Conformational changes of membrane-bound (Na+-K+)-ATPase as revealed by antibody inhibition

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

Right-side-out vesicles of pig kidney microsomes and amino-acid-sequence-specific antibodies were used to probe the sidedness of the C-terminus and the N-terminus of the catalytic alpha subunit of Na+/K(+)-ATPase. Polyclonal antibodies were raised in rabbits against the peptide corresponding to the N-terminal sequence GRDKYEPAAVSE (peptide 1-12) and against peptides corresponding to the C-terminal sequences IFVYDEVRKLIIRRR (peptide 991-1005) and RPGGWVEKETYY (peptide 1005-1016). These antibodies were purified by affinity chromatography on the respective peptide-Sepharose columns. Moreover, antibodies against the N-terminal dodecapeptide GRDKYEPAAVSE were obtained by affinity purification from heteroclonal antibodies against the alpha subunit of pork kidney Na+/K(+)-ATPase. These antibodies reacted with native as well as SDS-denaturated Na+/K(+)-ATPase. When the antibodies were used to probe the sidedness of the sequences in right-side-out vesicles of pig kidney microsomes, the N-terminal peptide 1-12 as well as the C-terminal peptides 991-1005 and 1005-1016 were found on the cytosolic side. Concanavalin A, however, which interacts with the beta subunit, a glycoprotein, reacted with the outside of right-side-out vesicles.

A radioimmunoassay to screen for antibodies to native conformational antigens and analyse ligand-induced structural states of antigenic proteins

A radioimmunoassay is described in which antigenic protein was immobilized by incubating nitrocellulose filters of defined diameter with antigen-containing solutions. The amount of adsorbed antigen increased in a linear fashion over a wide range of antigen concentrations. The antigen-antibody reactions and the indicator reactions were performed by incubating the filters with appropriate solutions. During the test any contact of the antigen with air was avoided. Antigenic sites which are sensitive to protein denaturation by drying could be detected with the assay. The assay was also used to screen hybridoma supernatants for antibodies directed against Na+ cotransport proteins from renal brush-border membranes. Monoclonal antibodies were selected which showed different binding characteristics depending on whether or not substrates of Na+ cotransporters were present. Since binding of several antibodies was altered by two different substrates and not by non-transported control substances, these monoclonal antibodies were believed to interact with more than one transport system. One of the antibodies, which showed different antibody binding after addition of D-glucose or L-lactate, bound to a polypeptide component of the renal Na+-D-glucose cotransporter and was able to inhibit Na+ gradient-dependent D-glucose uptake in brush-border membrane vesicles (Koepsell, Korn, Raszeja-Specht, Bernotat-Danielowski, Ollig, 1988, J. Biol. Chem., in press). To investigate the effects of D-glucose and L-lactate on the binding of this antibody concentration dependence was measured. High and low affinity binding sites for D-glucose and L-lactate were characterized thereby demonstrating that the radioimmunoassay permits investigations of the properties of high and low affinity substrate binding sites.

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.

Partial immunochemical identity between (Na+ + K+)-ATPase and a membrane component extractable with chloroform: methanol

An IgG fraction prepared from an antiserum against a holoenzyme preparation of (Na+ + K+)-ATPase precipitated a single antigen when samples of holoenzyme were subjected to crossed immunoelectrophoresis but precipitated an additional, immunochemically-related antigen when a plasma membrane-enriched fraction was subjected to crossed immunoelectrophoresis under the same conditions. The immunochemically-related antigen could be extracted from the plasma membrane fraction with CHCl3:CH3OH.

Effect of pH and different substrates on the electrokinetic properties of (Na+, K+)-ATPase vesicles

Some biophysical properties of a (Na+, K+)-ATPase preparation from guinea-pig kidney have been analysed. The recently developed technique of laser Doppler spectroscopy was applied to measure particle mobility under electrophoretic conditions. The following results were obtained: 1. magnesium ions at pH 7.3 decrease the mobility of the ATPase containing vesicles by binding to negatively charged surface groups. At pH 3.3 the competitive binding of protons causes a shift of the mobility vs. [Mg2+] curve to higher values of [Mg2+], 2. binding of ATP at pH 7.3 (Kd = 0.9 X 10(-4) M for (mM 1 NaCl, 0.2 KCl, 0.1 MgCl2, 0.1 Tris) was measured as an increase in particle mobility depending also on [Mg2+]. At pH 3.3 also unspecific ATP-binding occurred, 3. ITP and GTP had the same Kd value as ATP; ADP a slightly lower one (Kd = 1.2 X 10(-4) M). Tris-H3PO4 (Kd = 2.6 X 10(-4) M) was also able to increase particle mobility, but only at higher concentrations and not to the same extent as ATP; AMP induced only very small changes, 4. from the mobility-pH curve an isoelectric point of 4.1 is derived (buffer: 1 mM NaCl, 0.2 mM KCl, 0.1 mM MgCl2, 0.1 mM Tris). In the presence of 0.9 mM ATP the isoelectric point is shifted to 3.2. As the electrophoretic mobility is directly proportional to the net charge of the vesicles, the results may be interpreted as changes in surface charge density, originating from both a conformational change of the ATPase polypeptide and a decrease in vesicle size.

