Purification and characterization of (Na+, K+)-ATPase. V. Conformational changes in the enzyme Transitions between the Na-form and the K-form studied with tryptic digestion as a tool

A 19-kDa C-terminal tryptic fragment of the alpha chain of Na/K-ATPase is essential for occlusion and transport of cations

Tryptic digestion of pig renal Na/K-ATPase in the presence of Rb and absence of Ca ions removes about half of the protein but leaves a stable 19-kDa membrane-embedded fragment derived from the alpha chain, a largely intact beta chain, and essentially normal Rb- and Na-occlusion capacity. Subsequent digestion with trypsin in the presence of Ca or absence of Rb ions leads to rapid loss of the 19-kDa fragment and a parallel loss of Rb occlusion, demonstrating that the fragment is essential for occlusion. The N-terminal sequence of the 19-kDa fragment is Asn-Pro-Lys-Thr-Asp-Lys-Leu-Val-Asn-Glu-Arg-Leu-Ile-Ser-Met-Ala, beginning at residue 830 and extending toward the C terminus. Membranes containing the 19-kDa fragment have the following functional properties. (i) ATP-dependent functions are absent. (ii) The apparent affinity for occluding Rb is unchanged, the affinity for Na is lower than in the control enzyme, and activation is now strongly sigmoidal rather than hyperbolic. (iii) Membranes containing the 19-kDa fragment can be reconstituted into phospholipid vesicles and sustain slow Rb-Rb exchange. Thus the transport pathway is retained. We conclude that cation occlusion sites and the transport pathway within transmembrane segments are quite separate from the ATP binding site, located on the cytoplasmic domain of the alpha chain. Interactions between cation and ATP sites, the heart of active transport, must be indirect--mediated, presumably, by conformational changes of the protein.

Mechanism of Na+/K(+)-ATPase activation by trypsin and kallikrein

The mechanism of the Na+/K(+)-ATPase activation by trypsin (from bovine pancreas) and kallikrein (from human plasma) was investigated on enzyme preparations from different sources (beef heart and dog kidney) and at different degrees of purification (beef heart). Kallikrein was effective on both beef and dog enzymes, whereas trypsin stimulated only the beef-heart Na+/K(+)-ATPase. The extent of activation by the proteinases was inversely related to the degree of purification (maximal enzyme activation about 60 and 20% on the partially purified and the more purified enzymes, respectively). Enzyme activation was observed up to 0.5-0.6 microgram/ml of proteinase. At higher concentrations the activation decreased and was converted into inhibition at proteinase concentrations above 1.0 micrograms/ml. Na+/K(+)-ATPase stimulation was due to an increase in the Vmax of the enzyme reaction. Km for ATP remained unaffected. The activating effect was favoured by sodium and counteracted by potassium. Accordingly, Na(+)-ATPase activity was stimulated to a greater extent (up to 350%), whereas K(+)-dependent p-nitrophenylphosphatase activity proved to be insensitive to the actions of the proteinases. The Na+/K(+)-ATPase stimulation by both proteinases was antagonized by either ouabain or canrenone, two drugs that bind on the extracellular side of the Na+/K(+)-ATPase molecule. On the contrary, the enzyme inactivation observed at high proteinase concentrations was not counteracted by these two drugs. The stimulation of either Na+/K(+)- or Na(+)-ATPase activity was shown to be an irreversible effect without any significant protein degradation detectable by SDS gel electrophoresis. The results obtained suggest that proteinases exert their stimulatory effects by interacting preferentially with the E2 conformation of Na+/K(+)-ATPase at site(s) located on the extracellular moiety of the enzyme.

Differential effects of substrates on three transport modes of the Na+/K(+)-ATPase

With a purified Na+/K(+)-ATPase preparation reconstituted into phospholipid vesicles, Na+/K+, Na+/Na+, and uncoupled Na+ transport were studied using three nucleotides and five substrates of the K(+)-phosphatase reaction that this enzyme also catalyzes. For Na+/K+ exchange, CTP was half as effective as ATP and GTP one-twentieth; of the phosphatase substrates only carbamyl phosphate and 3-O-methylfluorescein phosphate produced significant transport and at merely 1% of the rate with ATP. For Na+/Na+ exchange, comparable rates of transport were produced by ATP, CTP, carbamyl phosphate and acetyl phosphate, although the actual rate of transport with ATP was only 2.4% of that for Na+/K+ exchange; slower rates occurred with GTP (69%), 3-O-methylfluorescein phosphate (51%), and nitrophenyl phosphate (33%). Only umbelliferone phosphate was ineffective. For uncoupled Na+ transport results similar to those for Na+/Na+ exchange were obtained, but the actual rate of transport was still slower, 1.4% of that for Na+/K+ exchange. Thus, not only nucleotides but a variety of phosphatase substrates (which are phosphoric acid mixed anhydrides) can phosphorylate the enzyme at the high-affinity substrate site to form the E1P intermediate of the reaction sequence. Oligomycin inhibited Na+/K+ exchange with ATP by half, but with carbamyl phosphate not at all; with CTP the inhibition was intermediate, one-fourth. By contrast, oligomycin inhibited Na+/Na+ exchange by one-fifth with all three substrates. A quantitative, steady-state kinetic model accounts for the relative magnitudes of Na+/K+ and Na+/Na+ exchanges with ATP, CTP, and carbamyl phosphate as substrates, as well as the extents of inhibition by oligomycin. The model requires that even when Na+ substitutes for K+ a slow step in the reaction sequence is the E2 to E1 conformational transition.

