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

The Switch and Reciprocating Models for the Function of ABC Multidrug Exporters: Perspectives on Recent Research

ATP-binding cassette (ABC) transporters comprise a large superfamily of primary active transporters, which are integral membrane proteins that couple energy to the uphill vectorial transport of substrates across cellular membranes, with concomitant hydrolysis of ATP. ABC transporters are found in all living organisms, coordinating mostly import in prokaryotes and export in eukaryotes. Unlike the highly conserved nucleotide binding domains (NBDs), sequence conservation in the transmembrane domains (TMDs) is low, with their divergent nature likely reflecting a need to accommodate a wide range of substrate types in terms of mass and polarity. An explosion in high resolution structural analysis over the past decade and a half has produced a wealth of structural information for ABCs. Based on the structures, a general mechanism for ABC transporters has been proposed, known as the Switch or Alternating Access Model, which holds that the NBDs are widely separated, with the TMDs and NBDs together forming an intracellular-facing inverted "V" shape. Binding of two ATPs and the substrate to the inward-facing conformation induces a transition to an outward conformation. Despite this apparent progress, certainty around the transport mechanism for any given ABC remains elusive. How substrate binding and transport is coupled to ATP binding and hydrolysis is not known, and there is a large body of biochemical and biophysical data that is at odds with the widely separated NBDs being a functional physiological state. An alternative Constant Contact model has been proposed in which the two NBSs operate 180 degrees out of phase with respect to ATP hydrolysis, with the NBDs remaining in close proximity throughout the transport cycle and operating in an asymmetric allosteric manner. The two models are discussed in the light of recent nuclear magnetic resonance and hydrogen-deuterium exchange mass spectrometry analyses of three ABC exporters.

Nucleotide binding is the critical regulator of ABCG2 conformational transitions

ABCG2 is an exporter-type ABC protein that can expel numerous chemically unrelated xeno- and endobiotics from cells. When expressed in tumor cells or tumor stem cells, ABCG2 confers multidrug resistance, contributing to the failure of chemotherapy. Molecular details orchestrating substrate translocation and ATP hydrolysis remain elusive. Here, we present methods to concomitantly investigate substrate and nucleotide binding by ABCG2 in cells. Using the conformation-sensitive antibody 5D3, we show that the switch from the inward-facing (IF) to the outward-facing (OF) conformation of ABCG2 is induced by nucleotide binding. IF-OF transition is facilitated by substrates, and hindered by the inhibitor Ko143. Direct measurements of 5D3 and substrate binding to ABCG2 indicate that the high-to-low affinity switch of the drug binding site coincides with the transition from the IF to the OF conformation. Low substrate binding persists in the post-hydrolysis state, supporting that dissociation of the ATP hydrolysis products is required to reset the high substrate affinity IF conformation of ABCG2.

Human ABCB1 with an ABCB11-like degenerate nucleotide binding site maintains transport activity by avoiding nucleotide occlusion

Several ABC exporters carry a degenerate nucleotide binding site (NBS) that is unable to hydrolyze ATP at a rate sufficient for sustaining transport activity. A hallmark of a degenerate NBS is the lack of the catalytic glutamate in the Walker B motif in the nucleotide binding domain (NBD). The multidrug resistance transporter ABCB1 (P-glycoprotein) has two canonical NBSs, and mutation of the catalytic glutamate E556 in NBS1 renders ABCB1 transport-incompetent. In contrast, the closely related bile salt export pump ABCB11 (BSEP), which shares 49% sequence identity with ABCB1, naturally contains a methionine in place of the catalytic glutamate. The NBD-NBD interfaces of ABCB1 and ABCB11 differ only in four residues, all within NBS1. Mutation of the catalytic glutamate in ABCB1 results in the occlusion of ATP in NBS1, leading to the arrest of the transport cycle. Here we show that despite the catalytic glutamate mutation (E556M), ABCB1 regains its ATP-dependent transport activity, when three additional diverging residues are also replaced. Molecular dynamics simulations revealed that the rescue of ATPase activity is due to the modified geometry of NBS1, resulting in a weaker interaction with ATP, which allows the quadruple mutant to evade the conformationally locked pre-hydrolytic state to proceed to ATP-driven transport. In summary, we show that ABCB1 can be transformed into an active transporter with only one functional catalytic site by preventing the formation of the ATP-locked pre-hydrolytic state in the non-canonical site.

ABC transporters are one of the largest membrane protein superfamilies, present in all organisms from archaea to humans. They transport a wide range of molecules including amino acids, sugars, vitamins, nucleotides, peptides, lipids, metabolites, antibiotics, and xenobiotics. ABC transporters energize substrate transport by hydrolyzing ATP in two symmetrically arranged nucleotide binding sites (NBSs). The human multidrug resistance transporter ABCB1 has two active NBSs, and it is generally believed that integrity and cooperation of both sites are needed for transport. Several human ABC transporters, such as the bile salt transporter ABCB11, have one degenerate NBS, which has significantly reduced ATPase activity. Interestingly, unilateral mutations affecting one of the two NBSs completely abolish the function of symmetrical ABC transporters. Here we engineered an ABCB1 variant with a degenerate, ABCB11-like NBS1, which can nevertheless transport substrates. Our results indicate that ABCB1 can mediate active transport with a single active site, questioning the validity of models assuming strictly alternating catalysis.

In vivo FRET analyses reveal a role of ATP hydrolysis-associated conformational changes in human P-glycoprotein

P-glycoprotein (P-gp; also known as MDR1 or ABCB1) is an ATP-driven multidrug transporter that extrudes various hydrophobic toxic compounds to the extracellular space. P-gp consists of two transmembrane domains (TMDs) that form the substrate translocation pathway and two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP. At least two P-gp states are required for transport. In the inward-facing (pre-drug transport) conformation, the two NBDs are separated, and the two TMDs are open to the intracellular side; in the outward-facing (post-drug transport) conformation, the NBDs are dimerized, and the TMDs are slightly open to the extracellular side. ATP binding and hydrolysis cause conformational changes between the inward-facing and the outward-facing conformations, and these changes help translocate substrates across the membrane. However, how ATP hydrolysis is coupled to these conformational changes remains unclear. In this study, we used a new FRET sensor that detects conformational changes in P-gp to investigate the role of ATP binding and hydrolysis during the conformational changes of human P-gp in living HEK293 cells. We show that ATP binding causes the conformational change to the outward-facing state and that ATP hydrolysis and subsequent release of γ-phosphate from both NBDs allow the outward-facing state to return to the original inward-facing state. The findings of our study underscore the utility of using FRET analysis in living cells to elucidate the function of membrane proteins such as multidrug transporters.

