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Living cells require membranes and membrane transporters for the maintenance of life. After decades of biochemical scrutiny, the structures and molecular mechanisms by which membrane transporters catalyze transmembrane solute movements are beginning to be understood. The plasma membrane proton-translocating adenosine triphosphatase (ATPase) is an archetype of the P-type ATPase family of membrane transporters, which are important in a wide variety of cellular processes. The H+-ATPase has been crystallized and its structure determined to a resolution of 8 angstrom in the membrane plane. When considered together with the large body of biochemical information that has been accumulated for this transporter, and for enzymes in general, this new structural information is providing tantalizing insights regarding the molecular mechanism of active ion transport catalyzed by this enzyme.

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Large single three-dimensional crystals of the dodecylmaltoside complex of the Neurospora crassa plasma membrane H(+)-ATPase (H(+) P-ATPase) can be grown in polyethylene-glycol-containing solutions optimized for moderate supersaturation of both the protein surfaces and detergent micellar region. Large two-dimensional H(+) P-ATPase crystals also grow on the surface of such mixtures and on carbon films located at such surfaces. Electron crystallographic analysis of the two-dimensional crystals grown on carbon films has recently elucidated the structure of the H(+) P-ATPase at a resolution of 0.8 nm in the membrane plane. The two-dimensional crystals comprise two offset layers of ring-shaped ATPase hexamers with their exocytoplasmic surfaces face to face. Side-to-side interactions between the cytoplasmic regions of the hexamers in each layer can be seen, and an interaction between identical exocytoplasmic loops in opposing hexamer layers holds the two layers together. Detergent rings around the membrane-embedded region of the hexamers are clearly visible, and detergent-detergent interactions between the rings are also apparent. The crystal packing forces thus comprise both protein-protein and detergent-detergent interactions, supporting the validity of the original crystallization strategy. Ten transmembrane helices in each ATPase monomer are well-defined in the structure map. They are all relatively straight, closely packed, moderately tilted at various angles with respect to a plane normal to the membrane surface and average approximately 3.5 nm in length. The transmembrane helix region is connected in at least three places to the larger cytoplasmic region, which comprises several discrete domains separated by relatively wide, deep clefts. Previous work has shown that the H(+) P-ATPase undergoes substantial conformational changes during its catalytic cycle that are not changes in secondary structure. Importantly, the results of hydrogen/deuterium exchange experiments indicate that these conformational changes are probably rigid-body interdomain movements that lead to cleft closure. When interpreted within the framework of established principles of enzyme catalysis, this information on the structure and dynamics of the H(+) P-ATPase molecule provides the basis of a rational model for the sequence of events that occurs as the ATPase proceeds through its transport cycle. The forces that drive the sequence can also be clearly stipulated. However, an understanding of the molecular mechanism of ion transport catalyzed by the H(+) P-ATPase awaits an atomic resolution structure.

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The P-type ATPases are integral membrane proteins that generate essential transmembrane ion gradients in virtually all living cells. The structures of two of these have recently been elucidated at a resolution of 8 A. When considered together with the large body of biochemical information that has accrued for these transporters and for enzymes in general, this new structural information is providing tantalizing insights regarding the molecular mechanism of active ion transport catalyzed by these proteins.

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Large two-dimensional crystals of H+-ATPase, a 100 kDa integral membrane protein, were grown directly onto the carbon surface of an electron microscope grid. This procedure prevented the fragmentation that is normally observed upon transfer of the crystals from the air-water interface to a continuous carbon support film. Crystals grown by this method measure approximately 5 microm across and have a thickness of approximately 240 A. They are of better quality than the monolayers previously obtained at the air-water interface, yielding structure factors to at least 8 A in-plane resolution by electron image processing. Unlike most other two-dimensional crystals of membrane proteins they do not contain a lipid bilayer, but consist of detergent-protein micelles of H+-ATPase hexamers tightly packed on a trigonal lattice. The crystals belong to the two-sided plane group p321 (a=b=165 A), containing two layers of hexamers related by an in-plane axis of 2-fold symmetry. The protein is in contact with the carbon surface through its large, hydrophilic 70 kDa cytoplasmic portion, yet due to the presence of detergent in the crystallizing buffer, the hydrophobicity of the carbon surface does not appear to affect crystal formation. Surface crystallisation may be a useful method for other proteins which form fragile two-dimensional crystals, in particular if conditions for obtaining three-dimensional crystals are known, but their quality or stability is insufficient for X-ray structure determination.

