Characterization of the human multidrug resistance protein containing mutations in the ATP-binding cassette signature region

Lipopolysaccharide transport involves long-range coupling between cytoplasmic and periplasmic domains of the LptB2FGC extractor

The cell surface of the Gram-negative cell envelope contains lipopolysaccharide (LPS) molecules, which form a permeability barrier against hydrophobic antibiotics. The LPS transport (Lpt) machine composed of LptB2FGCADE forms a proteinaceous trans-envelope bridge that allows for the rapid and specific transport of newly synthesized LPS from the inner membrane (IM) to the outer membrane (OM). This transport is powered from the IM by the ATP-binding cassette transporter LptB2FGC. The ATP-driven cycling between closed- and open-dimer states of the ATPase LptB2 is coupled to the extraction of LPS by the transmembrane domains LptFG. However, the mechanism by which LPS moves from a substrate-binding cavity formed by LptFG at the IM to the first component of the periplasmic bridge, the periplasmic β-jellyroll domain of LptF, is poorly understood. To better understand how LptB2FGC functions in Escherichia coli, we searched for suppressors of a defective LptB variant. We found that defects in LptB2 can be suppressed by both structural modifications to the core oligosaccharide of LPS and changes in various regions of LptFG, including a periplasmic loop in LptF that connects the substrate-binding cavity in LptFG to the periplasmic β-jellyroll domain of LptF. These novel suppressors suggest that interactions between the core oligosaccharide of LPS and periplasmic regions in the transporter influence the rate of LPS extraction by LptB2FGC. Together, our genetic data reveal a path for the bi-directional coupling between LptB2 and LptFG that extends from the cytoplasm to the entrance to the periplasmic bridge of the transporter.IMPORTANCEGram-negative bacteria are intrinsically resistant to many antibiotics due to the presence of lipopolysaccharide (LPS) at their cell surface. LPS is transported from its site of synthesis at the inner membrane to the outer membrane by the Lpt machine. Lpt proteins form a transporter that spans the entire envelope and is thought to function similarly to a PEZ candy dispenser. This trans-envelope machine is powered by the cytoplasmic LptB ATPase through a poorly understood mechanism. Using genetic analyses in Escherichia coli, we found that LPS transport involves long-ranging bi-directional coupling across cellular compartments between cytoplasmic LptB and periplasmic regions of the Lpt transporter. This knowledge could be exploited in developing antimicrobials that overcome the permeability barrier imposed by LPS.

Expression and phylogeny of multidrug resistance protein 2 and 4 in African white backed vulture (Gyps africanus)

Diclofenac toxicity in old world vultures is well described in the literature by both the severity of the toxicity induced and the speed of death. While the mechanism of toxicity remains unknown at present, the necropsy signs of gout suggests primary renal involvement at the level of the uric acid excretory pathways. From information in the chicken and man, uric acid excretion is known to be a complex process that involves a combination of glomerular filtration and active tubular excretion. For the proximal convoluted tubules excretion occurs as a two-step process with the basolateral cell membrane using the organic anion transporters and the apical membrane using the multidrug resistant protein to transport uric acid from the blood into the tubular fluid. With uric acid excretion seemingly inhibited by diclofenac, it becomes important to characterize these transporter mechanism at the species level. With no information being available on the molecular characterization/expression of MRPs of Gyps africanus, for this study we used next generation sequencing, and Sanger sequencing on the renal tissue of African white backed vulture (AWB), as the first step to establish if the MRPs gene are expressed in AWB. In silico analysis was conducted using different software to ascertain the function of the latter genes. The sequencing results revealed that the MRP2 and MRP4 are expressed in AWB vultures. Phylogeny of avian MRPs genes confirms that vultures and eagles are closely related, which could be attributed to having the same ancestral genes and foraging behavior. In silico analysis confirmed the transcribed proteins would transports anionic compounds and glucose.

LptB-LptF coupling mediates the closure of the substrate-binding cavity in the LptB2 FGC transporter through a rigid-body mechanism to extract LPS

Lipopolysaccharides (LPS) are essential envelope components in many Gram-negative bacteria and provide intrinsic resistance to antibiotics. LPS molecules are synthesized in the inner membrane and then transported to the cell surface by the LPS transport (Lpt) machinery. In this system, the ATP-binding cassette (ABC) transporter LptB₂ FGC extracts LPS from the inner membrane and places it onto a periplasmic protein bridge through a poorly understood mechanism. Here, we show that residue E86 of LptB is essential for coupling the function of this ATPase to that of its partners LptFG, specifically at the step where ATP binding drives the closure of the LptB dimer and the collapse of the LPS-binding cavity in LptFG that moves LPS to the Lpt periplasmic bridge. We also show that defects caused by changing residue E86 are suppressed by mutations altering either LPS structure or transmembrane helices in LptG. Furthermore, these suppressors also fix defects in the coupling helix of LptF, but not of LptG. Together, these results support a transport mechanism in which the ATP-driven movements of LptB and those of the substrate-binding cavity in LptFG are bi-directionally coordinated through the rigid-body coupling, with LptF's coupling helix being important in coordinating cavity collapse with LptB dimerization.

Single-nucleotide polymorphisms in a short basic motif in the ABC transporter ABCG2 disable its trafficking out of endoplasmic reticulum and reduce cell resistance to anticancer drugs

ATP-binding cassette (ABC) subfamily G member 2 (ABCG2) belongs to the ABC transporter superfamily and has been implicated in multidrug resistance of cancers. Although the structure and function of ABCG2 have been extensively studied, little is known about its biogenesis and the regulation thereof. In this study, using mutagenesis and several biochemical analyses, we show that the positive charges in the vicinity of the RKR motif downstream of the ABC signature drive trafficking of nascent ABCG2 out of the endoplasmic reticulum (ER) onto plasma membranes. Substitutions of and naturally occurring single-nucleotide polymorphisms within these positively charged residues disabled the trafficking of ABCG2 out of the ER. A representative ABCG2 variant in which the RKR motif had been altered underwent increased ER stress-associated degradation. We also found that unlike WT ABCG2, genetic ABCG2 RKR variants have disrupted normal maturation and do not reduce accumulation of the anticancer drug mitoxantrone and no longer confer resistance to the drug. We conclude that the positive charges downstream of the ABC signature motif critically regulate ABCG2 trafficking and maturation. We propose that single-nucleotide polymorphisms of these residues reduce ABCG2 expression via ER stress-associated degradation pathway and may contribute to reduced cancer drug resistance, improving the success of cancer chemotherapy.