Chemical modification as an approach to elucidation of sodium pump structure-function relations

Chemical modification of specific residues in enzymes, with the characterization of the type of inhibition and properties of the modified activity, is an established approach in structure-function studies of proteins. This strategy has become more productive in recent years with the advances made in obtaining primary sequence information from gene-cloning technologies. This article discusses the application of chemical modification procedures to the study of the Na(+)-K(+)-ATPase protein. A wide array of information has become available about the kinetics, enzyme structure, and various conformational states as a result of the combined use of inhibitors, ligands, modifiers, and proteolytic enzymes. We will review a variety of reagents and approaches that have been employed to arrive at structure-function correlates and discuss critically the limits and ambiguities in the type of information obtained from these methodologies. Chemical modification of the Na(+)-pump protein has already provided a body of data and will, we anticipate, guide the efforts of mutagenesis studies in the future when suitable expression systems become available.

Minimum enzyme unit for Na+/K+-ATPase is the alpha beta-protomer. Determination by low-angle laser light scattering photometry coupled with high-performance gel chromatography for substantially simultaneous measurement of ATPase activity and molecular weight

The oligomeric state of canine renal NA+/K+ -ATPase solubilized by octaethylene glycol n-dodecyl ether (C12E8) was studied by means of low-angle laser light scattering photometry coupled with high-performance gel chromatography (HPGC). At around 0 degree C the solubilized enzyme was separated into the (alpha beta)2-diprotomeric and alpha beta-protomeric protein components with Mr values of 302,000 +/- 10,000 and 156,000 +/- 4,000, respectively, in approximately equal quantities. As the temperature of chromatography was increased toward 20 degrees C, the two protein components converged into a single major component. The Mr of this component depended on the monovalent cation included in the elution buffer, and was 255,000 or 300,000 in the presence of 0.1 M NaCl or 0.1 M KCl, respectively. A computer simulation technique showed that the solubilized enzyme was in a dissociation-association equilibrium of 2 protomers = diprotomer at 20 degrees C, and the difference in apparent Mr of the solubilized enzyme between the two species of monovalent cation was interpreted by an association constant (Ka) in the presence of 0.1 M KCl that was about 50-fold larger than in the presence of 0.1 M NaCl. In order to measure ATPase activity and Mr of the solubilized enzyme simultaneously, a TSKgel G3000SW column had been equilibrated and was eluted with an elution buffer containing 0.30 mg/ml C12E8 and 60 microgram/ml phosphatidylserine (bovine brain) as well as the ligands necessary for the enzyme to exhibit the activity at pH 7.0 and 20 degrees C. The solubilized enzyme was always eluted as a single protein component irrespective of the the amount of the protein applied to the column, ranging between 240 and 10 microgram. The Mr of the protein component, however, decreased from 214,000 and 158,000 with the decrease of the protein amount. The specific ATPase activity, however, remained constant at a level of 64 +/- 4% of that of the membrane-bound enzyme even in the range of protein concentration sufficiently low as to allow the enzyme to exist only in the protomeric form. Thus, the alpha beta-protomer is concluded to be the minimum functional unit for the ATPase activity. The value of Ka obtained from the concentration-dependent dissociation curve was 5 . 10(5) M-1 for the enzyme turning over, and 1.1 . 10(7) M-1 for the enzyme inhibited with ouabain. It was discussed, based on the values of Ka obtained, that the enzyme would exist as the diprotomer or the higher oligomer in the membrane.

Solvent effects on substrate and phosphate interactions with the (Na+ + K+)-ATPase

(Na+ + K+)-ATPase activity of a dog kidney enzyme preparation was markedly inhibited by 10-30% (v/v) dimethyl sulfoxide (Me2SO) and ethylene glycol (Et(OH)2); moreover, Me2SO produced a pattern of uncompetitive inhibition toward ATP. However, K+-nitrophenylphosphatase activity was stimulated by 10-20% Me2SO and Et(OH)2 but was inhibited by 30-50%. Me2SO decreased the Km for this substrate but had little effect on the Vmax below 30% (at which concentration Vmax was then reduced). Me2SO also reduced the Ki for Pi and acetyl phosphate as competitors toward nitrophenyl phosphate but increased the Ki for ATP, CTP and 2-O-methylfluorescein phosphate as competitors. Me2SO inhibited K+-acetylphosphatase activity, although it also reduced the Km for that substrate. Finally, Me2SO increased the rate of enzyme inactivation by fluoride and beryllium. These observations are interpreted in terms of the E1P to E2P transition of the reaction sequence being associated with an increased hydrophobicity of the active site, and of Me2SO mimicking such effects by decreasing water activity: (i) primarily to stabilize the covalent E2P intermediate, through differential solvation of reactants and products, and thereby inhibiting the (Na+ + K+)-ATPase reaction and acting as a dead-end inhibitor to produce the pattern of uncompetitive inhibition; inhibiting the K+-acetylphosphatase reaction that also passes through an E2P intermediate; but not inhibiting (at lower Me2SO concentrations) the K+-nitrophenylphosphatase reaction that does not pass through such an intermediate; and (ii) secondarily to favor partitioning of Pi and non-nucleotide phosphates into the hydrophobic active site, thereby decreasing the Km for nitrophenyl phosphate and acetyl phosphate, the Ki for Pi and acetyl phosphate in the K+-nitrophenylphosphatase reaction, accelerating inactivation by fluoride and beryllium acting as phosphate analogs, and, at higher concentrations, inhibiting the K+-nitrophenylphosphatase reaction by stabilizing the non-covalent E2.P intermediate of that reaction. In addition, Me2SO may decrease binding at the adenine pocket of the low-affinity substrate site, represented as an increased Ki for ATP, CTP and 3-O-methylfluorescein phosphate.

