Structure/function analysis of the amino-terminal region of the 1 and 2 subunits of Na,K-ATPase

Existence and importance of Na+K+-ATPase in the plasma membrane of boar spermatozoa

Sodium-potassium-ATPase (Na+K+-ATPase), a target to treat congestive heart failure, is the only known receptor for cardiac glycosides implicated in intracellular signaling and additionally functions enzymatically in ion transport. Spermatozoa need transmembrane ion transport and signaling to fertilize, and Na+K+-ATPase is identified here for the first time in boar spermatozoa. Head plasma membrane (HPM) isolated from boar spermatozoa was confirmed pure by marker enzymes acid and alkaline phosphatase (218 ± 23% and 245 ± 38% enrichment, respectively, versus whole spermatozoa). Western immunoblotting detected α and β subunits (isoforms α1, α3, β1, β2, and β3) in different concentrations in whole spermatozoa and HPM. Immunofluorescence of intact sperm only detected α3 on the post-equatorial exterior membrane; methanol-permeabilized sperm also had α3 post-equatorially and other isoforms on the acrosomal ridge and cap. Mass spectrometry confirmed the presence of all isoforms in HPM. Incubating boar sperm in capacitating media to induce the physiological changes preceding fertilization significantly increased the percentage of capacitated sperm compared to 0 h control (33.0 ± 2.6% vs. 19.2 ± 2.6% capacitated sperm, respectively; p = 0.014) and altered the β2 immunofluorescence pattern. These results demonstrate the presence of Na+K+-ATPase in boar sperm HPM and that it changes during capacitation.

Involvement of the alpha-subunit N-terminus in the mechanism of the Na+,K+-ATPase

Previous studies have shown that cytoplasmic K+ release and the associated E2 → E1 conformational change of the Na+,K+-ATPase is a major rate-determining step of the enzyme's ion pumping cycle and hence a prime site of acute regulatory intervention. From the ionic strength dependence of the enzyme's distribution between the E2 and E1 states, it has also been found that E2 is stabilized by an electrostatic attraction. Any disruption of this electrostatic attraction would, thus, have profound effects on the rate of ion pumping. The aim of this paper is to identify the location of this interaction. Using enhanced-sampling molecular dynamics simulations with a predicted N-terminal structure added to the X-ray crystal structure of the Na+,K+-ATPase, a previously postulated salt bridge between Lys32 and Glu233 (rat sequence numbering) of the enzyme's α-subunit can be excluded. The residues never approach closely enough to form a salt bridge. In contrast, strong interactions with anionic lipid head groups were seen. To investigate the possibility of a protein-lipid interaction experimentally, the surface charge density of Na+,K+-ATPase-containing membrane fragments was estimated from zeta potential measurements to be 0.019 (± 0.001) C m-2. This is in good agreement with the charge density previously determined to be responsible for stabilization of the E2 state of 0.023 (± 0.009) C m-2 and the membrane charge density estimated here from published electron-microscopic images of 0.018C m-2. The results are, therefore, consistent with an interaction of the Na+,K+-ATPase α-subunit N-terminus with negatively-charged lipid head groups of the neighbouring cytoplasmic membrane surface as the origin of the electrostatic interaction stabilising the E2 state.

Cholesterol depletion inhibits Na+,K+-ATPase activity in a near-native membrane environment

Cholesterol's effects on Na+,K+-ATPase reconstituted in phospholipid vesicles have been extensively studied. However, previous studies have reported both cholesterol-mediated stimulation and inhibition of Na+,K+-ATPase activity. Here, using partial reaction kinetics determined via stopped-flow experiments, we studied cholesterol's effect on Na+,K+-ATPase in a near-native environment in which purified membrane fragments were depleted of cholesterol with methyl-β-cyclodextrin (mβCD). The mβCD-treated Na+,K+-ATPase had significantly reduced overall activity and exhibited decreased observed rate constants for ATP phosphorylation (ENa3+ → E2P, i.e. phosphorylation by ATP and Na+ occlusion from the cytoplasm) and K+ deocclusion with subsequent intracellular Na+ binding (E2K2+ → E1Na3+). However, cholesterol depletion did not affect the observed rate constant for K+ occlusion by phosphorylated Na+,K+-ATPase on the extracellular face and subsequent dephosphorylation (E2P → E2K2+). Thus, partial reactions involving cation binding and release at the protein's intracellular side were most dependent on cholesterol. Fluorescence measurements with the probe eosin indicated that cholesterol depletion stabilizes the unphosphorylated E2 state relative to E1, and the cholesterol depletion-induced slowing of ATP phosphorylation kinetics was consistent with partial conversion of Na+,K+-ATPase into the E2 state, requiring a slow E2 → E1 transition before the phosphorylation. Molecular dynamics simulations of Na+,K+-ATPase in membranes with 40 mol % cholesterol revealed cholesterol interaction sites that differ markedly among protein conformations. They further indicated state-dependent effects on membrane shape, with the E2 state being likely disfavored in cholesterol-rich bilayers relative to the E1P state because of a greater hydrophobic mismatch. In summary, cholesterol extraction from membranes significantly decreases Na+,K+-ATPase steady-state activity.

Evolutionary Analysis of the Lysine-Rich N-terminal Cytoplasmic Domains of the Gastric H+,K+-ATPase and the Na+,K+-ATPase

The catalytic α-subunits of both the Na⁺,K⁺-ATPase and the gastric H⁺,K⁺-ATPase possess lysine-rich N-termini which project into the cytoplasm. Due to conflicting experimental results, it is currently unclear whether the N-termini play a role in ion pump function or regulation, and, if they do, by what mechanism. Comparison of the lysine frequencies of the N-termini of both proteins with those of all of their extramembrane domains showed that the N-terminal lysine frequencies are far higher than one would expect simply from exposure to the aqueous solvent. The lysine frequency was found to vary significantly between different vertebrate classes, but this is due predominantly to a change in N-terminal length. As evidenced by a comparison between fish and mammals, an evolutionary trend towards an increase of the length of the N-terminus of the H⁺,K⁺-ATPase on going from an ancestral fish to mammals could be identified. This evolutionary trend supports the hypothesis that the N-terminus is important in ion pump function or regulation. In placental mammals, one of the lysines is replaced by serine (Ser-27), which is a target for protein kinase C. In most other animal species, a lysine occupies this position and hence no protein kinase C target is present. Interaction with protein kinase C is thus not the primary role of the lysine-rich N-terminus. The disordered structure of the N-terminus may, via increased flexibility, facilitate interaction with another binding partner, e.g. the surrounding membrane, or help to stabilise particular enzyme conformations via the increased entropy it produces.

