated MRP1 versions containing either the MDR-like core alone or the core plus linker L0, produced in the baculovirus-insect (Sf9) cell system. Their function was examined in isolated membrane vesicles. Full-length MRP1 showed ATP-dependent, vanadate-sensitive accumulation of leukotriene C4 and N-ethylmaleimide glutathione. In addition, leukotriene C4-stimulated, vanadate-dependent nucleotide occlusion was detected. The MDR-like core was virtually inactive. Co-expression of the core with the N-terminal region including L0 fully restored MRP1 function. Unexpectedly, a truncated MRP1 mutant lacking the entire TMD0 region but still containing L0 behaved like wild-type MRP1 in vesicle uptake and nucleotide trapping experiments. We also expressed the MRP1 constructs in polarized canine kidney derived MDCKII cells. Like wild-type MRP1, the MRP1 protein without the TMD0 region was routed to the lateral plasma membrane and transported dinitrophenyl glutathione and daunorubicin. The TMD0L0 and the MRP1 minus TMD0L0 remained in an intracellular compartment. Taken together, these experiments strongly suggest that the TMD0 region is neither required for the transport function of MRP1 nor for its proper routing to the plasma membrane.