Glutamate 779, an intramembrane carboxyl, is essential for monovalent cation binding by the Na,K-ATPase

Photolabile amiloride derivatives as cation site probes of the Na,K-ATPase

Treatment of purified canine renal Na,K-ATPase with a range of photoactivatable amiloride derivatives results in inhibition of ATPase activity prior to illumination. Inhibition by amiloride derivatives substituted on a guanidium N could not be prevented by the presence of either K or Na; however, these cations could protect the enzyme against inhibition by derivatives substituted on the 5-position of the pyrazine ring. In the case of 5-(N-ethyl-[2'-methoxy-4'-nitrobenzyl])amiloride (NENMBA), the presence of monovalent cations (Na, K, and Rb) protected the enzyme effectively against inhibition, with concentrations in the millimolar range. ATP did not prevent inhibition; furthermore, native and NENMBA-treated enzyme exhibited normal levels of high affinity [3H]ADP (and hence ATP) binding. The rate of inhibition increased with increasing concentrations of NENMBA. Extensive washing of NENMBA-inhibited enzyme did not restore ATPase activity, showing that NENMBA has an extremely slow off-rate for dissociation from its inhibitory site. Partially inhibited enzyme could be rapidly pelleted and resuspended in NENMBA-free buffer and inhibition was observed to continue, albeit at a somewhat diminished rate, suggesting that NENMBA gains access to its inhibitory site after partitioning into the lipid phase rather than directly from the aqueous solution. Photolysis of NENMBA-inhibited enzyme resulted in covalent incorporation of the reagent into the alpha-subunit of the Na,K-ATPase, as observed by separation of labeled protein on a Laemmli gel and Western analysis using a polyclonal amiloride antibody. Almost all of the covalent labeling could be prevented by the presence of Rb in the incubation and labeling medium. These results suggest that NENMBA inhibits the Na, K-ATPase by disruption of the cation transport domain rather than the catalytic domain of the enzyme and that it promises to be a useful tool for cation site localization.

Substitutions of glutamate 781 in the Na,K-ATPase alpha subunit demonstrate reduced cation selectivity and an increased affinity for ATP

The intramembrane Glu781 residue of the Na,K-ATPase alpha subunit has been postulated to have a role in the binding and/or occlusion of cations. To ascertain the role of Glu781, the residue was substituted with an aspartate, alanine, or lysine residue and the mutant Na,K-ATPases were coexpressed with the native beta 1 subunit in Sf9 insect cells using the baculovirus expression system. All alpha mutants are able to efficiently assemble with the beta 1 subunit and produce catalytically competent Na,K-ATPase molecules with hydrolytic activities comparable to that of the wild-type enzyme. Analysis of the kinetic properties of the mutated enzymes showed a decrease in apparent affinity for K+ compared to wild-type Na,K-ATPase, with the lysine and alanine substitutions displaying the greatest reduction. All Na,K-ATPase mutants demonstrated a significant increase in apparent affinity for ATP compared to wild-type Na,K-ATPase, while the sensitivity to the cardiotonic inhibitor, ouabain, was unchanged. The dependence on Na+, however, differs among the mutant enzymes with both the Glu781-->Asp and Glu781-->Ala mutants displaying a decrease in the apparent affinity for the cation, while the Glu781-->Lys mutant exhibits a modest increase. Furthermore, in the absence of K+, the Glu781-->Ala mutant displays a Na(+)-ATPase activity and a cellular Na+ influx suggesting that Na+ is substituting for K+ at the extracellular binding sites. The observation that trypsin digestion of the Glu781-->Ala mutant in Na+ medium produces a K(+)-stabilized tryptic fragment also intimates a decreased capacity of the mutant to discriminate between Na+ and K+ at the extracellular loading sites. All together, these data implicate Glu781 of the Na,K-ATPase alpha subunit as an important coordinate of cation selectivity and activation, although the modest effect of Glu781-->Lys substitution seemingly precludes direct involvement of the residue in the cation binding process. In addition, the fifth membrane segment is proposed to represent an important communicative link between the extramembraneous ATP binding domain and the cation transport regions of the Na,K-ATPase.

