Epitope mapping by amino-acid-sequence-specific antibodies reveals that both ends of the alpha subunit of Na+/K(+)-ATPase are located on the cytoplasmic side of the membrane

Identified and potential internalization signals involved in trafficking and regulation of Na+/K+ ATPase activity

The sodium-potassium pump (NKA) or Na+/K+ ATPase consumes around 30-40% of the total energy expenditure of the animal cell on the generation of the sodium and potassium electrochemical gradients that regulate various electrolyte and nutrient transport processes. The vital role of this protein entails proper spatial and temporal regulation of its activity through modulatory mechanisms involving its expression, localization, enzymatic activity, and protein-protein interactions. The residence of the NKA at the plasma membrane is compulsory for its action as an antiporter. Despite the huge body of literature reporting on its trafficking between the cell membrane and intracellular compartments, the mechanisms controlling the trafficking process are by far the least understood. Among the molecular determinants of the plasma membrane proteins trafficking are intrinsic sequence-based endocytic motifs. In this review, we (i) summarize previous reports linking the regulation of Na+/K+ ATPase trafficking and/or plasma membrane residence to its activity, with particular emphasis on the endocytic signals in the Na+/K+ ATPase alpha-subunit, (ii) map additional potential internalization signals within Na+/K+ ATPase catalytic alpha-subunit, based on canonical and noncanonical endocytic motifs reported in the literature, (iii) pinpoint known and potential phosphorylation sites associated with NKA trafficking, (iv) highlight our recent studies on Na+/K+ ATPase trafficking and PGE2-mediated Na+/K+ ATPase modulation in intestine, liver, and kidney cells.

The sodium pump. Its molecular properties and mechanics of ion transport

The sodium pump (Na(+)/K(+)-ATPase; sodium- and potassium-activated adenosine 5'-triphosphatase; EC 3.6.1.37) has been under investigation for more than four decades. During this time, the knowledge about the structure and properties of the enzyme has increased to such an extent that specialized groups have formed within this field that focus on specific aspects of the active ion transport catalyzed by this enzyme. Taking this into account, this review, while somewhat speculative, is an attempt to summarize the information regarding the enzymology of the sodium pump with the hope of providing to interested readers from outside the field a concentrated overview and to readers from related fields a guide in their search for gathering specific information concerning the structure, function, and enzymology of this enzyme.

A hybrid between Na+,K+-ATPase and H+,K+-ATPase is sensitive to palytoxin, ouabain, and SCH 28080

Na(+),K(+)-ATPase is inhibited by cardiac glycosides such as ouabain, and palytoxin, which do not inhibit gastric H(+),K(+)-ATPase. Gastric H(+),K(+)-ATPase is inhibited by SCH28080, which has no effect on Na(+),K(+)-ATPase. The goal of the current study was to identify amino acid sequences of the gastric proton-potassium pump that are involved in recognition of the pump-specific inhibitor SCH 28080. A chimeric polypeptide consisting of the rat sodium pump alpha3 subunit with the peptide Gln(905)-Val(930) of the gastric proton pump alpha subunit substituted in place of the original Asn(886)-Ala(911) sequence was expressed together with the gastric beta subunit in the yeast Saccharomyces cerevisiae. Yeast cells that express this subunit combination are sensitive to palytoxin, which interacts specifically with the sodium pump, and lose intracellular K(+) ions. The palytoxin-induced K(+) efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proton pump-specific inhibitor SCH 28080. The IC(50) for SCH 28080 inhibition of palytoxin-induced K(+) efflux is 14.3 +/- 2.4 microm, which is similar to the K(i) for SCH 28080 inhibition of ATP hydrolysis by the gastric H(+),K(+)-ATPase. In contrast, palytoxin-induced K(+) efflux from cells expressing either the native alpha3 and beta1 subunits of the sodium pump or the alpha3 subunit of the sodium pump together with the beta subunit of the gastric proton pump is inhibited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensitivity by the chimera indicates that the Gln(905)-Val(930) peptide of the gastric proton pump is likely to be involved in the interactions of the gastric proton-potassium pump with SCH 28080.