Specific Sites in the Cytoplasmic N Terminus Modulate Conformational Transitions of the Na,K-ATPase

The cytoplasmic N terminus of the Na,K-ATPase is a highly charged and flexible structure that comprises three predicted helical regions including H1 spanning residues 27 to 33 and H2 spanning residues 42 to 50. Previous deletion mutagenesis experiments showed that deletion of residues up to and including most of H2 shifts the E(1)/E(2) conformational equilibrium toward E(1). The present study describes a clustered charge-to-alanine mutagenesis approach designed to delineate specific sites within the N terminus that modulate the steady-state E(1) <--> E(2) and E(1)P <--> E(2)P poise. Criteria to assess shifts in poise include (i) sensitivity to inhibition by inorganic orthovanadate to assess overall poise; (ii) K(+)-sensitivity of Na-ATPase measured at micromolar ATP to assess changes in the E(2)(K) + ATP --> E(1) x ATP + K(+) rate; (iii) K'(ATP) for low-affinity ATP binding at the latter step; (iv) overall catalytic turnover, and (v) the E(1)P --> E(2)P transition. The results of alanine replacements in H1 (31KKE) suggest that this site stabilizes E(2)P and to a lesser extent E(2). In H2, residues within 47HRK have a role in stabilizing E(2) but not E(2)P as revealed with double mutants 31KKE --> AAA/47H --> A and 31KKE --> AAA/47HRK --> AAA. Taken together, these observations suggest that sites 31KKE in H1 and 47HRK in H2 have distinct roles in modulating the enzyme's conformational transitions during the catalytic cycle of the enzyme.

New roles for an old enzyme: Na,K-ATPase emerges as an interesting drug target

Na,K-ATPase (NKA) is well known for its role as a maintainer of electrolyte and fluid balance in cells, organs and whole body. Exciting new findings have revealed additional fundamental roles for NKA as a signal transducer and modulator of growth, apoptosis, cell adhesion and motility. The signal transduction function can be triggered by the binding of ouabain, the mammalian analogue of digitalis to NKA. The catalytic subunit of NKA exists in different forms and mutations in two of the forms that are expressed in brain can give rise to migraine, epilepsy and Parkinsonism-like symptoms. This review will present these new aspects of NKA and their clinical implications.

Low Doses of Ouabain Protect from Serum Deprivation–Triggered Apoptosis and Stimulate Kidney Cell Proliferation via Activation of NF-κB

It now generally is agreed that Na,K-ATPase, in addition to its role in the maintenance of Na+ and K+ gradients across the cell membrane, plays a role in communicating information from the extracellular environment to intracellular signaling pathways. It was reported recently that interaction between ouabain-bound Na,K-ATPase and the 1,4,5-trisphosphate receptor (IP3R) triggers slow calcium oscillations and activation of NF-kappaB. Here it is demonstrated that this signaling pathway can serve to prevent cell death and promote cell growth. Rat renal proximal tubular cells in primary culture first were grown in the presence of 10% serum and then exposed to 0.2% serum for 24 h to induce apoptosis. Serum starvation increased the apoptotic index from 1.21 +/- 0.26 to 14.01 +/- 1.17%. Ouabain in concentrations that did not inhibit Na,K-ATPase activity (1 to 10 nM) completely abolished the apoptotic effect of serum starvation. Ouabain protection from apoptosis was not observed when release of calcium from intracellular stores via the IP3R was prevented. It was shown that the NH2 terminal tail of the Na,K-ATPase alpha subunit plays a key role in ouabain-triggered calcium oscillations. It was found that ouabain did not protect from apoptosis in serum-deprived cells that expressed a mutant Na,K-ATPase alpha subunit with deletion of the NH2 terminal tail. Ouabain exposure (10 nM for 24 h) significantly increased translocation of NF-kappaB from cytoplasm to nucleus. Helenalin, an inhibitor of NF-kappaB, abolished the antiapoptotic effect of ouabain. Ouabain (0.1 to 10 nM) also was found to stimulate proliferation and increase the viability of kidney cells. These effects were abolished when release of calcium via the IP3R was prevented.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

Inhibition of Na,K-ATPase by Dopamine in Proximal Tubule Epithelial Cells

In the current report we review the results that lay grounds for the model of intracellular sodium-mediated dopamine-induced endocytosis of Na,K-ATPase. Under conditions of a high salt diet, dopamine activates PKCzeta, which phosphorylates NKA alpha1 Ser-18. The phosphorylation produces a conformational change of alpha1 NH2-terminus, which through interaction with other domains of alpha1 exposes PI3K- and AP-2-binding domains. PI3K bound to the NKA alpha1 induces the recruitment and activation of other proteins involved in endocytosis, and PI3K-generated 3-phosphoinositides affect the local cytoskeleton and modify the biophysical conditions of the membrane for development of clathrin-coated pits. Plasma membrane phosphorylated NKA is internalized to specialized intracellular compartments where the NKA will be dephosphorylated. The NKA internalization results in a reduced Na+ transport by proximal tubule epithelial cells.

Curcumin modulation of Na,K-ATPase: phosphoenzyme accumulation, decreased K+ occlusion, and inhibition of hydrolytic activity

1 Curcumin, the major constitute of tumeric, is an important nutraceutical that has been shown to be useful in the treatment of many diseases. As an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, curcumin was shown to correct cystic fibrosis (CF) defects in some model systems, whereas others have reported no or little effects on CF after curcumin treatment, suggesting that curcumin effect is not due to simple inhibition of the Ca(2+)-ATPase. 2 We tested the hypothesis that curcumin may modulate other members of the P(2)-type ATPase superfamily by studying the effects of curcumin on the activity and kinetic properties of the Na,K-ATPase. 3 Curcumin treatment inhibited Na,K-ATPase activity in a dose-dependent manner (K(0.5) approximately 14.6 microM). Curcumin decreased the apparent affinity of Na,K-ATPase for K(+) and increased it for Na(+) and ATP. Kinetic analyses indicated that curcumin induces a three-fold reduction in the rate of E1P --> E2P transition, thereby increasing the steady-state phosphoenzyme level. Curcumin treatment significantly abrogated K(+) occlusion to the enzyme as evidenced from kinetic and proteolytic cleavage experiments. Curcumin also significantly decreased the vanadate sensitivity of the enzyme. 4 Thus, curcumin partially blocks the K(+) occlusion site, and induces a constitutive shift in the conformational equilibrium of the enzyme, towards the E1 conformation. 5 The physiological consequences of curcumin treatment previously reported in different epithelial model systems may, at least in part, be related to the direct effects of curcumin on Na,K-ATPase activity.