In vitro transport of methotrexate by Drosophila Multidrug Resistance-associated Protein

Methotrexate (MTX) is a widely used chemotherapeutic agent, immune suppressant and antimalarial drug. It is a substrate of several human ABC proteins that confer multidrug resistance to cancer cells and determine compartmentalization of a wide range of physiological metabolites and endo or xenobiotics, by their primary active transport across biological membranes. The substrate specificity and tissue distribution of these promiscuous human ABC transporters show a high degree of redundancy, providing robustness to these key physiological and pharmacological processes, such as the elimination of toxins, e.g. methotrexate from the body. A similar network of proteins capable of transporting methotrexate has been recently suggested to exist in Drosophila melanogaster. One of the key players of this putative network is Drosophila Multidrug-resistance Associated Protein (DMRP). DMRP has been shown to be a highly active and promiscuous ABC transporter, capable of transporting various organic anions. Here we provide the first direct evidence that DMRP, expressed alone in a heterologous system lacking other, potentially functionally overlapping D. melanogaster organic anion transporters, is indeed able to transport methotrexate. Our in vitro results support the hypothesized but debated role of DMRP in in vivo methotrexate excretion.

A single active catalytic site is sufficient to promote transport in P-glycoprotein

P-glycoprotein (Pgp) is an ABC transporter responsible for the ATP-dependent efflux of chemotherapeutic compounds from multidrug resistant cancer cells. Better understanding of the molecular mechanism of Pgp-mediated transport could promote rational drug design to circumvent multidrug resistance. By measuring drug binding affinity and reactivity to a conformation-sensitive antibody we show here that nucleotide binding drives Pgp from a high to a low substrate-affinity state and this switch coincides with the flip from the inward- to the outward-facing conformation. Furthermore, the outward-facing conformation survives ATP hydrolysis: the post-hydrolytic complex is stabilized by vanadate, and the slow recovery from this state requires two functional catalytic sites. The catalytically inactive double Walker A mutant is stabilized in a high substrate affinity inward-open conformation, but mutants with one intact catalytic center preserve their ability to hydrolyze ATP and to promote drug transport, suggesting that the two catalytic sites are randomly recruited for ATP hydrolysis.

Snapshots of ligand entry, malleable binding and induced helical movement in P-glycoprotein

P-glycoprotein (P-gp) is a transporter of great clinical and pharmacological significance. Several structural studies of P-gp and its homologs have provided insights into its transport cycle, but questions remain regarding how P-gp recognizes diverse substrates and how substrate binding is coupled to ATP hydrolysis. Here, four new P-gp co-crystal structures with a series of rationally designed ligands are presented. It is observed that the binding of certain ligands, including an ATP-hydrolysis stimulator, produces a large conformational change in the fourth transmembrane helix, which is positioned to potentially transmit a signal to the nucleotide-binding domains. A new ligand-binding site on the surface of P-gp facing the inner leaflet of the membrane is also described, providing vital insights regarding the entry mechanism of hydrophobic drugs and lipids into P-gp. These results represent significant advances in the understanding of how P-gp and related transporters bind and export a plethora of metabolites, antibiotics and clinically approved and pipeline drugs.

Substrate binding stabilizes a pre-translocation intermediate in the ATP-binding cassette transport protein MsbA

ATP-binding cassette (ABC) transporters belong to one of the largest protein superfamilies that expands from prokaryotes to man. Recent x-ray crystal structures of bacterial and mammalian ABC exporters suggest a common alternating access mechanism of substrate transport, which has also been biochemically substantiated. However, the current model does not yet explain the coupling between substrate binding and ATP hydrolysis that underlies ATP-dependent substrate transport. In our studies on the homodimeric multidrug/lipid A ABC exporter MsbA from Escherichia coli, we performed cysteine cross-linking, fluorescence energy transfer, and cysteine accessibility studies on two reporter positions, near the nucleotide-binding domains and in the membrane domains, for transporter embedded in a biological membrane. Our results suggest for the first time that substrate binding by MsbA stimulates the maximum rate of ATP hydrolysis by facilitating the dimerization of nucleotide-binding domains in a state, which is markedly distinct from the previously described nucleotide-free, inward-facing and nucleotide-bound, outward-facing conformations of ABC exporters and which binds ATP.

Evidence for modulatory sites at the lipid-protein interface of the human multidrug transporter P-glycoprotein

The human multidrug transporter P-glycoprotein (Pgp or ABCB1) sets up pharmacological barriers to many clinically important drugs, a therapeutic remedy for which has yet to be formulated. For the rational design of mechanism-based inhibitors (or modulators), it is necessary to map the potential sites for modulator interaction and understand their modes of communication with the other functional domains of Pgp. In this study, combining directed mutagenesis with homology modeling, we provide evidence of two modulator-specific sites at the lipid protein interface of Pgp. Targeting 21 variant positions in the COOH-terminal transmembrane (TM) regions, we find residues M948 (in TM11) and F983, M986, V988, and Q990 (all four in TM12) critically involved in substrate-site modulation by a thioxanthene-based allosteric modulator cis-(Z)-flupentixol. Interestingly, for ATP-site modulation by the same modulator, only two (M948 and Q990) of those four residues appear indispensable, together with two additional residues, T837 and I864 in TM9 and TM10, respectively, suggesting independent modes of communication linking the allosteric site with the substrate binding and ATPase domains. None of the seven residues identified prove to be critical for modulation of the substrate or ATP sites by Pgp modulators that are transported by the pump, such as cyclosporin A or verapamil, indicating their specificity for cis-(Z)-flupentixol. On the other hand, ATP-site modulation by verapamil proves to be highly sensitive to replacement at positions F716 (in TM7) and I765 (in TM8), and to a more moderate extent at I764 and L772 (both in TM8). Homology modeling based on the known crystal structures of the bacterial multidrug transporter SAV1866 and the mouse Pgp homologue maps the identified residues primarily at the lipid-protein interface of Pgp, in two spatially distinct modulator-specific clusters. The two modulatory sites demonstrate negative synergism in influencing ATP hydrolysis, consolidating their spatial distinctness. Because Pgp is known to recruit drug molecules directly from the lipid bilayer, identification of modulatory sites at the lipid-protein interface and at the same time outside the conventional central drug binding cavity is mechanistically revealing.

The choreography of multidrug export

Multidrug transporters have a crucial role in causing the drug resistance that can arise in infectious micro-organisms and tumours. These integral membrane proteins mediate the export of a broad range of unrelated compounds from cells, including antibiotics and anticancer agents, thus reducing the concentration of these compounds to subtoxic levels in target cells. In spite of intensive research, it is not clear exactly how multidrug transporters work. The present review focuses on recent advancements in the biochemistry and structural biology of bacterial and human multidrug ABC (ATP-binding cassette) transporters. These advancements point to a common mechanism in which polyspecific drug-binding surfaces in the membrane domains are alternately exposed to the inside and outside surface of the membrane in response to the ATP-driven dimerization of nucleotide-binding domains and their dissociation following ATP hydrolysis.

Role of multidrug transporters in neurotherapeutics

Acquired resistance to antibiotics and other chemotherapeutic agents is a major problem in the practice of neurology and other branches of medicine. There are several mechanisms by which drug resistance is acquired. Multidrug transporters are important glycoproteins located in the cell membrane that actively transport small lipophilic molecules from one side of the cell membrane to the other, most often from the inside to the outside of a cell. They have important protective role yet may prove inconvenient in chemotherapy. In epilepsy and other disorders this mechanism augments the elimination of drugs from their target cells and leads to drug resistance. In this review, we have discussed the biochemical characteristics of multidrug transporters and the mechanisms by which these membrane bound proteins transport their target molecules from one side to the other side of the cell membrane. We have also briefly discussed the application of this knowledge in the understanding of drug resistance in various clinical situations with particular reference to neurological disorders. These proteins located in the placenta have important role in preventing the transplacental movement of drugs in to the fetus which may result in congenital malformations or other defects. The molecular genetic mechanisms that govern the expression of these important proteins are discussed briefly. The potential scope to develop targeted chemotherapeutic agents is also discussed.

