Overexpression and purification of the carboxyl-terminal nucleotide-binding domain from mouse P-glycoprotein. Strategic location of a tryptophan residue

A review of TNP-ATP in protein binding studies: benefits and pitfalls

We review 50 years of use of 2',3'-O-trinitrophenyl (TNP)-ATP, a fluorescently tagged ATP analog. It has been extensively used to detect binding interactions of ATP to proteins and to measure parameters of those interactions such as the dissociation constant, K_d, or inhibitor dissociation constant, K_i. TNP-ATP has also found use in other applications, for example, as a fluorescence marker in microscopy, as a FRET pair, or as an antagonist (e.g., of P2X receptors). However, its use in protein binding studies has limitations because the TNP moiety often enhances binding affinity, and the fluorescence changes that occur with binding can be masked or mimicked in unexpected ways. The goal of this review is to provide a clear perspective of the pros and cons of using TNP-ATP to allow for better experimental design and less ambiguous data in future experiments using TNP-ATP and other TNP nucleotides.

Interactions and cooperativity between P-glycoprotein structural domains determined by thermal unfolding provides insights into its solution structure and function

Structural changes in mouse P-glycoprotein (Pgp) induced by thermal unfolding were studied by differential scanning calorimetry (DSC), circular dichroism and fluorescence spectroscopy to gain insight into the solution conformation(s) of this ABC transporter that may not be apparent from current crystal structures. DSC of reconstituted Pgp showed two thermal unfolding transitions in the absence of MgATP, suggesting that each transition involved the cooperative unfolding of two or more interacting structural domains. A low calorimetric unfolding enthalpy and minimal structural changes were observed, which are hallmarks of the thermal unfolding of α-helical membrane proteins, because generally only the extramembranous regions undergo significant unfolding. Nucleotide binding increased the unfolding temperature of both transitions to the same extent, suggesting that one nucleotide binding domain (NBD) unfolds with each transition. Combined with the results from the two isolated NBDs, we propose that each DSC transition represents the cooperative unfolding of one NBD and the two contacting intracellular loops. Further, the presence of two transitions in both apo and MgATP bound wild-type Pgp suggests the NBD-dimeric conformation is transient, and that Pgp resides predominantly in the crystallographically observed inward-facing conformation with NBDs separated, even under conditions supporting continuous MgATP hydrolysis. In contrast, DSC of the vanadate-trapped MgADP·Pgp complex and the MgATP-bound catalytically inactive mutant, E552A/E1197A, show an additional transition at much higher temperature, corresponding to the unfolding of the nucleotide-trapped NBD-dimeric outward-facing conformation. The collective results indicate a strong preference for an NBD dissociated, inward-facing conformation of Pgp.

Abcb1 in Pigs: Molecular cloning, tissues distribution, functional analysis, and its effect on pharmacokinetics of enrofloxacin

P-glycoprotein (P-gp) is one of the best-known ATP-dependent efflux transporters, contributing to differences in pharmacokinetics and drug-drug interactions. Until now, studies on pig P-gp have been scarce. In our studies, the full-length porcine P-gp cDNA was cloned and expressed in a Madin-Darby Canine Kidney (MDCK) cell line. P-gp expression was then determined in tissues and its role in the pharmacokinetics of oral enrofloxacin in pigs was studied. The coding region of pig Abcb1 gene was 3,861 bp, encoding 1,286 amino acid residues (Mw = 141,966). Phylogenetic analysis indicated a close evolutionary relationship between porcine P-gp and those of cow and sheep. Pig P-gp was successfully stably overexpressed in MDCK cells and had efflux activity for rhodamine 123, a substrate of P-gp. Tissue distribution analysis indicated that P-gp was highly expressed in brain capillaries, small intestine, and liver. In MDCK-pAbcb1 cells, enrofloxacin was transported by P-gp with net efflux ratio of 2.48 and the efflux function was blocked by P-gp inhibitor verapamil. High expression of P-gp in the small intestine could modify the pharmacokinetics of orally administrated enrofloxacin by increasing the Cmax, AUC and Ka, which was demonstrated using verapamil, an inhibitor of P-gp.

Boundary of the nucleotide-binding domain of Streptococcus ComA based on functional and structural analysis

The ATP-binding cassette (ABC) transporter ComA is a key molecule essential for the first step of the quorum-sensing system of Streptococcus. The nucleotide binding domains (NBD) of Streptococcus mutans ComA with different N termini, NBD1 (amino acid residues 495-760), NBD2 (517-760), and NBD3 (528-760), were expressed, purified, and characterized. The shortest NBD3 corresponds to the region commonly defined as NBD in the database searches of ABC transporters. A kinetic analysis showed that the extra N-terminal region conferred a significantly higher ATP hydrolytic activity on the NBD at a neutral pH. Gel-filtration, X-ray crystallography, and mutational analyses suggest that at least four to five residues beyond the N-terminal boundary of NBD3 indeed participate in stabilizing the protein scaffold of the domain structure, thereby facilitating the ATP-dependent dimerization of NBD which is a prerequisite to the catalysis. These findings, together with the presence of a highly conserved glycine residue in this region, support the redefinition of the N-terminal boundary of the NBD of these types of ABC exporters.

Interaction of LDS-751 with the drug-binding site of P-glycoprotein: a Trp fluorescence steady-state and lifetime study

P-glycoprotein (ABCB1) is an ATP-driven efflux pump which binds drugs within a large flexible binding pocket. Intrinsic Trp fluorescence was used to probe the interactions of LDS-751 (2-[4-(4-[dimethylamino]phenyl)-1,3-butadienyl]-3-ethylbenzo-thiazolium perchlorate) with purified P-glycoprotein, using steady-state/lifetime measurements and collisional quenching. The fast decay component of P-glycoprotein intrinsic fluorescence (tau(1)=0.97 ns) was unaffected by LDS-751 binding, while the slow decay component (tau(2)=4.02 ns) was quenched by dynamic and static mechanisms. Both the wavelength-dependence of the decay kinetics, and the time-resolved emission spectra, suggested the existence of excited-state relaxation processes within the protein matrix on the nanosecond time-scale, which were altered by LDS-751 binding. The fast decay component, which is more solvent-exposed, can be attributed to cytosolic/extracellular Trp residues, while the slow decay component likely arises from more buried transmembrane Trp residues. Interaction of a drug with the binding pocket of P-glycoprotein thus affects its molecular structure and fast dynamics.

Functional characterization of the nucleotide binding domain of the Cryptosporidium parvum CpABC4 transporter: an iron-sulfur cluster transporter homolog

In a previous study, we showed that the Cryptosporidium parvum ATP half-transporter CpABC4 (cgd1_1350) transcript was up-regulated in response to drug treatment with paromomycin and cyclosporine A in an in vitro infection model. CpABC4 may be directly or indirectly involved in the metabolic interactions between host and parasite in response to drug treatment and/or be involved in the intrinsic resistance to chemotherapy. In order to characterize the catalytic site of this transporter, an extended region of the nucleotide-binding domain of CpABC4 (H6-1350NBD) was expressed and purified as an N-terminal hexahistidine-tagged protein in E. coli. The presence of a single tryptophan residue enabled the intrinsic fluorescence to be monitored in response to binding of different compounds. A dose-dependent quenching of the domain's intrinsic fluorescence was observed with its natural substrate, ATP and the fluorescent analogue TNP-ATP. A similar effect was observed with progesterone as well as the flavonoids quercetin and silibinin, previously shown to inhibit parasite development in a cell-based assay. The purified domain also exhibited ATPase activity in the nanomolar range, which further confirmed correct folding and activity of the recombinant domain. The H6-1350NBD serves as a tool to test and design stereospecific inhibitors of the catalytic site, as well as other compounds that bind elsewhere in the domain that may indirectly interact with the catalytic site of the NBD of the CpABC4 transporter.

