Na,K-ATPase extracellular surface probed with a monoclonal antibody that enhances ouabain binding

Multiple digitalis receptors A molecular perspective

The Na,K-ATPase is the only established receptor for cardiac glycosides like digoxin or ouabain. There are now known to be three different isoforms of its principal subunit. These isoforms can differ from one another in their intrinsic affinity for cardiac glycosides. Recent work examines the molecular structure of the binding site. The relative level of expression of the isoforms in cardiac tissue is modified in several developmental, hormonal, and pathological states, contributing to alterations in the digitalis sensitivity of the tissue.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Crystal structure of the sodium-potassium pump (Na+,K+-ATPase) with bound potassium and ouabain

The sodium-potassium pump (Na(+),K(+)-ATPase) is responsible for establishing Na(+) and K(+) concentration gradients across the plasma membrane and therefore plays an essential role in, for instance, generating action potentials. Cardiac glycosides, prescribed for congestive heart failure for more than 2 centuries, are efficient inhibitors of this ATPase. Here we describe a crystal structure of Na(+),K(+)-ATPase with bound ouabain, a representative cardiac glycoside, at 2.8 A resolution in a state analogous to E2.2K(+).Pi. Ouabain is deeply inserted into the transmembrane domain with the lactone ring very close to the bound K(+), in marked contrast to previous models. Due to antagonism between ouabain and K(+), the structure represents a low-affinity ouabain-bound state. Yet, most of the mutagenesis data obtained with the high-affinity state are readily explained by the present crystal structure, indicating that the binding site for ouabain is essentially the same. According to a homology model for the high affinity state, it is a closure of the binding cavity that confers a high affinity.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Multiplicity of expression of FXYD proteins in mammalian cells: dynamic exchange of phospholemman and gamma-subunit in response to stress

Functional properties of Na-K-ATPase can be modified by association with FXYD proteins, expressed in a tissue-specific manner. Here we show that expression of FXYDs in cell lines does not necessarily parallel the expression pattern of FXYDs in the tissue(s) from which the cells originate. While being expressed only in lacis cells in the juxtaglomerular apparatus and in blood vessels in kidney, FXYD1 was abundant in renal cell lines of proximal tubule origin (NRK-52E, LLC-PK1, and OK cells). Authenticity of FXYD1 as a part of Na-K-ATPase in NRK-52E cells was demonstrated by co-purification, co-immunoprecipitation, and co-localization. Induction of FXYD2 by hypertonicity (500 mosmol/kgH(2)O with NaCl for 48 h or adaptation to 700 mosmol/kgH(2)O) correlated with downregulation of FXYD1 at mRNA and protein levels. The response to hypertonicity was influenced by serum factors and entailed, first, dephosphorylation of FXYD1 at Ser(68) (1-5 h) and, second, induction of FXYD2a and a decrease in FXYD1 with longer exposure. FXYD1 was completely replaced with FXYD2a in cells adapted to 700 mosmol/kgH(2)O and showed a significantly decreased sodium affinity. Thus dephosphorylation of FXYD1 followed by exchange of regulatory subunits is utilized to make a smooth transition of properties of Na-K-ATPase. We also observed expression of mRNA for multiple FXYDs in various cell lines. The expression was dynamic and responsive to physiological stimuli. Moreover, we demonstrated expression of FXYD5 protein in HEK-293 and HeLa cells. The data imply that FXYDs are obligatory rather than auxiliary components of Na-K-ATPase, and their interchangeability underlies responses of Na-K-ATPase to cellular stress.

Evidence that the H1‐H2 domain of α1 subunit of (Na++K+)‐ATPase participates in the regulation of cardiac contraction

(Na++K+)-ATPase (NKA) plays an important role in ion homeostasis and regulates cardiac contraction. To understand the molecular basis of its cardiac regulatory functions, we investigated whether the primary structure of the H1-H2 domain in alpha-1 (alpha1) subunit of the enzyme plays a role in myocardial contractile regulation. Here we show that site-specific binding to this 1 H1-H2 domain with a targeted antibody (SSA78) markedly augments intracellular Ca2+ transients and contraction of rat ventricular cardiomyocytes without inactivating NKA. In vivo SSA78 infusion in mice results in a positive inotropic effect with enhanced contractile function yet no change in relaxation, indicating a direct cardiac effect linked to the H1-H2 domain. Competitive immunofluorescent staining and flow cytometry reveal that SSA78 binding is antagonized by ouabain, supporting the interaction of SSA78 at one of the glycoside-effecter sites. These new findings suggest that the H1-H2 domain of 1 subunit of NKA is a critical determinant of enzyme biologic activity, which couples to enhanced myocyte calcium transient and inotropic action.

De novo biosynthesis and radiolabeling of mammalian digitalis-like factors

BACKGROUND

Digoxin-like immunoreactive factors (DLIFs) are endogenous mammalian cardenolides with structural features similar to those of the plant-derived digitalis compounds. DLIFs and their structurally related forms (Dh-DLIFs) may serve as effectors of ion-transport activity mediated by their interaction with Na,K-ATPase and thus play a role as a new hormonal axis. Although some evidence implicates the adrenal gland as a tissue source for the DLIFs, little is known about the biosynthetic pathway producing these compounds. We now demonstrate de novo biosynthesis of DLIF by incorporation of radioactive carbon ((14)C) into the structures of both DLIF and Dh-DLIF.

METHODS

We used a combination of reversed-phase HPLC techniques to separate the radioactive DLIF components after incorporation of (14)C into their structure by use of either [1,2-(14)C]acetic acid or [4-(14)C]cholesterol as precursors and a Y-1 mouse adrenocortical tumor cell line. We also stimulated and suppressed production of steroidogenesis by use of cAMP analogs and Mevastatin, respectively, to demonstrate the dependence of DLIF production on the cholesterol-dependent biosynthetic pathway. A combination of chromatographic mobility, immunoassays specific for digoxin and dihydrodigoxin, and deglycosylation using 5-sulfosalicylic acid were used to identify the DLIF and Dh-DLIF components.

