P-glycoprotein shows strong catalytic cooperativity between the two nucleotide sites

Selective Accumulation of Near Infrared-Labeled Multivalent Quinidine Copolymers in Tumors Overexpressing P-Glycoprotein: Potential for Noninvasive Diagnostic Imaging

P-glycoprotein (P-gp) is a promiscuous small molecule transporter whose overexpression in cancer is associated with multidrug resistance (MDR). In these instances, anticancer drugs can select for P-gp-overexpressing cells, leading to cancer recurrence with an MDR phenotype. To avoid selection for MDR cancers and inform individual patient treatment plans, it is critical to noninvasively identify P-gp-overexpressing tumors prior to administration of chemotherapy. We report the facile free radical copolymerization of quinidine, a competitive inhibitor of P-gp, and acrylic acid to generate multiplexed polymeric P-gp-targeted imaging agents with tunable quinidine content. Copolymer targeting was demonstrated in a nude mouse xenograft model. In xenografts overexpressing P-gp, copolymer distribution was enhanced over two-fold compared to the negative control of poly(acrylic acid) regardless of quinidine content. In contrast, accumulation of the copolymers in xenografts lacking P-gp was equivalent to poly(acrylic acid). This work forms the foundation for a unique approach toward the phenotype-specific noninvasive imaging of MDR tumors and is the first in vivo demonstration of copolymer accumulation through the active targeting of P-gp.

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.

Structure, Assembly, and Function of Tripartite Efflux and Type 1 Secretion Systems in Gram-Negative Bacteria

Tripartite efflux pumps and the related type 1 secretion systems (T1SSs) in Gram-negative organisms are diverse in function, energization, and structural organization. They form continuous conduits spanning both the inner and the outer membrane and are composed of three principal components-the energized inner membrane transporters (belonging to ABC, RND, and MFS families), the outer membrane factor channel-like proteins, and linking the two, the periplasmic adaptor proteins (PAPs), also known as the membrane fusion proteins (MFPs). In this review we summarize the recent advances in understanding of structural biology, function, and regulation of these systems, highlighting the previously undescribed role of PAPs in providing a common architectural scaffold across diverse families of transporters. Despite being built from a limited number of basic structural domains, these complexes present a staggering variety of architectures. While key insights have been derived from the RND transporter systems, a closer inspection of the operation and structural organization of different tripartite systems reveals unexpected analogies between them, including those formed around MFS- and ATP-driven transporters, suggesting that they operate around basic common principles. Based on that we are proposing a new integrated model of PAP-mediated communication within the conformational cycling of tripartite systems, which could be expanded to other types of assemblies.

Conformational changes in the nucleotide-binding domains of P-glycoprotein induced by ATP hydrolysis

P-glycoprotein (Pgp) is a member of the ABC transporter superfamily with high physiological importance. Pgp nucleotide-binding domains (NBDs) drive the transport cycle through ATP binding and hydrolysis. We use molecular dynamics simulations to investigate the ATP hydrolysis-induced conformational changes in NBDs. Five systems, including all possible ATP/ADP combinations in the NBDs and the APO system, are simulated. ATP/ADP exchange induces conformational changes mostly within the conserved signature motif of the NBDs, resulting in relative orientational changes in the NBDs. Nucleotide removal leads to additional orientational changes in the NBDs, allowing their dissociation. Furthermore, we capture putative hydrolysis-competent configurations in which the conserved glutamate in the Walker-B motif acts as a catalytic base capturing a water molecule likely initiating ATP hydrolysis.

Characterization of the nucleotide-binding domain NsrF from the BceAB-type ABC-transporter NsrFP from the human pathogen Streptococcus agalactiae

Treatment of bacterial infections is a great challenge of our era due to the various resistance mechanisms against antibiotics. Antimicrobial peptides are considered to be potential novel compound as antibiotic treatment. However, some bacteria, especially many human pathogens, are inherently resistant to these compounds, due to the expression of BceAB-type ABC transporters. This rather new transporter family is not very well studied. Here, we report the first full characterization of the nucleotide binding domain of a BceAB type transporter from Streptococcus agalactiae, namely SaNsrF of the transporter SaNsrFP, which confers resistance against nisin and gallidermin. We determined the NTP hydrolysis kinetics and used molecular modeling and simulations in combination with small angle X-ray scattering to obtain structural models of the SaNsrF monomer and dimer. The fact that the SaNsrF_H202A variant displayed no ATPase activity was rationalized in terms of changes of the structural dynamics of the dimeric interface. Kinetic data show a clear preference for ATP as a substrate, and the prediction of binding modes allowed us to explain this selectivity over other NTPs.

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.

Invited review: Architectures and mechanisms of ATP binding cassette proteins

ATP binding cassette (ABC) ATPases form chemo-mechanical engines and switches that function in a broad range of biological processes. Most prominently, a very large family of integral membrane NTPases-ABC transporters-catalyzes the import or export of a diverse molecules across membranes. ABC proteins are also important components of the chromosome segregation, recombination, and DNA repair machineries and regulate or catalyze critical steps of ribosomal protein synthesis. Recent structural and mechanistic studies draw interesting architectural and mechanistic parallels between diverse ABC proteins. Here, I review this state of our understanding how NTP-dependent conformational changes of ABC proteins drive diverse biological processes. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 492-504, 2016.

Interdomain regulation of the ATPase activity of the ABC transporter haemolysin B from Escherichia coli

Type 1 secretion systems (T1SS) transport a wide range of substrates across both membranes of Gram-negative bacteria and are composed of an outer membrane protein, a membrane fusion protein and an ABC (ATP-binding cassette) transporter. The ABC transporter HlyB (haemolysin B) is part of a T1SS catalysing the export of the toxin HlyA in E. coli. HlyB consists of the canonical transmembrane and nucleotide-binding domains. Additionally, HlyB contains an N-terminal CLD (C39-peptidase-like domain) that interacts with the transport substrate, but its functional relevance is still not precisely defined. In the present paper, we describe the purification and biochemical characterization of detergent-solubilized HlyB in the presence of its transport substrate. Our results exhibit a positive co-operativity in ATP hydrolysis. We characterized further the influence of the CLD on kinetic parameters by using an HlyB variant lacking the CLD (HlyB∆CLD). The biochemical parameters of HlyB∆CLD revealed an increased basal maximum velocity but no change in substrate-binding affinity in comparison with full-length HlyB. We also assigned a distinct interaction of the CLD and a transport substrate (HlyA1), leading to an inhibition of HlyB hydrolytic activity at low HlyA1 concentrations. At higher HlyA1 concentrations, we observed a stimulation of the hydrolytic activities of both HlyB and HlyB∆CLD, which was completely independent of the interaction of HlyA1 with the CLD. Notably, all observed effects on ATPase activity, which were also analysed in detail by mass spectrometry, were independent of the HlyA1 secretion signal. These results assign an interdomain regulatory role for the CLD modulating the hydrolytic activity of HlyB.

