Reconstitution of drug-stimulated ATPase activity following co-expression of each half of human P-glycoprotein as separate polypeptides

Participation of ABC-transporters in lipid metabolism and pathogenesis of atherosclerosis

Atherosclerosis is one of the key causes of morbidity and mortality worldwide. It is known that a leading role in the development and progression of atherosclerosis is played by a violation of lipid metabolism. ABC transporters provide lipid cell homeostasis, performing a number of transport functions - moving lipids inside the cell, in the plasma membrane, and also removing lipids from the cell. In a large group of ABC transporters, about 20 take part in lipid homeostasis, playing, among other things, an important role in the pathogenesis of atherosclerosis. It was shown that cholesterol is not only a substrate for a number of ABC transporters, but also able to modulate their activity. Regulation of activity is carried out due to specific lipid-protein interactions.

GALNT14 Involves the Regulation of Multidrug Resistance in Breast Cancer Cells

GALNT14 is a member of N-acetylgalactosaminyltransferase enzyme family and mediates breast cancer cell development. Here, we find that GALNT14 regulates multidrug resistance (MDR) in breast cancer. The expression of GALNT14 is associated with MDR in breast cancer. Higher level of GALNT14 facilitates MCF-7 cells to resist Adriamycin, whereas knockdown of GALNT14 sensitizes cells to Adriamycin. Moreover, the expression of GALNT14 associates with the expression of P-gp, the efflux pump localized on the cell membrane, which could be the underlying mechanism of how GALNT14 induces MDR. In-depth analysis shows that GALNT14 regulates the stability of P-gp. Finally, GALNT14 associates with higher level of P-gp in chemotherapy-resistant human breast cancer tissues. Taken together, our studies reveal a molecular mechanism in breast cancer MDR.

Coarse-grained molecular dynamics simulations reveal lipid access pathways in P-glycoprotein

P-glycoprotein (P-gp) exports a broad range of dissimilar compounds, including drugs, lipids, and lipid-like molecules. Because of its substrate promiscuity, P-gp is a key player in the development of cancer multidrug resistance. Although P-gp is one of the most studied ABC transporters, the mechanism by which its substrates access the cavity remains unclear. In this study, we perform coarse-grained molecular dynamics simulations to explore possible lipid access pathways in the inward-facing conformation of P-gp embedded in bilayers of different lipid compositions. In the inward-facing orientation, only lipids from the lower leaflet access the cavity of the transporter. We identify positively charged residues at the portals of P-gp that favor lipid entrance to the cavity, as well as lipid-binding sites at the portals and within the cavity, which is in good agreement with previous experimental studies. This work includes several examples of lipid pathways for phosphatidylcholine and phosphatidylethanolamine lipids that help elucidate the molecular mechanism of lipid binding in P-gp.

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.

Attachment of a 'molecular spring' restores drug-stimulated ATPase activity to P-glycoprotein lacking both Q loop glutamines

P-glycoprotein (P-gp) is an ABC (ATP-Binding Cassette) drug pump that is clinically important because it confers multidrug resistance. Drugs bind at the interface between the transmembrane domains to activate ATPase activity at the two nucleotide-binding domains (NBDs). Drug transport involves ATP-dependent conformational changes between inward- (open, NBDs far apart) and outward-facing (closed, NBDs close together) conformations. Recently, it was reported that the conserved glutamines residues (Gln475 in NBD1 and Gln1118 in NBD2) in the Q loops of P-gp when mutated to alanine completely inhibited the drug-stimulated ATPase activity. It is unknown why the glutamine residues (Gln475 and Gln1118) in the Q loops of the NBDs of P-gp are required for drug-stimulated ATPase activity. Here we show that introduction of these mutations into the L175C/N820C mutant (L175C/N820C/Q475A/Q1118A) also abolished drug-stimulated ATPase activity. The ATPase activity was restored however, when the L175C/N820C/Q475A/Q1118A mutant was cross-linked with a flexible disulfide cross-linker. These results suggest that both Q-loop glutamines are not required for ATP hydrolysis and they might function as part of a spring-like mechanism in facilitating the open (inactive) to closed (active) conformational change during ATP hydrolysis. The molecular spring-like action of the Q-loop glutamines during drug-stimulated ATPase activity is likely mimicked by the attachment of the flexible cross-linker.

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

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

P-glycoprotein ATPase activity requires lipids to activate a switch at the first transmission interface

P-glycoprotein (P-gp) is an ABC (ATP-Binding Cassette) drug pump. A common feature of ABC proteins is that they are organized into two wings. Each wing contains a transmembrane domain (TMD) and a nucleotide-binding domain (NBD). Drug substrates and ATP bind at the interface between the TMDs and NBDs, respectively. Drug transport involves ATP-dependent conformational changes between inward- (open, NBDs far apart) and outward-facing (closed, NBDs close together) conformations. P-gps crystallized in the presence of detergent show an open structure. Human P-gp is inactive in detergent but basal ATPase activity is restored upon addition of lipids. The lipids might cause closure of the wings to bring the NBDs close together to allow ATP hydrolysis. We show however, that cross-linking the wings together did not activate ATPase activity when lipids were absent suggesting that lipids may induce other structural changes required for ATPase activity. We then tested the effect of lipids on disulfide cross-linking of mutants at the first transmission interface between intracellular loop 4 (TMD2) and NBD1. Mutants L443C/S909C and L443C/R905C but not G471C/S909C and V472C/S909C were cross-linked with oxidant when in membranes. The mutants were then purified and cross-linked with or without lipids. Mutants G471C/S909C and V472C/S909C cross-linked only in the absence of lipids whereas mutants L443C/S909C and L443C/R905C were cross-linked only in the presence of lipids. The results suggest that lipids activate a switch at the first transmission interface and that the structure of P-gp is different in detergents and lipids.

Tariquidar inhibits P-glycoprotein drug efflux but activates ATPase activity by blocking transition to an open conformation

P-glycoprotein (P-gp, ABCB1) is a drug pump that confers multidrug resistance. Inhibition of P-gp would improve chemotherapy. Tariquidar is a potent P-gp inhibitor but its mechanism is unknown. Here, we tested our prediction that tariquidar inhibits P-gp cycling between the open and closed states during the catalytic cycle. Transition of P-gp to an open state can be monitored in intact cells using reporter cysteines introduced into extracellular loops 1 (A80C) and 4 (R741C). Residues A80C/R741C come close enough (<7Å) to spontaneously cross-link in the open conformation (<7Å) but are widely separated (>30Å) in the closed conformation. Cross-linking of A80C/R741C can be readily detected because it causes the mutant protein to migrate slower on SDS-PAGE gels. We tested whether drug substrates or inhibitors could inhibit cross-linking of the mutant. It was found that only tariquidar blocked A80C/R741C cross-linking. Tariquidar was also a more potent pharmacological chaperone than other P-gp substrates/modulators such as cyclosporine A. Only tariquidar promoted maturation of misprocessed mutant F804D to yield mature P-gp. Tariquidar interacted with the transmembrane domains because it could rescue a misprocessed truncation mutant lacking the nucleotide-binding domains. These results show that tariquidar is a potent pharmacological chaperone and inhibits P-gp drug efflux by blocking transition to the open state during the catalytic cycle.