Motility, heat, and lactate production in ejaculated bovine sperm

Effects of various inhibitors on motility, heat, and lactate production of ejaculated bovine sperm were determined in the presence of antimycin A and rotenone. erythro-9-[3-(2-Hydroxynonyl)]adenine (EHNA) and polyvinylpyrrolidone (PVP-360) stopped motility and reduced heat or lactate production by 30-50%. Carbodiimides resulted in loss of motility and a reduction of metabolism by 60-75%. Quercetin treatment, which enhanced rather than inhibited motility, depressed heat and lactate production by 50-60%. Since mechanical immobilization reduced heat production by only 30%, the question arises as to what other cellular processes are major contributors to the energy budget. Inhibitors of ion flux had little-to-no effect on heat or lactate production, suggesting that neither mitochondrial nor Na+/K+ ATPases were major ATP-requiring processes. Calcium flux at the plasma membrane also was minimal and previous reports eliminated glycolytic substrate cycling as major consuming processes for ATP. Although quercetin inhibited lactate production in intact cells, no effect of quercetin on cell-free glycolysis and the ATPase activities of isolated dynein was detected. Quercetin did, however, inhibit ATPase activity of plasma membrane, suggesting that this unidentified ATPase may contribute to the formation of ADP and Pi required for lactate production by the intact cell. We propose (a) that the bioenergetic costs of motility are divided between regulatory events and dynein-microtubule interaction (dynein ATPase), (b) that some of the membrane-related processes may be "inefficient," and (c) that quercetin may render these steps more "efficient," in a manner analogous to its action on the Na+/K+ pump of Ehrlich ascites tumor cells.

Binding of monovalent cations induces large changes in the secondary structure of Na+,K+-ATPase as probed by Raman spectroscopy

Raman spectra of active Na+,K+-ATPase from pig kidney in media containing Na+ (E1), K+ (E2) or without exogenous ions (E1 conformation) were recorded in order to calculate the changes in the enzyme's secondary structure induced by binding of monovalent cations. It is demonstrated that: (i) K+ binding to the E1 form of the enzyme leads to conversion of approximately 100 peptide groups from the beta-structure to alpha-helical conformation; (ii) the transition is reversible and fully reproducible in the E1----E2----E1 and E2----E1----E2 experimental schemes. Predictional calculations revealed polypeptide chain segments involved in the alpha----beta transformations. These segments reside mainly in the two highly conserved regions of the alpha-subunit in the cytoplasmic domain of Na+,K+-ATPase. A possible role for the beta-subunit is discussed.

Role of the Na,K-ATPase beta-subunit in the cellular accumulation and maturation of the enzyme as assessed by glycosylation inhibitors

No functional role could yet be established for the glycosylated beta-subunit of the Na,K-ATPase. In this study, we describe the intracellular processing of the beta-subunit as a glycoprotein in toad bladder cells and the consequences of its structural perturbation with glycosylation inhibitors on the cellular expression of the alpha- and beta-subunits and on the structural and functional maturation of the enzyme. Controlled trypsinolysis of homogenates from pulse-labeled cells reveals that the beta-subunit is subjected to glycosylation-dependent structural rearrangements during its intracellular routing. Inhibition of correct terminal glycosylation of the beta-subunit with deoxynojirimycin or swainsonine has no effect on the trypsin sensitivity of the alpha-subunit, its ability to perform cation-dependent conformation changes or the cellular Na,K-ATPase activity. Acquisition of core-sugars is sufficient for the enzyme to assume its catalytic functions. On the other hand, complete inhibition of glycosylation with tunicamycin leads to a destabilization of both the beta- and the alpha-subunits as judged by their higher trypsin sensitivity. In addition, tunicamycin treatment results in a decrease of the amount of newly synthesized beta- and alpha-subunit indicating that a glycoprotein, possibly the beta-subunit itself, plays a role in the efficient accumulation of the alpha-subunit in the endoplasmic reticulum.

Conformational states of (K+ + H+)-ATPase studied using tryptic digestion as a tool

The (K+ + H+)-ATPase from gastric mucosa has been treated by limited proteolytic digestion with trypsin to study the conformational states of the enzyme. The existence of a K+- and an ATP-form of the enzyme follows from the kinetics of inactivation and from the specific cleavage products. In the presence of K+ the 95 kDa chain is cleaved into two fragments of 56 and 42 kDa, whereas in the presence of ATP fragments of 67 and 35 kDa are formed. When Mg2+ is present during tryptic digestion cleavage products which are specific for both the ATP- and the K+-form of the enzyme are yielded. In analogy to ATP, Mg2+ is able to convert the enzyme from a K+-conformation to a more protected form. Moreover Mg2+ supports the protecting effect of ATP against tryptic inactivation. The K0.5 for ATP is lowered from 1.6 mM (no Mg2+) to 0.2 mM in the presence of 10 mM Mg2+. Mg2+, which in previous studies has been shown to induce a specific conformation, apparently induces a conformation different from the K+-form of the enzyme and has ATP-like effects on the enzyme. In addition it has been found that in the initial rapid phase of the digestion process the K+-ATPase activity is interrupted at a step which is very likely the interconversion of the phosphoenzyme forms E1P and E2P, since neither the K+-stimulated p-nitrophenylphosphatase activity nor the phosphorylation of the enzyme are inhibited in this phase. During the tryptic digestion in the presence of K+ there is a good correlation between the residual ATPase activity and the amount of the catalytic subunit left, suggesting that the latter is homogeneous. After tryptic digestion in the presence of K+, phosphorylation only occurs in the 42 kDa and not in the 56 kDa band. The same experiments in the presence of ATP yield only phosphorylation in the 67 kDa band and not in the 35 kDa band. A provisional model for the structure of the catalytic subunit is given.