Analysis of the relationship between P-glycoprotein and abamectin resistance in Tetranychus cinnabarinus (Boisduval)

Abamectin is an effective acaricide and widely used in the control of Tetranychus cinnabarinus. With the increase of control failures, it is however important to clarify the resistance mechanism to improve the control of this mite. P-glycoprotein (Pgp) is an ATP-dependent drug efflux pump for xenobiotic compounds and is involved in multidrug resistance. In this study, the results showed that verapamil, the specific inhibitor of Pgp, could enhance the lethal effect of abamectin on mites, and this effect is more enhanced in abamectin-resistant strain (AbR, mortality increased 74.51%) than that in susceptible strain (SS, 19.91%). Further analysis showed that the activity of Pgp ATPase in AbR was significantly higher (1.65-fold) than that in SS. After exposure to sublethal concentration of abamectin, the ATPase activity in AbR was significantly increased 1.43-fold to that in control; but there was no significant difference in SS after treatment. Two Pgp gene sequences (TcPgp1 and TcPgp2) from ABCB subfamily were characterized, and their expressions were much more sensitive to abamectin's stimulation in AbR strain than SS. These findings indicate a direct relationship between Pgp and abamectin resistance, and abamectin-induced Pgp expression may be involved in the modulation of abamectin efflux in T. cinnabarinus.

ATP-Binding Cassette Transporter Structure Changes Detected by Intramolecular Fluorescence Energy Transfer for High-Throughput Screening

Multidrug resistance protein 1 (MRP1) actively transports a wide variety of drugs out of cells. To quantify MRP1 structural dynamics, we engineered a "two-color MRP1" construct by fusing green fluorescent protein (GFP) and TagRFP to MRP1 nucleotide-binding domains NBD1 and NBD2, respectively. The recombinant MRP1 protein expressed and trafficked normally to the plasma membrane. Two-color MRP1 transport activity was normal, as shown by vesicular transport of [(3)H]17β-estradiol-17-β-(D-glucuronide) and doxorubicin efflux in AAV-293 cells. We quantified fluorescence resonance energy transfer (FRET) from GFP to TagRFP as an index of NBD conformational changes. Our results show that ATP binding induces a large-amplitude conformational change that brings the NBDs into closer proximity. FRET was further increased by substrate in the presence of ATP but not by substrate alone. The data suggest that substrate binding is required to achieve a fully closed and compact structure. ATP analogs bind MRP1 with reduced apparent affinity, inducing a partially closed conformation. The results demonstrate the utility of the two-color MRP1 construct for investigating ATP-binding cassette transporter structural dynamics, and it holds great promise for high-throughput screening of chemical libraries for unknown activators, inhibitors, or transportable substrates of MRP1.

Kinetic validation of the models for P-glycoprotein ATP hydrolysis and vanadate-induced trapping. Proposal for additional steps

P-Glycoprotein, a member of the ATP-binding cassette (ABC) superfamily, is a multidrug transporter responsible for cellular efflux of hundreds of structurally unrelated compounds, including natural products, many clinically used drugs and anti-cancer agents. Expression of P-glycoprotein has been linked to multidrug resistance in human cancers. ABC transporters are driven by ATP hydrolysis at their two cytoplasmic nucleotide-binding domains, which interact to form a closed ATP-bound sandwich dimer. Intimate knowledge of the catalytic cycle of these proteins is clearly essential for understanding their mechanism of action. P-Glycoprotein has been proposed to hydrolyse ATP by an alternating mechanism, for which there is substantial experimental evidence, including inhibition of catalytic activity by trapping of ortho-vanadate at one nucleotide-binding domain, and the observation of an asymmetric occluded state. Despite many studies of P-glycoprotein ATPase activity over the past 20 years, no comprehensive kinetic analysis has yet been carried out, and some puzzling features of its behaviour remain unexplained. In this work, we have built several progressively more complex kinetic models, and then carried out simulations and detailed analysis, to test the validity of the proposed reaction pathway employed by P-glycoprotein for ATP hydrolysis. To establish kinetic parameters for the catalytic cycle, we made use of the large amount of published data on ATP hydrolysis by hamster P-glycoprotein, both purified and in membrane vesicles. The proposed kinetic scheme(s) include a high affinity priming reaction for binding of the first ATP molecule, and an independent pathway for ADP binding outside the main catalytic cycle. They can reproduce to varying degrees the observed behavior of the protein's ATPase activity and its inhibition by ortho-vanadate. The results provide new insights into the mode of action of P-glycoprotein, and some hypotheses about the nature of the occluded nucleotide-bound state.

A reciprocating twin-channel model for ABC transporters

ABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane (TM) domains that bind substrate, and two ATP-binding cassettes, which use the cell's energy currency to couple substrate translocation to ATP hydrolysis. Despite the availability of over a dozen resolved structures and a wealth of biochemical and biophysical data, this field is bedeviled by controversy and long-standing mechanistic questions remain unresolved. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the ATP-binding cassettes dimerize upon binding two ATP molecules, and thence dissociate upon sequential ATP hydrolysis. This cycle of nucleotide-binding domain (NBD) dimerization and dissociation is coupled to a switch between inward- or outward facing conformations of a single TM channel; this alternating access enables substrate binding on one face of the membrane and its release at the other. Notwithstanding widespread acceptance of the Switch Model, there is substantial evidence that the NBDs do not separate very much, if at all, and thus physical separation of the ATP cassettes observed in crystallographic structures may be an artefact. An alternative Constant Contact Model has been proposed, in which ATP hydrolysis occurs alternately at the two ATP-binding sites, with one of the sites remaining closed and containing occluded nucleotide at all times. In this model, the cassettes remain in contact and the active sites swing open in an alternately seesawing motion. Whilst the concept of NBD association/dissociation in the Switch Model is naturally compatible with a single alternating-access channel, the asymmetric functioning proposed by the Constant Contact model suggests an alternating or reciprocating function in the TMDs. Here, a new model for the function of ABC transporters is proposed in which the sequence of ATP binding, hydrolysis, and product release in each active site is directly coupled to the analogous sequence of substrate binding, translocation and release in one of two functionally separate substrate translocation pathways. Each translocation pathway functions 180° out of phase. A wide and diverse selection of data for both ABC importers and exporters is examined, and the ability of the Switch and Reciprocating Models to explain the data is compared and contrasted. This analysis shows that not only can the Reciprocating Model readily explain the data; it also suggests straightforward explanations for the function of a number of atypical ABC transporters. This study represents the most coherent and complete attempt at an all-encompassing scheme to explain how these important proteins work, one that is consistent with sound biochemical and biophysical evidence.