RESULTS

With cholesterol as precursor, the cells produced DLIF (7.5 mCi/mmol) with a labeling efficiency of 10%, whereas with acetate the cells produced DLIF (72.2 mCi/mmol) with a labeling efficiency of 0.08% of the total DLIF produced. The radiolabeled DLIF and Dh-DLIF molecules had identical chromatographic mobilities and stoichiometric removal of sugars as the previously characterized DLIFs isolated from different mammalian species and tissues. With radioactive cholesterol as precursor, the (14)C was incorporated into the DLIF-genin portion of the compounds and not the sugars. Interestingly, treatment of Y-1 cells with 8-bromoadenosine 3':5'-cAMP to stimulate steroidogenesis did not increase production of DLIF or Dh-DLIF but did increase production of progesterone. Mevastatin (5 micromol), an inhibitor of the enzyme hydroxymethylglutaryl-CoA reductase and thus of cholesterol biosynthesis, gave an 85% decrease in the production of (14)C-DLIF and progesterone, but only a modest 15% decrease in (14)C-Dh-DLIF production.

CONCLUSIONS

These data demonstrate that the adrenal cell has the cellular machinery necessary for de novo biosynthesis of DLIF and Dh-DLIF starting from a simple carbon pool and also support the concept that cholesterol is a major precursor of the DLIF compounds. This cell culture model provides a source of radiolabeled DLIF compounds for future experimental work.

Distribution and oligomeric association of splice forms of Na(+)-K(+)-ATPase regulatory gamma-subunit in rat kidney

Renal Na(+)-K(+)-ATPase is associated with the gamma-subunit (FXYD2), a single-span membrane protein that modifies ATPase properties. There are two splice variants with different amino termini, gamma(a) and gamma(b). Both were found in the inner stripe of the outer medulla in the thick ascending limb. Coimmunoprecipitation with each other and the alpha-subunit indicated that they were associated in macromolecular complexes. Association was controlled by ligands that affect Na(+)-K(+)-ATPase conformation. In the cortex, the proportion of the gamma(b)-subunit was markedly lower, and the gamma(a)-subunit predominated in isolated proximal tubule cells. By immunofluorescence, the gamma(b)-subunit was detected in the superficial cortex only in the distal convoluted tubule and connecting tubule, which are rich in Na(+)-K(+)-ATPase but comprise a minor fraction of cortex mass. In the outer stripe of the outer medulla and for a short distance in the deep cortex, the thick ascending limb predominantly expressed the gamma(b)-subunit. Because different mechanisms maintain and regulate Na(+) homeostasis in different nephron segments, the splice forms of the gamma-subunit may have evolved to control the renal Na(+) pump through pump properties, gene expression, or both.

Predicted location and limited accessibility of protein kinase A phosphorylation site on Na-K-ATPase

Regulation of Na-K-ATPase by cAMP-dependent protein kinase occurs in a variety of tissues. Phosphorylation of the enzyme's catalytic subunit at a classical phosphorylation consensus motif has been observed with purified enzyme. Demonstration of phosphorylation at the same site in normal living cells or tissues has been more difficult, however, making it uncertain that the Na-K-ATPase is a direct physiological substrate of the kinase. Recently, the structure of the homologous sarco(endo)plasmic reticulum Ca-ATPase (SERCA1a) has been determined at 2.6 A resolution (Toyoshima C, Nakasako M, Nomura H, and Ogawa H. Nature 405: 647-655, 2000.), and the Na-K- ATPase should have the same fold. Here, the Na-K-ATPase sequence has been aligned with the Ca-ATPase structure to examine the predicted disposition of the phosphorylation site. The location is close to the membrane and partially buried by adjacent loops, and the site is unlikely to be accessible to the kinase in this conformation. Conditions that may expose the site or further bury it are discussed to highlight the issues facing future research on regulation of Na-K-ATPase by cAMP-dependent pathways.

High-affinity ouabain binding by a chimeric gastric H+,K+-ATPase containing transmembrane hairpins M3-M4 and M5-M6 of the alpha 1-subunit of rat Na+,K+-ATPase

Na(+),K(+)-ATPase and gastric H(+),K(+)-ATPase are two related enzymes that are responsible for active cation transport. Na(+), K(+)-ATPase activity is inhibited specifically by ouabain, whereas H(+),K(+)-ATPase is insensitive to this drug. Because it is not known which parts of the catalytic subunit of Na(+),K(+)-ATPase are responsible for ouabain binding, we prepared chimeras in which small parts of the alpha-subunit of H(+),K(+)-ATPase were replaced by their counterparts of the alpha(1)-subunit of rat Na(+),K(+)-ATPase. A chimeric enzyme in which transmembrane segments 5 and 6 of H(+), K(+)-ATPase were replaced by those of Na(+),K(+)-ATPase could form a phosphorylated intermediate, but hardly showed a K(+)-stimulated dephosphorylation reaction. When transmembrane segments 3 and 4 of Na(+),K(+)-ATPase were also included in this chimeric ATPase, K(+)-stimulated dephosphorylation became apparent. This suggests that there is a direct interaction between the hairpins M3-M4 and M5-M6. Remarkably, this chimeric enzyme, HN34/56, had obtained a high-affinity ouabain-binding site, whereas the rat Na(+), K(+)-ATPase, from which the hairpins originate, has a low affinity for ouabain. The low affinity of the rat Na(+),K(+)-ATPase previously had been attributed to the presence of two charged amino acids in the extracellular domain between M1 and M2. In the HN34/56 chimera, the M1/M2 loop, however, originates from H(+),K(+)-ATPase, which has two polar uncharged amino acids on this position. Placement of two charged amino acids in the M1/M2 loop of chimera HN34/56 results in a decreased ouabain affinity. This indicates that although the M1/M2 loop affects the ouabain affinity, binding occurs when the M3/M4 and M5/M6 hairpins of Na(+),K(+)-ATPase are present.

Isoform-specific effects of charged residues at borders of the M1-M2 loop of the Na,K-ATPase alpha subunit

The Na,K-ATPase is specifically inhibited by the cardiac glycoside, ouabain. Via a largely undefined mechanism, the ouabain affinity of the Na,K-ATPase can be manipulated by mutating the residues at the borders of the first extracellular (M1-M2) loop of the alpha subunit [Price, E. M., Rice, D. A., and Lingrel, J. B. (1990) J. Biol. Chem. 265, 6638-6641]. To address this issue, we compared the effects of two combinations of charged residues at the M1-M2 loop border, R113, D124 and D113,R124 (numbered according to the rat alpha1 subunit), on the ouabain sensitivity of the alpha1 and alpha2 isoforms. We report that ouabain sensitivity is dependent not only upon the identity of the residues at the M1-M2 loop border but also upon the context into which they are introduced. Furthermore, at low concentrations of ATP, the identity of the residues at the M1-M2 loop border affects the regulation of ATP hydrolysis by potassium in an isoform-specific manner. Analysis of chimeric alpha subunits reveals that the effects of potassium are determined primarily by the interaction of the N-terminus and M1-M2 loop with the C-terminal third of the alpha subunit. M1-M2 loop border residues may, therefore, influence ouabain sensitivity indirectly by altering the stability or structure of the intermediate of the Na,K-ATPase catalytic cycle which is competent to bind ouabain.