Maturation of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport

The protease sensitivity of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport along the exocytic pathway has been investigated in two amphibian epithelial cell lines. Controlled trypsinolysis followed by immunoprecipitation of cell homogenates or microsomal fractions from [35S]methionine pulse-chased A6 kidney cells revealed distinct cleavage patterns by SDS-PAGE. Shortly after synthesis (7-min pulse), the 98-kD alpha-subunit is fully sensitive to trypsin digestion and is cleaved into a 35-kD membrane-bound and a 27.5-kD soluble peptide. With a 15-min pulse, 10% of the newly synthesized polypeptide becomes resistant to trypsin digestion. With longer chase time, the proportion of protease-resistant alpha-subunit further increases. Concomitantly, the alpha-subunit acquires the ability to undergo cation-dependent conformational transitions, as reflected by distinct tryptic digest patterns in the presence of Na+ or K+. Similar results were obtained in TBM cells, a toad bladder cell line. Our data indicate that the catalytic subunit of Na,K-ATPase is structurally rearranged during intracellular transport from its site of synthesis to its site of action at the cell surface, a modification which might mark the functional maturation of the enzyme.

Structural organization of alpha-subunit from purified and microsomal toad kidney (Na+ + K+)-ATPase as assessed by controlled trypsinolysis

The membrane organization of the alpha-subunit of purified (Na+ + K+)-ATPase ((Na+ + K+)-dependent adenosine triphosphate phosphorylase, EC 3.6.1.3) and of the microsomal enzyme of the kidney of the toad Bufo marinus was compared by using controlled trypsinolysis. With both enzyme preparations, digestions performed in the presence of Na+ yielded a 73 kDa fragment and in the presence of K+ a 56 kDa, a 40 kDa and small amounts of a 83 kDa fragment from the 96 kDa alpha-subunit. In contrast to mammalian preparations (Jørgensen, P.L. (1975) Biochim. Biophys. Acta 401, 399-415), trypsinolysis of the purified amphibian enzyme led to a biphasic loss of (Na+ + K+)-ATPase activity in the presence of both Na+ and K+. These data could be correlated with an early rapid cleavage of 3 kDa from the alpha-subunit in both ionic conditions and a slower degradation of the remaining 93 kDa polypeptide. On the other hand, in the microsomal enzyme, a 3 kDa shift of the alpha-subunit could only be produced in the presence of Na+. Our data indicate that (1) purification of the amphibian enzyme with detergent does not influence the overall topology of the alpha-subunit but produces a distinct structural alteration of its N-terminus and (2) the amphibian kidney enzyme responds to cations with similar conformational transitions as the mammalian kidney enzyme. In addition, anti alpha-serum used on digested enzyme samples revealed on immunoblots that the 40 kDa fragment was better recognized than the 56 kDa fragment. It is concluded that the NH2-terminal of the alpha-subunit contains more antigenic sites than the COOH-terminal domain in agreement with the results of Farley et al. (Farley, R.A., Ochoa, G.T. and Kudrow, A. (1986) Am. J. Physiol. 250, C896-C906).

Rb+ occlusion in renal (Na+ + K+)-ATPase characterized with a simple manual assay

This paper describes properties of a simple manual assay for Rb+ occlusion on renal (Na+ + K+)-ATPase. Rb+ occlusion is measured by applying the enzyme plus Rb+ (86Rb) mixture to a Dowex-50 cation exchange column at 0 degree C, and eluting the enzyme with occluded Rb+ using an ice-cold sucrose solution. The enzyme-Rb+ complex is quite stable at 0 degree C. This method is useful for measuring Rb+ occlusion under equilibrium binding conditions and slow rates of dissociation of the enzyme-Rb+ complex. The stoichiometry of Rb+ occluded per phosphorylation site is 2. Rb+ saturation curves are strictly hyperbolic, suggesting that the two Rb+ sites have very different affinities, one in the micromolar range and one in the tens of millimolar range. ATP shifts the Rb+ saturation curves to the right (control K0.5 100-200 microM; plus ATP, K0.5 0.8-1.4 mM, in a 100 mM Tris-HCl medium, pH 7.0) and reduces the maximal level occluded (control approx. 4 nmol/mg; plus ATP approx. 3 nmol/mg protein). Thus, as expected, ATP shifts the E(1)2Rb+-E2(2Rb+)occ equilibrium towards E1. Sodium ions at concentrations of up to 30 mM compete with the rubidium ions, KNa = 1.86 mM in the Tris-HCl medium. Na+ at higher concentrations (30-100 mM) has an added non-competitive antagonistic effect. At room temperature, Rb+ dissociates slowly from the enzyme, kobs = 0.08 s-1, in the presence of either Rb+ (20 mM) or Na, (100 mM). As expected, dissociation is greatly accelerated by ATP, the rate being to fast to be measured by this technique. (Na+ + K+)-ATPase proteolyzed selectively by chymotrypsin in a Na+ medium, occludes Rb+. For control and proteolyzed (Na+ + K+)-ATPase the Rb+ saturation curves are similar and the rates of dissociation of the enzyme-Rb+ complex are identical. The chymotryptic split appears to disrupt antagonistic interactions between cation and ATP binding domains, while the E1-E2 conformational transition of the unphosphorylated protein probably remains.

Sodium ions protect a membrane transport protein from proteolysis

Na+-dependent alanine transport activity in vesicles prepared from pigeon erythrocyte membranes was examined after exposure of the vesicles to some proteinases under various conditions. The presence of sodium ions during proteolysis affords considerable protection of alanine transport activity from the inhibitory action of the proteinases. The concentration of sodium ions required for half-maximum protection is greater than that needed for half-maximum activation of alanine uptake. The site of protective action could be at either or both surfaces of the membrane because the vesicles are very permeable to sodium ions. Neither measurement of residual protein content nor analysis by polyacrylamide gel electrophoresis revealed any differences in the extent of protein degradation occurring in the presence and absence of sodium ions, suggesting that the transporter constitutes only a minor membrane component. We conclude that sodium ions probably induce a conformation change in the transporter.