Molecular cloning and characterization of a P-glycoprotein from the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

Macrocyclic lactones such as abamectin and ivermectin constitute an important class of broad-spectrum insecticides. Widespread resistance to synthetic insecticides, including abamectin and ivermectin, poses a serious threat to the management of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), a major pest of cruciferous plants worldwide. P-glycoprotein (Pgp), a member of the ABC transporter superfamily, plays a crucial role in the removal of amphiphilic xenobiotics, suggesting a mechanism for drug resistance in target organisms. In this study, PxPgp1, a putative Pgp gene from P. xylostella, was cloned and characterized. The open reading frame (ORF) of PxPgp1 consists of 3774 nucleotides, which encodes a 1257-amino acid peptide. The deduced PxPgp1 protein possesses structural characteristics of a typical Pgp, and clusters within the insect ABCB1. PxPgp1 was expressed throughout all developmental stages, and showed the highest expression level in adult males. PxPgp1 was highly expressed in midgut, malpighian tubules and testes. Elevated expression of PxPgp1 was observed in P. xylostella strains after they were exposed to the abamectin treatment. In addition, the constitutive expressions of PxPgp1 were significantly higher in laboratory-selected and field-collected resistant strains in comparison to their susceptible counterpart.

Mechanism of the ABC transporter ATPase domains: catalytic models and the biochemical and biophysical record

ABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane domains that bind substrate and form the channel, and two ATP-binding cassettes, which bind and hydrolyze ATP to energize the translocase function. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the nucleotide binding domains are proposed to dimerise upon binding of two ATP molecules, and thence dissociate upon sequential hydrolysis of the ATP. This idea appears consistent with crystal structures of both isolated subunits and whole transporters, as well as with a significant body of biochemical data. Nonetheless, an alternative Constant Contact Model has been proposed, in which the nucleotide binding domains do not fully dissociate, and ATP hydrolysis occurs alternately at each of the two active sites. Here, we review the biochemical and biophysical data relating to the ABC catalytic mechanism, to show how they may be construed as consistent with a Constant Contact Model, and to assess to what extent they support the Switch Model.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

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.

Anti-proliferative activity and suppression of P-glycoprotein by (-)-antofine, a natural phenanthroindolizidine alkaloid, in paclitaxel-resistant human lung cancer cells

Multidrug resistance (MDR) is a major obstacle in effective chemotherapy for cancer patients. The expression of P-glycoprotein (P-gp) in cancer cells is highly correlated with resistance to chemotherapeutic drugs. (-)-Antofine, a phenanthroindolizidine alkaloid derived from Cynanchum paniculatum, inhibits the growth of various human cancer cells. In this study, we further explored the potential of (-)-antofine to overcome the resistance induced by anti-cancer drugs. To this end, we established the paclitaxel-resistant human lung cancer cell line A549-PA by gradually exposing A549 cells to increasing concentrations of paclitaxel. As a result, the A549-PA cells acquired resistance against paclitaxel treatment and had an IC50 that was more than 200 times that of the parental A549 cells. (-)-Antofine, however, effectively suppressed the growth of both the parental and drug-resistant cells. Additional studies revealed that the anti-proliferative activity of (-)-antofine in A549-PA cells is accompanied by a down-regulation of P-gp mRNA and protein expression. The effect of reversing the multidrug resistance of A549-PA cells via (-)-antofine treatment was demonstrated an increase in intracellular rhodamine-123 accumulation, measured using FACS analysis. These findings suggest an additional chemotherapeutic value of (-)-antofine, that is, regulation of cancer cell drug resistance, in addition to its antitumor effect.

In vitro folding and assembly of the Escherichia coli ATP-binding cassette transporter, BtuCD

Studies on membrane protein folding have focused on monomeric α-helical proteins and a major challenge is to extend this work to larger oligomeric membrane proteins. Here, we study the Escherichia coli (E. coli) ATP-binding cassette (ABC) transporter that imports vitamin B(12) (the BtuCD protein) and use it as a model system for investigating the folding and assembly of a tetrameric membrane protein complex. Our work takes advantage of the modular organization of BtuCD, which consists of two transmembrane protein subunits, BtuC, and two cytoplasmically located nucleotide-binding protein subunits, BtuD. We show that the BtuCD transporter can be re-assembled from both prefolded and partly unfolded, urea denatured BtuC and BtuD subunits. The in vitro re-assembly leads to a BtuCD complex with the correct, native, BtuC and BtuD subunit stoichiometry. The highest rates of ATP hydrolysis were achieved for BtuCD re-assembled from partly unfolded subunits. This supports the idea of cooperative folding and assembly of the constituent protein subunits of the BtuCD transporter. BtuCD folding also provides an opportunity to investigate how a protein that contains both membrane-bound and aqueous subunits coordinates the folding requirements of the hydrophobic and hydrophilic subunits.

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

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

Review. Structure and mechanism of ATP-binding cassette transporters

ATP-binding cassette (ABC) transporters constitute a large superfamily of integral membrane proteins that includes both importers and exporters. In recent years, several structures of complete ABC transporters have been determined by X-ray crystallography. These structures suggest a mechanism by which binding and hydrolysis of ATP by the cytoplasmic, nucleotide-binding domains control the conformation of the transmembrane domains and therefore which side of the membrane the translocation pathway is exposed to. A basic, conserved two-state mechanism can explain active transport of both ABC importers and ABC exporters, but various questions remain unresolved. In this article, I will review some of the crystal structures and the mechanistic insight gained from them. Future challenges for a better understanding of the mechanism of ABC transporters will be outlined.