Combining Mutations That Inhibit Two Distinct Steps of the ATP Hydrolysis Cycle Restores Wild-Type Function in the Lipopolysaccharide Transporter and Shows that ATP Binding Triggers Transport

Gram-negative bacteria are naturally resistant to many antibiotics because their surface is covered by the glycolipid LPS. Newly synthesized LPS is transported across the cell envelope by the multiprotein Lpt machinery, which includes LptB₂FGC, an unusual ABC transporter that extracts LPS from the inner membrane. Like in other ABC transporters, the LptB₂FGC transport cycle is driven by the cyclical conformational changes that a cytoplasmic, dimeric ATPase, LptB, undergoes when binding and hydrolyzing ATP. How these conformational changes are controlled in ABC transporters is poorly understood. Here, we identified two lethal changes in LptB that, when combined, remarkably restore wild-type transport function. Biochemical studies revealed that the two changes affect different steps in the transport cycle, having opposing, lethal effects on LptB’s dimerization cycle. Our work provides mechanistic details about the LptB₂FGC extractor that could be used to develop Lpt inhibitors that would overcome the innate antibiotic resistance of Gram-negative bacteria.

ATP-binding cassette (ABC) transporters constitute a large family of proteins present in all domains of life. They are powered by dynamic ATPases that harness energy from binding and hydrolyzing ATP through a cycle that involves the closing and reopening of their two ATP-binding domains. The LptB₂FGC exporter is an essential ABC transporter that assembles lipopolysaccharides (LPS) on the surface of Gram-negative bacteria to form a permeability barrier against many antibiotics. LptB₂FGC extracts newly synthesized LPS molecules from the inner membrane and powers their transport across the periplasm and through the outer membrane. How LptB₂FGC functions remains poorly understood. Here, we show that the C-terminal domain of the dimeric LptB ATPase is essential for LPS transport in Escherichia coli. Specific changes in the C-terminal domain of LptB cause LPS transport defects that can be repaired by intragenic suppressors altering the ATP-binding domains. Surprisingly, we found that each of two lethal changes in the ATP-binding and C-terminal domains of LptB, when present in combined form, suppressed the defects associated with the other to restore LPS transport to wild-type levels both in vivo and in vitro. We present biochemical evidence explaining the effect that each of these mutations has on LptB function and how the observed cosuppression results from the opposing lethal effects these changes have on the dimerization state of the LptB ATPase. We therefore propose that these sites modulate the closing and reopening of the LptB dimer, providing insight into how the LptB₂FGC transporter cycles to export LPS to the cell surface and how to inhibit this essential envelope biogenesis process.

The linker region of breast cancer resistance protein ABCG2 is critical for coupling of ATP-dependent drug transport

The ATP-binding cassette (ABC) transporters of class G display a different domain organisation than P-glycoprotein/ABCB1 and bacterial homologues with a nucleotide-binding domain preceding the transmembrane domain. The linker region connecting these domains is unique and its function and structure cannot be predicted. Sequence analysis revealed that the human ABCG2 linker contains a LSGGE sequence, homologous to the canonical C-motif/ABC signature present in all ABC nucleotide-binding domains. Predictions of disorder and of secondary structures indicated that this C2-sequence was highly mobile and located between an α-helix and a loop similarly to the C-motif. Point mutations of the two first residues of the C2-sequence fully abolished the transport-coupled ATPase activity, and led to the complete loss of cell resistance to mitoxantrone. The interaction with potent, selective and non-competitive, ABCG2 inhibitors was also significantly altered upon mutation. These results suggest an important mechanistic role for the C2-sequence of the ABCG2 linker region in ATP binding and/or hydrolysis coupled to drug efflux.

A multi-system approach assessing the interaction of anticonvulsants with P-gp

30% of epilepsy patients receiving antiepileptic drugs (AEDs) are not fully controlled by therapy. The drug transporter hypothesis for refractory epilepsy proposes that P-gp is over expressed at the epileptic focus with a role of P-gp in extruding AEDs from the brain. However, there is controversy regarding whether all AEDs are substrates for this transporter. Our aim was to investigate transport of phenytoin, lamotrigine and carbamazepine by using seven in-vitro transport models. Uptake assays in CEM/VBL cell lines, oocytes expressing human P-gp and an immortalised human brain endothelial cell line (hCMEC/D3) were carried out. Concentration equilibrium transport assays were performed in Caco-2, MDCKII ±P-gp and LLC-PK1±P-gp in the absence or presence of tariquidar, an inhibitor of P-gp. Finally, primary porcine brain endothelial cells were used to determine the apparent permeability (P_app) of the three AEDs in the absence or presence of P-gp inhibitors. We detected weak transport of phenytoin in two of the transport systems (MDCK and LLC-PK1 cells transfected with human P-gp) but not in the remaining five. No P-gp interaction was observed for lamotrigine or carbamazepine in any of the seven validated in-vitro transport models. Neither lamotrigine nor carbamazepine was a substrate for P-gp in any of the model systems tested. Our data suggest that P-gp is unlikely to contribute to the pathogenesis of refractory epilepsy through transport of carbamazepine or lamotrigine.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