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Phosphorylation of the alpha-subunits of the Na+/K+-ATPase from mammalian kidneys and Xenopus oocytes by cGMP-dependent protein kinase results in stimulation of ATPase activity

Phosphorylation of Na+/K+-ATPase by cGMP-dependent protein kinase (PKG) has been studied in enzymes purified from pig, dog, sheep and rat kidneys, and in Xenopus oocytes. PKG phosphorylates the alpha-subunits of all animal species investigated. Phosphorylation of the beta-subunit was not observed. The stoichiometry of phosphorylation estimated for pig, sheep and dog renal Na+/K+-ATPase is 3.5, 2.2 and 2.1 mol Pi per mol alpha-subunit, respectively. Proteolytic fingerprinting of the pig alpha1-subunits phosphorylated by PKG using specific antibodies raised against N-terminus or C-terminus reveals that phosphorylation sites are located within the intracellular loop of the alpha-subunit between the 35 kDa N-terminal and 27 kDa C-terminal fragments. Phosphorylation sites within the alpha1-subunit of the purified Na+/K+-ATPase do not appear to be easily accessible for PKG since incorporation of Pi requires 0.2% of Triton X-100. Administration of cGMP and PKG in the presence of 5 mm ATP, which prevents inactivation of the Na+/K+-ATPase by detergent, leads to stimulation of hydrolytic activity by 61%. Administration of 50 microm of cGMP or dbcGMP in yolk-free homogenates of Xenopus oocytes leads to stimulation of ouabain-dependent ATPase activity by 130-198% and to incorporation of 33P into the alpha-subunit without the detergent. Hence, PKG plays regulatory role in active transmembraneous transport of Na+ and K+ via phosphorylation of the catalytic subunit of the Na+/K+-ATPase.

A specific binding protein for cardiac glycosides exists in bovine serum

Searching for a binding protein in blood, which may be involved in the specific transport of cardiac glycosides to their receptor sites on the sodium pump, we isolated a cardiac glycoside-binding protein (CGBG) of 26 kDa from the globulin fraction of bovine serum by affinity chromatography and on a ouabain-Sepharose 4B column by a purification factor of 5000. The cardiac glycoside-binding globulin was labeled specifically and covalently by the protein-reactive digoxigenin derivative HDMA (N-hydroxysuccimidyldigoxigenin-3-O-methylcarbonyl-epsilon-+ ++aminocapro ate). Even very high concentrations of other steroids, such as estrogen, testosterone, progesterone, and cortisone, did not prevent HDMA-labeling (at 5 and 100 nM) of CGBG, but the cardenolides ouabain and digoxin or the bufadienolide proscillaridin A did so. CGBG is a homodimer of two 26-kDa subunits forming disulfide bonds, since HDMA labeling of a protein of 53 kDa was observed in SDS-polyacrylamide gel electrophoresis when beta-mercaptoethanol was absent during SDS denaturation. The N-terminal amino acid sequence K-D-V-Y-R-A-P-D-G-T-Q-S-A showed no sequence similarity with proteins recorded in gene and protein sequence data banks. A 90-kDa cytosolic CGBG exists in bovine kidneys and reacts with antibodies against CGBG. Binding of ouabain to the cardiac glycoside-binding globulin was monitored by quenching of intrinsic tryptophan fluorescence. Such studies reveal two negatively cooperative ouabain binding sites with Kd' of 1.52 nM and Kd' = 75 nM and with an interaction factor of 50 using a Koshland-Némethy-Filmer model. The demonstration of a cardiac glycoside-binding globulin in plasma is consistent with the recent finding of endogenous cardiac glycosides in mammals.

