Rate limitation of the Na(+),K(+)-ATPase pump cycle

Consensus, controversies, and conundrums of P4-ATPases: The emerging face of eukaryotic lipid flippases

The cryo-EM resolution revolution has heralded a new era in our understanding of eukaryotic lipid flippases with a rapidly growing number of high-resolution structures. Flippases belong to the P4 family of ATPases (type IV P-type ATPases) that largely follow the reaction cycle proposed for the more extensively studied cation-transporting P-type ATPases. However, unlike the canonical P-type ATPases, no flippase cargos are transported in the phosphorylation half-reaction. Instead of being released into the intracellular or extracellular milieu, lipid cargos are transported to their destination at the inner leaflet of the membrane. Recent flippase structures have revealed multiple conformational states during the lipid transport cycle. Nonetheless, critical conformational states capturing the lipid cargo "in transit" are still missing. In this review, we highlight the amazing structural advances of these lipid transporters, discuss various perspectives on catalytic and regulatory mechanisms in the literature, and shed light on future directions in further deciphering the detailed molecular mechanisms of lipid flipping.

Molecular determinants for cold adaptation in an Antarctic Na+/K+-ATPase

Enzymes from ectotherms living in chronically cold environments have evolved structural innovations to overcome the effects of temperature on catalysis. Cold adaptation of soluble enzymes is driven by changes within their primary structure or the aqueous milieu. For membrane-embedded enzymes, like the Na+/K+-ATPase, the situation is different because changes to the lipid bilayer in which they operate may also be relevant. Although much attention has been focused on thermal adaptation within lipid bilayers, relatively little is known about the contribution of structural changes within membrane-bound enzymes themselves. The identification of specific mutations that confer temperature compensation is complicated by the presence of neutral mutations, which can be more numerous. In the present study, we identified specific amino acids in a Na+/K+-ATPase from an Antarctic octopus that underlie cold resistance. Our approach was to generate chimeras between an Antarctic clone and a temperate ortholog and then study their temperature sensitivities in Xenopus oocytes using an electrophysiological approach. We identified 12 positions in the Antarctic Na+/K+-ATPase that, when transferred to the temperate ortholog, were sufficient to confer cold tolerance. Furthermore, although all 12 Antarctic mutations were required for the full phenotype, a single leucine in the third transmembrane segment (M3) imparted most of it. Mutations that confer cold resistance are mostly in transmembrane segments, at positions that face the lipid bilayer. We propose that the interface between a transmembrane enzyme and the lipid bilayer is a critical determinant of temperature sensitivity and, accordingly, has been a prime evolutionary target for thermal adaptation.

Disease mutations of human α3 Na+/K+-ATPase define extracellular Na+ binding/occlusion kinetics at ion binding site III

Na⁺/K⁺-ATPase, which creates transmembrane electrochemical gradients by exchanging 3 Na⁺ for 2 K⁺, is central to the pathogenesis of neurological diseases such as alternating hemiplegia of childhood. Although Na⁺/K⁺-ATPase has 3 distinct ion binding sites I-III, the difficulty of distinguishing ion binding events at each site from the others hinders kinetic study of these transitions. Here, we show that binding of Na⁺ at each site in the human α3 Na⁺/K⁺-ATPase can be resolved using extracellular Na⁺-mediated transient currents. When Na⁺/K⁺-ATPase is constrained to bind and release only Na⁺, three kinetic components: fast, medium, and slow, can be isolated, presumably corresponding to the protein dynamics associated with the binding (or release depending on the voltage step direction) and the occlusion (or deocclusion) of each of the 3 Na⁺. Patient-derived mutations of residues which coordinate Na⁺ at site III exclusively impact the slow component, demonstrating that site III is crucial for deocclusion and release of the first Na⁺ into the extracellular milieu. These results advance understanding of Na⁺/K⁺-ATPase mutation pathogenesis and provide a foundation for study of individual ions' binding kinetics.

Structural basis for gating mechanism of the human sodium-potassium pump

P2-type ATPase sodium-potassium pumps (Na⁺/K⁺-ATPases) are ion-transporting enzymes that use ATP to transport Na⁺ and K⁺ on opposite sides of the lipid bilayer against their electrochemical gradients to maintain ion concentration gradients across the membranes in all animal cells. Despite the available molecular architecture of the Na⁺/K⁺-ATPases, a complete molecular mechanism by which the Na⁺ and K⁺ ions access into and are released from the pump remains unknown. Here we report five cryo-electron microscopy (cryo-EM) structures of the human alpha3 Na⁺/K⁺-ATPase in its cytoplasmic side-open (E1), ATP-bound cytoplasmic side-open (E1•ATP), ADP-AlF₄⁻ trapped Na⁺-occluded (E1•P-ADP), BeF₃⁻ trapped exoplasmic side-open (E2P) and MgF₄²⁻ trapped K⁺-occluded (E2•P_i) states. Our work reveals the atomically resolved structural detail of the cytoplasmic gating mechanism of the Na⁺/K⁺-ATPase.

Through cryo-EM analysis, here authors reveal conformational rearrangements that are critical for the gating mechanism of the human alpha3 Na+/K+−ATPase

Dual-Cell Patch-Clamp Recording Revealed a Mechanism for a Ribbon Synapse to Process Both Digital and Analog Inputs and Outputs

A chemical synapse is either an action potential (AP) synapse or a graded potential (GP) synapse but not both. This study investigated how signals passed the glutamatergic synapse between the rod photoreceptor and its postsynaptic hyperpolarizing bipolar cells (HBCs) and light responses of retinal neurons with dual-cell and single-cell patch-clamp recording techniques. The results showed that scotopic lights evoked GPs in rods, whose depolarizing Phase 3 associated with the light offset also evoked APs of a duration of 241.8 ms and a slope of 4.5 mV/ms. The depolarization speed of Phase 3 (Speed) was 0.0001–0.0111 mV/ms and 0.103–0.469 mV/ms for rods and cones, respectively. On pairs of recorded rods and HBCs, only the depolarizing limbs of square waves applied to rods evoked clear currents in HBCs which reversed at −6.1 mV, indicating cation currents. We further used stimuli that simulated the rod light response to stimulate rods and recorded the rod-evoked excitatory current (rdEPSC) in HBCs. The normalized amplitude (R/R_max), delay, and rising slope of rdEPSCs were differentially exponentially correlated with the Speed (all p < 0.001). For the Speed < 0.1 mV/ms, R/R_max grew while the delay and duration reduced slowly; for the Speed between 0.1 and 0.4 mV/ms, R/R_max grew fast while the delay and duration dramatically decreased; for the Speed > 0.4 mV/ms, R/R_max reached the plateau, while the delay and duration approached the minimum, resembling digital signals. The rdEPSC peak was left-shifted and much faster than currents in rods. The scotopic-light-offset-associated major and minor cation currents in retinal ganglion cells (RGCs), the gigantic excitatory transient currents (GTECs) in HBCs, and APs and Phase 3 in rods showed comparable light-intensity-related locations. The data demonstrate that the rod-HBC synapse is a perfect synapse that can differentially decode and code analog and digital signals to process enormously varied rod and coupled-cone inputs.