Genetic probing of the stalk segments associated with M2 and M3 of the plasma membrane H+-ATPase from Saccharomyces cerevisiae

The stalk region of the H+-ATPase from Saccharomyces cerevisiae has been proposed to play a role in coupling ATP hydrolysis to proton transport. Genetic probing was used to examine the role of stalk segments S2 and S3, associated with M2 and M3, respectively. Saturation mutagenesis was used to explore the role of side group character at position Ile183 in S2, at which an alanine substitution was shown previously to partially uncouple the enzyme (Wang, G., Tamas, M. J., Hall, M. J., Pascual-Ahuir, A., and Perlin, D. S. (1996) J. Biol. Chem. 271, 25438-25445). Diverse side group substitutions were tolerated at this position, although three substitutions, I183N, I183R, and I183Y required second site mutations at the C terminus of the enzyme for stabilization. Substitution of glycine and proline at Ile183 resulted in lethal phenotypes, suggesting that the backbone may be more important than side group at this position. Proline/glycine mutagenesis was used to study additional sites in S2 and S3. The substitution of proline at Gly186 resulted in a lethal phenotype, whereas substitutions in S3 of proline or serine at Gly270 and proline or glycine at Thr287 resulted in viable mutants. Mutations G270P and T287P resulted in mutant enzymes that produced pronounced growth defects and ATP hydrolysis rates that were 35% and 60% lower than wild type enzyme, respectively. The mutant enzymes transported protons at rates consistent with their ATPase activity, suggesting that the growth defects observed were due to a reduced rate of ATP hydrolysis and not to uncoupling of proton transport. The prominent growth phenotypes produced by mutations G270P and T287P permitted the isolation of suppressor mutations, which restored wild type growth. Most of the suppressors either replaced the primary site mutation with alanine or restored the wild type residue by ectopic recombination with PMA2, both of which restore alpha-helical tendency. This study suggests that maintaining alpha-helical character is essential to S2 and may play an important role in S3.

Extensive random mutagenesis analysis of the Na+/K+-ATPase alpha subunit identifies known and previously unidentified amino acid residues that alter ouabain sensitivity--implications for ouabain binding

Random mutagenesis with ouabain selection has been used to comprehensively scan the extracellular and transmembrane domains of the alpha1 subunit of the sheep Na+/K+-ATPase for amino acid residues that alter ouabain sensitivity. The four random mutant libraries used in this study include all of the transmembrane and extracellular regions of the molecule as well as 75% of the cytoplasmic domains. Through an extensive number of HeLa cell transfections of these libraries and subsequent ouabain selection, 24 ouabain-resistant clones have been identified. All previously described amino acids that confer ouabain resistance were identified, confirming the completeness of this random mutagenesis screen. The amino acid substitutions that confer the greatest ouabain resistance, such as Gln111-->Arg, Asp121-->Gly, Asp121-->Glu, Asn122-->Asp, and Thr797-->Ala were identified more than once in this study. This extensive survey of the extracellular and transmembrane regions of the Na+/K+-ATPase molecule has identified two new regions of the molecule that affect ouabain sensitivity: the H4 and the H10 transmembrane regions. The new substitutions identified in this study are Leu330-->Gln, Ala331-->Gly, Thr338-->Ala, and Thr338-->Asn in the H4 transmembrane domain and Phe982-->Ser in the H10 transmembrane domain. These substitutions confer modest increases in the concentration of cardiac glycoside needed to produce 50% inhibition of activity (IC50 values), 3.1-7.9-fold difference. The results of this extensive screening of the Na+/K+-ATPase alpha1 subunit to identify amino acids residues that are important in ouabain sensitivity further supports our hypothesis that the H1-H2 and H4-H8 regions represent the major binding sites for the cardiac glycoside class of drugs.

Amino acids Val115-Ile126 of rat gastric H(+)-K(+)-ATPase confer high affinity for Sch-28080 to Na(+)-K(+)-ATPase

Na(+)-K(+)-ATPase is inhibited by cardiac glycosides and is insensitive to Sch-28080, an inhibitor of gastric H(+)-K(+)-ATPase. Gastric H(+)-K(+)-ATPase is not inhibited by cardiac glycosides. Both ouabain and, Sch-28080 binding are inhibited by K+, and it has been suggested that the inhibitors bind to corresponding regions on the alpha-subunit of each ion pump. For identification of regions of each pump that interact with the specific inhibitors, chimeric alpha-subunits consisting of selected regions from Na(+)-K(+)-ATPase and gastric H(+)-K(+)-ATPase have been prepared. One chimera (gM1/2) has been constructed from cDNA of the sheep alpha1-subunit of Na(+)-K(+)-ATPase by replacement of the last 12 amino acids of the first predicted transmembrane region (Ile99-Ile110) with corresponding amino acids from rat gastric H(+)-K(+)-ATPase. gM1/2 was expressed in yeast cells together with either the rat Na(+)-K(+)-ATPase beta 1-subunit (NK beta 1) or rat gastric H(+)-K(+)-ATPase beta-subunit (HK beta). Western blots show that the expression level of the chimeric alpha-subunit was comparable to the Na(+)-K(+)-ATPase alpha 1. Ouabain binds with high affinity to gM1/2+NK beta 1 [ouabain binding affinity (Kd) = 9.5 nM] but not to gM1/2+HK beta. The Kd for ouabain binding to Na(+)-K(+)-ATPase was 7.8 nM. Na(+)-K(+)-ATPase activity of gM1/2+NK beta 1 was inhibited both by ouabain and Sch-28080. The 50% inhibition concentration for Sch-28080 was 20-60 nM. Sch-28080 at 10 microM did not inhibit Mg(2+)- and Pi-dependent ouabain binding to gM1/2+NK beta 1. Ouabain (0.75 mM) inhibited palytoxin-induced K+ efflux from yeast cells expressing either gM1/2+NK beta 1 or gM1/2+NK beta, and Sch-28080 increased the palytoxin-induced K+ efflux from yeast cells expressing gM1/2+NK beta 1 or gM1/2+HK beta. These results implicate a small number of amino acids in the first transmembrane part of gastric H(+)-K(+)-ATPase as partial determinants of the sensitivity to Sch-28080. The data also suggest that ouabain and Sch-28080 do not bind to the same site on the chimera.