Isoform-specific interactions of Na,K-ATPase subunits are mediated via extracellular domains and carbohydrates

The functional unit of the Na,K-ATPase consists of a catalytic alpha subunit noncovalently linked with a glycoprotein subunit, beta. Using ouabain binding assays and immunoprecipitation of rodent alpha/beta complexes, we show here that all six possible isozymes between three alpha and two beta isoforms can be formed in Xenopus oocytes. Two isoform-specific differences in alpha/beta interactions are observed: (i) alpha1/beta1 and alpha2/beta2 complexes, in contrast to alpha1/beta2 complexes, are stable against Triton X-100-mediated dissociation, and (ii) beta2 subunits must carry N-glycans to combine with alpha1 but not with alpha2. The interacting surfaces are mainly exposed to the extracellular side because coexpression of a truncated beta1 subunit comprising the ectodomain results in assembly with alpha1 and alpha2, but not with alpha3; the beta2 ectodomain combines with alpha2 only. A chimera consisting of 81% and 19% of the alpha1 N terminus and alpha2 C terminus, respectively, behaves like alpha2 and coprecipitates with the beta2 ectodomain. In contrast, the reciprocal chimera does not coprecipitate with the beta2 ectodomain. These results provide evidence for a selective interaction of Na,K-ATPase alpha and beta subunits.

Transmembrane topology of alpha- and beta-subunits of Na+,K+-ATPase derived from beta-galactosidase fusion proteins expressed in yeast

Various models of the transmembrane topology of the Na+,K+-ATPase predict either 8 or 10 membrane spans for the alpha-subunit and one to three membrane spans for the beta-subunit. Structure/function analysis, however, requires precise knowledge about the folding of enzymes. Therefore, the intention of this work was to establish a transmembrane topology model for the subunits of Na+,K+-ATPase. The bacterial enzyme beta-galactosidase was fused to the C termini of truncated alpha- and beta-subunits of Na+,K+-ATPase. Fusions were generated at Arg60 (LTTAR60), Glu116 (AATEE116), Ala247 (VEGTA247), Leu311 (YTWEL311), Ala444 (VAGDA444), Ala789 (IFIIA789), Met809 (LGTDM809), Asp884 (RVTWD884), Ile946 (MKNKI946), and Arg972 (GVALR972) of the sheep alpha1-subunit and at Pro236 (LGGYP236) of the dog beta-subunit. The fusion constructs were expressed in yeast cells for studies on the localization of the fused reporter enzyme. Activity measurements of the reporter enzyme revealed that only intracellular fusion sites lead to active beta-galactosidase. Indirect immunofluorescence microscopy with cells expressing alpha1/beta-galactosidase and beta/beta-galactosidase hybrid proteins demonstrated that inactive beta-galactosidase is associated with the yeast plasma membrane and can be detected from the extracellular side. The data obtained suggest that Pro236 of the beta-subunit is located on the extracellular surface, corresponding to a model with one transmembrane segment, and that the alpha-subunit of the Na+,K+-ATPase consists of 10 membrane-associated spans. They also suggest that a stretch of the alpha1-subunit between membrane spans M7 and M8 might be hidden within the membrane, surrounded by the other hydrophobic spans, in analogy to the P-loop of Na+ or K+ channels and to the "hourglass" structure of water channels.

Epitope topology of Na,K-ATPase alpha subunit analyzed in basolateral cell membranes of rat kidney tubules

For topological analysis of integral membrane protein in situ, we used a novel immunoelectron microscopic technique, SDS-digested freeze-fracture replica labeling (SDS-FRL), and oligopeptide-specific antibodies to clarify the sidedness of Na,K-ATPase alpha subunit epitopes in basolateral cell membranes of kidney tubules. Unfixed tissue slices from rat kidney outer medulla were frozen with liquid helium, freeze-fractured, and replicated. After digestion with SDS to solubilize unfractured membranes and cytoplasm, the platinum/carbon replicas, along with attached cytoplasmic and exoplasmic membrane halves, were processed for immunocytochemistry. Immunogold labeling using antibodies against the N-terminus (Gly1-His13), Leu815-Gln828 and the C-terminus (Ile1002-Tyr1006) was superimposed on the images of the electron microscope protoplasmic fracture face of the basolateral plasma membranes, thus demonstrating cytoplasmic locations of these epitopes. On the contrary, SDS-FRL showed specific binding of Asn889-Gln903 to cross-fractured basolateral plasma membranes suggesting that this epitope is located on the extracellular side of the membrane.