Conformational change induced by ATP binding in the multidrug ATP-binding cassette transporter BmrA

ATP-binding cassette (ABC) transporters are involved in the transport of a wide variety of substrates, and ATP-driven dimerization of their nucleotide binding domains (NBDs) has been suggested to be one of the most energetic steps of their catalytic cycle. Taking advantage of the propensity of BmrA, a bacterial multidrug resistance ABC transporter, to form stable, highly ordered ring-shaped structures [Chami et al. (2002) J. Mol. Biol. 315, 1075-1085], we show here that addition of ATP in the presence of Mg2+ prevented ring formation or destroyed the previously formed rings. To pinpoint the catalytic step responsible for such an effect, two classes of hydrolysis-deficient mutants were further studied. In contrast to hydrolytically inactive glutamate mutants that behaved essentially as the wild-type, lysine Walker A mutants formed ring-shaped structures even in the presence of ATP-Mg. Although the latter mutants still bound ATP-Mg, and even slowly hydrolyzed it for the K380R mutant, they were most likely unable to undergo a proper NBD dimerization upon ATP-Mg addition. The ATP-driven dimerization step, which was still permitted in glutamate mutants and led to a stable conformation suitable to monitor the growth of 2D crystals, appeared therefore responsible for destabilization of the BmrA ring structures. Our results provide direct visual evidence that the ATP-induced NBD dimerization triggers a conformational change large enough in BmrA to destabilize the rings, which is consistent with the assumption that this step might constitute the "power stroke" for ABC transporters.

Shedding light on drug transport: structure and function of the P-glycoprotein multidrug transporter (ABCB1)

P-glycoprotein (Pgp; ABCB1), a member of the ATP-binding cassette (ABC) superfamily, exports structurally diverse hydrophobic compounds from the cell, driven by ATP hydrolysis. Pgp expression has been linked to the efflux of chemotherapeutic drugs in human cancers, leading to multidrug resistance (MDR). The protein also plays an important physiological role in limiting drug uptake in the gut and entry into the brain. Substrates partition into the lipid bilayer before interacting with Pgp, which has been proposed to function as a hydrophobic vacuum cleaner. Low- and medium-resolution structural models of Pgp suggest that the 2 nucleotide-binding domains are closely associated to form a nucleotide sandwich dimer. Pgp is an outwardly directed flippase for fluorescent phospholipid and glycosphingolipid derivatives, which suggests that it may also translocate drug molecules from the inner to the outer membrane leaflet. The ATPase catalytic cycle of the protein is thought to proceed via an alternating site mechanism, although the details are not understood. The lipid bilayer plays an important role in Pgp function, and may regulate both the binding and transport of drugs. This review focuses on the structure and function of Pgp, and highlights the importance of fluorescence spectroscopic techniques in exploring the molecular details of this enigmatic transporter.

Overexpression, purification and characterization of a hexahistidine-tagged recombinant extended nucleotide-binding domain 1 (NBD1) of the Cryptosporidium parvum CpABC3 for rational drug design

Its natural resistance to antiprotozoal chemotherapy characterizes the intestinal protozoan parasite Cryptosporidium parvum and the P-glycoprotein-related multidrug resistance proteins such as CpABC3 could be involved. In order to design and study specific inhibitors of the CpABC3 nucleotide-binding domain, a hexahistidine-tagged recombinant protein encompassing the N-terminal cytosolic NBD1 domain was overexpressed in E. coli and purified. The 45 kDa H6-NBD1 displayed intrinsic fluorescent properties consistent with the presence of two Trp residues in a hydrophobic environment. The binding of ATP and the fluorescent analogue TNP-ATP produced a dose-dependent quenching as well as progesterone and the flavone quercetin. The extrinsic fluorescence of TNP-ATP was enhanced upon binding to H6-NBD1, which was only partially displaced by the natural substrate ATP. The recombinant protein hydrolyzed ATP (K(m)=145.4+/-18.2 microM), but ADP (K(m)=4.3+/-0.6mM) and AMP (K(m)=5.4+/-1.5 microM) were also substrates. TNP-ATP is a competitive inhibitor of the catalytic activity (K(i)=36.6+/-4.5 microM), but quercetin and progesterone were not inhibitors, evidencing different binding sites. The recombinant C. parvum H6-NBD1 should be a valuable tool for rational drug design and will allow the discrimination between specific inhibitors of the catalytic site and molecules binding to other sites.

Sensitivity of P-glycoprotein tryptophan residues to benzodiazepines and ATP interaction

Plasma membrane P-glycoprotein is a member of the ATP-binding cassette family of membrane transporters. In the present study tryptophan intrinsic fluorescence was used to understand the P-glycoprotein response to three benzodiazepines (bromazepam, chlordiazepoxide and flurazepam) in the presence and absence of ATP. Fluorescence emission spectra showed a red shift on the maximal emission wavelength upon interaction of P-glycoprotein with all benzodiazepines. Benzodiazepine association with nucleotide-bound P-glycoprotein also showed this trend and the quenching profile was attributed to a sphere-of-action model, for static fluorescence. Furthermore, quenching data of benzodiazepine-bound P-glycoprotein with ATP were concentration dependent and saturable, indicating that nucleotide binds to P-glycoprotein whether drug is present or not. These results seems in agreement with the proposal of the ATP-switch model by Higgins and Linton, where substrate binding to the transporters initiates the transport cycle by increasing the ATP binding affinity.

Characterization of Cdr1p, a major multidrug efflux protein of Candida albicans: purified protein is amenable to intrinsic fluorescence analysis

Candida drug resistance protein 1 (Cdr1p), an ATP-dependent drug efflux pump, confers multidrug resistance in immunocompromised and debilitated patients. A member of the ATP-binding cassette (ABC) superfamily of membrane transporters, Cdr1p contains two nucleotide binding/utilization sites (NBDs) and two transmembrane domains (TMDs). We had earlier characterized Cdr1p by its overexpression as a GFP-tagged fusion protein that elicits oligomycin-sensitive ATPase activity and is linked to drug extrusion. However, it is essential to have highly purified Cdr1p to understand the detailed molecular basis of structure and functions of this protein. In this study, we have developed a two-step purification protocol using stably overexpressed His-tagged Cdr1p in Saccharomyces cerevisiae. Purified Cdr1p exhibited divalent cation-dependent ATPase activity [approximately 1.2 micromol (mg of protein)(-)(1) min(-)(1)] with an apparent K(M) in the range of 1.8 to 2.1 mM and V(max) between 1.0 and 1.4 micromol (mg of protein)(-)(1) min(-)(1). Unlike its close homologue human P-gp/MDR1, purified Cdr1p only moderately displayed drug stimulated ATPase activity. By exploiting intrinsic fluorescence intensity of purified Cdr1p, which contains 24 tryptophan residues, we could monitor defined conformational changes upon substrate drug and ATP binding. It is observed that ATP binding to Cdr1p (K(d) = approximately 1.7 mM) is not a prerequisite for drug binding, and both the mechanisms of drug as well as ATP binding, which induce specific conformational changes, occur independent of each other. Our study for the first time provides a catalytically active purified ABC transporter from a fungal pathogen, which is amenable to fluorescence measurements and thus would be useful in understanding the molecular basis of antifungal transport.

Rescue of Folding Defects in ABC Transporters Using Pharmacological Chaperones

The ATP-binding cassette (ABC) family of membrane transport proteins is the largest class of transporters in humans (48 members). The majority of ABC transporters function at the cell surface. Therefore, defective folding and trafficking of the protein to the cell surface can lead to serious health problems. The classic example is cystic fibrosis (CF). In most CF patients, there is a deletion of Phe508 in the CFTR protein (DeltaF508 CFTR) that results in defective folding and intracellular retention of the protein (processing mutant). A potential treatment for most patients with CF would be to use a ligand(s) of CFTR that acts a pharmacological chaperone to correct the folding defect. The feasibility of such an approach was first demonstrated with the multidrug transporter P-glycoprotein (P-gp), an ABC transporter, and a sister protein of CFTR. It was found that P-gps with mutations at sites equivalent to those found in CFTR processing mutants were rescued when they were expressed in the presence of drug substrates or modulators of P-gp. These compounds acted as pharmacological chaperones and functioned by promoting interactions among the various domains in the protein during the folding process. Several groups have attempted to identify compounds that could rescue the folding defect in DeltaF508 CFTR. The best compound identified through high-throughout screening is a quinazoline derivative (CFcor-325). Expression of DeltaF508 CFTR as well as other CFTR processing mutants in the presence of 1 muM CFcor-325 promoted folding and trafficking of the mutant proteins to the cell surface in an active conformation. Therefore, CFcor-325 and other quinazoline derivates could be important therapeutic compounds for the treatment of CF.