Order-disorder transitions of cytoplasmic N-termini in the mechanisms of P-type ATPases

Membrane protein structure and function are modulated via interactions with their lipid environment. This is particularly true for integral membrane pumps, the P-type ATPases. These ATPases play vital roles in cell physiology, where they are associated with the transport of cations and lipids, thereby generating and maintaining crucial (electro-)chemical potential gradients across the membrane. Several pumps (Na⁺, K⁺-ATPase, H⁺, K⁺-ATPase and the plasma membrane Ca²⁺-ATPase) which are located in the asymmetric animal plasma membrane have been found to possess polybasic (lysine-rich) domains on their cytoplasmic surfaces, which are thought to act as phosphatidylserine (PS) binding domains. In contrast, the sarcoplasmic reticulum Ca²⁺-ATPase, located within an intracellular organelle membrane, does not possess such a domain. Here we focus on the lysine-rich N-termini of the plasma-membrane-bound Na⁺, K⁺- and H⁺, K⁺-ATPases. Synthetic peptides corresponding to the N-termini of these proteins were found, via quartz crystal microbalance and circular dichroism measurements, to interact via an electrostatic interaction with PS-containing membranes, thereby undergoing an increase in helical or other secondary structure content. As well as influencing ion pumping activity, it is proposed that this interaction could provide a mechanism for sensing the lipid asymmetry of the plasma membrane, which changes drastically when a cell undergoes apoptosis, i.e. programmed cell death. Thus, polybasic regions of plasma membrane-bound ion pumps could potentially perform the function of a "death sensor", signalling to a cell to reduce pumping activity and save energy.

The interaction of Na+, K+, and phosphate with the gastric H,K-ATPase. Kinetics of E1-E2 conformational changes assessed by eosin fluorescence measurements

H,K-ATPase and Na,K-ATPase show the highest degree of sequence similarity among all other members of the P-type ATPases family. To explore their common features in terms of ligand binding, we evaluated conformational transitions due to the binding of Na⁺, K⁺ and Pi in the H,K-ATPase, and compared the results with those obtained for the Na,K-ATPase. This work shows that eosin fluorescence time courses provide a reasonably precise method to study the kinetics of the E1-E2 conformational changes in the H,K-ATPase. We found that, although Na⁺ shifts the equilibrium toward the E1 conformation and seems to compete with H⁺ in ATPase activity assays, it was neither possible to isolate a Na⁺-occluded state, nor to reveal an influx of Na⁺ related to H,K-ATPase activity. The high rate of the E2K → E1 transition found for the H,K-ATPase, which is not compatible with the presence of a K⁺-occluded form, agrees with the negligible level of occluded Rb⁺ (used as a K⁺ congener) found in the absence of added ligands. The use of vanadate and fluorinated metals to induce E2P-like states increased the level of occluded Rb⁺ and suggests that-during dephosphorylation-the probability of K⁺ to remain occluded increases from the E2P-ground to the E2P-product state. From kinetic experiments we found an unexpected increase in the values of k_obs for E2P formation with [Pi]; consequently, to obey the Albers-Post model, the binding of Pi to the E2 state cannot be a rapid-equilibrium reaction.

The Physiology of the Gastric Parietal Cell

Parietal cells are responsible for gastric acid secretion, which aids in the digestion of food, absorption of minerals, and control of harmful bacteria. However, a fine balance of activators and inhibitors of parietal cell-mediated acid secretion is required to ensure proper digestion of food, while preventing damage to the gastric and duodenal mucosa. As a result, parietal cell secretion is highly regulated through numerous mechanisms including the vagus nerve, gastrin, histamine, ghrelin, somatostatin, glucagon-like peptide 1, and other agonists and antagonists. The tight regulation of parietal cells ensures the proper secretion of HCl. The H+-K+-ATPase enzyme expressed in parietal cells regulates the exchange of cytoplasmic H+ for extracellular K+. The H+ secreted into the gastric lumen by the H+-K+-ATPase combines with luminal Cl- to form gastric acid, HCl. Inhibition of the H+-K+-ATPase is the most efficacious method of preventing harmful gastric acid secretion. Proton pump inhibitors and potassium competitive acid blockers are widely used therapeutically to inhibit acid secretion. Stimulated delivery of the H+-K+-ATPase to the parietal cell apical surface requires the fusion of intracellular tubulovesicles with the overlying secretory canaliculus, a process that represents the most prominent example of apical membrane recycling. In addition to their unique ability to secrete gastric acid, parietal cells also play an important role in gastric mucosal homeostasis through the secretion of multiple growth factor molecules. The gastric parietal cell therefore plays multiple roles in gastric secretion and protection as well as coordination of physiological repair.

Polarity of the ATP binding site of the Na+,K+-ATPase, gastric H+,K+-ATPase and sarcoplasmic reticulum Ca2+-ATPase

A fluorescence ratiometric method utilizing the probe eosin Y is presented for estimating the ATP binding site polarity of P-type ATPases in different conformational states. The method has been calibrated by measurements in a series of alcohols and tested using complexation of eosin Y with methyl-β-cyclodextrin. The results obtained with the Na⁺,K⁺-, H⁺,K⁺- and sarcoplasmic reticulum Ca²⁺-ATPases indicate that the ATP binding site, to which eosin is known to bind, is significantly more polar in the case of the Na⁺,K⁺- and H⁺,K⁺-ATPases compared to the Ca²⁺-ATPase. This result was found to be consistent with docking calculations of eosin with the E2 conformational state of the Na⁺,K⁺-ATPase and the Ca²⁺-ATPase. Fluorescence experiments showed that eosin binds significantly more strongly to the E1 conformation of the Na⁺,K⁺-ATPase than the E2 conformation, but in the case of the Ca²⁺-ATPase both fluorescence experiments and docking calculations showed no significant difference in binding affinity between the two conformations. This result could be due to the fact that, in contrast to the Na⁺,K⁺- and H⁺,K⁺-ATPases, the E2-E1 transition of the Ca²⁺-ATPase does not involve the movement of a lysine-rich N-terminal tail which may affect the overall enzyme conformation. Consistent with this hypothesis, the eosin affinity of the E1 conformation of the Na⁺,K⁺-ATPase was significantly reduced after N-terminal truncation. It is suggested that changes in conformational entropy of the N-terminal tail of the Na⁺, K⁺- and the H⁺,K⁺-ATPases during the E2-E1 transition could affect the thermodynamic stability of the E1 conformation and hence its ATP binding affinity.

General and specific interactions of the phospholipid bilayer with P-type ATPases

Protein structure and function are modulated via interactions with their environment, representing both the surrounding aqueous media and lipid membranes that have an active role in shaping the structural topology of membrane proteins. Compared to a decade ago, there is now an abundance of crystal structural data on membrane proteins, which together with their functional studies have enhanced our understanding of the salient features of lipid-protein interactions. It is now important to recognize that membrane proteins are regulated by both (1) general lipid-protein interactions, where the general physicochemical properties of the lipid environment affect the conformational flexibility of a membrane protein, and (2) by specific lipid-protein interactions, where lipid molecules directly interact via chemical interactions with specific lipid-binding sites located on the protein. However, due to local differences in membrane composition, thickness, and lipid packing, local membrane physical properties and hence the associated lipid-protein interactions also differ due to membrane location, even for the same protein. Such a phenomenon has been shown to be true for one family of integral membrane ion pumps, the P2-type adenosine triphosphatases (ATPases). Despite being highly homologous, individual members of this family have distinct structural and functional activity and are an excellent candidate to highlight how the local membrane physical properties and specific lipid-protein interactions play a vital role in facilitating the structural rearrangements of these proteins necessary for their activity. Hence in this review, we focus on both the general and specific lipid-protein interactions and will mostly discuss the structure-function relationships of the following P2-type ATPases, Na⁺,K⁺-ATPase (NKA), gastric H⁺,K⁺-ATPase (HKA), and sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), in concurrence with their lipid environment.