Ins and outs of the ABCG2 multidrug transporter: an update on in vitro functional assays

The major aim of this chapter is to provide a critical overview of the in vitro methods available for studying the function of the ABCG2 multidrug transporter protein. When describing the most applicable assay systems, in each case we present a short overview relevant to ABC multidrug transporters in general, and then we concentrate on the tools applicable to analysis of substrate-drug interactions, the effects of potential activators and inhibitors, and the role of polymorphisms of the ABCG2 transporter. Throughout this chapter we focus on recently developed assay systems, which may provide new possibilities for analyzing the pharmacological aspects of this medically important protein.

Utilization of membrane vesicle preparations to study drug-ABC transporter interactions

BACKGROUND

The last 15 years have marked an expansion in our understanding of how ABC transporters modulate the pharmacokinetic properties of drugs. Assays based on different membrane preparations were one of the first methods developed to study ABC transporters. Later, they turned out to be valuable tools to gain insight into the nature of drug-ABC transporter interactions.

OBJECTIVES

Membranes prepared from different sources have been used and characterized; based on the biochemical characteristics of the transport process, a number of different assay types have been developed.

METHODS

This review focuses on the current experiences on how different membrane-based assays can be utilized in pharmaceutical R&D. Sources of membrane preparations, available assay types and correlation studies between different in-vitro and in-vivo methods are discussed.

RESULTS/CONCLUSION

Membrane-based assays are valuable tools in drug discovery to characterize drug-ABC transporter interactions.

CLC-0 and CFTR: Chloride Channels Evolved From Transporters

CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl−channels play important roles in Cl−transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl−channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.

A mutation of the H-loop selectively affects rhodamine transport by the yeast multidrug ABC transporter Pdr5

The yeast ABC transporter Pdr5 plays a major role in drug resistance against a large number of structurally unrelated compounds. Although Pdr5 has been extensively studied, many important aspects regarding its molecular mechanisms remain unresolved. For example, a striking degeneration of conserved amino acid residues exists in the nucleotide binding domains (NBDs), but their functional relevance is unknown. Here, we performed in vivo and in vitro experiments to address the functional asymmetry of NBDs. It became evident by ATPase activity and drug transport studies that catalysis at only one of the two NBD composite sites is crucial for protein function. Furthermore, mutations of the proposed "catalytic carboxylate" (E1036) and the "catalytic dyad histidine" (H1068) were characterized. Although a mutation of the glutamate abolished ATPase activity and substrate transport, mutation of H1068 had no influence on ATP consumption. However, the H1068A mutation abolished rhodamine transport in vivo and in vitro, while leaving the transport of other substrates unaffected. By contrast to mammalian P-glycoprotein (P-gp), the ATPase activity of yeast Pdr5 is not stimulated by the addition of substrates, indicating that Pdr5 is an uncoupled ABC transporter that constantly hydrolyses ATP to ensure active substrate transport. Taken together, our data provide important insights into the molecular mechanism of Pdr5 and suggest that not solely the transmembrane domains dictate substrate selection.

Mutational analysis of conserved aromatic residues in the A-loop of the ABC transporter ABCB1A (mouse Mdr3)

The A-loop is a recently described conserved region in the NBDs of ABC transporters [Ambudkar, S.V., Kim, I.-W., Xia, D. and Sauna, Z.E. (2006) The A-loop, a novel conserved aromatic acid subdomain upstream of the Walker A motif in ABC transporters, is critical for ATP binding. FEBS Lett. 580, 1049-1055; Kim, I.W., Peng, X.H., Sauna, Z.E., FitzGerald, P.C., Xia, D., Muller, M., Nandigama, K. and Ambudkar, S.V. (2006) The conserved tyrosine residues 401 and 1044 in ATP sites of human P-glycoprotein are critical for ATP binding and hydrolysis: evidence for a conserved subdomain, the A-loop in the ATP-binding cassette. Biochemistry 45, 7605-7616]. In mouse P-glycoprotein (Abcb1a), the aromatic residue of the A-loop in both NBDs is a tyrosine: Y397 in NBD1 and Y1040 in NBD2. Another tyrosine residue (618 in NBD1 and 1263 in NBD2) also appears to lie in proximity to the ATP molecule. We have mutated residues Y397, Y618, Y1040, and Y1263 to tryptophan and analyzed the effect of these substitutions on transport properties, ATP binding, and ATP hydrolysis by Abcb1a (mouse Mdr3). Y618W and Y1263W enzymes had catalytic characteristics similar to WT Abcb1a. On the other hand, Y397W and Y1040W showed impaired transport and greatly reduced ATPase activity, including a approximately 10-fold increase in Km for MgATP. Thus, Y397 and Y1040 play an important role in Abcb1a catalysis.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

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.

Modulator-induced interference in functional cross talk between the substrate and the ATP sites of human P-glycoprotein

The human P-glycoprotein (Pgp, ABCB1) is an ATP-dependent efflux pump for structurally unrelated hydrophobic compounds, conferring simultaneous resistance to and restricting bioavailability of several anticancer and antimicrobial agents. Drug transport by Pgp requires a coordinated communication between its substrate binding/translocating pathway (substrate site) and the nucleotide binding domains (NBDs or ATP sites). In this study, we demonstrate that certain thioxanthene-based Pgp modulators, such as cis-(Z)-flupentixol and its closely related analogues, effectively disrupt molecular cross talk between the substrate, and the ATP, sites without affecting the basic functional aspects of the two domains, such as substrate recognition, binding, and hydrolysis of ATP and dissociation of ADP following ATP hydrolysis. The allosteric modulator cis-(Z)-flupentixol has no effect on [alpha-(32)P]-8-azido-ATP binding to Pgp under nonhydrolytic conditions or on the K(m) for ATP during ATP hydrolysis. Both hydrolysis of ATP and vanadate-induced [alpha-(32)P]-8-azido-ADP trapping (following [alpha-(32)P]-8-azido-ATP breakdown) by Pgp are stimulated by the modulator. However, the ability of Pgp substrates (such as prazosin) to stimulate ATP hydrolysis and facilitate vanadate-induced trapping of [alpha-(32)P]-8-azido-ADP is substantially affected in the presence of cis-(Z)-flupentixol. Substrate recognition by Pgp as determined by [(125)I]iodoarylazidoprazosin ([(125)I]IAAP) binding both in the presence and in the absence of ATP is facilitated by the modulator, whereas substrate dissociation in response to vanadate trapping is considerably affected in its presence. In the Pgp F983A mutant, which is impaired in modulation by cis-(Z)-flupentixol, the modulator has a minimal effect on substrate-stimulated ATP hydrolysis as well as on substrate dissociation coupled to vanadate trapping. Finally, cis-(Z)-flupentixol has no effect on dissociation of [alpha-(32)P]-8-azido-ADP (or ADP) from vanadate-trapped Pgp, which is essential for subsequent rounds of ATP hydrolysis. Taken together, our results demonstrate a distinct mechanism of Pgp modulation that involves allosteric disruption of molecular cross talk between the substrate, and the ATP, sites without any direct interference with their individual functions.