Topology of the Na,K-ATPase. Evidence for externalization of a labile transmembrane structure during heating

The topological organization of the Na,K-ATPase alpha subunit is controversial. Detection of extracellular proteolytic cleavage sites would help define the topology, and so attempts were made to find conditions and proteases that would permit digestion of Na,K-ATPase in sealed right-side-out vesicles from renal medulla. The beta subunit is predominantly extracellular and could mask the surface of the alpha subunit. Most of the tested proteases cleaved beta, and some digested it extensively. However, without further disruption of structure, there was still no digestion of the alpha subunit. Reduction (at 50 degrees C) of disulfide bonds that might stabilize the beta subunit fragments, or heating alone at 55 degrees C, permitted tryptic digestion of alpha at a site close to the C terminus, while simultaneously increasing digestion of beta. A 90-kDa N-terminal fragment of alpha was recovered, but the C-terminal fragment was further digested. Heating and reduction resulted in the extracellular exposure of a protein kinase A phosphorylation site, Ser-938, and the C terminus, both of which have been proposed to be located on the intracellular surface. At the same time, access to a distant protein kinase C phosphorylation site was not increased. The data suggest that the harsh treatment simultaneously resulted in alteration of the beta subunit and the extrusion of a segment of alpha that normally spans the membrane, without causing complete denaturation or opening the sealed vesicles. Preincubation with Rb+ was protective, consistent with prior evidence that it stabilizes the protein segments in the C-terminal third of alpha. We conclude that this portion of the alpha subunit contains a transmembrane structure with unique lability to heating.

Determination of the sidedness of the carboxy-terminus of the Na+/K(+)ATPase alpha-subunit using lactoperoxidase iodination

The orientation of the carboxy-terminal pair of tyrosines of the Na+/K(+)-ATPase alpha-subunit with respect to the plane of the plasma membrane was determined. The approach was based on lactoperoxidase-catalysed radioiodination of the tyrosine residues accessible on the surface of the enzyme molecule in intact cells of a pig kidney embryonic cell line and those accessible in a broken plasma membrane fraction and in isolated membrane-bound Na+/K(+)-ATPase. The labeled alpha-subunit was isolated by SDS gel electrophoresis followed by electroblotting. Then the COOH-terminal amino acids were hydrolyzed by carboxypeptidases B and Y. Radioactivity and quantitative analysis of the protein and released amino acids showed that the COOH-terminal tyrosine residues of the alpha-subunit were only accessible to modification only when lactoperoxidase had access to the inner side of the plasma membrane. Therefore, the COOH-terminus of the Na+/K(+)-ATPase alpha-subunit is located on the cytoplasmic surface of the pump molecule and its polypeptide chain must have an even number of transmembrane segments.

Affinity labeling of a sulfhydryl group in the cardiacglycoside receptor site of Na+/K(+)-ATPase by N-hydroxysuccinimidyl derivatives of digoxigenin