Reconstitution in phospholipid vesicles of calcium-activated potassium channel from outer renal medulla

A barium-sensitive Ca-activated K+ channel in the luminal membrane of the tubule cells in thick ascending limb of Henle's loop is required for maintenance of the lumen positive transepithelial potential and may be important for regulation of NaCl reabsorption. In this paper we examine if the K+ channel can be solubilized and reconstituted into phospholipid vesicles with preservation of its native properties. The K+ channel in luminal plasma membrane vesicles can be quantitatively solubilized in CHAPS at a detergent/protein ratio of 3. For reconstitution, detergent is removed by passage over a column of Sephadex G 50 (coarse). K+-channel activity is assayed by measurement of 86Rb+ uptake against a large opposing K+ gradient. The reconstituted K+ channel is activated by Ca2+ in the physiological range of concentration (K1/2 approximately 2 X 10(-7) M at pH 7.2) as found for the K+ channel in native plasma membrane vesicles and shows the same sensitivity to inhibitors (Ba2+, trifluoperazine, calmidazolium, quinidine) and to protons. Reconstitution of the K+ channel into phospholipid vesicles with full preservation of its native properties is an essential step towards isolation and purification of the K+-channel protein. Titration with Ca2+ shows that most of the active K+ channels in reconstituted vesicles have their cytoplasmic aspect facing outward in contrast to the orientation in plasma membrane vesicles, which requires also addition of Ca2+ ionophore in order to observe Ca2+ stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotide specificity of the E2K----E1K transition in (Na+ + K+)-ATPase as probed with tryptic inactivation and fragmentation

The nucleotide specificity for the E2K----E1K conformational transition in (Na+ + K+)-ATPase as the key step for overall hydrolytic activity and coupled cation transport has been investigated. Use has been made of tryptic inactivation, which is biexponential in time for the enzyme in the presence of Na+ with or without nucleotides (E1 conformation) and monoexponential in the presence of K+ (E2 conformation). ATP, AdoPP[NH]P and CTP in order of decreasing effectivity induce the biphasic tryptic inactivation pattern in the presence of K+. Their order of effectivity is inversely related to the rate constant of the second (slow) phase of inactivation. In the presence of K+ and either ITP or GTP tryptic inactivation remains monoexponential, indicating that these nucleotides cannot drive the E2K----E1K transition. Tryptic inactivation has been compared with tryptic fragmentation of the alpha-subunit (apparent mol. wt. 94 kDa) of (Na+ + K+)-ATPase. In the E1 conformation (Na+ present) a 71 kDa fragment is formed during the second phase of inactivation. In the E2 conformation (K+ present) the alpha-subunit is split to fragments of 41 and 52 kDa. In the presence of K+ and ATP, ADP, AdoPP[NH]P or CTP the 71 kDa fragment is formed in amounts which decrease in the order ATP approximately equal to ADP greater than AdoPP[NH]P greater than CTP. In the presence of K+ and AMP, ITP or GTP the 71 kDa fragment is absent and only the E2 fragments are formed. From these and literature data we arrive at a specificity order for the E2K----E1K transition of ATP greater than ADP greater than AdoPP[NH]P greater than CTP greater than ITP = GTP = AMP. The same order holds for K+ transport in the K+-K+ exchange and for overall hydrolytic activity (Na+ + K+ present) with the natural nucleoside triphosphates as substrates. This marks the E2K----E1K transition as the step in the reaction mechanism that determines nucleotide specificity for (Na+ + K+)-activated hydrolysis and coupled cation transport.

Tryptic and chymotryptic cleavage sites in sequence of alpha-subunit of (Na+ + K+)-ATPase from outer medulla of mammalian kidney

Localization of selective proteolytic splits in alpha-subunit of (Na+ + K+)-ATPase is important for understanding the mechanism of active Na+,K+-transport. Proteolytic fragments of alpha-subunit from pig kidney were purified by chromatography in NaDodSO4 on TSK 3000 SW columns. NH2-terminal amino acid sequences of fragments as determined in a gas phase sequenator were unambiguously located within the total sequence of alpha-subunit from sheep kidney (Shull, C.E., et al. (1985) Nature 316, 691-695) and pig kidney (Ovchinnikov, Y.A., et al. (1985) Proc. Acad. Sci. USSR 285, 1490-1495). The primary chymotryptic split in the E1-form is located between Leu-266 and Ala-267 while the tryptic cleavage site appears to be between Arg-262 and Ile-263 (Bond 3). Tryptic cleavage in the initial fast phase of inactivation of the E1-form is located between Lys-30 and Glu-31 (Bond 2). In the E2-form, primary tryptic cleavage is between Arg-438 and Ala-439 (Bond 1). Chymotryptic cleavage between Leu-266 and Ala-267 stabilizes the E1-form of the protein without affecting the sites for binding of cations or nucleotides. Titration of fluorescence responses demonstrates the importance of the NH2-terminal for E1-E2 transition. Protonation of His-13 facilitates transition from E1- to E2-forms of the protein. Removal of His-13 after cleavage of bond 2 can explain the increase in apparent affinity of the cleaved enzyme for Na+ and the shift in poise of E1-E2 equilibrium in direction of E1-forms. The NH2-terminal sequence in renal alpha-subunit is not conserved in alpha + from rat neurolemma or in alpha-subunit from Torpedo or brine shrimp. A regulatory function of the NH2-terminal part of the alpha-subunit may thus be a unique feature of the alpha-subunit in (Na+ + K+)-ATPase from mammalian kidney.

Circular dichroism of the two major conformational states of mammalian (Na+ + K+)-ATPase

No alteration in the circular dichroic spectrum of fully active, membrane-bound (Na+ + K+)-ATPase is observed when the protein is cycled between the two major conformational states, E1 and E2. This finding is in agreement with the infrared study by Chetverin and Brazhnikov (J. Biol. Chem. 260 (1985) 7817) and demonstrates that any difference in secondary structure between the two conformers must be less than 2%.