Alterations in the alpha2 isoform of Na,K-ATPase associated with familial hemiplegic migraine type 2

A number of missense mutations in the Na,K-ATPase alpha2 catalytic subunit have been identified in familial hemiplegic migraine with aura. Two alleles (L764P and W887R) showed loss-of-function, whereas a third (T345A) is fully functional but with altered Na,K-ATPase kinetics. This study describes two additional mutants, R689Q and M731T, originally identified by Vanmolkot et al. [Vanmolkot, K. R., et al. (2003) Ann. Neurol. 54, 360-366], which we show here to also be functional and kinetically altered. Both mutants have reduced catalytic turnover and increased apparent affinity for extracellular K(+). For both R689Q and M731T, sensitivity to vanadate inhibition is decreased, suggesting that the steady-state E(1) <==> E(2) poise of the enzyme is shifted toward E(1). Whereas the K'(ATP) is not affected by the R689Q replacement, the M731T mutant has an increase in apparent affinity for ATP. Analysis of the structural changes effected by T345A, R689Q, and M731T mutations, based on homologous replacements in the known crystal structure of the sarcoplasmic reticulum Ca-ATPase, provides insights into the molecular bases for the kinetic alterations. It is suggested that the disease phenotype is the consequence of lowered molecular activity of the alpha2 pump isoform due to either decreased K(+) affinity (T345A) or catalytic turnover (R689Q and M731T), thus causing a delay in extracellular K(+) clearance and/or altered localized Ca(2+) handling/signaling secondary to reduced activity in colocalized Na(+)/Ca(2+) exchange.

Thr-774 (transmembrane segment M5), Val-920 (M8), and Glu-954 (M9) are involved in Na+ transport, and Gln-923 (M8) is essential for Na,K-ATPase activity

The highly conserved amino acids of rat Na,K-ATPase, Thr-774 in the transmembrane helices M5, Val-920 and Gln-923 in M8, and Glu-953 and Glu-954 in M9, the side chains of which appear to be in close proximity, were mutated, and the resulting proteins, T774A, E953A/K, and E954A/K, V920E and Q923N/E/D/L, were expressed in HeLa cells. Ouabain-resistant cell lines were obtained from T774A, V920E, E953A, and E954A, whereas Q923N/E/D/L, E953K, and E954K could only be transiently expressed as fusion proteins with an enhanced green fluorescent protein. The apparent K0.5 values for Na+, as estimated by the Na+-dependent phosphoenzyme formation (K0.5(Na,EP)) or Na,K-ATPase activity (K(0.5)(Na,ATPase)), were increased by around 2 approximately 8-fold in the case of T774A, V920E, and E954A. The apparent K0.5 values for K+, as estimated by the Na,K-ATPase (K0.5(K,ATPase)) or p-nitrophenylphosphatase activity (K0.5(K,pNPPase)), were affected only slightly by the 3 mutations, except that V920E showed a 1.7-fold increase in the K0.5(K,ATPase). The apparent K0.5 values for ATP (K0.5(EP)), as estimated by phosphorylation (a high affinity ATP effect), were increased by 1.6 approximately 2.6-fold in the case of T774A, V920E, and E954A. Those estimated by Na,K-ATPase activity (K0.5(ATPase)) and ATP-induced inhibition (K(i,0.5)(pNPPase)) of K-pNPPase activity (low affinity ATP effects) were, respectively, increased by 1.8-fold and unchanged in the case of T774A but decreased by 2- and 4.8-fold in the case of V920E and were slightly changed and increased by 1.7-fold in the case of E954A. The E953A showed little significant change in the apparent affinities. These results suggest that Gln-923 in M8 is crucial for the active transport of Na+ and/or K+ across membranes and that the side chain oxygen atom of Thr-774 in M5, the methyl group(s) of Val-920 in M8, and the carboxyl oxygen(s) of Glu-954 in M9 mainly play some role in the transport of Na+ and also in the high and low affinity ATP effects rather than the transport of K+.

Deletion of the Na/K-ATPase alpha1-subunit gene (Atp1a1) does not prevent cavitation of the preimplantation mouse embryo

Increases in Na/K-ATPase activity occur concurrently with the onset of cavitation and are associated with increases in Na(+)-pump subunit mRNA and protein expression. We have hypothesized that the alpha1-isozyme of the Na/K-ATPase is required to mediate blastocyst formation. We have tested this hypothesis by characterizing preimplantation development in mice with a targeted disruption of the Na/K-ATPase alpha1-subunit (Atp1a1) using embryos acquired from matings between Atp1a1 heterozygous mice. Mouse embryos homozygous for a null mutation in the Na/K-ATPase alpha1-subunit gene are able to undergo compaction and cavitation. These findings demonstrate that trophectoderm transport mechanisms are maintained in the absence of the predominant isozyme of the Na(+)-pump that has previously been localized to the basolateral membranes of mammalian trophectoderm cells. The presence of multiple isoforms of Na/K-ATPase alpha- and beta-subunits at the time of cavitation suggests that there may be a degree of genetic redundancy amongst isoforms of the catalytic alpha-subunit that allows blastocyst formation to progress in the absence of the alpha1-subunit.

Kinetic Alterations due to a Missense Mutation in the Na,K-ATPase α2 Subunit Cause Familial Hemiplegic Migraine Type 2

A number of missense mutations in the ATP1A2 gene, which encodes the Na,K-ATPase alpha2 subunit, have been identified in familial hemiplegic migraine with aura. Loss of function and haploinsufficiency have been the suggested mechanisms in mutants for which functional analysis has been reported. This paper describes a kinetic analysis of mutant T345A, recently identified in a detailed genetic analysis of a large Finnish family (Kaunisto, M. A., Harno, H., Vanmolkot, K. R., Gargus, J. J., Sun, G., Hamalainen, E., Liukkonen, E., Kallela, M., van den Maagdenberg, A. M., Frants, R. R., Farkkila, M., Palotie, A., and Wessman, M. (2004) Neurogenetics 5, 141-146). Introducing T345A into the conserved rat alpha2 enzyme does not alter cell growth or catalytic turnover but causes a substantial decrease in apparent K+ affinity (2-fold increase in K0.5(K+)). In view of the location of Thr-345 in the cytoplasmic stalk domain adjacent to transmembrane segment 4, the 2-fold increase in K0.5(K+) is probably due to T345A replacement altering K+ occlusion/deocclusion. Faster K+ deocclusion of the mutant via the E2(K) + ATP --> E1.ATP + K+ partial reaction is evidenced in (i) a marked increase (300%) in K+ stimulation of Na-ATPase at micromolar ATP, (ii) a 4-fold decrease in KATP, and (iii) only a modest increase (approximately 3-fold) in I50 for vanadate, which was used as a probe of the steady state E1/E2 conformational equilibrium. We suggest that the decreased apparent K+ affinity is the basis for a reduced rate of extracellular K+ removal, which delays the recovery phase of nerve impulse transmission in the central nervous system and, thereby, the clinical picture of migraine with aura. This is the first demonstration of a mutation that leads to a disease associated with a kinetically altered but fully functional Na,K-ATPase, refining the molecular mechanism of pathogenesis in familial hemiplegic migraine.

