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

In vivo saturation of the transport of vinblastine and colchicine by P-glycoprotein at the rat blood-brain barrier

PURPOSE

To determine concentration-dependent P-gp-mediated efflux across the luminal membrane of endothelial cells at the blood-brain barrier (BBB) in rats.

METHODS

The transport of radiolabeled colchicine and vinblastine across the rat BBB was measured with or without PSC833, a well known P-gp inhibitor, and within a wide range of colchicine and vinblastine concentration by an in situ brain perfusion. Thus, the difference of brain transport achieved with or without PSC833 gives the P-gp-mediated efflux component of the compound transported through the rat BBB. Cerebral vascular volume was determined by coperfusion with labeled sucrose in all experiments.

RESULTS

Sucrose perfusion indicated that the vascular space was close to normal in all the studies, indicating that the BBB remained intact. P-gp limited the uptake of both colchicine and vinblastine, but the compounds differ in that vinblastine inhibited its own transport. Vinblastine transport was well fitted by a Hill equation giving IC50 at approximately 71 microM, a Hill coefficient (n) approximately 2, and a maximal efflux velocity Jmax of approximately 9 pmol s(-1) g(-1) of brain.

CONCLUSIONS

P-gp at the rat BBB may carry out both capacity-limited and capacity-unlimited transport, depending on the substrate, with pharmacotoxicologic significance for drug brain disposition and risk of drug-drug interactions.

Biochemical basis of polyvalency as a strategy for enhancing the efficacy of P-glycoprotein (ABCB1) modulators: stipiamide homodimers separated with defined-length spacers reverse drug efflux with greater efficacy

Human P-glycoprotein (Pgp) is as an ATP-dependent efflux pump for a variety of chemotherapeutic drugs. The aim of this study is to evaluate whether Pgp modulators can be engineered to exhibit high-affinity binding using polyvalency. Five bivalent homodimeric polyenes based on stipiamide linked with polyethylene glycol ethers in the range of 3-50 A were synthesized and quantitatively characterized for their effect on Pgp function. The stipiamide homodimers displaced [(125)I]iodoarylazidoprazoin (IAAP), an analogue of the Pgp substrate prazosin. A minimal spacer of 11 A is necessary for inhibition of IAAP labeling, beyond which there is an inverse correlation between the length of the spacer and the IC(50) for the displacement of IAAP. ATP hydrolysis by Pgp on the other hand is stimulated by the dimers with spacers of up to 22 A, whereas dimers with longer spacers inhibit ATP hydrolysis. Finally, the homodimers reverse Pgp-mediated drug efflux in intact cells overexpressing Pgp, and 11 A is a threshold beyond which the effectiveness of the homodimers increases exponentially and levels off at 33 A. We demonstrate that dimerization and identification of an optimal spacer length increase by 11-fold the affinity of stipiamide, and this is reflected in the efficacy with which Pgp-mediated drug efflux is reversed. These results suggest that polyvalency could be a useful strategy for the development of more potent Pgp modulators.

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

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

The molecular basis of the action of disulfiram as a modulator of the multidrug resistance-linked ATP binding cassette transporters MDR1 (ABCB1) and MRP1 (ABCC1)

