A model for the stepwise radiation inactivation of the α2-dimer of Na,K-ATPase

Ral-GTPase interacts with the β1 subunit of Na+/K+-ATPase and is activated upon inhibition of the Na+/K+pumpThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Na+/K+-ATPase functions as both an ion pump and a signal transducer. Cardiac glycosides partially inhibit Na+/K+-ATPase, causing activation of multiple interrelated growth pathways via the Na+/K+-ATPase/c-Src/epidermal growth factor receptor complex. Such pathways include Ras/MEK/ERK and Ral/RalGDS cascades, which can lead to cardiac hypertrophy. In search of novel Ral-GTPase binding proteins, we used RalB as the bait to screen a human testes cDNA expression library using the yeast 2-hybrid system. The results demonstrated that 1 of the RalB interacting clones represented the C-terminal region of the β1 subunit of Na+/K+-ATPase. Further analysis using the yeast 2-hybrid system and full-length β1 subunit of Na+/K+-ATPase confirmed the interaction with RalA and RalB. In vitro binding and pull-down assays demonstrated that the β1 subunit of Na+/K+-ATPase interacts directly with RalA and RalB. Ral-GTP pull-down assays demonstrated that short-term ouabain treatment of A7r5 cells, a rat aorta smooth muscle cell line, caused activation of Ral GTPase. Maximal activation was observed 10 min after ouabain treatment. Ouabain-mediated Ral activation was inhibited upon the stimulation of Na+/K+-ATPase activity by Ang II. We propose that Ral GTPase is involved in the signal transducing function of Na+/K+-ATPase and provides a possible molecular mechanism connecting Ral to cardiac hypertrophy during diseased conditions.

Large-scale preparation of sodium-potassium ATPase from kidney outer medulla

BACKGROUND

Large amounts of Na,K-ATPase are needed for studies involving protein chemistry. Preparation of Na,K-ATPase from kidney by the widely used, rapid procedure of Jørgensen (Biochim Biophys Acta 356:36-52, 1974; Methods Enzymol 156:29-43, 1988) includes labor-intensive dissection of tissue from the outer medulla and centrifugation into a step gradient of sucrose solution.

METHODS

In a large-scale modification presented here, tissue was dissected with a surgical instrument, a rongeur, and centrifugation was simply a five times repeated differential centrifugation. The procedure took seven days and 68 person-hours of work.

RESULTS

The yield of activity from 26 kg of whole kidneys was 6600 units (micromol Pi/min) in one preparation. The amount of protein was 240 mg and the specific activity was 28 micromol Pi/min per mg protein.

CONCLUSIONS

There is a significant saving of labor to obtain a product with a specific activity similar to that commonly obtained. The microsomal fraction may be useful for preparing other membrane proteins from the outer medulla.

Structure-function relationships of Na(+), K(+), ATP, or Mg(2+) binding and energy transduction in Na,K-ATPase

The focus of this article is on progress in establishing structure-function relationships through site-directed mutagenesis and direct binding assay of Tl(+), Rb(+), K(+), Na(+), Mg(2+) or free ATP at equilibrium in Na,K-ATPase. Direct binding may identify residues coordinating cations in the E(2)[2K] or E(1)P[3Na] forms of the ping-pong reaction sequence and allow estimates of their contributions to the change of Gibbs free energy of binding. This is required to understand the molecular basis for the pronounced Na/K selectivity at the cytoplasmic and extracellular surfaces. Intramembrane Glu(327) in transmembrane segment M4, Glu(779) in M5, Asp(804) and Asp(808) in M6 are essential for tight binding of K(+) and Na(+). Asn(324) and Glu(327) in M4, Thr(774), Asn(776), and Glu(779) in 771-YTLTSNIPEITP of M5 contribute to Na(+)/K(+) selectivity. Free ATP binding identifies Arg(544) as essential for high affinity binding of ATP or ADP. In the 708-TGDGVND segment, mutations of Asp(710) or Asn(713) do not interfere with free ATP binding. Asp(710) is essential and Asn(713) is important for coordination of Mg(2+) in the E(1)P[3Na] complex, but they do not contribute to Mg(2+) binding in the E(2)P-ouabain complex. Transition to the E(2)P form involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713) and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369).

