Incorporation of C12E8-solubilized Na+,K+-ATPase into liposomes: determination of sidedness and orientation

General and specific lipid-protein interactions in Na,K-ATPase

The molecular activity of Na,K-ATPase and other P2 ATPases like Ca(2+)-ATPase is influenced by the lipid environment via both general (physical) and specific (chemical) interactions. Whereas the general effects of bilayer structure on membrane protein function are fairly well described and understood, the importance of the specific interactions has only been realized within the last decade due particularly to the growing field of membrane protein crystallization, which has shed new light on the molecular details of specific lipid-protein interactions. It is a remarkable observation that specific lipid-protein interactions seem to be evolutionarily conserved, and conformations of specifically bound lipids at the lipid-protein surface within the membrane are similar in crystal structures determined with different techniques and sources of the protein, despite the rather weak lipid-protein interaction energy. Studies of purified detergent-soluble recombinant αβ or αβFXYD Na,K-ATPase complexes reveal three separate functional effects of phospholipids and cholesterol with characteristic structural selectivity. The observations suggest that these three effects are exerted at separate binding sites for phophatidylserine/cholesterol (stabilizing), polyunsaturated phosphatidylethanolamine (stimulatory), and saturated PC or sphingomyelin/cholesterol (inhibitory), which may be located within three lipid-binding pockets identified in recent crystal structures of Na,K-ATPase. The findings point to a central role of direct and specific interactions of different phospholipids and cholesterol in determining both stability and molecular activity of Na,K-ATPase and possible implications for physiological regulation by membrane lipid composition. This article is part of a special issue titled "Lipid-Protein Interactions."

Intrinsic reaction-cycle time scale of Na+,K+-ATPase manifests itself in the lipid-protein interactions of nonequilibrium membranes

Interaction between integral membrane proteins and the lipid-bilayer component of biological membranes is expected to mutually influence the proteins and the membrane. We present quantitative evidence of a manifestation of the lipid-protein interactions in liposomal membranes, reconstituted with actively pumping Na(+),K(+)-ATPase, in terms of nonequilibrium shape fluctuations that contain a relaxation time, τ, which is robust and independent of the specific fluctuation modes of the membrane. In the case of pumping Na(+)-ions, analysis of the flicker-noise temporal correlation spectrum of the liposomes leads to τ ~/= 0.5 s, comparing favorably with an intrinsic reaction-cycle time of about 0.4 s from enzymology.

Lipid bilayer composition affects transmembrane protein orientation and function

Sperm membranes change in structure and composition upon ejaculation to undergo capacitation, a molecular transformation which enables spermatozoa to undergo the acrosome reaction and be capable of fertilization. Changes to the membrane environment including lipid composition, specifically lipid microdomains, may be responsible for enabling capacitation. To study the effect of lipid environment on proteins, liposomes were created using lipids extracted from bull sperm membranes, with or without a protein (Na⁺ K⁺-ATPase or α-amylase). Protein incorporation, function, and orientation were determined. Fluorescence resonance energy transfer (FRET) confirmed protein inclusion in the lipid bilayer, and protein function was confirmed using a colourometric assay of phosphate production from ATP cleavage. In the native lipid liposomes, ATPase was oriented with the β subunit facing the outer leaflet, while changing the lipid composition to 50% native lipids and 50% exogenous lipids significantly altered this orientation of Na⁺ K⁺-ATPase within the membranes.

L-lysine uptake in giant vesicles from cardiac ventricular sarcolemma: two components of cationic amino acid transport