Cell signaling microdomain with Na,K-ATPase and inositol 1,4,5-trisphosphate receptor generates calcium oscillations

Recent studies indicate novel roles for the ubiquitous ion pump, Na,K-ATPase, in addition to its function as a key regulator of intracellular sodium and potassium concentration. We have previously demonstrated that ouabain, the endogenous ligand of Na,K-ATPase, can trigger intracellular Ca2+ oscillations, a versatile intracellular signal controlling a diverse range of cellular processes. Here we report that Na,K-ATPase and inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) form a cell signaling microdomain that, in the presence of ouabain, generates slow Ca2+ oscillations in renal cells. Using fluorescent resonance energy transfer (FRET) measurements, we detected a close spatial proximity between Na,K-ATPase and InsP3R. Ouabain significantly enhanced FRET between Na,K-ATPase and InsP3R. The FRET effect and ouabain-induced Ca2+ oscillations were not observed following disruption of the actin cytoskeleton. Partial truncation of the NH2 terminus of Na,K-ATPase catalytic alpha1-subunit abolished Ca2+ oscillations and downstream activation of NF-kappaB. Ouabain-induced Ca2+ oscillations occurred in cells expressing an InsP3 sponge and were hence independent of InsP3 generation. Thus, we present a novel principle for a cell signaling microdomain where an ion pump serves as a receptor.

The amino acid sequence 442GDASE446 in Na/K-ATPase is an important motif in forming the high and low affinity ATP binding pockets

A highly conserved amino acid sequence 442GDASE446 in the ATP binding pocket of rat Na/K-ATPase was mutated, and the resulting proteins, G442A, G442P, D443A, S445A, and E446A, were expressed in HeLa cells to investigate the effect of individual ligands on Na/K-ATPase. The apparent Km for the high and low affinity ATP effects was estimated by ATP concentration dependence for the formation of the Na-dependent phosphoenzyme (Kmh) and Na/K-ATPase activity (Kml). The apparent Km for p-nitrophenylphosphate (pNPP) for K-dependent-pNPPase (KmP) and its inhibition by ATP (Ki,0.5) and the apparent Km for Mg2+, Na+, K+, and vanadate in Na/K-ATPase were also estimated. For all the mutants, the value for ATP was approximately 2-10-fold larger than that of the wild type. While the turnover number for Na/K-ATPase activity were unaffected or reduced by 20 approximately 50% in mutants G442(A/P) and D443A. Although both affinities for ATP effects were reduced as a result of the mutations, the ratio, Kml Kmh, for each mutant was 1.3 approximately 3.7, indicating that these mutations had a greater impact on the low affinity ATP effect than on the high affinity effect. Each KmP value with the turnover number suggests that these mutations favor the binding of pNPP over that of ATP. These data and others indicate that the sequence 442GDASE446 in the ATP binding pocket is an important motif that it is involved in both the high and low affinity ATP effects rather than in free Mg2+, Na+, and K+ effects.

Regulation of Na,K-ATPase by PLMS, the phospholemman-like protein from shark: molecular cloning, sequence, expression, cellular distribution, and functional effects of PLMS

In Na,K-ATPase membrane preparations from shark rectal glands, we have previously identified an FXYD domain-containing protein, phospholemman-like protein from shark, PLMS. This protein was shown to associate and modulate shark Na,K-ATPase activity in vitro. Here we describe the complete coding sequence, expression, and cellular localization of PLMS in the rectal gland of the shark Squalus acanthias. The mature protein contained 74 amino acids, including the N-terminal FXYD motif and a C-terminal protein kinase multisite phosphorylation motif. The sequence is preceded by a 20 amino acid candidate cleavable signal sequence. Immunogold labeling of the Na,K-ATPase alpha-subunit and PLMS showed the presence of alpha and PLMS in the basolateral membranes of the rectal gland cells and suggested their partial colocalization. Furthermore, through controlled proteolysis, the C terminus of PLMS containing the protein kinase phosphorylation domain can be specifically cleaved. Removal of this domain resulted in stimulation of maximal Na,K-ATPase activity, as well as several partial reactions. Both the E1 approximately P --> E2-P reaction, which is partially rate-limiting in shark, and the K+ deocclusion reaction, E2(K) --> E1, are accelerated. The latter may explain the finding that the apparent Na+ affinity was increased by the specific C-terminal PLMS truncation. Thus, these data are consistent with a model where interaction of the phosphorylation domain of PLMS with the Na,K-ATPase alpha-subunit is important for the modulation of shark Na,K-ATPase activity.

Structural basis for alpha1 versus alpha2 isoform-distinct behavior of the Na,K-ATPase

We showed earlier that the kinetic behavior of the alpha2 isoform of the Na,K-ATPase differs from the ubiquitous alpha1 isoform primarily by a shift in the steady-state E(1)/E(2) equilibrium of alpha2 in favor of E(1) form(s). The aim of the present study was to identify regions of the alpha chain that confer the alpha1/alpha2 distinct behavior using a mutagenesis and chimera approach. Criteria to assess shifts in conformational equilibrium included (i) K(+) sensitivity of Na-ATPase measured at micromolar ATP, under which condition E(2)(K(+)) --> E(1) + K(+) becomes rate-limiting, (ii) changes in K'(ATP) for low affinity ATP binding, (iii) vanadate sensitivity of Na,K-ATPase activity, and (iv) the rate of the partial reaction E(1)P --> E(2)P. We first confirmed that interactions between the cytoplasmic domains of alpha2 that modulate conformational shifts are fundamentally similar to those of alpha1, suggesting that the predilection of alpha2 for E(1) state(s) is due to differences in primary structure of the two isoforms. Kinetic behavior of the alpha1/alpha2 chimeras indicates that the difference in E(1)/E(2) poise of the two isoforms cannot be accounted for by their notably distinct N termini, but rather by the front segment extending from the cytoplasmic N terminus to the C-terminal end of the extracellular loop between transmembranes 3 and 4, with a lesser contribution of the alpha1/alpha2 divergent portion within the M4-M5 loop near the ATP binding domain. In addition, we show that the E(1) shift of alpha2 results primarily from differences in the conformational transition of the dephosphoenzyme, (E(2)(K(+)) --> E(1) + K(+)), rather than phosphoenzyme (E(1)P --> E(2)P).