New light on multidrug binding by an ATP-binding-cassette transporter

ATP-binding-cassette (ABC) multidrug transporters confer multidrug resistance to pathogenic microorganisms and human tumour cells by mediating the extrusion of structurally unrelated chemotherapeutic drugs from the cell. The molecular basis by which ABC multidrug transporters bind and transport drugs is far from clear. Genetic analyses during the past 14 years reveal that the replacement of many individual amino acids in mammalian multidrug resistance P-glycoproteins can affect cellular resistance to drugs, but these studies have failed to identify specific regions in the primary amino acid sequence that are part of a defined drug-binding pocket. The recent publication of an X-ray crystallographic structure of the bacterial P-glycoprotein homologue MsbA and an MsbA-based homology model of human P-glycoprotein creates an opportunity to compare the original mutagenesis data with the three-dimensional structures of transporters. Our comparisons reveal that mutations that alter specificity are present in three-dimensional 'hotspot' regions in the membrane domains of P-glycoprotein.

Insight in eukaryotic ABC transporter function by mutation analysis

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

Functional asymmetry of nucleotide-binding domains in ABCG5 and ABCG8

The ATP-binding cassette half-transporters ABCG5 (G5) and ABCG8 (G8) promote secretion of neutral sterols into bile, a major pathway for elimination of sterols. Mutations in either ABCG5 or ABCG8 cause sitosterolemia, a recessive disorder characterized by impaired biliary and intestinal sterol secretion, sterol accumulation, and premature atherosclerosis. The mechanism by which the G5G8 heterodimer couples ATP hydrolysis to sterol transport is not known. Here we examined the roles of the Walker A, Walker B, and signature motifs in the nucleotide-binding domains (NBD) of G5 and G8 using recombinant adenoviruses to reconstitute biliary sterol transport in G5G8-deficient mice. Mutant forms of each half-transporter were co-expressed with their wild-type partners. Mutations at crucial residues in the Walker A and Walker B domains of G5 prevented biliary sterol secretion, whereas mutations of the corresponding residues in G8 did not. The opposite result was obtained when mutations were introduced into the signature motif; mutations in the signature domain of G8 prevented sterol transport, but substitution of the corresponding residues in G5 did not. Taken together, these findings indicate that the NBDs of G5 and G8 are not functionally equivalent. The integrity of the canonical NBD formed by the Walker A and Walker B motifs of G5 and the signature motif of G8 is essential for G5G8-mediated sterol transport. In contrast, mutations in key residues of the NBD formed by the Walker A and B motifs of G8 and the signature sequence of G5 did not affect sterol secretion.

Involvement of the "occluded nucleotide conformation" of P-glycoprotein in the catalytic pathway

We found recently that the combined mutation of both "catalytic carboxylate" residues (E552A/E1197A) in mouse P-glycoprotein (Pgp) arrested the protein in an "occluded nucleotide conformation", possibly a stabilized dimer of nucleotide-binding domains (NBDs), that binds MgATP tightly at stoichiometry of 1 mol/mol Pgp [Tombline, G., Bartholomew, L., Urbatsch, I. L., and Senior, A. E. (2004) J. Biol. Chem. 279, 31212-31220]. Here, we further examine this conformation in respect to its potential involvement in the catalytic pathway. The occluded nucleotide conformation is promoted by drugs. Verapamil markedly accelerated the rate of tight binding of MgATP, whereas it did not effect the rate of dissociation. Mutations in "Q-loop" residues that are thought to interfere with communication between drug and catalytic sites prevented the occluded nucleotide conformation, as did covalent reagents N-ethylmaleimide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, which are known to inhibit ATP hydrolysis by reacting in catalytic sites. Mutations of Walker A Ser and Lys residues in combination with E552A/E1197A had the same effect, showing that interaction of these conserved residues with MgATP is required to stabilize the occluded nucleotide conformation. We present an enzymatic scheme that incorporates this conformation. We propose that upon initial loose binding of MgATP at two nucleotide-binding domains (NBDs), together with drug binding, the NBDs dimerize to form the occluded conformation, with one tightly bound MgATP committed to hydrolysis. The pathway progresses such that the tightly bound MgATP enters the transition state and is hydrolyzed. This work suggests that small molecules or peptides that interact at the NBD dimer interface might effectively disable Pgp catalysis.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Effect of Walker A mutation (K86M) on oligomerization and surface targeting of the multidrug resistance transporter ABCG2

The ATP binding cassette (ABC) half-transporter ABCG2 (MXR/BCRP/ABCP) is associated with mitoxantrone resistance accompanied by cross-resistance to a broad spectrum of cytotoxic drugs. Here we investigate the functional consequences of mutating a highly conserved lysine in the Walker A motif of the nucleotide binding domain (NBD) known to be critical for ATP binding and/or hydrolysis in ABC transporters. The mutant (ABCG2-K86M) was inactive as expected but was expressed at similar levels as the wild-type (wt) protein. The mutation did not affect the predicted oligomerization properties of the transporter; hence, co-immunoprecipitation experiments using differentially tagged transporters showed evidence for oligomerization of both ABCG2-wt and of ABCG2-wt with ABCG2-K86M. We also obtained evidence that both ABCG2-wt and ABCG2-K86M exist in the cells as disulfide-linked dimers. Moreover, measurement of prazosin-stimulated ATPase activity revealed a dominant-negative effect of ABCG2-K86M on ABCG2-wt function in co-transfected HEK293 cells. This is consistent with the requirement for at least two active NBDs for transporter activity and suggests that the transporter is a functional dimer. Finally, we analyzed targeting of ABCG2-wt and ABCG2-K86M and observed that they localize to two distinct subcellular compartments: ABCG2-wt targets the cell surface whereas ABCG2-K86M is targeted to the Golgi apparatus followed by retrieval to the endoplasmic reticulum. This suggests an as yet unknown role of the NBDs in assisting proper surface targeting of ABC transporters.

Function-dependent conformational changes of the ABCG2 multidrug transporter modify its interaction with a monoclonal antibody on the cell surface

The human ABCG2 protein is an important primary active transporter for hydrophobic compounds in several cell types, and its overexpression causes multidrug resistance in tumors. A monoclonal antibody (5D3) recognizes this protein on the cell surface. In ABCG2-expressing cells 5D3 antibody showed a saturable labeling and inhibited ABCG2 transport and ATPase function. However, at low antibody concentrations 5D3 binding to intact cells depended on the actual conformation of the ABCG2 protein. ATP depletion or the addition of the ABCG2 inhibitor Ko143 significantly increased, whereas the vanadate-induced arrest of ABCG2 strongly decreased 5D3 binding. The binding of the 5D3 antibody to a non-functional ABCG2 catalytic center mutant (K86M) in intact cells was not affected by the addition of vanadate but still increased with the addition of Ko143. In isolated membrane fragments the ligand modulation of 5D3 binding to ABCG2 could be analyzed in detail. In this case 5D3 binding was maximum in the presence of ATP, ADP, or Ko143, whereas the non-hydrolysable ATP analog, adenosine 5'-(beta,gamma-imido)triphosphate (AMP-PNP), and nucleotide trapping by vanadate decreased antibody binding. In membranes expressing the ABCG2-K86M mutant, ATP, ADP, and AMP-PNP decreased, whereas Ko143 increased 5D3 binding. Based on these data we suggest that the 5D3 antibody can be used as a sensitive tool to reveal intramolecular changes, reflecting ATP binding, the formation of a catalytic intermediate, or substrate inhibition within the transport cycle of the ABCG2 protein.