Directed evolution of P-glycoprotein cysteines reveals site-specific, non-conservative substitutions that preserve multidrug resistance

Pgp (P-glycoprotein) is a prototype ABC (ATP-binding-cassette) transporter involved in multidrug resistance of cancer. We used directed evolution to replace six cytoplasmic Cys (cysteine) residues in Pgp with all 20 standard amino acids and selected for active mutants. From a pool of 75000 transformants for each block of three Cys, we identified multiple mutants that preserved drug resistance and yeast mating activity. The most frequent substitutions were glycine and serine for Cys427 (24 and 20%, respectively) and Cys1070 (37 and 25%) of the Walker A motifs in the NBDs (nucleotide-binding domains), Cys1223 in NBD2 (25 and 8%) and Cys638 in the linker region (24 and 16%), whereas close-by Cys669 tolerated glycine (16%) and alanine (14%), but not serine (absent). Cys1121 in NBD2 showed a clear preference for positively charged arginine (38%) suggesting a salt bridge with Glu269 in the ICL2 (intracellular loop 2) may stabilize domain interactions. In contrast, three Cys residues in transmembrane α-helices could be successfully replaced by alanine. The resulting CL (Cys-less) Pgp was fully active in yeast cells, and purified proteins displayed drug-stimulated ATPase activities indistinguishable from WT (wild-type) Pgp. Overall, directed evolution identified site-specific, non-conservative Cys substitutions that allowed building of a robust CL Pgp, an invaluable new tool for future functional and structural studies, and that may guide the construction of other CL proteins where alanine and serine have proven unsuccessful.

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.

Structural characterization of two metastable ATP-bound states of P-glycoprotein

ATP Binding Cassette (ABC) transporters couple the binding and hydrolysis of ATP to the transport of substrate molecules across the membrane. The mechanism by which ATP binding and/or hydrolysis drives the conformational changes associated with substrate transport has not yet been characterized fully. Here, changes in the conformation of the ABC export protein P-glycoprotein on ATP binding are examined in a series of molecular dynamics simulations. When one molecule of ATP is placed at the ATP binding site associated with each of the two nucleotide binding domains (NBDs), the membrane-embedded P-glycoprotein crystal structure adopts two distinct metastable conformations. In one, each ATP molecule interacts primarily with the Walker A motif of the corresponding NBD. In the other, the ATP molecules interacts with both Walker A motif of one NBD and the Signature motif of the opposite NBD inducing the partial dimerization of the NBDs. This interaction is more extensive in one of the two ATP binding site, leading to an asymmetric structure. The overall conformation of the transmembrane domains is not altered in either of these metastable states, indicating that the conformational changes associated with ATP binding observed in the simulations in the absence of substrate do not lead to the outward-facing conformation and thus would be insufficient in themselves to drive transport. Nevertheless, the metastable intermediate ATP-bound conformations observed are compatible with a wide range of experimental cross-linking data demonstrating the simulations do capture physiologically important conformations. Analysis of the interaction between ATP and its cofactor Mg(2+) with each NBD indicates that the coordination of ATP and Mg(2+) differs between the two NBDs. The role structural asymmetry may play in ATP binding and hydrolysis is discussed. Furthermore, we demonstrate that our results are not heavily influenced by the crystal structure chosen for initiation of the simulations.

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.

ABC transporters and their role in nucleoside and nucleotide drug resistance

ATP-binding cassette (ABC) transporters confer drug resistance against a wide range of chemotherapeutic agents, including nucleoside and nucleotide based drugs. While nucleoside based drugs have been used for many years in the treatment of solid and hematological malignancies as well as viral and autoimmune diseases, the potential contribution of ABC transporters has only recently been recognized. This neglect is likely because activation of nucleoside derivatives require an initial carrier-mediated uptake step followed by phosphorylation by nucleoside kinases, and defects in uptake or kinase activation were considered the primary mechanisms of nucleoside drug resistance. However, recent studies demonstrate that members of the ABCC transporter subfamily reduce the intracellular concentration of monophosphorylated nucleoside drugs. In addition to the ABCC subfamily members, ABCG2 has been shown to transport nucleoside drugs and nucleoside-monophosphate derivatives of clinically relevant nucleoside drugs such as cytarabine, cladribine, and clofarabine to name a few. This review will discuss ABC transporters and how they interact with other processes affecting the efficacy of nucleoside based drugs.

Multidrug efflux pumps and cancer stem cells: insights into multidrug resistance and therapeutic development

Stem cells possess the dual properties of self-renewal and pluripotency. Self-renewal affords these populations the luxury of self-propagation, whereas pluripotency allows them to produce the multitude of cell types found in the body. Protection of the stem cell population from damage or death is critical because these cells need to remain intact throughout the life of an organism. The principal mechanism of protection is through expression of multifunctional efflux transporters--the adenosine triphosphate-binding cassette (ABC) transporters that are the "guardians" of the stem cell population. Ironically, it has been shown that these ABC efflux pumps also afford protection to cancer stem cells (CSCs), shielding them from the adverse effects of chemotherapeutic insult. It is therefore imperative to gain a better understanding of the mechanisms involved in the resistance of stem cells to chemotherapy, which could lead to the discovery of new therapeutic targets and improvement of current anticancer strategies.

Correctors enhance maturation of DeltaF508 CFTR by promoting interactions between the two halves of the molecule

Deletion of Phe508 in cystic fibrosis transmembrane conductance regulator (DeltaF508 CFTR) causes cystic fibrosis. CFTR consists of two homologous halves with each containing a nucleotide-binding domain (NBD) and a transmembrane domain (TMD). DeltaF508 CFTR appears to be trapped in an incompletely folded state. Small molecules (correctors) promote folding of DeltaF508 CFTR with relatively low efficiency. Understanding the mechanism of repair may lead to the development of more effective correctors. Here we tested the effect of correctors and the DeltaF508 mutation on interactions between the halves of CFTR when expressed as separate polypeptides. Glycosylation of C-half CFTR was defective when expressed alone as a mixture of core and unglycosylated proteins was detected. Coexpression of C-half CFTR with either wild-type N-half or DeltaF508/N-half CFTR, however, increased the amount of core-glycosylated protein, but only coexpression with wild-type N-half promoted maturation of C-half CFTR (Endo H resistant). This suggested that the DeltaF508 mutation inhibited some interactions between N-half and C-half CFTRs. Interaction of A52-tagged wild-type N-half or DeltaF508/N-half CFTR with histidine-tagged C-half CFTR was then followed by nickel-chelate chromatography. Coexpression of A52-tagged wild-type N-half or DeltaF508/N-half CFTR with histidine-tagged C-half CFTR resulted in the wild-type N-half CFTR but not DeltaF508/N-half CFTR protein being retained on the column. Coexpression of DeltaF508/N-half and C-half CFTR in the presence correctors VX-325 and corr-4a, however, restored interactions between the two halves. An interaction that was restored was that between NBD1 and TMD2 as the correctors restored cross-linking of mutant DeltaF508/NBD1(V510C)/TMD2(A1067C). Therefore, correctors promote proper interactions between the two halves of CFTR.