Cationic L-amino acids enter cardiac-muscle cells through carrier-mediated transport. To study this process in detail, L-[(14)C]lysine uptake experiments were conducted within a 10(3)-fold range of L-lysine concentrations in giant sarcolemmal vesicles prepared from rat cardiac ventricles. Vesicles had a surface-to-volume ratio comparable with that of an epithelial cell, thus representing a suitable system for initial uptake rate studies. Two Na(+)-independent, N-ethylmaleimide-sensitive uptake components were found, one with high apparent affinity (K(m)=222+/-71 microM) and low transport capacity (V(max)=121+/-36 pmol/min per mg of vesicle protein) and the other with low apparent affinity (K(m)=16+/-4 mM) and high capacity (V(max)=4.0+/-0.4 nmol/min per mg of vesicle protein). L-Lysine uptake mediated by both components was stimulated by the presence of intravesicular L-lysine as well as by valinomycin-induced membrane hyperpolarization. Altogether, this behaviour is consistent with the functional properties of the CAT-1 and CAT-2A members of the system y(+) family of cationic amino acid transporters. Furthermore, mRNA transcripts for these two carrier proteins were identified in freshly isolated rat cardiac myocytes, the amount of CAT-1 mRNA, relative to beta-actin, being 33-fold larger than that of CAT-2A. These two transporters appear to function simultaneously as a homoeostatic device that supplies cardiac-muscle cells with cationic amino acids under a variety of metabolic conditions. Analysis of two carriers acting in parallel with such an array of kinetic parameters shows significant activity of the low-affinity component even at amino acid plasma levels far below its K(m).

Integrative Technology for the Twenty-First Century

Presented is the concept of Integrative Technology, the intersection of the precision assembly of matter (nanotechnology), coupled with the functional building blocks of nature (biotechnology), and fused by the network flow of spatiotemporal information (informatics). The power of Integrative Technology is illuminated through an illustrative example; the engineering of nano-sized excitable vesicles with the ability to intrinsically process information. The fusion of the tools of nanotechnology and biotechnology to produce excitable vesicles is described, as is the mechanics of information flow that ultimately lead to the manifestations of emergent higher-order behavior. Finally, the potential of using systems engineered and produced from nanoscale components to create complex systems and materials that manifest embedded functional behavior is discussed.

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.

Direct activation of gastric H,K-ATPase by N-terminal protein kinase C phosphorylation. Comparison of the acute regulation mechanisms of H,K-ATPase and Na,K-ATPase

In this study we compared the protein kinase dependent regulation of gastric H,K-ATPase and Na,K-ATPase. The protein kinase A/protein kinase C (PKA/PKC) phosphorylation profile of H,K-ATPase was very similar to the one found in the Na,K-ATPase. PKC phosphorylation was taking place in the N-terminal part of the alpha-subunit with a stoichiometry of approximately 0.6 mol Pi/mole alpha-subunit. PKA phosphorylation was in the C-terminal part and required detergent, as is also found for the Na,K-ATPase. The stoichiometry of PKA-induced phosphorylation was approximately 0.7 mol Pi/mole alpha-subunit. Controlled proteolysis of the N-terminus abolished PKC phosphorylation of native H,K-ATPase. However, after detergent treatment additional C-terminal PKC sites became exposed located at the beginning of the M5M6 hairpin and at the cytoplasmic L89 loop close to the inner face of the plasma membrane. N-terminal PKC phosphorylation of native H,K-ATPase alpha-subunit was found to stimulate the maximal enzyme activity by 40-80% at saturating ATP, depending on pH. Thus, a direct modulation of enzyme activity by PKC phosphorylation could be demonstrated that may be additional to the well-known regulation of acid secretion by recruitment of H,K-ATPase to the apical membranes of the parietal cells. Moreover, a distinct difference in the regulation of H,K-ATPase and Na,K-ATPase is the apparent absence of any small regulatory proteins associated with the H,K-ATPase.

Modulation of Na,K-ATPase by associated small transmembrane regulatory proteins and by lipids

The effects of phospholipid acyl chain length (n(c)) and cholesterol on Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. The optimal hydrophobic thickness of the lipid bilayer decreases from n(c) = 22 to 18 in the presence of 40 mol% cholesterol. Hydrophobic matching as well as specific interactions of cholesterol with the phosphorylation/dephosphorylation reactions is found to be important. A novel regulatory protein has been identified in Na,K-ATPase membrane preparations from the shark (phospholemmanlike protein from shark, PLMS) with significant homology to phospholemman (PLM), the major protein kinase substrate in myocardium. Both are members of the FXYD gene family. Another member of this family is the Na,K-ATPase gamma subunit indicating that these proteins may be specific regulators of the Na,K-ATPase. A regulatory mechanism is described in which association/dissociation of PLMS with the Na,K-ATPase is governed by its phosphorylation by protein kinases.