Structure/function analysis of Na(+)-K(+)-ATPase central isoform-specific region: involvement in PKC regulation

Specific functions served by the various Na(+)-K(+)-ATPase alpha-isoforms are likely to originate in regions of structural divergence within their primary structures. The isoforms are nearly identical, with the exception of the NH(2) terminus and a 10-residue region near the center of each molecule (isoform-specific region; ISR). Although the NH(2) terminus has been clearly identified as a source of isoform functional diversity, other regions seem to be involved. We investigated whether the central ISR could also contribute to isoform variability. We constructed chimeric molecules in which the central ISRs of rat alpha(1)- and alpha(2)-isoforms were exchanged. After stable transfection into opossum kidney cells, the chimeras were characterized for two properties known to differ dramatically among the isoforms: their K(+) deocclusion pattern and their response to PKC activation. Comparisons with rat full-length alpha(1)- and alpha(2)-isoforms expressed under the same conditions suggest an involvement of the central ISR in the response to PKC but not in K(+) deocclusion.

Menkes copper-translocating P-type ATPase (ATP7A): biochemical and cell biology properties, and role in Menkes disease

The Menkes copper-translocating P-type ATPase (ATP7A; MNK) is a ubiquitous protein that regulates the absorption of copper in the gastrointestinal tract. Inside cells the protein has a dual function: it delivers copper to cuproenzymes in the Golgi compartment and effluxes excess copper. The latter property is achieved through copper-dependent vesicular trafficking of the Menkes protein to the plasma membrane of the cell. The trafficking mechanism and catalytic activity combine to facilitate absorption and intercellular transport of copper. The mechanism of catalysis and copper-dependent trafficking of the Menkes protein are the subjects of this review. Menkes disease, a systemic copper deficiency disorder, is caused by mutations in the gene encoding the Menkes protein. The effect of these mutations on the catalytic cycle and the cell biology of the Menkes protein, as well as predictions of the effect of particular mutant MNKs on observed Menkes disease symptoms will also be discussed.

New insights into the role of the N terminus in conformational transitions of the Na,K-ATPase

The deletion of 32 residues from the N terminus of the alpha1 catalytic subunit of the rat Na,K-ATPase (mutant alpha1M32) shifts the E(1)/E(2) conformational equilibrium toward E(1), and the combination of this deletion with mutation E233K in the M2-M3 loop acts synergistically to shift the conformation further toward E(1) (Boxenbaum, N., Daly, S. E., Javaid, Z. Z., Lane, L. K., and Blostein, R. (1998) J. Biol. Chem. 273, 23086-23092). To delimit the region of the cytoplasmic N terminus involved in these interactions, the consequences of a series of N-terminal deletions of alpha1 beyond Delta32 were evaluated. Criteria to assess shifts in conformational equilibrium were based on effects of perturbation of the entire catalytic cycle ((i) sensitivity to vanadate inhibition, (ii) K(+) sensitivity of Na-ATPase measured at micromolar ATP, (iii) changes in K'(ATP), and (iv) catalytic turnover), as well as estimates of the rates of the conformational transitions of phospho- and dephosphoenzyme (E(1)P --> E(2)P and E(2)(K(+)) --> E(1) + K(+)). The results show that, compared with alpha1M32, the deletion of up to 40 residues (alpha1M40) further shifts the poise toward E(1). Remarkably, further deletions (mutants alpha1M46, alpha1M49, and alpha1M56) reverse the effect, such that these mutants increasingly resemble the wild type alpha1. These results suggest novel intramolecular interactions involving domains within the N terminus that impact the manner in which the N terminus/M2-M3 loop regulatory domain interacts with the M4-M5 catalytic loop to effect E(1) <--> E(2) transitions.

Role of conserved TGDGVND-loop in Mg2+ binding, phosphorylation, and energy transfer in Na,K-ATPase

In the P-domain, the 369-DKTGTLT and the 709-GDGVNDSPALKK segment are highly conserved during evolution of P-type E1-E2-ATPase pumps irrespective of their cation specificities. The focus of this article is on evaluation of the role of the amino acid residues in the P domain of the alpha subunit of Na,K-ATPase for the E1P[3Na]--> E2P[2Na] conversion, the K+-activated dephosphorylation, and the transmission of these changes to and from the cation binding sites. Mutations of residues in the TGDGVND loop show that Asp710 is essential, and Asn713 is important, for Mg2+ binding and formation of the high-energy MgE1P[3Na] intermediate. In contrast Asp710 and Asp713 do not contribute to Mg2+ binding in the E2P-ouabain complex. Transition to E2P thus involves a shift of Mg2+ coordination away from Asp710 and Asn713 and the two residues become more important for K+-activated hydrolysis of the acyl phosphate bond at Asp369. Transmission of structural changes between the P-domain and cation sites in the membrane domain is evaluated in light of the protein structure, and the information from proteolytic or metal-catalyzed cleavage and mutagenesis studies.

Mechanistic basis for kinetic differences between the rat alpha 1, alpha 2, and alpha 3 isoforms of the Na,K-ATPase

Previous studies showed that the alpha 1, alpha 2, and alpha 3 isoforms of the catalytic subunit of the Na,K-ATPase differ in their apparent affinities for the ligands ATP, Na(+), and K(+). For the rat isoforms transfected into HeLa cells, K'(ATP) for ATP binding at its low affinity site is lower for alpha 2 and alpha 3 compared with alpha 1; relative to alpha 1 and alpha 2, alpha 3 has a higher K'(Na) and lower K'(K) (Jewell, E. A., and Lingrel, J. B (1991) J. Biol. Chem. 266, 16925--16930; Munzer, J. S., Daly, S. E., Jewell-Motz, E. A., Lingrel, J. B, and Blostein, R. (1994) J. Biol. Chem. 269, 16668--16676). The experiments described in the present study provide insight into the mechanistic basis for these differences. The results show that alpha 2 differs from alpha1 primarily by a shift in the E(1) E(2) equilibrium in favor of E(1) form(s) as evidenced by (i) a approximately 20-fold increase in IC(50) for vanadate, (ii) decreased catalytic turnover, and (iii) notable stability of Na,K-ATPase activity at acidic pH. In contrast, despite its lower K'(ATP) compared with alpha 1, the E(1) E(2) poise of alpha 3 is not shifted toward E(1). Distinct intrinsic interactions with Na(+) ions are underscored by the marked selectivity for Na(+) over Li(+) of alpha 3 compared with either alpha1 or alpha 2 and higher K'(Na) for cytoplasmic Na(+), which persists over a 100-fold range in proton concentration, independent of the presence of K(+). The kinetic analysis also suggests alpha 3-specific differences in relative rates of partial reactions, which impact this isoform's distinct apparent affinities for both Na(+) and K(+).

