Studies of the antigenic properties of the catalytic and glycoprotein subunits of Na+,K+-ATPase

Na(+)-K(+)-ATPase alpha(2)-isoform expression in guinea pig hearts during transition from compensation to decompensation

Disturbance in ionic gradient across sarcolemma may lead to arrhythmias. Because Na(+)-K(+)-ATPase regulates intracellular Na(+) and K(+) concentrations, and therefore intracellular Ca(2+) concentration homeostasis, our aim was to determine whether changes in the Na(+)-K(+)-ATPase alpha-isoforms in guinea pigs during transition from compensated (CLVH) to decompensated left ventricular hypertrophy (DLVH) were concomitant with arrhythmias. After 12- and 20-mo aortic stenosis, CLVH and DLVH were characterized by increased mean arterial pressure (30% and 52.7%, respectively). DLVH differed from CLVH by significantly increased end-diastolic pressure (34%), decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (-75%), and increased Na(+)/Ca(2+) exchanger (25%) mRNA levels and by the occurrence of ventricular arrhythmias. The alpha-isoform (mRNA and protein levels) was significantly lower in DLVH (2.2 +/- 0.2- and 1. 4 +/- 0.15-fold, respectively, vs. control) than in CLVH (3.5 +/- 0. 4- and 2.2 +/- 0.13-fold, respectively) and was present in sarcolemma and T tubules. Changes in the levels of alpha(1)- and alpha(3)-isoform in CLVH and DLVH appear physiologically irrelevant. We suggest that the increased level of alpha(2)-isoform in CLVH may participate in compensation, whereas its relative decrease in DLVH may enhance decompensation and arrhythmias.

Identification of antigenic sites on the Na+/K(+)-ATPase beta-subunit: their sequences and the effects of thiol reduction upon their structure

In contrast to the catalytic (alpha) subunit of the Na+/K(+)-ATPase holoenzyme, the glycoprotein (beta) subunit has proven to be a poor antigen for monoclonal antibody (Mab) production. However, in this work six Mabs directed against the beta-subunit of the lamb kidney holoenzyme have been isolated. These Mabs all recognize the holoenzyme, but their 'in solution' binding affinities for deglycosylated enzyme or isolated beta are generally at least 10-fold higher. Species specificity mapping, antibody patterns of binding to beta-fragments and competition binding studies indicated that there were only three distinct epitopes, with two antibodies binding in the NH2-terminal half (epitopes I and II) and 4 Mabs binding at the same or overlapping site (III) in the -COOH terminal half of beta. DNA sequence analysis of isolated collections of bacteriophage M13 that contain a 15 amino-acid 'epitope library' insert in the pIII protein, which enables them to bind to the antibodies, revealed the residues KYRDS (amino acids 111-115) and LETYP (amino acids 197-201) to be the deduced sequences for the epitopes of Mabs M19-P7-E5 (II) and M17-P5-F11 (III), respectively. The epitope I site was not, however, identified. Further studies showed that antibody binding to these three determinant sites had no affect on the Na+/K(+)-ATPase and K(+)-stimulated p-nitrophenylphosphatase (pNPPase) activities of either holoenzyme or deglycosylated enzyme, nor any affect on the cation- (Na+, K+ or Mg2+) and ouabain-induced conformational changes monitored with FITC-labeled deglycosylated enzyme. Interestingly, anti-beta Mab access to the three epitopes was increased following beta-mercaptoethanol inactivation of the holoenzyme, but this thiol reduction abolished the binding of two conformation-sensitive anti-alpha Mabs to the enzyme. These results are consistent with the previous suggestion of Kirley ((1990) J. Biol. Chem. 265, 4227-4232) that the beta-disulfide linkages not only maintain beta-structure but they are critical for maintaining alpha-conformation and holoenzyme activity.

Immunochemical studies of (Na+ + K+)-ATPase using site-specific, synthetic peptide directed antibodies

Rabbit antisera were raised against a series of synthetic peptides corresponding to regions of the alpha subunit of lamb kidney (Na+ + K+)-ATPase which chemical labeling studies and hydropathy plots of the amino-acid sequence suggest are exposed, accessible regions of the enzyme and may comprise the cation selectivity region, the ATP and cardiac glycoside binding sites, and the phosphorylation site. Five of six peptides tested (11-15 residues in length) were immunogenic and the antisera to four peptides recognized the intact, electroblotted (Western blot analysis) alpha subunit. Immunization with peptides conjugated to keyhole limpet haemocyanin (KLH) produced antipeptide antibodies for seven of nine conjugates. Antisera to four peptide conjugates recognized the native enzyme, confirming predictions that these sequence regions are exposed regions of the holoenzyme. In addition, a collection of four polyclonal antisera and five monoclonal antibodies raised to native holoenzyme were tested for their ability to bind to the peptide conjugates. In this way, two NH2-terminal sequence regions (1-12 and 16-30) and the putative ATP-binding site region (496-506) were identified as epitopes of the native enzyme. These results confirm some aspects of the transmembrane folding models proposed by Shull et al. and Kawakami et al. for the membrane-bound (Na+ + K+)-ATPase.