Nucleotides and transported substrates modulate different steps of the ATPase catalytic cycle of MRP1 multidrug transporter

The human ABC (ATP-binding cassette) transporter MRP1 (human multidrug-resistance-associated protein 1; ABCC1) is involved in the cellular extrusion of conjugated metabolites and causes multidrug resistance in tumour cells. The transport of substrate molecules by ABC proteins is energized by ATP hydrolysis, performed by two co-operating ABC units. Orthovanadate (Vi), a non-covalent inhibitor of the ABC ATPases, was found to catalyse a photo-oxidative cleavage of various ATP-binding proteins. In the present study, we have identified three Vi-cleavage sites within MRP1, and found that the cleavage reactions were variably modulated by the presence of nucleotides and by transported substrates. We concluded that Vi cleavage of MRP1 at Site I detects conformational changes due to the binding of MgATP. In contrast, Site II could be identified as part of the substrate-modulated catalytic cycle, probably containing an MRP1.MgADP.Vi transition-state-like complex. Cleavage at Site III was modulated by both the binding and hydrolysis of MgATP, in a biphasic pattern, which was also affected by the presence of transported substrates. We detected two different allosteric effects and found that they control two consecutive steps of the MRP1 ATPase catalytic cycle. Nucleotide binding to the low-affinity site accelerated the formation of the pre-hydrolytic intermediate in the other catalytic centre. Interaction of the transporter with its transported substrates stimulated a later reaction of the hydrolytic cycle, the formation of the post-hydrolytic intermediate, which could be detected in both catalytic sites by the experimental strategy used.

The Role of the Conserved Glycines of ATP-binding Cassette Signature Motifs of MRP1 in the Communication between the Substrate-binding Site and the Catalytic Centers

A key element of the structural model of ABC-ATP-ases is the interaction of the two ABC domains. They complement each other's active sites in a way that the ABC signature motif (LSGGQ) of one subunit interacts with the gamma-phosphate of the ATP, bound at the Walker motifs of the opposite subunit. In the present study, the conserved glycines in the fourth position of the LSGGQ motifs of human MRP1 were substituted for aspartic acids (G771D and G1433D), the mutants were expressed in Sf9 insect cells, and the nucleotideas well as the transported substrate-protein interactions were studied. We found that these transport- and ATPase-incompetent mutants showed no nucleotide trapping under any of the conditions examined. However, when measuring the effect of nucleotide and transported substrates on the vanadate-induced cleavage reactions, we found that the effect of substrates on the cleavage reactions was significantly different in the mutant MRP1 proteins than in the wild type. Although the transported substrates (e.g. etoposide + oxidized glutathione) stimulated the formation of the posthydrolytic complex in the wild type, this reaction was inhibited in the signature mutants. Our study also revealed that a similar mutation in the ABC signature of either ABC unit resulted in the same effect. We suggest that the conserved glycine residues in both LSGGQ segments are part of the conformational network, which is responsible for the accelerated hydrolytic activity upon interaction of the protein with its transported substrates. This intramolecular communication between the substrate-binding site and the catalytic centers is assumed to be a general feature of the molecular mechanism of ABC transporters.

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.

Synergy between conserved ABC signature Ser residues in P-glycoprotein catalysis

Functional roles of the two ABC signature sequences ("LSGGQ") in the N- and C-terminal nucleotide binding domains of P-glycoprotein were studied by mutating the conserved Ser residues to Ala. The two single mutants (S528A; S1173A) each impaired ATPase activity mildly, and showed generally symmetrical effects on function, consistent with equivalent mechanistic roles of the two nucleotide sites. Synergy between the two mutations when combined was remarkable and resulted in strong catalytic impairment. The Ser residues are not involved significantly in MgATP- or MgADP-binding or in interdomain communication between catalytic sites and drug binding sites. Retention of product MgADP is not the cause of reduced turnover. Mutation of Ser to Ala reduced the strength of interaction with the chemical transition state specifically, as shown by vanadate-ADP and beryllium fluoride-ADP trapping experiments. Therefore, the two conserved ABC signature motif Ser residues of P-glycoprotein cooperatively accelerate ATP hydrolysis via chemical transition state interaction. Because the transition state complex is currently believed to form in the dimerized state of the nucleotide binding domains, one may also conclude that both Ser-OH are necessary for correct formation of the dimer state.

Identification of two different states of P-glycoprotein in its catalytic cycle: role of the linker region in the transition between these two states

P-glycoprotein (Pgp) is a drug-translocating ATPase responsible for multidrug resistance in cancer. Although it is well-established that Pgp exhibits drug-dependent ATPase and ATP-dependent drug transport functions, the mechanism by which these two reactions are coupled remains unclear. We have shown recently that proteolytic cleavage of the linker region, which joins the NH2 and COOH halves of the Pgp molecule, results in a Pgp form that exhibits drug-independent and -dependent ATPase activities (Nuti et al., (2000) Biochemistry 39, 3424-3432; Nuti, S. L., and Rao, U. S. (2002) J. Biol. Chem. 277, 29417-29423). To understand the mechanism underlying this phenomenon, we used the procedure of vanadate-mediated trapping of the Pgp transport cycle intermediates to determine the steps in the catalytic cycle that are being regulated by the linker region. We show that vanadate stably traps Pgp under two different conditions, one in the presence of ATP alone and the other in the presence of ATP and drug, suggesting the existence of two Pgp conformations. These two conformations, one mediating basal and the other drug-stimulated ATPase reactions, represent different transport cycle intermediates of Pgp, because arresting Pgp in either conformation prevents the catalytic cycle from proceeding to completion. The results also show that these two conformations are uncoupled and appear simultaneously in Pgp that was cleaved in the linker region. These results together suggest that Pgp assumes at least two distinct conformational states, which catalyze two ATP hydrolysis events in the drug transport cycle, and the linker region mediates the transition between these two states of Pgp.