Identification of residues in the drug translocation pathway of the human multidrug resistance P-glycoprotein by arginine mutagenesis

P-glycoprotein (P-gp, ATP-binding cassette B1) is a drug pump that extracts toxic drug substrates from the plasma membrane and catalyzes their ATP-dependent efflux. To map the residues in the drug translocation pathway, we performed arginine-scanning mutagenesis on all transmembrane (TM) segments (total = 237 residues) of a P-gp processing mutant (G251V) defective in folding (15% maturation efficiency) (glycosylation state used to monitor folding). The rationale was that arginines introduced into the drug-binding sites would mimic drug rescue and enhance maturation of wild-type or processing mutants of P-gp. It was found that 38 of the 89 mutants that matured had enhanced maturation. Enhancer mutations were found in 11 of the 12 TM segments with the largest number found in TMs 6 and 12 (seven in each), TMs that are critical for P-gp-drug substrate interactions. Modeling of the TM segments showed that the enhancer arginines were found on the hydrophilic face, whereas inhibitory arginines were located on a hydrophobic face that may be in contact with the lipid bilayer. It was found that many of the enhancer arginines caused large alterations in P-gp-drug interactions in ATPase assays. For example, mutants A302R (TM5), L339R (TM6), G872R (TM10), F942R (TM11), Q946R (TM11), V982R (TM12), and S993R (TM12) reduced the apparent affinity for verapamil by approximately 10-fold, whereas the F336R (TM6) and M986R (TM12) mutations caused at least a 10-fold increase in apparent affinity for rhodamine B. The results suggest that P-gp contains a large aqueous-filled drug translocation pathway with multiple drug-binding sites that can accommodate the bulky arginine side chains to promote folding of the protein.

Interaction of tomato lectin with ABC transporter in cancer cells: glycosylation confers functional conformation of P-gp

Phospho-glycoprotein (P-gp) is a polytopic plasma membrane protein whose overexpression causes multidrug resistance (MDR) responsible for the failure of cancer chemotherapy. P-gp 170 is a member of the ATP-binding cassette (ABC) transporter superfamily and has two potentially interesting regions for drugs interfering with its efflux function, namely the oligosaccharides on the first extracellular loop with unknown function and the two intracellular ATP-binding regions providing the energy for drug efflux function. The polylactoseamine oligosaccharides on the first loop can specifically bind the tomato lectin (TL). The P-gp efflux activities of TL-pre-treated MDR resistant cells were measured in the presence of structurally unrelated resistance modifiers such as phenothiazines, terpenoids and carotenoids. The inhibition of efflux activity was measured via the increased rhodamine uptake by mouse lymphoma cells transfected in human MDR1 gene and in human brain capillary endothelial cells. The tested resistance modifiers inhibit the function of ABC transporter resulting in increased R123 accumulation in MDR1 expressing cells. TL prevented the inhibitory action of phenothiazine and verapamil on brain capillary endothelial and MDR1-lymphoma cells, presumably due to the stabilization of the functional active conformation of P-gp. Our results indicate that the polylactosamine chains of P-gp are part of the functionally active protein conformation.

Functional role of the linker region in purified human P-glycoprotein

Human P-glycoprotein (P-gp), which conveys multidrug resistance, is an ATP-dependent drug efflux pump that transports a wide variety of structurally unrelated compounds out of cells. P-gp possesses a 'linker region' of approximately 75 amino acids that connects two homologous halves, each of which contain a transmembrane domain followed by a nucleotide-binding domain. To investigate the role of the linker region, purified human P-gp was cleaved by proteases at the linker region and then compared with native P-gp. Based on a verapamil-stimulated ATP hydrolase assay, size-exclusion chromatography analysis and a thermo-stability assay, cleavage of the P-gp linker did not directly affect the preservation of the overall structure or the catalytic process in ATP hydrolysis. However, linker cleavage increased the k(cat) values both with substrate (k(sub)) and without substrate (k(basal)), but decreased the k(sub)/k(basal) values of all 10 tested substrates. The former result indicates that cleaving the linker activates P-gp, while the latter result suggests that the linker region maintains the tightness of coupling between the ATP hydrolase reaction and substrate recognition. Inspection of structures of the P-gp homolog, MsbA, suggests that linker-cleaved P-gp has increased ATP hydrolase activity because the linker interferes with a conformational change that accompanies the ATP hydrolase reaction. Moreover, linker cleavage affected the specificity constants [k(sub)/K(m(D))] for some substrates (i.e. linker cleavage probably shifts the substrate specificity profile of P-gp). Thus, this result also suggests that the linker region regulates the inherent substrate specificity of P-gp.

Molecular models of human P-glycoprotein in two different catalytic states

Background

P-glycoprotein belongs to the family of ATP-binding cassette proteins which hydrolyze ATP to catalyse the translocation of their substrates through membranes. This protein extrudes a large range of components out of cells, especially therapeutic agents causing a phenomenon known as multidrug resistance. Because of its clinical interest, its activity and transport function have been largely characterized by various biochemical studies. In the absence of a high-resolution structure of P-glycoprotein, homology modeling is a useful tool to help interpretation of experimental data and potentially guide experimental studies.

Results

We present here three-dimensional models of two different catalytic states of P-glycoprotein that were developed based on the crystal structures of two bacterial multidrug transporters. Our models are supported by a large body of biochemical data. Measured inter-residue distances correlate well with distances derived from cross-linking data. The nucleotide-free model features a large cavity detected in the protein core into which ligands of different size were successfully docked. The locations of docked ligands compare favorably with those suggested by drug binding site mutants.

Conclusion

Our models can interpret the effects of several mutants in the nucleotide-binding domains (NBDs), within the transmembrane domains (TMDs) or at the NBD:TMD interface. The docking results suggest that the protein has multiple binding sites in agreement with experimental evidence. The nucleotide-bound models are exploited to propose different pathways of signal transmission upon ATP binding/hydrolysis which could lead to the elaboration of conformational changes needed for substrate translocation. We identified a cluster of aromatic residues located at the interface between the NBD and the TMD in opposite halves of the molecule which may contribute to this signal transmission.

Our models may characterize different steps in the catalytic cycle and may be important tools to understand the structure-function relationship of P-glycoprotein.

Processing mutations disrupt interactions between the nucleotide binding and transmembrane domains of P-glycoprotein and the cystic fibrosis transmembrane conductance regulator (CFTR)

P-glycoprotein (P-gp, ABCB1) is an ATP-dependent drug pump. Each of its two homologous halves contains a transmembrane domain (TMD) that has six transmembrane (TM) segments and a nucleotide-binding domain (NBD). Determining how the two halves interact may provide insight into the folding of P-gp as the drug-binding pocket and nucleotide-binding sites are predicted to be at the interface between the two halves. Here, we present evidence for NBD1-TMD2 and NBD2-TMD1 interactions. We also show that TMD-NBD interactions in immature and mature P-gp can be affected by the presence of a processing mutation. We found that the NBD-TMD mutants L443C(NBD1)/S909C(TMD2) and A266C(TMD1)/F1086C(NBD2) could be cross-linked at 0 degrees C with oxidant (copper phenanthroline). Cross-linking was inhibited by vanadate-trapping of nucleotide. The presence of a processing mutation (G268V/L443C(NBD1)/S909C(TMD2); L1260A/A266C(TMD1)/F1086C(NBD2)) resulted in the synthesis of the immature (150 kDa) protein as the major product and the mutants could not be cross-linked with copper phenanthroline. Expression of the processing mutants in the presence of a pharmacological chaperone (cyclosporin A), however, resulted in the expression of mature (170 kDa) protein at the cell surface that could be cross-linked. Similarly, CFTR mutants A274C(TMD1)/L1260C(NBD2) and V510C(NBD1)/A1067C(TMD2) could be cross-linked at 0 degrees C with copper phenanthroline. Introduction of DeltaF508 mutation in these mutants, however, resulted in the synthesis of immature CFTR that could not be cross-linked. These results suggest that establishment of NBD interactions with the opposite TMD is a key step in folding of ABC transporters.