The overexpression of multidrug resistance protein 1 (MDR1) and multidrug resistance protein 1 (MRP1) gene products is a major cause of multidrug resistance in cancer cells. A recent study suggested that disulfiram, a drug used to treat alcoholism, might act as a modulator of P-glycoprotein. In this study, we investigated the molecular and chemical basis of disulfiram as a multidrug resistance modulator. We demonstrate that in intact cells, disulfiram reverses either MDR1- or MRP1-mediated efflux of fluorescent drug substrates. Disulfiram inhibits ATP hydrolysis and the binding of [alpha-32P]8-azidoATP to P-glycoprotein and MRP1, with inhibition curves comparable with those of N-ethylmaleimide, a cysteine-modifying agent. However, if the ATP sites are protected with excess ATP, disulfiram stimulates ATP hydrolysis by both transporters in a concentration-dependent manner. Thus, in addition to modifying cysteines at the ATP sites, disulfiram may interact with the drug-substrate binding site. We demonstrate that disulfiram, but not N-ethylmaleimide, inhibits in a concentration-dependent manner the photoaffinity labeling of the multidrug transporter with 125I-iodoarylazidoprazosin and [3H]azidopine. This suggests that the interaction of disulfiram with the drug-binding site is independent of its role as a cysteine-modifying agent. Finally, we have exploited MRP4 (ABCC4) to demonstrate that disulfiram can inhibit ATP binding by forming disulfide bonds between cysteines located in the vicinity of, although not in, the active site. Taken together, our results suggest that disulfiram has unique molecular interactions with both the ATP and/or drug-substrate binding sites of multiple ATP binding cassette transporters, which are associated with drug resistance, and it is potentially an attractive agent to combat multidrug resistance.

The role of ABC transporters in clinical practice

Drug resistance remains one of the primary causes of suboptimal outcomes in cancer therapy. ATP-binding cassette (ABC) transporters are a family of transporter proteins that contribute to drug resistance via ATP-dependent drug efflux pumps. P-glycoprotein (P-gp), encoded by the MDR1 gene, is an ABC transporter normally involved in the excretion of toxins from cells. It also confers resistance to certain chemotherapeutic agents. P-gp is overexpressed at baseline in chemotherapy-resistant tumors, such as colon and kidney cancers, and is upregulated after disease progression following chemotherapy in malignancies such as leukemia and breast cancer. Other transporter proteins mediating drug resistance include those in the multidrug-resistance-associated protein (MRP) family, notably MRP1, and ABCG2. These transporters are also involved in normal physiologic functions. The expressions of MRP family members and ABCG2 have not been well worked out in cancer. Increased drug accumulation and drug resistance reversal with P-gp inhibitors have been well documented in vitro, but only suggested in clinical trials. Limitations in the design of early resistance reversal trials contributed to disappointing results. Despite this, three randomized trials have shown statistically significant benefits with the use of a P-gp inhibitor in combination with chemotherapy. Improved diagnostic techniques aimed at the selection of patients with tumors that express P-gp should result in more successful outcomes. Further optimism is warranted with the advent of potent, nontoxic inhibitors and new treatment strategies, including the combination of new targeted therapies with therapies aimed at the prevention of drug resistance.

The distinct nucleotide binding states of the transporter associated with antigen processing (TAP) are regulated by the nonhomologous C-terminal tails of TAP1 and TAP2

The transporter associated with antigen processing (TAP) delivers peptides into the lumen of the endoplasmic reticulum for binding onto major histocompatibility complex class I molecules. TAP comprises two polypeptides, TAP1 and TAP2, each with an N-terminal transmembrane domain and a C-terminal cytosolic nucleotide binding domain (NBD). The two NBDs have distinct intrinsic nucleotide binding properties. In the resting state of TAP, the NBD1 has a much higher binding activity for ATP than the NBD2, while the binding of ADP to the two NBDs is equivalent. To attribute the different nucleotide binding behaviour of NBD1 and NBD2 to specific sequences, we generated chimeric TAP1 and TAP2 polypeptides in which either the nonhomologous C-terminal tails downstream of the Walker B motif, or the core NBDs which are enclosed by the conserved Walker A and B motifs, were reciprocally exchanged. Our biochemical and functional studies on the different TAP chimeras show that the distinct nucleotide binding behaviour of TAP1 and TAP2 is controlled by the nonhomologous C-terminal tails of the two TAP chains. In addition, our data suggest that the C-terminal tail of TAP2 is required for a functional transporter by regulating ATP binding. Further experiments indicate that ATP binding to NBD2 is important because it prevents simultaneous uptake of ATP by TAP1. We propose that the C-terminal tails of TAP1 and TAP2 play a crucial regulatory role in the coordination of nucleotide binding and ATP hydrolysis by TAP.