Functional characterization of N-terminal nucleotide binding domain (NBD-1) of a major ABC drug transporter Cdr1p of Candida albicans: uncommon but conserved Trp326 of Walker B is important for ATP binding

Using purified N-terminal NBD (NBD-512) domain of Cdr1p, a major multidrug extrusion pump of human pathogenic yeast Candida albicans, we show the relevance of the unique positioning of an atypical Trp326 residue. Similar to Cys193 in Walker A, Trp326 in the Walker B motif of Cdr1p is also a conserved feature of other fungal ATP Binding Cassette (ABC) transporters. By employing fluorescence spectroscopy, chemical modification, and site-directed mutagenesis, we demonstrate that of the five Trp residues in the NBD-512 domain, Trp326 alone is important for nucleotide binding and subsequent conformational changes within the domain. Furthermore, mutation of Trp326 to Ala results in an increased K(M) without appreciably affecting V(max) of ATPase activity. Thus, Trp326 in NBD-512 appears to be important for nucleotide binding and not for its hydrolysis. Additionally, the role of Trp326 in ATP binding is independent of the presence of the adjacent well-conserved Asp327 residue which, like Cys193, has a catalytic role in ATP hydrolysis. Considering that Trp326 of Cdr1p is a typical feature of fungal transporters alone, our study suggests that these ABC transporters may reflect mechanistic differences with regard to nucleotide binding and hydrolysis as compared to their counterparts of non-fungal origin.

Definition of the domain boundaries is critical to the expression of the nucleotide-binding domains of P-glycoprotein

Heterologous expression of domains of eukaryotic proteins is frequently associated with formation of inclusion bodies, consisting of aggregated mis-folded protein. This phenomenon has proved a significant barrier to the characterization of domains of eukaryotic ATP binding cassette (ABC) transporters. We hypothesized that the solubility of heterologously expressed nucleotide binding domains (NBDs) of ABC transporters is dependent on the definition of the domain boundaries. In this paper we have defined a core NBD, and tested the effect of extensions to and deletions of this core domain on protein expression. Of 10 NBDs constructed, only one was expressed as a soluble protein in Escherichia coli, with expression of the remaining NBDs being associated with inclusion body formation. The soluble NBD protein we have obtained corresponds to residues 386-632 of P-glycoprotein and represents an optimally defined domain. The NBD has been isolated and purified to 95% homogeneity by a two-step purification protocol, involving affinity chromatography and gel filtration. Although showing no detectable ATP hydrolysis, the protein retains specific ATP binding and has a secondary structure compatible with X-ray crystallographic data on bacterial NBDs. We have interpreted our results in terms of homology models, which suggest that the N-terminal NBD of P-glycoprotein can be produced as a stable, correctly folded, isolate domain with judicious design of the expression construct.

Purification and characterization of the N-terminal nucleotide binding domain of an ABC drug transporter of Candida albicans: uncommon cysteine 193 of Walker A is critical for ATP hydrolysis

The Candida drug resistance protein Cdr1p (approximately 170 kDa) is a member of ATP binding cassette (ABC) superfamily of drug transporters, characterized by the presence of 2 nucleotide binding domains (NBD) and 12 transmembrane segments (TMS). NBDs of these transporters are the hub of ATP hydrolysis activity, and their sequence contains a conserved Walker A motif (GxxGxGKS/T). Mutations of the lysine residue within this motif abrogate the ability of NBDs to hydrolyze ATP. Interestingly, the sequence alignments of Cdr1p NBDs with other bacterial and eukaryotic transporters reveal that its N-terminal NBD contains an unusual Walker A sequence (GRPGAGCST), as the invariant lysine is replaced by a cysteine. In an attempt to understand the significance of this uncommon positioning of cysteine within the Walker A motif, we for the first time have purified and characterized the N-terminal NBD (encompassing first N-terminal 512 amino acids) of Cdr1p as well as its C193A mutant protein. The purified NBD-512 protein could exist as an independent functional general ribonucleoside triphosphatase with strong divalent cation dependence. It exhibited ATPase activity with an apparent K(m) in the 0.8-1.0 mM range and V(max) in the range of 147-160 nmol min(-)(1) (mg of protein)(-)(1). NBD-512-associated ATPase activity was also sensitive to inhibitors such as vanadate, azide, and NEM. The Mut-NBD-512 protein (C193A) showed a severe impairment in its ability to hydrolyze ATP (95%); however, no significant effect on ATP (TNP-ATP) binding was observed. Our results show that C193 is critical for N-terminal NBD-mediated ATP hydrolysis and represents a unique feature distinguishing the ATP-dependent functionality of the ABC transporters of fungi from those found in bacteria and other eukaryotes.

Detection of structural and functional asymmetries in P-glycoprotein by combining mutagenesis and H/D exchange measurements

During the last few years, Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR) has become one of the most powerful methods to determine the structure of biological materials and in particular of components of biological membranes, like proteins which cannot be studied by X-ray crystallography and NMR. Indeed, ATR-FTIR method requires little amount of material, gives valuable information about the secondary structure, orientation and tertiary structure changes in peptides and proteins. Moreover, this technique can be used in the presence of lipids and hence provides an excellent tool to study membrane proteins in their natural environment. In this review, we describe how structural information about the catalytic cycle of membrane proteins can be gained by combining ATR-FTIR spectroscopy and mutagenesis. In particular, results obtained about the structure and function of the nucleotide binding domains (NBD) of P-glycoprotein (Pgp), a multidrug transporter involved in cancer cells resistance to chemotherapy, are described.

A new experimental approach to detect long-range conformational changes transmitted between the membrane and cytosolic domains of LmrA, a bacterial multidrug transporter

LmrA confers multidrug resistance to Lactococcus lactis by mediating the extrusion of antibiotics, out of the bacterial membrane, using the energy derived from ATP hydrolysis. Cooperation between the cytosolic and membrane-embedded domains plays a crucial role in regulating the transport ATPase cycle of this protein. In order to demonstrate the existence of a structural coupling required for the cross-talk between drug transport and ATP hydrolysis, we studied specifically the dynamic changes occurring in the membrane-embedded and cytosolic domains of LmrA by combining infrared linear dichroic spectrum measurements in the course of H/D exchange with Trp fluorescence quenching by a water-soluble attenuator. This new experimental approach, which is of general interest in the study of membrane proteins, detects long-range conformational changes, transmitted between the membrane-embedded and cytosolic regions of LmrA. On the one hand, nucleotide binding and hydrolysis in the cytosolic nucleotide binding domain cause a repacking of the transmembrane helices. On the other hand, drug binding to the transmembrane helices affects both the structure of the cytosolic regions and the ATPase activity of the nucleotide binding domain.