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Electron cryocrystallography of precipitant-induced two-dimensional surface crystals of the neurospora plasma membrane H+ - ATPase and tubular crystals of the sarcoplasmic reticulum Ca(2+)-ATPase has recently yielded structure maps for these ion transporters at a resolution of about 8 A. The membrane-embedded regions of these closely related enzymes are similar, but the cytoplasmic regions appear to be significantly different.

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Similar to the recombinant cystic fibrosis transmembrane conductance regulator (CFTR) expressed in Sf9 insect cells, underglycosylated CFTR expressed in yeast is not effectively solubilized by a variety of commonly used detergents, requiring instead harsh alkali and SDS treatments, which would denature most proteins. Moreover, solubilized CFTR has a strong tendency to aggregate and form high-molecular-weight aggregates during subsequent purification. We report here that the mild detergent, lysophosphatidylglycerol (LPG), is a very effective detergent for solubilizing the CFTR expressed in both yeast and Sf9 insect cells. LPG solubilizes nearly 100% of the CFTR in yeast in the absence of NaCl and none in the presence of 1 M NaCl. It is also very potent in preventing aggregation of the CFTR during subsequent purification. Exploiting these characteristics, a rapid simple procedure for the purification of functional recombinant CFTR expressed in yeast has been developed. It includes selective CFTR solubilization in the presence and the absence of NaCl followed by nickel-chelate chromatography of His-tagged CFTR. The CFTR produced by this procedure is about 70% pure. Purified CFTR molecules were reconstituted into liposomes and then fused to planar lipid bilayers for single-channel recording. The reconstituted CFTR exhibits regulatory chloride channel activities with a slope conductance of 7.1 pS and a reversal potential of -32 mV. The effectiveness and simplicity of this new purification procedure for the CFTR should greatly facilitate a variety of biochemical and biophysical studies of this important protein. Furthermore, the potency of LPG in solubilizing the notoriously intractable underglycosylated CFTR suggest that this detergent may be useful for solubilizing the CFTR from other sources and for other difficult membrane proteins as well.

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The H+-ATPase from the plasma membrane of Neurospora crassa is an integral membrane protein of relative molecular mass 100K, which belongs to the P-type ATPase family that includes the plasma membrane Na+/K+-ATPase and the sarcoplasmic reticulum Ca2+-ATPase. The H+-ATPase pumps protons across the cell's plasma membrane using ATP as an energy source, generating a membrane potential in excess of 200mV. Despite the importance of P-type ATPases in controlling membrane potential and intracellular ion concentrations, little is known about the molecular mechanism they use for ion transport. This is largely due to the difficulty in growing well ordered crystals and the resulting lack of detail in the three-dimensional structure of these large membrane proteins. We have now obtained a three-dimensional map of the H+-ATPase by electron crystallography of two-dimensional crystals grown directly on electron microscope grids. At an in-plane resolution of 8 A, this map reveals ten membrane-spanning alpha-helices in the membrane domain, and four major cytoplasmic domains in the open conformation of the enzyme without bound ligands.