Substrate-dependent breast cancer resistance protein (Bcrp1/Abcg2)-mediated interactions: consideration of multiple binding sites in in vitro assay design

In vitro assays are frequently used for the screening of substrates and inhibitors of transporter-mediated efflux. Examining directional flux across Madin-Darby canine kidney (MDCK) II cell monolayers that overexpress a transporter protein is particularly useful in identifying whether or not a candidate compound is an inhibitor or substrate for that transport system. Studies that use a single substrate or inhibitor in competition assays can be challenging to interpret because of the possible multiple mechanisms involved in substrate/inhibitor-protein interactions. During our previous studies of substrate-inhibitor-transporter interactions, we observed differences in breast cancer resistance protein (BCRP) inhibition, depending on the substrate and the inhibitor. Therefore, we investigated BCRP-mediated interactions with a 4 x 4 matrix of substrates and inhibitors using monolayers formed from MDCKII cells transfected with murine BCRP (Bcrp1/Abcg2). The selective BCRP inhibitor 3-(6-isobutyl-9-methoxy-1,4-dioxo-1,2,3,4,6,7,12,12a-octahydropyrazino [1',2':1,6] pyrido [3,4-b]indol-3-yl)-propionic acid tert-butyl ester (Ko143) effectively inhibited the Bcrp1-mediated transport of all substrates examined. However, N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918), nelfinavir, and Pluronic P85 exhibited differences in inhibition depending on the substrate examined. Our findings support recent reports suggesting that the interactions of substrate molecules with BCRP involve multiple binding regions in the protein. The nucleoside substrates zidovudine and abacavir seem to bind to a region on BCRP that may have little or no overlap with the binding regions of either prazosin or imatinib. In conclusion, the choice of substrate or inhibitor molecules for an in vitro assay system can be crucial for the optimal design of experiments to evaluate transporter-mediated drug-drug interactions.

Uptake or extrusion: crystal structures of full ABC transporters suggest a common mechanism

ATP-binding cassette (ABC) transporters are integral membrane proteins that move diverse substrates across cellular membranes. ABC importers catalyse the uptake of essential nutrients from the environment, whereas ABC exporters facilitate the extrusion of various compounds, including drugs and antibiotics, from the cytoplasm. How ABC transporters couple ATP hydrolysis to the transport reaction has long remained unclear. The recent crystal structures of four complete ABC transporters suggest that a key step of the molecular mechanism is conserved in importers and exporters. Whereas binding of ATP promotes an outward-facing conformation, the release of the hydrolysis products ADP and phosphate promotes an inward-facing conformation. This basic scheme can in principle explain ATP-driven drug export and binding protein-dependent nutrient uptake.

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

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

Crystal structure of a multi-domain immunophilin from Arabidopsis thaliana: a paradigm for regulation of plant ABC transporters

FKBP42 is a membrane-anchored immunophilin playing a critical role in morphogenesis and development of higher plants. We present the X-ray structure of the cytoplasmic portion of FKBP42 comprising both the FKBP-like domain and the TPR domain at 2.85 A resolution. The data shed light on the probable binding modes of key interaction partners, including HSP90 and two classes of ABC transporters. The resulting models provide a structural background for further investigation of the unique biological properties of this protein.

Structure of a bacterial multidrug ABC transporter

Multidrug transporters of the ABC family facilitate the export of diverse cytotoxic drugs across cell membranes. This is clinically relevant, as tumour cells may become resistant to agents used in chemotherapy. To understand the molecular basis of this process, we have determined the 3.0 A crystal structure of a bacterial ABC transporter (Sav1866) from Staphylococcus aureus. The homodimeric protein consists of 12 transmembrane helices in an arrangement that is consistent with cross-linking studies and electron microscopic imaging of the human multidrug resistance protein MDR1, but critically different from that reported for the bacterial lipid flippase MsbA. The observed, outward-facing conformation reflects the ATP-bound state, with the two nucleotide-binding domains in close contact and the two transmembrane domains forming a central cavity--presumably the drug translocation pathway--that is shielded from the inner leaflet of the lipid bilayer and from the cytoplasm, but exposed to the outer leaflet and the extracellular space.

Preface: the concept and consequences of multidrug resistance

The problem of multidrug resistance (MDR) in human cancers led to the discovery 30 years ago of a single protein P-glycoprotein (P-gp), capable of mediating resistance to multiple structurally diverse drugs. P-gp became the archetypal eukaryotic ABC transporter gene, and studies of P-gp and related ABC transporters in both eukaryotes and bacteria have led to a basic mechanistic understanding of the molecular basis of MDR. Particular milestones along the way have been the identification of the homology between P-gp and bacterial transport proteins, the purification and functional reconstitution of P-gp into synthetic lipid systems, and the development of targeted therapies that attempt to overcome MDR by inhibiting P-gp. This preface places into this context some of the less well-explored themes developed in the MDR field, particularly various alternative models of P-gp action, evidence for parallel physiological roles for P-gp, and the unusual relationship between the substrate recognition capabilities of ABC transporters and their evolutionary history.

Quantitative evaluation of isothiocyanates as substrates and inhibitors of P-glycoprotein

The ATP-binding cassette transporter P-glycoprotein (P-gp) exerts a critical role in the systemic disposition of, and exposure to, lipophilic and amphipathic drugs, carcinogens, toxins and other xenobiotics. The ability of P-gp to transfer a wide variety of structurally unrelated compounds from the cell interior across the membrane bilayer remains intriguing. Since dietary chemicals in cruciferous and several other foods appear to exert anticarcinogenic effects by inducing phase II enzymes and inhibiting some phase I enzymes, the isothiocyanate constituents are frequently studied for interactions with various biomacromolecules as well as cytotoxins or isolated cells. Several prominent dietary isothiocyanates were characterized for their interaction with P-gp and their specific effects on the P-gp export activity of several marker substrates. Some of these compounds inhibit the active P-gp-mediated efflux of the fluorescent markers LDS-751 and daunorubicin with low potency, with the most potent among them, phenethyl isothiocyanate, inhibiting transport of the LDS-751 substrate with an IC(50) of approximately 240 microM. Overall, these isothiocyanates are unlikely to impede the xenobiotic defence function of P-gp even in the intestine where the concentrations are potentially high.

ABC transporters as multidrug resistance mechanisms and the development of chemosensitizers for their reversal

One of the major problems related with anticancer chemotherapy is resistance against anticancer drugs. The ATP-binding cassette (ABC) transporters are a family of transporter proteins that are responsible for drug resistance and a low bioavailability of drugs by pumping a variety of drugs out cells at the expense of ATP hydrolysis. One strategy for reversal of the resistance of tumor cells expressing ABC transporters is combined use of anticancer drugs with chemosensitizers. In this review, the physiological functions and structures of ABC transporters, and the development of chemosensitizers are described focusing on well-known proteins including P-glycoprotein, multidrug resistance associated protein, and breast cancer resistance protein.