Transmembrane topology of alpha- and beta-subunits of Na+,K+-ATPase derived from beta-galactosidase fusion proteins expressed in yeast

Various models of the transmembrane topology of the Na+,K+-ATPase predict either 8 or 10 membrane spans for the alpha-subunit and one to three membrane spans for the beta-subunit. Structure/function analysis, however, requires precise knowledge about the folding of enzymes. Therefore, the intention of this work was to establish a transmembrane topology model for the subunits of Na+,K+-ATPase. The bacterial enzyme beta-galactosidase was fused to the C termini of truncated alpha- and beta-subunits of Na+,K+-ATPase. Fusions were generated at Arg60 (LTTAR60), Glu116 (AATEE116), Ala247 (VEGTA247), Leu311 (YTWEL311), Ala444 (VAGDA444), Ala789 (IFIIA789), Met809 (LGTDM809), Asp884 (RVTWD884), Ile946 (MKNKI946), and Arg972 (GVALR972) of the sheep alpha1-subunit and at Pro236 (LGGYP236) of the dog beta-subunit. The fusion constructs were expressed in yeast cells for studies on the localization of the fused reporter enzyme. Activity measurements of the reporter enzyme revealed that only intracellular fusion sites lead to active beta-galactosidase. Indirect immunofluorescence microscopy with cells expressing alpha1/beta-galactosidase and beta/beta-galactosidase hybrid proteins demonstrated that inactive beta-galactosidase is associated with the yeast plasma membrane and can be detected from the extracellular side. The data obtained suggest that Pro236 of the beta-subunit is located on the extracellular surface, corresponding to a model with one transmembrane segment, and that the alpha-subunit of the Na+,K+-ATPase consists of 10 membrane-associated spans. They also suggest that a stretch of the alpha1-subunit between membrane spans M7 and M8 might be hidden within the membrane, surrounded by the other hydrophobic spans, in analogy to the P-loop of Na+ or K+ channels and to the "hourglass" structure of water channels.

Probing conserved regions of the cytoplasmic LOOP1 segment linking transmembrane segments 2 and 3 of the Saccharomyces cerevisiae plasma membrane H+-ATPase

Genetic probing was used to examine conserved amino acid clusters in the first cytoplasmic loop domain (LOOP1) linking transmembrane segments 2 and 3 of the plasma membrane H+-ATPase from Saccharomyces cerevisiae. Deletion of the LOOP1 region in PMA1 resulted in a defective enzyme. Scanning alanine mutagenesis of conserved residues produced lethal cell phenotypes in 14 of 26 amino acids, suggesting major enzyme defects. Most viable mutants showed growth characteristics that were comparable to wild type. Two mutations, I183A and D185A, produced reduced growth rates, hygromycin B resistance, and low pH sensitivity, which are phenotypes associated with defects in the H+-ATPase. However, both mutant enzymes displayed near-normal kinetics for ATP hydrolysis in vitro. Localized random mutagenesis was also performed at sites Glu195, Val196, and Ile210, which all showed lethal phenotypes upon conversion to alanine. Amino acids with polar side groups could substitute for Glu195, while Val196 could not tolerate polar side group moieties. Nine mutations at Ile210 proved lethal, including K, R, E, P, H, N, V, G, and A, while functional enzyme was obtained with S, C, M, and L. Normal rates and extents of pH gradient formation were observed for all mutant enzymes, except I183A and D185A. Detailed analysis of the I183A enzyme indicated that it hydrolyzed ATP like wild type, but it appeared to inefficiently couple ATP hydrolysis to proton transport. In total, these results affirm that conserved amino acids in LOOP1 are important to H+-ATPase function, and purturbations in this region can alter the efficiency of energy coupling.

Localization of alpha 1,2,3-subunit isoforms of Na,K-ATPase in cultured neonatal and adult rat myocardium: the immunofluorescence and immunocytochemical study

By indirect immunofluorescence and preembedding peroxidase-diaminobenzidine technique the localization of polyclonal and monoclonal antibodies against alpha 1, alpha 2 and alpha 3 isoforms of the Na,K-ATPase were studied in rat myocardium. The alpha 1-subunit was identified predominantly on sarcolemma of cultured myocytes, neonatal, as well as adult cardiocytes. The alpha 2 signal was localized around nuclei of cultured cardiocytes, very weak signals were seen in neonatal and more intense signal, were dispersed throughout the adult myocytes. The alpha 3-subunit immunoreactivity was weak and localized in cell processes connecting individual cultured cells, on sarcolemma and intercalated discs of neonatal cells and very weak in adult working myocytes. Cytochemically demonstrated ouabain resistant Na,K-ATPase localized in junctional sarcoplasmic reticulum may represent alpha 1 isoenzyme which is directly involved in modulation of action potential fluxes.

Random mutagenesis of the sheep Na,K-ATPase alpha1 subunit generating the ouabain-resistant mutant L793P

The polymerase chain reaction was used to randomly mutagenize a cDNA cassette encoding amino acids 691-946 of the sheep Na,K-ATPase alpha subunit. The mutagenized cassettes were used to replace the wild-type region in the full-length cDNA, and pools of mutants were transfected into HeLa cells. After the generation of resistant cells via selection in 0.5 microM ouabain, polymerase chain reaction was used to amplify the mutagenized cassette from the genomic DNA of the stable transfectants. Sequence analysis of the polymerase chain reaction product revealed three amino acid substitutions: I729V, L793P, and K836R. Subsequent site-directed mutagenesis experiments showed that only L793P was important for resistance. To elucidate the role of L793 in ouabain inhibition, additional mutations at this position were prepared. L793A and L793I mutants were constructed and expressed in HeLa cells. Only L793A survived selection using ouabain, which suggested that resistance is not due to the specific substitution of leucine with proline. To explore the mechanism of resistance, apparent affinities of the L793P mutant for sodium and potassium were compared to the wild-type HeLa pump. Although the apparent affinities were comparable for sodium, the mutant had a 2-fold higher apparent affinity for potassium. This suggests that the mechanism of ouabain insensitivity of L793P is due to a perturbation in the region of the enzyme that may include the K+ binding site.