Effect of cysteine mutagenesis on the function and disulfide bond formation of human ABCG2

ABCG2 is a member of the ATP-binding cassette (ABC) transporter superfamily. Its overexpression causes multidrug resistance in cancer chemotherapy. Based on its apparent half size in sequence when compared with other traditional ABC transporters, ABCG2 has been thought to exist and function as a homodimer linked by intermolecular disulfide bonds. However, recent evidence suggests that ABCG2 may exist as a higher form of oligomers due to noncovalent interactions. In this study, we attempted to create a cysless mutant ABCG2 as a tool for further characterization of this molecule. However, we found that the cysless mutant ABCG2 is well expressed but not functional. Mapping of the cysteine residues showed that three cysteine residues (Cys284, Cys374, and Cys438) are required concurrently for the function of ABCG2 and potentially for intramolecular disulfide bond formation. We also found that the cysteine residues (Cys592, Cys603, and Cys608) in the third extracellular loop are involved in forming intermolecular disulfide bonds and that mutation of these residues does not affect the expression or drug transport activity of human ABCG2. Thus, we conclude that Cys284, Cys374, and Cys438, which may be involved in intramolecular disulfide bond formation, are concurrently required for ABCG2 function, whereas Cys592, Cys603, and Cys608, potentially involved in intermolecular disulfide bond formation, are not required.

CLC-0 and CFTR: Chloride Channels Evolved From Transporters

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

The human multidrug resistance protein 4 (MRP4, ABCC4): functional analysis of a highly polymorphic gene

ABCC4 encodes multidrug resistance protein 4 (MRP4), a member of the ATP-binding cassette family of membrane transporters involved in the efflux of endogenous and xenobiotic molecules. The aims of this study were to identify single nucleotide polymorphisms of ABCC4 and to functionally characterize selected nonsynonymous variants. Resequencing was performed in a large ethnically diverse population. Ten nonsynonymous variants were selected for analysis of transport function based on allele frequencies and evolutionary conservation. The reference and variant MRP4 cDNAs were constructed by site-directed mutagenesis and transiently transfected into human embryonic kidney cells (HEK 293T). The function of MRP4 variants was compared by measuring the intracellular accumulation of two antiviral agents, azidothymidine (AZT) and adefovir (PMEA). A total of 98 variants were identified in the coding and flanking intronic regions of ABCC4. Of these, 43 variants are in the coding region, and 22 are nonsynonymous. In a functional screen of ten variants, there was no evidence for a complete loss of function allele. However, two variants (G187W and G487E) showed a significantly reduced function compared to reference with both substrates, as evidenced by higher intracellular accumulation of AZT and PMEA compared to the reference MRP4 (43 and 69% increase in accumulation for G187W compared with the reference MRP4, with AZT and PMEA, respectively). The G187W variant also showed decreased expression following transient transfection of HEK 293T cells. Further studies are required to assess the clinical significance of this altered function and expression and to evaluate substrate specificity of this functional change.

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

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

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

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

Insight in eukaryotic ABC transporter function by mutation analysis

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

The power of the pump: mechanisms of action of P-glycoprotein (ABCB1)

Members of the superfamily of ATP-binding cassette (ABC) transporters mediate the movement of a variety of substrates including simple ions, complex lipids and xenobiotics. At least 18 ABC transport proteins are associated with disease conditions. P-glycoprotein (Pgp, ABCB1) is the archetypical mammalian ABC transport protein and its mechanism of action has received considerable attention. There is strong biochemical evidence that Pgp moves molecular cargo against a concentration gradient using the energy of ATP hydrolysis. However, the molecular details of how the energy of ATP hydrolysis is coupled to transport remain in dispute and it has not been possible to reconcile the data from various laboratories into a single model. The functional unit of Pgp consists of two nucleotide binding domains (NBDs) and two trans-membrane domains which are involved in the transport of drug substrates. Considerable progress has been made in recent years in characterizing these functionally and spatially distinct domains of Pgp. In addition, our understanding of the domains has been augmented by the resolution of structures of several non-mammalian ABC proteins. This review considers: (i) the role of specific conserved amino acids in ATP hydrolysis mediated by Pgp; (ii) emerging insights into the dimensions of the drug binding pocket and the interactions between Pgp and the transport substrates and (iii) our current understanding of the mechanisms of coupling between energy derived from ATP binding and/or hydrolysis and efflux of drug substrates.

CFTR channel opening by ATP-driven tight dimerization of its nucleotide-binding domains

ABC (ATP-binding cassette) proteins constitute a large family of membrane proteins that actively transport a broad range of substrates. Cystic fibrosis transmembrane conductance regulator (CFTR), the protein dysfunctional in cystic fibrosis, is unique among ABC proteins in that its transmembrane domains comprise an ion channel. Opening and closing of the pore have been linked to ATP binding and hydrolysis at CFTR's two nucleotide-binding domains, NBD1 and NBD2 (see, for example, refs 1, 2). Isolated NBDs of prokaryotic ABC proteins dimerize upon binding ATP, and hydrolysis of the ATP causes dimer dissociation. Here, using single-channel recording methods on intact CFTR molecules, we directly follow opening and closing of the channel gates, and relate these occurrences to ATP-mediated events in the NBDs. We find that energetic coupling between two CFTR residues, expected to lie on opposite sides of its predicted NBD1-NBD2 dimer interface, changes in concert with channel gating status. The two monitored side chains are independent of each other in closed channels but become coupled as the channels open. The results directly link ATP-driven tight dimerization of CFTR's cytoplasmic nucleotide-binding domains to opening of the ion channel in the transmembrane domains. This establishes a molecular mechanism, involving dynamic restructuring of the NBD dimer interface, that is probably common to all members of the ABC protein superfamily.