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.

Na,K-ATPase α-subunit conformation determines glutathionylation efficiency

The functioning of the N, K-ATPase depends on the redox status of cells and its activity is inhibited by oxidative stress and hypoxia. We previously found that redox sensitivity of the Na,K-ATPase is mediated by glutathionylation of the α-subunit. An increase in the level of glutathionylation of cysteine residues in the Na,K-ATPase α-subunit under stressful conditions leads to a decrease in the activity of the enzyme and a change in its receptor function. The structure of the Na,K-ATPase undergoes significant conformational changes during functioning. The effects of enzyme conformation on its ability to undergo glutathionylation are not clear. Here we show that the highest level of glutathionylation in the α-subunit of Na,K-ATPase is achieved in the E1 (Na⁺-induced) conformation. The transition of the Na,K-ATPase to the E2 (K⁺-induced) conformation leads to a decrease in the efficiency of glutathionylation. The lowest efficiency of Na,K-ATPase glutathionylation was observed in the E2P and E2P ouabain states. According to molecular modelling data, the maximum number of cysteine residues available for glutathionylation are present in the E1P conformation. In the E2P conformation, the main functional cysteine residues (Cys204, Cys242, Cys452, and Cys456) are buried from the solvent, which makes them inaccessible for glutathionylation. Thus, the efficiency of α-subunit glutathionylation depends on enzyme conformation, which is altered by bound ligands and proteins. A shift in the E1/E2 equilibrium towards prevalence of E1 can lead to better access for the relevant ligands and proteins to the binding site located in the Na,K-ATPase α-subunit. Na,K-ATPase.

Electrostatic Stabilization Plays a Central Role in Autoinhibitory Regulation of the Na+,K+-ATPase

The Na⁺,K⁺-ATPase is present in the plasma membrane of all animal cells. It plays a crucial role in maintaining the Na⁺ and K⁺ electrochemical potential gradients across the membrane, which are essential in numerous physiological processes, e.g., nerve, muscle, and kidney function. Its cellular activity must, therefore, be under tight metabolic control. Consideration of eosin fluorescence and stopped-flow kinetic data indicates that the enzyme's E2 conformation is stabilized by electrostatic interactions, most likely between the N-terminus of the protein's catalytic α-subunit and the adjacent membrane. The electrostatic interactions can be screened by increasing ionic strength, leading to a more evenly balanced equilibrium between the E1 and E2 conformations. This represents an ideal situation for effective regulation of the Na⁺,K⁺-ATPase's enzymatic activity, because protein modifications, which perturb this equilibrium in either direction, can then easily lead to activation or inhibition. The effect of ionic strength on the E1:E2 distribution and the enzyme's kinetics can be mathematically described by the Gouy-Chapman theory of the electrical double layer. Weakening of the electrostatic interactions and a shift toward E1 causes a significant increase in the rate of phosphorylation of the enzyme by ATP. Electrostatic stabilization of the Na⁺,K⁺-ATPase's E2 conformation, thus, could play an important role in regulating the enzyme's physiological catalytic turnover.

Stimulation, inhibition, or stabilization of Na,K-ATPase caused by specific lipid interactions at distinct sites

The activity of membrane proteins such as Na,K-ATPase depends strongly on the surrounding lipid environment. Interactions can be annular, depending on the physical properties of the membrane, or specific with lipids bound in pockets between transmembrane domains. This paper describes three specific lipid-protein interactions using purified recombinant Na,K-ATPase. (a) Thermal stability of the Na,K-ATPase depends crucially on a specific interaction with 18:0/18:1 phosphatidylserine (1-stearoyl-2-oleoyl-sn-glycero-3-phospho-L-serine; SOPS) and cholesterol, which strongly amplifies stabilization. We show here that cholesterol associates with SOPS, FXYD1, and the α subunit between trans-membrane segments αTM8 and -10 to stabilize the protein. (b) Polyunsaturated neutral lipids stimulate Na,K-ATPase turnover by >60%. A screen of the lipid specificity showed that 18:0/20:4 and 18:0/22:6 phosphatidylethanolamine (PE) are the optimal phospholipids for this effect. (c) Saturated phosphatidylcholine and sphingomyelin, but not saturated phosphatidylserine or PE, inhibit Na,K-ATPase activity by 70-80%. This effect depends strongly on the presence of cholesterol. Analysis of the Na,K-ATPase activity and E1-E2 conformational transitions reveals the kinetic mechanisms of these effects. Both stimulatory and inhibitory lipids poise the conformational equilibrium toward E2, but their detailed mechanisms of action are different. PE accelerates the rate of E1 → E2P but does not affect E2(2K)ATP → E13NaATP, whereas sphingomyelin inhibits the rate of E2(2K)ATP → E13NaATP, with very little effect on E1 → E2P. We discuss these lipid effects in relation to recent crystal structures of Na,K-ATPase and propose that there are three separate sites for the specific lipid interactions, with potential physiological roles to regulate activity and stability of the pump.

How do astrocytes participate in neural plasticity?

Work over the past 20 years has implicated electrically nonexcitable astrocytes in complex neural functions. Despite controversies, it is increasingly clear that many, if not all, neural processes involve astrocytes. This review critically examines past work to identify the commonalities among the many published studies of neuroglia signaling. Although several studies have shown that astrocytes can impact short-term and long-term synaptic plasticity, further work is required to determine the requirement for astrocytic Ca(2+) and other second messengers in these processes. One of the roadblocks to the field advancing at a rapid pace has been technical. We predict that the novel experimental tools that have emerged in recent years will accelerate the field and likely disclose an entirely novel path of neuroglia signaling within the near future.

Extracellular allosteric Na(+) binding to the Na(+),K(+)-ATPase in cardiac myocytes

Whole-cell patch-clamp measurements of the current, Ip, produced by the Na(+),K(+)-ATPase across the plasma membrane of rabbit cardiac myocytes show an increase in Ip over the extracellular Na(+) concentration range 0-50 mM. This is not predicted by the classical Albers-Post scheme of the Na(+),K(+)-ATPase mechanism, where extracellular Na(+) should act as a competitive inhibitor of extracellular K(+) binding, which is necessary for the stimulation of enzyme dephosphorylation and the pumping of K(+) ions into the cytoplasm. The increase in Ip is consistent with Na(+) binding to an extracellular allosteric site, independent of the ion transport sites, and an increase in turnover via an acceleration of the rate-determining release of K(+) to the cytoplasm, E2(K(+))2 → E1 + 2K(+). At normal physiological concentrations of extracellular Na(+) of 140 mM, it is to be expected that binding of Na(+) to the allosteric site would be nearly saturated. Its purpose would seem to be simply to optimize the enzyme's ion pumping rate under its normal physiological conditions. Based on published crystal structures, a possible location of the allosteric site is within a cleft between the α- and β-subunits of the enzyme.