Nucleotide dissociation from NBD1 promotes solute transport by MRP1

MRP1 transports glutathione-S-conjugated solutes in an ATP-dependent manner by utilizing its two NBDs to bind and hydrolyze ATP. We have found that ATP binding to NBD1 plays a regulatory role whereas ATP hydrolysis at NBD2 plays a dominant role in ATP-dependent LTC4 transport. However, whether ATP hydrolysis at NBD1 is required for the transport was not clear. We now report that ATP hydrolysis at NBD1 may not be essential for transport, but that the dissociation of the NBD1-bound nucleotide facilitates ATP-dependent LTC4 transport. These conclusions are supported by the following results. The substitution of the putative catalytic E1455 with a non-acidic residue in NBD2 greatly decreases the ATPase activity of NBD2 and the ATP-dependent LTC4 transport, indicating that E1455 participates in ATP hydrolysis. The mutation of the corresponding D793 residue in NBD1 to a different acidic residue has little effect on ATP-dependent LTC4 transport. The replacement of D793 with a non-acidic residue, such as D793L or D793N, increases the rate of ATP-dependent LTC4 transport. Along with their higher transport activities, their Michaelis constant Kms (ATP) are also higher than that of wild-type. Coincident with their higher Kms (ATP), their Kds derived from ATP binding are also higher than that of wild-type, implying that the rate of dissociation of the bound nucleotide from the mutated NBD1 is faster than that of wild-type. Therefore, regardless of whether the bound ATP at NBD1 is hydrolyzed or not, the release of the bound nucleotide from NBD1 may bring the molecule back to its original conformation and facilitate the protein to start a new cycle of ATP-dependent solute transport.

Functional Interactions Between Nucleotide Binding Domains and Leukotriene C4Binding Sites of Multidrug Resistance Protein 1 (ABCC1)

Multidrug resistance protein 1 (MRP1) is a member of the "C" branch of the ATP-binding cassette transporter superfamily. The NH(2)-proximal nucleotide-binding domain (NBD1) of MRP1 differs functionally from its COOH-proximal domain (NBD2). NBD1 displays intrinsic high-affinity ATP binding and little ATPase activity. In contrast, ATP binding to NBD2 is strongly dependent on nucleotide binding by NBD1, and NBD2 is more hydrolytically active. We have demonstrated that occupancy of NBD2 by ATP or ADP markedly decreased substrate binding by MRP1. We have further explored the relationship between nucleotide and substrate binding by examining the effects of various ATP analogs and ADP trapping, as well as mutations in conserved functional elements in the NBDs, on the ability of MRP1 to bind the photoactivatable, high-affinity substrate cysteinyl leukotriene C(4) (LTC(4))(.) Overall, the results support a model in which occupancy of both NBD1 and NBD2 by ATP results in the formation of a low-affinity conformation of the protein. However, nonhydrolyzable ATP analogs (beta,gamma-imidoadenosine 5'-triphosphate and adenylylmethylene diphosphonate) failed to substitute for ATP or adenosine 5'-O-(thiotriphosphate) (ATPgammaS) in decreasing LTC(4) photolabeling. Furthermore, mutations of the signature sequence in either NBD that had no apparent effect on azido-ATP binding abrogated the formation of a low-affinity substrate binding state in the presence of ATP or ATPgammaS. We suggest that the effect of these mutations, and possibly the failure of some ATP analogs to decrease LTC(4) binding, may be attributable to an inability to elicit a conformational change in the NBDs that involves interactions between the signature sequence and the gamma-phosphate of the bound nucleotide.

P-glycoprotein retains function when reconstituted into a sphingolipid- and cholesterol-rich environment

P-glycoprotein (P-gp) appears to be associated within specialized raftlike membrane microdomains. The activity of P-gp is sensitive to its lipid environment, and a functional association in raft microdomains will require that P-gp retains activity in the microenvironment. Purified hamster P-gp was reconstituted in liposomes comprising sphingomyelin and cholesterol, both highly enriched in membrane microdomains and known to impart a liquid-ordered phase to bilayers. The activity of P-gp was compared with that of proteoliposomes composed of crude egg phosphatidylcholine (unsaturated) or dipalmitoyl phosphatidylcholine (saturated) in the presence or absence of cholesterol. The maximal rate of ATP hydrolysis was not significantly altered by the nature of the lipid species. However, the potencies of nicardipine and XR9576 to modulate the ATPase activity of P-gp were increased in the sphingolipid-based proteoliposomes. The drug-P-gp interaction was investigated by measurement of the rates of [(3)H]XR9576 association and dissociation from the transporter. The lipid environment of P-gp did not affect these kinetic parameters of drug binding. In summary, P-gp retains function in liquid-ordered cholesterol and sphingolipid model membranes in which the communication between the transmembrane and the nucleotide binding domains after drug binding to the protein is more efficient.

Highly Altered Protein Expression Profile in the Adriamycin Resistant MCF-7 Cell Line

The protein expression pattern in the cytosol fraction of the adriamycin resistant MCF-7 cell line (MCF-7/ADR) was compared to that of the parental MCF-7 cell line using two-dimensional gel electrophoresis and mass spectrometry. Twenty proteins with altered abundances were identified and studied in MCF-7/ADR. Both up regulation and down regulation are characterized. The most striking differences were found for proteins that were uniquely expressed in this cell line and not detectable in the parental MCF-7 cell line. These proteins include annexin I, the neuronal ubiquitin carboxyl hydrolase isoenzyme L-1 (also known as PGP9.5), glutathione-S-transferase pi class, nicotinamide N-methyltransferase, and interleukin-18 precursor. On the other hand, catechol-O-methyltransferase was expressed in the parental cell line, but was not detected in the adriamycin resistant cell line. This protein expression pattern was unique to MCF-7/ADR and not observed in MCF-7 cell lines selected for resistant to etoposide, mitoxantrone or melphalan.

