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

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.

Structure of Ycf1p reveals the transmembrane domain TMD0 and the regulatory region of ABCC transporters

ATP binding cassette (ABC) proteins typically function in active transport of solutes across membranes. The ABC core structure is composed of two transmembrane domains (TMD1 and TMD2) and two cytosolic nucleotide binding domains (NBD1 and NBD2). Some members of the C-subfamily of ABC (ABCC) proteins, including human multidrug resistance proteins (MRPs), also possess an N-terminal transmembrane domain (TMD0) that contains five transmembrane α-helices and is connected to the ABC core by the L0 linker. While TMD0 was resolved in SUR1, the atypical ABCC protein that is part of the hetero-octameric ATP-sensitive K+ channel, little is known about the structure of TMD0 in monomeric ABC transporters. Here, we present the structure of yeast cadmium factor 1 protein (Ycf1p), a homolog of human MRP1, determined by electron cryo-microscopy (cryo-EM). A comparison of Ycf1p, SUR1, and a structure of MRP1 that showed TMD0 at low resolution demonstrates that TMD0 can adopt different orientations relative to the ABC core, including a ∼145° rotation between Ycf1p and SUR1. The cryo-EM map also reveals that segments of the regulatory (R) region, which links NBD1 to TMD2 and was poorly resolved in earlier ABCC structures, interacts with the L0 linker, NBD1, and TMD2. These interactions, combined with fluorescence quenching experiments of isolated NBD1 with and without the R region, suggest how posttranslational modifications of the R region modulate ABC protein activity. Mapping known mutations from MRP2 and MRP6 onto the Ycf1p structure explains how mutations involving TMD0 and the R region of these proteins lead to disease.

Function of essential chloride and arginine residue in nucleotide binding to vesicular nucleotide transporter

Vesicular nucleotide transporter (VNUT) plays a key role in purinergic signalling through its ability to transport nucleotides. VNUT belongs to the SLC17 family, which includes vesicular glutamate transporters (VGLUTs) and Type I Na+/phosphate cotransporters. All of these transporters exhibit membrane potential and Cl--dependent organic anion transport activity and have essential arginine in the transmembrane region. Previously, we reported that ketoacids inhibit these transporters through modulation of Cl- activation. Although this regulation is important to control signal transmission, the mechanisms underlying Cl--dependent regulation are unclear. Here, we examined the functional roles of Cl- and essential arginine residue on ATP binding to VNUT using the fluorescent ATP analogue trinitrophenyl-ATP (TNP-ATP). The fluorescence of TNP-ATP was enhanced by VNUT, whereas no enhancement was observed by VGLUT. Concentration-dependence curves showed that TNP-ATP was a high-affinity fluorescent probe for VNUT, with a Kd of 4.8 μM. TNP-ATP binding was competitive to ATP and showed similar specificity to transport activity. Addition of Cl- and ketoacids did not affect the apparent affinity for TNP-ATP. The Arg119 to Ala mutant retained TNP-ATP binding ability with slightly reduced affinity. Overall, these results indicated that Cl- and essential arginine were not important for ATP binding.

Exploring movement and energy in human P-glycoprotein conformational rearrangement

Human P-glycoprotein (P-gp), a kind of ATP-Binding Cassette transporter, can export a diverse variety of anti-cancer drugs out of the tumor cell. Its overexpression is one of the main reasons for the multidrug resistance (MDR) of tumor cells. It has been confirmed that during the substrate transport process, P-gp experiences a large-scale structural rearrangement from the inward- to outward-facing states. However, the mechanism of how the nucleotide-binding domains (NBDs) control the transmembrane domains (TMDs) to open towards the periplasm in the outward-facing state has not yet been fully characterized. Herein, targeted molecular dynamics simulations were performed to explore the conformational rearrangement of human P-gp. The results show that the allosteric process proceeds in a coupled way, and first the transition is driven by the NBDs, and then transmitted to the cytoplasmic parts of TMDs, finally to the periplasmic parts. The trajectories show that besides the translational motions, the NBDs undergo a rotation movement, which mainly occurs in xy plane and ensures the formation of the correct ATP-binding pockets. The analyses on the interaction energies between the six structure segments (cICLs) from the TMDs and NBDs reveal that their subtle energy differences play an important role in causing the periplasmic parts of the transmembrane helices to separate from each other in the established directions and in appropriate amplitudes. This conclusion can explain the two experimental phenomena about human P-gp in some extent. These studies have provided a detailed exploration into human P-gp rearrangement process and given an energy insight into the TMD reorientation during P-gp transition.

Hyperinsulinism-Causing Mutations Cause Multiple Molecular Defects in SUR1 NBD1

The sulfonylurea receptor 1 (SUR1) protein forms the regulatory subunit in ATP sensitive K⁺ (K_ATP) channels in the pancreas. SUR proteins are members of the ATP binding cassette (ABC) superfamily of proteins. Binding and hydrolysis of MgATP at the SUR nucleotide binding domains (NBDs) lead to channel opening. Pancreatic K_ATP channels play an important role in insulin secretion. SUR1 mutations that result in increased levels of channel opening ultimately inhibit insulin secretion and lead to neonatal diabetes. In contrast, SUR1 mutations that disrupt trafficking and/or decrease gating of K_ATP channels cause congenital hyperinsulinism, where oversecretion of insulin occurs even in the presence of low glucose levels. Here, we present data on the effects of specific congenital hyperinsulinism-causing mutations (G716V, R842G, and K890T) located in different regions of the first nucleotide binding domain (NBD1). Nuclear magnetic resonance (NMR) and fluorescence data indicate that the K890T mutation affects residues throughout NBD1, including residues that bind MgATP, NBD2, and coupling helices. The mutations also decrease the MgATP binding affinity of NBD1. Size exclusion and NMR data indicate that the G716V and R842G mutations cause aggregation of NBD1 in vitro, possibly because of destabilization of the domain. These data describe structural characterization of SUR1 NBD1 and shed light on the underlying molecular basis of mutations that cause congenital hyperinsulinism.