Location of major antibody binding domains on alpha-subunit of dog kidney Na+-K+-ATPase

The locations of binding sites on the alpha-subunit of dog kidney Na+-K+-ATPase for both monoclonal antibodies and antibodies from polyclonal antisera have been determined. Three distinct regions of the alpha-subunit, all located within the amino terminal half of the polypeptide, were recognized by the antibodies: a region near the amino terminus of the polypeptide and two regions that are separated by a site for trypsin cleavage of the ATPase in KCl. No significant binding of antibodies to the carboxy terminal region of the alpha-subunit was detected. The binding sites for the antibodies are located within regions of the polypeptide predicted to be exposed within the cytoplasm of the cell (P. L. Jorgensen, S. J. D. Karlish, and C. Gitler, J. Biol. Chem. 257: 7435-7442, 1982). This prediction was verified by the demonstration that the antibodies did not react with Na+-K+-ATPase in tight right-side-out vesicles but would bind to the protein after the vesicles had been disrupted with detergent. A model for the folding of the alpha-subunit through the membrane, based on these data, is presented.

Tryptic modification of red-cell sodium pump behaviour

Inside-out membrane vesicles derived from human red cells were used to probe the effects of controlled tryptic digestion on the sodium pump as it exists in situ. Digestion of the enzyme in its E1 conformation resulted in several alterations which are generally similar to those reported for the purified kidney enzyme, namely (i) greater loss in overall hydrolytic activity compared to level of phosphoenzyme intermediate and (ii) cleavage of the alpha-subunit by trypsin as well as chymotrypsin at the cytoplasmic surface to yield a fragment of approx. 78 kDa. Tryptic digestion effected similar rates of inactivation of pump-mediated Na+-K+(Rb+) exchange, (ATP- plus ADP)-dependent Na+-Na+ exchange and, in the absence extracellular alkali cation, 'uncoupled' Na+ flux (Na+/0 flux). Alteration in the Na+:Rb+(K+) stoichiometry following trypsin cleavage could not be detected. The conformational transitions of phosphoenzyme and dephosphoenzyme are affected similarly by trypsin, as evidenced by similar inactivation rates of reactions through the 'forward' sequence involving the E1P to E2P transition as well as through the 'reserve' sequence involving the E1 to E2 transition.

Effects of ATP and monovalent cations on Mg2+ inhibition of (Na,K)-ATPase

The hydrolysis of ATP catalyzed by purified (Na,K)-ATPase from pig kidney was more sensitive to Mg2+ inhibition when measured in the presence of saturating Na+ and K+ concentrations [(Na,K)-ATPase] than in the presence of Na+ alone, either at saturating [(Na,Na)-ATPase] or limiting [(Na,0)-ATPase] Na+ concentrations. This was observed at two extreme concentrations of ATP (3 mM where the low-affinity site is involved and 3 microM where only the catalytic site is relevant), although Mg2+ inhibition was higher at low ATP concentration. In the case of (Na,Na)-ATPase activity, inhibition was barely observed even at 10 mM free Mg2+ when ATP was 3 mM. When (Na,K)-ATPase activity was measured at different fixed K+ concentrations the apparent Ki for Mg2+ inhibition was lower at higher monovalent cation concentration. When K+ was replaced by its congeners (Rb+, NH+4, Li+), Mg2+ inhibition was more pronounced in those cases in which the dephosphorylating cation forms a tighter enzyme-cation complex after dephosphorylation. This effect was independent of the ATP concentration, although inhibition was more marked at lower ATP for all the dephosphorylating cations. The K0.5 for ATP activation at its low-affinity site, when measured in the presence of different dephosphorylating cations, increased following the sequence Rb+ greater than K+ greater than NH+4 greater than Li+ greater than none. The K0.5 values were lower with 0.05 mM than with 10 mM free Mg2+ but the order was not modified. The trypsin inactivation pattern of (Na,K)-ATPase indicated that Mg2+ kept the enzyme in an E1 state. Addition of K+ changed the inactivation into that observed with the E2 enzyme form. On the other hand, K+ kept the enzyme in an E2 state and addition of Mg2+ changed it to an E1 form. The K0.5 for KCl-induced E1-to-E2 transformation (observed by trypsin inactivation profile) in the presence of 3 mM MgCl2 was about 0.9 mM. These results concur with two mechanisms for free Mg2+ inhibition of (Na,K)-ATPase: "product" and dead-end. The first would result from Mg2+ interaction with the enzyme in the E2(K) occluded state whereas the second would be brought about by a Mg2+-enzyme complex with the enzyme in an E1 state.

Chymotryptic cleavage of alpha-subunit in E1-forms of renal (Na+ + K+)-ATPase: effects on enzymatic properties, ligand binding and cation exchange

Chymotrypsin in NaCl medium at low ionic strength rapidly cleaves a bond in the N-terminal half of the alpha-subunit of pure membrane-bound (Na+ + K+)-ATPase from outer renal medulla. Secondary cleavage is very slow and the alpha-subunit can be converted almost quantitatively to a 78 kDa fragment. The sensitive bond is exposed to cleavage when the protein is stabilized in the E1 form by binding of Na+ or nucleotides. The bond is protected in medium containing KCl (E2K form), but it is exposed when ADP or ATP are added (E1KATP form). Fluorescence analysis and examination of ligand binding and enzymatic properties of the cleaved protein demonstrate that cleavage of the bond stabilizes the protein in the E1 form with sites for tight binding of nucleotides and cations exposed to the medium. About two 86Rb ions are bound per cleaved alpha-subunit with normal affinity (Kd = 9 microM). The bound Rb+ is not displaced by ATP or ADP. The nucleotide-potassium antagonism is abolished and ATP is bound with high affinity both in NaCl and in KCl media. Na+-dependent phosphorylation is quantitatively recovered in the 78 kDa fragment, but the affinity for binding of [48V]vanadate is very low after cleavage. ADP-ATP exchange is stimulated 4-5-fold by cleavage; while nucleotide dependent Na+-Na+, K+-K+, or Na+-K+ exchange are abolished. Cleavage with chymotrypsin in NaCl at the N-terminal side of the phosphorylated residue thus stabilizes the E1 form of the protein and abolishes cation exchange and conformational transitions in the protein although binding of cations, nucleotides and phosphate is preserved. In contrast, cleavage with trypsin in KCl at the C-terminal side of the phosphorylated residue does not interfere with E1-E2 transitions and Na+-Na+ or K+-K+ exchange. This data support the notion that cation exchange and E1-E2 transitions are thightly coupled.