Contribution to Tl+, K+, and Na+ binding of Asn776, Ser775, Thr774, Thr772, and Tyr771 in cytoplasmic part of fifth transmembrane segment in alpha-subunit of renal Na,K-ATPase

The sequence Y771TLTSNIPEIT781P in the fifth transmembrane segment of the alpha-subunit of Na,K-ATPase is unique among cation pump proteins. Here, in search of the molecular basis for Na,K specificity, alanine and conservative substitutions were directed to six oxygen-carrying residues in this segment. The contribution of the residues to cation binding was estimated from direct binding of Tl+ [Nielsen, et al. (1998) Biochemistry 37, 1961-1968], K+ displacement of ATP binding at equilibrium, and Na+-dependent phosphorylation from ATP in the presence of oligomycin. As an intrinsic control, substitution of Thr781 had no effect on Tl+(K+) or Na+ binding. There are several novel observations from this work. First, the carboxamide group of Asn776 is equally important for binding Tl+(K+) or Na+, whereas a shift of the position of the carboxamide of Asn776 (Asn776Gln) causes a large depression of Na+ binding without affecting the binding of Tl+(K+). Second, Thr774 is important for Na+ selectivity because removal of the hydroxyl group reduces the binding of Na+ with no effect on binding of Tl+(K+). Removal of the methyl groups of Thr774 or Thr772 reduces binding of both Tl+(K+) and Na+, whereas the hydroxyl group of Thr772 does not contribute to cation binding. Furthermore, the hydroxyl groups of Ser775 and Tyr771 are important for binding both Tl+(K+) and Na+. The data suggest that rotating or tilting of the cytoplasmic part of the fifth transmembrane segment may adapt distances between coordinating groups and contribute to the distinctive Na+/K+ selectivity of the pump.

Importance of intramembrane carboxylic acids for occlusion of K+ ions at equilibrium in renal Na,K-ATPase

Site-directed mutagenesis and assay of Rb+ and Tl+ occlusion in recombinant Na,K-ATPase from yeast were combined to establish structure-function relationships of amino acid side chains involved in high-affinity occlusion of K+ in the E2[2K] form. The wild-type yeast enzyme was capable of occluding 2 Rb+ or Tl+ ions/ouabain binding site or alpha 1 beta 1 unit with high apparent affinity (Kd(Tl+) = 7 +/- 2 microM), like the purified Na,K-ATPase from pig kidney. Mutations of Glu327(Gln,Asp), Asp804(Asn, Glu), Asp808(Asn, Glu) and Glu779(Asp) abolished high-affinity occlusion of Rb+ or Tl+ ions. The substitution of Glu779 for Gln reduced the occlusion capacity to 1 Tl+ ion/alpha 1 beta 1-unit with a 3-fold decrease of the apparent affinity for the ion (Kd(Tl+) = 24 +/- 8 microM). These effects on occlusion were closely correlated to effects of the mutations on K0.5(K+) for K+ displacement of ATP binding. Each of the four carboxylate residues Glu327, Glu779, and Asp804 or Asp808 in transmembrane segments 4, 5, and 6 is therefore essential for high-affinity occlusion of K+ in the E2[2K] form. These residues either may engage directly in cation coordination or they may be important for formation or stability of the occlusion cavity.