How Intrinsic Dynamics Mediates the Allosteric Mechanism in the ABC Transporter Nucleotide Binding Domain Dimer

A protein's architecture facilitates specific motions-intrinsic dynamic modes-that are employed to effect function. Here we used molecular dynamics (MD) simulations to investigate the dynamics of the MJ0796 ABC transporter nucleotide-binding domain (NBD). ABC transporter NBDs form a rotationally symmetric dimer whereby two equivalent active sites are formed at their interface; in complex with a dimer of transmembrane domains they hydrolyze ATP to energize translocation of substrates across cellular membranes. Our data suggest the ABC NBD's ensemble of functional states can be understood predominately in terms of conformational changes between its major subdomains, occurring along two orthogonal dynamic modes. The data show that ligands and oligomeric interactions modulate the equilibrium conformation of the NBD with respect to these motions, suggesting that allostery is achieved by affecting the energetic profile along these two modes. The observed dynamics and allostery integrate consonantly and logically within a mechanistic framework for the ABC NBD dimer, which is supported by a large body of experimental and theoretical data, providing a higher resolution view of the enzyme's dynamic cycle. Our study shows how valuable mechanistic inferences can be derived from accessible short-time scale MD simulations of an enzyme's substructures.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Detergent-free purification of ABC (ATP-binding-cassette) transporters

ABC (ATP-binding-cassette) transporters carry out many vital functions and are involved in numerous diseases, but study of the structure and function of these proteins is often hampered by their large size and membrane location. Membrane protein purification usually utilizes detergents to solubilize the protein from the membrane, effectively removing it from its native lipid environment. Subsequently, lipids have to be added back and detergent removed to reconstitute the protein into a lipid bilayer. In the present study, we present the application of a new methodology for the extraction and purification of ABC transporters without the use of detergent, instead, using a copolymer, SMA (polystyrene-co-maleic acid). SMA inserts into a bilayer and assembles into discrete particles, essentially solubilizing the membrane into small discs of bilayer encircled by a polymer, termed SMALPs (SMA lipid particles). We show that this polymer can extract several eukaryotic ABC transporters, P-glycoprotein (ABCB1), MRP1 (multidrug-resistance protein 1; ABCC1), MRP4 (ABCC4), ABCG2 and CFTR (cystic fibrosis transmembrane conductance regulator; ABCC7), from a range of different expression systems. The SMALP-encapsulated ABC transporters can be purified by affinity chromatography, and are able to bind ligands comparably with those in native membranes or detergent micelles. A greater degree of purity and enhanced stability is seen compared with detergent solubilization. The present study demonstrates that eukaryotic ABC transporters can be extracted and purified without ever being removed from their lipid bilayer environment, opening up a wide range of possibilities for the future study of their structure and function.

NMR and fluorescence studies of drug binding to the first nucleotide binding domain of SUR2A

ATP sensitive potassium (K(ATP)) channels are composed of four copies of a pore-forming inward rectifying potassium channel (Kir6.1 or Kir6.2) and four copies of a sulfonylurea receptor (SUR1, SUR2A, or SUR2B) that surround the pore. SUR proteins are members of the ATP-binding cassette (ABC) superfamily of proteins. Binding of MgATP at the SUR nucleotide binding domains (NBDs) results in NBD dimerization, and hydrolysis of MgATP at the NBDs leads to channel opening. The SUR proteins also mediate interactions with K(ATP) channel openers (KCOs) that activate the channel, with KCO binding and/or activation involving residues in the transmembrane helices and cytoplasmic loops of the SUR proteins. Because the cytoplasmic loops make extensive interactions with the NBDs, we hypothesized that the NBDs may also be involved in KCO binding. Here, we report nuclear magnetic resonance (NMR) spectroscopy studies that demonstrate a specific interaction of the KCO pinacidil with the first nucleotide binding domain (NBD1) from SUR2A, the regulatory SUR protein in cardiac K(ATP) channels. Intrinsic tryptophan fluorescence titrations also demonstrate binding of pinacidil to SUR2A NBD1, and fluorescent nucleotide binding studies show that pinacidil binding increases the affinity of SUR2A NBD1 for ATP. In contrast, the KCO diazoxide does not interact with SUR2A NBD1 under the same conditions. NMR relaxation experiments and size exclusion chromatography indicate that SUR2A NBD1 is monomeric under the conditions used in drug binding studies. These studies identify additional binding sites for commonly used KCOs and provide a foundation for testing binding of drugs to the SUR NBDs.

Molecular analysis of the multidrug transporter, P-glycoprotein

Inherent or acquired resistance of tumor cells to cytotoxic drugs represents a major limitation to the successful chemotherapeutic treatment of cancer. During the past three decades dramatic progress has been made in the understanding of the molecular basis of this phenomenon. Analyses of drug-selected tumor cells which exhibit simultaneous resistance to structurally unrelated anti-cancer drugs have led to the discovery of the human MDR1 gene product, P-glycoprotein, as one of the mechanisms responsible for multidrug resistance. Overexpression of this 170 kDa N-glycosylated plasma membrane protein in mammalian cells has been associated with ATP-dependent reduced drug accumulation, suggesting that P-glycoprotein may act as an energy-dependent drug efflux pump. P-glycoprotein consists of two highly homologous halves each of which contains a transmembrane domain and an ATP binding fold. This overall architecture is characteristic for members of the ATP-binding cassette or ABC superfamily of transporters. Cell biological, molecular genetic and biochemical approaches have been used for structure-function studies of P-glycoprotein and analysis of its mechanism of action. This review summarizes the current status of knowledge on the domain organization, topology and higher order structure of P-glycoprotein, the location of drug- and ATP binding sites within P-glycoprotein, its ATPase and drug transport activities, its possible functions as an ion channel, ATP channel and lipid transporter, its potential role in cholesterol biosynthesis, and the effects of phosphorylation on P-glycoprotein activity.