Chymotryptic digestion of the cytoplasmic domain of the beta subunit of Na/K-ATPase alters kinetics of occlusion of Rb+ ions

This paper demonstrates that specific chymotryptic digestion of the cytoplasmic domain of the beta subunit of Na/K-ATPase leads to changes in the kinetics of occlusion of Rb+ ions. The experiments utilize extensively trypsinized Na/K-ATPase, "19-kDa membranes," which lack cytoplasmic loops of the alpha subunit, whereas membrane-embedded fragments (a COOH-terminal 19 kDa and three fragments of 8.1-11.7 kDa) containing transmembrane segments and extracellular loops are intact. The beta subunit is partially split into NH2- and COOH-terminal fragments of 16 and approximately 50 kDa, respectively. Cation occlusion and ouabain binding are preserved. The 19-kDa membranes were incubated, at 37 degrees C, with a selection of proteases, in the presence of Rb+ ions. In these conditions, only alpha-chymotrypsin destroyed the ability to occlude Rb+ ions. This process was associated with truncation of the 16-kDa fragment of the beta subunit in two stages. In the first stage, chymotrypsin removed 10 residues from the 16-kDa fragment to form a 15-kDa fragment (NH2-terminal Ile15) and 4 or 6 residues from the NH2 terminus of the alpha subunit fragment beginning at Asp68. In these membranes Rb+ occlusion was still intact at 37 degrees C. Strikingly, however, deocclusion of two Rb+ ions, which is characteristically biphasic in 19-kDa membranes, displayed deocclusion kinetic with mainly one fast phase. These membranes also showed a much lower affinity for Rb+ ions compared with 19-kDa membranes; and, consistent with the lower Rb+ affinity, Rb+ ions, at nonsaturating concentrations, protected less well against thermal inactivation of Rb+ occlusion. In the second stage, the 15-kDa fragment was truncated further to a 14-kDa fragment (NH2-terminal Leu24), followed by thermal destabilization of Rb+ occlusion even at high concentrations of Rb+ ions. Eventually, the thermally inactivated complex of fragments of alpha and beta subunits was digested to the limit peptides. The results suggest that the cytoplasmic domain of the beta subunit interacts with that of the alpha subunit, possibly with residues leading into the first transmembrane segment, and controls access of Rb+ ions into or out of the occlusion sites.

Probing the cytoplasmic LOOP1 domain of the yeast plasma membrane H(+)-ATPase by targeted factor Xa proteolysis

The cytoplasmic domain linking transmembrane segments 2 and 3 (LOOP1) of the yeast H(+)-ATPase was probed by the introduction of unique factor Xa recognition sites. Three sites, I170EGR, I254EGR and I275EGR, representing different structural regions of the LOOP1 domain, were engineered by site-specific mutagenesis of the PMA1 gene. In each case, multiple amino acid substitutions were required to form the factor Xa sites, which enabled an analysis of clustered mutations. Both I170EGR and I275EGR-containing mutants grew at normal rates, but showed prominent growth resistance to hygromycin B and sensitivity to low external pH. The engineered I254EGR site within the predicted beta-strand region produced a recessive lethal phenotype, indicating that mutations G254I and F257R were not tolerated. Mutant I170EGR- and I275EGR-containing enzymes showed relatively normal Km and Vmax values, but they displayed a strong insensitivity to inhibition by vanadate. An I170EGR/I275EGR double mutant was more significantly perturbed showing a reduced Vmax and pronounced vanadate insensitivity. The I170EGR site within the putative alpha-helical stalk region was cleaved to a maximum of 10% by factor Xa under non-denaturing conditions resulting in a characteristic 81 kDa fragment, whereas the I275EGR site, near the end of the beta-strand region, showed about 30-35% cleavage with the appearance of a 70 kDa fragment. A I170EGR/I275EGR double mutant enzyme showed about 55-60% cleavage. The cleavage profile for the mutant enzymes was enhanced under denaturing conditions, but was unaffected by MgATP or MgATP plus vanadate. Cleavage at the I275EGR position had no adverse effects on ATP hydrolysis or proton transport by the H(+)-ATPase making it unlikely that this localized region of LOOP1 influences coupling. Overall, these results suggest that the local region encompassing I275EGR is accessible to factor Xa, while the region around I170EGR appears buried. Although there is no evidence for gross molecular motion at either site, the effects of multiple amino acid substitutions in these regions suggest that the LOOP1 domain is conformationally active, and that perturbations in this domain affect the distribution of conformational intermediates during steady-state catalysis.

Subcellular distribution of sodium pump isoform subunits in mammalian cardiac myocytes

The cardiac Na+ pump (Na+ -K+ -ATPase) provides the driving force for the Na+/Ca2+ exchanger, a determinant of intracellular Ca2+ stores. Three Na+ pump alpha-catalytic subunit isoforms are expressed in human heart, alpha1 and alpha2 are expressed in rat heart, and only alpha1 is expressed in guinea pig heart. The objective of this study was to determine whether there are isoform-specific patterns of expression in the transverse tubules (T tubules) vs. the peripheral sarcolemma. In adult rat cardiomyocytes, anti-alpha1-specific antibodies labeled the T tubules more intensely than the peripheral sarcolemma, in which labeling was patchy, the same pattern reported for distribution of the Na+/Ca2+ exchanger (J. S. Frank, G. Mottino, D. Reid, R. S. Molday, and K. D. Philipson, J. Cell Biol. 117: 337-345, 1992), whereas anti-alpha2- and anti-beta1-antibodies uniformly labeled T tubules and peripheral sarcolemma. In guinea pig cardiomyocytes, an anti-alpha-antibody against an extracellular epitope evenly labeled the peripheral sarcolemma and T tubules, and immunogold labeling demonstrated coincidence of alpha-subunits and intramembranous particles in sarcolemma. In summary, Na+ pumps are located in both peripheral sarcolemma and T tubules of cardiomyocytes expressing either multiple or single Na+ pump isoforms.