Subunit interactions in the Na,K-ATPase explored with the yeast two-hybrid system

Subunit interactions of the alpha1- and beta1-subunits of the chicken Na,K-ATPase were explored with the yeast two-hybrid system. Gal4-fusion proteins containing domains of the alpha1- and beta1-subunits were designed for examining both intersubunit and intrasubunit protein-protein interactions. Regions of the alpha- and beta-subunits known to be involved in alpha-beta-subunit assembly were positive in two-hybrid assay, supporting the validity of the assays. A library of beta-subunit ectodomains with C-terminal truncations was screened to find the maximal truncation retaining an interaction with the alpha-subunit extracellular H7H8 loop (where H7 refers to the seventh membrane span, and so on). The maximal truncation removed all the cysteines involved in disulfide bridges, leaving only 63 amino acids of the beta-subunit ectodomain. Scanning alanine mutagenesis led to identification of an evolutionarily conserved sequence of four amino acids (SYGQ) in the extracellular H7H8 loop of the alpha-subunit that is crucial to alpha-beta-intersubunit interactions. Oligomerization studies with single domains failed to detect self-association of either of the two large cytosolic loops (H2H3 and H4H5) within the alpha-subunit. However, evidence was found for an interaction between these two cytoplasmic loops.

Interactions of phosphorylation and dimerizing domains of the alpha-subunits of Na+/K(+)-ATPase

Chemical cross-linking studies are among a number of experimental approaches that have suggested the functional significance of higher association states of alpha,beta-protomers of Na+/K(+)-ATPase. Formation of the phosphointermediate of the enzyme on Asp369 of the alpha-subunit is known to induce oxidative cross-linking of the alpha-subunits catalyzed by Cu(2+)-phenanthroline. To localize the phosphorylation-induced alpha,alpha-interface, we cleaved alpha at Arg438-Ala439 by controlled proteolysis and exposed the partially cleaved enzyme to the cross-linking reagent. In addition to the alpha,alpha-dimer, two other phosphorylation-induced cross-linked products were obtained. Using gel electrophoretic resolution of the cross-linked 32P-labeled enzyme, N-terminal analyses of the products, and their reactivities with sequence-specific antibodies, the two products were identified as a homodimer of the C-terminal 64-kDa fragment of alpha and a heterodimer of alpha and the 64-kDa peptide. The latter dimer was also obtained when the cross-linked alpha,alpha-dimer was formed first and then subjected to proteolysis. The findings localize the dimerizing domain to the C-terminal side of Ala439 and indicate that intersubunit proximities of dimerizing domains are regulated by phosphorylation-dephosphorylation of Asp369 during the reaction cycle of the enzyme.

Regulation of yeast Golgi glycosylation. Guanosine diphosphatase functions as a homodimer in the membrane

The Golgi lumenal GDPase plays an important role in the mannosylation of proteins and lipids of Saccharomyces cerevisiae by regulating the amount of GDP-mannose available in the Golgi lumen. The enzyme makes available GMP as an antiporter to be coupled with entry of GDP-mannose into the Golgi lumen from the cytosol. Using radiation inactivation and target analysis, we have now determined the functional molecular mass of the GDPase within the Golgi membrane and whether or not the enzyme has functional associations with other Golgi membrane proteins, including mannosyltransferases and the GDP-mannose transporter. The functional size of the GDPase was found to be approximately twice the estimated structural target size of the protein; this strongly suggests that the GDPase protein in situ functions as homodimer and does not require association with other membrane proteins for its function.

Phosphorylation/dephosphorylation of reconstituted shark Na+,K(+)-ATPase: one phosphorylation site per alpha beta protomer

In the present investigation reconstitution of Na+,K(+)-ATPase increases the number of phosphorylation sites (EP) of solubilized enzyme from 4.2 +/- 0.3 nmol/mg to 6.9 +/- 0.6 nmol/mg. The latter figure corresponds to one phosphorylation site per alpha beta-promoter. A cholesterol content > 10 mol% in the liposome bilayer and a high extracellular [Na+] are necessary to obtain this high value. Spontaneous dephosphorylation after maximum phosphorylation in Na+ is biphasic both in solubilized enzyme and after reconstitution. The rate of dephosphorylation compares with the specific hydrolytic Na(+)-ATPase activity measured at exactly identical conditions for all three preparations assuming parallel dephosphorylation of at least two phosphointermediates. The distribution of EP-species is found to vary among the three enzyme preparation used, i.e., membrane bound, solubilized, and reconstituted Na+,K(+)-ATPase, however in all the equilibrium is strongly poised away from the E1P-form.