Perspectives on the structure-function of ABC transporters: the Switch and Constant Contact models

ABC transporters constitute one of the largest protein families across the kingdoms of archaea, eubacteria and eukarya. They couple ATP hydrolysis to vectorial translocation of diverse substrates across membranes. The ABC transporter architecture comprises two transmembrane domains and two cytosolic ATP-binding cassettes. During 2002-2012, nine prokaryotic ABC transporter structures and two eukaryotic structures have been solved to medium resolution. Despite a wealth of biochemical, biophysical, and structural data, fundamental questions remain regarding the coupling of ATP hydrolysis to unidirectional substrate translocation, and the mechanistic suite of steps involved. The mechanics of the ATP cassette dimer is defined most popularly by the 'Switch Model', which proposes that hydrolysis in each protomer is sequential, and that as the sites are freed of nucleotide, the protomers lose contact across a large solvent-filled gap of 20-30 Å; as captured in several X-ray solved structures. Our 'Constant Contact' model for the operational mechanics of ATP binding and hydrolysis in the ATP-binding cassettes is derived from the 'alternating sites' model, proposed in 1995, and which requires an intrinsic asymmetry in the ATP sites, but does not require the partner protomers to lose contact. Thus one of the most debated issues regarding the function of ABC transporters is whether the cooperative mechanics of ATP hydrolysis requires the ATP cassettes to separate or remain in constant contact and this dilemma is discussed at length in this review.

Role of the D-loops in allosteric control of ATP hydrolysis in an ABC transporter

ABC transporters couple ATP hydrolysis to movement of substrates across cell membranes. They comprise two transmembrane domains and two cytosolic nucleotide-binding domains forming two active sites that hydrolyze ATP cooperatively. The mechanism of ATP hydrolysis is controversial and the structural dynamic basis of its allosteric control unknown. Here we report molecular dynamics simulations of the ATP/apo and ATP/ADP states of the bacterial ABC exporter Sav1866, in which the cytoplasmic region of the protein was simulated in explicit water for 150 ns. In the simulation of the ATP/apo state, we observed, for the first time, conformers of the active site with the canonical geometry for an in-line nucleophilic attack on the ATP γ-phosphate. The conserved glutamate immediately downstream of the Walker B motif is the catalytic base, forming a dyad with the H-loop histidine, whereas the Q-loop glutamine has an organizing role. Each D-loop provides a coordinating residue of the attacking water, and comparison with the simulation of the ATP/ADP state suggests that via their flexibility, the D-loops modulate formation of the hydrolysis-competent state. A global switch involving a coupling helix delineates the signal transmission route by which allosteric control of ATP hydrolysis in ABC transporters is mediated.

Design and synthesis of ATP-based nucleotide analogues and profiling of nucleotide-binding proteins

Two nucleotide-based probes were designed and synthesized in order to enrich samples for specific classes of proteins by affinity-based protein profiling. We focused on the profiling of adenine nucleotide-binding proteins. Two properties were considered in the design of the probes: the bait needs to bind adenine nucleotide-binding proteins with high affinity and carry a second functional group suitable and easily accessible for coupling to a chromatography resin. For this purpose, we synthesized p-biotinyl amidobenzoic acid-ATP (p-BABA-ATP) and p-biotinyl aminomethylbenzoic acid-ATP (p-BAMBA-ATP). p-BABA-ATP and p-BAMBA-ATP both bind to ATP-binding cassette (ABC) proteins with at least 10-fold higher affinity than ATP. Several ABC transporters could be enriched using p-BABA-ATP or p-BAMBA-ATP.

Synthesis and modulation properties of imidazo[4,5-b]pyridin-7-one and indazole-4,7-dione derivatives towards the Cryptosporidium parvum CpABC3 transporter

The syntheses of new N-polysubstituted imidazo[4,5-b]pyridine-7-one (IP, 5 and 8a-8f) and indazole-4,7-dione (ID, 9 and 10) derivatives are described. The binding affinity of IP and ID towards the recombinant Nucleotide Binding Domain NBD1 of Cryptosporidium parvum CpABC3 was evaluated by intrinsic fluorescence quenching. IP induced a moderate quenching of the intrinsic fluorescence of H6-NBD1 whereas IDs 9 and 10 showed a binding affinity comparable to the ATP analogue TNP-ATP. In addition, 8d, 8e and 10 were shown to be competitive inhibitors of the ATPase activity, but with low affinity. These compounds could thus act like some flavonoid derivatives, which can partly overlap both the nucleotide-binding site and the adjacent hydrophobic steroid-binding region of mammalian P-glycoproteins.

The ABC transporter MsbA interacts with lipid A and amphipathic drugs at different sites

MsbA is an essential ABC (ATP-binding cassette) transporter involved in lipid A transport across the cytoplasmic membrane of Gram-negative bacteria. The protein has also been linked to efflux of amphipathic drugs. Purified wild-type MsbA was labelled stoichiometrically with the fluorescent probe MIANS [2-(4′-maleimidylanilino)naphthalene-6-sulfonic acid] on C315, which is located within the intracellular domain connecting transmembrane helix 6 and the nucleotide-binding domain. MsbA–MIANS displayed high ATPase activity, and its folding and stability were unchanged. The initial rate of MsbA labelling by MIANS was reduced in the presence of amphipathic drugs, suggesting that binding of these compounds alters the protein conformation. The fluorescence of MsbA–MIANS was saturably quenched by nucleotides, lipid A and various drugs, and estimates of the K_d values for binding fell in the range of 0.35–10 μM. Lipid A and daunorubicin were able to bind to MsbA–MIANS simultaneously, implying that they occupy different binding sites. The effects of nucleotide and lipid A/daunorubicin binding were additive, and binding was not ordered. The K_d of MsbA for binding lipid A was substantially decreased when the daunorubicin binding site was occupied first, and prior binding of nucleotide also modulated lipid A binding affinity. These results indicate that MsbA contains two substrate-binding sites that communicate with both the nucleotide-binding domain and with each other. One is a high affinity binding site for the physiological substrate, lipid A, and the other site interacts with drugs with comparable affinity. Thus MsbA may function as both a lipid flippase and a multidrug transporter.

