Three-dimensional structure of renal Na,K-ATPase determined by electron microscopy of membrane crystals

A structural overview of the plasma membrane Na+,K+-ATPase and H+-ATPase ion pumps

Plasma membrane ATPases are primary active transporters of cations that maintain steep concentration gradients. The ion gradients and membrane potentials derived from them form the basis for a range of essential cellular processes, in particular Na(+)-dependent and proton-dependent secondary transport systems that are responsible for uptake and extrusion of metabolites and other ions. The ion gradients are also both directly and indirectly used to control pH homeostasis and to regulate cell volume. The plasma membrane H(+)-ATPase maintains a proton gradient in plants and fungi and the Na(+),K(+)-ATPase maintains a Na(+) and K(+) gradient in animal cells. Structural information provides insight into the function of these two distinct but related P-type pumps.

Renal Na,K-ATPase structure from cryo-electron microscopy of two-dimensional crystals

The molecular structure of Na,K-ATPase was determined by electron crystallography from two-dimensional crystals induced in purified membranes isolated from the outer medulla of pig kidney. The P2 type unit cell contains two protomers in the E(2) conformation, each of them with a size of 65 x 75 x 150 A(3). The alpha, beta, and gamma subunits in the membrane crystals were demonstrated in the crystals with Western blotting and related to distinct domains in the density map. The alpha subunit corresponds to most of the density in the transmembrane region as well as to the large hydrophilic headpiece on the cytoplasmic side of the membrane. The headpiece is divided into three separated domains. One of these gives rise to an elongated projection onto the membrane plane, while the putative nucleotide binding and phosphorylation domains form compact densities in the rest of the cytoplasmic part of the structure. Density on the extracellular face corresponds to the protein part of the beta subunit. Ten helices from the catalytic a subunit correspond to two groups of distinct densities in the transmembrane region. The structure of the lipid bilayer spanning part also suggests positions for the transmembrane helices from the beta and gamma subunits. The overall structure of the alpha subunit of Na,K-ATPase as determined here by cryo-electron microscopy is similar to the X-ray structure of Ca-ATPase. However, conformational changes between the E(1) and E(2) forms are suggested by different relative positions of cytoplasmic domains.

Three-dimensional structure of renal Na,K-ATPase from cryo-electron microscopy of two-dimensional crystals

The structure of Na, K-ATPase was determined by electron crystallography at 9.5 A from multiple small 2-D crystals induced in purified membranes isolated from the outer medulla of pig kidney. The density map shows a protomer stabilized in the E(2) conformation which extends approximately 65 A x 75 A x 150 A in the asymmetric unit of the P2 type unit cell. The alpha, beta, and gamma subunits were demonstrated in the membrane crystals with Western blotting and related to distinct domains in the density map. The alpha subunit corresponds to most of the density in the transmembrane region as well as the large hydrophilic headpiece on the cytoplasmic side of the membrane. The headpiece is divided into three separated domains, which are similar in overall shape to the domains of the calcium pump of the sarcoplasmic reticulum. One of these domains gives rise to a characteristic elongated projection onto the membrane plane while the putative nucleotide binding and phosphorylation domains form comparatively compact densities in the rest of the cytoplasmic part of the structure. Density on the extracellular face corresponds to the protein part of the beta subunit and is located as an extension of the transmembrane region perpendicular to the membrane plane. The structure of the lipid bilayer spanning part suggests the positions for the transmembrane helix from the beta subunit as well as the small gamma subunit present in this Na,K-ATPase. Two groups of ten helices from the catalytic alpha subunit corresponds to the remaining density in the transmembrane region. The present results demonstrate distinct similarities between the structure of the alpha subunit of Na,K-ATPase as determined here by cryo-electron microscopy and the reported X-ray structure of Ca-ATPase. However, conformational changes between the E(1) and E(2) forms are suggested by different relative positions of cytoplasmatic domains.