Modulation of Na,K-ATPase and Na-ATPase activity by phospholipids and cholesterol. I. Steady-state kinetics

The effects of phospholipid acyl chain length (n(c)), degree of acyl chain saturation, and cholesterol on Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. The optimal acyl chain length of monounsaturated phosphatidylcholine in the absence of cholesterol was found to be 22 but decreased to 18 in the presence of 40 mol % cholesterol. This indicates that the hydrophobic matching of the lipid bilayer and the transmembrane hydrophobic core of the membrane protein is a crucial parameter in supporting optimal Na,K-ATPase activity. In addition, the increased bilayer order induced by both cholesterol and saturated phospholipids could be important for the conformational mobility of the Na,K-ATPase changing the distribution of conformations. Lipid fluidity was important for several parameters of reconstitution, e.g., the amount of protein inserted and the orientation in the liposomes. The temperature dependence of the Na,K-ATPase as well of the Na-ATPase reactions depends both on phospholipid acyl chain length and on cholesterol. Cholesterol increased significantly both the enthalpy of activation and entropy of activation for Na,K-ATPase activity and Na-ATPase activity of Na,K-ATPase reconstituted with monounsaturated phospholipids. In the presence of cholesterol the free energy of activation was minimum at a lipid acyl chain length of 18, the same that supported maximum turnover. In the case of ATPase reconstituted without cholesterol, the minimum free energy of activation and the maximum turnover both shifted to longer acyl chain lengths of about 22.

Studies on lipids and the activity of Na,K-ATPase in lens fibre cells

Na,K-ATPase was studied in the two cell types that make up the lens of the eye. Membrane material was isolated from lens fibre cells, which make up the bulk of the lens cell mass, and also from lens epithelial cells, which are present only as a monolayer on the anterior lens surface. Judged by immunoblotting, greater amounts of Na,K-ATPase alpha1 and beta1 polypeptides were found in fibre cell membrane material than in epithelial cell membrane material. However, the NA,K-ATPase activity in epithelial cell membrane material was 20 times that measured in fibre cell membrane material. In 86Rb uptake experiments with intact lenses, ouabain-inhibitable 86Rb uptake was observed for lens epithelium but not for lens fibres. These findings are consistent with a low Na,K-ATPase activity in lens fibre cells even though these cells express a considerable amount of Na,K-ATPase alpha1 and beta1 polypeptides. The lipid composition of lens fibre cell membranes causes them to be more ordered than epithelial cell membranes; this was confirmed by measurements of the infrared CH2 symmetric stretching band frequency. Because lipid composition can influence Na,K-ATPase activity, experiments were conducted to determine whether the activity of Na,K-ATPase alpha1 beta1 is inhibited by lens fibre lipid. However, no significant difference in Na,K-ATPase activity was detected when Na,K-ATPase alpha1 beta1 was purified from rabbit kidney and then reconstituted with lipid that had been isolated from either lens epithelium or lens fibre cells. These studies indicate that lens fibre cells contain both Na,K-ATPase alpha1 and beta1 polypeptides but have low Na,K-ATPase activity. However, the results do not support the notion that this is due to the lipid composition of lens fibre cell membranes.

Functional regulation of reconstituted Na,K-ATPase by protein kinase A phosphorylation

Reconstituted Na+,K+-ATPase from either pig kidney or shark rectal glands was phosphorylated by cAMP dependent protein kinase, PKA. The stoichiometry was approximately 0.9 mol P(i)/mol alpha-subunit in the pig kidney enzyme and approximately 0.2 mol P(i)/mol alpha-subunit in the shark enzyme. In shark, Na+,K+-ATPase PKA phosphorylation increased the maximum hydrolytic activity for cytoplasmic Na+ activation and extracellular K+ activation without affecting the apparent K(m) values. In contrast, no significant functional effect after PKA phosphorylation was observed in pig kidney Na+,K+-ATPase.