Na+/K(+)-ATPase from pig kidney is inactivated by protein-reactive N-hydroxysuccinimidyl derivatives of digoxigenin. Like digoxigenin, its protein-reactive derivatives N-hydroxysuccinimidyl digoxigenin-3-methylcarbonyl-epsilon-aminocaproate (HDMA), 3-amino-3-deoxydigoxigenin hemisuccinimide succinimidyl ester (ADHS), 3-iodoacetylamino-3-deoxydigoxigenin (IAD) and digoxigenin-3-O-succinyl-[2-(N-maleimido)]ethylamide (DSME) inhibited the sodium pump in the presence of Na+, Mg2+ and ATP. At 37 degrees C, half-maximal inhibition of Na+/K(+)-ATPase was seen by HDMA at 0.47 microM, by ADHS at 5.8 microM, by IAD at 8 microM and by DSME at 94 microM. Thus, all compounds bind to the cardiac steroid receptor site of Na+/K(+)-ATPase. Affinity labeling of the alpha subunit by 'front door' or 'back door' phosphorylation was only seen with HDMA or ADHS in the range 0.1 microM. Excess of ouabain protected against affinity labeling. All the other protein-reactive derivatives of digoxigenin labeled the enzyme independent of the formation of a phosphointermediate at much higher concentrations. This labeling was not suppressed by an excess of ouabain. Tryptic hydrolysis of the HDMA-modified Na+/K(+)-ATPase gave peptides of the apparent molecular masses 20, 12.5 and 11.2 kDa. The 11.2-kDa and 12.5-kDa peptides started amino-terminally with Asp68, and the 20-kDa peptide with Asp24. Thus, the HDMA-labeled peptides originate from the cardioactive steroid-binding site formed by the first and second transmembrane helix. N-Hydroxysuccinimidyl esters such as HDMA are normally thought to modify lysine and arginine residues covalently. Since such residues do not exist in the putative cardiac glycoside-binding site, the possibility of a thioester formation of the digoxigenin derivatives HDMA and ADHS with Cys104 in the H1 transmembrane domain was tested. In fact, hydroxylaminolysis led to the release of the covalently bound HDMA, and the formation of a free sulfhydryl group. This could be labeled by [2-14C]ICH2COOH. We therefore propose, consistent with a recent conclusion from a site-directed mutagenesis experiment [Canessa, C. M., Horisberger, J.-D., Louvard, D. & Rossier, B. C. (1992) EMBO J. 11, 1681-1687], that a cysteine residue (probably Cys104) participates in the structure and function of the cardiac glycoside binding.

Structure, transmembrane topology and helix packing of P-type ion pumps

Electron microscopy has recently provided improved structures for P-type ion pumps. In the case of Ca(2+)-ATPase, the use of unstained specimens revealed the structure of the transmembrane domain. The composition of this domain has been controversial due to the variety of methods used to study the number and exact locations of transmembrane crossings within the sequence. After reviewing the results from several members of the family, we found a consensus for 10 transmembrane segments, and also that 10 helices fitted well into the structure of Ca(2+)-ATPase. Thus, we present the most detailed model for transmembrane structure so far, in the hope of stimulating more precise experimental strategies.

C-terminal topology of gastric H+,K(+)-ATPase

An antibody was prepared against a peptide corresponding to residues 1024-1034 (the putative C-terminus) of the alpha-subunit of hog gastric H+,K(+)-ATPase. The antibody bound to a 95 kDa band of H+,K(+)-ATPase that was solubilized in SDS, but not to that of Na+,K(+)-ATPase. It also bound to products of tryptic digestion that included C-terminal fragments of the H+,K(+)-ATPase alpha-subunit. The same amount of the antibody bound to both intact (tight) and lyophilized (leaky) inside-out gastric vesicles, indicating that its epitope is present on the cytosolic side of the vesicles. This finding was further confirmed by using fluorescence-immunolocalization techniques and streptolysin-O to permeabilize newt oxyntic cells. Stimulation of isolated newt oxyntic cells with dibutyryl cyclic AMP induces fusion of tubulovesicles with the apical membrane, so that the luminal domains of the H+,K(+)-ATPase alpha-subunit directly face the cell-suspension medium. The antibody did not bind to the stimulated intact cell, but bound to cells permeabilized with streptolysin-O, indicating that it binds from the cytoplasmic side to the C-terminus of the H+,K(+)-ATPase alpha-subunit in apical and tubulovesicular membrane, and also that the H+,K(+)-ATPase alpha-subunit has an even number of transmembrane domains.