Ion-transporting properties and ATPase activity of (Na+ + K+)-ATPase large subunit incorporated into bilayer lipid membranes

A purified (Na+ + K+)-ATPase large subunit obtained from microsomes by water-alcohol extraction was incorporated into a bilayer lipid membrane. The protein formed in the membrane conductance channels which were sensitive to ouabain and selective for monovalent cations. ATP activated these channels in the presence of sodium and potassium ions. When sodium ions were eliminated ATP did not change the conductance of the modified membrane whereas p-nitrophenyl phosphate increased it. The (Na+ + K+)-ATPase large subunit incorporated into bilayer lipid membrane possessed an ATPase activity. The presence of a potential on the membrane was a necessary condition for the enzyme incorporated into a bilayer lipid membrane to show high ATPase activity. Increasing the potential above 100 mV resulted in the closing of conductance channels.

Immunochemical comparison of cardiac glycoside-sensitive (lamb) and -insensitive (rat) kidney (Na+ + K+)-ATPase

The immunological cross-reactivity of the ouabain-sensitive lamb kidney and the ouabain-insensitive rat kidney (Na+ + K+)-ATPase (EC 3.6.1.37) was examined using polyclonal and monoclonal antibodies. Studies using rabbit antisera prepared against both the lamb kidney and rat kidney holoenzymes showed the existence of substantial antigenic differences as well as similarities between the holoenzymes and the respective denatured alpha and beta subunits of these two enzymes. Quantitation of the extent of cross-reactivity using holoenzyme-directed antibodies showed a 40-60% cross-reactivity. In addition, rabbit antisera monospecific to the purified, denatured alpha and beta subunits of the lamb kidney enzyme showed about a 50% cross-reactivity towards the respective subunit of the rat enzyme. In contrast to the cross-reactivity observed using the polyclonal antibodies, six monoclonal antibodies specific for the alpha subunit of the lamb holoenzyme exhibited no cross-reactivity with the rat holoenzyme. Four of these monoclonal antibodies, however, showed substantial cross-reactivity with rat alpha subunit as resolved by SDS-polyacrylamide gel electrophoresis. A fifth antibody did not bind to the denatured alpha subunit of either the lamb or the rat enzyme. Another monoclonal antibody (M7-PB-E9), which is specific for an epitope previously implicated in the regulation of both ATP and ouabain binding to (Na+ + K+)-ATPase (Ball, W.J., Jr. (1984) Biochemistry 2275-2281) was found to bind to the denatured lamb alpha but not to the rat alpha. This antibody has identified a region of the lamb alpha that has an altered amino acid sequence in the ouabain-insensitive rat enzyme. These immunological studies indicate that there are substantial antigenic differences between the lamb and rat kidney (Na+ + K+)-ATPases. The majority of these antigenic differences appear to be due to variations in the tertiary structures rather than to variations in the primary structures of the alpha subunits.

Structural studies on H+,K+-ATPase: determination of the NH2-terminal amino acid sequence and immunological cross-reactivity with Na+,K+-ATPase

The NH2-terminal amino acid sequence of the 100 kilodalton subunit of porcine gastric H+,K+-ATPase has been determined to be YKAENYELYQVELGPGP. Although the NH2-terminal region of this protein is not similar to the same region of the lamb kidney Na+,K+-ATPase catalytic subunit, other regions of these ATPase proteins appear to be homologous. Both monoclonal and polyclonal antibodies raised to lamb kidney Na+,K+-ATPase and its alpha, but not beta, subunit cross-react with the 100 kilodalton protein of H+,K+-ATPase.

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.

Recognition of sodium- and potassium-dependent adenosine triphosphatase on mouse lymphoid cells by means of a monoclonal antibody

Previous evidence has established the similarity between (Na+ + K+)-ATPase (ATP phosphohydrolase, EC.3.6.1.3) and the antigen recognized by the rat antimouse monoclonal antibody anti-BSP-3. This antibody has been used for investigation of the surface expression and biochemical analysis of the enzyme in different mouse lymphoid populations. The BSP-3 determinant is found on almost all thymocytes and concanavalin A-induced thymocytes, to a lesser extent on bone marrow cells and also on a minor population of spleen cells. Spleen cells from athymic mice are negative. The (Na+ + K+)-ATPase purified from mouse thymus by affinity chromatography migrates in SDS-polyacrylamide gels in the form of two polypeptide chains of 105 000 and 51 000 daltons. Chains of the same molecular weight, fractionated on SDS-PAGE from microsomes of mouse thymuses, are shown to react with subunit-specific polyclonal antisera against ATPase in immunoblotting experiments. Immunoprecipitation with anti-BSP-3 from surface iodinated thymocytes yields only the small subunit. Comparison of the chains isolated from thymus and brain shows molecular weight differences in both subunits. These results, and variations in the reactivity pattern of the anti-BSP-3 antibody on several cell types, may indicate a possible heterogeneity of the (Na+ + K+)-ATPase expressed by various tissues and cells.