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

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

Cholesterol modulation of molecular activity of reconstituted shark Na+,K(+)-ATPase

The cholesterol content of liposome bilayers has been varied between 0-40 mol% to study the effects on reconstituted Na+,K(+)-ATPase. The maximum hydrolytic activity of reconstituted Na+,K(+)-ATPase was increased by cholesterol at concentrations above 10 mol% for both the physiological Na+/K(+)-exchange reactions, as well as for the partial reactions Na+/Na(+)-exchange and uncoupled Na+ efflux. Omission of cholesterol from the liposome bilayer modified the activation by cytoplasmic Na+, indicating effects on both Vmax and on the Na(+)-affinity. Several other kinetic parameters were found to be strongly influenced as well, most notable the steady-state phosphorylation level, and the characteristics of the phosphorylation/dephosphorylation reactions. These results indicate that cholesterol interacts directly with the Na+,K(+)-ATPase as an essential effector perhaps by affecting its conformational mobility or monomer interaction.

Investigation of reconstitution of the Na, K-ATPase in lipid vesicles

Vesicles containing Na,K-ATPase were prepared by a dialysis method in buffers with various concentrations of K+ and Na+ ions. Ion-exchange chromatography has been used to separate proteoliposomes into protein-depleted and protein-rich fractions. The pumping activity of reconstituted ion pumps has been determined in the different fractions of the vesicle preparation using voltage-dependent fluorescence dyes. This method allowed to characterise vesicle fractions by a quantity which is proportional to the average number of pumps per vesicle with an active (inside-out) orientation. It could be shown that both, the amount of enzymatic active protein and the orientation of Na,K-ATPase in the vesicle lipid bilayer, is partially controlled by the Na+ and K+ concentration in the buffer during vesicle formation. High Na+ concentrations preferentially maintain the E1 conformation of the enzyme, which is less stable against denaturation during the dialysis, but displays a higher percentage of inside-out orientation of the transport-active protein. High K+ concentrations maintain the E2 conformation of the enzyme, which is stable against denaturation during the dialysis, but leads to a random orientation of the pump during dialysis.

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.

A voltage-activated cation transport pathway associated with the sodium pump

In proteoliposomes containing reconstituted shark Na,K-ATPase, inside positive potentials open a cation conductance characterized by a voltage-dependence very similar to that found in mammalian erythrocytes. In both proteoliposomes and erythrocytes, the voltage-activated pathway is inhibited by external oligomycin, which traps the Na,K-ATPase in a Na-occluded E1 form. These results indicate that a cation permeable pathway, activated by inside positive potentials, can be ascribed to the Na-K pump--possibly through interaction with its gating mechanism.

The effect of cytoplasmic K+ on the activity of the Na+/K(+)-ATPase

Experiments with the reconstituted (Na+ + K+)-ATPase show that besides the ATP-dependent cytoplasmic Na(+)-K+ competition for Na+ activation there is a high affinity inhibitory effect of cytoplasmic K+. In contrast to the high affinity K+ inhibition seen with the unsided preparation at a low ATP especially at a low temperature, the high affinity inhibition by cytoplasmic K+ does not disappear when the ATP concentration an-or the temperature is increased. The high affinity inhibition by cytoplasmic K+ is also observed with Cs+, Li+ or K+ as the extracellular cation, but the fractional inhibition is much less pronounced than with Na+ as the extracellular cation. The results suggest that either there are two populations of enzyme, one with the normal ATP dependent cytoplasmic Na(+)-K+ competition, and another which due to the preparative procedure has lost this ATP sensitivity. Or that the normal enzyme has two pathways for the transition from E2-P to E1ATP. One on which the enzyme with the translocated ion binds cytoplasmic K+ with a high affinity but not ATP, and another on which ATP is bound but not K+. A kinetic model which can accommodate this is suggested.