Phosphatase activity of (Na+ + K+)-ATPase. Ligand interactions and related enzyme forms

The prevailing conformations of partially purified pig kidney (Na+ + K+)-ATPase interacting with ligands related to its phosphatase activity were determined following time-dependent trypsin digestion and inactivation as well as the amounts of Rb+ or Ca2+ bound to the enzyme after passage through cation-exchange resin columns. In the presence of 150 mM choline chloride, alone or with 3 mM MgCl2, 3 mM MnCl2 or 1 mM CaCl2, the major enzyme conformation was E1. Similar forms were seen with 5 mM p-nitrophenyl phosphate with and without 3 mM MgCl2. KCl, at 0.5 mM or 150 mM, produced an E2 enzyme state; the effects of 0.5 mM KCl were completely counteracted by 5 mM p-nitrophenyl phosphate. Under optimal conditions for phosphatase activity (3 mM MgCL2/5 mM p-nitrophenyl phosphate/10 mM KCl) the (Na+ + K+)-ATPase was in the E2 state. At low ionic strength and 20 degrees C and under 85% of maximal RbCl-stimulated phosphatase turnover (1 mM RbCl/3 mM MgCl2/5 mM p-nitrophenyl phosphate) no Rb+ occlusion could be detected. Ca2+, at low ionic strength and in the presence of 3 mM MgCl2, stimulated an ouabain-sensitive phosphatase activity. The rates of hydrolysis obtained wit 1 mM CaCl2 were similar to those seen with 0.5 mM KCl; under both conditions, similar patterns of trypsin digestion and inactivation of the enzyme were obtained. On the other hand, Ca2+ could not mimic Rb+ in its ability to induce an E2-occluding state. These results suggest that during phosphatase activity of (Na+ + K+)-ATPase, the most abundant form is a non-occluding E2 and that at least one of the mechanisms of potassium stimulation of that activity it to take the enzyme into the E2 state.

Multiple ion-dependent and substrate-dependent Na+/K+-ATPase conformational states. Transient and steady-state kinetic studies

The hydrolysis of beta-(2-furyl)acryloyl phosphate (FAP), catalyzed by the Na+/K+-ATPase, is faster than the catalyzed hydrolysis of ATP. This is due to catalyzed hydrolysis of the pseudosubstrate by K+-dependent states of the enzyme, thus bypassing the Na+-dependent enzyme states that are required and are rate limiting in ATP hydrolysis. Unlike ATP, FAP is a positive effector of the E2 state. A study of FAP hydrolysis permits a detailed analysis of later steps in the overall ion translocation-ATP hydrolysis pathway. During the steady state of FAP hydrolysis in the presence of K+, substantial phosphoryl-enzyme is formed, as is indicated by the covalent incorporation of 32P from [32P]FAP. A comparison of the phosphoryl-enzyme yield with the rate of overall hydrolysis reveals that at 25 degrees C the phosphoryl-enzyme formed is all kinetically competent. Both the yield of phosphoryl-enzyme and the rate of overall hydrolysis of FAP are [K+] dependent. The transition E1 in equilibrium E2 is also [K+] dependent, but the rate of transition is differently affected by [K+] than are the above-mentioned two processes. Two distinct roles for K+ are indicated, as an effector of the E1-E2 equilibrium and as a "catalyst" in the hydrolysis of the E2-P. In contrast to the results at 25 degrees C, a virtually stoichiometric yield of phosphoryl-enzyme occurs at 0 degree C in the presence of Na+ and the absence of K+. At lower concentrations of K+ and in the presence of Na+, the hydrolysis of FAP at 0 degree C proceeds substantially through the E1-E2 pathway characteristic of ATP hydrolysis. The selectivity of FAP for the E2-K+-dependent pathway is due to the thermal inactivation of E1 at 25 degrees C in the absence of ATP or ATP analogues, even at high concentrations of Na+. These results emphasize the existence of multiple functional "E1" and "E2" states in the overall ATPase-ion translocation pathway.

Sodium-induced conformation changes in membrane transport proteins

In the presence of KCl, tryptic digestion of vesicles derived from pigeon erythrocyte membranes inactivates sodium-dependent uptake of alanine by the vesicles, whereas digestion in the presence of NaCl does not. Extensive degradation of vesicle proteins occurs under both conditions. Similarly, the extent of inhibition by N-ethylmaleimide of the sodium-dependent influxes of both glycine and alanine into human erythrocytes is greater when the cells are exposed to the thiol reagent in the presence of KCl than when NaCl is used. These observations are interpreted as providing evidence for sodium-induced conformation changes in these transport proteins.

K+-dependent 3-O-methylfluorescein phosphatase activity in crude homogenate of rodent heart ventricle: effect of K+ depletion and changes in thyroid status

The total Na+-K+-ATPase activity in rodent heart ventricle was quantified by determination of K+-dependent, ouabain suppressible 3-O-methylfluorescein phosphatase activity. A K+-dependent phosphatase activity of 0.80 mumol/min per g wet wt was obtained from crude homogenate of heart ventricle from 4-week-old guinea pigs. The anticipated association between K+-dependent phosphatase activity and Na+-K+-activated ATP hydrolysis could be clearly demonstrated in the crude homogenate. Based upon a molecular activity of 550/min the corresponding cardiac glycoside receptor concentration in the crude homogenate was 1450 pmol/g wet wt. In crude homogenate of heart ventricle from 3-month-old rats the K+-dependent phosphatase activity was 1.16 mumol/min per g wet wt. Following 4 weeks of K+ depletion of the rats, a decrease of 13% in total K+ content of the heart ventricle was seen. This was associated with a 14% decrease in K+-dependent phosphatase activity. The induction of hypo- and hyperthyroidism for 3 weeks in rats was followed by a decrease of 27% and an increase of 13% in K+-dependent phosphatase activity, respectively.