Staphylococcus aureus HrtA is an ATPase required for protection against heme toxicity and prevention of a transcriptional heme stress response

During systemic infection, Staphylococcus aureus acquires nutrient iron from heme, the cofactor of vertebrate myoglobin and hemoglobin. Upon exposure to heme, S. aureus up-regulates the expression of the heme-regulated transporter, HrtAB. Strains lacking hrtAB exhibit increased sensitivity to heme toxicity, and upon heme exposure they elaborate a secreted protein response that interferes with the recruitment of neutrophils to the site of infection. Taken together, these results have led to the suggestion that hrtAB encodes an efflux system responsible for relieving the toxic effects of accumulated heme. Here we extend these observations by demonstrating that HrtA is the ATPase component of the HrtAB transport system. We show that HrtA is an Mn(2+)/Mg(2+)-dependent ATPase that functions at an optimal pH of 7.5 and exhibits in vitro temperature dependence uncommon to ABC transporter ATPases. Furthermore, we identify conserved residues within HrtA that are required for in vitro ATPase activity and are essential for the functionality of HrtA in vivo. Finally, we show that heme induces an alteration in the gene expression pattern of S. aureus Delta hrtA, implying the presence of a novel transcriptional regulatory mechanism responsible for the previously described immunomodulatory characteristics of hrtA mutants exposed to heme.

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.

Molecular dynamics simulations of E. coli MsbA transmembrane domain: formation of a semipore structure

The human P-glycoprotein (MDR1/P-gp) is an ATP-binding cassette (ABC) transporter involved in cellular response to chemical stress and failures of anticancer chemotherapy. In the absence of a high-resolution structure for P-gp, we were interested in the closest P-gp homolog for which a crystal structure is available: the bacterial ABC transporter MsbA. Here we present the molecular dynamics simulations performed on the transmembrane domain of the open-state MsbA in a bilayer composed of palmitoyl oleoyl phosphatidylethanolamine lipids. The system studied contained more than 90,000 atoms and was simulated for 50 ns. This simulation shows that the open-state structure of MsbA can be stable in a membrane environment and provides invaluable insights into the structural relationships between the protein and its surrounding lipids. This study reveals the formation of a semipore-like structure stabilized by two key phospholipids which interact with the hinge region of the protein during the entire simulation. Multiple sequence alignments of ABC transporters reveal that one of the residues involved in the interaction with these two phospholipids are under a strong selection pressure specifically applied on the bacterial homologs of MsbA. Hence, comparison of molecular dynamics simulation and phylogenetic data appears as a powerful approach to investigate the functional relevance of molecular events occurring during simulations.

Biochemical and pharmacological properties of an allosteric modulator site of the human P-glycoprotein (ABCB1)

The drug-transport function of the human P-glycoprotein (Pgp or ABCB1) is inhibited by a number of structurally unrelated compounds, known as modulators or reversing agents. Among them, the thioxanthene derivative flupentixol inhibits Pgp-mediated drug transport by an allosteric mechanism. Unlike most other Pgp modulators, the cis isomer of flupentixol [cis-(Z)-flupentixol] facilitates interaction of Pgp with its transport-substrate [125I]iodoarylazidoprazosin (or [125I]IAAP), yet inhibits transport. In this study, we show that the flupentixol site acts as a common site of interaction for the tricyclic ring-containing modulators thioxanthenes and phenothiazines. The allosteric stimulation of [125I]IAAP binding to Pgp occurs independent of the phosphorylation status of the transporter. Stimulation is retained in purified Pgp reconstituted into proteoliposomes, suggesting no involvement of any other cellular protein in the phenomenon. However, perturbation of the lipid environment of the reconstituted Pgp by nonionic detergent octylglucoside abolishes stimulation by cis-(Z)-flupentixol of [125I]IAAP binding. Extensive trypsin digestion of the [125I]IAAP-labeled Pgp generates a 5.5 kDa fragment with 80% of the stimulated level of labeling associated with it. Sensitivity to inhibition by transport-substrate vinblastine and competitive modulator cyclosporin A suggests that the elevated level of [125I]IAAP binding to the fragment represents a functionally relevant interaction with the substrate site of Pgp. In summary, we demonstrate that allosteric modulation by cis-(Z)-flupentixol is mediated through its interaction with Pgp at a site specific for tricyclic ring-containing Pgp modulators of thioxanthene and phenothiazine backbone, independent of other cellular components and the phosphorylation status of the protein.

Recent progress in understanding the mechanism of P-glycoprotein-mediated drug efflux

P-glycoprotein (P-gp) is an ATP-dependent drug pump that can transport a broad range of hydrophobic compounds out of the cell. The protein is clinically important because of its contribution to the phenomenon of multidrug resistance during AIDS/HIV and cancer chemotherapy. P-gp is a member of the ATP-binding cassette (ABC) family of proteins. It is a single polypeptide that contains two repeats joined by a linker region. Each repeat has a transmembrane domain consisting of six transmembrane segments followed by a hydrophilic domain containing the nucleotide-binding domain. In this mini-review, we discuss recent progress in determining the structure and mechanism of human P-glycoprotein.

The Dileucine Motif at the COOH Terminus of Human Multidrug Resistance P-glycoprotein Is Important for Folding but Not Activity

P-glycoprotein (P-gp, ABCB1) actively transports a broad range of cytotoxic compounds out of the cell. The COOH terminus of P-gp contains a dileucine motif (Leu(1260)-Leu(1261)) and a conserved phenylalanine (Phe(1268)). Similar residues in SUR1 (ABCC8) were reported to be important plasma membrane-targeting signals (Sharma, N., Crane, A., Clement, J. P. t., Gonzalez, G., Babenko, A. P., Bryan, J., and Aguilar-Bryan, L. (1999) J. Biol. Chem. 274, 20628-20632). Here, we used alanine-scanning mutagenesis to test whether these residues were essential for trafficking of P-gp to the cell surface. Mutant L1260A expressed a 150-kDa immature protein that did not reach the cell surface and was sensitive to digestion by Endo H(f). By contrast, mutants L1261A, F1268A, and wild-type P-gps expressed the 170-kDa mature proteins at the cell surface. Mutation of Leu(1260) to Gly, Ile, Trp, Lys, or Glu also resulted in the expression of the 150-kDa immature protein. All of the mutants, however, expressed the 170-kDa protein in the presence of the drug substrate/specific chemical chaperone cyclosporin A. Mutant L1260A P-gp exhibited drug-stimulated ATPase activities similar to that of wild-type enzyme after rescue with cyclosporin A. Deletion of the last 22 amino acids (Q(1259)-Q(1280)) also caused misprocessing. The mutant, however, was rescued by expression in the presence of cyclosporin A and conferred resistance to colchicine in transfected cells. These results show that the dileucine motif is not a plasma membrane targeting signal. The COOH terminus is required for proper folding of P-gp but not for activity.