Benzodiazepine-mediated structural changes in the multidrug transporter P-glycoprotein: an intrinsic fluorescence quenching analysis

P-glycoprotein expressed in Pichia pastoris was used to study the drug binding sites of different benzodiazepines. The effect of bromazepam, chlordiazepoxide, diazepam and flurazepam on P-glycoprotein structure was investigated by measuring the intrinsic fluorescence of the transporter tryptophan residues. Purified mouse mdr1a transporter in mixed micelles of 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid and 1,2-dimiristoyl-sn-glycerol-3-phosphocholine emitted fluorescence at 340 nm indicative of the fluorophores in a relatively apolar environment. Acrylamide and iodide ion were used as collisional quenchers toward distinct regions of the transporter, the protein and the interface protein-surface, respectively. Binding of ATP induced conformational changes at the protein surface level in accordance with the location of the nucleotide binding sites. Bromazepam interaction with the transporter was located at the protein-surface interface, diazepam at the membrane region and chlordiazepoxide at the protein surface. Only the flurazepam interaction site was not detected by the quenchers used. All benzodiazepines were able to elicit reorientation of the protein fluorophores on the P-glycoprotein-ATP complex.

ABC multidrug transporters: structure, function and role in chemoresistance

Three ATP-binding cassette (ABC)-superfamily multidrug efflux pumps are known to be responsible for chemoresistance; P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2 (BCRP). These transporters play an important role in normal physiology by protecting tissues from toxic xenobiotics and endogenous metabolites. Hydrophobic amphipathic compounds, including many clinically used drugs, interact with the substrate-binding pocket of these proteins via flexible hydrophobic and H-bonding interactions. These efflux pumps are expressed in many human tumors, where they likely contribute to resistance to chemotherapy treatment. However, the use of efflux-pump modulators in clinical cancer treatment has proved disappointing. Single nucleotide polymorphisms in ABC drug-efflux pumps may play a role in responses to drug therapy and disease susceptibility. The effect of various genotypes and haplotypes on the expression and function of these proteins is not yet clear, and their true impact remains controversial.

Structure-based interpretation of the mutagenesis database for the nucleotide binding domains of P-glycoprotein

P-glycoprotein (P-gp) is the most intensively studied eukaryotic ATP binding cassette (ABC) transporter, due to its involvement in the multidrug resistance phenotype of a number of cancers. In common with most ABC transporters, P-gp is comprised of two transmembrane domains (TMDs) and two nucleotide binding domains (NBD), the latter coupling ATP hydrolysis with substrate transport (efflux in the case of P-gp). Biochemical investigations over the past twenty years have attempted to unlock mechanistic aspects of P-glycoprotein through scanning and site-directed mutagenesis of both the TMDs and the NBDs. Contemporaneously, crystallographers have elucidated the atomic structure of numerous ABC transporter NBDs, as well as the intact structure (i.e. NBDs and TMDs) of a distantly related ABC-exporter Sav1866. Significantly, the structure of P-gp remains unknown, and only low resolution electron microscopy data exists. Within the current manuscript we employ crystallographic data for homologous proteins, and a molecular model for P-gp, to perform a structural interpretation of the existing "mutagenesis database" for P-gp NBDs. Consequently, this will enable testable predictions to be made that will result in further in-roads into our understanding of this clinically important drug pump.

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.

Conformational and functional characterization of trapped complexes of the P-glycoprotein multidrug transporter

The Pgp (P-glycoprotein) multidrug transporter couples ATP hydrolysis at two cytoplasmic NBDs (nucleotide-binding domains) to the transport of hydrophobic compounds. Orthovanadate (V(i)) and fluoroaluminate (AlF(x)) trap nucleotide in one NBD by forming stable catalytically inactive complexes (Pgp-M2+-ADP-X), which are proposed to resemble the catalytic transition state, whereas the complex formed by beryllium fluoride (BeF(x)) is proposed to resemble the ground state. We studied the trapped complexes formed via incubation of Pgp with ATP (catalytically forward) or ADP (reverse) and V(i), BeF(x) or AlF(x) using Mg2+ or Co2+ as the bivalent cation. Quenching of intrinsic Pgp tryptophan fluorescence by acrylamide, iodide and caesium indicated that conformational changes took place upon formation of the trapped complexes. Trapping with V(i) and ATP led to a 6-fold increase in the acrylamide quenching constant, K(SV), suggesting that large conformational changes take place in the Pgp transmembrane regions on trapping in the forward direction. Trapping with V(i) and ADP gave only a small change in quenching, indicating that the forward- and reverse-trapped complexes are different. TNP (trinitrophenyl)-ATP/TNP-ADP interacted with all of the trapped complexes, however, the fluorescence enhancement differed for the trapped states, suggesting a change in polarity in the nucleotide-binding sites. The nucleotide-binding site of the BeF(x)-trapped complex was much more polar than that of the V(i) and AlF(x) complexes. Functionally, all the trapped complexes were able to bind drugs and TNP-nucleotides with unchanged affinity compared with native Pgp.

Structure and function of ABC transporters: the ATP switch provides flexible control

ATP-binding cassette (ABC) transporters are ubiquitous integral membrane proteins that facilitate the transbilayer movement of ligands. They comprise, minimally, two transmembrane domains, which impart ligand specificity, and two nucleotide-binding domains (NBDs), which power the transport cycle. Almost 25 years of biochemistry is reviewed in light of the recent structure analyses resulting in the ATP-switch model for function in which the NBDs switch between a dimeric conformation, closed around two molecules of ATP, and a nucleotide-free, dimeric 'open' conformation. The flexibility of this switching mechanism has evolved to provide different kinetic control for different transporters and has also been co-opted to diverse functions other than transmembrane transport.