P-glycoprotein: from genomics to mechanism

Resistance to chemically different natural product anti-cancer drugs (multidrug resistance, or MDR) results from decreased drug accumulation, resulting from expression of one or more ATP-dependent efflux pumps. The first of these to be identified was P-glycoprotein (P-gp), the product of the human MDR1 gene, localized to chromosome 7q21. P-gp is a member of the large ATP-binding cassette (ABC) family of proteins. Although its crystallographic 3-D structure is yet to be determined, sequence analysis and comparison to other ABC family members suggest a structure consisting of two transmembrane (TM) domains, each with six TM segments, and two nucleotide-binding domains. In the epithelial cells of the gastrointestinal tract, liver, and kidney, and capillaries of the brain, testes, and ovaries, P-gp acts as a barrier to the uptake of xenobiotics, and promotes their excretion in the bile and urine. Polymorphisms in the MDR1 gene may affect the pharmacokinetics of many commonly used drugs, including anticancer drugs. Substrate recognition of many different drugs occurs within the TM domains in multiple-overlapping binding sites. We have proposed a model for how ATP energizes transfer of substrates from these binding sites on P-gp to the outside of the cell, which accounts for the apparent stoichiometry of two ATPs hydrolysed per molecule of drug transported. Understanding of the biology, genetics, and biochemistry of P-gp promises to improve the treatment of cancer and explain the pharmacokinetics of many commonly used drugs.

Peptides induce ATP hydrolysis at both subunits of the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) plays a key role in the adaptive immune response by pumping antigenic peptides into the endoplasmic reticulum for subsequent loading of major histocompatibility complex class I molecules. TAP is a heterodimer consisting of TAP1 and TAP2. Each subunit is composed of a transmembrane domain and a nucleotide-binding domain, which energizes the peptide transport. To analyze ATP hydrolysis of each subunit we developed a method of trapping 8-azido-nucleotides to TAP in the presence of phosphate transition state analogs followed by photocross-linking, immunoprecipitation, and high resolution SDS-PAGE. Strikingly, trapping of both TAP subunits by beryllium fluoride is peptide-specific. The peptide concentration required for half-maximal trapping is identical for TAP1 and TAP2 and directly correlates with the peptide binding affinity. Only a background level of trapping was observed for low affinity peptides or in the presence of the herpes simplex viral protein ICP47, which specifically blocks peptide binding to TAP. Importantly, the peptide-induced trapped state is reached after ATP hydrolysis and not in a backward reaction of ADP binding and trapping. In the trapped state, TAP can neither bind nor exchange nucleotides, whereas peptide binding is not affected. In summary, these data support the model that peptide binding induces a conformation that triggers ATP hydrolysis in both subunits of the TAP complex within the catalytic cycle.

P-glycoprotein catalytic mechanism: studies of the ADP-vanadate inhibited state

Kinetics of inhibition of ATPase activity of pure mouse Mdr3 P-glycoprotein upon incubation with MgADP and vanadate were studied along with the trapping of [14C]ADP in presence of vanadate. The presence of verapamil strongly magnified both effects. Inhibition of ATPase was also increased by several other drugs known to bind to drug-binding sites. Inhibition by ADP-vanadate was slow and depended cooperatively on nucleotide binding. Stoichiometry of [14C]ADP trapping by vanadate was 1 mol/mol P-glycoprotein at full inhibition. Catalytic site mutants prevented [14C]ADP trapping, whereas interdomain signal communication mutants reduced it in approximate correlation with their effects upon drug stimulation of ATPase. In explanation of the results, we propose that a "closed conformation" involving dimerization and interdigitation of the two nucleotide-binding domains is necessary to allow inhibition by ADP-vanadate. The results suggest that such a conformation occurs naturally during ATP hydrolysis. It is proposed that in order for the catalytic transition state to form, the two nucleotide-binding domains dimerize to form an integrated single entity containing two bound ATP with just one of the two ATP being hydrolyzed per dimerization event.

Characterization of drug transport, ATP hydrolysis, and nucleotide trapping by the human ABCG2 multidrug transporter. Modulation of substrate specificity by a point mutation

The overexpression of the human ATP-binding cassette half-transporter, ABCG2 (placenta-specific ABC transporter, mitoxantrone resistance-associated protein, breast cancer resistance protein), causes multidrug resistance in tumor cells. An altered drug resistance profile and substrate recognition were suggested for wild-type ABCG2 and its mutant variants (R482G and R482T); the mutations were found in drug-selected tumor cells. In order to characterize the different human ABCG2 transporters without possible endogenous dimerization partners, we expressed these proteins and a catalytic center mutant (K86M) in Sf9 insect cells. Transport activity was followed in intact cells, whereas the ATP binding and hydrolytic properties of ABCG2 were studied in isolated cell membranes. We found that the K86M mutant had no transport or ATP hydrolytic activity, although its ATP binding was retained. The wild-type ABCG2 and its variants, R482G and R482T, showed characteristically different drug and dye transport activities; mitoxantrone and Hoechst 33342 were transported by all transporters, whereas rhodamine 123 was only pumped by the R482G and R482T mutants. In each case, ABCG2-dependent transport was blocked by the specific inhibitor, fumitremorgin C. A relatively high basal ABCG2-ATPase, inhibited by fumitremorgin C, was observed in all active proteins, but specific drug stimulation could only be observed in the case of R482G and R482T mutants. We found that ABCG2 is capable of a vanadate-dependent adenine nucleotide trapping. Nucleotide trapping was stimulated by the transported compounds in the R482G and R482T variants but not in the wild-type ABCG2. These experiments document the applicability of the Sf9 expression system for parallel, quantitative examination of the specific transport and ATP hydrolytic properties of different ABCG2 proteins and demonstrate significant differences in their substrate interactions.

Importance of the conserved Walker B glutamate residues, 556 and 1201, for the completion of the catalytic cycle of ATP hydrolysis by human P-glycoprotein (ABCB1)

The human MDR1 (ABCB1) gene product, P-glycoprotein (Pgp), functions as an ATP-dependent efflux pump for a variety of chemotherapeutic drugs. In this study, we assessed the role of conserved glutamate residues in the Walker B domain of the two ATP sites (E556 and E1201, respectively) during the catalytic cycle of human Pgp. The mutant Pgps (E556Q, E556A, E1201Q, E1201A, E556/1201Q, and E556/1201A) were characterized using a vaccinia virus based expression system. Although steady-state ATP hydrolysis and drug transport activities were abrogated in both E556Q and E1201Q mutant Pgps, [alpha-(32)P]-8-azidoADP was trapped in the presence of vanadate (Vi), and the release of trapped [alpha-(32)P]-8-azidoADP occurred to a similar extent as in wild-type Pgp. This indicates that these mutations do not affect either the first hydrolysis event or the ADP release step. Similar results were also obtained when Glu residues were replaced with Ala (E556A and E1201A). Following the first hydrolysis event and release of [alpha-(32)P]-8-azidoADP, both E556Q and E1201Q mutant Pgps failed to undergo another cycle of Vi-induced [alpha-(32)P]-8-azidoADP trapping. Interestingly, the double mutants E556/1201Q and E556/1201A trapped [alpha-(32)P]-8-azidoADP even in the absence of Vi, and the occluded nucleotide was not released after incubation at 37 degrees C for an extended period. In addition, the properties of transition state conformation of the double mutants generated in the absence of Vi were found to be similar to that of the wild-type protein trapped in the presence of Vi (Pgp x [alpha-(32)P]-8-azidoADP xVi). Thus, in contrast to the single mutants, the double mutants appear to be defective in the ADP release step. In aggregate, these data suggest that E556 and E1201 residues in the Walker B domains may not be critical as catalytic carboxylates for the cleavage of the bond between the gamma-P and the beta-P of ATP during hydrolysis but are essential for the second ATP hydrolysis step and completion of the catalytic cycle.