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A system for expression and facile purification of the human P-glycoprotein (Pgp) from the yeast Saccharomyces cerevisiae is described. The wild-type human mdr1 cDNA was cloned into a high copy number yeast expression vector under the control of the constitutive promoter of the yeast plasma membrane H+-ATPase. Western blots of membranes from the stable transformants confirmed that the Pgp is expressed in yeast cells in amounts approximately 0.4% of the total yeast membrane protein. Density gradient sedimentation analysis of the yeast membranes indicated that the expressed Pgp is localized in the plasma membrane. Yeast cells transformed with the Pgp expression plasmid acquire increased resistance to valinomycin, suggesting that the expressed Pgp is properly folded and functional. The expressed Pgp can be solubilized from the yeast membranes with lysophosphatidylcholine, and when tagged with ten histidines at its C-terminus, can be readily purified to about 90% homogeneity by Ni2+ affinity chromatography. About 50 microg of the Pgp can be purified from 20 mg of crude yeast membranes. The purified human Pgp exhibits a verapamil-stimulated ATPase activity and the maximal activity is 2.5 +/- 0.5 micromol/min per mg of Pgp, suggesting that the purified Pgp from yeast is highly functional. The Pgp expressed in yeast has the same electrophoretic mobility (ca. 130 kDa) as the Pgp produced in Sf9 insect cells and is unaffected by N-glycosidase treatment, suggesting that it is not glycosylated. Because of the relative ease of growing yeast in massive quantities this expression system appears to be excellent for producing this membrane transporter at levels sufficient for further biochemical and biophysical studies, and for site-directed mutagenesis studies as well.

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Recombinant human cystic fibrosis transmembrane conductance regulator (CFTR) has been produced in a Saccharomyces cerevisiae expression system used previously to produce transport ATPases with high yields. The arrangement of the bases in the region immediately upstream from the ATG start codon of the CFTR is extremely important for high expression levels. The maximal CFTR expression level is about 5-10% of that in Sf9 insect cells as judged by comparison of immunoblots. Upon sucrose gradient centrifugation, the majority of the CFTR is found in a light vesicle fraction separated from the yeast plasma membrane in a heavier fraction. It thus appears that most of expressed CFTR is not directed to the plasma membrane in this system. CFTR expressed in yeast has the same mobility (ca. 140 kDa) as recombinant CFTR produced in Sf9 cells in a high resolution SDS-PAGE gel before and after N-glycosidase F treatment, suggesting that it is not glycosylated. The channel function of the expressed CFTR was measured by an isotope flux assay in isolated yeast membrane vesicles and single channel recording following reconstitution into planar lipid bilayers. In the isotope flux assay, protein kinase A (PKA) increased the rate of 125I- uptake by about 30% in membrane vesicles containing the CFTR, but not in control membranes. The single channel recordings showed that a PKA-activated small conductance anion channel (8 pS) with a linear I-V relationship was present in the CFTR membranes, but not in control membranes. These results show that the human CFTR has been expressed in functional form in yeast. With the reasonably high yield and the ability to grow massive quantities of yeast at low cost, this CFTR expression system may provide a valuable new source of starting material for purification of large quantities of the CFTR for biochemical studies.

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A high-yield yeast expression system for site-directed mutagenesis of the Neurospora crassa plasma membrane H(+)-ATPase has recently been reported (Mahanty, S. K., Rao, U. S., Nicholas, R. A., and Scarborough, G. A. (1994) J. Biol. Chem. 269, 17705-17712). Using this system, each of the eight cysteine residues in the ATPase was changed to a serine or an alanine residue, producing strains C148S and C148A, C376S and C376A, C409S and C409A, C472S and C472A, C532S and C532A, C545S and C545A, C840S and C840A, and C869S and C869A, respectively. With the exception of C376S and C532S, all of the mutant ATPases are able to support the growth of yeast cells to different extents, indicating that they are functional. The C376S and C532S enzymes appear to be non-functional. After solubilization of the functional mutant ATPase molecules from isolated membranes with lysolecithin, all behaved similar to the native enzyme when subjected to glycerol density gradient centrifugation, indicating that they fold in a natural manner. The kinetic properties of these mutant enzymes were also similar to the native ATPase with the exception of C409A, which has a substantially higher Km. These results clearly indicate that none of the eight cysteine residues in the H(+)-ATPase molecule are essential for ATPase activity, but that Cys376, Cys409, and Cys532 may be in or near important sites. They also demonstrate that the previously described disulfide bridge between Cys148 and Cys840 or Cys869 plays no obvious role in the structure or function of this membrane transport enzyme.