Characterization of Saccharomyces cerevisiae Atm1p: functional studies of an ABC7 type transporter

Saccharomyces cerevisiae Atm1p has been cloned, over-expressed and purified from a yeast expression system. The sequence includes both the soluble ATPase and transmembrane-spanning domains. With the introduction of an N-terminal Kozak sequence and a C-terminal (His)(6)-tag, a yield of 1 mg of Atm1p was obtained from 3 g wet yeast cells, which is comparable to other membrane-associated proteins isolated from eukaryotic expression systems. The ATPase activity of Atm1p is sensitive to sodium vanadate, a P-type ATPase inhibitor, with an IC(50) of 4 microM. MgADP is a product inhibitor for Atm1p with an IC(50) of 0.9 mM. The Michaelis-Menten constants V(max), K(M) and k(cat) of Atm1p were measured as 8.7+/-0.3 microM/min, 107+/-16 microM and 1.24+/-0.06 min(-1), respectively. A plot of ATPase activity versus concentration of Atm1p exhibits a nonlinear relationship, suggesting an allosteric response and an important role for the transmembrane domain in mediating both ATP hydrolysis and MgADP release. The metal dependence of Atm1p ATPase activity demonstrated a reactivity order of Mg(2+)>Mn(2+)>Co(2+), while each divalent ion was found to be inhibitory at higher concentrations. The activation and inhibitory effect of phospholipids suggest that formation of a lipid-micelle complex is important for enzymatic activity and stability. Structural analysis of Atm1p by CD spectroscopy suggested a similarity of secondary structure to that found for other members of this ABC protein family.

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

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

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.

The translocation mechanism of P-glycoprotein

Multidrug transporters are involved in mediating the failure of chemotherapy in treating several serious diseases. The archetypal multidrug transporter P-glycoprotein (P-gp) confers resistance to a large number of chemically and functionally unrelated anti-cancer drugs by mediating efflux from cancer cells. The ability to efflux such a large number of drugs remains a biological enigma and the lack of mechanistic understanding of the translocation pathway used by P-gp prevents rational design of compounds to inhibit its function. The translocation pathway is critically dependent on ATP hydrolysis and drug interaction with P-gp is possible at one of a multitude of allosterically linked binding sites. However, aspects such as coupling stoichiometry, molecular properties of binding sites and the nature of conformational changes remain unresolved or the centre of considerable controversy. The present review attempts to utilise the available data to generate a detailed sequence of events in the translocation pathway for this dexterous protein.

Celastraceae sesquiterpenes as a new class of modulators that bind specifically to human P-glycoprotein and reverse cellular multidrug resistance

Overexpression of ABCB1 (MDR1) P-glycoprotein, a multidrug efflux pump, is one mechanism by which tumor cells may develop multidrug resistance (MDR), preventing the successful chemotherapeutic treatment of cancer. Sesquiterpenes from Celastraceae family are natural compounds shown previously to reverse MDR in several human cancer cell lines and Leishmania strains. However, their molecular mechanism of reversion has not been characterized. In the present work, we have studied the ability of 28 dihydro-beta-agarofuran sesquiterpenes to reverse the P-glycoprotein-dependent MDR phenotype and elucidated their molecular mechanism of action. Cytotoxicity assays using human MDR1-transfected NIH-3T3 cells allowed us to select the most potent sesquiterpenes reversing the in vitro resistance to daunomycin and vinblastine. Flow cytometry experiments showed that the above active compounds specifically inhibited drug transport activity of P-glycoprotein in a saturable, concentration-dependent manner (K(i) down to 0.24 +/- 0.01 micromol/L) but not that of ABCC1 (multidrug resistance protein 1; MRP1), ABCC2 (MRP2), and ABCG2 (breast cancer resistance protein; BCRP) transporters. Moreover, sesquiterpenes inhibited at submicromolar concentrations the P-glycoprotein-mediated transport of [(3)H]colchicine and tetramethylrosamine in plasma membrane from CH(R)B30 cells and P-glycoprotein-enriched proteoliposomes, supporting that P-glycoprotein is their molecular target. Photoaffinity labeling in plasma membrane and fluorescence spectroscopy experiments with purified protein suggested that sesquiterpenes interact with transmembrane domains of P-glycoprotein. Finally, sesquiterpenes modulated P-glycoprotein ATPase-activity in a biphasic, concentration-dependent manner: they stimulated at very low concentrations but inhibited ATPase activity as noncompetitive inhibitors at higher concentrations. Sesquiterpenes from Celastraceae are promising P-glycoprotein modulators with potential applications in cancer chemotherapy because of their MDR reversal potency and specificity for P-glycoprotein.

The ATP switch model for ABC transporters

ABC transporters mediate active translocation of a diverse range of molecules across all cell membranes. They comprise two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Recent biochemical, structural and genetic studies have led to the ATP-switch model in which ATP binding and ATP hydrolysis, respectively, induce formation and dissociation of an NBD dimer. This provides an exquisitely regulated switch that induces conformational changes in the TMDs to mediate membrane transport.

Cross-talk between the paired domain and the homeodomain of Pax3: DNA binding by each domain causes a structural change in the other domain, supporting interdependence for DNA Binding

The Pax3 protein has two DNA binding domains, a Paired domain (PD) and a paired-type Homeo domain (HD). Although the PD and HD can bind to cognate DNA sequences when expressed individually, genetic and biochemical data indicate that the two domains are functionally interdependent in intact Pax3. The mechanistic basis of this functional interdependence is unknown and was studied by protease sensitivity. Pax3 was modified by the creation of Factor Xa cleavage sites at discrete locations in the PD, the HD, and in the linker segment joining the PD and the HD (Xa172, Xa189, and Xa216) in individual Pax3 mutants. The effect of Factor Xa insertions on protein stability and on DNA binding by the PD and the HD was measured using specific target site sequences. Independent insertions at position 100 in the linker separating the first from the second helix-turn-helix motif of the PD and at position 216 immediately upstream of the HD were found to be readily accessible to Factor Xa cleavage. The effect of DNA binding by the PD or the HD on accessibility of Factor Xa sites inserted in the same or in the other domain was monitored and quantitated for multiple mutants bearing different numbers of Xa sites at each position. In general, DNA binding reduced accessibility of all sites, suggesting a more compact and less solvent-exposed structure of DNA-bound versus DNA-free Pax3. Results of dose response and time course experiments were consistent and showed that DNA binding by the PD not only caused a local structural change in the PD but also caused a conformational change in the HD (P3OPT binding to Xa216 mutants); similarly, DNA binding by the HD also caused a conformational change in the PD (P2 binding to Xa100 mutants). These results provide a structural basis for the functional interdependence of the two DNA binding domains of Pax3.