The thermal unfolding and domain structure of Na+/K+-exchanging ATPase. A scanning calorimetry study

The thermal unfolding and domain structure of Na+/K+-ATPase from pig kidney were studied by high-sensitivity differential scanning calorimetry (HS-DSC). The excess heat capacity function of Na+/K+-ATPase displays the unfolding of three cooperative domains with midpoint transition temperatures (Td) of 320.6, 327.5, 331.5 K, respectively. The domain with Td = 327.5 K was identified as corresponding to the beta subunit, while two other domains belong to the alpha subunit. The thermal unfolding of the low-temperature domain leads to large changes in the amplitude of the short-circuit current, but has no effect on the ATP hydrolysing activity. Furthermore, dithiothreitol or 2-mercaptoethanol treatment causes destruction of this domain, accompanied by significant disruption of the ion transporting function and a 25% loss of ATPase activity. The observed total unfolding enthalpy of the protein is rather low (approximately 12 J.g-1), suggesting that thermal denaturation of Na+/K+-ATPase does not lead to complete unfolding of the entire molecule. Presumably, transmembrane segments retain most of their secondary structure upon thermal denaturation. The binding of physiological ligands results in a pronounced increase in the conformational stability of both enzyme subunits.

Structure of Na+,K+-ATPase at 11-A resolution: comparison with Ca2+-ATPase in E1 and E2 states

Na+,K+-ATPase is a heterodimer of alpha and beta subunits and a member of the P-type ATPase family of ion pumps. Here we present an 11-A structure of the heterodimer determined from electron micrographs of unstained frozen-hydrated tubular crystals. For this reconstruction, the enzyme was isolated from supraorbital glands of salt-adapted ducks and was crystallized within the native membranes. Crystallization conditions fixed Na+,K+-ATPase in the vanadate-inhibited E2 conformation, and the crystals had p1 symmetry. A large number of helical symmetries were observed, so a three-dimensional structure was calculated by averaging both Fourier-Bessel coefficients and real-space structures of data from the different symmetries. The resulting structure clearly reveals cytoplasmic, transmembrane, and extracellular regions of the molecule with densities separately attributable to alpha and beta subunits. The overall shape bears a remarkable resemblance to the E2 structure of rabbit sarcoplasmic reticulum Ca2+-ATPase. After aligning these two structures, atomic coordinates for Ca2+-ATPase were fit to Na+,K+-ATPase, and several flexible surface loops, which fit the map poorly, were associated with sequences that differ in the two pumps. Nevertheless, cytoplasmic domains were very similarly arranged, suggesting that the E2-to-E1 conformational change postulated for Ca2+-ATPase probably applies to Na+,K+-ATPase as well as other P-type ATPases.

Reconstitution of detergent-solubilized Na,K-ATPase and formation of two-dimensional crystals

Very pure, detergent-solubilized Na,K-ATPase from dog or lamb kidneys has been successfully reconstituted at high protein-to-lipid weight ratios. Studies have been conducted to establish the orientation of the Na,K-ATPase molecules in the reconstituted membranes and to assess the functional activity and the conformational state of the reconstituted enzyme. Results indicate that reincorporation of the Na,K-ATPase molecules in the lipid bilayer is unidirectional and that the reconstituted enzyme retains its functional and structural integrity. Two-dimensional crystals have been induced in these preparations by vanadate ions. The arrays, with a dimeric structure in the unit cell, have a morphology similar to that of the crystals that had previously formed in the native membranes. Filtered images show that in projection, the molecule had an asymmetrical mass distribution, which at the resolution of 2.5 nm is identical to that of the earlier crystals. These sheets, although small, represent the first crystals of Na, K-ATPase to be formed by reconstitution. We expect that optimization of the reconstitution and crystallization parameters will lead to larger and better-ordered sheets, suitable for electron crystallography.

Secondary structure of the membrane-bound domains of H+,K+-ATPase and Ca2+-ATPase, a comparison by FTIR after proteolysis treatment of the native membranes

The sarcoplasmic reticulum Ca2+-ATPase and the gastric H+,K+-ATPase were cleaved under three different proteolysis conditions. After elimination of the protease and of the cleaved peptides, the vesicles containing the membrane-bound peptides of the ATPases were studied by Fourier transform attenuated total reflection infrared spectroscopy. In the harsher proteolysis conditions, the membrane-associated domain of the Ca2+-ATPase represented about 20% of the protein and was mainly constituted of alpha-helices. Polarized infrared spectroscopy showed that these alpha-helices were mainly oriented perpendicular to the membrane. However, only 10-20% of the H+,K+-ATPase was cleaved. The remaining, membrane-associated domain of the protein contained about 30% of alpha-helices and 30% of beta-sheet structures. The alpha-helices adopted a mainly transmembrane orientation. While the data on the Ca2+-ATPase are in general agreement with the current model of the protein, our results indicate that caution must be used in choosing this protein as a general structural model for all P-type ATPases. The protease-resistant, membrane-associated domain of the H+K+-ATPase is indeed much larger than predicted and also contained beta-sheet structures.