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Large, well-ordered 2-D crystals of the dodecylmaltoside complex of the Neurospora crassa plasma membrane H(+)-ATPase grow rapidly on the surface of a polyethylene glycol-containing mixture similar to that originally developed for growing 3-D crystals of this integral membrane transport protein. Negative stain electron microscopy of the crystals shows that many are single layers. Cryoelectron microscopy of unstained specimens indicates that the crystals have a p6 layer group with unit cell dimensions of a = b = 167 A. Image processing of selected electron micrographs has yielded a projection map at 10.3 A resolution. The repeating unit of the ATPase crystals comprises six 100 kDa ATPase monomers arranged in a symmetrical ring. The individual monomers in projection are shaped like a boot. These results provide the first indications of the molecular structure of the H(+)-ATPase molecule. They also establish the feasibility of precipitant-induced surface growth as a rapid, simple alternative to conventional methods for obtaining 2-D crystals of the integral membrane proteins useful for structure analysis.

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The purified H(+)-ATPase of the Neurospora crassa plasma membrane has been reconstituted by a gel filtration method into lipidic vesicles using sodium deoxycholate as the detergent. Reconstitution was performed for lipid/ATPase ratios ranging from 1000:1 to 5:1 (w/w). Whatever the lipid/ATPase ratio, the ATPase molecules completely associate with the lipid vesicles. The ATPase specific activity is identical for all proteoliposomes regardless of the lipid/ATPase ratio, but the H+ transport decreases at high protein/lipid ratios, suggesting that the proteoliposomes are more leaky to H+ as the amount of protein inserted into the lipidic membrane increases. Analysis of the fragments generated by trypsin proteolysis in the presence and in the absence of MgATP+ vanadate indicate that most of the reconstituted ATPase molecules are able to assume the transition state of the enzyme dephosphorylation reaction, and are therefore functional. The orientation (inside-out or rightside-out) of the ATPase molecules in the vesicles is independent of the lipid/ATPase ratio chosen for the reconstitution. For all the lipid/ATPase ratios tested, most of the ATPase molecules (> 99%) expose their cytoplasmic side to the outside of the reconstituted proteoliposomes. The size of the vesicles increases parallel to the ATPase amount. Although the H+ leakiness of our preparation at low lipid/protein ratios prevents proton pumping measurements, the reconstitution procedure described here has the main advantage on other procedures to allow the obtention of vesicles at high protein-to-lipid ratios, facilitating further structural characterization of the ATPase by biochemical and biophysical techniques. Therefore, the procedure described here could be of general interest in the field of membrane protein study.

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The human multidrug resistance protein, or P-glycoprotein (Pgp), exhibits a high-capacity drug-dependent ATP hydrolytic activity that is a direct reflection of its drug transport capability. This activity is readily measured in membranes isolated from cultured insect cells infected with a baculovirus carrying the human mdr1 cDNA. The drug-stimulated ATPase activity is a useful alternative to conventional screening systems for identifying high-affinity drug substrates of the Pgp with potential clinical value as chemosensitizers for tumor cells that have become drug resistant. Using this assay system, a variety of drugs have been directly shown to interact with the Pgp. Many of the drugs stimulate the Pgp ATPase activity, but certain drugs bind tightly to the drug-binding site of the Pgp without eliciting ATP hydrolysis. Either class of drugs may be useful as chemosensitizing agents. The baculovirus/insect cell Pgp ATPase assay system may also facilitate future studies of the molecular structure and mechanism of the Pgp.

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Attenuated total reflection Fourier transform infrared spectroscopy of hydrated films of the Neurospora crassa plasma membrane H(+)-ATPase has been used to monitor the alpha-helix and beta-sheet contents and amide hydrogen exchange rates of the enzyme in the absence of ligands or locked in several stages of the enzyme catalytic cycle by MgADP, Mg-vanadate, and MgATP-vanadate. No difference larger than 2% was found in the alpha-helix or beta-sheet content of the H(+)-ATPase in different conformational states. However, when the rate of hydrogen/deuterium exchange monitored by the evolution of the area of amide II and amide II' is decomposed into three components, the number of amide protons characterized by a short exchange rate (1.1 min) falls from 38% of the protein amide protons (or 37% in the presence of Mg2+ alone) to 24-27% in the presence of Mg-vanadate and MgATP-vanadate and to 19% in the presence of MgADP. These results suggest that the conformational changes known to occur when the H(+)-ATPase interacts with the above ligands are predominantly tertiary structure changes.