Fluorescent and colored trinitrophenylated analogs of ATP and GTP

Fluorescent and colored trinitrophenylated (TNP) analogs of ATP and GTP can interact with nucleotide-requiring enzymes and proteins as a substitute for the parent nucleotide. These analogs have strong binding affinities for most nucleotide-requiring systems. Their bindings are easily detected by absorption and fluorescence changes in the visible region. Recent years have seen dramatic developments in the application of the TNP nucleotide analogs as spectroscopic probes for the study on the nucleotide-interacting properties of various enzymes and proteins including their mutants. This review is intended as a broad overview of currently extensively used applications of the nucleotide analogs in various biological systems.

Functionally similar vanadate-induced 8-azidoadenosine 5'-[alpha-(32)P]Diphosphate-trapped transition state intermediates of human P-glycoprotin are generated in the absence and presence of ATP hydrolysis

P-glycoprotein (Pgp) is an ATP-dependent drug efflux pump whose overexpression confers multidrug resistance to cancer cells. Pgp exhibits a robust drug substrate-stimulable ATPase activity, and vanadate (Vi) blocks this activity effectively by trapping Pgp nucleotide in a non-covalent stable transition state conformation. In this study we compare Vi-induced [alpha-(32)P]8-azido-ADP trapping into Pgp in the presence of [alpha-(32)P]8-azido-ATP (with ATP hydrolysis) or [alpha-(32)P]8-azido-ADP (without ATP hydrolysis). Vi mimics P(i) to trap the nucleotide tenaciously in the Pgp.[alpha-(32)P]8-azido-ADP.Vi conformation in either condition. Thus, by using [alpha-(32)P]8-azido-ADP we show that the Vi-induced transition state of Pgp can be generated even in the absence of ATP hydrolysis. Furthermore, half-maximal trapping of nucleotide into Pgp in the presence of Vi occurs at similar concentrations of [alpha-(32)P]8-azido-ATP or [alpha-(32)P]8-azido-ADP. The trapped [alpha-(32)P]8-azido-ADP is almost equally distributed between the N- and the C-terminal ATP sites of Pgp in both conditions. Additionally, point mutations in the Walker B domain of either the N- (D555N) or C (D1200N)-terminal ATP sites that arrest ATP hydrolysis and Vi-induced trapping also show abrogation of [alpha-(32)P]8-azido-ADP trapping into Pgp in the absence of hydrolysis. These data suggest that both ATP sites are dependent on each other for function and that each site exhibits similar affinity for 8-azido-ATP (ATP) or 8-azido-ADP (ADP). Similarly, Pgp in the transition state conformation generated with either ADP or ATP exhibits drastically reduced affinity for the binding of analogues of drug substrate ([(125)I]iodoarylazidoprazosin) as well as nucleotide (2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate). Analyses of Arrhenius plots show that trapping of Pgp with [alpha-(32)P]8-azido-ADP (in the absence of hydrolysis) displays an approximately 2.5-fold higher energy of activation (152 kJ/mol) compared with that observed when the transition state intermediate is generated through hydrolysis of [alpha-(32)P]8-azido-ATP (62 kJ/mol). In aggregate, these results demonstrate that the Pgp.[alpha-(32)P]8-azido-ADP (or ADP).Vi transition state complexes generated either in the absence of or accompanying [alpha-(32)P]8-azido-ATP hydrolysis are functionally indistinguishable.

Molecular genetic analysis and biochemical characterization of mammalian P-glycoproteins involved in multidrug resistance

A variety of human cancers become resistant or are intrinsically resistant to treatment with conventional drug therapies. This phenomenon is due in large part to the overexpression of a 170 kDa plasma membrane ATP-dependent pump known as the multidrug resistance transporter or P-glycoprotein. P-glycoprotein is a member of the large ATP binding cassette (ABC) superfamily of membrane transporters. This review focuses on the use of structure-function analyses to elucidate further the mechanism of action of mammalian P-glycoproteins. Ultimately, a complete understanding of the mechanism is important for the development of novel strategies for the treatment of many human cancers.

Expression and function of P-glycoprotein in rats with carbon tetrachloride-induced acute hepatic failure

Acute hepatic failure was induced experimentally in rats by intraperitoneal injection of 2.5 mL kg(-1) carbon tetrachloride (CCl4), and the effects on the expression and function of P-glycoprotein in the liver, kidney and brain were evaluated. The CCl4 injection significantly increased the indicators of hepatic function (glutamate oxaloacetate transaminase, glutamate pyruvate transaminase), but not of renal function (blood urea nitrogen, glomerular filtration rate). In rats with acute hepatic failure, the hepatic P-glycoprotein concentration increased 1.5-fold and the ATP concentration decreased to approximately 40% that in control rats. In contrast, P-glycoprotein concentrations in the kidney and brain and ATP concentrations in the kidney remained unchanged. The in-vivo P-glycoprotein function in these tissues was suppressed as evaluated by biliary and renal secretory clearances and brain distribution of rhodamine 123, a P-glycoprotein substrate. These findings suggest that factors other than P-glycoprotein concentration are involved in the systemic suppression of P-glycoprotein function in diseased rats. In Caco-2 cells, plasma collected from CCl4-treated rats exhibited a greater inhibitory effect on P-glycoprotein-mediated transport of rhodamine 123 than that from control rats, suggesting the accumulation of an endogenous P-glycoprotein substrate/inhibitor in the plasma of diseased rats. In fact, the plasma concentration of corticosterone, an endogenous P-glycoprotein substrate, increased 2-fold in CCl4-treated rats compared with control rats. It was demonstrated that P-glycoprotein function is systemically suppressed in rats with CCl4-induced acute hepatic failure, not only in the target organ (liver), but also in other organs (kidney and brain), although the P-glycoprotein concentration remained unchanged in the kidney and brain, and increased in the liver. In the systemic suppression of the P-glycoprotein function in the diseased state, the alteration of plasma concentrations or components of endogenous P-glycoprotein-related compounds, such as corticosterone, would likely be involved.