Two-dimensional crystallization of Ca-ATPase by detergent removal

By using Bio-Beads as a detergent-removing agent, it has been possible to produce detergent-depleted two-dimensional crystals of purified Ca-ATPase. The crystallinity and morphology of these different crystals were analyzed by electron microscopy under different experimental conditions. A lipid-to-protein ratio below 0.4 w/w was required for crystal formation. The rate of detergent removal critically affected crystal morphology, and large multilamellar crystalline sheets or wide unilamellar tubes were generated upon slow or fast detergent removal, respectively. Electron crystallographic analysis indicated unit cell parameters of a = 159 A, b = 54 A, and gamma = 90 degrees for both types of crystals, and projection maps at 15-A resolution were consistent with Ca-ATPase molecules alternately facing the two sides of the membrane. Crystal formation was also affected by the protein conformation. Indeed, tubular and multilamellar crystals both required the presence of Ca2+; the presence of ADP gave rise to another type of packing within the unit cell (a = 86 A, b = 77 A, and gamma = 90 degrees), while maintaining a bipolar orientation of the molecules within the bilayer. All of the results are discussed in terms of nucleation and crystal growth, and a model of crystallogenesis is proposed that may be generally true for asymmetrical proteins with a large hydrophilic cytoplasmic domain.

How to make tubular crystals by reconstitution of detergent-solubilized Ca2(+)-ATPase

In an attempt to better define the parameters governing reconstitution and two-dimensional crystallization of membrane proteins, we have studied Ca2(+)-ATPase from rabbit sarcoplasmic reticulum. This ion pump forms vanadate-induced crystals in its native membrane and has previously been reconstituted at high lipid-to-protein ratios for functional studies. We have characterized the reconstitution of purified Ca2(+)-ATPase at low lipid-to-protein ratios and discovered procedures that produce long, tubular crystals suitable for helical reconstruction. C12E8 (n-dodecyl-octaethylene-glycol monoether) was used to fully solubilize various mixtures of lipid and purified Ca2(+)-ATPase, and BioBeads were then used to remove the C12E8. Slow removal resulted in two populations of vesicles, and the proteoliposome population was separated from the liposome population on a sucrose density gradient. These proteoliposomes had a lipid-to-protein ratio of 1:2, and virtually 100% of molecules faced the outside of vesicles, as determined by fluorescein isothiocyanate labeling. Cycles of freeze-thaw caused considerable aggregation of these proteoliposomes, and, if phosphatidyl ethanolamine and phosphatidic acid were included, or if the bilayers were doped with small amounts of C12E8, vanadate-induced tubular crystals grew from the aggregates. Thus our procedure comprised two steps-reconstitution followed by crystallization-allowing us to consider mechanisms of bilayer formation separately from those of crystallization and tube formation.

Three-dimensional structure of the porcine gastric H,K-ATPase from negatively stained crystals

A low-resolution three-dimensional model of membrane-bound H,K-ATPase from pig gastric mucosa has been reconstructed by electron microscopy and image processing of two-dimensional crystals in negative stain. The crystal formation is induced by magnesium and vanadate, which stabilize the E2 conformation of the enzyme. The unit cell, with a size of a = b = 123 A, gamma = 90 degrees, has tetragonal p4 symmetry. There are four separate alpha beta protomers within each unit cell. The high-contrast region is limited to the cytoplasmic part of the protein. The total volume of the observed asymmetric protein domain corresponds to a molecular mass of 80-90 kDa. It consists mainly of a large pear-shaped domain measuring 60 x 45 A2, with a height of 50 A as measured perpendicular to the membrane plane. A small stalk segment, 20 A in length, forms a connection to the transmembrane region.