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A simple system for high yield expression of the neurospora plasma membrane H(+)-ATPase is described. Two neurospora H(+)-ATPase cDNAs differing only in a few bases preceding the coding region were cloned into a high copy number yeast expression vector under the control of the constitutive promoter of the yeast plasma membrane H(+)-ATPase, and the resulting plasmids were used to transform Saccharomyces cerevisiae strain RS-72, which requires a plasmid-borne functional plasma membrane H(+)-ATPase for growth in glucose medium (Villalba, J. M., Palmgren, M. G., Berberian, G. E., Ferguson, C., and Serrano, R. (1992) J. Biol. Chem. 267, 12341-12349. Both plasmids supported growth of the cells, indicating that the neurospora ATPase is expressed in functional form in yeast. Western blots of membranes from the transformants confirmed that the neurospora ATPase is expressed in the yeast cells, with production in the range of several percent of the yeast membrane protein. Importantly, when the expressed, recombinant neurospora ATPase molecules are solubilized from the membranes with lysolecithin and subjected to glycerol gradient centrifugation, they migrate to a position indistinguishable from that of the native ATPase and display a comparable specific ATPase activity, indicating that the great majority of the recombinant neurospora ATPase molecules produced in yeast fold in a natural manner. This expression system thus appears to be ideal for site-directed mutagenesis studies of the neurospora ATPase molecule.

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Large single crystals of the dodecylmaltoside (DDM) complex of a polytopic integral membrane transport protein, the Neurospora plasma membrane H(+)-ATPase, have been obtained using an approach that attempts to take into account the possibly radically different physicochemical properties of the protein surfaces and the detergent micellar collar. The overall goal of the crystallization strategy employed was to identify conditions in which the protein surfaces of the DDM-ATPase complex are moderately insoluble and in which the DDM micellar collar is also near its solubility limit. The first step was to screen a variety of commonly used protein precipitants for those that were able to induce the aggregation of pure DDM micelles. The concentration at which any precipitant induced DDM micellar aggregation was hoped to be close to the concentration at which it might induce insolubility of the detergent micellar collar of the DDM-ATPase complex. Of the nine precipitants tried, seven, all polyethylene glycols (PEGs), were able to induce DDM micelle insolubility. The seven PEGs were then tested for their effect on the solubility of the DDM-ATPase complex at a concentration slightly below that necessary to induce DDM micellar aggregation. Three of the PEGs caused extensive precipitation of the ATPase at this concentration and were, therefore, shelved. The other four PEGs did not induce precipitation at the concentration employed and were subsequently used at this concentration for crystallization trials in which the protein concentration was varied. Encouragingly, crystalline plates of the ATPase were obtained for each of the four PEGs tried, indicating that the overall approach may be valid. Unfortunately, the crystals obtained were visibly flawed, suggesting that the correct balance of protein surface and DDM micelle insolubility had not yet been reached. The ionic strength of the crystallization trials was then raised, which was known from other experiments to render the protein surfaces of the ATPase less soluble while having no effect on the DDM micellar aggregation point. For one of the PEGs, PEG 4000, this brought on a new, well formed hexagonal crystal habit. Subsequent optimization of the initial conditions has yielded large single hexagonal crystals of the H(+)-ATPase roughly 0.4 x 0.4 x 0.15 mm in size, holding promise for exploration of the structure of the ATPase by X-ray diffraction analysis.