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

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

Functional non-equivalence of ATP-binding cassette signature motifs in the transporter associated with antigen processing (TAP)

The transporter associated with antigen processing (TAP) is a key component of the cellular immune system. As a member of the ATP-binding cassette (ABC) superfamily, TAP hydrolyzes ATP to energize the transport of peptides from the cytosol into the lumen of the endoplasmic reticulum. TAP is composed of TAP1 and TAP2, each containing a transmembrane domain and a nucleotide-binding domain (NBD). Here we investigated the role of the ABC signature motif (C-loop) on the functional non-equivalence of the NBDs, which contain a canonical C-loop (LSGGQ) for TAP1 and a degenerate C-loop (LAAGQ) for TAP2. Mutation of the leucine or glycine (LSGGQ) in TAP1 fully abolished peptide transport. However, TAP complexes with equivalent mutations in TAP2 still showed residual peptide transport activity. To elucidate the origin of the asymmetry of the NBDs of TAP, we further examined TAP complexes with exchanged C-loops. Strikingly, the chimera with two canonical C-loops showed the highest transport rate whereas the chimera with two degenerate C-loops had the lowest transport rate, demonstrating that the ABC signature motifs control peptide transport efficiency. All single site mutants and chimeras showed similar activities in peptide or ATP binding, implying that these mutations affect the ATPase activity of TAP. In addition, these results prove that the serine of the C-loop is not essential for TAP function but rather coordinates, together with other residues of the C-loop, the ATP hydrolysis in both nucleotide-binding sites.

Function of the ABC signature sequences in the human multidrug resistance protein 1

Human multidrug resistance protein 1 (MRP1) is a membrane ATP-binding cassette transporter that confers multidrug resistance to tumor cells by effluxing intracellular drugs in an ATP-dependent manner. The mechanisms by which transport occurs and by which ATP hydrolysis is coupled to drug transport are not fully elucidated. In particular, the function of the signature sequences in the nucleotide binding domains (NBDs) of MRP1 is unknown. We therefore investigated the effect of mutation of the signature sequences (G771D and G1433D) and of the Walker A motifs (K684M and K1333M) in the NBDs on the 8-azido-[alpha-32P]ATP photolabeling and 8-azido-[alpha-32P]ADP vanadate trapping of MRP1. Both mutations in the Walker A motif almost completely inhibited the labeling of the mutated NBD with 8-azido-[alpha-32P]ATP but not the labeling of the other intact NBD. In contrast, the G771D mutation in the signature sequence of NBD1 enhanced the labeling of NBD1 but slightly decreased the labeling of NBD2. The G1433D mutation in the signature motif of NBD2 enhanced the labeling of NBD2 but did not affect the labeling of NBD1. These effects were all substrate-independent. Photolabeling of NBD2 and a very slight photolableing of NBD1 were detectable under vanadate trapping conditions with 8-azido-[alpha-32P]ATP. Trapping at both NBD1 and NBD2 was almost completely inhibited by K684M and K1333M mutations and by the K684M/K1333M double mutation. The G771D mutation completely inhibited trapping at NBD2 and considerably inhibited trapping at NBD1. However, whereas the G1433D mutation also considerably inhibited trapping at NBD1, it only partially inhibited trapping of NBD2, and the trapping could still be enhanced by leukotriene C4. Our findings suggest that both signature sequences of MRP1 are involved in ATP hydrolysis and must be intact for the ATP hydrolysis and the transport by MRP1.

Communication between the nucleotide binding domains of P-glycoprotein occurs via conformational changes that involve residue 508

Our aim is to provide molecular understanding of the mechanisms underlying the (i) interaction between the two nucleotide binding domains (NBDs) and (ii) coupling between NBDs and transmembrane domains within P-glycoprotein (Pgp) during a transport cycle. To facilitate this, we have introduced a number of unique cysteine residues at surface exposed positions (E393C, S452C, I500C, N508C, and K578C) in the N-terminal NBD of Pgp, which had previously been engineered to remove endogenous cysteines. Positions of the mutations were designed using a model based on crystallographic features of prokaryotic NBDs. The single cysteine mutants were expressed in insect cells using recombinant baculovirus and the proteins purified by metal affinity chromatography by virtue of a polyhistidine tag. None of the introduced cysteine residues perturbed the function of Pgp as judged by the characteristics of drug stimulated ATP hydrolysis. The role of residues at each of the introduced sites in the catalytic cycle of Pgp was investigated by the effect of covalent conjugation with N-ethyl-maleimide (NEM). All but one mutation (K578C) was accessible to labeling with [(3)H]-NEM. However, perturbation of ATPase activity was only observed for the derivitized N508C isoform. The principle functional manifestation was a marked inhibition of the "basal" rate of ATP hydrolysis. Neither the extent nor potency to which a range of drugs could affect the ATPase activity were altered in the NEM conjugated N508C isoform. The results imply that the accessibility of residue 508, located in the alpha-helical subdomain of NBD1 in Pgp, is altered by the conformational changes that occur during ATP hydrolysis.

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 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.

Role of the N-terminal transmembrane region of the multidrug resistance protein MRP2 in routing to the apical membrane in MDCKII cells

In polarized cells, the multidrug resistance protein MRP2 is localized in the apical plasma membrane, whereas MRP1, another multidrug resistance protein (MRP) family member, is localized in the basolateral membrane. MRP1 and MRP2 are thought to contain an N-terminal region of five transmembrane segments (TMD(0)) coupled to 2 times six transmembrane segments via an intracellular loop (L(0)). We previously demonstrated for MRP1 that a mutant lacking TMD(0) but still containing L(0), called L(0)DeltaMRP1, was functional and routed to the lateral plasma membrane. To investigate the role of the TMD(0)L(0) region of MRP2 in routing to the apical membrane, we generated mutants similar to those made for MRP1. In contrast to L(0)DeltaMRP1, L(0)DeltaMRP2 was associated with an intracellular compartment, most likely endosomes. Co-expression with TMD(0), however, resulted in apical localization of L(0)DeltaMRP2 and transport activity. Uptake experiments with vesicles containing L(0)DeltaMRP2 demonstrated that the molecule is able to transport LTC(4). An MRP2 mutant without TMD(0)L(0), DeltaMRP2, was only core-glycosylated and localized intracellularly. Co-expression of DeltaMRP2 with TMD(0)L(0) resulted in an increased protein level of DeltaMRP2, full glycosylation of the protein, routing to the apical membrane, and transport activity. Our results suggest that the TMD(0) region is required for routing to or stable association with the apical membrane.