Combined mutation of catalytic glutamate residues in the two nucleotide binding domains of P-glycoprotein generates a conformation that binds ATP and ADP tightly

Combined mutation of "catalytic carboxylates" in both nucleotide binding domains (NBDs) of P-glycoprotein generates a conformation capable of tight binding of 8-azido-ADP (Sauna, Z. E., Müller, M., Peng, X. H., and Ambudkar, S. V. (2002) Biochemistry 41, 13989-14000). Here we characterized this conformation using pure mouse MDR3 P-glycoprotein and natural MgATP and MgADP. Mutants E552A/E1197A, E552Q/E1197Q, E552D/E1197D, and E552K/E1197K had low but real ATPase activity in the order Ala > Gln > Asp > Lys, emphasizing the requirement for Glu stereochemistry. Mutant E552A/E1197A bound MgATP and MgADP (1 mol/mol) with K(d) 9.2 and 92 microm, showed strong temperature sensitivity of MgATP binding and equal dissociation rates for MgATP and MgADP. With MgATP as the added ligand, 80% of bound nucleotide was in the form of ATP. None of these parameters was vanadate-sensitive. The other mutants showed lower stoichiometry of MgATP and MgADP binding, in the order Ala > Gln > Asp > Lys. We conclude that the E552A/E1197A mutation arrests the enzyme in a conformation, likely a stabilized NBD dimer, which occludes nucleotide, shows preferential binding of ATP, does not progress to a normal vanadate-sensitive transition state, but hydrolyzes ATP and releases ADP slowly. Impairment of turnover is primarily due to inability to form the normal transition state rather than to slow ADP release. The Gln, Asp, and Lys mutants are less effective at stabilizing the occluded nucleotide, putative dimeric NBD, conformation. We envisage that in wild-type the occluded nucleotide conformation occurs transiently after MgATP binds to both NBDs with associated dimerization, and before progression to the transition state.

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.

Molecular modeling correctly predicts the functional importance of Phe594 in transmembrane helix 11 of the multidrug resistance protein, MRP1 (ABCC1)

The human ATP-binding cassette (ABC) transporter, multidrug resistance protein 1 (MRP1/ABCC1), confers resistance to a broad range of anti-cancer agents and transports a variety of organic anions. At present, essentially no structural data exists for MRP1 that might be used to elucidate its mechanism of transport. Consequently, we have applied a modeling strategy incorporating crystal and indirect structural data from other ABC transporters to construct a model of the transmembrane domains of the core region of MRP1 that includes the amino acid side chains. Three conserved Trp residues and one non-conserved Tyr residue, shown previously to be of functional importance (Koike, K., Oleschuk, C. J., Haimeur, A., Olsen, S. L., Deeley, R. G., and Cole, S. P. C. (2002) J. Biol. Chem. 277, 49495-49503), were found to line the "pore" in our model proximal to the membrane cytosol interface. A fifth aromatic residue (Phe594) was identified that, with the Trp and Tyr residues, completed a ring or "basket" of aromatic amino acids and, accordingly, we postulated that it would also be of functional importance. To test this idea, MRP1-Phe594 mutants were expressed in human embryonic kidney cells, and their properties were examined using membrane vesicles. Substitution of Phe594 with Ala substantially reduced or eliminated the transport of five organic anion substrates by MRP1 and abrogated the binding of leukotriene C4. On the other hand, the conservatively substituted F594W and F594Y mutants remained transport competent, although significant substrate- and substitution-specific changes were observed. These studies provide some structural insight into a possible substrate binding/transport site of MRP1 at the beginning of a putative substrate translocation pathway and demonstrate the usefulness of modeling for directing structure-function analyses of this transporter.

Role of carboxylate residues adjacent to the conserved core Walker B motifs in the catalytic cycle of multidrug resistance protein 1 (ABCC1)

MRP1 belongs to subfamily "C" of the ABC transporter superfamily. The nucleotide-binding domains (NBDs) of the C family members are relatively divergent compared with many ABC proteins. They also differ in their ability to bind and hydrolyze ATP. In MRP1, NBD1 binds ATP with high affinity, whereas NBD2 is hydrolytically more active. Furthermore, ATP binding and/or hydrolysis by NBD2 of MRP1, but not NBD1, is required for MRP1 to shift from a high to low affinity substrate binding state. Little is known of the structural basis for these functional differences. One minor structural difference between NBDs is the presence of Asp COOH-terminal to the conserved core Walker B motif in NBD1, rather than the more commonly found Glu present in NBD2. We show that the presence of Asp or Glu following the Walker B motif profoundly affects the ability of the NBDs to bind, hydrolyze, and release nucleotide. An Asp to Glu mutation in NBD1 enhances its hydrolytic capacity and affinity for ADP but markedly decreases transport activity. In contrast, mutations that eliminate the negative charge of the Asp side chain have little effect. The decrease in transport caused by the Asp to Glu mutation in NBD1 is associated with an inability of MRP1 to shift from high to low affinity substrate binding states. In contrast, mutation of Glu to Asp markedly increases the affinity of NBD2 for ATP while decreasing its ability to hydrolyze ATP and to release ADP. This mutation eliminates transport activity but potentiates the conversion from a high to low affinity binding state in the presence of nucleotide. These observations are discussed in the context of catalytic models proposed for MRP1 and other ABC drug transport proteins.

ATP binding to the first nucleotide binding domain of multidrug resistance-associated protein plays a regulatory role at low nucleotide concentration, whereas ATP hydrolysis at the second plays a dominant role in ATP-dependent leukotriene C4 transport

Multidrug resistance-associated protein (MRP1) transports solutes in an ATP dependent manner by utilizing its two nonequivalent nucleotide binding domains (NBDs) to bind and hydrolyze ATP. The two NBDs possess different properties (Gao, M., Cui, H. R., Loe, D. W., Grant, C. E., Almquist, K. C., Cole, S. P., and Deeley, R. G. (2000) J. Biol. Chem. 275, 13098-13108; Hou, Y., Cui, L., Riordan, J. R., and Chang, X. (2000) J. Biol. Chem. 275, 20280-20287) and may play different roles during solute transport. We now report that NBD1 has moderately higher affinity for ATP than NBD2. The consequence of this difference is that the overall Kd value for wild-type MRP1 is mainly determined by ATP binding at NBD1. This conclusion is supported by the following: 1) mutation of the cysteine residue at 682 to alanine (C682A) in Walker A motif in NBD1 decreases the Kd value, indicating increased affinity for ATP; 2) mutation of the alanine residue at 1331 to cysteine (A1331C) in the Walker A motif of NBD2 does not have an effect on the Kd value; and 3) photolabeling of the protein with a cysteine residue in the Walker A motif of NBD1 is much more sensitive to N-ethylmaleimide modification than the protein with a cysteine residue in the Walker A motif of NBD2. In contrast, the Km for ATP in support of LTC4 transport is mainly determined by ATP hydrolysis at NBD2. This conclusion is supported by the following: 1) although mutation of A1331C does not have an effect on the Kd value, the Km values measured from LTC4 transport by proteins with this mutation in NBD2 are much higher than the proteins with wild-type NBD2, implying that the A1331C mutation affects ATP binding/hydrolysis at NBD2; and 2) ATP-dependent LTC4 transport by the protein with a cysteine residue in the Walker A motif of NBD2 is much more sensitive to N-ethylmaleimide modification than the protein with a cysteine residue in the Walker A motif of NBD1. Our previous results indicated that ATP binding at NBD1 at low concentration enhanced ATP binding/hydrolysis at NBD2. All of these results support the notion that ATP binding at NBD1 at low concentration plays a more important regulatory role than the binding at high ATP concentration and that ATP hydrolysis at NBD2 plays a dominant role in the ATP-dependent LTC4 transport.