Cytoplasmic segment interactions in the alpha1-subunit of the rat Na+, K+-atpase

The currently accepted topographical model for the organization of the alpha-subunit of the Na+, K+-ATPase in the membrane considers that the protein has ten transmembrane segments and six cytoplasmic loops. Evidence of interaction between the cytoplasmic regions may contribute to a better understanding of the structure/function relationship of this protein. In this study, the first four cytoplasmic segments (C1, C2, C3 and C4) of the rat alpha1 subunit were expressed in Escherichia Coli. The large cytoplasmic loop between transmembrane segments four and five (C3) retained its native structure as demonstrated by the ability of ATP to protect against chemical modification by Fluorescein 5-isothiocyanate (FITC). Interaction studies were conducted by an overlay assay (Far Western blots) and surface plasmon resonance technology. We observed that C3 interacts with the N-terminal segment of the Na+, K+-ATPase, C1; and that both C1 and C3 interact with the cytoplasmic segments C2 and C4.

Stalk segment 5 of the yeast plasma membrane H+-ATPase: mutational evidence for a role in glucose regulation

In P(2)-type ATPases, a stalk region connects the cytoplasmic part of the molecule, which binds and hydrolyzes ATP, to the membrane-embedded part through which cations are pumped. The present study has used cysteine scanning mutagenesis to examine structure-function relationships within stalk segment 5 (S5) of the yeast plasma-membrane H(+)-ATPase. Of 29 Cys mutants that were made and examined, two (G670C and R682C) were blocked in biogenesis, presumably due to protein misfolding. In addition, one mutant (S681C) had very low ATPase activity, and another (F685C) displayed a 40-fold decrease in sensitivity to orthovanadate, reflecting a shift in equilibrium from the E(2) conformational state toward E(1). By far the most striking group of mutants (F666C, L671C, I674C, A677C, I684C, R687C, and Y689C) were constitutively activated even in the absence of glucose, with rates of ATP hydrolysis and kinetic properties normally seen only in glucose-metabolizing cells. Previous work has suggested that activation of the wild-type H(+)-ATPase results from kinase-mediated phosphorylation in the auto-inhibitory C-terminal region of the 100-kDa polypeptide. The seven residues identified in the present study are located on one face of the S5 alpha-helix, consistent with the idea that mutations along this face serve to release the auto-inhibition.

The regulation of catalytic activity of the menkes copper-translocating P-type ATPase. Role of high affinity copper-binding sites

The Menkes protein is a transmembrane copper translocating P-type ATPase. Mutations in the Menkes gene that affect the function of the Menkes protein may cause Menkes disease in humans, which is associated with severe systemic copper deficiency. The catalytic mechanism of the Menkes protein, including the formation of transient acylphosphate, is poorly understood. We transfected and overexpressed wild-type and targeted mutant Menkes protein in yeast and investigated its transient acyl phosphorylation. We demonstrated that the Menkes protein is transiently phosphorylated by ATP in a copper-specific and copper-dependent manner and appears to undergo conformational changes in accordance with the classical P-type ATPase model. Our data suggest that the catalytic cycle of the Menkes protein begins with the binding of copper to high affinity binding sites in the transmembrane channel, followed by ATP binding and transient phosphorylation. We propose that putative copper-binding sites at the N-terminal domain of the Menkes protein are important as sensors of low concentrations of copper but are not essential for the overall catalytic activity.

Stalk segment 4 of the yeast plasma membrane H+-ATPase. Mutational evidence for a role in the E1-E2 conformational change

In the P(2)-type ATPases, there is growing evidence that four alpha-helical stalk segments connect the cytoplasmic part of the molecule, responsible for ATP binding and hydrolysis, to the membrane-embedded part that mediates cation transport. The present study has focused on stalk segment 4, which displays a significant degree of sequence conservation among P(2)-ATPases. When site-directed mutants in this region of the yeast plasma membrane H(+)-ATPase were constructed and expressed in secretory vesicles, more than half of the amino acid substitutions led to a severalfold decrease in the rate of ATP hydrolysis, although they had little or no effect on the coupling between hydrolysis and transport. Strikingly, mutant ATPases bearing single substitutions of 13 consecutive residues from Ile-359 through Gly-371 were highly resistant to inorganic orthovanadate, with IC(50) values at least 10-fold above those seen in the wild-type enzyme. Most of the same mutants also displayed a significant reduction in the K(m) for MgATP and an increase in the pH optimum for ATP hydrolysis. Taken together, these changes in kinetic behavior point to a shift in equilibrium from the E(2) conformation of the ATPase toward the E(1) conformation. The residues from Ile-359 through Gly-371 would occupy three full turns of an alpha-helix, suggesting that this portion of stalk segment 4 may provide a conformationally active link between catalytic sites in the cytoplasm and cation-binding sites in the membrane.

K(+)/Na(+) antagonism at cytoplasmic sites of Na(+)-K(+)-ATPase: a tissue-specific mechanism of sodium pump regulation

Tissue-distinct interactions of the Na(+)-K(+)-ATPase with Na(+) and K(+), independent of isoform-specific properties, were reported previously (A. G. Therien, N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol. Chem. 271: 7104-7112, 1996). In this paper, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K(+), namely K(+) inhibition at cytoplasmic Na(+) sites. Our results show that the order of susceptibilities of alpha(1) pumps of various rat tissues to K(+)/Na(+) antagonism, represented by the ratio of the apparent affinity for Na(+) binding at cytoplasmic activation sites in the absence of K(+) to the affinity constant for K(+) as a competitive inhibitor of Na(+) binding at cytoplasmic sites, is red blood cell < axolemma approximately rat alpha(1)-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K(+) binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K(+) binding/occlusion and the apparent affinity for K(+) as a competitive inhibitor of Na(+) activation, the order for both parameters being heart >/= kidney > small intestine approximately rat alpha(1)-transfected HeLa cells. The notion that modulations in cytoplasmic K(+)/Na(+) antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K(+)/Na(+) antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.