Mutations in the linker domain of NBD2 of SUR inhibit transduction but not nucleotide binding

ATP-sensitive potassium (K(ATP)) channels are composed of an ATP-binding cassette (ABC) protein (SUR1, SUR2A or SUR2B) and an inwardly rectifying K(+) channel (Kir6.1 or Kir6.2). Like other ABC proteins, the nucleotide binding domains (NBDs) of SUR contain a highly conserved "signature sequence" (the linker, LSGGQ) whose function is unclear. Mutation of the conserved serine to arginine in the linker of NBD1 (S1R) or NBD2 (S2R) did not alter the ability of ATP or ADP (100 microM) to displace 8-azido-[(32)P]ATP binding to SUR1, or abolish ATP hydrolysis at NBD2. We co-expressed Kir6.2 with wild-type or mutant SUR in Xenopus oocytes and recorded the resulting currents in inside-out macropatches. The S1R mutation in SUR1, SUR2A or SUR2B reduced K(ATP) current activation by 100 microM MgADP, whereas the S2R mutation in SUR1 or SUR2B (but not SUR2A) abolished MgADP activation completely. The linker mutations also reduced (S1R) or abolished (S2R) MgATP-dependent activation of Kir6.2-R50G co-expressed with SUR1 or SUR2B. These results suggest that the linker serines are not required for nucleotide binding but may be involved in transducing nucleotide binding into channel activation.

The use of a novel taxane-based P-glycoprotein inhibitor to identify mutations that alter the interaction of the protein with paclitaxel

Murine thymoma cell lines expressing mutated forms of the mdr1b P-glycoprotein were isolated using a novel taxane-based P-glycoprotein inhibitor tRA-96023 (SB-RA-31012). The selection strategy required resistance to a combination of tRA-96023 and colchicine. Five mutations were identified (N350I, I862F, L865F, L868W, and A933T) that reduce the capacity of tRA-96023 to inhibit P-glycoprotein-dependent drug resistance. These mutations also result in a loss of paclitaxel resistance ranging from 47 to 100%. Four mutations are located in the second half of the protein, within or near the proposed transmembrane segment (TMS) 10--11 regions. The fifth mutation (N350I) is within the first half of the protein, proximal (cytoplasmic) to TMS 6. The variant cell line expressing the L868W mutation was subjected to a second round of selection involving tRA-96023 and the toxic drug puromycin. This resulted in the isolation of a cell line expressing a P-glycoprotein with a double mutation. The additional mutation (N988D) is located within TMS 12 and conveys further decreases in resistance to paclitaxel and the capacity of tRA-96023 to inhibit drug resistance. Taken together, the results indicate a significant contribution by the TMS 10--12 portion of the protein to the recognition and transport of taxanes and give evidence that the cytoplasmic region proximal to TMS 6 also plays a role in taxane interactions with P-glycoproteins. Interestingly, mutations within TMS 6 and 12 were found to cause a partial loss of PSC-833 inhibitor activity, suggesting that these regions participate in the interactions with cyclosporin and its derivatives.

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

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

P-Glycoprotein conformational changes detected by antibody competition

Conformational changes accompanying P-glycoprotein (Pgp) mediated drug transport are reflected by changes in the avidity of certain monoclonal antibodies (mAbs). More of the UIC2 mAb binds to Pgp-expressing cells in the presence of substrates or modulators [Mechetner, E.B., Schott, B., Morse, S.B., Stein, W., Druley, T., Dvis, K.A., Tsuruo, T. & Roninson, I.B. (1997) Proc. Natl Acad. Sci. USA 94, 12908-12913], while the binding of other mAbs (e.g. MM12.10, MRK16, 4E3) is not conformation sensitive. Pre-staining of Pgp+ cells with UIC2 decreased the subsequent binding of MM12.10 mAb by about 30-40%, suggesting that there are Pgp molecules available for both UIC2 and MM12.10, and others accessible only for MM12.10. In the presence of certain substrates/modulators such as vinblastin, cyclosporin A or valinomycin, the MM12.10 reactivity was completely abolished by preincubation with UIC2. However, verapamil, Tween-80 and nifedipine did not influence the ratio of bound mAbs significantly. This is the first assay to our knowledge, sharply distinguishing two classes of modulators. The conformational changes accompanying the mAb competition phenomenon appear to be closely related, though not identical to those accompanying the UIC2-shift, as suggested by the simultaneous assessment of the two phenomena.