Topology of Na,K-ATPase alpha subunit epitopes analyzed with oligopeptide-specific antibodies and double-labeling immunoelectron microscopy

Using four oligopeptide-specific polyclonal antibodies, we mapped the alpha subunit of Na,K-ATPase by double-labeling immunoelectron microscopy combined with negative staining. The results show that the epitopes of the N-terminus (Gly1-His13), C-terminus (Ile1002-Tyr1016) and Leu815-Gln828 are located on the same face of crystallized Na,K-ATPase membranes from pig kidney, whereas the epitope Asn889-Gln903 is present on the opposite side. The present study demonstrates the cytoplasmic location of C-terminus and that Leu815-Gln828 is exposed on the cytoplasmic and Asn889-Gln903 on the extracellular side. The results are consistent with an eight- or ten-segment model, and support the existence of an M5/M6 loop and the presence of one transmembrane segment between Leu815-Gln828 and Asn889-Gln903.

Transmembrane organization of the Na,K-ATPase determined by epitope addition

The Na,K-ATPase is a membrane-associated enzyme that establishes the internal Na+/K+ environment of most animal cells. The catalytic (alpha) subunit of the Na,K-ATPase contains multiple transmembrane segments, but the number and location of these domains has not been clearly established. We have used epitope addition to determine the transmembrane topology of the alpha subunit. An immunoreactive peptide was inserted into various regions of the cDNA encoding the rat alpha 1 subunit, and the constructs were expressed in transfected mammalian cells. The intra- or extracellular location of the epitope tags was determined by immunofluorescence analysis. Our results indicate that the amino and carboxyl termini of the alpha subunit are situated intracellularly, and the polypeptide is likely to possess eight membrane-spanning segments. The systematic application of epitope tagging may be useful for analyzing the topology of membrane proteins of unknown structure.

Mutation of a conserved proline residue in the beta-subunit ectodomain prevents Na(+)-K(+)-ATPase oligomerization

A highly conserved sequence motif (4 tyrosines and 1 proline: YYPYY) of the Na(+)-K(+)-adenosinetriphosphatase (ATPase) beta 1-subunit ectodomain has been mutagenized to study its possible role in alpha/beta-assembly and sodium pump function. Single as well as double tyrosine mutants (tyrosine to phenylalanine: Y to F) of Xenopus laevis beta 1-subunits are able to associate with alpha 1-subunits and form functional Na-K pumps at the plasma membrane that are indistinguishable from wild-type alpha 1, beta 1-Na-K pumps (as assessed by measurements of ouabain binding, 86Rb flux, Na-K pump current, and activation by external potassium). In contrast, a single proline mutation (proline to glycine: P244G) reduced by > 90% the proper assembly and function of Na(+)-K(+)-ATPase, despite a normal rate of synthesis and core glycosylation. Our data indicate that proline-244 plays a critical role in the proper folding of the beta-subunit and its ability to associate efficiently with the alpha 1-subunit in the endoplasmic reticulum.

Antibody epitope mapping of the gastric H+/K(+)-ATPase

Several antibodies against the gastric H+/K(+)-ATPase were analysed for the topological and sequence location of their epitopes. Topological mapping was done by comparing indirect immunofluorescent staining in intact and permeabilised rat parietal cells. Epitope definition was by Western analysis of intact and of trypsin or V8-proteinase-fragmented hog gastric ATPase combined with N-terminal sequencing of the fragments; by Western analysis of fragments of rabbit alpha subunit expressed in Escherichia coli; by analysis of rabbit alpha and beta subunits expressed in baculovirus-transfected SF 9 cells and by ELISA assay of synthetic octamers of one region of the hog alpha subunit. It was confirmed that the monoclonal antibody, mAb 95-111, recognised a cytoplasmic region between M4 and M5, close to the ATP-binding domain. The major epitope for monoclonal antibody mAb 12-18 was also in this region, but a second epitope was confirmed to be present in the M7/M8 region. The monoclonal antibody, mAb 146-14, was shown to recognise an extracytoplasmic epitope dependent on intact disulfide bonds, present in the rat and the rabbit, but absent in the hog beta subunit, due to non-conservative amino-acid substitutions. This antibody also recognised an epitope present in the alpha subunit of the H+/K(+)-ATPase at the M7 extracytoplasmic interface, perhaps indicating structural association of these two regions. The polyclonal antibody, pAb39, raised against the C-terminal portion of the enzyme, reacted only with the cytoplasmic surface of the H+/K(+)-ATPase, showing that the alpha subunit of the enzyme has an even number of membrane spanning segments.