Mutations of Ser-23 of the alpha1 subunit of the rat Na+/K+-ATPase to negatively charged amino acid residues mimic the functional effect of PKC-mediated phosphorylation

The Na+/K+-ATPase is a target protein for protein kinase C (PKC). The PKC-mediated phosphorylation of the rat alpha1 subunit at Ser-23 results in the inhibition of its transport function. To understand the molecular basis of the inhibition by PKC, the Ser-23 in the rat alpha1 subunit has been replaced by negatively (Asp, Glu) or positively (Lys) charged, or uncharged (Gln, Ala) residues, and the mutants were expressed in Xenopus oocytes. Ouabain-specific 86Rb uptake and pump-generated current as well as sensitivity to ouabain and to external K+ have been investigated. When Ser-23 was replaced by the negatively charged residues, transport function was inhibited, and simultaneously synthesis of the alpha subunits was enhanced. In addition, if Ser-23 was substituted by Glu, the K(I) value for inhibition of transport by ouabain was drastically increased from 46.5 microM to 1.05 mM. The data suggest that insertion of a negative charge within the N-terminus of alpha subunit of the Na+/K+-ATPase due to phosphorylation of Ser-23 plays an important role in the PKC-mediated inhibition of transport function.

The energy transduction mechanism of Na,K-ATPase studied with iron-catalyzed oxidative cleavage

This paper extends our recent report on specific iron-catalyzed oxidative cleavages of renal Na,K-ATPase and effects of E1 left arrow over right arrow E2 conformational transitions (Goldshleger, R. , and Karlish, S. J. D. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 9596-9601). The experiments indicate that only peptide bonds close to a bound Fe2+ ion are cleaved, and provide evidence on proximity of the different cleavage positions in the native enzyme. A sequence HFIH near trans-membrane segment M3 appears to be involved in Fe2+ binding. Previously we hypothesized that E2 and E1 conformations are characterized by formation or relaxation of interactions within the alpha subunit at or near highly conserved sequences, TGES in the minor cytoplasmic loop and CSDK, MVTGD, and VNDSPALKK in the major cytoplasmic loop. This concept has been tested by examining iron-catalyzed cleavage in both non-phosphorylated and phosphorylated conformations and effects of phosphate, vanadate, and ouabain. The results imply that both E1 left arrow over right arrow E2 and E1P left arrow over right arrow E2P transitions are indeed associated with formation and relaxation of interactions between cytoplasmic domains, comprising the minor loop plus N-terminal tail leading into M1 and major loop, respectively. Furthermore, it appears that either non-covalently or covalently bound phosphate bind near CSDK and MVTGD, and Mg2+ ions may bind to residues within TGES and VNDSPALKK and to bound phosphate. Thus cytoplasmic domain interactions seem to occur within or near the active site. We discuss the relationship between structural changes in the cytoplasmic domain and movements of trans-membrane segments that lead to cation transport. Presumably conformation-dependent formation and relaxation of domain interactions underlie energy transduction in all P-type pumps.

The nongastric H+-K+-ATPases: molecular and functional properties

The Na-K/H-K-ATPase gene family is divided in three subgroups including the Na-K-ATPases, mainly involved in whole body and cellular ion homeostasis, the gastric H-K-ATPase involved in gastric fluid acidification, and the newly described nongastric H-K-ATPases for which the identification of physiological roles is still in its infancy. The first member of this last subfamily was first identified in 1992, rapidly followed by the molecular cloning of several other members. The relationship between each member remains unclear. The functional properties of these H-K-ATPases have been studied after their ex vivo expression in various functional expression systems, including the Xenopus laevis oocyte, the insect Sf9 cell line, and the human HEK 293 cells. All these H-K-ATPase alpha-subunits appear to encode H-K-ATPases when exogenously expressed in such expression systems. Recent data suggest that these H-K-ATPases could also transport Na+ in exchange for K+, revealing a complex cation transport selectivity. Moreover, they display a unique pharmacological profile compared with the canonical Na-K-ATPases or the gastric H-K-ATPase. In addition to their molecular and functional characterizations, a major goal is to correlate the molecular expression of these cloned H-K-ATPases with the native K-ATPases activities described in vivo. This appears to be more complex than anticipated. The discrepancies between the functional data obtained by exogenous expression of the nongastric H-K-ATPases and the physiological data obtained in native organs could have several explanations as discussed in the present review. Extensive studies will be required in the future to better understand the physiological role of these H-K-ATPases, especially in disease processes including ionic or acid-base disorders.

Expression and functional role of the gamma subunit of the Na, K-ATPase in mammalian cells

The functional role of the gamma subunit of the Na,K-ATPase was studied using rat gamma cDNA-transfected HEK-293 cells and an antiserum (gammaC33) specific for gamma. Although the sequence for gamma was verified and shown to be larger (7237 Da) than first reported, it still comprises a single initiator methionine despite the expression of a gammaC33-reactive doublet on immunoblots. Kinetic analysis of the enzyme of transfected compared with control cells and of gammaC33-treated kidney pumps shows that gamma regulates the apparent affinity for ATP. Thus, gamma-transfected cells have a decreased K'ATP as shown in measurements of (i) K'ATP of Na,K-ATPase activity and (ii) K+ inhibition of Na-ATPase at 1 microM ATP. Consistent with the behavior of gamma-transfected cells, gammaC33 pretreatment increases K'ATP of the kidney enzyme and K+ inhibition (1 microM ATP) of both kidney and gamma-transfected cells. These results are consistent with previous findings that an antiserum raised against the pig gamma subunit stabilizes the E2(K) form of the enzyme (Therien, A. G., Goldshleger, R., Karlish, S. J., and Blostein, R. (1997) J. Biol. Chem. 272, 32628-32634). Overall, our data demonstrate that gamma is a tissue (kidney)-specific regulator of the Na,K-ATPase that can increase the apparent affinity of the enzyme for ATP in a manner that is reversible by anti-gamma antiserum.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Changes in steady-state conformational equilibrium resulting from cytoplasmic mutations of the Na,K-ATPase alpha-subunit

Mutations comprising either deletion of 32 amino acids from the NH2 terminus (alpha1M32) or a Glu233 --> Lys substitution in the first M2-M3 cytoplasmic loop (E233K) of the alpha1-subunit of the Na, K-ATPase result in a shift in the steady-state E1 left arrow over right arrow E2 conformational equilibrium toward E1 form(s). In the present study, the functional consequences of both NH2-terminal deletion and Glu233 substitution provide evidence for mutual interactions of these cytoplasmic regions. Following transfection and selection of HeLa cells expressing the ouabain-resistant alpha1M32E233K double mutant, growth was markedly reduced unless the K+ concentration in the culture medium was increased to at least 10 mM. Marked changes effected by this double mutation included 1) a 15-fold reduction in catalytic turnover (Vmax/EPmax), 2) a 70-fold increase in apparent affinity for ATP, 3) a marked decrease in vanadate sensitivity, and 4) marked (approximately 10-fold) K+ activation of the Na-ATPase activity measured at micromolar ATP under which condition the E2(K) --> --> E1 pathway is normally (alpha1) rate-limiting and K+ is inhibitory. The decrease in catalytic turnover was associated with a 5-fold decrease in Vmax and a compensatory approximately 3-fold increase in expressed alpha1M32E233K protein. In contrast to the behavior of either alpha1M32 or E233K, alpha1M32E233K also showed alterations in apparent cation affinities. K'Na was decreased approximately 2-fold and K'K was increased approximately 2-fold. The importance of the charge at residue 233 is underscored by the consequences of single and double mutations comprising either a conservative change (E233D) or neutral substitution (E233Q). Thus, whereas mutation to a positively charged residue (E233K) causes a drastic change in enzymatic behavior, a conservative change causes only a minor change and the neutral substitution, an intermediate effect. Overall, the combined effects of the NH2-terminal deletion and the Glu233 substitutions are synergistic rather than additive, consistent with an interaction between the NH2-terminal region, the first cytoplasmic loop, and possibly the large M4-M5 cytoplasmic loop bearing the nucleotide binding and phosphorylation sites.