Novel aromatic isothiouronium derivatives which act as high affinity competitive antagonists of alkali metal cations on Na/K-ATPase

This paper describes properties of a novel family of aromatic isothiouronium derivatives, which act as Na(+)-like competitive antagonists on renal Na/K-ATPase. The derivatives are reversible competitors of Rb+ and Na+ occlusion. Ki values of the most potent compounds, 1-bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU) and 1,3-dibromo-2,4,6-tris(methylisothiouronium)benzene(Br2-TITU ), 0.65 and 0.32 microM, respectively, are 15-30-fold lower than Ki values of the bis-guanidinium derivatives described previously (David, P., Mayan, H., Cohen, H., Tal, D. M., and Karlish, S. J. D. (1992) J. Biol. Chem. 267, 1141-1149), and represent the lowest reported values for cation antagonists. Using fluorescein-labeled Na/K-ATPase, all derivatives have been shown to stabilize the E1 conformation when bound at high affinity sites (i.e. they are sodium-like). In addition, in one condition (10 mM Tris-HCl, pH 8.1), high concentrations of Br-TITU (KD approximately 10 microM) appear to stabilize an E2 conformation. We propose a model which allows for simultaneous binding of the antagonists to high affinity cytoplasmic sites and low affinity sites, which may be at the extracellular surface. Blockage of cation occlusion by the isothiouronium derivatives at the cytoplasmic surface probably occurs at the entrance to the occlusion sites, which is recognized both by Na+ antagonists and by Na+ or K+ ions. Unlike the alkali metal cations, the Na+ antagonists are not occluded or transported (see also Or, E., David, P., Shainskaya, A., Tal, D. M., and Karlish, S. J. D. (1993) J. Biol. Chem. 268, 16929-16937). The isothiouronium derivatives appear to be promising candidates for further development as affinity labels of cation binding domains, for kinetic analysis of isoforms or mutated Na/K pumps, or as probes of other cation transport proteins.

Substitutions of serine 775 in the alpha subunit of the Na,K-ATPase selectively disrupt K+ high affinity activation without affecting Na+ interaction

The functional role of serine 775, predicted to be located in the fifth transmembrane segment of the alpha subunit of the Na,K-ATPase (YTLTSNIPE), was studied using site-directed mutagenesis, expression, and kinetic analysis. Substitutions S775A, S775C, and S775Y were introduced into an ouabain-resistant alpha 1 sheep isoform and expressed in HeLa cells. cDNAs carrying substitutions S775C and S775A produced ouabain-resistant colonies only when extracellular K+ was increased from 5.4 mM to 10 or 20 mM, respectively. No ouabain-resistant colonies were obtained for substitutions S775Y at any tested K+ concentration. Kinetic characterization of S775C and S775A substituted enzymes showed expression levels higher than control enzyme, reduced Vmax and turnover, and normal phosphorylation and high affinity ATP binding. Dephosphorylation experiments indicated that S775A substituted enzyme is insensitive to ADP but readily dephosphorylated by K+. The K+ K1/2 values for the activation of the Na,K-ATPase were markedly altered, with S775C displaying a 13-fold increase and S775A exhibiting a 31-fold increase. These large changes in the Na,K-ATPase affinity for K+ are consistent with the participation of this amino acid in binding K+ during the translocation of this cation. Substitutions of Ser775 did not change Na+ affinity, indicating that this residue is likely not involved in Na+ binding and occlusion. These data show that the electronegative oxygen and the small side chain of Ser775 are required for efficient enzyme function. Moreover, these results suggest Ser775 plays a distinct role in K+ transport and not in Na+ interactions, revealing a possible mechanism for the enzymatic differentiation of these cations by the Na,K-ATPase.

The influence of beta subunit structure on the interaction of Na+/K(+)-ATPase complexes with Na+. A chimeric beta subunit reduces the Na+ dependence of phosphoenzyme formation from ATP