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Multidrug-resistant (MDR) tumor cells reduce the toxicity of antineoplastic drugs by an energy-dependent active efflux mechanism mediated by the MDR1 gene product, the P-glycoprotein (Pgp). Pgp expressed in cultured Sf9 insect cells has been shown to exhibit a high capacity ATPase activity in the presence of a variety of drugs known to be transported by the Pgp (Sarkadi et al., J Biol Chem 267: 4854-4858, 1992). The strict dependence of the Pgp ATPase activity on the presence of transport substrates indicates that the drug-stimulated ATPase activity is a direct reflection of the drug transport function of the Pgp. In the present study, this system has been utilized to investigate the possibility that antiestrogens and steroid hormones are transported by the Pgp. Antiestrogens such as tamoxifen, metabolites of tamoxifen (4-hydroxytamoxifen and N-desmethyltamoxifen), droloxifen, and toremifene stimulated the Pgp ATPase activity, and the maximum stimulation obtained with these agents equalled the maximal stimulation obtained by the best known MDR chemosensitizer, verapamil. Clomifene, nafoxidine and diethylstilbestrol also stimulated the Pgp ATPase activity, with maximal activations 75, 60 and 45% of the verapamil stimulation, respectively. Different degrees of stimulation of the Pgp ATPase activity were also obtained in the presence of steroid hormones such as progesterone, beta-estradiol, hydrocortisone, and corticosterone. Among these, progesterone is a potent inducer of the Pgp ATPase activity; at 50 microM, this hormone stimulated the Pgp ATPase activity as effectively as verapamil. These results suggest that the antiestrogens and steroid hormones that are known to reverse the multidrug-resistant phenotype do so by directly interacting with Pgp, thus interfering with its anticancer drug-extruding activity.

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The interactions between the human P-glycoprotein (Pgp) and two different types of immunosuppressant drugs known to modulate multidrug resistance in tumor cells have been directly investigated using our newly developed drug-stimulated ATPase assay for Pgp function. The macrolides FK506 and FK520 stimulate the Pgp-ATPase activity with affinities in the 100 nM range, nearly 10 times higher than that of verapamil, a well known Pgp substrate. On the other hand, the cyclic peptides cyclosporin A and dihydrocyclosporin C do not stimulate the Pgp-ATPase activity at all. They do, however, act as potent competitive inhibitors of verapamil-stimulated Pgp-ATPase activity, with affinity constants in the 20-25 nM range. Thus, although these two classes of immunosuppressant drugs affect the Pgp in different ways, they both probably interact with high affinity at the transported drug binding site(s) of the Pgp, which would explain their ability to resensitize multidrug-resistant cells to the killing action of certain antitumor drugs. Possible implications of these findings for Pgp function, cancer chemotherapy, and immunosuppression are discussed.

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The structure of the Neurospora crassa plasma membrane H(+)-ATPase has been investigated using a variety of chemical and physiochemical techniques. The transmembrane topography of the H(+)-ATPase has been elucidated by a direct, protein chemical approach. Reconstituted proteoliposomes containing purified H(+)-ATPase molecules oriented predominantly with their cytoplasmic surface facing outward were treated with trypsin, and the numerous peptides released were purified by HPLC and subjected to amino acid sequence analysis. In this way, seventeen released peptides were unequivocally identified as located on the cytoplasmic side of the membrane, and numerous intervening segments could be inferred to be cytoplasmically located by virtue of the fact that they are too short to cross the membrane and return between sequences established to be cytoplasmically located. Additionally, three large membrane-embedded segments of the H(+)-ATPase were isolated using our recently developed methods for purifying hydrophobic peptides, and identified by amino acid sequence analysis. This information established the topographical location of virtually all of the 919 residues in the H(+)-ATPase molecule, allowing the formulation of a reasonably detailed model for the transmembrane topography of the H(+)-ATPase polypeptide chain. Separate studies of the cysteine chemistry of the H(+)-ATPase have demonstrated the existence of a single disulfide bridge in the molecule, linking the NH2- and COOH-terminal membrane-embedded domains. And, analyses of the circular dichroism and infrared spectra of the purified H(+)-ATPase have elucidated the secondary structure composition of the molecule. A first-generation model for the tertiary structure of the H(+)-ATPase based on this information and other considerations is presented.

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