Evidence that Ser775 in the alpha subunit of the Na,K-ATPase is a residue in the cation binding pocket

Substitution of alanine for Ser775 in a ouabain-resistant alpha1 sheep isoform causes a 30-fold decrease in apparent affinity for K+ as an activator of the Na,K-ATPase, as well as an increase in apparent affinity for ATP (Arguello, J. M., and Lingrel, J. B (1995) J. Biol. Chem. 270, 22764-22771). This study was carried out to determine whether Ser775 is a direct cation-ligating residue or whether the change in apparent affinity for K+ is secondary to a conformational alteration as evidenced in the change in ATP affinity, with the following results. Kinetics of K+(Rb+) influx into intact cells show that the change is due to a change in K+ interaction at the extracellular surface. The K+ dependence of formation of K+-occluded enzyme (E2(K)) and of the rate of formation of deoccluded enzyme from E2(K) indicate that the Ser775 --> Ala mutation results in a marked increase (>/=30-fold) in rate of release of K+ from E2(K). The high affinity Na+-like competitive antagonist 1,3-dibromo2,4,6-tris-(methylisothiouronium)benzene (Br2TITU), which interacts with the E1 conformation and blocks cytoplasmic cation binding (Hoving, S., Bar-Shimon, M., Tijmes, J. J. , Tal, D. M., and Karlish, S. J. D. (1995) J. Biol. Chem. 270, 29788-29793), inhibits Na+-ATPase of the mutant less than the control enzyme. With intact cells, Br2TITU acts as a competitive inhibitor of extracellular K+ activation of both the mutant and control enzymes. In this case, the mutant was more sensitive to inhibition. With vanadate as a probe of conformation, a difference in conformational equilibrium between the mutant and control enzymes could not be detected under turnover conditions (Na+- ATPase) in the absence of K+. These results indicate that the increase in apparent affinity for ATP effected by the Ser775 --> Ala mutation is secondary to a change in intrinsic cation affinity/selectivity. The large change in affinity for extracellular K+ compared with cytoplasmic Na+ and to Br2TITU binding supports the conclusion that the serine hydroxyl is either part of the K+-gate structure or a direct cation-ligating residue that is shared by at least one Na+ ion, albeit with less consequence on rate constants for Na+ binding or release compared with K+.

Epitope and mimotope for an antibody to the Na, K-ATPase

The epitope of a monoclonal antibody specific for the alpha 2 isoform of the Na,K-ATPase was determined and its accessibility in native enzyme was examined. Protein fragmentation with N-chlorosuccinimide, formic acid, trypsin, and leucine aminopeptidase indicated binding near the Na,K-ATPase N-terminus but did not unambiguously delineate the extent of the epitope. The ability of the antibody to bind to denatured enzyme made it a good candidate for screening a random peptide library displayed on M13 phage, but the consensus sequence that emerged was not found in the Na,K-ATPase, Full-length cDNA for the Na,K-ATPase was randomly fragmented and cloned into beta-galactosidase to create a lambda gt11 expression library; screening with the antibody yielded a set of overlaps spanning 23 amino acids at the N-terminus. Chimeras of Na,K-ATPase alpha 1 and alpha 2 narrowed down the epitope to 14-19 amino acids. The antibody did not recognize fusion proteins constructed with shorter segments of this epitope. It did recognize a fusion protein containing the M13 library consensus sequence, however, indicating that this sequence, which is rich in proline and hydrophobic amino acids (FPPNFLFPPPP), was a mimotope. The natural epitope, unique to the Na,K-ATPase alpha 2 isoform, was GREYSPAATTAENG. Reconstitution of antibody binding in a foreign context such as M13 PIII protein or beta-galactosidase thus required a relatively large number of amino acids, indicating that antibody mapping approaches must allow for epitopes of significant size. The epitope was accessible in native enzyme and exposed on the cytoplasmic side, documenting the surface exposure of a stretch of amino acids at the N-terminus, where the Na,K-ATPase isoforms differ most.

Functional consequences of a posttransfection mutation in the H2-H3 cytoplasmic loop of the alpha subunit of Na,K-ATPase

During kinetic studies of mutant rat Na,K-ATPases, we identified a spontaneous mutation in the first cytoplasmic loop between transmembrane helices 2 and 3 (H2-H3 loop) which results in a functional enzyme with distinct Na,K-ATPase kinetics. The mutant cDNA contained a single G950 to A substitution, which resulted in the replacement of glutamate at 233 with a lysine (E233K). E233K and alpha1 cDNAs were transfected into HeLa cells and their kinetic behavior was compared. Transport studies carried out under physiological conditions with intact cells indicate that the E233K mutant and alpha1 have similar apparent affinities for cytoplasmic Na+ and extracellular K+. In contrast, distinct kinetic properties are observed when ATPase activity is assayed under conditions (low ATP concentration) in which the K+ deocclusion pathway of the reaction is rate-limiting. At 1 microM ATP K+ inhibits Na+-ATPase of alpha1, but activates Na+-ATPase of E233K. This distinctive behavior of E233K is due to its faster rate of formation of dephosphoenzyme (E1) from K+-occluded enzyme (E2(K)), as well as 6-fold higher affinity for ATP at the low affinity ATP binding site. A lower ratio of Vmax to maximal level of phosphoenzyme indicates that E233K has a lower catalytic turnover than alpha1. These distinct kinetics of E233K suggest a shift in its E1/E2 conformational equilibrium toward E1. Furthermore, the importance of the H2-H3 loop in coupling conformational changes to ATP hydrolysis is underscored by a marked (2 orders of magnitude) reduction in vanadate sensitivity effected by this Glu233 --> Lys mutation.