The epitope for the inhibitory antibody M7-PB-E9 contains Ser-646 and Asp-652 of the sheep Na+,K(+)-ATPase alpha-subunit

The binding of monoclonal antibody M7-PB-E9 to the alpha-subunit of Na+,K(+)-ATPase partially inhibits enzyme activity (35%) in competition with ATP, while in the presence of magnesium it stimulates the rate of ouabain binding severalfold [Ball, W. J. (1984) Biochemistry 23, 2275-2281]. These effects have been shown to result from an antibody-induced shifting of the enzyme's E1 <==> E2 conformational equilibrium to the right that affects all enzyme-ligand interactions except that with Mg2+ [Abbott, A.J., & Ball, W.J. (1992) Biochemistry 31, 11236-11243]. In order to identify the location of the M7-PB-E9 epitope, proteolytic fragments of the lamb kidney enzyme were generated and the immunoreactive alpha fragments were identified by Western blot analyses. These studies revealed a 47-kDa tryptic fragment, which bound both M7-PB-E9 and a -COOH terminus specific antisera and NH2-terminal sequencing showed to originate at Ala-590. Digestion with Staphylococcus aureus V8 protease produced a 36-kDa -COOH-terminus fragment which originated at Gly-697 and did not contain the antibody epitope. Thus the intracellular sequence region Ala-590 to Gly-697 was shown to contain the antibody epitope. When M7-PB-E9's ability to recognize the alpha subunits from various species and tissues was determined and correlated with available sequencing data, only Ser-646 was present in the highly reactive lamb, pig, and avian kidney alpha 1 proteins and altered (Asn) in the poorly recognized Xenopus and rat kidney and Torpedo electroplax organ enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of thiol-topography in Na,K-ATPase using labelling with different maleimide nitroxide derivatives

Spin-label EPR spectroscopy of shark rectal gland Na,K-ATPase modified at cysteine residues with a variety of maleimide-nitroxide derivatives is used to characterize the different classes of sulphydryl groups. The spin-labelled derivatives vary with respect to charge and lipophilicity, and the chemical reactivity towards modification and inactivation of the Na,K-ATPase is dependent on these properties. Ascorbate is used to reduce the spin-labels in situ, and the kinetics of reduction of the protein-bound spin-labels are found also to depend on the nature of the maleimide-nitroxide derivative. The Na,K-ATPase is labelled either at Class I groups (with retention of enzymatic activity) or at Class II groups (where the enzymatic activity is lost). Although Class I groups are labelled more readily than are Class II groups they are only slightly more susceptible to reduction by ascorbate than the Class II groups, indicating no major difference in environment. The spectral difference observed between immobilized and mobile spin-labels with both Class I and Class II groups labelling is not reflected in widely different reduction kinetics for these two spectral components. Solubilization of the enzyme in an active form does not change the protein structure in terms of increased accessibility of the SH-groups to reduction by ascorbate. The results are discussed in terms of the location of the different SH-groups and the origins of the differences in mobility evident in the EPR spectra of the spin-labelled SH-groups.

Determination of the sidedness of the C-terminal region of the gastric H,K-ATPase alpha subunit

It cannot be predicted from hydropathy analysis whether the C-terminal end of the alpha subunit of the gastric H,K-ATPase is cytoplasmic or extracytoplasmic. The sideness of the C-terminal amino acids was determined by taking advantage of the two C-terminal tyrosines in the primary sequence of the enzyme. Intact, cytoplasmic side out vesicles derived from hog gastric mucosa or detergent solubilized vesicles were iodinated by the lactoperoxidase method and then the C-terminal amino acids hydrolyzed by carboxypeptidase Y. The alpha and beta subunits were separated by SDS gel electrophoresis. The level of iodination of the alpha subunit following solubilization was about three fold greater than when intact vesicles were iodinated, and the beta subunit was iodinated only when solubilized enzyme was used. Carboxypeptidase Y removed 28 +/- 4% of the radioactivity from the alpha subunit iodinated in intact vesicles. These data are consistent with a cytoplasmic location of the C-terminal amino acids of the alpha subunit and with a mostly extracytoplasmic location of the amino acids of the beta subunit.