The mechanism of action of multidrug-resistance-linked P-glycoprotein

P-glycoprotein (Pgp), the ATP-binding cassette (ABC) transporter, confers multidrug resistance to cancer cells by extruding cytotoxic natural product amphipathic drugs using the energy of ATP hydrolysis. Our studies are directed toward understanding the mechanism of action of Pgp and recent work deals with the assessment of interaction between substrate and ATP sites and elucidation of the catalytic cycle of ATP hydrolysis. The kinetic analyses of ATP hydrolysis by reconstituted purified Pgp suggest that ADP release is the rate-limiting step in the catalytic cycle and the substrates exert their effect by modulating ADP release. In addition, we provide evidence for two distinct roles for ATP hydrolysis in a single turnover of Pgp, one in the transport of drug and the other in effecting conformational changes so as to reset the transporter for the next catalytic cycle. Detailed kinetic measurements determined that both nucleotide-binding domains behave symmetrically and during individual hydrolysis events the ATP sites are recruited in a random manner. Furthermore, only one nucleotide site hydrolyzes ATP at any given time, causing (in this site) a conformational change that drastically decreases (>30-fold) the affinity of the second site for ATP-binding. Thus, the blocking of ATP-binding to the second site while the first one is in catalytic conformation appears to be the basis for the alternate catalytic cycle of ATP hydrolysis by Pgp, and this may be applicable as well to other ABC transporters linked with the development of multidrug resistance.

Characterization of the ATPase cycle of human ABCA1: implications for its function as a regulator rather than an active transporter

ABCA1 plays a key role in cellular cholesterol and phospholipid traffic. To explore the biochemical properties of this membrane protein we applied a Baculovirus-insect cell expression system. We found that human ABCA1 in isolated membranes showed a specific, Mg(2+)-dependent ATP binding but had no measurable ATPase activity. Nevertheless, conformational changes in ABCA1 could be demonstrated by nucleotide occlusion, even without arresting the catalytic cycle by phosphate-mimicking anions. Addition of potential lipid substrates or lipid acceptors (apolipoprotein A-I) did not modify the ATPase activity or nucleotide occlusion by ABCA1. Our data indicate that ATP hydrolysis by ABCA1 occurs at a very low rate, suggesting that ABCA1 may not function as an effective active transporter as previously assumed. In the light of the observed conformational changes we propose a regulatory function for human ABCA1.

Evidence for the vectorial nature of drug (substrate)-stimulated ATP hydrolysis by human P-glycoprotein

P-glycoprotein (Pgp), the ATP-binding cassette multidrug transporter, exhibits a drug (substrate)-stimulatable ATPase activity, and vanadate (Vi) inhibits this activity by stably trapping the nucleoside diphosphate in the Pgp.ADP.Vi conformation. We recently demonstrated that Vi-induced 8-azido-[alpha-(32)P]ADP trapping into Pgp in the absence of substrate occurs both in the presence of 8-azido-[alpha-(32)P]ATP (following 8-azido-ATP hydrolysis) or 8-azido-[alpha-(32)P]ADP (without hydrolysis) and, the transition state intermediates generated under either condition are functionally indistinguishable. In this study, we compare the effect of substrates on Vi-induced 8-azido-[alpha-(32)P]ADP trapping into Pgp under both non-hydrolysis and hydrolysis conditions. We demonstrate that whereas substrates stimulate the Vi-induced trapping of 8-azido-[alpha-(32)P]ADP under hydrolysis conditions, they strongly inhibit Vi-induced trapping under non-hydrolysis conditions. This inhibition is concentration-dependent, follows first order kinetics, and is effected by drastically decreasing the affinity of nucleoside diphosphate for Pgp during trapping. However, substrates do not affect the binding of nucleoside diphosphate in the absence of Vi, indicating that the substrate-induced conformation exerts its effect at a step distinct from nucleoside diphosphate-binding. Our results demonstrate that during the catalytic cycle of Pgp, although the transition state, Pgp x ADP x P(i) (Vi), can be generated both via the hydrolysis of ATP or by directly providing ADP to the system, in the presence of substrate the reaction is driven in the forward direction, i.e. hydrolysis of ATP. These data suggest that substrate-stimulated ATP hydrolysis by Pgp is a vectorial process.

Distinct functions of the ATP binding cassettes of transporters associated with antigen processing: a mutational analysis of Walker A and B sequences

The transporters associated with antigen processing (TAP1/TAP2) provide peptides to MHC class I molecules in the endoplasmic reticulum. Like other ATP-binding cassette proteins, TAP uses ATP hydrolysis to power transport. We have studied peptide binding to as well as translocation by TAP proteins with mutations in the Walker A and B sequences that are known to mediate ATP binding and hydrolysis. We show that a mutation in the TAP1 Walker B sequence reported to abrogate class I expression by a lung tumor does not affect ATP binding affinity, suggesting a defect restricted to ATP hydrolysis. This mutation reduces peptide transport by only 50%, suggesting that TAP function can be highly limiting for antigen presentation in non-lymphoid cells. Single substitutions in Walker A sequences (TAP1K544A, TAP2K509A), or their complete replacements, abrogate nucleotide binding to each subunit. Although all of these mutations abrogate peptide transport, they reveal distinct roles for nucleotide binding to the two transporter subunits in TAP folding and in regulation of peptide substrate affinity, respectively. Alteration of the TAP1 Walker A motif can have strong effects on TAP1 and thereby TAP complex folding. However, TAP1 Walker A mutations compatible with correct folding do not affect peptide binding. In contrast, abrogation of the TAP2 nucleotide binding capacity has little or no effect on TAP folding but eliminates peptide binding to TAP at 37 degrees C in the presence of nucleotides. Thus, nucleotide binding to TAP2 but not to TAP1 is a prerequisite for peptide binding to TAP. Based on these results, we propose a model in which nucleotide and peptide release from TAP are coupled and followed by ATP binding to TAP2, which induces high peptide affinity and initiates the transport cycle.

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.

Cysteines 431 and 1074 are responsible for inhibitory disulfide cross-linking between the two nucleotide-binding sites in human P-glycoprotein

Human wild-type and Cys-less P-glycoproteins were expressed in Pichia pastoris and purified in high yield in detergent-soluble form. Both ran on SDS gels as a single 140-kDa band in the presence of reducing agent and showed strong verapamil-stimulated ATPase activity in the presence of added lipid. The wild type showed spontaneous formation of higher molecular mass species in the absence of reducing agent, and its ATPase was activated by dithiothreitol. Oxidation with Cu(2+) generated the same higher molecular mass species, primarily at 200 and approximately 300 kDa, in high yield. Cross-linking was reversed by dithiothreitol and prevented by pretreatment with N-ethylmaleimide. Using proteins containing different combinations of naturally occurring Cys residues, it was demonstrated that an inhibitory intramolecular disulfide bond forms between Cys-431 and Cys-1074 (located in the Walker A sequences of nucleotide-binding sites 1 and 2, respectively), giving rise to the 200-kDa species. In addition, dimeric P-glycoprotein species ( approximately 300 kDa) form by intermolecular disulfide bonding between Cys-431 and Cys-1074. The ready formation of the intramolecular disulfide between Cys-431 and Cys-1074 establishes that the two nucleotide-binding sites of P-glycoprotein are structurally very close and capable of intimate functional interaction, consistent with available information on the catalytic mechanism. Formation of such a disulfide in vivo could, in principle, underlie a regulatory mechanism and might provide a means of intervention to inhibit P-glycoprotein.