FRET analysis indicates that the two ATPase active sites of the P-glycoprotein multidrug transporter are closely associated

Members of the ABC superfamily carry out the transport of various molecules and ions across cellular membranes, powered by ATP hydrolysis. Substantial evidence indicates that the two catalytic sites of the nucleotide binding domains function in a highly cooperative, alternating sites mode, which suggests the possibility that they interact with each other physically. In this study, fluorescence energy transfer experiments were used to estimate the distance between two fluors, each covalently linked to a highly conserved Cys residue (Cys428 and Cys1071) within the Walker A motif of the catalytic site. The vanadate.ADP.Mg(2+) complex was trapped in one catalytic site of membrane-bound or highly purified P-glycoprotein, and the other site was labeled with MIANS [2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid]. Following loss of the trapped vanadate complex, the newly vacant site was then labeled with NBD-Cl (7-chloro-4-nitrobenzo-2-oxa-1,3-diazole). The fluorescence properties of the singly labeled P-glycoproteins showed that no energy transfer occurred between MIANS (the donor) and NBD (the acceptor) when they were simply mixed together. On the other hand, the fluorescence emission of the MIANS group in doubly labeled P-glycoprotein was highly quenched as a result of energy transfer to NBD, leading to an estimate of a donor-acceptor separation distance of approximately 16 A for P-glycoprotein labeled in the native plasma membrane and approximately 22 A for P-glycoprotein labeled in detergent solution. The separation of the two fluorophores is compatible with the recently reported crystal structure of the Rad50cd dimer, but not with that of the HisP dimer. These results suggest that the two catalytic sites of the P-glycoprotein nucleotide binding domains are relatively close together, which would facilitate cooperation between them during the catalytic cycle.

ABC drug transporters: hereditary polymorphisms and pharmacological impact in MDR1, MRP1 and MRP2

Transport by ATP-dependent efflux pumps, such as P-glycoprotein (PGP) and multi-drug resistance related proteins (MRPs), influences bioavailability and disposition of drugs. These efflux pumps serve as defence mechanisms and determine bioavailability and CNS concentrations of many drugs. However, despite the fact that substantial data have been accumulated on the structure, function and pharmacological role of ABC transporters and even though modification of PGP function is an important mechanism of drug interactions and adverse effects in humans, there is a striking lack of data on variability of the underlying genes. This review focuses on the human drug transporter proteins PGP (MDR1) and the multi-drug resistance proteins MRP1 and MRP2. An overview is provided of pharmacologically relevant genetic, structural and functional data as well as on hereditary polymorphisms, their phenotypical consequences and pharmacological implications.

Vectors with hidden cloning sites

A general strategy is described for using the cleavage site of restriction enzymes in vectors for cloning regardless of how many sites the given enzymes have in the vector. The application of this method allows one to open any vector at its cloning site with protruding ends which can be compatible with almost every commercially available Class II restriction enzyme. By employing this method, the laborious construction of new vectors can be simplified considerably. This general strategy is based on the known ability of Class IIS restriction enzymes to cut any sequence located outside of their recognition site; the introduction of a linker containing recognition site(s) for Class IIS restriction enzyme(s), not present originally in the vector, gives rise to the possibility of opening the vector so as to produce overhangs of arbitrary sequence. In particular, when a symmetrical short sequence representing the protruding end of any Class II enzyme is situated at the cutting position of the Class IIS enzyme, cleavage with the Class IIS enzyme exposes the hitherto hidden, "unique" cloning site. This technique is demonstrated by cloning the cDNA of the multidrug resistance protein to an expression vector.

The detergent-soluble maltose transporter is activated by maltose binding protein and verapamil

The maltose transporter FGK2 complex of Escherichia coli was purified with the aid of a glutathione S-transferase molecular tag. In contrast to the membrane-associated form of the complex, which requires liganded maltose binding protein (MBP) for ATPase activity, the purified detergent-soluble complex exhibited a very high level of ATPase activity. This uncoupled activity was not due to dissociation of the MalK ATPase subunit from the integral membrane protein MalF and MalG subunits. The detergent-soluble ATPase activity of the complex could be further stimulated by wild-type MBP but not by a signaling-defective mutant MBP. Wild-type MBP increased the V(max) of the ATPase 2.7-fold but had no effect on the K(m) of the enzyme for ATP. When the detergent-soluble complex was reconstituted in proteoliposomes, it returned to being dependent on MBP for activation of ATPase, consistent with the idea that the structural changes induced in the complex by detergent that result in activation of the ATPase are reversible. The uncoupled ATPase activity resembled the membrane-bound activity of the complex also with respect to sensitivity to NaN(3), as well as a mercurial, p-chloromercuribenzosulfonic acid. Verapamil, a compound that activates the ATPase activity of the multiple drug resistance P-glycoprotein, activated the maltose transporter ATPase as well. The activation of this bacterial transporter by verapamil suggests that a structural feature that is conserved among both eukaryotic and prokaryotic ATP binding cassette transporters is responsible for this activation.

Determining the structure and mechanism of the human multidrug resistance P-glycoprotein using cysteine-scanning mutagenesis and thiol-modification techniques

The multidrug resistance P-glycoprotein is an ATP-dependent drug pump that extrudes a broad range of hydrophobic compounds out of cells. Its physiological role is likely to protect us from exogenous and endogenous toxins. The protein is important because it contributes to the phenomenon of multidrug resistance during AIDS and cancer chemotherapy. We have used cysteine-scanning mutagenesis and thiol-modification techniques to map the topology of the protein, show that both nucleotide-binding domains are essential for activity, examine packing of the transmembrane segments, map the drug-binding site, and show that there is cross-talk between the ATP-binding sites and the transmembrane segments.

Functional consequences of mutations in the conserved 'signature sequence' of the ATP-binding-cassette protein MalK