Constraints on models for the folding of the Na,K-ATPase

We have attempted to bring together in graphic fashion the available evidence on the structure of the Na,K-ATPase and the H,K-ATPase. There appears to be much room for modification of the existing models for transmembrane folding. More sites on each side of the membrane need to be identified. Whether these will be antibody epitopes, sites of covalent modification, or tags inserted by mutagenesis is less important than that there be many of them and that each be verified by alternative approaches. If any single principle has emerged from the study of the topography of membrane proteins, it is that it is easy to reach conclusions too soon.

Definition of surface-exposed and trans-membranous regions of the (Ca(2+)-Mg2+)-ATPase of sarcoplasmic reticulum using anti-peptide antibodies

Peptides have been synthesized representing parts of the transduction, phosphorylation, nucleotide-binding and hinge domains of the (Ca(2+)-Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR), and corresponding to segments of all of the postulated short inter-membranous loops of the (Ca(2+)-Mg2+)-ATPase (residues 77-88, 277-287, 780-791, 808-818, 915-924 and 949-958). A number of antibodies raised to these peptides have been shown to bind to the ATPase, defining surface-exposed regions. Many of these are concentrated in the phosphorylation and nucleotide-binding domains, suggesting that these domains could be exposed on the top surface of the ATPase. The cytoplasmic location of the loop containing residues 808-818 was confirmed by the finding that proteinase K treatment of intact SR vesicles enhanced the binding of antibodies against this segment. These findings support the 10-alpha-helix model of the ATPase. These results also suggest that only inter-membranous loops larger than about 20 residues are likely to be detected by immunological methods in transmembranous proteins. Binding of anti-peptide antibodies to proteolytic fragments of the ATPase has been used to define the domain structure of the enzyme. Some of the anti-peptide antibodies have been characterized by studying their binding to sets of hexameric peptides synthesized on plastic pegs. A wide pattern of responses is observed, with a restricted range of epitopes being recognized by each anti-peptide antibody.

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

The Na,K-ATPase

The energy dependent exchange of cytoplasmic Na+ for extracellular K+ in mammalian cells is due to a membrane bound enzyme system, the Na,K-ATPase. The exchange sustains a gradient for Na+ into and for K+ out of the cell, and this is used as an energy source for creation of the membrane potential, for its de- and repolarisation, for regulation of cytoplasmic ionic composition and for transepithelial transport. The Na,K-ATPase consists of two membrane spanning polypeptides, an alpha-subunit of 112-kD and a beta-subunit, which is a glycoprotein of 35-kD. The catalytic properties are associated with the alpha-subunit, which has the binding domain for ATP and the cations. In the review, attention will be given to the biochemical characterization of the reaction mechanism underlying the coupling between hydrolysis of the substate ATP and transport of Na+ and K+.

Three-dimensional structure of Na,K-ATPase determined from membrane crystals induced by cobalt-tetrammine-ATP

The three-dimensional structure of Na,K-ATPase has been analyzed with electron microscopy and image processing. The enzyme, purified from pig kidney outer medulla, was arranged in a new form of tetragonal two-dimensional membrane crystals after incubation with cobalt-tetrammine-ATP, a stable MgATP complex analogue. Each continuous protein domain, as delineated by negative stain, consists of two alpha beta-protomers related by a dyad axis. The two rod-like regions are connected by a bridge displaced about 20 A away from the center of the structure toward the lipid bilayer. The domain connecting the two promoters is more constricted and closer to the center of the structure in the Co(NH3)4ATP-induced crystals than in the vanadate-induced p21 crystals. These observations suggest that the difference between previously analyzed dimers of two-dimensional p21 crystals induced with vanadate/magnesium and dimers of p4 crystals induced with Co(NH3)4ATP reflects two different conformational states of the enzyme.