Purification and properties of TrwB, a hexameric, ATP-binding integral membrane protein essential for R388 plasmid conjugation

TrwB is an integral membrane protein linking the relaxosome to the DNA transport apparatus in plasmid R388 conjugation. Native TrwB has been purified in monomeric and hexameric forms, in the presence of dodecylmaltoside from overexpressing bacterial cells. A truncated protein (TrwBDeltaN70) that lacked the transmembrane domain could be purified only in the monomeric form. Electron microscopy images revealed the hexameric structure and were in fact superimposable to the previously published atomic structure for TrwBDeltaN70. In addition, the electron micrographs showed an appendix, approximately 25 A wide, corresponding to the transmembrane region of TrwB. TrwB was located in the bacterial inner membrane in agreement with its proposed coupling role. Purified TrwB hexamers and monomers bound tightly the fluorescent ATP analogue TNP-ATP. A mutant in the Walker A motif, TrwB-K136T, was equally purified and found to bind TNP-ATP with a similar affinity to that of the wild type. However, the TNP-ATP affinity of TrwBDeltaN70 was significantly reduced in comparison with the TrwB hexamers. Competition experiments in which ATP was used to displace TNP-ATP gave an estimate of ATP binding by TrwB (K(d)((ATP)) = 0.48 mm for hexamers). The transmembrane domain appears to be involved in TrwB protein hexamerization and also influences its nucleotide binding properties.

Recent advances in the discovery of flavonoids and analogs with high-affinity binding to P-glycoprotein responsible for cancer cell multidrug resistance

P-glycoprotein (P-gp) is a plasma membrane glycoprotein that confers multidrug resistance on cells by virtue of its ability to exclude cytotoxic drugs in an ATP-dependent manner. The most commonly considered hypothesis is that P-gp acts as an ATP-driven drug-export pump, the mechanism of which is not understood in detail. Therefore, a tremendous effort is being made to find out modulator molecules to inhibit P-gp. We have been developing flavonoid derivatives as a new class of promising modulators using a new in vitro rational-screening assay based on measurements of the binding-affinity toward the C-terminal nucleotide-binding domain (NBD2) of P-gp. This review is focused on our results obtained with a variety of flavonoids. Structure-activity relationships of flavonoids as potential MDR modulators are reported.

Nucleotide triphosphatase activity of the N-terminal nucleotide-binding domains of the multidrug resistance proteins P-glycoprotein and MRP1

The multidrug resistance proteins P-glycoprotein (Pgp) and MRP1 are drug-efflux pumps. In this study, we compared the nucleotide triphosphatase activities of the isolated N-terminal nucleotide binding domains (NBD1) of Pgp and MRP1, and explored the potential role of the phosphorylation target domain of Pgp on the regulation of Pgp NBD1 ATPase activity. We found that: (1) the NBD1s of Pgp and MRP1 have ATPase and GTPase activities, (2) the K(m)s of Pgp NBD1 for ATP and GTP hydrolysis are identical, while the K(m) of MRP1 NBD1 for ATP is lower than that for GTP, and (3) phosphorylation of MLD by PKA or PKC produces a marginal increase of V(max) for ATP hydrolysis, without affecting the affinity for ATP. These results show efficient GTP hydrolysis by the NBD1s of Pgp and MRP1, and a minor role of phosphorylation in the control of Pgp NBD1 ATPase activity.

S-decyl-glutathione nonspecifically stimulates the ATPase activity of the nucleotide-binding domains of the human multidrug resistance-associated protein, MRP1 (ABCC1)

The human multidrug resistance-associated protein(MRP1) is an ATP-dependent efflux pump that transports anionic conjugates, and hydrophobic compounds in a glutathione dependent manner. Similar to the other, well-characterized multidrug transporter P-gp, MRP1 comprises two nucleotide-binding domains (NBDs) in addition to transmembrane domains. However, whereas the NBDs of P-gp have been shown to be functionally equivalent, those of MRP1 differ significantly. The isolated NBDs of MRP1 have been characterized in Escherichia coli as fusions with either the glutathione-S-transferase (GST) or the maltose-binding domain (MBP). The nonfused NBD1 was obtained by cleavage of the fusion protein with thrombin. The GST-fused forms of NBD1 and NBD2 hydrolyzed ATP with an apparent K(m) of 340 microm and a V(max) of 6.0 nmol P(I) x mg-1 x min-1, and a K(m) of 910 microm ATP and a V(max) of 7.5 nmol P(I) x mg-1 x min-1, respectively. Remarkably, S-decyl-glutathione, a conjugate specifically transported by MRP1 and MRP2, was able to stimulate the ATPase activities of the isolated NBDs more than 2-fold in a concentration-dependent manner. However,the stimulation of the ATPase activity was found to coincide with the formation of micelles by S-decyl-glutathione. Equivalent stimulation of ATPase activity could be obtained by surfactants with similar critical micelle concentrations.

The carboxyl termini of K(ATP) channels bind nucleotides

ATP-sensitive potassium (K(ATP)) channels are expressed in many excitable, as well as epithelial, cells and couple metabolic changes to modulation of cell activity. ATP regulation of K(ATP) channel activity may involve direct binding of this nucleotide to the pore-forming inward rectifier (Kir) subunit despite the lack of known nucleotide-binding motifs. To examine this possibility, we assessed the binding of the fluorescent ATP analogue, 2',3'-O-(2,4,6-trinitrophenylcyclo-hexadienylidene)adenosine 5'-triphosphate (TNP-ATP) to maltose-binding fusion proteins of the NH(2)- and COOH-terminal cytosolic regions of the three known K(ATP) channels (Kir1.1, Kir6.1, and Kir6.2) as well as to the COOH-terminal region of an ATP-insensitive inward rectifier K(+) channel (Kir2.1). We show direct binding of TNP-ATP to the COOH termini of all three known K(ATP) channels but not to the COOH terminus of the ATP-insensitive channel, Kir2.1. TNP-ATP binding was specific for the COOH termini of K(ATP) channels because this nucleotide did not bind to the NH(2) termini of Kir1.1 or Kir6.1. The affinities for TNP-ATP binding to K(ATP) COOH termini of Kir1.1, Kir6.1, and Kir6.2 were similar. Binding was abolished by denaturing with 4 m urea or SDS and enhanced by reduction in pH. TNP-ATP to protein stoichiometries were similar for all K(ATP) COOH-terminal proteins with 1 mol of TNP-ATP binding/mole of protein. Competition of TNP-ATP binding to the Kir1.1 COOH terminus by MgATP was complex with both Mg(2+) and MgATP effects. Glutaraldehyde cross-linking demonstrated the multimerization potential of these COOH termini, suggesting that these cytosolic segments may directly interact in intact tetrameric channels. Thus, the COOH termini of K(ATP) tetrameric channels contain the nucleotide-binding pockets of these metabolically regulated channels with four potential nucleotide-binding sites/channel tetramer.

Structural and functional asymmetry of the nucleotide-binding domains of P-glycoprotein investigated by attenuated total reflection Fourier transform infrared spectroscopy

The dynamic changes occurring during the catalytic cycle of MDR3 P-glycoprotein (Pgp) and the role of each nucleotide-binding domain (NBD) in the transport process were investigated using attenuated total reflection Fourier transform infrared spectroscopy. For this purpose, wild-type Pgp and two mutations of homologous residues in each NBD were studied. On the one hand, we demonstrate here that, during its catalytic cycle, Pgp does not undergo secondary structure changes, but only modifications in its stability and accessibility to the external environment. On the other hand, amide H/D exchange kinetics demonstrate that homologous mutations in the two NBDs affect, in a different way, the dynamic properties of Pgp and also the dynamic changes occurring during ATP hydrolysis. These observations led to the conclusion that the NBDs have an asymmetric structure and different functions in the catalytic cycle of Pgp. Our data suggest that the release of drug from the membrane into the extracellular environment is due to decreased stability and/or increased accessibility to the external medium of the membrane-embedded drug-binding site(s). NBD1 would play an important role in this first restructuring of the membrane-embedded domains. NBD2 would be directly implicated in the subsequent restructuring of the membrane-embedded binding sites by which they recover their initial stability and accessibility to the membrane. It is proposed that this restructuring step would allow the binding and transport of another molecule of substrate.