A method to measure hydrolytic activity of adenosinetriphosphatases (ATPases)

The detection of small amounts (nanomoles) of inorganic phosphate has a great interest in biochemistry. In particular, phosphate detection is useful to evaluate the rate of hydrolysis of phosphatases, that are enzymes able to remove phosphate from their substrate by hydrolytic cleavage. The hydrolysis rate is correlated to enzyme activity, an extremely important functional parameter. Among phosphatases there are the cation transporting adenosinetriphosphatases (ATPases), that produce inorganic phosphate by cleavage of the γ-phosphate of ATP. These membrane transporters have many fundamental physiological roles and are emerging as potential drug targets. ATPase hydrolytic activity is measured to test enzyme functionality, but it also provides useful information on possible inhibitory effects of molecules that interfere with the hydrolytic process. We have optimized a molybdenum-based protocol that makes use of potassium antimony (III) oxide tartrate (originally employed for phosphate detection in environmental analysis) to allow its use with phosphatase enzymes. In particular, the method was successfully applied to native and recombinant ATPases to demonstrate its reliability, validity, sensitivity and versatility. Our method introduces significant improvements to well-established experimental assays, which are currently employed for ATPase activity measurements. Therefore, it may be valuable in biochemical and biomedical investigations of ATPase enzymes, in combination with more specific tests, as well as in high throughput drug screening.

Kinetic comparisons of heart and kidney Na+,K(+)-ATPases

Most kinetic measurements of the partial reactions of Na(+),K(+)-ATPase have been conducted on enzyme from mammalian kidney. Here we present a kinetic model that is based on the available equilibrium and kinetic parameters of purified kidney enzyme, and allows predictions of its steady-state turnover and pump current in intact cells as a function of ion and ATP concentrations and the membrane voltage. Using this model, we calculated the expected dependence of the pump current on voltage and extracellular Na(+) concentration. The simulations indicate a lower voltage dependence at negative potentials of the kidney enzyme in comparison with heart muscle Na(+),K(+)-ATPase, in agreement with experimental results. The voltage dependence is enhanced at high extracellular Na(+) concentrations. This effect can be explained by a voltage-dependent depopulation of extracellular K(+) ion binding sites on the E2P state and an increase in the proportion of enzyme in the E1P(Na(+))(3) state in the steady state. This causes a decrease in the effective rate constant for occlusion of K(+) by the E2P state and hence a drop in turnover. Around a membrane potential of zero, negligible voltage dependence is observed because the voltage-independent E2(K(+))(2) → E1 + 2K(+) transition is the major rate-determining step.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Pumping ions

1. This is a concise review of the field of ion pumping from the perspective of the authors. 2. The period covered spans the discovery of Na(+) and K(+) concentration gradients across animal cell membranes by Carl Schmidt in the 1850s, through the isolation of the Na(+) /K(+) -ATPase by Skou in 1957 (for which he was awarded the 1997 Nobel Prize in Chemistry), to the publication of the first crystal structure of the enzyme in 2007 and beyond. 3. Contributions of the authors' research group to the resolution of the questions of the mechanism of the allosteric role of ATP within the Na(+) /K(+) -ATPase reaction cycle and how protomeric versus diprotomeric states of the enzyme influence its kinetics are discussed within the context of the research field. 4. The results obtained indicate that the Na(+) /K(+) -ATPase has a single ATP binding site, which can be catalytic or allosteric in different parts of the enzyme's reaction cycle. 5. The long-running controversy over whether P-type ATPases function as protomers or diprotomers can be resolved in the case of the Na(+) /K(+) -ATPase by an ATP-induced dissociation of (αβ)(2) diprotomers into separate αβ protomers. 6. Kinetic data suggest that protein-protein interactions between the two αβ protomers within an (αβ)(2) diprotomer result in a much lower enzymatic turnover (i.e. a lower gear) when only one of the α-subunits of the diprotomer has bound ATP. The inactive αβ protomer within the diprotomer can be thought of as causing a drag on the active protomer.

Astrocytes modulate neural network activity by Ca²+-dependent uptake of extracellular K+

Astrocytes are electrically nonexcitable cells that display increases in cytosolic calcium ion (Ca²+) in response to various neurotransmitters and neuromodulators. However, the physiological role of astrocytic Ca²+ signaling remains controversial. We show here that astrocytic Ca²+ signaling ex vivo and in vivo stimulated the Na+,K+-ATPase (Na+- and K+-dependent adenosine triphosphatase), leading to a transient decrease in the extracellular potassium ion (K+) concentration. This in turn led to neuronal hyperpolarization and suppressed baseline excitatory synaptic activity, detected as a reduced frequency of excitatory postsynaptic currents. Synaptic failures decreased in parallel, leading to an increase in synaptic fidelity. The net result was that astrocytes, through active uptake of K+, improved the signal-to-noise ratio of synaptic transmission. Active control of the extracellular K+ concentration thus provides astrocytes with a simple yet powerful mechanism to rapidly modulate network activity.

Artifact versus reality--how astrocytes contribute to synaptic events

The neuronal doctrine, developed a century ago regards neuronal networks as the sole substrate of higher brain function. Recent advances in glial physiology have promoted an alternative hypothesis, which places information processing in the brain into integrated neuronal-glial networks utilizing both binary (neuronal action potentials) and analogue (diffusional propagation of second messengers/metabolites through gap junctions or transmitters through the interstitial space) signal encoding. It has been proposed that the feed-forward and feed-back communication between these two types of neural cells, which underlies information transfer and processing, is accomplished by the release of neurotransmitters from neuronal terminals as well as from astroglial processes. Understanding of this subject, however, remains incomplete and important questions and controversies require resolution. Here we propose that the primary function of perisynaptic glial processes is to create an "astroglial cradle" that shields the synapse from a multitude of extrasynaptic signaling events and provides for multifaceted support and long-term plasticity of synaptic contacts through variety of mechanisms, which may not necessarily involve the release of "glio" transmitters.

Kinetics of K(+) occlusion by the phosphoenzyme of the Na(+),K(+)-ATPase

Investigations of K(+)-occlusion by the phosphoenzyme of Na(+),K(+)-ATPase from shark rectal gland and pig kidney by stopped-flow fluorimetry reveal major differences in the kinetics of the two enzymes. In the case of the pig enzyme, a single K(+)-occlusion step could be resolved with a rate constant of 342 (± 26) s⁻¹. However, in the case of the shark enzyme, two consecutive K(+)-occlusions were detected with rate constants of 391 (± 19) s⁻¹ and 48 (± 2) s⁻¹ at 24°C and pH 7.4. A conformational change of the phosphoenzyme associated with K(+)-occlusion is, thus, the major rate-determining step of the shark enzyme under saturating concentrations of all substrates, whereas for the pig enzyme the major rate-determining step under the same conditions is the E2 → E1 transition and its associated K(+) deocclusion and release to the cytoplasm. The differences in rate constants of the K(+) occlusion reactions of the two enzymes are paralleled by compensating changes to the rate constant for the E2 → E1 transition, which explains why the differences in the enzymes' kinetic behaviors have not previously been identified.