Role of glycine-534 and glycine-1179 of human multidrug resistance protein (MDR1) in drug-mediated control of ATP hydrolysis

The human multidrug resistance protein (MDR1) (P-glycoprotein), a member of the ATP-binding cassette (ABC) family, causes multidrug resistance by an active transport mechanism, which keeps the intracellular level of hydrophobic compounds below a cell-killing threshold. Human MDR1 variants with mutations affecting a conserved glycine residue within the ABC signature of either or both ABC units (G534D, G534V, G1179D and G534D/G1179D) were expressed and characterized in Spodoptera frugiperda (Sf9) cell membranes. These mutations caused a loss of measurable ATPase activity but still allowed ATP binding and the formation of a transition-state intermediate (nucleotide trapping). In contrast with the wild-type protein, in which substrate drugs accelerate nucleotide trapping, in the ABC signature mutants nucleotide trapping was inhibited by MDR1-substrate drugs, suggesting a miscommunication between the drug-binding site(s) and the catalytic domains. Equivalent mutations of the two catalytic sites resulted in a similar effect, indicating the functional equivalence of the two sites. On the basis of these results and recent structural information on an ABC-ABC dimer [Hopfner, Karcher, Shin, Craig, Arthur, Carney and Tainer (2000) Cell 101, 789-800], we propose a key role of these glycine residues in the interdomain communication regulating drug-induced ATP hydrolysis.

Perturbation of the pore of the cystic fibrosis transmembrane conductance regulator (CFTR) inhibits its atpase activity

Mutations in the cystic fibrosis gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) lead to altered chloride (Cl(-)) flux in affected epithelial tissues. CFTR is a Cl(-) channel that is regulated by phosphorylation, nucleotide binding, and hydrolysis. However, the molecular basis for the functional regulation of wild type and mutant CFTR remains poorly understood. CFTR possesses two nucleotide binding domains, a phosphorylation-dependent regulatory domain, and two transmembrane domains that comprise the pore through which Cl(-) permeates. Mutations of residues lining the channel pore (e.g. R347D) are typically thought to cause disease by altering the interaction of Cl(-) with the pore. However, in the present study we show that the R347D mutation and diphenylamine-2-carboxylate (an open pore inhibitor) also inhibit CFTR ATPase activity, revealing a novel mechanism for cross-talk from the pore to the catalytic domains. In both cases, the reduction in ATPase correlates with a decrease in nucleotide turnover rather than affinity. Finally, we demonstrate that glutathione (GSH) inhibits CFTR ATPase and that this inhibition is altered in the CFTR-R347D variant. These findings suggest that cross-talk between the pore and nucleotide binding domains of CFTR may be important in the in vivo regulation of CFTR in health and disease.

Analysis of MDR1 P-glycoprotein conformational changes in permeabilized cells using differential immunoreactivity

The reactivity of the ATP-dependent multidrug transporter P-glycoprotein (Pgp) with the conformation-sensitive monoclonal antibody UIC2 is increased in the presence of Pgp transport substrates, ATP-depleting agents, or mutations that reduce the level of nucleotide binding by Pgp. We have investigated the effects of nucleotides and vinblastine, a Pgp transport substrate, on the UIC2 reactivity of Pgp in cells permeabilized by Staphylococcus aureus alpha-toxin. ATP, ADP, and nonhydrolyzable ATP analogues decreased the UIC2 reactivity; this effect was potentiated by vanadate, a nucleotide-trapping agent. The Hill number for the nucleotide-induced conformational transition was 2 for ATP and ADP but 1 for nonhydrolyzable ATP analogues. The Hill numbers for ATP and ADP were decreased to 1 by mutations in one of the two nucleotide binding sites of Pgp, whereas mutation of both sites greatly diminished the overall effect of nucleotides. Vinblastine reversed the decrease in the UIC2 reactivity brought about by all the nucleotides, including nonhydrolyzable analogues; this effect of vinblastine was blocked by vanadate. These data indicate that UIC2-detectable conformational changes of Pgp are driven by binding and debinding of nucleotides, that nucleotide hydrolysis affects the Hill number for its Pgp interactions, and that Pgp transport substrates promote nucleotide dissociation from Pgp. These findings are consistent with a conventional E1/E2 model that explains conformational transitions of a transporter protein through a series of linked equilibria.

Functional reassembly of ATP-dependent xenobiotic transport by the N- and C-terminal domains of RLIP76 and identification of ATP binding sequences

We have recently shown that RLIP76, a Ral-binding, GTPase-activating protein, is an ATP-dependent transporter of doxorubicin (DOX) as well as glutathione conjugates [Awasthi, S., et al. (2000) Biochemistry 39, 9327-9334]. RLIP76 overexpressed in human cells or transformed E. coli undergoes proteolysis to yield several fragments, including two prominent peptides, N-RLIP76(1-367) and C-RLIP76(410-655), from the N- and C-terminal domains, respectively. To investigate whether the fragmentation of RLIP76 has any relevance to its transport function, we have studied the characteristics of these two peptide fragments. Recombinant N-RLIP76(1-367) and C-RLIP76(410-655) were purified from overexpressing transformed E. coli. While N-RLIP76(1-367) readily underwent proteolysis, showing SDS-gel patterns similar to those of RLIP76, C-RLIP76(410-655) was resistant to such degradation. Both N-RLIP76(1-367) and C-RLIP76(410-655) had ATPase activity (K(m) for ATP, 2.5 and 2.0 mM, respectively) which was stimulated by DNP-SG, DOX, and colchicine (COL). ATP binding to both peptides was confirmed by photoaffinity labeling with 8-azido-ATP that was increased in the presence of compounds that stimulated their ATPase activity. Photoaffinity labeling was also increased in the presence of vanadate, indicating trapping of a reaction intermediate in the ATP binding site. The ATP binding sites in N-RLIP76(1-367) and C-RLIP76(410-655) were identified to be (69)GKKKGK(74) and (418)GGIKDLSK(425), respectively. Mutation of K(74) and K(425) to M residues, in N-RLIP76(1-367) and C-RLIP76(410-655), respectively, abrogated their ATPase activity as well as azido-ATP labeling. Proteoliposomes reconstituted with either N-RLIP76(1-367) or C-RLIP76(410-655) alone did not catalyze ATP-dependent transport of DOX or COL. However, proteoliposomes reconstituted with a mixture of N-RLIP76(1-367) and C-RLIP76(410-655) mediated such transport. Proteoliposomes reconstituted with the mixture of mutant peptides lacking ATPase activity did not exhibit transport activity. Present studies have identified the ATP binding sites in RLIP76, and show that DOX and COL transport can be reconstituted by two fragments of RLIP76.