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.

Characterization of the catalytic cycle of ATP hydrolysis by human P-glycoprotein. The two ATP hydrolysis events in a single catalytic cycle are kinetically similar but affect different functional outcomes

P-glycoprotein (Pgp) is a plasma membrane protein whose overexpression confers multidrug resistance to tumor cells by extruding amphipathic natural product cytotoxic drugs using the energy of ATP. An elucidation of the catalytic cycle of Pgp would help design rational strategies to combat multidrug resistance and to further our understanding of the mechanism of ATP-binding cassette transporters. We have recently reported (Sauna, Z. E., and Ambudkar, S. V. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 2515-2520) that there are two independent ATP hydrolysis events in a single catalytic cycle of Pgp. In this study we exploit the vanadate (Vi)-induced transition state conformation of Pgp (Pgp.ADP.Vi) to address the question of what are the effects of ATP hydrolysis on the nucleotide-binding site. We find that at the end of the first hydrolysis event there is a drastic decrease in the affinity of nucleotide for Pgp coincident with decreased substrate binding. Release of occluded dinucleotide is adequate for the next hydrolysis event to occur but is not sufficient for the recovery of substrate binding. Whereas the two hydrolysis events have different functional outcomes vis à vis the substrate, they show comparable t(12) for both incorporation and release of nucleotide, and the affinities for [alpha-(32)P]8-azido-ATP during Vi-induced trapping are identical. In addition, the incorporation of [alpha-(32)P]8-azido-ADP in two ATP sites during both hydrolysis events is also similar. These data demonstrate that during individual hydrolysis events, the ATP sites are recruited in a random manner, and only one site is utilized at any given time because of the conformational change in the catalytic site that drastically reduces the affinity of the second ATP site for nucleotide binding. In aggregate, these findings provide an explanation for the alternate catalysis of ATP hydrolysis and offer a mechanistic framework to elucidate events at both the substrate- and nucleotide-binding sites in the catalytic cycle of Pgp.

The multi-structural feature of the multidrug resistance gene product P-glycoprotein: implications for its mechanism of action (hypothesis)

P-glycoprotein is a member of the ATP-binding cassette (ABC) transport superfamily. It plays an important role in the development of multidrug resistance in cancers by effluxing a wide variety of anticancer drugs. A large amount of information on the structure and function of P-glycoprotein has been accumulated over recent years from studies using molecular, biochemical, and biophysical approaches. It remains unclear, however, how this protein folds in membranes and how it transports such a wide variety of hydrophobic compounds. This paper highlights the recent progress in the structural and biogenesis aspects of P-glycoprotein. A model mechanism of P-glycoprotein action is proposed as a hypothesis that is based on recent progress in studying the topological folding of P-glycoprotein.

Alteration of the adenine nucleotide response and increased Rubisco activation activity of Arabidopsis rubisco activase by site-directed mutagenesis

Arabidopsis Rubisco was activated in vitro at rates 2- to 3-fold greater by recombinant Arabidopsis 43-kD Rubisco activase with the amino acid replacements Q111E and Q111D in a phosphate-binding loop, G-G-K-G-Q-G-K-S. However, these two mutant enzymes had only slightly greater rates of ATP hydrolysis. Activities of the Q111D enzyme were much less sensitive and those of Q111E were somewhat less sensitive to inhibition by ADP. Both mutant enzymes exhibited higher Rubisco activation activities over the physiological range of ADP to ATP ratios. Enzymes with non-polar, polar, and basic residues substituted at position Gln-111 exhibited rates of Rubisco activation less than the wild-type enzyme. Estimates of the relative affinity of the wild type and the Q111D, Q111E, and Q111S enzymes for adenosine nucleotides by a variety of methods revealed that the nucleotide affinities were the most diminished in the Q111D enzyme. The temperature stability of the Q111D and Q111E enzymes did not differ markedly from that of the 43-kD recombinant wild-type enzyme, which is somewhat thermolabile. The Q111D and Q111E enzymes, expressed in planta, may provide a means to better define the role of the ADP to ATP ratio in the regulation of Rubisco activation and photosynthesis rate.

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.

The packing of the transmembrane segments of human multidrug resistance P-glycoprotein is revealed by disulfide cross-linking analysis

Residues from several transmembrane (TM) segments of P-glycoprotein (P-gp) likely form the drug-binding site(s). To determine the organization of the TM segments, pairs of cysteine residues were introduced into the predicted TM segments of a Cys-less P-gp, and the mutant protein was subjected to oxidative cross-linking. In SDS gels, the cross-linked product migrated with a slower mobility than the native protein. The cross-linked products were not detected in the presence of dithiothreitol. Cross-linking was observed in 12 of 125 mutants. The pattern of cross-linking suggested that TM6 is close to TMs 10, 11, and 12, while TM12 is close to TMs 4, 5, and 6. In some mutants the presence of drug substrate colchicine, verapamil, cyclosporin A, or vinblastine either enhanced or inhibited cross-linking. Cross-linking was inhibited in the presence of ATP plus vanadate. These results suggest that the TM segments critical for drug binding must be close to each other and exhibit different conformational changes in response to binding of drug substrate or vanadate trapping of nucleotide. Based on these results, we propose a model for the arrangement of the TM segments.

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.