Dual mechanisms of allosteric acceleration of the Na(+),K(+)-ATPase by ATP

Investigations of the E2 --> E1 conformational change of Na(+),K(+)-ATPase from shark rectal gland and pig kidney via the stopped-flow technique have revealed major differences in the kinetics and mechanisms of the two enzymes. Mammalian kidney Na(+),K(+)-ATPase appears to exist in a diprotomeric (alphabeta)(2) state in the absence of ATP, with protein-protein interactions between the alpha-subunits causing an inhibition of the transition, which occurs as a two-step process: E2:E2 --> E2:E1 --> E1:E1. This is evidenced by a biphasicity in the observed kinetics. Binding of ATP to the E1 or E2 states causes the kinetics to become monophasic and accelerate, which can be explained by an ATP-induced dissociation of the diprotomer into separate alphabeta protomers and relief of the preexisting inhibition. In the case of enzyme from shark rectal gland, the observed kinetics are monophasic at all ATP concentrations, indicating a monoprotomeric enzyme; however, an acceleration of the E2 --> E1 transition by ATP still occurs, to a maximum rate constant of 182 (+/- 6) s(-1). This indicates that ATP has two separate mechanisms whereby it accelerates the E2 --> E1 transition of Na(+),K(+)-ATPase alphabeta protomers and (alphabeta)(2) diprotomers.

General requirement for harvesting antennae at ca and h channels and transporters

The production and dissipation of energy in cells is intimately linked to the movement of small molecules in and out of enzymes, channels, and transporters. An analytical model of diffusion was described previously, which was used to estimate local effects of these proteins acting as molecular sources. The present article describes a simple but more general model, which can be used to estimate the local impact of proteins acting as molecular sinks. The results show that the enzymes, transporters, and channels, whose substrates are present at relatively high concentrations like ATP, Na⁺, glucose, lactate, and pyruvate, do not operate fast enough to deplete their vicinity to a meaningful extent, supporting the notion that for these molecules the cytosol is a well-mixed compartment. One specific consequence of this analysis is that the well-documented cross-talk existing between the Na⁺/K⁺ ATPase and the glycolytic machinery should not be explained by putative changes in local ATP concentration. In contrast, Ca2⁺ and H⁺ transporters like the Na⁺/Ca2⁺ exchanger NCX and the Na⁺/H⁺ exchanger NHE, show experimental rates of transport that are two to three orders of magnitude faster than the rates at which the aqueous phase may possibly feed their binding sites. This paradoxical result implies that Ca2⁺ and H⁺ transporters do not extract their substrates directly from the bulk cytosol, but from an intermediate “harvesting” compartment located between the aqueous phase and the transport site.

Electrogenic plasma membrane H+-ATPase activity using voltage sensitive dyes

Fast responding voltage sensitive dyes, RH421 and di-4-ASPBS, were used to study the electrogenic properties of plant plasma membrane proton pumps on sealed plasma membrane vesicles extracted by two-phase partitioning from Beta vulgaris and Avena sativa cv Swan root material. Fluorescence spectroscopy in the presence of the dye RH421 (10.8 nM) was sufficiently sensitive to detect electrogenic activity of the extracted plant vesicles. The dye detection system could detect inhibition of electrogenic activity of vesicles by vanadate (75 μM) and stimulation by nigericin (0.5 μM). The newly developed dye di-4-ASPBS was less sensitive to detecting the electrogenic proton pump activity. This study represents an important innovation in plant biophysics as this class of fast responding voltage sensitive dyes have never to our knowledge been used to study electrogenic proton pump activity derived from plant membranes and represents a novel approach for carrying out such studies.

Interaction of ATP with the phosphoenzyme of the Na+,K+-ATPase

The interaction of ATP with the phosphoenzyme of Na(+),K(+)-ATPase from pig kidney, rabbit kidney, and shark rectal gland was investigated using the voltage-sensitive fluorescent probe RH421. In each case, ATP concentrations >or=100 microM caused a drop in fluorescence intensity, which, because RH421 is sensitive to the formation of enzyme in the E2P state, can be attributed to ATP binding to the E2P phosphoenzyme. Simulations of the experimental behavior using kinetic models based on either a monomeric or a dimeric enzyme mechanism yielded a K(d) for ATP binding in the range 140-500 muM. Steady-state activity measurements and independent measurements of the phosphoenzyme level via a radioactive assay indicated that ATP binding to E2P causes a deceleration in its dephosphorylation when acting in the Na(+)-ATPase mode, i.e., in the absence of K(+) ions. Both the ATP-induced drop in RH421 fluorescence and the effect on the dephosphorylation reaction could be attributed to an inhibition of dissociation from the E2P(Na(+))(3) state of the one Na(+) ion necessary to allow dephosphorylation. Stopped-flow studies on the shark enzyme indicated that the ATP-induced inhibition of dephosphorylation is abolished in the presence of 1 mM KCl. A possible physiological role of allosteric binding of ATP to the phosphoenzyme could be to stabilize the E2P state and stop the enzyme running backward, which would cause dissipation of the Na(+) electrochemical potential gradient and the resynthesis of ATP from ADP. ATP binding to E2P could also fix ATP within the enzyme ready to phosphorylate it in the subsequent turnover.

Changes in electrostatic surface potential of Na+/K+-ATPase cytoplasmic headpiece induced by cytoplasmic ligand(s) binding

A set of single-tryptophan mutants of the Na(+)/K(+)-ATPase isolated, large cytoplasmic loop connecting transmembrane helices M4 and M5 (C45) was prepared to monitor effects of the natural cytoplasmic ligands (i.e., Mg(2+) and/or ATP) binding. We introduced a novel method for the monitoring of the changes in the electrostatic surface potential (ESP) induced by ligand binding, using the quenching of the intrinsic tryptophan fluorescence by acrylamide or iodide. This approach opens a new way to understanding the interactions within the proteins. Our experiments revealed that the C45 conformation in the presence of the ATP (without magnesium) substantially differed from the conformation in the presence of Mg(2+) or MgATP or in the absence of any ligand not only in the sense of geometry but also in the sense of the ESP. Notably, the set of ESP-sensitive residues was different from the set of geometry-sensitive residues. Moreover, our data indicate that the effect of the ligand binding is not restricted only to the close environment of the binding site and that the information is in fact transmitted also to the distal parts of the molecule. This property could be important for the communication between the cytoplasmic headpiece and the cation binding sites located within the transmembrane domain.

Electrogenic ion pumps investigated on a solid supported membrane: comparison of current and voltage measurements

Current and voltage measurements were performed on Na,K-ATPase and sarcoplasmic reticulum (SR) Ca-ATPase. Measurements of current transients under short-circuit conditions and of voltage transients under open-circuit conditions were carried out by employing a solid supported membrane (SSM). Purified membrane fragments containing Na,K-ATPase or native SR vesicles were adsorbed on a SSM and were activated by performing substrate concentration jumps. Current and voltage transients were recorded in the external circuit. They are related to pump activity and can be attributed to electrogenic events in the reaction cycles of the two enzymes. While current transients of very small amplitude are difficult to detect, the corresponding voltage transients can be measured with higher accuracy because of a much more favorable signal-to-noise ratio. Therefore, voltage measurements are preferable for the investigation of slow processes generating low current signals, e.g., for the analysis of low turnover transporters.