Sequence requirements of the ATP-binding site within the C-terminal nucleotide-binding domain of mouse P-glycoprotein: structure-activity relationships for flavonoid binding

Sequence requirements of the ATP-binding site within the C-terminal nucleotide-binding domain (NBD2) of mouse P-glycoprotein were investigated by using two recombinantly expressed soluble proteins of different lengths and photoactive ATP analogues, 8-azidoadenosine triphosphate (8N(3)-ATP) and 2',3',4'-O-(2,4,6-trinitrophenyl)-8-azidoadenosine triphosphate (TNP-8N(3)-ATP). The two proteins, Thr(1044)-Thr(1224) (NBD2(short)) and Lys(1025)-Ser(1276) (NBD2(long)), both incorporated the four consensus sequences of ABC (ATP-binding cassette) transporters, Walker A and B motifs, the Q-loop, and the ABC signature, while differing in N-terminal and C-terminal extensions. Radioactive photolabeling of both proteins was characterized by hyperbolic dependence on nucleotide concentration and high-affinity binding with K(0.5)(8N(3)-ATP) = 36-37 microM and K(0.5)(TNP-8N(3)-ATP) = 0.8-2.6 microM and was maximal at acidic pH. Photolabeling was strongly inhibited by TNP-ATP (K(D) = 0.1-5 microM) and ATP (K(D) = 0.5-2.7 mM). Since flavonoids display bifunctional interactions at the ATP-binding site and a vicinal steroid-interacting hydrophobic sequence [Conseil, G., Baubichon-Cortay, H., Dayan, G., Jault, J.-M., Barron, D., and Di Pietro, A. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 9831-9836], a series of 30 flavonoids from different classes were investigated for structure-activity relationships toward binding to the ATP site, monitored by protection against photolabeling. The 3-OH and aromaticity of conjugated rings A and C appeared important, whereas opening of ring C abolished the binding in all but one case. It can be concluded that the benzopyrone portion of the flavonoids binds at the adenyl site and the phenyl ring B at the ribosyl site. The Walker A and B motifs, intervening sequences, and small segments on both sides are sufficient to constitute the ATP site.

Exploring the structure and function of the P-glycoprotein multidrug transporter using fluorescence spectroscopic tools

P-glycoprotein is an ABC protein that functions as an efflux pump for multiple drugs, natural products and peptides. It is proposed to operate as a hydrophobic vacuum cleaner, expelling non-polar compounds from the membrane bilayer to the exterior, driven by the energy of ATP hydrolysis. The nucleotide-binding domains of P-glycoprotein appear to operate by an alternating sites mechanism to power drug transport. In recent years, purification and functional reconstitution of the protein has allowed the application of fluorescence spectroscopic techniques. This approach has led to insights into the structural architecture of the P-glycoprotein molecule, and a more detailed understanding of the way in which it interacts with nucleotides and drugs.

C-Isoprenylation of flavonoids enhances binding affinity toward P-glycoprotein and modulation of cancer cell chemoresistance

Previous studies have shown that flavones bind to P-glycoprotein (Pgp) with higher affinity than isoflavones, flavanones, and glycosylated derivatives. In the present work, a series of C- or O-substituted hydrophobic derivatives of chrysin were synthesized to further investigate structural requirements of the A ring toward Pgp modulation. Increasing hydrophobicity at either position 6, 8, or 7 increased the affinity of in vitro binding to a purified cytosolic domain of Pgp, but only benzyl and 3,3-dimethylallyl C-substitution produced a high maximal quenching of the protein intrinsic fluorescence. Inhibition of membrane Pgp within leukemic cells, characterized by intracellular drug accumulation, was specifically produced by isoprenylated derivatives, with 8-(3,3-dimethylallyl)chrysin being even more efficient than the commonly used cyclosporin A.

Protein kinase C effectors bind to multidrug ABC transporters and inhibit their activity

P-Glycoprotein and homologous multidrug transporters contain a phosphorylatable linker sequence that was proposed to control drug efflux on the basis that it was indeed phosphorylated in vitro and in vivo, and that inhibitors of protein kinase C (PKC) inhibited both P-glycoprotein phosphorylation and activity. However, site-directed mutagenesis of all phosphorylatable residues did not alter the drug resistance. The present work shows that PKC effectors are able to bind directly to multidrug transporters, from either cancer cells (mouse P-glycoprotein), yeast (Saccharomyces cerevisiae Pdr5p), or protozoan parasite (Leishmania tropica ltmdr1), and to inhibit their energy-dependent drug-efflux activity. The binding of staurosporine and derivatives such as CGP 41251 is prevented by preincubation with ATP, suggesting at least partial interaction at the ATP-binding site. In contrast, more hydrophobic compounds such as calphostin C and CGP 42700 bind outside the ATP-binding site and strongly interfere with drug interaction. A direct correlation is obtained between the efficiencies of PKC effectors to inhibit energy-dependent interaction of rhodamine 6G with yeast Pdr5p, to promote intracellular drug accumulation in various multidrug resistant cells, and to chemosensitize growth of resistant cells. The noncompetitive inhibition by PKC effectors of rhodamine 6G interaction with Pdr5p suggests that the binding might interfere with signal transduction between nucleotide hydrolysis and drug interaction. The overall results indicate that the multidrug transporters from different species display common features for interaction with PKC inhibitors. The hydrophobic derivative of staurosporine, CGP 42700, constitutes a potentially powerful modulator of P-glycoprotein-mediated multidrug resistance.

Intrinsic fluorescence of the P-glycoprotein multidrug transporter: sensitivity of tryptophan residues to binding of drugs and nucleotides

P-glycoprotein is a member of the ATP binding cassette family of membrane proteins, and acts as an ATP-driven efflux pump for a diverse group of hydrophobic drugs, natural products, and peptides. The side chains of aromatic amino acids have been proposed to play an important role in recognition and binding of substrates by P-glycoprotein. Steady-state and lifetime fluorescence techniques were used to probe the environment of the 11 tryptophan residues within purified functional P-glycoprotein, and their response to binding of nucleotides and substrates. The emission spectrum of P-glycoprotein indicated that these residues are present in a relatively nonpolar environment, and time-resolved experiments showed the existence of at least two lifetimes. Quenching studies with acrylamide and iodide indicated that those tryptophan residues predominantly contributing to fluorescence emission are buried within the protein structure. Only small differences in Stern-Volmer quenching constants were noted on binding of nucleotides and drugs, arguing against large changes in tryptophan accessibility following substrate binding. P-glycoprotein fluorescence was highly quenched on binding of fluorescent nucleotides, and moderately quenched by ATP, ADP, and AMP-PNP, suggesting that the site for nucleotide binding is located relatively close to tryptophan residues. Drugs, modulators, hydrophobic peptides, and nucleotides quenched the fluorescence of P-glycoprotein in a saturable fashion, allowing estimation of dissociation constants. Many compounds exhibited biphasic quenching, suggesting the existence of multiple drug binding sites. The quenching observed for many substrates was attributable largely to resonance energy transfer, indicating that these compounds may be located close to tryptophan residues within, or adjacent to, the membrane-bound domains. Thus, the regions of P-glycoprotein involved in nucleotide and drug binding appear to be packed together compactly, which would facilitate coupling of ATP hydrolysis to drug transport.

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.

Prenyl-flavonoids as potent inhibitors of the Pdr5p multidrug ABC transporter from Saccharomyces cerevisiae

The Pdr5p multidrug ABC ("ATP-binding cassette) transporter was highly overexpressed in plasma membranes from a yeast strain exhibiting both pdr1-3 gain-of-function mutation in the transcription factor-encoding gene PDR1 and disruption of genes encoding other plasma membrane ABC transporters. Solubilized and purified Pdr5p displayed a tryptophan-characteristic intrinsic fluorescence, whose quenching was used to monitor interactions with substrates and effectors. The transporter exhibited a magnesium-dependent binding affinity for ATP and its fluorescent analogue 2'(3')-N-methylanthraniloyl-ATP, producing a marked fluorescence resonance-energy transfer. It also bound a series of known drug substrates and modulators. Interestingly, yeast Pdr5p interacted with flavonoids recently found to bind to cancer cell P-glycoprotein and to the protozoan parasite multidrug transporter. The extent of high-affinity binding of prenyl-flavonoids to purified Pdr5p was correlated to their efficiency to inhibit energy-dependent quenching of rhodamine 6G fluorescence catalyzed by Pdr5p-enriched plasma membranes. The hydrophobic flavonoid derivative 6-(3, 3-dimethylallyl)galangin was the most efficient, with a K(i) of 0.18 microM for competitive inhibition of the MgATP-dependent quenching of rhodamine 6G fluorescence. In contrast, inhibition of either ATP or UTP hydrolysis occurred at much higher concentrations and appeared to be noncompetitive. Prenyl-flavonoids therefore behave as potent inhibitors of drug binding to the yeast Pdr5p ABC transporter.