The carbohydrate moieties of the beta-subunit of Na+, K(+)-ATPase: their lateral motions and proximity to the cardiac glycoside site

The beta-subunit associated with the catalytic (alpha) subunit of the mammalian Na+, K(+) -ATPase is a transmembrane glycoprotein with three extracellularly located N-glycosylation sites. Although beta appears to be essential for a functional enzyme, the role of beta and its sugars remains unknown. In these studies, steady-state and dynamic fluorescence measurements of the fluorophore lucifer yellow (LY) covalently linked to the carbohydrate chains of beta have demonstrated that the bound probes are highly solvent exposed but restricted in their diffusional motions. Furthermore, the probes' environments on beta were not altered by Na+ or K+ or ouabain-induced enzyme conformational changes, but both divalent cation and oligomycin addition evoked modest changes in LY fluorescence. Frequency domain measurements reflecting the Förster fluorescence energy transfer (FET) occurring between anthroylouabain (AO) bound to the cardiac glycoside receptor site on alpha and the carbohydrate-linked LY demonstrated their close proximity (18 A). Additional FET determinations made between LY as donor and erythrosin-5-isothiocyanate, covalently bound at the enzyme's putative ATP binding site domain, indicated that a distance of about 85 A separates these two regions and that this distance is reduced upon divalent cation binding and increased upon the Na+E1-->K+E2 conformational transition. These data suggest a model for the localization of the terminal moieties of the oligosaccharides that places them, on average, about 18 A from the AO binding site and this distance or less from the extracellular membrane surface.

Structure, transmembrane topology and helix packing of P-type ion pumps

Electron microscopy has recently provided improved structures for P-type ion pumps. In the case of Ca(2+)-ATPase, the use of unstained specimens revealed the structure of the transmembrane domain. The composition of this domain has been controversial due to the variety of methods used to study the number and exact locations of transmembrane crossings within the sequence. After reviewing the results from several members of the family, we found a consensus for 10 transmembrane segments, and also that 10 helices fitted well into the structure of Ca(2+)-ATPase. Thus, we present the most detailed model for transmembrane structure so far, in the hope of stimulating more precise experimental strategies.

Local translational diffusion rates of membranous Na+,K(+)-ATPase measured by saturation transfer ESR spectroscopy

Diffusion-controlled Heisenberg spin exchange between spin-labeled Na+,K(+)-ATPase [ATP phosphohydrolase (Na+/K(+)-transporting), EC 3.6.1.37] proteins has been studied by saturation transfer ESR spectroscopy in reconstituted membranes. Na+,K(+)-ATPase from the salt gland of Squalus acanthias was solubilized in a polyoxyethylene ether detergent, octa(ethylene glycol) dodecyl monoether. Part of the solubilized enzyme was covalently spin-labeled with a nitroxide derivative of indanedione and recombined with various proportions of the unlabeled enzyme while the native lipid/protein ratio was maintained. Purified membranes were then reconstituted from the various samples by precipitation with divalent ions. The reciprocal integrated intensities of the saturation transfer ESR spectra were found to increase linearly with the fraction of protein that was spin-labeled, and the gradient of the concentration dependence increased with increasing temperature over the range 4 degrees-25 degrees C. Comparison with theoretical analyses of the effects of weak Heisenberg spin exchange [Marsh, D. & Horváth, L. I. (1992) J. Magn. Reson. 97, 13-26] suggests that the effects on the saturation transfer ESR intensity are attributable to short-range diffusional collisions between the spin-labeled protein molecules. The effective value of the local translational diffusion coefficient is 1.8-2.9 microns2.s-1 at 15 degrees C, depending on the diffusion model used, which is much larger than the values obtained for the long-range diffusion coefficient in cells by photobleaching techniques. The temperature dependence of the translational diffusion is larger than expected but correlates with the anomalous temperature dependence of the rotational diffusion observed in the same system.

Na,K-ATPase in crystalline form investigated by scanning force microscopy

Na,K-ATPase has been isolated in purified membrane fragments from kidney tissue and crystallized by phospholipase treatment to obtain two-dimensional, membrane-bound protein crystals. Scanning force microscopy has been used to identify and analyze the topography of the membrane fragments. Specific patterns in accordance with electron microscopic images have been found. In biological material under physiological conditions the scanning force is a crucial parameter for the resulting image at high resolution.