Genetic probing of the first and second transmembrane helices of the plasma membrane H(+)-ATPase from Saccharomyces cerevisiae

Structural features of the putative helical hairpin region comprising transmembrane segments 1 (TM1) and 2 (TM2) of the yeast plasma membrane H(+)-ATPase were probed by site-directed mutagenesis. The importance of phenylalanine residues Phe-116, Phe-119, Phe-120, Phe-126, Phe-144, Phe-159, and Phe-163 was explored by alanine replacement mutagenesis. It was found that substitutions at all positions, except Phe-120 and Phe-144, produced viable enzymes, although a range of cellular growth phenotypes were observed like hygromycin B resistance and low pH sensitivity, which are linked to in vivo action of the H(+)-ATPase. Lethal positions Phe-120 and Phe-144, could be replaced with tryptophan to produce viable enzyme, although the F144W mutant was highly perturbed. ATP hydrolysis measurements showed that Km was not significantly altered for most mutant enzymes, whereas Vmax was moderately reduced with two mutants, F144W and F163A, showing less than 50% of the normal activity. Double Phe-->Ala mutations in TM1 and TM2 were constructed to examine whether such substitutions would result in a higher degree of enzyme destabilization. Mutant F116A/F119A was viable and gave a normal phenotype, while F159A/F163A was not viable. Other double mutants, F116A/F159A and F119AF/159A, which are predicted to lie juxtaposed on TM1 and TM2, produced non-functional enzymes. However, a viable F119V/F159A mutant was isolated and showed hygromycin B resistance. These results suggest that double mutations eliminating 2 phenylalanine residues strongly destabilize the enzyme. A putative proline kink at Gly-122/Pro-123 in TM1 is not essential for enzyme action since these residues could be variously substituted (G122A or G122N; P123A, P123G, or P123F) producing viable enzymes with moderate effects on in vitro ATP hydrolysis or proton transport. However, several substitutions produced prominent growth phenotypes, suggesting that local perturbations were occurring. The location of Pro-123 is important because Gly-122 and Pro-123 could not be exchanged. In addition, a double Pro-Pro created by a G122P mutation was lethal, suggesting that maintenance of an alpha-helical structure is important. Other mutations in the hairpin, including modification of a buried charged residue, E129A, were not critical for enzyme action. These data are consistent with the view that the helical hairpin comprising TM1 and TM2 has important structural determinants that contribute to its overall stability and flexibility.

Labeling of a cysteine in the cardiotonic glycoside binding site by the steroid derivative HDMA

The digoxigenin derivative N-hydroxysuccinimidyl digoxigenin-3-O-methylcarbonyl-epsilon-aminocaproate (HDMA) has been shown to covalently label the ouabain binding site of the Na,K-ATPase epsilon subunit [Antolovic et al. (1995) Eur. J. Biochem. 227, 61-67]. In the present study we observed both, labeling and inactivation of the activity, of wild type Na,K-ATPase overexpressed in Xenopus oocyte. In contrast, no significant inhibition and no labeling could be detected when a Cys-113 of the first transmembrane segment was mutated to serine, although the affinity of this mutant for digoxigenin or HDMA measured in acute inhibition experiments was similar to the wild type. This indicates that after docking of its genin moiety, HDMA can form a thioester bond with Cys-113.

Four hydrophobic segments in the NH2-terminal third (H1-H4) of Na,K-ATPase alpha subunit alternately initiate and halt membrane translocation of the newly synthesized polypeptide

Transmembrane disposition of the NH2-terminal third of the Na,K-ATPase alpha subunit was studied using an experimental approach that involved in vitro endoplasmic reticulum membrane insertion of chimeras. These chimeras consisted of four truncated amino-terminal segments of the alpha subunit linked at amino acid residues 126, 179, 313, and 439 to chloramphenicol acetyltransferase (CAT), a reporter protein, that contains a consensus sequence for N-linked glycosylation. The fusion sites were located after one of the four hydrophobic segments (H1-H4). The results showed that the chimeras in which the alpha subunit was truncated at positions 126 and 313 were glycosylated, and the glycosylated peptides were protected by membranes from proteolysis. However, the other two chimeras were not glycosylated and the inserted peptides were digested by protease into fragments which did not immunoprecipitate with anti-CAT. These results clearly demonstrate that hydrophobic segments H1 and H3 function as signal/anchor type II, and H2 and H4 function as halt transfer signals. Furthermore, membrane insertion of the NH2-terminal third of Na,K-ATPase alpha subunit is achieved by a series of alternate signal/anchor type II and halt transfer sequences.

Topology of the Na,K-ATPase. Evidence for externalization of a labile transmembrane structure during heating

The topological organization of the Na,K-ATPase alpha subunit is controversial. Detection of extracellular proteolytic cleavage sites would help define the topology, and so attempts were made to find conditions and proteases that would permit digestion of Na,K-ATPase in sealed right-side-out vesicles from renal medulla. The beta subunit is predominantly extracellular and could mask the surface of the alpha subunit. Most of the tested proteases cleaved beta, and some digested it extensively. However, without further disruption of structure, there was still no digestion of the alpha subunit. Reduction (at 50 degrees C) of disulfide bonds that might stabilize the beta subunit fragments, or heating alone at 55 degrees C, permitted tryptic digestion of alpha at a site close to the C terminus, while simultaneously increasing digestion of beta. A 90-kDa N-terminal fragment of alpha was recovered, but the C-terminal fragment was further digested. Heating and reduction resulted in the extracellular exposure of a protein kinase A phosphorylation site, Ser-938, and the C terminus, both of which have been proposed to be located on the intracellular surface. At the same time, access to a distant protein kinase C phosphorylation site was not increased. The data suggest that the harsh treatment simultaneously resulted in alteration of the beta subunit and the extrusion of a segment of alpha that normally spans the membrane, without causing complete denaturation or opening the sealed vesicles. Preincubation with Rb+ was protective, consistent with prior evidence that it stabilizes the protein segments in the C-terminal third of alpha. We conclude that this portion of the alpha subunit contains a transmembrane structure with unique lability to heating.

Determination of the sidedness of the carboxy-terminus of the Na+/K(+)ATPase alpha-subunit using lactoperoxidase iodination

The orientation of the carboxy-terminal pair of tyrosines of the Na+/K(+)-ATPase alpha-subunit with respect to the plane of the plasma membrane was determined. The approach was based on lactoperoxidase-catalysed radioiodination of the tyrosine residues accessible on the surface of the enzyme molecule in intact cells of a pig kidney embryonic cell line and those accessible in a broken plasma membrane fraction and in isolated membrane-bound Na+/K(+)-ATPase. The labeled alpha-subunit was isolated by SDS gel electrophoresis followed by electroblotting. Then the COOH-terminal amino acids were hydrolyzed by carboxypeptidases B and Y. Radioactivity and quantitative analysis of the protein and released amino acids showed that the COOH-terminal tyrosine residues of the alpha-subunit were only accessible to modification only when lactoperoxidase had access to the inner side of the plasma membrane. Therefore, the COOH-terminus of the Na+/K(+)-ATPase alpha-subunit is located on the cytoplasmic surface of the pump molecule and its polypeptide chain must have an even number of transmembrane segments.