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.

ATP binding, not hydrolysis, at the first nucleotide-binding domain of multidrug resistance-associated protein MRP1 enhances ADP.Vi trapping at the second domain

Multidrug resistance-associated protein (MRP1) transports solutes in an ATP-dependent manner by utilizing its two nonequivalent nucleotide binding domains (NBDs) to bind and hydrolyze ATP. We found that ATP binding to the first NBD of MRP1 increases binding and trapping of ADP at the second domain (Hou, Y., Cui, L., Riordan, J. R., and Chang, X. (2002) J. Biol. Chem. 277, 5110-5119). These results were interpreted as indicating that the binding of ATP at NBD1 causes a conformational change in the molecule and increases the affinity for ATP at NBD2. However, we did not distinguish between the possibilities that the enhancement of ADP trapping might be caused by either ATP binding alone or hydrolysis. We now report the following. 1) ATP has a much lesser effect at 0 degrees C than at 37 degrees C. 2) After hexokinase treatment, the nonhydrolyzable ATP analogue, adenyl 5'-(yl iminodiphosphate), does not enhance ADP trapping. 3) Another nonhydrolyzable ATP analogue, adenosine 5'-(beta,gamma-methylene)triphosphate, whether hexokinase-treated or not, causes a slight enhancement. 4) In contrast, the hexokinase-treated poorly hydrolyzable ATP analogue, adenosine 5'-O-(thiotriphosphate) (ATPgammaS), enhances ADP trapping to a similar extent as ATP under conditions in which ATPgammaS should not be hydrolyzed. We conclude that: 1) ATP hydrolysis is not required to enhance ADP trapping by MRP1 protein; 2) with nucleotides having appropriate structure such as ATP or ATPgammaS, binding alone can enhance ADP trapping by MRP1; 3) the stimulatory effect on ADP trapping is greatly diminished when the MRP1 protein is in a "frozen state" (0 degrees C); and 4) the steric structure of the nucleotide gamma-phosphate is crucial in determining whether binding of the nucleotide to NBD1 of MRP1 protein can induce the conformational change that influences nucleotide trapping at NBD2.

On the mechanism of MgATP-dependent gating of CFTR Cl- channels

CFTR, the product of the gene mutated in cystic fibrosis, is an ATPase that functions as a Cl(-) channel in which bursts of openings separate relatively long interburst closed times (tauib). Channel gating is controlled by phosphorylation and MgATP, but the underlying molecular mechanisms remain controversial. To investigate them, we expressed CFTR channels in Xenopus oocytes and examined, in excised patches, how gating kinetics of phosphorylated channels were affected by changes in [MgATP], by alterations in the chemical structure of the activating nucleotide, and by mutations expected to impair nucleotide hydrolysis and/or diminish nucleotide binding affinity. The rate of opening to a burst (1/tauib) was a saturable function of [MgATP], but apparent affinity was reduced by mutations in either of CFTR's nucleotide binding domains (NBDs): K464A in NBD1, and K1250A or D1370N in NBD2. Burst duration of neither wild-type nor mutant channels was much influenced by [MgATP]. Poorly hydrolyzable nucleotide analogs, MgAMPPNP, MgAMPPCP, and MgATPgammaS, could open CFTR channels, but only to a maximal rate of opening approximately 20-fold lower than attained by MgATP acting on the same channels. NBD2 catalytic site mutations K1250A, D1370N, and E1371S were found to prolong open bursts. Corresponding NBD1 mutations did not affect timing of burst termination in normal, hydrolytic conditions. However, when hydrolysis at NBD2 was impaired, the NBD1 mutation K464A shortened the prolonged open bursts. In light of recent biochemical and structural data, the results suggest that: nucleotide binding to both NBDs precedes channel opening; at saturating nucleotide concentrations the rate of opening to a burst is influenced by the structure of the phosphate chain of the activating nucleotide; normal, rapid exit from bursts occurs after hydrolysis of the nucleotide at NBD2, without requiring a further nucleotide binding step; if hydrolysis at NBD2 is prevented, exit from bursts occurs through a slower pathway, the rate of which is modulated by the structure of the NBD1 catalytic site and its bound nucleotide. Based on these and other results, we propose a mechanism linking hydrolytic and gating cycles via ATP-driven dimerization of CFTR's NBDs.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

P-glycoprotein inhibits caspase-8 activation but not formation of the death inducing signal complex (disc) following Fas ligation

Previous studies by our laboratory have shown that the drug transporter protein P-glycoprotein, P-gp, can specifically inhibit Fas-induced caspase-3 activation and apoptosis. Importantly, inhibition of both caspase-3 activation and cell death could be reversed by pharmacological and antibody inhibitors of P-gp function. However, the molecular mechanisms underpinning P-gp-mediated resistance to Fas-induced cell death and caspase activation remained unknown. We therefore sought to identify the point(s) within the death receptor pathway at which P-gp exerted its inhibitory effect and to determine whether the ATPase activity of P-gp was required. Structure-function analysis determined that ATP hydrolysis was necessary for P-gp to confer resistance to Fas-induced caspase activation and cell death. Importantly, although both FADD and caspase-8 were recruited to the Death Inducing Signal Complex (DISC) in wild-type P-gp expressing cells following Fas ligation, subsequent activation of caspase-8 at the DISC was inhibited. The ability of P-gp to inhibit caspase-8 activation was also ATP dependent. These studies demonstrate that P-gp inhibits Fas-induced caspase-8 activation but not formation of the DISC and that this activity of P-gp is dependent on ATP hydrolysis.