Processing mutations located throughout the human multidrug resistance P-glycoprotein disrupt interactions between the nucleotide binding domains

The most common cause of cystic fibrosis is misfolding of the cystic fibrosis transmembrane conductance regulator (CFTR) protein because of deletion of residue Phe-508 (DeltaF508). P-glycoprotein (P-gp) is an ideal model protein for studying how mutations disrupt folding of ATP-binding cassette proteins such as CFTR because specific chemical chaperones can be used to correct folding defects. Interactions between the nucleotide binding domains (NBDs) are critical because ATP binds at the interface between the NBDs. Here, we used disulfide cross-linking between cysteines in the Walker A sites and the LSGGQ signature sequences to test whether processing mutations located throughout P-gp disrupted interactions between the NBDs. We found that mutations present in the cytoplasmic loops, transmembrane segments, and linker regions or deletion of Tyr-490 (equivalent to Phe-508 in CFTR) inhibited cross-linking between the NBDs. Deletion of Phe-508 in the P-gp/CFTR chimera also inhibited cross-linking between the NBDs. Cross-linking was restored, however, when the mutants were expressed in the presence of the chemical chaperone cyclosporin A. The "rescued" mutants exhibited drug-stimulated ATPase activity, and cross-linking between the NBDs was inhibited by vanadate trapping of nucleotide. These results together with our previous findings (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2002) J. Biol. Chem. 277, 27585-27588) indicate that processing mutations disrupt interactions among all four domains. It appears that cross-talk between the cytoplasmic and the transmembrane domains is required for establishment of proper domain-domain interactions that occur during folding of ATP-binding cassette protein transporters.

Uptake inhibitors but not substrates induce protease resistance in extracellular loop two of the dopamine transporter

Changes in protease sensitivity of extracellular loop two (EL2) of the dopamine transporter (DAT) during inhibitor and substrate binding were examined using trypsin proteolysis and epitope-specific immunoblotting. In control rat striatal membranes, proteolysis of DAT in a restricted region of EL2 was produced by 0.001 to 10 microg/ml trypsin. However, in the presence of the dopamine uptake blockers [2-(diphenylmethoxyl) ethyl]-4-(3phenylpropyl) piperazine (GBR 12909), mazindol, 2beta-carbomethoxy-3beta-(4-flourophenyl)tropane (beta-CFT), nomifensine, benztropine, or (-)-cocaine, 100- to 1000-fold higher concentrations of trypsin were required to produce comparable levels of proteolysis. Protease resistance induced by ligands was correlated with their affinity for DAT binding, was not observed with Zn2+, (+)-cocaine, or inhibitors of norepinephrine or serotonin transporters, and was not caused by altered catalytic activity of trypsin. Together, these results support the hypothesis that the interaction of uptake inhibitors with DAT induces a protease-resistant conformation in EL2. In contrast, binding of substrates did not induce protease resistance in EL2, suggesting that substrates and inhibitors interact with DAT differently during binding. To assess the effects of EL2 proteolysis on DAT function, the binding and transport properties of trypsin-digested DAT were assayed with [3H]CFT and [3H]dopamine. Digestion decreased the Bmax for binding and the Vmax for uptake in amounts that were proportional to the extent of proteolysis, indicating that the structural integrity of EL2 is required for maintenance of both DAT binding and transport functions. Together this data provides novel information about inhibitor and substrate interactions at EL2, possibly relating the protease resistant DAT conformation to a mechanism of transport inhibition.

Val133 and Cys137 in transmembrane segment 2 are close to Arg935 and Gly939 in transmembrane segment 11 of human P-glycoprotein

P-glycoprotein (P-gp; ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The protein has two homologous halves joined by a linker region. Each half consists of a transmembrane (TM) domain with six TM segments and a nucleotide-binding domain. The drug substrate-binding pocket is at the interface between the TM segments in each half of the protein. Preliminary studies suggested that the arrangement of the two halves of P-gp shows rotational symmetry (i.e. "head-to-tail" arrangement). Here, we tested this model by determining whether the cytoplasmic ends of TM2 and TM3 in the N-terminal half are in close contact with TM11 in the C-terminal half. Mutants containing a pair of cysteines in TM2/TM11 or TM3/TM11 were subjected to oxidative cross-linking with copper phenanthroline. Two of the 110 TM2/TM11 mutants, V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C (TM11), were cross-linked at 4 degrees C, when thermal motion is reduced. Cross-linking was specific since no cross-linked product was detected in the 100 double Cys TM3/TM11 mutants. Vanadate trapping of nucleotide or the presence of some drug substrates inhibited cross-linking of mutants V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C(TM11). Cross-linking of TM2 and TM11 also blocked drug-stimulated ATPase activity. The close proximity of TM2/TM11 and TM5/TM8 (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2004) J. Biol. Chem. 279, 7692-7697) indicates that these regions between the two halves must enclose the drug-binding pocket at the cytoplasmic side of P-gp. They may form the "hinges" required for conformational changes during the transport cycle.

Disulfide cross-linking analysis shows that transmembrane segments 5 and 8 of human P-glycoprotein are close together on the cytoplasmic side of the membrane

Human P-glycoprotein (P-gp) transports a wide variety of structurally diverse compounds out of the cell. Knowledge about the packing of the transmembrane (TM) segments is essential for understanding the mechanism of drug recognition and transport. We used cysteine-scanning mutagenesis and disulfide cross-linking analysis to determine which TM segment in the COOH half of P-gp was close to TMs 5 and 6 since these segments in the NH(2) half are important for drug binding. An active Cys-less P-gp mutant cDNA was used to generate 240 double cysteine mutants that contained 1 cysteine in TMs 5 or 6 and another in TMs 7 or 8. The mutants were subjected to oxidative cross-linking analysis. No disulfide cross-linking was observed in the 140 TM6/TM7 or TM6/TM8 mutants. By contrast, cross-linking was detected in several P-gp TM5/TM8 mutants. At 4 degrees C, when thermal motion is low, P-gp mutants N296C(TM5)/G774C(TM8), I299C(TM5)/F770C(TM8), I299C(TM5)/G774C(TM8), and G300C(TM5)/F770C(TM8) showed extensive cross-linking with oxidant. These mutants retained drug-stimulated ATPase activity, but their activities were inhibited after treatment with oxidant. Similarly, disulfide cross-linking was inhibited by vanadate trapping of nucleotide. These results indicate that significant conformational changes must occur between TMs 5 and 8 during ATP hydrolysis. We revised the rotational symmetry model for TM packing based on our results and by comparison to the crystal structure of MsbA (Chang, G. (2003) J. Mol. Biol. 330, 419-430) such that TM5 is adjacent to TM8, TM2 is adjacent to TM11, and TMs 1 and 7 are next to TMs 6 and 12, respectively.