Structural changes in the catalytic cycle of the Na+,K+-ATPase studied by infrared spectroscopy

Pig kidney Na(+),K(+)-ATPase was studied by means of reaction-induced infrared difference spectroscopy. The reaction from E1Na(3)(+) to an E2P state was initiated by photolysis of P(3)-1-(2-nitrophenyl)ethyl ATP (NPE caged ATP) in samples that contained 3 mM free Mg(2+) and 130 mM NaCl at pH 7.5. Release of ATP from caged ATP produced highly detailed infrared difference spectra indicating structural changes of the Na(+),K(+)-ATPase. The observed transient state of the enzyme accumulated within seconds after ATP release and decayed on a timescale of minutes at 15 degrees C. Several controls ensured that the observed difference signals were due to structural changes of the Na(+),K(+)-ATPase. Samples that additionally contained 20 mM KCl showed similar spectra but less intense difference bands. The absorbance changes observed in the amide I region, reflecting conformational changes of the protein backbone, corresponded to only 0.3% of the maximum absorbance. Thus the net change of secondary structure was concluded to be very small, which is in line with movement of rigid protein segments during the catalytic cycle. Despite their small amplitude, the amide I signals unambiguously reveal the involvement of several secondary structure elements in the conformational change. Similarities and dissimilarities to corresponding spectra of the Ca(2+)-ATPase and H(+),K(+)-ATPase are discussed, and suggest characteristic bands for the E1 and E2 conformations at 1641 and 1661 cm(-1), respectively, for alphabeta heterodimeric ATPases. The spectra further indicate the participation of protonated carboxyl groups or lipid carbonyl groups in the reaction from E1Na(3)(+) to an E2P state. A negative band at 1730 cm(-1) is in line with the presence of a protonated Asp or Glu residue that coordinates Na(+) in E1Na(3)(+). Infrared signals were also detected in the absorption regions of ionized carboxyl groups.

Mechanism of Mg2+ binding in the Na+,K+-ATPase

The Mg(2+) dependence of the kinetics of the phosphorylation and conformational changes of Na(+),K(+)-ATPase was investigated via the stopped-flow technique using the fluorescent label RH421. The enzyme was preequilibrated in buffer containing 130 mM NaCl to stabilize the E1(Na(+))(3) state. On mixing with ATP, a fluorescence increase was observed. Two exponential functions were necessary to fit the data. Both phases displayed an increase in their observed rate constants with increasing Mg(2+) to saturating values of 195 (+/- 6) s(-1) and 54 (+/- 8) s(-1) for the fast and slow phases, respectively. The fast phase was attributed to enzyme conversion into the E2MgP state. The slow phase was attributed to relaxation of the dephosphorylation/rephosphorylation (by ATP) equilibrium and the buildup of some enzyme in the E2Mg state. Taking into account competition from free ATP, the dissociation constant (K(d)) of Mg(2+) interaction with the E1ATP(Na(+))(3) state was estimated as 0.069 (+/- 0.010) mM. This is virtually identical to the estimated value of the K(d) of Mg(2+)-ATP interaction in solution. Within the enzyme-ATP-Mg(2+) complex, the actual K(d) for Mg(2+) binding can be attributed primarily to complexation by ATP itself, with no apparent contribution from coordination by residues of the enzyme environment in the E1 conformation.

ATP binding equilibria of the Na(+),K(+)-ATPase

Reported values of the dissociation constant, K(d), of ATP with the E1 conformation of the Na(+),K(+)-ATPase fall in two distinct ranges depending on how it is measured. Equilibrium binding studies yield values of 0.1-0.6 microM, whereas presteady-state kinetic studies yield values of 3-14 microM. It is unacceptable that K(d) varies with the experimental method of its determination. Using simulations of the expected equilibrium behavior for different binding models based on thermodynamic data obtained from isothermal titration calorimetry we show that this apparent discrepancy can be explained in part by the presence in presteady-state kinetic studies of excess Mg(2+) ions, which compete with the enzyme for the available ATP. Another important contributing factor is an inaccurate assumption in the majority of presteady-state kinetic studies of a rapid relaxation of the ATP binding reaction on the time scale of the subsequent phosphorylation. However, these two factors alone are insufficient to explain the previously observed presteady-state kinetic behavior. In addition one must assume that there are two E1-ATP binding equilibria. Because crystal structures of P-type ATPases indicate only a single bound ATP per alpha-subunit, the only explanation consistent with both crystal structural and kinetic data is that the enzyme exists as an (alphabeta)(2) diprotomer, with protein-protein interactions between adjacent alpha-subunits producing two ATP affinities. We propose that in equilibrium measurements the measured K(d) is due to binding of ATP to one alpha-subunit, whereas in presteady-state kinetic studies, the measured apparent K(d) is due to the binding of ATP to both alpha-subunits within the diprotomer.

Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

Cholesterol-dependent interaction of polyunsaturated phospholipids with Na,K-ATPase

Polyunsaturated phospholipids such as 16:0-22:6 PC and 22:6 PC both stabilized the E1 conformation and inhibited turnover of Na,K-ATPase reconstituted into 18:1 PC or 18:1 PC/cholesterol liposomes. The inhibition increases in the order 22:6 PC > 16:0-22:6 PC both in the presence and in the absence of cholesterol, but is most pronounced in the absence of cholesterol. The inhibition of Na,K-ATPase turnover may thus correlate with the capability of polyunsaturated phospholipids and cholesterol to induce liquid-disordered and liquid-ordered lipid phases, respectively. In the presence of cholesterol 16:0-22:6 PC and 22:6 PC both increase the apparent Na+ affinity and change the K+ inhibition observed at low ATP concentration into activation. These effects on Na,K-ATPase kinetics can be explained by the ability of polyunsaturated phospholipids to induce lateral phase separation from cholesterol, which may be partially excluded from interaction with the Na,K-ATPase/lipid interface. Finally, inclusion of polyunsaturated phospholipids may induce changes in the bilayer hydrophobic thickness, which will increase the hydrophobic mismatch between lipids and protein.

Two gears of pumping by the sodium pump

The kinetics of the phosphorylation and subsequent conformational change of Na(+),K(+)-ATPase was investigated via the stopped-flow technique using the fluorescent label RH421 (pH 7.4, 24 degrees C). The enzyme was preequilibrated in buffer containing 130 mM NaCl to stabilize the E1(Na(+))(3) state. On mixing with ATP in the presence of Mg(2+), a fluorescence increase occurred, due to enzyme conversion into the E2P state. The fluorescence change accelerated with increasing ATP concentration until a saturating limit in the hundreds of micromolar range. The amplitude of the fluorescence change (DeltaF/F(0)) increased to 0.98 at 50 microM ATP. DeltaF/F(0) then decreased to 0.82 at 500 microM. The decrease was attributed to an ATP-induced allosteric acceleration of the dephosphorylation reaction. The ATP concentration dependence of the time course and the amplitude of the fluorescence change could not be explained by either a one-site monomeric enzyme model or by a two-pool model. All of the data could be explained by an (alphabeta)(2) dimeric model, in which the enzyme cycles at a low rate with ATP hydrolysis by one alpha-subunit or at a high rate with ATP hydrolysis by both alpha-subunits. Thus, we propose a two-gear bicyclic model to replace the classical monomeric Albers-Post model for kidney Na(+),K(+)-ATPase.