Analysis of the properties of the N-terminal nucleotide-binding domain of human P-glycoprotein

Human P-glycoprotein, the MDR1 gene product, requires both Mg(2+)-ATP binding and hydrolysis to function as a drug transporter; however, the mechanism(s) defining these events is not understood. In the present study, we explored the nature of Mg(2+)-ATP binding in the N-terminal nucleotide-binding domain of human P-glycoprotein and identified the minimal functional unit required for specific ATP binding. Recombinant proteins encompassing amino acids within the region beginning at 348 and ending at 707 were expressed in Escherichia coli, purified from inclusion bodies under denaturing conditions, and renatured by rapid dilution. The ability of ATP to interact with these proteins was examined by use of the photoactive ATP analogue [alpha-(32)P]-8-azido-ATP. Photoaffinity labeling of recombinant proteins identified the region between amino acids 375 and 635 as the region necessary to obtain specific ATP-binding properties. Specific protein labeling was saturable, enhanced by Mg(2+), and inhibited by ATP. Recombinant proteins confined within the region beginning at amino acid 392 and ending at amino acid 590 demonstrated nonspecific [alpha-(32)P]-8-azido-ATP labeling. Nonspecific labeling was not enhanced by Mg(2+) and was inhibited only by high concentrations of ATP. Using a D555N mutated protein, we found that the conserved aspartate residue in the Walker B motif plays a role in magnesium-enhanced ATP-binding. Taken together, these data define the region of the N-terminal nucleotide-binding domain of P-glycoprotein that is required for specific ATP binding and suggest that magnesium may play a role in stabilizing the ATP-binding site.

Insights into the structure and substrate interactions of the P-glycoprotein multidrug transporter from spectroscopic studies

The P-glycoprotein multidrug transporter is a 170-kDa efflux pump which exports a diverse group of natural products, chemotherapeutic drugs, and hydrophobic peptides across the plasma membrane, driven by ATP hydrolysis. The transporter has been proposed to interact with its drug substrates within the membrane environment; however, much remains to be learned about the nature and number of the drug binding site(s). The two nucleotide binding domains are responsible for ATP binding and hydrolysis, which is coupled to drug movement across the membrane. In recent years, P-glycoprotein has been purified and functionally reconstituted in amounts large enough to allow biophysical studies. The use of spectroscopic techniques has led to insights into both its secondary and tertiary structure, and its interaction with nucleotides and drugs. In this review, we will summarise what has been learned by application to purified P-glycoprotein of fluorescence spectroscopy, circular dichroism spectroscopy and infra-red spectroscopy.

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.

Synthesis and biological activity of 4-alkoxy chalcones: potential hydrophobic modulators of P-glycoprotein-mediated multidrug resistance

A series of 4-alkoxy-2',4',6'-trihydroxychalcones have been synthesized and evaluated for their ability to inhibit P-glycoprotein-mediated multidrug resistance (MDR) by direct binding to a purified protein domain containing an ATP-binding site and a modulator-interacting region. The introduction of hydrophobic alkoxy groups at position 4 led to much more active compounds as compared to the parent chalcone. The binding affinity increased as a function of the chain length, up to the octyloxy derivative for which a K(D) of 20 nM was obtained.

The transmembrane domains of the human multidrug resistance P-glycoprotein are sufficient to mediate drug binding and trafficking to the cell surface

The human multidrug resistance P-glycoprotein (P-gp) is organized in two tandem repeats with each repeat consisting of an N-terminal hydrophobic domain containing six potential transmembrane segments followed by a hydrophilic domain containing a nucleotide-binding fold. A series of deletion mutants together with an in vivo drug-binding assay were used to test whether the deletion mutants interacted with substrates or were transported to the cell surface. We found that a deletion mutant consisting of only the transmembrane domains (residues 1-379 plus 681-1025) retained the ability to interact with drug substrates. In the absence of drug substrates, the deletion mutant was sensitive to trypsin and endoglycosidase H. Expression in the presence of verapamil, vinblastine, capsaicin, or cyclosporin A, however, resulted in a mutant protein that was resistant to trypsin and endoglycosidase H. The mutant was then detected at the cell surface and was sensitive to digestion by endoglycosidase F. By contrast, the N-terminal transmembrane domain (residues 1-379) alone did not interact with drug substrates, since it was sensitive to only endoglycosidase H and was not detected at the cell surface. These results show that the nucleotide-binding domains are not required for interaction of P-gp with substrate or for trafficking of P-gp to the cell surface.

P-glycoprotein-mediated resistance to chemotherapy in cancer cells: using recombinant cytosolic domains to establish structure-function relationships

Resistance to chemotherapy in cancer cells is mainly mediated by overexpression of P-glycoprotein (Pgp), a plasma membrane ATP-binding cassette (ABC) transporter which extrudes cytotoxic drugs at the expense of ATP hydrolysis. Pgp consists of two homologous halves each containing a transmembrane domain and a cytosolic nucleotide-binding domain (NBD) which contains two consensus Walker motifs, A and B, involved in ATP binding and hydrolysis. The protein also contains an S signature characteristic of ABC transporters. The molecular mechanism of Pgp-mediated drug transport is not known. Since the transporter has an extraordinarily broad substrate specificity, its cellular function has been described as a "hydrophobic vacuum cleaner". The limited knowledge about the mechanism of Pgp, partly due to the lack of a high-resolution structure, is well reflected in the failure to efficiently inhibit its activity in cancer cells and thus to reverse multidrug resistance (MDR). In contrast to the difficulties encountered when studying the full-length Pgp, the recombinant NBDs can be obtained in large amounts as soluble proteins. The biochemical and biophysical characterization of recombinant NBDs is shown here to provide a suitable alternative route to establish structure-function relationships. NBDs were shown to bind ATP and analogues as well as potent modulators of MDR, such as hydrophobic steroids, at a region close to the ATP site. Interestingly, flavonoids also bind to NBDs with high affinity. Their binding site partly overlaps both the ATP-binding site and the steroid-interacting region. Therefore flavonoids constitute a new promising class of bifunctional modulators of Pgp.

Monoclonal antibodies that inhibit the transport function of the 190-kDa multidrug resistance protein, MRP. Localization of their epitopes to the nucleotide-binding domains of the protein

Multidrug resistance in tumor cells is often accompanied by overexpression of multidrug resistance protein (MRP), a 190-kDa transmembrane protein that belongs to the ATP-binding cassette superfamily of transport proteins. MRP mediates ATP-dependent transport of a variety of conjugated organic anions and can also transport several unmodified xenobiotics in a glutathione-dependent manner. To facilitate structure-function studies of MRP, we have generated a panel of MRP-specific monoclonal antibodies (mAbs). Four of these mAbs, QCRL-2, -3, -4, and -6, bind intracellular conformation-dependent epitopes, and we have shown that they can inhibit the transport of several MRP substrates. Binding competition and immunoprecipitation assays indicated that mAbs QCRL-4 and -6 probably recognize the same detergent-sensitive epitope in MRP, whereas mAbs QCRL-2, -3, and -4 each bind distinct, non-overlapping epitopes. Fab fragments inhibit transport as effectively as the intact mAbs, suggesting that inhibition results from direct interactions of the mAbs with MRP. Immunodot blot and immunoprecipitation analyses revealed that the minimal regions of MRP sufficient for full reactivity of mAbs QCRL-2 and -3 are amino acids 617-858 and 617-932, respectively, which encompass the NH2-proximal nucleotide-binding domain (NBD). In contrast, the epitope bound by mAb QCRL-4 localized to amino acids 1294-1531, a region that contains the COOH-proximal NBD. However, none of the mAbs inhibited photolabeling of intact MRP with 8-azido-[alpha-32P]ATP. This suggests that rather than preventing nucleotide binding, the mAbs inhibit transport by interfering with substrate binding or by trapping MRP in a conformation that does not allow transport to occur. Our results also demonstrate for the first time that the NBDs of MRP can be expressed as soluble polypeptides that retain a native conformation.