Methanethiosulfonate Derivatives of Rhodamine and Verapamil Activate Human P-glycoprotein at Different Sites

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) actively extrudes a broad range of potentially cytotoxic compounds out of the cell. Key steps in understanding the transport process are binding of drug substrates in the transmembrane domains, initiation of ATPase activity, and subsequent drug efflux. We used cysteine-scanning mutagenesis of the transmembrane segment residues and reaction with the thiol-reactive drug substrate analog of rhodamine, methane-thiosulfonate-rhodamine (MTS-rhodamine), to test whether P-gp could be trapped in an activated state with high levels of ATPase activity. The presence of such an activated P-gp could be used to further investigate P-gp-drug substrate interactions. Single cysteine mutants (149) were treated with MTS-rhodamine, and ATPase activities were determined after removal of unreacted MTS-rhodamine. One mutant, F343C(TM6), showed a 5.8-fold increase in activity after reaction with MTS-rhodamine. Pre-treatment of mutant F343C with rhodamine B protected it from activation by MTS-rhodamine, indicating that residue Cys-343 contributes to the rhodamine-binding site. The ATPase activity of MTS-rhodamine-treated mutant F343C, however, was not stimulated further by colchicine or calcein-AM. By contrast, verapamil and Hoechst 33342 stimulated and inhibited, respectively, the ATPase activity of the MTS-rhodamine-treated mutant F343C. These results indicate that the MTS-rhodamine binding site overlaps that of colchicine and calcein-AM but not that of verapamil and Hoechst 33342 within the common drug-binding pocket.

Functional interaction between the two halves of the photoreceptor-specific ATP binding cassette protein ABCR (ABCA4). Evidence for a non-exchangeable ADP in the first nucleotide binding domain

ABCR, also known as ABCA4, is a member of the superfamily of ATP binding cassette transporters that is believed to transport retinal or retinylidene-phosphatidylethanolamine across photoreceptor disk membranes. Mutations in the ABCR gene are responsible for Stargardt macular dystrophy and related retinal dystrophies that cause severe loss in vision. ABCR consists of two tandemly arranged halves each containing a membrane spanning segment followed by a large extracellular/lumen domain, a multi-spanning membrane domain, and a nucleotide binding domain (NBD). To define the role of each NBD, we examined the nucleotide binding and ATPase activities of the N and C halves of ABCR individually and co-expressed in COS-1 cells and derived from trypsin-cleaved ABCR in disk membranes. When disk membranes or membranes from co-transfected cells were photoaffinity labeled with 8-azido-ATP and 8-azido-ADP, only the NBD2 in the C-half bound and trapped the nucleotide. Co-expressed half-molecules displayed basal and retinal-stimulated ATPase activity similar to full-length ABCR. The individually expressed N-half displayed weak 8-azido-ATP labeling and low basal ATPase activity that was not stimulated by retinal, whereas the C-half did not bind ATP and exhibited little if any ATPase activity. Purified ABCR contained one tightly bound ADP, presumably in NBD1. Our results indicate that only NBD2 of ABCR binds and hydrolyzes ATP in the presence or absence of retinal. NBD1, containing a bound ADP, associates with NBD2 to play a crucial, non-catalytic role in ABCR function.

Simultaneous binding of two different drugs in the binding pocket of the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The drug-binding pocket of P-gp is located in the transmembrane domains. Although occupation of the drug-binding pocket by one molecule is sufficient to activate the ATPase activity of P-gp, the drug-binding pocket may be large enough to accommodate two different substrates at the same time. In this study, we used cysteine-scanning mutagenesis to test whether P-gp could simultaneously interact with the thiol-reactive drug substrate, Tris-(2-maleimidoethyl)amine (TMEA) and a second drug substrate. TMEA is a cross-linker substrate of P-gp that allowed us to test for stimulation of cross-linking by a second substrate such as calcein-acetoxymethyl ester, colchicine, demecolcine, cyclosporin A, rhodamine B, progesterone, and verapamil. We report that verapamil induced TMEA cross-linking of mutant F343C(TM6)/V982C(TM12). By contrast, no cross-linked product was detected in mutants F343C(TM6), V982C(TM12), or F343C(TM6)/V982C(TM12) in the presence of TMEA alone. The verapamil-stimulated ATPase activity of mutant F343C(TM6)/V982C(TM12) in the presence of TMEA decreased with increased cross-linking of the mutant protein. These results show that binding of verapamil must induce changes in the drug-binding pocket (induced-fit mechanism) resulting in exposure of residues F343C(TM6)/V982C(TM12) to TMEA. The results also indicate that the common drug-binding pocket in P-gp is large enough to accommodate both verapamil and TMEA simultaneously and suggests that the substrates must occupy different regions in the common drug-binding pocket.

Biochemical analysis of the yeast condensin Smc2/4 complex: an ATPase that promotes knotting of circular DNA

To better understand the contributions that the structural maintenance of chromosome proteins (SMCs) make to condensin activity, we have tested a number of biochemical, biophysical, and DNA-associated attributes of the Smc2p-Smc4p pair from budding yeast. Smc2p and Smc4p form a stable heterodimer, the "Smc2/4 complex," which upon analysis by sedimentation equilibrium appears to reversibly self-associate to form heterotetramers. Individually, neither Smc2p nor Smc4p hydrolyzes ATP; however, ATPase activity is recovered by equal molar mixing of both purified proteins. Hydrolysis activity is unaffected by the presence of DNA. Smc2/4 binds both linearized and circular plasmids, and the binding appears to be independent of adenylate nucleotide. High mole ratios of Smc2/4 to plasmid promote a geometric change in circular DNA that can be trapped as knots by type II topoisomerases but not as supercoils by a type I topoisomerase. Binding titration analyses reveal that two Smc2/4-DNA-bound states exist, one disrupted by and one resistant to salt challenge. Competition-displacement experiments show that Smc2/4-DNA-bound species formed at even high protein to DNA mole ratios remain reversible. Surprisingly, only linear and supercoiled DNA, not nicked-circular DNA, can completely displace Smc2/4 prebound to a labeled, nicked-circular DNA. To explain this geometry-dependent competition, we present two models of DNA binding by SMCs in which two DNA duplexes are captured within the inter-coil space of an Smc2/4 heterodimer. Based on these models, we propose a DNA displacement mechanism to explain how differences in geometry could affect the competitive potential of DNA.

