Molecular organization of subunits of electroplax (sodium plus potassium)--activated adenosine triphosphatase

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.

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

Antibodies were raised against isolated, delipidated catalytic [alpha] and glycoprotein [beta] subunits of the Na+,K+-dependent ATPase purified from lamb kidney medulla. The specificity of each antiserum was confirmed by agar double-diffusion precipitation, immunoelectrophoresis, and polyacrylamide gel electrophoresis. A solid phase adsorption assay was also employed to determine antibody binding titers and to further test the specificity of these antisera. Antibodies raised to the alpha subunit had a strong reactivity and similar titer values for both the holoenzyme and the alpha subunit and a low-affinity cross-reactivity with the beta subunit. In contrast, beta-subunit-directed antibodies had little reactivity or binding with the holoenzyme and a low-affinity cross-reactivity with the alpha subunit. Competition binding studies revealed that about 80% of the alpha-subunit-specific antibodies bound to the holoenzyme, indicating that similar sets of antigenic sites are exposed in the lipid-embedded holoenzyme complex and in the isolated alpha subunit. Competition binding studies also suggest that the subunit cross-reactivities of the antisera may not result from simple contamination of the respective antigens, but that there may be partial homologies of some antigenic sites. In addition, the beta-directed antibodies had no effect on Na+,K+-ATPase activity, while the alpha-directed antibodies were effective inhibitors of activity. This indicates that at least some functionally important antigenic sites of the alpha subunit may be unaltered by its isolation and delipidation.

Tryptic digest of the alpha subunit of lamb kidney (Na+ + K+)-ATPase

The Mr approximately equal to 100 000 alpha subunit was prepared from highly purified lamb kidney (Na+ + K+)-ATPase. Its N-terminal sequence is Gly-Arg-Asx-Lys-Tyr-Glu. The alpha subunit was S-carboxymethylated, succinylated, and cleaved at its 40 arginine residues with trypsin. Four major, well-differentiated peptide fractions (A to D) were obtained by chromatography of the digest on a Sephadex G-50 column. Fraction A eluted at the void volume of the column and contained aggregated, very hydrophobic peptides, possibly from regions of alpha that are buried within the membrane lipid bilayer in the native enzyme. Fractions B to D, which together accounted for about 75% of the total protein, contained water-soluble peptides. To test the feasibility of using antibodies to identify and purify specific peptides of alpha subunit, studies were carried out using antibodies to native (Na+ + K+)-ATPase. Carboxymethylation and succinylation did not significantly decrease total antibody binding to alpha subunit, although the affinity of the anti-(Na+ + K+)-ATPase antibodies for alpha subunit was reduced by about 50%. The tryptic peptides of alpha subunit also retain significant immunochemical reactivity. Fractions A, B and C (but not D) of the digest all bind antibodies. To characterize further the tryptic digest, 16 peptides from fraction D were isolated and sequence studies on these were carried out.

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.

Membrane asymmetry. A survey and critical appraisal of the methodology. I. Methods for assessing the asymmetric orientation and distribution of proteins

This and the companion article are aimed at surveying the methods used for the study of membrane asymmetry. The techniques employed for the assessment of the asymmetric distribution and orientation of membrane proteins are reviewed in this article, whereas those pertaining to the unequal distribution of lipids are detailed in the companion paper. The use of immunological techniques and lectins, functions of proteins and their perturbations, chemical reagents, enzymatic isotopic labeling and enzymatic cleavage of membrane proteins and physical techniques are discussed and illustrated using recent examples of their application. Whenever appropriate, problems involving crypticity and non-availability or non-reactivity of functional sites, relevant chemical functions or protein fragments to appropriate ligands, reagents or modifying enzymes are envisaged and possible modification of the exposure of proteins during preparation of ghosts and other drawbacks are discussed, the use of different techniques and control experiments in conjunction is recommended for a more realistic assessment of the distribution and orientation of proteins.

Inhibition of ouabain-binding to (Na+ + K+)ATPase by antibody against the catalytic subunit but not by antibody against the glycoprotein subunit

Antibodies against purified (Na+ + K+)ATPase from the rectal gland of Squalus acanthias, as well as against its catalytic subunit, inhibited ouabain binding by as much as 50%. However, antibodies against the glycoprotein subunit did not inhibit ouabain binding. These data suggest that binding of antibody against the catalytic subunit to the enzyme either covers the ouabain binding site or destroys its confirmation, while binding of antibody against the glycoprotein has no such effect.

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.