Allosteric effect of ATP on Na(+),K(+)-ATPase conformational kinetics

The kinetics of the E2 --> E1 conformational change of unphosphorylated Na+,K+-ATPase was investigated via the stopped-flow technique using the fluorescent label RH421 (pH 7.4, 24 degrees C). The enzyme was pre-equilibrated in a solution containing 25 mM histidine and 0.1 mM EDTA to stabilize the E2 conformation. When rabbit enzyme was mixed with 130 mM NaCl alone or with 130 mM NaCl and varying concentrations of Na2ATP simultaneously, a fluorescence decrease was observed. In the absence of ATP, the fluorescence decrease followed a biexponential time course, but at ATP concentrations after mixing of >or=50 microM, the fluorescence transient could be adequately fitted by a single exponential. On the basis of the agreement between theoretical simulations and experimental traces, we propose that in the absence of bound ATP the conformational transition occurs as a two step reversible process within a protein dimer, E2:E2 --> E2:E1 --> E1:E1. In the presence of 130 mM NaCl, the sum of the forward and backward rate constants for the E2:E2 --> E2:E1 and E2:E1 --> E1:E1 transitions were found to be 10.4 (+/-1.0) and 0.49 (+/-0.02) s-1, respectively. At saturating concentrations of ATP, however, the transition occurs in a single reversible step with the sum of its forward and backward rate constants equal to 35.2 (+/-0.3) s-1. It was found that ATP acting at a high affinity site (Kd approximately 0.25 microM), stimulated the reverse reaction, E1ATP --> E2ATP, in addition to its known allosteric low affinity (Kd approximately 71 microM) stimulation of the forward reaction, E2ATP --> E1ATP.

Fluorescence quenching by nucleotides of the plasma membrane H+-ATPase from Kluyveromyces lactis

The yeast plasma membrane H+-ATPase isolation procedure was improved; a highly pure enzyme (90-95%) was obtained after centrifugation on a trehalose concentration gradient. H+-ATPase kinetics was slightly cooperative: Hill number = 1.5, S0.5 = 800 microM ATP, and turnover number = 36 s-1. In contrast to those of other P-type ATPases, H+-ATPase fluorescence was highly sensitive to nucleotide binding; the fluorescence decreased 60% in the presence of both 5 mM ADP and AMP-PNP. Fluorescence titration with nucleotides allowed calculation of dissociation constants (Kd) from the binding site; Kd values for ATP and ADP were 700 and 800 microM, respectively. On the basis of amino acid sequence and homology model analysis, we propose that binding of the nucleotide to the N-domain is coupled to the movement of a loop beta structure and to the exposure of the Trp505 residue located in the loop. The recombinant N-domain also displayed a large hyperbolic fluorescence quenching when ATP binds; however, it displayed a higher affinity for ATP (Kd = 100 microM). We propose for P-type ATPases that structural movements during nucleotide binding could be followed if a Trp residue is properly located in the N-domain. Further, we propose the use of trehalose in enzyme purification protocols to increase the purity and quality of the isolated protein and to perform structural studies.

Modulation of FXYD interaction with Na,K-ATPase by anionic phospholipids and protein kinase phosphorylation

FXYD10 is a 74 amino acid small protein which regulates the activity of shark Na,K-ATPase. The lipid dependence of this regulatory interaction of FXYD10 with shark Na,K-ATPase was investigated using reconstitution into DOPC/cholesterol liposomes with or without the replacement of 20 mol % DOPC with anionic phospholipids. Specifically, the effects of the cytoplasmic domain of FXYD10, which contains the phosphorylation sites for protein kinases, on the kinetics of the Na,K-ATPase reaction were investigated by a comparison of the reconstituted native enzyme and the enzyme where 23 C-terminal amino acids of FXYD10 had been cleaved by mild, controlled trypsin treatment. Several kinetic properties of the Na,K-ATPase reaction cycle as well as the FXYD-regulation of Na,K-ATPase activity were found to be affected by acidic phospholipids like PI, PS, and PG. This takes into consideration the Na+ and K+ activation, the K+-deocclusion reaction, and the poise of the E1/E2 conformational equilibrium, whereas the ATP activation was unchanged. Anionic phospholipids increased the intermolecular cross-linking between the FXYD10 C-terminus (Cys74) and the Cys254 in the Na,K-ATPase A-domain. However, neither in the presence nor in the absence of anionic phospholipids did protein kinase phosphorylation of native FXYD10, which relieves the inhibition, affect such cross-linking. Together, this seems to indicate that phosphorylation involves only modest structural rearrangements between the cytoplasmic domain of FXYD10 and the Na,K-ATPase A-domain.

Modulation of reconstituted pig kidney Na+/K(+)-ATPase activity by cholesterol in endogenous lipid vesicles: role of lipid domains

Diverse experimental and theoretical evidence suggests that plasma membranes contain cholesterol-induced segregated domains that could play a key role in the modulation of membrane functions, including intrinsic enzyme activity. To gain insight into the role of cholesterol, we reconstituted pig kidney Na+/K+-ATPase into unilamellar vesicles of endogenous lipids mimicking the natural membrane and addressed the question of how modification of the cholesterol content could affect the ATPase activity via changes in the membrane lipid phase and in the protein structure and dynamics. We used steady-state and time-resolved fluorescence spectroscopy with the lipid phase probes DPH and Laurdan and the protein probe fluorescein and also used infrared spectroscopy using attenuated total reflectance. Upon modification of membrane cholesterol content, the ATPase activity did not change monotonically but instead exhibited abrupt changes resulting in two peaks at or close to critical cholesterol mole fractions (25 and 33.3 mol %) predicted by the superlattice or regular distribution model. Fluorescence parameters associated with the membrane probes also showed abrupt changes with peaks, coincident with the cholesterol concentrations associated with the peaks in the enzyme activity, while parameters associated with the protein probes also showed slight but abrupt changes resulting in dips at the same cholesterol concentrations. Notably, the IR amide I band maximum also showed spectral shifts, characterized by a frequency variation pattern with peaks at the same cholesterol concentrations. Overall, these results indicate that the lipid phase had slightly lower hydration, at or near the two critical cholesterol concentrations predicted by the superlattice theory. However, in the protein domains monitored there was a slight but significant hydration increase along with increased peptide backbone flexibility at these cholesterol concentrations. We propose that in the vicinity of the critical mole fractions, where superlattice formation can occur, minute changes in cholesterol concentration produce abrupt changes in the membrane organization, increasing interdomain surfaces. These changes, in turn, induce small changes in the protein's structure and dynamics, therefore acting to fine-tune the enzyme.

