Characterization of disulfide cross-links between fragments of proteolyzed Na,K-ATPase. Implications for spatial organization of trans-membrane helices

Unexpected Effects of K+ and Adenosine Triphosphate on the Thermal Stability of Na+,K+-ATPase

Na⁺,K⁺-ATPase is an integral membrane protein which couples ATP hydrolysis to the transport of three Na⁺ out and two K⁺ into the cell. The aim of this work is to characterize the effect of K⁺, ATP, and Mg²⁺ (essential activator) on the Na⁺,K⁺-ATPase thermal stability. Under all conditions tested, thermal inactivation of the enzyme is concomitant with a structural change involving the ATP binding site and membrane-associated regions. Both ligands exert a clear stabilizing effect due to both enthalpic and entropic contributions. Competition experiments between ATP and K⁺ showed that, when ATP is present, the inactivation rate coefficient exhibits a biphasic dependence on K⁺ concentration. At low [K⁺], destabilization of the enzyme is observed, while stabilization occurred at larger cation concentrations. This is not expected for a simple competition between the enzyme and two ligands that individually protect the enzyme. A model that includes enzyme species with none, one, or two K⁺ and/or one molecule of ATP bound explains the experimental data. We concluded that, despite both ligands stabilizing the enzyme, the species with one K⁺ and one ATP simultaneously bound is unstable.

Association of renal Na,K-ATPase alpha-subunit with the beta- and gamma-subunits based on cryoelectron microscopy

Na,K-ATPase transports Na(+) and K(+) across cell membranes and consists of alpha- and beta-subunits. Na,K-ATPase also associates with small FXYD proteins that regulate the activity of the pump. We have used cryoelectron microscopy of two-dimensional crystals including data to 8 A resolution to determine the three-dimensional (3-D) structure of renal Na,K-ATPase containing FXYD2, the gamma-subunit. A homology model for the alpha-subunit was calculated from a Ca(2+)-ATPase structure and used to locate the additional beta- and gamma-subunits present in the 3-D map of Na,K-ATPase. Based on the 3-D map, the beta-subunit is located close to transmembrane helices M8 and M10 and the gamma-subunit is adjacent to helices M2 and M9 of the alpha-subunit.

Janus model of the Na,K-ATPase beta-subunit transmembrane domain: distinct faces mediate alpha/beta assembly and beta-beta homo-oligomerization

Na,K-ATPase is a hetero-oligomer of alpha and beta-subunits. The Na,K-ATPase beta-subunit (Na,K-beta) is involved in both the regulation of ion transport activity, and in cell-cell adhesion. By structure prediction and evolutionary analysis, we identified two distinct faces on the Na,K-beta transmembrane domain (TMD) that could mediate protein-protein interactions: a glycine zipper motif and a conserved heptad repeat. Here, we show that the heptad repeat face is involved in the hetero-oligomeric interaction of Na,K-beta with Na,K-alpha, and the glycine zipper face is involved in the homo-oligomerization of Na,K-beta. Point mutations in the heptad repeat motif reduced Na,K-beta binding to Na,K-alpha, and Na,K-ATPase activity. Na,K-beta TMD homo-oligomerized in biological membranes, and mutation of the glycine zipper motif affected oligomerization and cell-cell adhesion. These results provide a structural basis for understanding how Na,K-beta links ion transport and cell-cell adhesion.

Fluorometric measurements of intermolecular distances between the alpha- and beta-subunits of the Na+/K+-ATPase

The Na+/K+-ATPase maintains the physiological Na+ and K+ gradients across the plasma membrane in most animal cells. The functional unit of the ion pump is comprised of two mandatory subunits including the alpha-subunit, which mediates ATP hydrolysis and ion translocation, as well as the beta-subunit, which acts as a chaperone to promote proper membrane insertion and trafficking in the plasma membrane. To examine the conformational dynamics between the alpha- and beta-subunits of the Na+/K+-ATPase during ion transport, we have used fluorescence resonance energy transfer, under voltage clamp conditions on Xenopus laevis oocytes, to differentiate between two models that have been proposed for the relative orientation of the alpha- and beta-subunits. These experiments were performed by measuring the time constant of irreversible donor fluorophore destruction with fluorescein-5-maleimide as the donor fluorophore and in the presence or absence of tetramethylrhodamine-6-maleimide as the acceptor fluorophore following labeling on the M3-M4 or M5-M6 loop of the alpha-subunit and the beta-subunit. We have also used fluorescence resonance energy transfer to investigate the relative movement between the two subunits as the ion pump shuttles between the two main conformational states (E1 and E2) as described by the Albers-Post scheme. The results from this study have identified a model for the orientation of the beta-subunit in relation to the alpha-subunit and suggest that the alpha- and beta-subunits move toward each other during the E2 to E1 conformational transition.

The beta subunit of the Na+/K+-ATPase follows the conformational state of the holoenzyme