Three-dimensional structure of Na,K-ATPase determined from membrane crystals induced by cobalt-tetrammine-ATP

The three-dimensional structure of Na,K-ATPase has been analyzed with electron microscopy and image processing. The enzyme, purified from pig kidney outer medulla, was arranged in a new form of tetragonal two-dimensional membrane crystals after incubation with cobalt-tetrammine-ATP, a stable MgATP complex analogue. Each continuous protein domain, as delineated by negative stain, consists of two alpha beta-protomers related by a dyad axis. The two rod-like regions are connected by a bridge displaced about 20 A away from the center of the structure toward the lipid bilayer. The domain connecting the two promoters is more constricted and closer to the center of the structure in the Co(NH3)4ATP-induced crystals than in the vanadate-induced p21 crystals. These observations suggest that the difference between previously analyzed dimers of two-dimensional p21 crystals induced with vanadate/magnesium and dimers of p4 crystals induced with Co(NH3)4ATP reflects two different conformational states of the enzyme.

Two-dimensional crystallization of a bacterial surface protein on lipid vesicles under controlled conditions

The solubilized surface protein of the Gram-negative bacterium Comamonas acidovorans was reconstituted on lipid vesicles by means of controlled dialysis. To this end, a multichamber dialysis apparatus was built which allows one to control the temperature and the dialysis rate, to apply various temperatures or buffer systems and sample conditions in a single experiment, and to monitor the turbidity of the sample by means of light scattering. The reconstitution conditions were optimized such that the surface protein formed two-dimensional crystals suitable for electron crystallography. The recrystallized surface protein arrays gave a resolution of approximately 1.3 nm in projection after correlation averaging of negatively stained preparations. The surface protein assembled into partially self-contained two-dimensional crystals which possess a strong shape-determining effect and formed cylinders and various cone-shaped vesicles. The development of the various vesicle forms is described in a model.

Configuration of subunits within crystals of Na, K-ATPase maintained in the frozen-hydrated state

Two-dimensional crystalline sheets of Na, K-ATPase were studied in the vitrified, frozen-hydrated state by electron microscopy and image processing. The technique of correlation averaging was used to determine the projected structure. The projection map shows asymmetry between the pair of "alpha beta" protomers comprising a dimer of Na, K-ATPase molecules. The two protomers differ in overall density as well as in shape. One protomer has an oblong shape, whereas the other with higher density has a head and a hook region. Such an asymmetry has not been reported by other laboratories. This asymmetry may either be due to the coexistence of two different conformations of the enzyme in the dimeric form or due to the simultaneous existence of two molecular species of Na, K-ATPase.

Coexistence of different forms of Na,K-ATPase in two-dimensional membrane crystals

Two-dimensional membrane crystals of renal Na,K-ATPase were analyzed by electron microscopy and image processing. The particular property of the crystals in this work was that they showed unit cell parameters similar to the previously studied p21 crystals but lacked the dyad axis as observed in nominal 0 degrees-projections. A three-dimensional reconstruction revealed that structural differences between alpha beta-units of the enzyme gave rise to the asymmetry. A high degree of two-fold rotational symmetry was observed in the middle of the structure while the protein units had different three-dimensional shapes at levels above and below the central sections. The simultaneous coexistence of different forms of Na,K-ATPase suggests that the conformational flexibility of the enzyme plays an important role in the pumping process.

Shift to the Na+ form of Na+/K+-transporting ATPase due to modification of the low-affinity ATP-binding site by Co(NH3)4ATP