Projection structure of P-glycoprotein by electron microscopy. Evidence for a closed conformation of the nucleotide binding domains

The structure of P-glycoprotein (Pgp) from mouse has been studied by electron microscopy and image analysis. Two-dimensional crystals of Pgp in a lipid bilayer were generated by reconstituting pure, detergent-solubilized protein containing a C-terminal six-histidine tag using the lipid monolayer technique. The crystals belong to plane group P1 with a = b = 104 +/- 2 A and gamma = 90 +/- 4 degrees. The projection structure of Pgp calculated at a resolution of 22 A shows two closely interacting protein domains that can be interpreted as the N- and C-terminal halves of the protein. The projection structure of Pgp is consistent with the recently published x-ray structure of MsbA, a lipid A flippase from Escherichia coli with high sequence homology to Pgp but only when the two MsbA subunits are rotated to bring their nucleotide binding domains together.

Elevation of P-glycoprotein function by a catechin in green tea

The ABC transporter P-glycoprotein (P-gp) exerts a critical role in the systemic disposition of and exposure to lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics. The ability of P-gp to transfer a wide variety of structurally unrelated compounds from the cell interior across the membrane bilayer remains intriguing. Since dietary chemicals in green tea (and several other foods) appear to exert anticarcinogenic effects by an unknown mechanism, the constituents are frequently studied for interactions with various biomacromolecules as well as cytotoxins or isolated cells. We characterized several green tea catechins for their interaction with P-gp and their specific effects on P-gp export activity of several marker substrates. Some of these compounds inhibit the active efflux of the fluorescent markers LDS-751 (LDS) and rhodamine 123 (Rho) with low potency. Remarkably, others of these catechins facilitate the P-gp-mediated transport of LDS without affecting daunorubicin (DNR) transport or Rho. Moreover, (-)epicatechin, though an inhibitor of Rho transport, can significantly enhance the active net transport of another P-gp marker substrate, LDS. This result indicates that (-)epicatechin may bind to and activate an allosteric site that enhances P-gp overall function or efficiency. Such a mechanism of heterotropic allosteric enhancement of P-gp could serve as chemoprotective to many cells and contribute to the purported anticarcinogenic effect of green tea consumption.

Auxin transport: ABC proteins join the club

The isolation of potential auxin carriers from Arabidopsis thaliana marks a breakthrough in the characterization of elements involved in auxin delivery. Current models suggest that asymmetrical localization of auxin uptake and efflux carriers within the plasma membrane control the establishment of auxin gradients via facilitated transport. Now, the analysis of mutants defective in Arabidopsis ABC proteins indicates that primary active transport might participate in the control of auxin homeostasis as well.

Tandem function of nucleotide binding domains confers competence to sulfonylurea receptor in gating ATP-sensitive K+ channels

Fundamental to the metabolic sensor function of ATP-sensitive K(+) (K(ATP)) channels is the sulfonylurea receptor. This ATP-binding cassette protein, which contains nucleotide binding domains (NBD1 and NBD2) with conserved Walker motifs, regulates the ATP sensitivity of the pore-forming Kir6.2 subunit. Although NBD2 hydrolyzes ATP, a property essential in K(ATP) channel gating, the role of NBD1, which has limited catalytic activity, if at all, remains less understood. Here, we provide functional evidence that cooperative interaction, rather than the independent contribution of each NBD, is critical for K(ATP) channel regulation. Gating of cardiac K(ATP) channels by distinct conformations in the NBD2 ATPase cycle, induced by gamma-phosphate analogs, was disrupted by point mutation not only of the Walker motif in NBD2 but also in NBD1. Cooling membrane patches to decelerate the intrinsic ATPase activity counteracted ATP-induced K(ATP) channel inhibition, an effect that mimicked stabilization of the MgADP-bound posthydrolytic state at NBD2 by the gamma-phosphate analog orthovanadate. Temperature-induced channel activation was abolished by mutations that either prevent stabilization of MgADP at NBD2 or ATP at NBD1. These findings provide a paradigm of K(ATP) channel gating based on integration of both NBDs into a functional unit within the multimeric channel complex.

ATP binding to the first nucleotide-binding domain of multidrug resistance protein MRP1 increases binding and hydrolysis of ATP and trapping of ADP at the second domain

Multidrug resistance protein (MRP1) utilizes two non-equivalent nucleotide-binding domains (NBDs) to bind and hydrolyze ATP. ATP hydrolysis by either one or both NBDs is essential to drive transport of solute. Mutations of either NBD1 or NBD2 reduce solute transport, but do not abolish it completely. How events at these two domains are coordinated during the transport cycle have not been fully elucidated. Earlier reports (Gao, M., Cui, H. R., Loe, D. W., Grant, C. E., Almquist, K. C., Cole, S. P., and Deeley, R. G. (2000) J. Biol. Chem. 275, 13098-13108; Hou, Y., Cui, L., Riordan, J. R., and Chang, X. (2000) J. Biol. Chem. 275, 20280-20287) indicate that intact ATP is observed bound at NBD1, whereas trapping of the ATP hydrolysis product, ADP, occurs predominantly at NBD2 and that trapping of ADP at NBD2 enhances ATP binding at NBD1 severalfold. This suggested transmission of a positive allosteric interaction from NBD2 to NBD1. To assess whether ATP binding at NBD1 can enhance the trapping of ADP at NBD2, photoaffinity labeling experiments with [alpha-(32)P]8-N(3)ADP were performed and revealed that when presented with this compound labeling of MRP1 occurred at both NBDs. However, upon addition of ATP, this labeling was enhanced 4-fold mainly at NBD2. Furthermore, the nonhydrolyzable ATP analogue, 5'-adenylylimidodiphosphate (AMP-PNP), bound preferentially to NBD1, but upon addition of a low concentration of 8-N(3)ATP, the binding at NBD2 increased severalfold. This suggested that the positive allosteric stimulation from NBD1 actually involves an increase in ATP binding at NBD2 and hydrolysis there leading to the trapping of ADP. Mutations of Walker A or B motifs in either NBD greatly reduced their ability to be labeled by [alpha-(32)P]8-N(3)ADP as well as by either [alpha-(32)P]- or [gamma-(32)P]8-N(3)ATP (Hou et al. (2000), see above). These mutations also strongly diminished the enhancement by ATP of [alpha-(32)P]8-N(3)ADP labeling and the transport activity of the protein. Taken together, these results demonstrate directly that events at NBD1 positively influence those at NBD2. The interactions between the two asymmetric NBDs of MRP1 protein may enhance the catalytic efficiency of the MRP1 protein and hence of its ATP-dependent transport of conjugated anions out of cells.