Correlation between the affinity of flavonoids binding to the cytosolic site of Leishmania tropica multidrug transporter and their efficiency to revert parasite resistance to daunomycin

The C-terminal nucleotide-binding domain (NBD2) of a P-glycoprotein-like transporter, encoded by the ltrmdr1 gene in Leishmania tropica and involved in parasite multidrug resistance (MDR), was overexpressed in Escherichia coli as a hexahistidine tagged protein and purified. The L. tropica recombinant domain efficiently bound fluorescent derivatives of ATP, the hydrophobic steroid analogue RU 486, and different classes of flavonoids with the following efficiency: flavone > flavanone > isoflavone > glucorhamnosyl-flavone > chromone. The affinity for flavones was dependent on the presence of hydroxyl groups at positions 5 and 3 and was further increased by a hydrophobic 1,1-dimethylallyl substituent at position 8. When flow cytometry was used to measure daunomycin accumulation in a MDR L. tropica line, a reversing effect was observed with flavones such as dimethylallyl-kaempferide at low concentration or apigenin at higher concentration, but neither with the glucorhamnosyl derivative rutin nor with the isoflavone genistein. The in vivo reversing effect of dimethylallyl-kaempferide was correlated to a high inhibition of MDR cell growth in the presence of daunomycin. The results suggest that flavone inhibition of both daunomycin efflux and parasite growth in the presence of the drug correlates to direct binding of the compound to cytosolic domain of the P-glycoprotein-like transporter.

Halogenated chalcones with high-affinity binding to P-glycoprotein: potential modulators of multidrug resistance

Previous studies have shown that flavonoids are modulators of the transmembrane P-glycoprotein (P-gp) which mediates cell multidrug resistance. Some structural elements have been identified which seem to contribute to these compounds' activity. In the present study, a series of halogenated chalcones was prepared to further explore the structural requirements for the P-gp modulation. Four halogenated chalcones have been synthesized and evaluated as potential modulators of P-gp-mediated multidrug resistance of cancer cells by in vitro assays using a purified recombinant domain of the transporter containing the modulator binding site. Halogenated chalcones exhibited high-affinity binding, the 2',4', 6'-trihydroxy-4-iodochalcone behaving as the most potent compound with a KD value in the nanomolar range.

Flavonoids: a class of modulators with bifunctional interactions at vicinal ATP- and steroid-binding sites on mouse P-glycoprotein

A hexahistidine-tagged C-terminal nucleotide-binding domain (H6-NBD2) from mouse P-glycoprotein was designed, overexpressed, and purified as a highly soluble recombinant protein. Intrinsic fluorescence of its single tryptophan residue allowed monitoring of high-affinity binding of 2'(3')-N-methylanthraniloyl-ATP (MANT-ATP), a fluorescent ATP derivative that induces a marked quenching correlated to fluorescence resonance-energy transfer. H6-NBD2 also bound all flavonoids known to modulate the multidrug resistance phenotype of P-glycoprotein-positive cancer cells, with similar affinities and relative efficiencies. Flavones (like quercetin or apigenin) bound more strongly than flavanones (naringenin), isoflavones (genistein), or glycosylated derivatives (rutin). Kaempferide, a 4'-methoxy 3,5,7-trihydroxy flavone, was even more reactive and induced a complete quenching of H6-NBD2 intrinsic fluorescence. Kaempferide binding was partly prevented by preincubation with ATP, or partly displaced upon ATP addition. Interestingly, kaempferide was also able to partly prevent the binding of the antiprogestin RU 486 to a hydrophobic region similar to that recently found, close to the ATP site, in the N-terminal cytosolic domain. Conversely, RU 486 partly prevented kaempferide binding, the effect being additive to the partial prevention by ATP. Furthermore, MANT-ATP binding, which occurred at the ATP site and extended to the vicinal steroid-interacting hydrophobic region, was completely prevented or displaced by kaempferide. All results indicate that flavonoids constitute a new class of modulators with bifunctional interactions at vicinal ATP-binding site and steroid-interacting region within a cytosolic domain of P-glycoprotein.

Secondary structure of P-glycoprotein investigated by circular dichroism and amino acid sequence analysis

P-glycoprotein (Pgp) is a plasma membrane protein known as an ATP-dependent drug-efflux pump that confers multidrug resistance to tumor cells. Structural analysis of Pgp was investigated by circular dichroism (CD) for the first time and in combination with amino acid sequence analysis. CD of highly purified Pgp from human, rat and murine Pgp-overexpressing drug resistant cells revealed slight variations in the spectral shape when recorded in the presence of dodecyl maltoside (DM). These species-dependent variations in CD shapes resulted from the interaction of the oligosaccharidic part with the protein core since they were abolished either in the presence of sodium dodecyl sulfate (SDS) or after deglycosylation, the latter not altering the Pgp ATP-dependent drug transport activity. Whatever the level of Pgp glycosylation and the detergent used (SDS or DM), the content in secondary structure deduced from deconvolution of CD spectra is almost the same for the three sources of Pgp and estimated to 43% alpha-helix, 16% beta-sheet, 15% beta-turn and 26% of other structures. These data, which constitute the first report of Pgp structure analysis by circular dichroism, are consistent with the 48% alpha-helix and 16% beta-sheets global contents predicted by using recently reported efficient secondary structure prediction methods. This consistency reinforces the reliability of the probable nature and localization of predicted Pgp secondary structure elements. This provides a good framework for precise 3D structure modeling of Pgp by homology with proteins of known 3D structure, as it is illustrated here for the A motifs of the ATP-binding domains of Pgp.

ATPase and multidrug transport activities of the overexpressed yeast ABC protein Yor1p

The Saccharomyces cerevisiae genome encodes 15 full-size ATP binding cassette transporters (ABC), of which PDR5, SNQ2, and YOR1 are known to be regulated by the transcription factors Pdr1p and Pdr3p (pleiotropic drug resistance). We have identified two new ABC transporter-encoding genes, PDR10 and PDR15, which were up-regulated by the PDR1-3 mutation. These genes, as well as four other ABC transporter-encoding genes, were deleted in order to study the properties of Yor1p. The PDR1-3 gain-of-function mutant was then used to overproduce Yor1p up to 10% of the total plasma membrane proteins. Overexpressed Yor1p was photolabeled by [gamma-32P]2', 3'-O-(2,4,6-trinitrophenyl)-8-azido-ATP (K0.5 = 45 microM) and inhibited by ATP (KD = 0.3 mM) in plasma membranes. Solubilization and partial purification on sucrose gradient allowed to detect significant Yor1p ATP hydrolysis activity (approximately 100 nmol of Pi.min-1.mg-1). This activity was phospholipid-dependent and sensitive to low concentrations of vanadate (I50 = 0.3 microM) and oligomycin (I50 = 8.5 microg/ml). In vivo, we observed a correlation between the amount of Yor1p in the plasma membrane and the level of resistance to oligomycin. We also demonstrated that Yor1p drives an energy-dependent, proton uncoupler-insensitive, cellular extrusion of rhodamine B. Furthermore, cells lacking both Yor1p and Pdr5p (but not Snq2p) showed increased accumulation of the fluorescent derivative of 1-myristoyl-2-[6-(NBD)aminocaproyl]phosphatidylethanolamine. Despite their different topologies, both Yor1p and Pdr5p mediated the ATP-dependent translocation of similar drugs and phospholipids across the yeast cell membrane. Both ABC transporters exhibit ATP hydrolysis in vitro, but Pdr5p ATPase activity is about 15 times higher than that of Yor1p, which may indicate mechanistic or regulatory differences between the two enzymes.