1. Inactivation of purified Na+/K+-transporting ATPase by the MgATP complex analogue Co(NH3)4ATP, which binds to the low-affinity ATP-binding site, results in the concomitant inhibition of the K+-activated p-nitrophenylphosphatase, which is considered to be a partial reaction catalyzed by the enzyme in the E2 conformational state. 2. Complete inactivation of Na+/K+-transporting ATPase by Co(NH3)4ATP does not alter the ADP/ATP exchange reaction which is considered to be part of the catalytic activity in the E1 conformation. 3. The enzyme binds eosin at the high-affinity ATP-binding site as measured by the change in eosin fluorescence. Eosin binding to the Co(NH3)4ATP-inactivated enzyme is, in contrast to the untreated enzyme, not stimulated by Na1. Inactivation by Co(NH3)4ATP increased the half-maximal opposing effect of K+ on eosin binding from 1.1 mM in the control to 43.2 mM in the almost completely inactive enzyme. No eosin fluorescence changes were observed when the Co(NH3)4ATP-inactivated enzyme was treated subsequently with CrATP. This MgATP complex analogue forms a stable complex at the high-affinity ATP-binding site. CrATP thus abolishes eosin binding. 4. It is concluded, that Co(NH3)4ATP interacts with Na+/K+-transporting ATPase in the E2 conformation and arrests it there. This affects eosin binding to the high-affinity ATP-binding site, since the K+ sensitivity is lost. A possible interpretation of these differing effects of Co(NH3)4ATP on partial reactions of Na+/K+-transporting ATPase is that the sodium pump works as an (alpha,beta)2 diprotomer. It is likely that the arrest of one alpha,beta promoter in the E2 conformational state by occupancy of the low-affinity ATP-binding site with Co(NH3)4ATP induces the Na+ form (E1 form) in the corresponding alpha,beta promoter, as is indicated by the unaffected ADP/ATP exchange and the response of the eosin fluorescence on Na+ and K+.

Digital image analysis of two-dimensional Na,K-ATPase crystals: dissimilarity between pump units

Two-dimensional crystals of purified Na,K-ATPase were induced by treatment with phospholipase-A2 and vanadate. The negatively stained crystals were imaged by electron microscopy and analysed by digital image processing. Two-dimensional averaged projections of the crystals were calculated by the technique of correlation analysis, utilizing SPIDER (System for Processing of Image Data in Electron microscopy and Related fields) image processing software. The calculated dimensions of the unit cell were found to be 13.3 X 4.59 nm with included angle of 98 degrees, comparable to those reported by others. However, the two protomers of the unit cell were found always to be dissimilar in shape and in orientation. All protomers of one side of the dimer ribbon had a triangular outline, and all protomers of the opposing side had a comma shape. This dissimilarity could be explained by two orientations of identical protomers: one orientation for one side of the dimer ribbon, and another orientation for the protomers of the opposing side of the ribbon. An alternative explanation is that the protomers of one side of the dimer ribbon are actually in a conformation different from that of the protomers of the opposing of the ribbon.

Three-dimensional structure of renal Na,K-ATPase determined from two-dimensional membrane crystals of the p1 form

Electron microscopy and image processing were used to reconstruct a three-dimensional model of membrane-bound monomeric renal Na,K-ATPase from negatively stained two-dimensional crystals of the p1 type. Correlation methods were applied to obtain projection averages which were aligned by a phase difference minimization procedure. The self-consistency of the reconstruction process was high as determined by correlation between experimental projections and projections of the calculated model. The three-dimensional model of the Na,K-ATPase promoter in the p1 crystal form contains three characteristic domains, a protein dense ellipsoid, a small globular stain deficient domain, and a connecting low-contrast region. The latter is thought to correspond to the lipid-penetrating part of the Na,K-ATPase promoter. The location of this domain gives the protein an asymmetric distribution in the bilayer so that it is exposed primarily on one side proposed to correspond to the intracellular face.

Demonstration of cooperating alpha subunits in working (Na+ + K+)-ATPase by the use of the MgATP complex analogue cobalt tetrammine ATP