The binding-protein-dependent maltose-transport system of enterobacteria, a member of the ATP-binding-cassette (ABC) transporter superfamily, is composed of two integral membrane proteins, MalF and MalG, and two copies of an ATPase subunit, MalK, which hydrolyze ATP, thus energizing the translocation process. Isolated MalK displays spontaneous ATPase activity, whereas in the assembled MalFGK2 complex, reconstituted in liposomes, ATP hydrolysis requires stimulation by the substrate-loaded extracellular maltose-binding protein, MalE. The ATPase domains of ABC transporters, including MalK, share a unique sequence motif ('LSGGQ', 'signature sequence' or 'linker peptide') with as yet unknown function. To elucidate its role in the transport process, we investigated the consequences of mutations affecting two highly conserved residues (G137, Q140) in the MalK-ATPase of Salmonella typhimurium, by biochemical means. Residues corresponding to Q140 in other ABC proteins have not yet been studied. All mutant alleles (G137--> A, V, T; Q140--> L, K, N) fail to restore a functional transport complex in vivo. In addition, the mutations increase the repressing activity of MalK on other maltose-regulated genes when compared with wild-type MalK. Purified variants of G137 have lost the ability to hydrolyze ATP but still display nucleotide-binding activity, albeit with reduced affinity. Binding of MgATP results in similar protection against trypsin, as observed with wild-type, indicating no major change in protein structure. In contrast, the variants of Q140 differ in their properties, depending on the chemical nature of the replacement residue. MalKQ140L fails to hydrolyze ATP and exhibits a strong intrinsic resistance to trypsin in the absence of MgATP, suggesting a drastically altered conformation. In contrast, the purified mutant proteins Q140K and Q140N display ATPase activities and MgATP-induced changes in the tryptic cleavage pattern similar to those of wild-type. However, mutant transport complexes containing the Q140K or Q140N variants, when studied in proteoliposomes, are severely impaired in MalE-maltose-stimulated ATPase activity. These results are discussed with respect to the crystal structure of the homologous HisP protein [Hung, L.-W., Wang, I.X., Nikaido, K., Liu, P.-Q., Ames, G.F.-L. & Kim, S.-H. (1998) Nature (London) 396, 703-707] and are interpreted in favor of a role of the signature sequence in activating the hydrolyzing activity of MalK upon substrate-initiated conformational changes in MalF/MalG.

Subunit interactions in ABC transporters: towards a functional architecture

The ABC superfamily is a diverse group of integral membrane proteins involved in the ATP-dependent transport of solutes across biological membranes in both prokaryotes and eukaryotes. Although ABC transporters have been studied for over 30 years, very little is known about the mechanism by which the energy of ATP hydrolysis is used to transport substrate across the membrane. The recent report of the high resolution crystal structure of HisP, the nucleotide-binding subunit of the histidine permease complex of Salmonella typhimurium, represents a significant breakthrough toward the elucidation of the mechanism of solute translocation by ABC transporters. In this review, we use data from the crystallographic structures of HisP and other nucleotide-binding proteins, combined with sequence analysis of a subset of atypical ABC transporters, to argue a new model for the dimerisation of the nucleotide-binding domains that embraces the notion that the C motif from one subunit forms part of the ATP-binding site in the opposite subunit. We incorporate this dimerisation of the ATP-binding domains into our recently reported beta-barrel model for P-glycoprotein and present a general model for the cooperative interaction of the two nucleotide-binding domains and the translocation of mechanical energy to the transmembrane domains in ABC transporters.

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

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

Functional domain analysis of the yeast ABC transporter Ycf1p by site-directed mutagenesis

The yeast cadmium factor (Ycf1p) is a vacuolar protein involved in resistance to Cd(2+) and to exogenous glutathione S-conjugate precursors in yeast. It belongs to the superfamily of ATP binding cassette transporters, which includes the human cystic fibrosis transmembrane conductance regulator and the multidrug resistance-associated protein. To examine the functional significance of conserved amino acid residues in Ycf1p, we performed an extensive mutational analysis. Twenty-two single amino acid substitutions or deletions were generated by site-directed mutagenesis in the nucleotide binding domains, the proposed regulatory domain, and the fourth cytoplasmic loop. Mutants were analyzed phenotypically by measuring their ability to grow in the presence of Cd(2+). Expression and subcellular localization of the mutant proteins were examined by immunodetection in vacuolar membranes. For functional characterization of the Ycf1p variants, the kinetic parameters of glutathione S-conjugated leukotriene C(4) transport were measured. Our analysis shows that residues Ile(711), Leu(712), Phe(713), Glu(927), and Gly(1413) are essential for Ycf1p expression. Five other amino acids, Gly(663), Gly(756), Asp(777), Gly(1306), and Gly(1311), are critical for Ycf1p function, and two residues, Glu(709) and Asp(821), are unnecessary for Ycf1p biogenesis and function. We also identify several regulatory domain mutants in which Cd(2+) tolerance of the mutant strain and transport activity of the protein are dissociated.

Identification of P-glycoprotein mutations causing a loss of steroid recognition and transport

P-glycoproteins transport a wide variety of hydrophobic compounds out of cells. While the diversity of transported molecules suggests a mechanism involving broad specificity, there is evidence of significant discrimination within given classes of molecules. One example of this behavior is transport of corticosteroids by the murine mdr1 P-glycoprotein. The presence of hydroxyl groups, associated with specific steroid carbon atoms, regulates the ability of corticosteroids to be transported. This specificity is demonstrated here by experiments measuring the ability of steroids to inhibit drug transport. The results indicate that a keto oxygen associated with the 3- and 20-carbon atoms, as well as a 17-carbon hydroxyl group, each acts to enhance steroidal P-glycoprotein inhibitory activity. Moreover, inhibitory steroids can be used for directed selection of variant cells, expressing mutated P-glycoproteins with a severely impaired ability to transport dexamethasone. The five mutations, reported here, are located within transmembrane domains 4-6, proximal to the cytoplasmic interface. The altered P-glycoproteins exhibit reduced capacity to be inhibited by specific steroids, suggesting decreased capacity to bind these molecules avidly. Studies comparing the relative inhibitory activity of a series of steroids indicate that these mutations alter recognition of the 17alpha-hydroxyl group and the 20-keto oxygen atom.