Imaging reversal of multidrug resistance in living mice with bioluminescence: MDR1 P-glycoprotein transports coelenterazine

Coelenterazine is widely distributed among marine organisms, producing bioluminescence by calcium-insensitive oxidation mediated by Renilla luciferase (Rluc) and calcium-dependent oxidation mediated by the photoprotein aequorin. Despite its abundance in nature and wide use of both proteins as reporters of gene expression and signal transduction, little is known about mechanisms of coelenterazine transport and cell permeation. Interestingly, coelenterazine analogues share structural and physiochemical properties of compounds transported by the multidrug resistance MDR1 P-glycoprotein (Pgp). Herein, we report that living cells stably transfected with a codon-humanized Rluc show coelenterazine-mediated bioluminescence in a highly MDR1 Pgp-modulated manner. In Pgp-expressing Rluc cells, low baseline bioluminescence could be fully enhanced (reversed) to non-Pgp matched control levels with potent and selective Pgp inhibitors. Therefore, using coelenterazine and noninvasive bioluminescence imaging in vivo, we could directly monitor tumor-specific Pgp transport inhibition in living mice. While enabling molecular imaging and high-throughput screening of drug resistance pathways, these data also raise concern for the indiscriminate use of Rluc and aequorin as reporters in intact cells or transgenic animals, wherein Pgp-mediated alterations in coelenterazine permeability may impact results.

2-Deoxy-d-glucose Increases the Efficacy of Adriamycin and Paclitaxel in Human Osteosarcoma and Non-Small Cell Lung Cancers In Vivo

Slow-growing cell populations located within solid tumors are difficult to target selectively because most cells in normal tissues also have low replication rates. However, a distinguishing feature between slow-growing normal and tumor cells is the hypoxic microenvironment of the latter, which makes them extraordinarily dependent on anaerobic glycolysis for survival. Previously, we have shown that hypoxic tumor cells exhibit increased sensitivity to inhibitors of glycolysis in three distinct in vitro models. Based on these results, we predicted that combination therapy of a chemotherapeutic agent to target rapidly dividing cells and a glycolytic inhibitor to target slow-growing tumor cells would have better efficacy than either agent alone. Here, we test this strategy in vivo using the glycolytic inhibitor 2-deoxy-D-glucose (2-DG) in combination with Adriamycin (ADR) or paclitaxel in nude mouse xenograft models of human osteosarcoma and non-small cell lung cancer. Nude mice implanted with osteosarcoma cells were divided into four groups as follows: (a) untreated controls; (b) mice treated with ADR alone; (c) mice treated with 2-DG alone; or (d) mice treated with a combination of ADR + 2-DG. Treatment began when tumors were either 50 or 300 mm(3) in volume. Starting with small or large tumors, the ADR + 2-DG combination treatment resulted in significantly slower tumor growth (and therefore longer survival) than the control, 2-DG, or ADR treatments (P < 0.0001). Similar beneficial effects of combination treatment were found with 2-DG and paclitaxel in the MV522 non-small cell lung cancer xenograft model. In summary, the treatment of tumors with both the glycolytic inhibitor 2-DG and ADR or paclitaxel results in a significant reduction in tumor growth compared with either agent alone. Overall, these results, combined with our in vitro data, provide a rationale for initiating clinical trials using glycolytic inhibitors in combination with chemotherapeutic agents to increase their therapeutic effectiveness.