Alloxan-induced diabetes reduces sarcolemmal Na+-K+ pump function in rabbit ventricular myocytes

The effect of diabetes on sarcolemmal Na(+)-K(+) pump function is important for our understanding of heart disease associated with diabetes and design of its treatment. We induced diabetes characterized by hyperglycemia but no other major metabolic disturbances in rabbits. Ventricular myocytes isolated from diabetic rabbits and controls were voltage clamped and internally perfused with the whole cell patch-clamp technique. Electrogenic Na(+)-K(+) pump current (I(p), arising from the 3:2 Na(+)-to-K(+) exchange ratio) was identified as the shift in holding current induced by Na(+)-K(+) pump blockade with 100 micromol/l ouabain in most experiments. There was no effect of diabetes on I(p) recorded when myocytes were perfused with pipette solutions containing 80 mmol/l Na(+) to nearly saturate intracellular Na(+)-K(+) pump sites. However, diabetes was associated with a significant decrease in I(p) measured when pipette solutions contained 10 mmol/l Na(+). The decrease was independent of membrane voltage but dependent on the intracellular concentration of K(+). There was no effect of diabetes on the sensitivity of I(p) to extracellular K(+). Pump inhibition was abolished by restoration of euglycemia or by in vivo angiotensin II receptor blockade with losartan. We conclude that diabetes induces sarcolemmal Na(+)-K(+) pump inhibition that can be reversed with pharmacological intervention.

Functional Significance of the Shark Na,K-ATPase N-Terminal Domain. Is the Structurally Variable N-Terminus Involved in Tissue-Specific Regulation by FXYD Proteins?

The proteolytic profile after mild controlled trypsin cleavage of shark rectal gland Na,K-ATPase was characterized and compared to that of pig kidney Na,K-ATPase, and conditions for achieving N-terminal cleavage of the alpha-subunit at the T(2) trypsin cleavage site were established. Using such conditions, the shark enzyme N-terminus was much more susceptible to proteolysis than the pig enzyme. Nevertheless, the maximum hydrolytic activity was almost unaffected for the shark enzyme, whereas it was significantly decreased for the pig kidney enzyme. The apparent ATP affinity was unchanged for shark but increased for pig enzyme after N-terminal truncation. The main common effect following N-terminal truncation of shark and pig Na,K-ATPase is a shift in the E(1)-E(2) conformational equilibrium toward E(1). The phosphorylation and the main rate-limiting E(2) --> E(1) step are both accelerated after N-terminal truncation of the shark enzyme, but decreased significantly in the pig kidney enzyme. Some of the kinetic differences, like the acceleration of the phosphorylation reaction, following N-terminal truncation of the two preparations may be due to the fact that under the conditions used for N-terminal truncation, the C-terminal domain of the FXYD regulatory protein of the shark enzyme, PLMS or FXYD10, was also cleaved, whereas the gamma or FXYD2 of the pig enzyme was not. In the shark enzyme, N-terminal truncation of the alpha-subunit abolished association of exogenous PLMS with the alpha-subunit and the functional interactions were abrogated. Moreover, PKC phosphorylation of the preparation, which relieves PLMS inhibition of Na,K-ATPase activity, exposed the N-terminal trypsin cleavage site. It is suggested that PLMS interacts functionally with the N-terminus of the shark Na,K-ATPase to control the E(1)-E(2) conformational transition of the enzyme and that such interactions may be controlled by regulatory protein kinase phosphorylation of the N-terminus. Such interactions are likely in shark enzyme where PLMS has been demonstrated by cross-linking to associate with the Na,K-ATPase A-domain.

The nitric oxide donor sodium nitroprusside stimulates the Na+-K+ pump in isolated rabbit cardiac myocytes

Nitric oxide (NO) affects the membrane Na(+)-K(+) pump in a tissue-dependent manner. Stimulation of intrinsic pump activity, stimulation secondary to NO-induced Na(+) influx into cells or inhibition has been reported. We used the whole-cell patch clamp technique to measure electrogenic Na(+)-K(+) pump current (I(p)) in rabbit ventricular myocytes. Myocytes were voltage clamped with wide-tipped patch pipettes to achieve optimal perfusion of the intracellular compartment, and I(p) was identified as the shift in holding current induced by 100 microm ouabain. The NO donor sodium nitroprusside (SNP) in concentrations of 1, 10, 50 or 100 microm induced a significant increase in I(p) when the intracellular compartment was perfused with pipette solutions containing 10 mm Na(+), a concentration near physiological levels. SNP had no effect when the pump was near-maximally activated by 80 mm Na(+) in pipette solutions. Stimulation persisted in the absence of extracellular Na(+), indicating its independence of transmembrane Na(+) influx. The SNP-induced pump stimulation was abolished by inhibition of soluble guanylyl cyclase (sGC) with 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, by inhibition of protein kinase G (PKG) with KT-5823 or by inhibition of protein phosphatase with okadaic acid. Inclusion of the non-hydrolysable cGMP analogue 8pCPT-cGMP, activated recombinant PKG or the sGC-activator YC-1 in patch pipette filling solutions reproduced the SNP-induced pump stimulation. Pump stimulation induced by YC-1 was dependent on the Na(+) concentration but not the K(+) concentration in pipette filling solutions, suggesting an altered sensitivity of the Na(+)-K(+) pump to intracellular Na(+).

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

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.

Modulation of Na,K-ATPase by phospholipids and cholesterol. II. Steady-state and presteady-state kinetics

The effects of phospholipid acyl chain length (n(c)) and cholesterol on several partial reactions of Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. This regards the E(1)/E(2) equilibrium, the phosphoenzyme level, and the K(+)-deocclusion reaction. In addition, the lipid effects on some steady-state properties were investigated. Finally, the effects of cholesterol on the temperature sensitivity of the phosphorylation and spontaneous dephosphorylation reactions were investigated. The fatty acid and cholesterol composition of the native Na,K-ATPase membrane preparation showed a remarkable similarity to the lipid composition known to support maximum hydrolytic capacity as determined from in vitro experiments. The main rate-determining step of the Na,K-ATPase reaction, the E(2) --> E(1) reaction, as well as several other partial reactions were accelerated by cholesterol. This regards the phosphorylation by ATP as well as the E(1) - P --> E(2)-P reaction. Moreover, cholesterol shifted the E(1)/E(2) equilibrium toward the E(1) conformation and increased the K(+)-deocclusion rate. Finally, cholesterol significantly affected the temperature sensitivity of the spontaneous dephosphorylation reaction and the phosphorylation by ATP. The effects of cholesterol were not completely equivalent to those induced by increasing the phospholipid acyl chain length, indicating that the cholesterol effects are not entirely caused by increasing the hydrophobic bilayer thickness, which indicates an additional mechanism of action on the Na,K-ATPase.

Themes in ion pump regulation

The Na,K-ATPase is an ion pump present in the plasma membrane. It is of vital importance for cell and body homeostasis and as such is under strict hormonal control. The molecular basis for the acute regulation of Na,K-ATPase is multisite phosphorylation by protein kinases that can alter its behavior. This includes direct effects on the Na,K-ATPase activity, regulation by membrane trafficking, and even dynamic regulation of interaction with regulatory proteins. In shark Na,K-ATPase, the latter includes functional interaction with a small hydrophobic protein of the FXYD protein family, phospholemman-like protein from shark, PLMS. This article summarizes our recent work on the mechanisms involved in the acute regulation of the Na,K-ATPase studied using a plasma membrane preparation from shark salt glands as an epithelial transport model.