Location and properties of the digitalis receptor site in Na+/K(+)-ATPase

Since 1985, several research groups have shown that a number of amino acids in the catalytic alpha-subunit of Na+/K(+)-ATPase more or less strongly modulate the affinity of a digitalis compound like ouabain to the enzyme. However, scrutiny of these findings by means of chimeric Na+/K(+)-ATPase constructs and monoclonal antibodies has recently revealed that the modulatory effect of most of these amino acids does not at all result from direct interaction with ouabain, but rather originates from long-range effects on the properties of the digitalis binding matrix. Starting from this knowledge, the present review brings together the various pieces of evidence pointing to the conclusion that the interface between two interacting alpha-subunits in the Na+/K(+)-ATPase protodimer (alpha beta)2 provides the cleft for inhibitory digitalis intercalation.

Affinity labeling of a sulfhydryl group in the cardiacglycoside receptor site of Na+/K(+)-ATPase by N-hydroxysuccinimidyl derivatives of digoxigenin

Na+/K(+)-ATPase from pig kidney is inactivated by protein-reactive N-hydroxysuccinimidyl derivatives of digoxigenin. Like digoxigenin, its protein-reactive derivatives N-hydroxysuccinimidyl digoxigenin-3-methylcarbonyl-epsilon-aminocaproate (HDMA), 3-amino-3-deoxydigoxigenin hemisuccinimide succinimidyl ester (ADHS), 3-iodoacetylamino-3-deoxydigoxigenin (IAD) and digoxigenin-3-O-succinyl-[2-(N-maleimido)]ethylamide (DSME) inhibited the sodium pump in the presence of Na+, Mg2+ and ATP. At 37 degrees C, half-maximal inhibition of Na+/K(+)-ATPase was seen by HDMA at 0.47 microM, by ADHS at 5.8 microM, by IAD at 8 microM and by DSME at 94 microM. Thus, all compounds bind to the cardiac steroid receptor site of Na+/K(+)-ATPase. Affinity labeling of the alpha subunit by 'front door' or 'back door' phosphorylation was only seen with HDMA or ADHS in the range 0.1 microM. Excess of ouabain protected against affinity labeling. All the other protein-reactive derivatives of digoxigenin labeled the enzyme independent of the formation of a phosphointermediate at much higher concentrations. This labeling was not suppressed by an excess of ouabain. Tryptic hydrolysis of the HDMA-modified Na+/K(+)-ATPase gave peptides of the apparent molecular masses 20, 12.5 and 11.2 kDa. The 11.2-kDa and 12.5-kDa peptides started amino-terminally with Asp68, and the 20-kDa peptide with Asp24. Thus, the HDMA-labeled peptides originate from the cardioactive steroid-binding site formed by the first and second transmembrane helix. N-Hydroxysuccinimidyl esters such as HDMA are normally thought to modify lysine and arginine residues covalently. Since such residues do not exist in the putative cardiac glycoside-binding site, the possibility of a thioester formation of the digoxigenin derivatives HDMA and ADHS with Cys104 in the H1 transmembrane domain was tested. In fact, hydroxylaminolysis led to the release of the covalently bound HDMA, and the formation of a free sulfhydryl group. This could be labeled by [2-14C]ICH2COOH. We therefore propose, consistent with a recent conclusion from a site-directed mutagenesis experiment [Canessa, C. M., Horisberger, J.-D., Louvard, D. & Rossier, B. C. (1992) EMBO J. 11, 1681-1687], that a cysteine residue (probably Cys104) participates in the structure and function of the cardiac glycoside binding.

Na,K-ATPase characterized in artificial membranes. 1. Predominant conformations and ion-fluxes associated with active and inhibited states

The Na,K-ATPase (NKA) system is the receptor for the cardioactive steroids of plant or animal origin. It is not yet known whether passive ion fluxes traverse the inactivated receptor and thereby contribute to the hormonal, pharmacological or toxic actions of these compounds. To look for putative passive ion-fluxes across the ouabain-NKA complex, we incorporated it into the artificial membrane of liposomes. Since this synthetic membrane is virtually impermeable to Na and K ions, the hypothetical ion-fluxes mediated by the NKA can be determined. E2-forms and E2-ouabain-forms of purified NKA were incorporated, in parallel, into separate liposome preparations and the permeability of the resulting E2-liposomes and E2-ouabain-liposomes to K, Na and Ca ions was compared. The E2-liposomes expressed a typical K-permeability which was not observed in the E2-ouabain-liposomes; the latter showed a slightly higher Na-permeability and a similar Ca-permeability as compared to the former. Thus, ouabain does not induce leaks for K or Ca ions in the NKA molecule.

Analysis of amino acid residues in the H5-H6 transmembrane and extracellular domains of Na,K-ATPase alpha subunit identifies threonine 797 as a determinant of ouabain sensitivity

Several amino acid residues of the alpha subunit of the Na,K-ATPase have been identified which alter ouabain sensitivity. These residues are located in the N-terminal half of the alpha 1 subunit suggesting that this portion of the molecule may represent the binding site for cardiac glycosides. However, not all extracellular and transmembrane regions have been investigated, including the H5-H6 membrane-spanning region. To determine if this region of the alpha subunit contributes to ouabain sensitivity, amino acids which have the potential to form hydrogen bonds were substituted with alanine, a non-hydrogen-bonding amino acid. cDNAs encoding enzyme containing these individual amino acid replacements were expressed in ouabain-sensitive HeLa cells, and the ability of the altered enzymes to confer ouabain resistance was examined. Nineteen amino acid substitutions were investigated. T797A (Thr 797 to Ala) was the only substitution which conferred ouabain resistance to sensitive HeLa cells. Three additional substitutions at this position (T797V, T797S, and T797D) were generated in order to examine the effects of the replacements of Thr 797 on ouabain inhibition of Na,K-ATPase activity. The T797V substitution conferred ouabain resistance, but T797S and T797D substitutions did not. The ouabain-resistant cell lines expressing the T797A and T797V substitutions exhibited Na,K-ATPase activity that was 60 and 70 times more resistant to ouabain than the endogenous HeLa or sheep enzymes. The absence of a hydroxyl group at amino acid 797 may be responsible for the reduced sensitivity of the enzyme with substitutions at this position.(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling a conformationally sensitive region of the membrane sector of the fungal plasma membrane proton pump