The activity of latent benzoperimidine esters to inhibit P-glycoprotein and multidrug resistance-associated protein 1 dependent efflux of pirarubicin from several lines of multidrug resistant tumor cells

Multidrug resistance of tumor cells is associated with the presence of membrane proteins responsible for the cytostatics export. Recently, we have synthesized a new family of benzoperimidines causing the futile cycle of MDR pumps. In this study, biological data for benzoperimidine esters are presented for selected cell lines: sensitive (HL-60, GLC4, K562), P-gp resistant (HL-60/VINC, K562/DX), MRP1 resistant (HL-60/DX) and MRP1/LRP resistant (GLC4/DX). Their ability to inhibit the efflux of anthracycline antitumor drug, pirarubicin and to restore its accumulation in MDR cells was studied using a spectrofluorometric method which allows to follow the uptake and efflux of fluorescent molecules by living cells. Benzoperimidine esters had high effectiveness in inhibiting pirarubicin efflux and in restoring its accumulation in resistant cells. In contrast, examined esters were less active in vitro in restoration of pirarubicin cytotoxicity towards resistant cells because an enzymatic cleavage of esters occurs in presence of serum esterases.

Structure and function of efflux pumps that confer resistance to drugs

Resistance to therapeutic drugs encompasses a diverse range of biological systems, which all have a human impact. From the relative simplicity of bacterial cells, fungi and protozoa to the complexity of human cancer cells, resistance has become problematic. Stated in its simplest terms, drug resistance decreases the chance of providing successful treatment against a plethora of diseases. Worryingly, it is a problem that is increasing, and consequently there is a pressing need to develop new and effective classes of drugs. This has provided a powerful stimulus in promoting research on drug resistance and, ultimately, it is hoped that this research will provide novel approaches that will allow the deliberate circumvention of well understood resistance mechanisms. A major mechanism of resistance in both microbes and cancer cells is the membrane protein-catalysed extrusion of drugs from the cell. Resistant cells exploit proton-driven antiporters and/or ATP-driven ABC (ATP-binding cassette) transporters to extrude cytotoxic drugs that usually enter the cell by passive diffusion. Although some of these drug efflux pumps transport specific substrates, many are transporters of multiple substrates. These multidrug pumps can often transport a variety of structurally unrelated hydrophobic compounds, ranging from dyes to lipids. If we are to nullify the effects of efflux-mediated drug resistance, we must first of all understand how these efflux pumps can accommodate a diverse range of compounds and, secondly, how conformational changes in these proteins are coupled to substrate translocation. These are key questions that must be addressed. In this review we report on the advances that have been made in understanding the structure and function of drug efflux pumps.

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

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

The conserved glutamate residue adjacent to the Walker-B motif is the catalytic base for ATP hydrolysis in the ATP-binding cassette transporter BmrA

ATP-binding cassette (ABC) proteins constitute one of the widest families in all organisms, whose P-glycoprotein involved in resistance of cancer cells to chemotherapy is an archetype member. Although three-dimensional structures of several nucleotide-binding domains of ABC proteins are now available, the catalytic mechanism triggering the functioning of these proteins still remains elusive. In particular, it has been postulated that ATP hydrolysis proceeds via an acid-base mechanism catalyzed by the Glu residue adjacent to the Walker-B motif (Geourjon, C., Orelle, C., Steinfels, E., Blanchet, C., Deléage, G., Di Pietro, A., and Jault, J. M. (2001) Trends Biochem. Sci. 26, 539-544), but the involvement of such residue as the catalytic base in ABC transporters was recently questioned (Sauna, Z. E., Muller, M., Peng, X. H., and Ambudkar, S. V. (2002) Biochemistry, 41, 13989-14000). The equivalent glutamate residue (Glu504) of a half-ABC transporter involved in multidrug resistance in Bacillus subtilis, BmrA (formerly known as YvcC), was therefore mutated to Asp, Ala, Gln, Ser, and Cys residues. All these mutants were fully devoid of ATPase activity, yet they showed a high level of vanadate-independent trapping of 8-N3-alpha-32P-labeled nucleotide(s), following preincubation with 8-N3-[alpha-32P]ATP. However, and in contrast to the wild-type enzyme, the use of 8-N3-[gamma-32P]ATP unequivocally showed that all the mutants trapped exclusively the triphosphate form of the analogue, suggesting that they were not able to perform even a single hydrolytic turnover. These results demonstrate that Glu504 is the catalytic base for ATP hydrolysis in BmrA, and it is proposed that equivalent glutamate residues in other ABC transporters play the same role.