Permanent activation of the human P-glycoprotein by covalent modification of a residue in the drug-binding site

The human multidrug resistance P-glycoprotein (ABCB1) transports a broad range of structurally diverse compounds out of the cell. The transport cycle involves coupling of drug binding in the transmembrane domains with ATP hydrolysis. Compounds such as verapamil stimulate ATPase activity. We used cysteine-scanning mutagenesis of the transmembrane segments and reaction with the thiol-reactive substrate analog of verapamil, methanethiosulfonate (MTS)-verapamil, to test whether it caused permanent activation of ATP hydrolysis. Here we report that one mutant, I306C(TM5) showed increased ATPase activity (8-fold higher than untreated) when treated with MTS-verapamil and isolated by nickel-chelate chromatography. Drug substrates that either enhance (calcein acetoxymethyl ester, demecolcine, and vinblastine) or inhibit (cyclosporin A and trans-(E)-flupentixol) ATPase activity of Cys-less or untreated mutant I306C P-glycoprotein did not affect the activity of MTS-verapamil-treated mutant I306C. Addition of dithiothreitol released the covalently attached verapamil, and ATPase activity returned to basal levels. Pretreatment with substrates such as cyclosporin A, demecolcine, verapamil, vinblastine, or colchicine prevented activation of mutant I306C by MTS-verapamil. The results suggest that MTS-verapamil reacts with I306C in a common drug-binding site. Covalent modification of I306C affects the long range linkage between the drug-binding site and the distal ATP-binding sites. This results in the permanent activation of ATP hydrolysis in the absence of transport. Trapping mutant I306C in a permanently activated state indicates that Ile-306 may be part of the signal to switch on ATP hydrolysis when the drug-binding site is occupied.

Role of P-glycoprotein in pharmacokinetics: clinical implications

P-glycoprotein, the most extensively studied ATP-binding cassette (ABC) transporter, functions as a biological barrier by extruding toxins and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-glycoprotein plays a significant role in drug absorption and disposition. Because of its localisation, P-glycoprotein appears to have a greater impact on limiting cellular uptake of drugs from blood circulation into brain and from intestinal lumen into epithelial cells than on enhancing the excretion of drugs out of hepatocytes and renal tubules into the adjacent luminal space. However, the relative contribution of intestinal P-glycoprotein to overall drug absorption is unlikely to be quantitatively important unless a very small oral dose is given, or the dissolution and diffusion rates of the drug are very slow. This is because P-glycoprotein transport activity becomes saturated by high concentrations of drug in the intestinal lumen. Because of its importance in pharmacokinetics, P-glycoprotein transport screening has been incorporated into the drug discovery process, aided by the availability of transgenic mdr knockout mice and in vitro cell systems. When applying in vitro and in vivo screening models to study P-glycoprotein function, there are two fundamental questions: (i) can in vitro data be accurately extrapolated to the in vivo situation; and (ii) can animal data be directly scaled up to humans? Current information from our laboratory suggests that in vivo P-glycoprotein activity for a given drug can be extrapolated reasonably well from in vitro data. On the other hand, there are significant species differences in P-glycoprotein transport activity between humans and animals, and the species differences appear to be substrate-dependent. Inhibition and induction of P-glycoprotein have been reported as the causes of drug-drug interactions. The potential risk of P-glycoprotein-mediated drug interactions may be greatly underestimated if only plasma concentration is monitored. From animal studies, it is clear that P-glycoprotein inhibition always has a much greater impact on tissue distribution, particularly with regard to the brain, than on plasma concentrations. Therefore, the potential risk of P-glycoprotein-mediated drug interactions should be assessed carefully. Because of overlapping substrate specificity between cytochrome P450 (CYP) 3A4 and P-glycoprotein, and because of similarities in P-glycoprotein and CYP3A4 inhibitors and inducers, many drug interactions involve both P-glycoprotein and CYP3A4. Unless the relative contribution of P-glycoprotein and CYP3A4 to drug interactions can be quantitatively estimated, care should be taken when exploring the underlying mechanism of such interactions.

Substrate-induced conformational changes in the transmembrane segments of human P-glycoprotein. Direct evidence for the substrate-induced fit mechanism for drug binding

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) is quite promiscuous in that it can transport a broad range of structurally diverse compounds out of the cell. We hypothesized that the transmembrane (TM) segments that constitute the drug-binding site are quite mobile such that drug binding occurs through a "substrate-induced fit" mechanism. Here, we used cysteine-scanning mutagenesis and oxidative cross-linking to test for substrate-induced changes in the TM segments. Pairs of cysteines were introduced into a Cys-less P-gp and the mutants treated with oxidant (copper phenanthroline) in the presence or absence of various drug substrates. We show that cyclosporin A promoted cross-linking between residues P350C(TM6)/G939C(TM11), while colchicine and demecolcine promoted cross-linking between residues P350C(TM6)/V991C(TM12). Progesterone promoted cross-linking between residues P350C(TM6)/A935C(TM11), P350C(TM6)/G939C(TM11), as well as between residues P350C(TM6)/V991C(TM12). Other substrates such as vinblastine, verapamil, cis-(Z)-flupenthixol or trans-(E)-flupenthixol did not induce cross-linking at these sites. These results provide direct evidence that the packing of the TM segments in the drug-binding site is changed when P-gp binds to a particular substrate. The induced-fit mechanism explains how P-gp can accommodate a broad range of compounds.

Overexpression, purification, and functional characterization of ATP-binding cassette transporters in the yeast, Pichia pastoris

The ATP-binding cassette (ABC) transporter superfamily is a large gene family that has been highly conserved throughout evolution. The physiological importance of these membrane transporters is highlighted by the large variety of substrates they transport, and by the observation that mutations in many of them cause heritable diseases in human. Likewise, overexpression of certain ABC transporters, such as P-glycoprotein and members of the multidrug resistance associated protein (MRP) family, is associated with multidrug resistance in various cells and organisms. Understanding the structure and molecular mechanisms of transport of the ABC transporters in normal tissues and their possibly altered function in human diseases requires large amounts of purified and active proteins. For this, efficient expression systems are needed. The methylotrophic yeast Pichia pastoris has proven to be an efficient and inexpensive experimental model for high-level expression of many proteins, including ABC transporters. In the present review, we will summarize recent advances on the use of this system for the expression, purification, and functional characterization of P-glycoprotein and two members of the MRP subfamily.

Drug-drug interaction mediated by inhibition and induction of P-glycoprotein

P-glycoprotein (P-gp), the most extensively studied ATP-binding cassette transporter, functions as a biological barrier by extruding toxic substances and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-gp plays a significant role in drug absorption and disposition. Like cytochrome P450 enzymes, inhibition and induction of P-gp have been reported as the causes of drug-drug interactions. Because many prototypic inhibitors and inducers affect both CYP3A4 and P-gp, many drug interactions caused by these inhibitors and inducers involve these two systems. Clinically, it is very difficult to quantitatively differentiate P-gp-mediated drug interactions versus CYP3A4-mediated drug interactions, unless their relative contributions can be accurately estimated. Therefore, care should be exercised when interpreting drug interaction data and exploring the underlying mechanisms of drug interactions.