Direct demonstration of structural changes in soluble, monomeric Ca2+-ATPase associated with Ca2+ release during the transport cycle

The time courses of changes in protein conformation and Ca2+ binding in the phosphorylated state of membrane-bound and soluble monomeric Ca2+-ATPase from sarcoplasmic reticulum have been examined at pH 8.0, 2 degrees C. The transition from ADP-sensitive to ADP-insensitive phosphoenzyme occurs in the soluble monomer as well as in membranous Ca2+-ATPase and is accompanied by an increase in fluorescence from 2',3'-O-(2,4,6-trinitrocyclohexyldienylidine)-adenosine diphosphate bound to the catalytic site and change in tryptic cleavage pattern. A decrease of Ca2+ affinity occurs simultaneously with the fluorescence rise, suggesting a single-step mechanism for energy transfer between the catalytic site and the Ca2+ transport sites. This is in accordance with the tryptic degradation pattern that suggests proximity between the phosphorylation site and Ca2+ transport sites on the peptide. The structural changes occurring in the soluble monomeric Ca2+-ATPase show that a single polypeptide chain is the functional unit in energy transduction.

Conformational states of sarcoplasmic reticulum Ca2+-ATPase as studied by proteolytic cleavage

Conformational states in sarcoplasmic reticulum Ca2+-ATPase have been examined by tryptic and chymotryptic cleavage. High affinity Ca2+ binding (E1 state) exposes a peptide bond in the A fragment of the polypeptide chain to trypsin. Absence of Ca2+ (E2 state) exposes bonds in the B fragment, which are protected by binding of Mg2+ or ATP. After phosphorylation from ATP the tryptic cleavage pattern depends on the predominant phosphoenzyme species present. ADP-sensitive E1P and ADP-insensitive E2P have cleavage patterns identical to those of unphosphorylated E1 and E2, respectively, indicating that two major conformational states are involved in Ca2+ translocation. The transition from E1P to E2P is inhibited by secondary tryptic splits in the A fragment, suggesting that parts of this fragment are of particular importance for the energy transduction process. The tryptic cleavage patterns of phosphorylated forms of detergent solubilized monomeric Ca2+-ATPase were similar to those of the membrane-bound enzyme, indicating that Ca2+ translocation depends mainly on structural changes within a single peptide chain. On the other hand, the protection of the second cleavage site as observed after vanadate binding to membranous Ca2+-ATPase could not be achieved in the soluble monomeric enzyme. Shielding of this peptide bond may therefore be due to protein-protein interactions in the semicrystalline state of the vanadate-bound Ca2+-ATPase in membranous form.

Na+-like effect of imidazole on the phosphorylation of (Na+ + K+)-ATPase

A high basal level of phosphorylation (approx. 70% of the optimal Na+-dependent phosphorylation level) is observed in 50 mM imidazole-HCl (pH 7.0), in the absence of added Na+ and K+ and the presence of 10-100 microM Mg2+. In 50 mM Tris-HCl (pH 7.0) the basal level is only 5%, irrespective of the Mg2+ concentration. Nevertheless, imidazole is a less effective activator of phosphorylation than Na+ (Km imidazole-H+ 5.9 mM, Km Na+ 2 mM under comparable conditions). Imidazole-activated phosphorylation is strongly pH dependent, being optimal at pH less than or equal to 7 and minimal at pH greater than or equal to 8, while Na+-activated phosphorylation is optimal at pH 7.4. This suggests that imidazole-H+ is the activating species. Imidazole facilitates Na+-stimulated phosphorylation. The Km for Na+ decreases from 0.63 mM at 5 mM imidazole-HCl to 0.21 mM at 50 mM imidazole-HCl (pH 7; 0.1 mM Mg2+ in all cases). Imidazole-activated phosphorylation is more sensitive to inhibition by K+ (I50 = 12.5 microM) than Na+-activated phosphorylation (I50 = 180 microM). Mg2+ antagonizes activation by imidazole-H+ and also inhibition by K+. The Ki value for Mg2+ (approx. 0.3 mM) is the same for the two antagonistic effects. Tris buffer (pH 7.0) inhibits imidazole-activated phosphorylation with an I50 value of 30 mM in 50 mM imidazole-HCl (pH 7.0) plus 0.1 mM Mg2+. We conclude that imidazole-H+, but not Tris-H+, can replace Na+ as an activator of ATP-dependent phosphorylation, primarily by shifting the E2----E1 transition to the right, leading to a phosphorylating E1 conformation which is different from that in Tris buffer.

Secondary structure and assembly mechanism of an oligomeric channel protein

The alpha-toxin of Staphylococcus aureus is secreted as a water-soluble, monomeric polypeptide (Mr 33 182) that can assemble into an oligomeric membrane channel. By chemical cross-linking, we have confirmed that the major form of the channel is a hexamer. The circular dichroism spectrum of this hexamer in detergent revealed that it contains a high proportion of beta-sheet that we deduce must lie within the lipid bilayer when the protein is associated with membranes. The circular dichroism spectrum of the monomeric toxin in the presence or absence of detergent was closely similar to the spectrum of the hexamer, suggesting that the secondary structure of the polypeptide is little changed on assembly. Results of experiments involving limited proteolysis of the monomer and hexamer are consistent with the idea that assembly involves the movement of two rigid domains about a hinge located near the midpoint of the polypeptide chain. The hydrophilic monomer is thereby converted to an amphipathic rod that becomes a subunit of the hexamer.