E1/E2 type cation transport ATPases: evidence for transient associations between protomers

E1/E2 type cation transport ATPases are known to exist in different conformeric states. Recent evidence characterizing these conformers in membrane is reviewed. A consensus view is proposed in which E2 conformers tend to form oligomeric complexes by lateral association between monomeric protomers and E1 conformers exhibit the opposite behaviour. It is suggested that transient associations between monomers during cation pump cycles may be a common feature of the ion translocation mechanism under physiological conditions.

Inactivation of the Na, K-ATPase by radiation-induced free radicals. Evidence for a radical-chain mechanism

Free radicals produced by water radiolysis were used to study the inactivation of the enzymatic activity of the Na,K-ATPase. A decrease of the activity to virtually zero with a mono-exponential dependence on the radiation dose was observed. The inactivation process is initiated by hydroxyl radicals. This was shown by the effect of appropriate radical scavengers such as t-butanol, formate and vitamin C. In all cases a significant increase in the characteristic D37 dose of inactivation was observed. Inactivation was found to show a so-called inverse dose-rate effect, i.e., the sensitivity of the enzyme to radical attack is increased if the dose rate is reduced. The data were found to agree with the relationship 1/D37 approximately 1/D1/2, which is known to be a strong indicator of a radical chain mechanism. This means that the inactivation, after initiation by single radicals, is amplified by a subsequent chain mechanism.

Biomechanical model of the P-type ion pumps of the cell

The mechanisms of the Na/K pump and of the primary Ca pumps of the cell have not yet been clarified. A biomechanical model of these so-called p-type ion pumps is proposed here. It is based on the assumption that the Na+ and Ca2+ ions are occluded by a contracting protein chain cooperating with the ATPase section of the pump. After transfer of the chain into the region of high Na+ or Ca2+ concentrations, the ions are released through stretching of the chain by the ATPase. In the backward transfer of the chain, a retrograde transport of Na+ ions is prevented through occlusion of K+ ions by another region of the same chain. In the case of Ca2+ ions, a similar effect is expected from hydrated Mg2+ ions. The two sections of the chain discriminate between the electrical field strength at the surface and the polarizability of the ions. The most likely mechanism for the transfer of the ion-binding chain is considered to involve a thermally induced transition of a pump dimer between two almost equivalent stable orientations in the membrane.

Damage to proteins due to the direct action of ionizing radiation

Proteins exposed to ionizing radiation suffer both reversible and irreversible effects. Reversible effects are defined as those which disappear in a short period of time after the removal of the radiation field and without further treatment of the sample. Irreversible effects are those which cause a permanent alteration in the structure of a protein.

Heterogeneity of pig kidney Na,K-ATPase as indicated by ADP- and ouabain-binding stoichiometry

A centrifugation method has been used for determination of [14C]ADP and [3H]ouabain binding to Na,K-ATPase from pig kidney with high specific activity. In the presence of K+, the fit of the [14C]ADP binding data to a two-site model gives a component with high affinity which accounts for 12 +/- 2% of the total sites. The figure is significantly different from 50%, i.e., two components of equal size cannot be assumed. This contrasts with a ratio between the sites of 1:1 obtained by the rate dialysis technique. The discrepancy may be due to the fact that the centrifugation method enables bound ADP to be determined at lower concentrations of free ligand. [3H]Ouabain binding in the absence of Na+ is compatible with a straight line in a Scatchard plot if the isotope is purified shortly before use. An unspecific binding of ouabain can be neglected if the concentration of free ouabain is not too high. In the presence of Na+, the isotherms become upward concave. An analysis of the binding data gives a 19:81% division, although equilibrium is not quite attained. This is a maximum value because the lack in equilibrium will be most pronounced at the small values of free ouabain. Thus the ADP-binding studies are supported. The finding here is in some agreement with the semiquantitative immunoassay showing that pig kidney enzyme contains the isoenzymes alpha 1, alpha 2 and alpha 3 in a proportion of 84:12:4, respectively. Determination of ADP- and ouabain-binding site stoichiometry favours a theory with one substrate site per (alpha beta)2.