The MgATP complex analogue cobalt-tetrammine-ATP [Co(NH3)4ATP] inactivates (Na+ + K+)-ATPase at 37 degrees C slowly in the absence of univalent cations. This inactivation occurs concomitantly with incorporation of radioactivity from [alpha-32P]Co(NH3)4ATP and from [gamma-32P]Co(NH3)4ATP into the alpha subunit. The kinetics of inactivation are consistent with the formation of a dissociable complex of Co(NH3)4ATP with the enzyme (E) followed by the phosphorylation of the enzyme: (Formula: see text). The dissociation constant of the enzyme-MgATP analogue complex at 37 degrees C is Kd = 500 microM, the inactivation rate constant k2 = 0.05 min-1. ATP protects the enzyme against the inactivation by Co(NH3)4ATP due to binding at a site from which it dissociates with a Kd of 360 microM. It is concluded, therefore, that Co(NH3)4ATP binds to the low-affinity ATP binding site of the E2 conformational state. K+, Na+ and Mg2+ protect the enzyme against the inactivation by Co(NH3)4ATP. Whilst Na+ or Mg2+ decrease the inactivation rate constant k2, K+ exerts its protective effect by increasing the dissociation constant of the enzyme.Co(NH3)4ATP complex. The Co(NH3)4ATP-inactivated (Na+ + K+)-ATPase, in contrast to the non-inactivated enzyme, incorporates [3H]ouabain. This indicates that the Co(NH3)4ATP-inactivated enzyme is stabilized in the E2 conformational state. Despite the inactivation of (Na+ + K+)-ATPase by Co(NH3)4ATP from the low-affinity ATP binding site, there is no change in the capacity of the high-affinity ATP binding site (Kd = 0.9 microM) nor of its capability to phosphorylate the enzyme Na+-dependently. Since (Na+ + K+)-ATPase is phosphorylated Na+-dependently from the high-affinity ATP binding site although the catalytic cycle is arrested in the E2 conformational state by specific modification of the low-affinity ATP binding site, it is concluded that both ATP binding sites coexist at the same time in the working sodium pump. This demonstration of interacting catalytic subunits in the E1 and E2 conformational states excludes the proposal that a single catalytic subunit catalyzes (Na+ + K+)-transport.

(Na+ + K+)-ATPase: on the number of the ATP sites of the functional unit

Questions concerning the number of the ATP sites of the functional unit of (Na+ + K+)-ATPase (i.e., the sodium pump) have been at the center of the controversies on the mechanisms of the catalytic and transport functions of the enzyme. When the available data pertaining to the number of these sites are examined without any assumptions regarding the reaction mechanism, it is evident that although some relevant observations may be explained either by a single site or by multiple ATP sites, the remaining data dictate the existence of multiple sites on the functional unit. Also, while from much of the data it is clear that the multiple sites of the unit enzyme represent the interacting catalytic sites of an oligomer, it is not possible to rule out the existence of a distinct regulatory site for ATP in addition to the interacting catalytic sites. Regardless of the ultimate fate of the regulatory site, any realistic approach to the resolution of the kinetic mechanism of the sodium pump should include the consideration of the established site-site interactions of the oligomer.

The three-dimensional structure of the Na,K-ATPase from electron microscopy

The structure of Na,K-ATPase has been studied by electron microscopy and image reconstruction. A three-dimensional structure of this enzyme has been obtained to an overall resolution of 2.5 nm using data from specimens of negatively stained dimer sheets tilted through a range of angles +/- 60 degrees. The reconstruction shows a complex mass distribution consisting of ribbons of paired molecules extending approximately 6.0 nm from the cytoplasmic side of the membrane. The molecular envelope consists of a massive "body" with "lobe" and "arm" structures projecting from it. The body has a columnar shape and is tilted with respect to the plane of the membrane. The region of interaction responsible for dimer formation is located between two bodies and is clearly visible in the reconstruction. It has been identified as a segment in the amino-terminal portion of the alpha subunit. The arms that interconnect the ribbons are located close to the membrane and are most probably formed by the beta subunits.

Application of the theory of enzyme subunit interactions to ATP-hydrolyzing enzymes. The case of Na,K-ATPase

The theory developed by T. L. Hill (1977, Proc. Natl. Acad. Sci. USA, 74:3632-3636) for enzyme interactions is applied to a dimeric enzyme, the subunits of which may each exist in three distinct states (as in a uni-bi kinetic mechanism). It is shown that when simultaneous binding of substrate to both subunits is excluded, the complex kinetic mechanism of the dimer reduces to a simpler scheme with two distinct, but analogous, cycles that are in principle separately observable in kinetic experiments. Because of the intersubunit interactions, which are explicitly taken into account, the two cycles have different Michaelis constants and maximal velocities. The model exhibits negative cooperativity and enhanced reactivity, relative to a monomeric enzyme. The theory is applied to Na,K-ATPase for which a complete, bicyclic, kinetic mechanism and rate constants are available. When taken together with other evidence, structural as well as functional, the striking similarity of the observed kinetics with that developed for a dimeric enzyme strongly suggests that the functional unit of Na,K-ATPase is a dimer. The free energy differences (calculated from the known rate constants) between intermediates are 6-16 kJ/mol, comparable, for example, to the free energy associated with the formation of a base pair in a nucleic acid double helix. The possible relevance of these results for other ATPases is briefly discussed.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)