Mutations in the white gene of Drosophila melanogaster affecting ABC transporters that determine eye colouration

The white, brown and scarlet genes of Drosophila melanogaster encode proteins which transport guanine or tryptophan (precursors of the red and brown eye colour pigments) and belong to the ABC transporter superfamily. Current models envisage that the white and brown gene products interact to form a guanine specific transporter, while white and scarlet gene products interact to form a tryptophan transporter. In this study, we report the nucleotide sequence of the coding regions of five white alleles isolated from flies with partially pigmented eyes. In all cases, single amino acid changes were identified, highlighting residues with roles in structure and/or function of the transporters. Mutations in w(cf) (G589E) and w(sat) (F590G) occur at the extracellular end of predicted transmembrane helix 5 and correlate with a major decrease in red pigments in the eyes, while brown pigments are near wild-type levels. Therefore, those residues have a more significant role in the guanine transporter than the tryptophan transporter. Mutations identified in w(crr) (H298N) and w(101) (G243S) affect amino acids which are highly conserved among the ABC transporter superfamily within the nucleotide binding domain. Both cause substantial and similar decreases of red and brown pigments indicating that both tryptophan and guanine transport are impaired. The mutation identified in w(Et87) alters an amino acid within an intracellular loop between transmembrane helices 2 and 3 of the predicted structure. Red and brown pigments are reduced to very low levels by this mutation indicating this loop region is important for the function of both guanine and tryptophan transporters.

Role of nucleotides and peptide substrate for stability and functional state of the human ABC family transporters associated with antigen processing

The transporters associated with antigen processing (TAP) belong to the family of ATP-binding cassette (ABC) transporters which share structural organization and use energy provided by ATP to translocate a large variety of solutes across cellular membranes. TAP is thought to hydrolyze ATP in order to deliver peptides to the endoplasmic reticulum where they can assemble with major histocompatibility complex class I molecules. However, initial binding of peptide substrates to TAP has been suggested to be ATP-independent. In this study, the effect of temperature, energetic nucleotides, and peptide on conformation and functional capacity of TAP proteins was examined. Incubation of insect cell microsomes overexpressing human TAP complexes or of human B cell microsomes at 37 degrees C induced a rapid and irreversible structural change that reduced dramatically TAP reactivity with antibodies to transmembrane and nucleotide-binding domains and abolished peptide binding and transport by TAP. These alterations were inhibited almost completely by di- or trinucleotides, and partially by high affinity peptides, suggesting that complete nucleotide dissociation inactivates TAP complexes. Experiments with isolated TAP subunits and fragments suggested that TAP complex stabilization by nucleotides may depend on their binding to the TAP1 subunit. Thus, the cellular level of functional TAP complexes may be regulated by nucleotide concentrations. It is speculated that this regulation may serve to prevent induction of autoimmunity by stressed cells with low energy levels.

Control of CFTR channel gating by phosphorylation and nucleotide hydrolysis

Control of CTFR Channel Gating by Phosphorylation and Nucleotide Hydrolysis. Physiol. Rev. 79, Suppl.: S77-S107, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is the protein product of the gene defective in cystic fibrosis, the most common lethal genetic disease among Caucasians. Unlike any other known ion channel, CFTR belongs to the ATP-binding cassette superfamily of transporters and, like all other family members, CFTR includes two cytoplasmic nucleotide-binding domains (NBDs), both of which bind and hydrolyze ATP. It appears that in a single open-close gating cycle, an individual CFTR channel hydrolyzes one ATP molecule at the NH2-terminal NBD to open the channel, and then binds and hydrolyzes a second ATP molecule at the COOH-terminal NBD to close the channel. This complex coordinated behavior of the two NBDs is orchestrated by multiple protein kinase A-dependent phosphorylation events, at least some of which occur within the third large cytoplasmic domain, called the regulatory domain. Two or more kinds of protein phosphatases selectively dephosphorylate distinct sites. Under appropriately controlled conditions of progressive phosphorylation or dephosphorylation, three functionally different phosphoforms of a single CFTR channel can be distinguished on the basis of channel opening and closing kinetics. Recording single CFTR channel currents affords an unprecedented opportunity to reproducibly examine, and manipulate, individual ATP hydrolysis cycles in a single molecule, in its natural environment, in real time.

Mutations in the nucleotide-binding sites of P-glycoprotein that affect substrate specificity modulate substrate-induced adenosine triphosphatase activity

The amino- and carboxy-terminal nucleotide-binding domains (NBD1 and NBD2) of P-glycoprotein (P-gp) share over 80% sequence identity. Almost all of NBD1 can be exchanged by corresponding NBD2 segments with no significant loss of function, except for a small segment around the Walker B motif. Within this segment, we identified two sets of residues [ERGA --> DKGT (522-525) and T578C] that, when replaced by their NBD2 counterparts, cause dramatic alterations of the substrate specificity of the protein [Beaudet, L., and Gros, P. (1995) J. Biol. Chem. 270, 17159-17170]. We wished to gain insight into the molecular basis of this defect. For this, we overexpressed the wild-type mouse Mdr3 and variants bearing single or double mutations at these positions in the yeast Pichia pastoris. P-gp-specific ATPase activity was measured in yeast plasma membrane preparations after detergent solubilization and reconstitution in Escherichia coli proteoliposomes. P-gp proteoliposomes from P. pastoris showed a strong verapamil- and valinomycin-stimulated ATPase activity, with characteristics (KM, Vmax) similar to those measured in mammalian cells. Mutations did not appear to affect the KM for Mg2+ATP ( approximately 0.4 mM), but maximum velocity (Vmax) of the drug-stimulated ATPase activity was severely affected in a substrate/modulator-specific fashion. Indeed, all mutants showed complete loss of verapamil-induced ATPase, while all retained at least some degree of valinomycin-induced ATPase activity. Photolabeling studies with [125I]iodoarylazidoprazosin, including competition with MDR drugs and modulators, suggested that drug binding was not affected in the mutants. The altered drug resistance profiles of the ERGA --> DKGT(522-525) and T578C mutants in vivo, together with the observed alterations in substrate-induced ATPase activity of these proteins, suggest that the residues involved may form part of a signal pathway between the membrane regions (substrate binding) and the ATP binding sites.

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

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