Investigation of subunit interactions by radiation inactivation: the case of Na+/K+-ATPase

The target size of Na+/K+-ATPase has been determined by radiation inactivation. To interpret the results, we have performed Monte Carlo simulations of the inactivation process. This seems to be a general method for the interpretation of such studies. The simulation revealed that radiation inactivation can distinguish between monoprotomeric and multiprotomeric models of enzyme action only if the measured reaction requires the actual co-operation of, rather than the mere co-existence of, different protomeres.

Phosphate binding and ATP-binding sites coexist in Na+/K(+)-transporting ATPase, as demonstrated by the inactivating MgPO4 complex analogue Co(NH3)4PO4

Tetrammine cobalt(III) phosphate [Co(NH3)4PO4] inactivates Na+/K(+)-ATPase in the E2 conformational state, dependent on time and concentration, according to Eqn (1): Co(NH3)4PO4 + E2 Kd in equilibrium E2.Co(NH3)4PO4k2----E'2.Co(NH3)4PO4. The inactivation rate constant k2 for the formation of a stable E'2.Co(NH3)4PO4 at 37 degrees C was 0.057 min-1; the dissociation constant, Kd = 300 microM. The activation energy for the inactivation process was 149 kJ/mol. ATP and the uncleavable adenosine 5'-[beta, gamma-methylene]triphosphate competed with Co(NH3)4PO4 for its binding site with Ks = 0.41 mM and 5 mM, respectively. MgPO4 competed with Co(NH3)4PO4 linearly, with Ks = 50 microM, as did phosphate (Ks = 16 mM) and Mg2+ (Ks = 160 microM). It is concluded that the MgPO4 analogue binds to the MgPO4-binding subsite of the low-affinity ATP-binding site (of the E2 conformation). Also, Na+ (Ks = 860 microM) protected the enzyme against inactivation in a competitive manner. From the intersecting (slope and intercept linear) noncompetitive effect of Na+ against the inactivation by Co(NH3)4PO4, apparent affinities of K+ for the free enzyme of 41 microM, and for the E.Co(NH3)4PO4 complex of 720 microM, were calculated. Binding of Co(NH3)4PO4 to the enzyme inactivated Na+/K(+)-ATPase and K(+)-activated phosphatase, and, moreover, prevented the occlusion of 86Rb+; however, the activity of the Na(+)-ATPase, the phosphorylation capacity of the high-affinity ATP-binding site and the ATP/ADP-exchange reaction remained unchanged. With Co(NH3)432PO4 a binding capacity of 135 pmol unit enzyme was found. Phosphorylation and complete inactivation of the enzyme with Co(NH3)432PO4 or the 32P-labelled tetramminecobalt ATP ([gamma-32P]Co(NH3)4ATP) at the low-affinity ATP-binding site, allowed (independent of the purity of the Na+/K(+)-ATPase preparation) a further incorporation of radioactivity from 32P-labelled tetraaquachromium(III) ATP ([gamma-32P]CrATP) to the high-affinity ATP-binding site with unchanged phosphorylation capacity. However, inactivation and phosphorylation of Na+/K(+)-ATPase by [gamma-32P]CrATP prevented the binding of Co(NH3)4 32PO4 or [gamma-32P]Co(NH3)4ATP to the enzyme. [gamma-32P]CO(NH3)4ATP and Co(NH3)432PO4 are mutually exclusive. The data are consistent with the assumption of a cooperation of catalytic subunits within an (alpha,beta)2-diprotomer, which change their interactions during the Na+/K(+)-pumping process. Our findings seem not to support a symmetrical Repke and Stein model of enzyme action.