Drug binding in human P-glycoprotein causes conformational changes in both nucleotide-binding domains

The human multidrug resistance P-glycoprotein (P-gp, ABCB1) uses ATP to transport many structurally diverse compounds out of the cell. It is an ABC transporter with two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Recently, we showed that the "LSGGQ" motif in one NBD ((531)LSGGQ(535) in NBD1; (1176)LSGGQ(1180) in NBD2) is adjacent to the "Walker A" sequence ((1070)GSSGCGKS(1077) in NBD2; (427)GNSGCGKS(434) in NBD1) in the other NBD (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2002) J. Biol. Chem. 277, 41303-41306). Drug substrates can stimulate or inhibit the ATPase activity of P-gp. Here, we report the effect of drug binding on cross-linking between the LSGGQ signature and Walker A sites (Cys(431)(NBD1)/C1176C(NBD2) and Cys(1074)(NBD2)/L531C(NBD1), respectively). Seven drug substrates (calcein-AM, demecolcine, cis(Z)-flupentixol, verapamil, cyclosporin A, Hoechst 33342, and trans(E)-flupentixol) were tested for their effect on oxidative cross-linking. Substrates that stimulated the ATPase activity of P-gp (calcein-AM, demecolcine, cis(Z)-flupentixol, and verapamil) increased the rate of cross-linking between Cys(431)(NBD1-Walker A)/C1176C(NBD2-LSGGQ) and between Cys(1074)(NBD2-Walker A)/L531C(NBD1-LSGGQ) when compared with cross-linking in the absence of drug substrate. By contrast, substrates that inhibited ATPase activity (cyclosporin A, Hoechst 33342, and trans(E)-flupentixol) decreased the rate of cross-linking. These results indicate that interaction between the LSGGQ motifs and Walker A sites must be essential for coupling drug binding to ATP hydrolysis. Drug binding in the transmembrane domains can induce long range conformational changes in the NBDs, such that compounds that stimulate or inhibit ATPase activity must decrease and increase, respectively, the distance between the Walker A and LSGGQ sequences.

Selective association of TRPC channel subunits in rat brain synaptosomes

TRPC genes encode a ubiquitous family of ion channel proteins responsible for Ca(2+) influx following stimulation of G-protein-coupled membrane receptors linked to phospholipase C. These channels may be localized to large multimeric signaling complexes via association with PDZ-containing scaffolding proteins. Based on sequence homology, the TRPC channel family can be divided into two major subgroups: TRPC1, -C4, and -C5 and TRPC3, -C6, and -C7. Although TRPC channels are thought to be tetramers, the actual subunit composition remains unknown. To determine subunit arrangement, individual TRPC channel pairs were heterologously expressed in Sf9 insect cells and immunoprecipitated using affinity-purified rabbit polyclonal antibodies specific for each channel subtype. Reciprocal co-immunoprecipitations showed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate but that cross-association between the two major subgroups does not occur. Additionally, the interaction between each TRPC channel and the PDZ-containing protein, INAD (protein responsible for the inactivation-no-after-potential Drosophila mutant), was examined. TRPC1, -C4, and -C5 co-immunoprecipitated with INAD, whereas TRPC3, -C6, and -C7 did not. To define channel subunit interactions in vivo, immunoprecipitations were performed from isolated rat brain synaptosomal preparations. The results revealed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate in both cortex and cerebellum but that cross-association between the two major subgroups does not occur. These results demonstrate that TRPC channels are present in nerve terminals and provide the first direct evidence for selective assembly of channel subunits in vivo.

Location of the rhodamine-binding site in the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp) pumps a wide variety of structurally diverse compounds out of the cell. It is an ATP-binding cassette transporter with two nucleotide-binding domains and two transmembrane (TM) domains. One class of compounds transported by P-gp is the rhodamine dyes. A P-gp deletion mutant (residues 1-379 plus 681-1025) with only the TM domains retained the ability to bind rhodamine. Therefore, to identify the residues involved in rhodamine binding, 252 mutants containing a cysteine in the predicted TM segments were generated and reacted with a thiol-reactive analog of rhodamine, methanethiosulfonate (MTS)-rhodamine. The activities of 28 mutants (in TMs 2-12) were inhibited by at least 50% after reaction with MTS-rhodamine. The activities of five mutants, I340C(TM6), A841C(TM9), L975C(TM12), V981C(TM12), and V982C(TM12), however, were significantly protected from inhibition by MTS-rhodamine by pretreatment with rhodamine B, indicating that residues in TMs 6, 9, and 12 contribute to the binding of rhodamine dyes. These results, together with those from previous labeling studies with other thiol-reactive compounds, dibromobimane, MTS-verapamil, and MTS-cross-linker substrates, indicate that common residues are involved in the binding of structurally different drug substrates and that P-gp has a common drug-binding site. The results support the "substrate-induced fit" hypothesis for drug binding.

The "LSGGQ" motif in each nucleotide-binding domain of human P-glycoprotein is adjacent to the opposing walker A sequence

The human multidrug resistance P-glycoprotein (P-gp, ABCB1), a member of the ATP-binding cassette (ABC) family of transport proteins, actively transports many cytotoxic compounds out of the cell. ABC transporters have two nucleotide-binding domains (NBD) and two transmembrane domains. The presence of the conserved "signature" sequence (LSGGQ) in each NBD is a unique feature in these transporters. The function of the signature sequences is unknown. In this study, we tested whether the signature sequences ((531)LSGGQ(535) in NBD1; (1176)LSGGQ(1180) in NBD2) in P-gp are in close proximity to the opposing Walker A consensus nucleotide-binding sequences ((1070)GSSGCGKS(1077) in NBD2; (427)GNSGCGKS(434) in NBD1). Pairs of cysteines were introduced into a Cys-less P-gp at the signature and "Walker A" sites and the mutant P-gps were subjected to oxidative cross-linking. At 4 degrees C, when thermal motion is low, P-gp mutants (L531C(Signature)/C1074(Walker A) and C431(Walker A)/L1176C(Signature) were cross-linked. Cross-linking inhibited the drug-stimulated ATPase activities of these two mutants. Their activities were restored, however, after addition of the reducing agent, dithiothreitol. Vanadate trapping of nucleotide at the ATP-binding sites prevented cross-linking of the mutants. These results indicate that the signature sequences are adjacent to the opposing Walker A site. They likely participate in forming the ATP-binding sites and are displaced upon ATP hydrolysis. The resulting conformational change may be the signal responsible for coupling ATP hydrolysis to drug transport by inducing conformational changes in the transmembrane segments.

Proteolytic Cleavage of the Linker Region of the Human P-glycoprotein Modulates Its ATPase Function

P-glycoprotein (Pgp), an anticancer drug-translocating ATPase, is responsible for multidrug resistance in cancer. We have previously shown (Nuti, S. L., Mehdi, A., and Rao, U. S. (2000) Biochemistry 39, 3424-3432) that tryptic cleavage of Pgp results in the activation of basal and drug-stimulated ATPase functions of Pgp. To understand this phenomenon, we determined the sites cleaved by trypsin and further examined whether the modulation of Pgp function is trypsin-specific or the result of proteolysis in general. The effects of chymotrypsin and proteinase K on Pgp ATPase function were studied. The results show that proteolysis of Pgp irrespective of the protease employed resulted in the activation of basal ATPase activity. However, drug-stimulated ATPase activities were differentially modulated. Immunoblot analysis of proteolytic digests indicated that, irrespective of the protease employed, Pgp was predominantly cleaved in the middle of the molecule. N-terminal amino acid sequencing of Pgp tryptic and chymotryptic peptides indicated Arg(680) and Leu(682) as the sites of cleavage, respectively. These two cleavage sites are part of the predicted linker region that joins the two halves of Pgp. Together, these results suggest that the linker region in Pgp is primarily accessible to protease action and that cleavage of this region modulates Pgp ATPase function.

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

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