High-affinity ouabain binding to Na+/K(+)-ATPase (sodium- and potassium-transport adenosine triphosphatase (EC 3.6.1.37)) requires phosphorylation of the alpha subunit of the enzyme either by ATP or by inorganic phosphate. For the native enzyme (alpha/beta 1), the ATP-dependent reaction proceeds about 4-fold more slowly in the absence of Na+ than when saturating concentrations of Na+ are present. Hybrid pumps were formed from either the alpha 1 or the alpha 3 subunit isoforms of Na+/K(+)-ATPase and a chimeric beta subunit containing the transmembrane segment of the Na+/K(+)-ATPase beta 1 isoform and the external domain of the gastric H+/K(+)-ATPase beta subunit (alpha/NH beta 1 complexes). In the absence of Na+, these complexes show a rate of ATP-dependent ouabain binding from approximately 75-100% of the rate seen in the presence of Na+ depending on buffer conditions. Nonhydrolyzable nucleotides or treatment of ATP with apyrase abolishes ouabain binding, demonstrating that ouabain binding to alpha/NH beta 1 complexes requires phosphorylation of the protein. Buffer ions inhibit ouabain binding by alpha/NH beta 1 in the absence of Na+ rather than promote ouabain binding, indicating that they are not substituting for sodium ions in the phosphorylation reaction. The pH dependence of ATP-dependent ouabain binding in the presence or absence of Na+ is similar, suggesting that protons are probably not substituting for Na+. Hybrid alpha/NH beta 1 pumps also show slightly higher apparent affinities (2-3-fold) for ATP, Na+, and ouabain; however, these are not sufficient to account for the increase in ouabain binding in the absence of Na+. In contrast to phosphoenzyme formation and ouabain binding by alpha/NH beta 1 complexes in the absence of Na+, ATPase activity, measured as release of phosphate from ATP, requires Na+. These data suggest that the transition from E1P to E2P during the catalytic cycle does not occur when the sodium binding sites are not occupied. Thus, the chimeric beta subunit reduces or eliminates the role of Na+ in phosphoenzyme formation from ATP, but Na+ binding or release by the enzyme is still required for ATP hydrolysis and release of phosphate.

Functional consequences of mutation Asn326-->Leu in the 4th transmembrane segment of the alpha-subunit of the rat kidney Na+, K(+)-ATPase

Site-specific mutagenesis was used to replace Asn326 in transmembrane segment M4 of the ouabain-insensitive alpha 1-isoform of rat kidney Na+, K(+)-ATPase. Mutant Asn326-->Leu was functional as demonstrated by the ability of COS cells expressing the mutant enzyme to grow in the presence of ouabain. In three independent assays encompassing Na+ titrations of Na+,K(+)-ATPase activity, Na(+)-ATPase activity, and phosphorylation from ATP, the Asn326-->Leu mutant displayed a reduced apparent affinity for Na+. By contrast, this mutant exhibited a slightly increased apparent affinity for K+ relative to the wild-type enzyme. In the presence of Na+ without K+, the Asn326-->Leu mutant hydrolyzed ATP at a high rate corresponding to 32% of the maximal Na+,K(+)-ATPase activity, and the rate of dephosphorylation of the phosphoenzyme intermediate was enhanced in the mutant relative to that of the wild-type enzyme. Oligomycin, known to stabilize the Na(+)-occluded phosphoenzyme intermediate, reduced the dephosphorylation rate of the mutant and increased the steady-state phosphoenzyme level formed by the mutant at least 3-fold, whereas an increase in the steady-state phosphoenzyme level of only 10-15% was determined for the wild-type enzyme. The molecular turnover number for the Na+,K(+)-ATPase reaction, calculated when the steady-state phosphoenzyme level obtained in the presence of oligomycin was taken as a measure of the concentration of active sites, was slightly reduced relative to that of the wild-type enzyme. The data are discussed in terms of a role for Asn326 in binding of cytoplasmic Na+ and in mediation of inhibition of dephosphorylation.

Mutational analysis of Glu771 of the Ca(2+)-ATPase of sarcoplasmic reticulum. Effect of positive charge on dephosphorylation

The glutamic acid residue Glu771 in the fifth transmembrane segment M5 of the Ca(2+)-ATPase of rabbit fast twitch muscle sarcoplasmic reticulum was substituted with lysine, alanine, and glycine by site-directed mutagenesis. Mutant Glu771-->Lys was unable to occlude Ca2+, and Ca2+ did not inhibit phosphorylation from P(i) or activate phosphorylation from ATP of this mutant. Mutants Glu771-->Ala and Glu771-->Gly were likewise unable to occlude Ca2+, but Ca2+ in the millimolar concentration range activated phosphorylation from ATP and inhibited phosphorylation from P(i) of these mutants. The dephosphorylation of the ADP-insensitive E2P phosphoenzyme intermediate of mutants Glu771-->Ala and Glu771-->Gly was found to be blocked, whereas the dephosphorylation proceeded rapidly for mutant Glu771-->Lys. This finding suggests a role of the positive charge of the lysine in induction of dephosphorylation, supporting the hypothesis that the side chain of Glu771 participates in the countertransport of two protons per Ca(2+)-ATPase cycle.