A molecular model for transmembrane segments 1 and 2 from the fungal proton pumping ATPase has been developed, and this structure is predicted to form a helical hairpin loop structure in the membrane. This region was selected because it is highly conformationally active and is believed to be an important site of action for clinically important therapeutics in related animal cell enzymes. The hairpin loop is predicted to form an asymmetric tightly packed structure that is stabilized by an N-cap between D140 and V142, by hydrogen bonding between residues in the turn region and the helices, and by pi-pi interactions between closely apposed aromatic residues. A short four-residue S-shaped turn is stabilized by hydrogen bonding but is predicted to be conformationally heterogeneous. The principal effect of mutations within the hairpin head region is to destabilize the local close packing of side groups which disrupts the pattern of hydrogen bonding in and around the turn region. Depending on the mutation, this causes either a localized or a more global distortion of the primary structure in the hairpin region. These altered structures may explain the effects of mutations in transmembrane segments 1 and 2 on ATP hydrolysis, sensitivity to vanadate, and electrogenic proton transport. The conformational sensitivity of the hairpin structure around the S-turn may also account for the effects of SCH28080 and possibly ouabain in blocking ATPase function in related animal cell enzymes. Finally, the model of transmembrane segments 1 and 2 serves as a template to position transmembrane segments 3 and 8. This model provides a new view of the H(+)-ATPase that promotes novel structure/function experimentation and could serve as the basis for a more detailed model of the membrane sector of this enzyme.

Fungal plasma membrane proton pumps as promising new antifungal targets

Fungi are widely dispersed in nature and frequently appear as pathogens in the animal and plant kingdoms. The incidence of opportunistic fungal infections in humans has increased due to the human immunodeficiency virus and the application of modern medical approaches that subvert natural protective barriers to infection. Also, fungal blights continue to threaten crops worldwide. As a result, new antifungal agents are needed to address these critical problems. Existing antifungals can be used to effectively treat most cases of topical infection caused by the opportunistic pathogen Candida albicans, which is the principal agent of nosocomially acquired fungal infections. However, life-threatening, disseminated Candida infections are treated with more modest success. Existing antifungals can be toxic or ineffective because of natural resistance or even induced resistance. This limited efficacy largely reflects the restricted range of cellular targets considered during the development of current antifungals. The advancement of highly selective fungicidal reagents requires the recognition of new essential cellular targets. The fungal plasma-membrane proton pump is a high-abundance essential enzyme with a number of well-understood molecular properties that should facilitate the development of new antifungals. The proton pump is important for intracellular pH regulation and the maintenance of electrochemical proton gradients needed for nutrient uptake. It is a member of the P-type class of ion-transport enzymes, which are present in nearly all external cellular membranes. Typical P-type enzymes such as the Na+,K(+)-ATPase and H+,K(+)-ATPase are well established as specific targets for surface-active cardiac glycosides and anti-ulcer therapeutics. The development of new classes of selective antifungals targeted to the proton pump will require exploitation of the well-characterized genetic, kinetic, topological, regulatory, and drug-interaction features of the fungal enzyme that discriminate it from related host P-type enzymes. New antifungal drugs of this type should be relevant to the control of fungal pathogens of medical and agricultural importance and may be applicable to the control of intracellular parasites that also depend on closely related proton pumps for survival.

Application of three-dimensional molecular hydrophobicity potential to the analysis of spatial organization of membrane domains in proteins. III. Modeling of intramembrane moiety of Na+, K(+)-ATPase

The most probable interlocation of transmembrane alpha-helices of Na+, K(+)-ATPase has been calculated by a computer-aided molecular simulation approach in the framework of models with eight and 10 helical peptides for the alpha-subunit. The method is based on the concept of three-dimensional molecular hydrophobicity potential (MHP) and provides valuable description of spatial hydrophobic properties of membrane-spanning segments as well as helix-helix packing interactions inside the membrane. Resulting model of the arrangement of intramembrane domain agrees with recent results on hydrophobic photolabeling of an intramembrane part of the beta-subunit and the sixth transmembrane segment of the alpha-subunit. It is also consistent with current ideas on hydrophobic organization of integral membrane proteins. Possible topology of a cation-binding site is discussed.

Substitution of transmembrane residues with hydrogen-bonding potential in the alpha subunit of Na,K-ATPase reveals alterations in ouabain sensitivity

The role of H-bonding amino acids as determinants of ouabain affinity in the Na,K-ATPase was examined. Site-directed mutagenesis was used to substitute 21 conserved amino acid residues in the sheep alpha-subunit transmembrane regions. The amino acids were changed from residues which possess side chains capable of forming H-bonds with specific cardiac glycoside moieties such as the lactone ring or sugar(s) to residues unable to participate in H-bonding. The effect of each of these amino acid replacements on the affinity of the Na,K-ATPase for ouabain was initially assessed by screening the altered enzymes for the ability to confer ouabain resistance when expressed in otherwise sensitive HeLa cells. Three of the substitutions (Tyr-108 to Ala, Cys-104 to Ala, and Cys-104 to Phe) were able to confer resistance to the normally sensitive HeLa cells. Stable cell lines, each expressing one of the altered enzymes, were further characterized in terms of ouabain-inhibitable cell growth and Na,K-ATPase activity. Cell lines expressing the alpha 1-isoform substitution Y108A, C104A, or C104F contained a Na,K-ATPase activity which gave an I50 value for enzyme inhibition 9-, 6-, and 150-fold greater, respectively, than the endogeneous HeLa or sheep enzyme. These data show that Tyr-108 and Cys-104 of the alpha subunit are determinants of ouabain affinity. Cys-104 has also been shown to be a determinant of ouabain sensitivity in Xenopus laevis [Canessa, C. M., Horisberger, J.-D., Louvard, D., & Rossier, B. C. (1992) EMBO J. 11, 1681-1687].(ABSTRACT TRUNCATED AT 250 WORDS)

Constraints on models for the folding of the Na,K-ATPase

We have attempted to bring together in graphic fashion the available evidence on the structure of the Na,K-ATPase and the H,K-ATPase. There appears to be much room for modification of the existing models for transmembrane folding. More sites on each side of the membrane need to be identified. Whether these will be antibody epitopes, sites of covalent modification, or tags inserted by mutagenesis is less important than that there be many of them and that each be verified by alternative approaches. If any single principle has emerged from the study of the topography of membrane proteins, it is that it is easy to reach conclusions too soon.