Blocking of Na+/K+ transport by the MgPO4 complex analogue Co(NH3)4PO4 leaves the Na+/Na(+)-exchange reaction of the sodium pump unaltered and shifts its high-affinity ATP-binding site to a Na(+)-like form

Inactivation of Na+/K(+)-ATPase activity by the MgPO4 complex analogue Co(NH3)4PO4 leads, in everted red blood cell vesicles, to the parallel inactivation of 22Na+/K+ flux and 86Rb/Rb+ exchange, but leaves the 22Na+/Na(+)-exchange activity and the uncoupled ATP-supported 22Na+ transport unaffected. Furthermore, inactivation of purified Na+/K(+)-ATPase by Co(NH3)4PO4 leads to a parallel decrease of the capacity of the [3H]ouabain receptor site, when binding was studied by the Mg2+/Pi-supported pathway (ouabain-enzyme complex II) but the capacity of the ouabain receptor site was unaltered, when the Na+/Mg2+/ATP-supported pathway (ouabain-enzyme complex I) was used. No change in the dissociation constants of either ouabain receptor complex was observed following inactivation of Na+/K(+)-ATPase. When eosin was used as a marker for the high-affinity ATP-binding site of the E1 conformation, formation of stable E'2.Co(NH3)4PO4 complex led to a shift in the high-affinity ATP-binding site towards the sodium form. This led to an increase in the dissociation constant of the enzyme complex with K+, from 1.4 mM with the unmodified enzyme to 280 mM with the Co(NH3)4PO4-inactivated enzyme. It was concluded, that the effects of Co(NH3)4PO4 on the partial activities of the sodium pump are difficult to reconcile with an alpha, beta-protomeric enzyme working according the Albers-Post scheme. The data are consistent with an alpha 2, beta 2 diprotomeric enzyme of interacting catalytic subunits working with a modified version of the Albers-Post model.

Radiation target sizes of the Na,K-ATPase and p-aminohippurate transport system in the basolateral membrane of renal proximal tubule

Basolateral membrane vesicles made from rabbit kidney proximal tubules were frozen and irradiated with a high energy electron beam and the effects of irradiation on Na,K-ATPase activity, p-aminohippurate (PAH) transport, the membrane diffusion barrier and vesicle volume were measured. The vesicle volume and diffusion barrier were not significantly changed by radiation exposure. Na,K-ATPase activity was inactivated as a simple exponential function of radiation dose. Target size analysis of the data yielded a molecular size of 267 +/- 17 kDa, consistent with its existence as a (alpha beta)2 dimer. The carrier-mediated PAH uptake by basolateral membrane vesicles was also inactivated as a function of radiation dose. A target molecular size of 74 +/- 16 kDa was calculated for the PAH transport system. This study is the first measurement of the functional size of the organic acid transport system based directly on flux measurements.

[32P]ATP synthesis in steady state from [32P]Pi and ADP by Na+/K(+)-ATPase from ox brain and pig kidney. Activation by K+

The ouabain-sensitive synthesis of [32P]ATP from [32P]Pi and ADP (vsyn) was measured in parallel with the ouabain-sensitive hydrolysis of [32P]ATP (vhy) at steady state, at varying concentrations of sodium, potassium, magnesium, inorganic phosphate, ADP, ATP and oligomycin, and at varying pH. Na+ was necessary for ATP synthesis, but vsyn was decreased by high sodium concentrations. Oligomycin, depending on the Na+ concentration, either decreased or did not affect vsyn. Potassium, at low concentrations (1-5 mM) increased vsyn at all magnesium and sodium concentrations tested, lower potassium concentrations being needed to activate vsyn at lower sodium concentrations. vsyn was optimal below pH 6.7, decreasing abruptly at higher values of pH. At pH 6.7, vsyn was a hyperbolic function of the concentration of inorganic phosphate. In the presence of potassium, half-maximal rate was obtained at [Pi] congruent to 40 mM, whereas a higher concentration was needed to obtain half-maximal rate in the absence of K+. In contrast, increasing the concentration of ADP caused a nonhyperbolic activation of vsyn, the pattern obtained in the presence of potassium being different from that obtained in its absence. Increasing the ATP concentration above 0.5 mM decreased vsyn. The data are used to elucidate (1) which reaction steps are involved in the ATP-synthesis catalysed by the Na+/K(+)-ATPase at steady state in the absence of ionic gradients and (2) the mechanism by which K+ ions stimulate the reaction.