P-glycoprotein: from genomics to mechanism

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

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

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

Transition state P-glycoprotein binds drugs and modulators with unchanged affinity, suggesting a concerted transport mechanism

The P-glycoprotein multidrug transporter is a plasma membrane efflux pump for hydrophobic natural products, drugs, and peptides, driven by ATP hydrolysis. Determination of the details of the catalytic cycle of P-glycoprotein is critical if we are to understand the mechanism of drug transport and design ways to inhibit it. It has been proposed that the vanadate-trapped transition state of P-glycoprotein (Pgp x ADP x V(i) x M(2+), where M(2+) is a divalent metal ion) has a very low affinity for drugs compared to resting state protein, thus leading to binding of substrate on the cytoplasmic side of the membrane and release of substrate to the extracellular medium (or the extracellular membrane leaflet). We have used several different fluorescence spectroscopic approaches to show that isolated purified P-glycoprotein, when trapped in a stable transition state with vanadate and either Co(2+)or Mg(2+), binds drugs with high affinity. For vinblastine, colchicine, rhodamine 123, and doxorubicin, the affinity of the vanadate-trapped transition state for drugs was only very slightly (less than 2-fold) lower than the binding affinity of resting state Pgp, whereas for the modulators cyclosporin A and verapamil and the substrate Hoechst 33342, the binding affinity was very similar for the two states. The drug binding affinity of the ADP-bound form of the transporter was also comparable to that of the unoccupied transporter. These results suggest that release of drug from the transporter during the catalytic cycle precedes formation of the transition state.

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

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

Transport ATPases in biological systems and relationship to human disease: a brief overview

Interest in the field of transport ATPases has grown dramatically during the past 20 years and gained considerable visibility for several reasons. First, it was shown that most transport ATPases can be lumped into only a few categories designated simply as P, V, F, and ABC types, the latter consisting of a large superfamily. Second, it has been shown that many transport ATPases have a clear relevance to human disease. Third, the field of transport ATPases has become rather advanced in the study of the reaction mechanisms and structure-function relationships associated with several of these enzymes. Finally, the Nobel committee recently recognized major accomplishments in this field of research. Here, the author provides a brief discussion of transport ATPases that are present in biological systems and their relevance or possible relevance to human disease.

The membrane-peripheral subunits of transhydrogenase from Entamoeba histolytica are functional only when dimerized

Unlike their bacterial and mammalian counterparts, the NADP(H)- and NAD(H)-binding components of proton-translocating transhydrogenase from the protozoan parasite Entamoeba histolytica (denoted ehdIII and ehdI, respectively) are tethered by a polypeptide linker. The recombinant tethered fragment, ehdIII-ehdI, was prepared without its membrane-spanning dII component. Dimers of ehdIII-ehdI catalyzed transhydrogenation, but monomers were inactive. The addition of ehdIII to ehdIII-ehdI monomers did not lead to an increase in the rate of transhydrogenation, showing that this inactivity is not the result of an unfavorable topology introduced by the linker. The addition of a bacterial dI to ehdIII-ehdI led to an increase in the rate of transhydrogenation, showing that the linker is flexible. A hybrid protein in which ehdIII is tethered to the bacterial dI (denoted ehdIII-rrdI) more readily formed active dimers. Data from small angle x-ray scattering by the hybrid dimers were fitted to models derived from the high-resolution crystal structure of the bacterial dI(2)dIII(1) complex (Cotton, N. P. J., White, S. A., Peake, S. J., McSweeney, S., and Jackson, J. B. (2001) Structure 9, 165-T176). The results show that the ehdIII-rrdI dimer is asymmetric; one dIII associates with dI, as in the bacterial complex, but the other is displaced. The results provide evidence for the alternating site, binding change model for proton translocation by intact transhydrogenase.