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

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.

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.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase is a pivotal residue for cation-induced conformational changes

The cation binding characteristics of the mutant E327A formed in the sheep alpha 1 isoform of the Na+,K(+)-ATPase were examined using [3H]ouabain binding as a function of monovalent cation concentrations. Equilibrium competition binding assays in the presence of Mg2+, inorganic phosphate and various amounts of unlabelled ouabain indicated that both wild-type sheep alpha 1 protein and the E327A mutant expressed in 3T3 cells had similar affinities for ouabain (KD = 1.53 and 1.31 nM respectively). Sodium inhibition of ouabain binding appeared competitive in both enzymes. However, binding of three Na+ ions was required to explain the steep character of the Na+ inhibition curve for the wild-type Na+,K(+)-ATPase (Ki = 12.8 +/- 1.6 mM), whereas the binding of two Na+ ions was detected for the mutant E327A (Ki = 19.2 +/- 2.5 mM). Potassium binding of [3H]ouabain binding displayed a partially competitive nature with Hill coefficients of 2 for both wild-type sheep alpha 1 (Ki = 0.743 +/- 0.044 mM) and E327A (Ki = 0.875 +/- 0.067 mM). At concentrations of K+ above 10 mM, the sheep alpha 1 competition curve levelled off whereas the inhibition curve for E327A displayed a stimulation in ouabain binding. This stimulation in [3H]ouabain binding also occurred with Rb+, Cs+ and Li+, but was never observed with choline or Na+, suggesting that this effect was not due to ionic strength. From these [3H]ouabain-binding studies, it is obvious that the mutant enzyme E327A in the presence of Mg2+, Pi and ouabain, interacts with monovalent cations in a unique fashion. One interpretation of these data is that the glutamic acid residue at position 327 is involved in a conformational transition induced by the binding of monovalent cations to the Na+,K+-ATPase and that this transition is inhibited by the mutation of E327A.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase stabilizes a K(+)-induced conformational change

By combining the tools of site-directed mutagenesis and [3H]ouabain binding, the functional role of glutamic acid 327 in the fourth transmembrane domain of the sheep alpha 1 isoform of Na+,K(+)-ATPase was examined with respect to its interactions with ouabain, Na+,K+,Mg2+, and inorganic phosphate. Using site-directed mutagenesis, this glutamic acid was substituted with alanine, aspartic acid, glutamine, and leucine. The mutant proteins were constructed in a sheep alpha 1 protein background such that [3H]ouabain binding could be utilized as a highly specific probe of the exogenous protein expressed in NIH 3T3 cells. Na+ competition of [3H]ouabain binding to the mutant forms of Na+,K(+)-ATPase revealed only slight alterations in their affinities for Na+ and in their abilities to undergo Na(+)-induced conformational changes which inhibit ouabain binding. In contrast, K+ competition of [3H]ouabain binding to all four mutant forms of Na+,K(+)-ATPase displayed severely altered interactions between these proteins and K+. Interestingly, [3H]ouabain binding to the mutant E327Q was not inhibited by the presence of K+. This mutant was previously reported to be functionally able to support cation transport with a 5-fold reduced K0.5 for K(+)-dependent ATPase activity (Jewell-Motz, E. A., and Lingrel, J.B. (1993) Biochemistry 32, 13523-13530; Vilsen, B. (1993) Biochemistry 32, 13340-13349). Thus, it appears that this glutamic acid in the fourth transmembrane domain may be important for stabilizing a K(+)-induced conformation within the catalytic cycle of Na+,K(+)-ATPase that is not rate-limiting in the overall ATPase cycle but that displays a greatly reduced affinity for ouabain.

Structure-function relationships of cation translocation by Ca(2+)- and Na+, K(+)-ATPases studied by site-directed mutagenesis

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channel-like structure. Three of the homologous Na+, K(+)-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca(2+)-ATPase residues appear to participate in the countertransport of H+, and two of the Na+, K(+)-ATPase residues to participate in the countertransport of K+. Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca(2+)-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+, K(+)-ATPase, may play a critical role in closing the gate to a transmembrane channel.