ABC-ATPases, adaptable energy generators fuelling transmembrane movement of a variety of molecules in organisms from bacteria to humans

The approximately 27 kDa ABC-ATPase, an extraordinarily conserved, unique type of ATPase, acts as a machine to fuel the movement across membranes of almost any type of molecule, from large polypeptides to small ions, via many different membrane-spanning proteins. A particular ABC-ATPase must therefore be tailor-made to function in a complex with its cognate membrane protein, forming a transport pathway appropriate for a specific type of molecule, or in the case of some ABC-transporters, several types of molecule. Molecules to be transported recognise their own transporter, bind and switch on the ATPase, which in turn activates or opens the transport pathway. ABC-dependent transport can be inwards across the membrane, or outwards to the cell exterior, and the ABC-ATPase can fuel transport through pathways which may involve a classical channel (CFTR), a "gateway" mechanism through a proteinacious chamber spanning the bilayer, or conceivably via a pathway at the protein-lipid interface of the outside of the membrane domain. This may be the case for drugs transported by Pgp, a multidrug resistance transporter. In this review, we try to identify the common fundamental principles which unite all ABC-transporters, including the basis of specificity for different transported compounds (allocrites), the interactions between the ATPase and membrane domains, activation of the ATPase and the coupling of consequent conformational changes, to the final movement of an allocrite through a given transport pathway. We discuss the so far limited structural information for the intact ABC-transporter complex and the exciting information from the first crystal structure of an ABC-ATPase. Finally, the action of specific transporters, CFTR (Cl- transport), Pgp, MRP and LmrA, all transporting many different drug molecules and HlyB transporting a large protein toxin are discussed.

Characterization of cultured dorsal root ganglion neuron P2X receptors

P2X receptors for adenosine 5'-triphosphate (ATP) comprise a family of ligand-gated cation channels with distinct characteristics which are dependent on the receptor subunits (P2X1-7) expressed, and the homomeric or heteromeric assembly of protein subunits in individual cells. We describe the properties of P2X receptors expressed by cultured adult rat dorsal root ganglion cells on the basis of the time course of responses to ATP, alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-meATP) and 2-methyl-thioadenosine 5'-triphosphate (2-meSATP), and using the antagonists 2',3'-O-(2,4,6-trinitrophenyl) ATP (TNP-ATP), a novel and highly selective purinoceptor antagonist, suramin and iso-pyridocalphosphate-6-azophenyl-2',5' disulphonic acid (PPADS). ATP (10 microM) evoked inward currents in approximately 95% of neurons tested and > 80% responded with a fast transient inward current that rapidly inactivated during the continued presence of ATP. Of the remaining neurons, approximately 4% showed a sustained response and approximately 10% showed a combination of transient and sustained components. Rapid application of ATP, alpha,beta-meATP and 2meSATP demonstrated these to be full agonists of the rapidly inactivating P2X receptor (pA50 values = 5.83, 5.86 and 5.55, respectively), whilst uridine triphosphate (UTP) and 1-beta,gamma-methyleneadenosine 5'-triphosphate (1-beta,gamma-meATP) were ineffective as agonists. These rapidly inactivating responses could be inhibited by TNP-ATP, suramin and PPADS (pIC50 = 9.5, 6.5, 6.4, respectively). Using inactivation protocols, we demonstrate the presence of homomeric P2X3-like receptors and non-inactivating P2X receptors, which indicates that individual subsets of adult dorsal root ganglion neurons have distinct P2X receptor phenotypes, and that individual DRG neurons may express multiple P2X receptor subtypes.

Linear and cyclic peptides as substrates and modulators of P-glycoprotein: peptide binding and effects on drug transport and accumulation

One cause of multidrug resistance (MDR) in human cancers is the overexpression of the P-glycoprotein multidrug transporter, a member of the ABC superfamily of membrane proteins. Natural products and chemotherapeutic drugs are pumped out of the cell by P-glycoprotein in an ATP-dependent fashion. There is growing evidence that many hydrophobic peptides are also P-glycoprotein substrates. With the use of a fluorescence-quenching assay, we have shown that some linear and cyclic hydrophobic peptides interact with P-glycoprotein, whereas others do not. The measured values of the quenching constant, Kq, for interaction of peptides with P-glycoprotein ranged from 200 nM for cyclosporine A to 138 microM for the tripeptide N-acetyl-leucyl-leucyl-norleucinal. Peptides that interacted with P-glycoprotein in the fluorescence assay also blocked colchicine transport into plasma membrane vesicles from MDR cells. The values of Dm, the peptide concentration causing 50% inhibition of drug uptake, were highly correlated with the values of Kq, over three orders of magnitude. The P-glycoprotein ATPase stimulation/inhibition profile of the peptides was not helpful in making a quantitative assessment of the ability of a peptide to interact with P-glycoprotein or to block drug transport. Some hydrophobic peptides were able to restore accumulation in MDR cells of the chemotherapeutic drug daunorubicin and the fluorescent dye rhodamine 123 to the levels observed in the drug-sensitive parent. Peptides that interacted with P-glycoprotein also displayed a relatively low overall toxicity to intact MDR cells, and inhibited drug transport at concentrations below the toxic range. Hydrophobic peptides should be given serious consideration for development as clinical chemosensitizing agents.

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.

ATP-binding-cassette (ABC) transport systems: functional and structural aspects of the ATP-hydrolyzing subunits/domains

Members of the superfamily of adenosine triphosphate (ATP)-binding-cassette (ABC) transport systems couple the hydrolysis of ATP to the translocation of solutes across a biological membrane. Recognized by their common modular organization and two sequence motifs that constitute a nucleotide binding fold, ABC transporters are widespread among all living organisms. They accomplish not only the uptake of nutrients in bacteria but are involved in diverse processes, such as signal transduction, protein secretion, drug and antibiotic resistance, antigen presentation, bacterial pathogenesis and sporulation. Moreover, some human inheritable diseases, like cystic fibrosis, adrenoleukodystrophy and Stargardt's disease are caused by defective ABC transport systems. Thus, albeit of major significance, details of the molecular mechanism by which these systems exert their functions are still poorly understood. In this review, recent data concerning the properties and putative role of the ATP-hydrolyzing subunits/domains are summarized and compared between bacterial and eukaryotic systems.