Binding of steroid modulators to recombinant cytosolic domain from mouse P-glycoprotein in close proximity to the ATP site

We recently found that recombinant NBD1 cytosolic domain corresponding to segment 395-581 of mouse mdr1 P-glycoprotein bound fluorescent 2'(3')-N-methylanthraniloyl-ATP (MANT-ATP) with high affinity [Dayan, G., Baubichon-Cortay, H., Jault, J.-M., Cortay, J. -C., Deléage, G., & Di Pietro, A. (1996) J. Biol. Chem. 271, 11652-11658]. The present work shows that a longer 371-705 domain (extended-NBD1), including tryptophan-696 as an intrinsic probe, which bound MANT-ATP with identical affinity, also interacted with steroids known to modulate anticancer drug efflux from P-glycoprotein-positive multidrug-resistant cells. Progesterone, which is not transported, its hydrophobic derivatives medroxyprogesterone acetate and megestrol acetate, and Delta6-progesterone produced nearly a 50% saturating quenching of the domain intrinsic fluorescence, with dissociation constants ranging from 53 to 18 microM. The even more hydrophobic antiprogestin RU 486 produced a complete quenching of tryptophan-696 fluorescence, in contrast to more hydrophilic derivatives of progesterone containing hydroxyl groups at positions 11, 16, 17, and 21 and known to be transported, which produced very little quenching. A similar differential interaction was observed with full-length purified P-glycoprotein. The steroid-binding region within extended-NBD1 appeared distinct from the nucleotide-binding site as the RU 486-induced quenching was neither prevented nor reversed by high ATP concentrations. In contrast, MANT-ATP binding was efficiently prevented or displaced by RU 486, suggesting that the hydrophobic MANT group of the bound nucleotide analogue overlaps, at least partially, the adjacent steroid-binding region revealed by RU 486.

Fluorescence spectroscopic studies on interactions between liver annexin VI and nucleotides--a possible role for a tryptophan residue

Annexin VI is a 68-kDa calcium-, phospholipid-, and cytoskeletal-element-binding protein, which has been implicated in various processes, including calcium release and sequestration in calcifying cartilage, in a receptor-mediated endocytosis in human fibroblasts, and in secretion from chromaffin granules. In these processes it was found that, in addition to Ca2+ and annexin, the presence of ATP is also a prerequisite. In the present report we show that annexin VI binds ATP and the binding of nucleotide to protein is accompanied by quenching of an intrinsic fluorescence of annexin VI, which was found to be specific for 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, GTP and ATP, and dependent on the annexin conformation. The nucleotide-binding site within an annexin VI molecule is likely to be close to the tryptophan-containing domain of annexin VI. We propose that ATP plays the role of a physiological ligand for annexin VI, and its binding to annexin VI may represent an alternative cellular mechanism for the regulation of annexin-membrane interactions coupled to overall energy transitions in the cell.

Interaction of annexins IV and VI with ATP. An alternative mechanism by which a cellular function of these calcium- and membrane-binding proteins is regulated

Annexin VI from porcine liver can be photoaffinity-labeled with 8-azido-[gamma-32P]ATP in a concentration-dependent, saturable manner. The extent of labeling varied with the concentration of calcium. The dissociation constant for the nucleotide was found to be in the range reported for ATP-binding proteins. The ATP analog, 2'-(or 3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-triphosphate, also bound to AnxVI, as indicated by shift in its fluorescence spectra in the presence of protein. Any significant 8-azido-ATP or TNP-ATP binding was not observed with AnxIV. ATP modulated the binding of AnxVI to erythrocyte membrane and increased the Ca2+ concentration required for half-maximal binding of AnxVI to F-actin.

Fluorescence studies on the nucleotide binding domains of the P-glycoprotein multidrug transporter

One of the major causes of multidrug resistance in human cancers is expression of the P-glycoprotein multidrug transporter, which acts as an efflux pump for a diverse range of natural products, chemotherapeutic drugs, and hydrophobic peptides. In the present study, fluorescence techniques were used to probe the nucleotide binding domains (NBD) of P-glycoprotein. The transporter was labeled at two conserved cysteine residues, one within each NBD, using the thiol-reactive fluor 2-(4'-maleimidylanilino)-naphthalene-6-sulfonic acid (MIANS), and collisional quenching was used to assess solvent accessibility of the bound probe. Acrylamide was a poor quencher, which suggests that MIANS is buried in a relatively inaccessible region of the protein. Iodide ion was a highly effective quencher, whereas Cs+ was not, demonstrating the presence of a positive charge in the region close to the ATP binding site. The fluorescent nucleotide derivative 2'(3')-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) was hydrolysed slowly by P-glycoprotein, with a V(max) approximately 20-fold lower than that for unmodified ATP, and a K(M) of 81 microM. TNP-ATP and TNP-ADP inhibited P-glycoprotein ATPase activity, indicating that they interact with the NBD, whereas TNP-AMP was a very poor inhibitor. When TNP-nucleotides bound to P-glycoprotein, their fluorescence intensity was enhanced in a concentration-dependent manner. Both TNP-ATP and TNP-ADP bound to P-glycoprotein with substantially higher affinity than ATP, with K(d) values of 43 and 36 microM, respectively. Addition of ATP led to only partial displacement of TNP-ATP. Resonance energy transfer was observed between cysteine-bound MIANS and TNP-ATP/ADP, which indicated that the two fluorescent groups are located close to each other within the catalytic site of P-glycoprotein.

Efficient purification and reconstitution of P-glycoprotein for functional and structural studies

Plasma membrane P-glycoprotein is known as an ATP-dependent drug efflux pump that confers multidrug resistance to tumor cells. None of the reported purification procedures worked properly for our P-glycoprotein-overproducing cell lines, i.e. murine lymphoid leukemia P388/ADR25, rat hepatoma AS30-D/COL10, and human lymphoblastic leukemia CEM/VLB5 cells. We have thus developed a general procedure for efficient purification of P-glycoprotein by combining solubilization with sodium dodecyl sulfate and chromatography on ceramic hydroxyapatite. This procedure was successful for the three cell lines and yielded 70% of the P-glycoprotein present in the starting plasma membranes with more than 99% purity. After exchanging sodium dodecyl sulfate into dodecyl maltoside and reconstitution into liposomes, purified P-glycoprotein exhibited a specific ATPase activity of about 200 nmol/min/mg, which was very similar to that obtained for P-glycoprotein solubilized and purified with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. This ATPase activity was sensitive to orthovanadate inhibition and stimulated by verapamil and other drugs. More importantly, drug transport properties of the reconstituted P-glycoprotein were comparable with those of P-glycoprotein embedded in plasma membranes. Since it is virtually devoid of lipids, this preparation is suitable for both functional and structural investigations.

Adenosine diphosphate as an intracellular regulator of insulin secretion

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels couple the cellular metabolic state to electrical activity and are a critical link between blood glucose concentration and pancreatic insulin secretion. A mutation in the second nucleotide-binding fold (NBF2) of the sulfonylurea receptor (SUR) of an individual diagnosed with persistent hyperinsulinemic hypoglycemia of infancy generated KATP channels that could be opened by diazoxide but not in response to metabolic inhibition. The hamster SUR, containing the analogous mutation, had normal ATP sensitivity, but unlike wild-type channels, inhibition by ATP was not antagonized by adenosine diphosphate (ADP). Additional mutations in NBF2 resulted in the same phenotype, whereas an equivalent mutation in NBF1 showed normal sensitivity to MgADP. Thus, by binding to SUR NBF2 and antagonizing ATP inhibition of KATP++ channels, intracellular MgADP may regulate insulin secretion.