Immunochemical evidence for a transmembrane orientation of both the (Na+, K+)-ATPase subunits

Antibodies were raised against the large catalytic subunit (apparent Mr 96000) and the glycoprotein (apparent Mr 60000) of the sodium- and potassium-dependent adenosine triphosphatase [(Na+, K+)-ATPase] from Bufo marinus. The specificity of each antiserum was assessed by two-dimensional immunoelectrophoresis using toad kidney microsomes or the purified holoenzyme as a source of antigen and by indirect immunoprecipitation of detergent-solubilized (Na+, K+)-ATPase subunits from radioiodinated or biosynthetically labeled kidney holoenzyme, microsomes, or postnuclear supernatant. The anticatalytic subunit serum reacted exclusively with a 96000-dalton protein. The antiserum to the glycoprotein was rendered specific to this subunit by absorption with purified catalytic subunit. The two antisera were agglutinating and lytic in the presence of complement when toad erythrocytes were used as targets, indicating that antigenic determinants of both subunits were exposed on the cell surface. The specific reactivities with surface-exposed antigenic determinants of both subunits could be absorbed with toad red blood cells. Such absorbed antisera still reacted with detergent-treated or untreated kidney microsomes, revealing the presence of cytoplasmic and/or intramembranous antigenic sites. Our immunochemical data demonstrate that the glycoprotein subunit of (Na+, K+)-ATPase spans the lipid bilayer and confirm the transmembrane orientation of the catalytic subunit postulated from functional studies.

Conformational changes of membrane-bound (Na+--K+)-ATPase as revealed by trypsin digestion

To distinguish ligand-induced structural states of the (Na+--K+)-ATPase, the purified membrane-bound enzyme isolated from rat kidneys was digested with trypsin in the presence of various combinations of Na+, K+, Mg++ and ATP. It was found that first the large and then the small polypeptide chain of the (Na+--K+)-ATPase was degraded, indicating that the lysine and arginine residues of the large chain are more exposed than are those of the small one. The (Na+--K+)-ATPase activity was inactivated in parallel with the degradation of the large polypeptide chain. After the degradation of the large polypeptide chain, about 75% of the (Na+--K+)-ATPase protein remained bound to the membrane, demonstrating that the split protein segments were only partially released. It was found that the combinations of ATP, Mg++, Na+ and K+ present during trypsin digestion influenced the time course and degree of degradation of the (Na+--K+)-ATPase protein. The degradations of the large and the small polypeptide chain were affected in parallel. Thus, certain ATP and ligand combinations influenced neither the degradation of the large nor the degradation of the small polypeptide chain, whereas by other combinations of ATP and ligands the degree of susceptibility of both polypeptide chains to trypsin was equally increased or reduced. In the absence of ATP the time course of trypsin digestion of the (Na+--K+)-ATPase was the same, whether Na+ or K+ was present. With low ATP concentrations (e.g., 0.1 mM), however, binding of Na+ or K+ led to different degradation patterns of the enzyme. If a high concentration of ATP (e.g. 10 mM) was present, Na+ and K+ also influenced the degradation pattern of the (Na+--K+)-ATPase, but differentially compared to that at low ATP concentrations, since the effects of Na+ and K+ were reversed. Furthermore, it was found that the degradation of the small chain was only influenced by certain combinations of ATP, Mg++, Na+ and K+ if the large chain was intact when the ligands were added to the enzyme. The described results demonstrate structural alterations of the (Na+--K+)-ATPase complex which are supposed to include a synchronous protrusion or retraction of both (Na+--K+)-ATPase subunits. The data further suggest that ATP and other ligands primarily alter the structure of the large (Na+--K+)-ATPase subunit. This structural alteration is presumed to lead to a synchronous movement of the small subunit of the enzyme. The structural state of the (Na+--K+)-ATPase is regulated by binding of Na+ or K+ to the enzyme-ATP complex. The effects of Na+ and K+ on the (Na+--K+)-ATPase structure are modulated by the ATP binding to "high affinity" and to "low affinity" ATP binding sites.