The Na+/K+-ATPase is a ubiquitous plasma membrane ion pump that utilizes ATP hydrolysis to regulate the intracellular concentration of Na+ and K+. It is comprised of at least two subunits, a large catalytic alpha subunit that mediates ATP hydrolysis and ion transport, and an ancillary beta subunit that is required for proper trafficking of the holoenzyme. Although processes mediated by the alpha subunit have been extensively studied, little is known about the participation of the beta subunit in conformational changes of the enzyme. To elucidate the role of the beta subunit during ion transport, extracellular amino acids proximal to the transmembrane region of the sheep beta1 subunit were individually replaced for cysteines. This enabled sulfhydryl-specific labeling with the environmentally sensitive fluorescent dye tetramethylrhodamine-6-maleimide (TMRM) upon expression in Xenopus oocytes. Investigation by voltage-clamp fluorometry identified three reporter positions on the beta1 subunit that responded with fluorescence changes to alterations in ionic conditions and/or membrane potential. These experiments for the first time show real-time detection of conformational rearrangements of the Na+/K+-ATPase through a fluorophore-labeled beta subunit. Simultaneous recording of presteady-state or stationary currents together with fluorescence signals enabled correlation of the observed environmental changes of the beta subunit to certain reaction steps of the Na+/K+-ATPase, which involve changes in the occupancy of the two principle conformational states, E1P and E2P. From these experiments, evidence is provided that the beta1-S62C mutant can be directly used to monitor the conformational state of the enzyme, while the F64C mutant reveals a relaxation process that is triggered by sodium transport but evolves on a much slower time scale. Finally, shifts in voltage dependence and kinetics observed for mutant K65C show that this charged lysine residue, which is conserved in beta1 isoforms, directly influences the effective potential that determines voltage dependence of extracellular cation binding and release.

Purification of Na+,K+-ATPase Expressed in Pichia pastoris Reveals an Essential Role of Phospholipid-Protein Interactions

Na+,K+-ATPase (porcine alpha/his10-beta) has been expressed in Pichia Pastoris, solubilized in n-dodecyl-beta-maltoside and purified to 70-80% purity by nickel-nitrilotriacetic acid chromatography combined with size exclusion chromatography. The recombinant protein is inactive if the purification is done without added phospholipids. The neutral phospholipid, dioleoylphosphatidylcholine, preserves Na+,K+-ATPase activity of protein prepared in a Na+-containing medium, but activity is lost in a K+-containing medium. By contrast, the acid phospholipid, dioleoylphosphatidylserine, preserves activity in either Na+- or K+-containing media. In optimal conditions activity is preserved for about 2 weeks at 0 degrees C. Both recombinant Na+,K+-ATPase and native pig kidney Na+,K+-ATPase, dissolved in n-dodecyl-beta-maltoside, appear to be mainly stable monomers (alpha/beta) as judged by size exclusion chromatography and sedimentation velocity. Na+,K+-ATPase activities at 37 degrees C of the size exclusion chromatography-purified recombinant and renal Na+,K+-ATPase are comparable but are lower than that of membrane-bound renal Na+,K+-ATPase. The beta subunit is expressed in Pichia Pastoris as two lightly glycosylated polypeptides and is quantitatively deglycosylated by endoglycosidase-H at 0 degrees C, to a single polypeptide. Deglycosylation inactivates Na+,K+-ATPase prepared with dioleoylphosphatidylcholine, whereas dioleoylphosphatidylserine protects after deglycosylation, and Na+,K+-ATPase activity is preserved. This work demonstrates an essential role of phospholipid interactions with Na+,K+-ATPase, including a direct interaction of dioleoylphosphatidylserine, and possibly another interaction of either the neutral or acid phospholipid. Additional lipid effects are likely. A role for the beta subunit in stabilizing conformations of Na+,K+-ATPase (or H+,K+-ATPase) with occluded K+ ions can also be inferred. Purified recombinant Na+,K+-ATPase could become an important experimental tool for various purposes, including, hopefully, structural work.

The carboxy terminus of the colonic H+,K+-ATPase α-subunit is required for stable β subunit assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of colonic H(+), K(+)-ATPase alpha-subunit (HKalpha(2)), for both function as well as integrity of assembly with beta1-Na(+), K(+)-ATPase.

METHODS

For this purpose, a mutation of 84 amino acids in the carboxy-terminus was created (DeltaHKalpha(2)) and HEK-293 cells were used as expression systems for functional studies using (86)Rb(+)-uptake, coimmunoprecipitation using specific antibodies and fluorescence microscopy using green fluorescent protein.

RESULTS

The results demonstrate that comparable levels of expression of HKalpha(2) and DeltaHKalpha(2) mRNA were observed when cells were cotransfected with beta1 subunit. However, the abundance of expression of full length HKalpha(2) protein exceeded that of the truncated protein DeltaHKalpha(2). Ouabain-sensitive (86)Rb(+)-uptake was present only in cells cotransfected with HKalpha(2)/beta(1), indicating that the mutation was incapable of sustaining functionality. Coimmunoprecipitation experiments demonstrated that HKalpha(2) protein was immunoprecipitated more abundantly than DeltaHKalpha(2) when coexpressed with beta1. The use of sucrose gradients and green fluorescence protein immunofluorescence demonstrated that while the DeltaHKalpha(2)/beta(1) complex was confined to the endoplasmic reticulum, the HKalpha(2)/beta(1) complex translocated to the plasma membrane.

CONCLUSION

Taken together, our results are consistent with the view that the carboxy-terminus of HKalpha(2) facilitates the proper folding of the HKalpha(2)/beta(1) complex allowing translocation of the heterodimer to the plasma membrane where potassium uptake occurs. Otherwise, the alpha/beta complex is destined for degradation.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Role of the self-association of beta subunits in the oligomeric structure of Na+/K+-ATPase

The two subunits of Na(+)/K(+)-ATPase that are essential for function are alpha and beta. Previous cross-linking studies on the oligomeric structure of the membrane-bound enzyme identified alpha,beta and alpha,alpha associations, but only the former and not the latter could be detected after solubilization. To study the possibility of direct beta,beta association, the purified membrane enzyme and a trypsin-digested enzyme that occludes cations and contains an essentially intact beta and fragments of alpha were subjected to oxidative cross-linking in the presence of Cu(2+)-phenanthroline. Resolution of products on polyacrylamide gels, N-terminal analysis and reactivity with anti-beta antibody showed that, in addition to previously identified products (e.g. alpha,alpha and alpha,beta dimers), a beta,beta dimer, most likely linked through intramembrane Cys(44) residues of two chains, is also formed. This dimer was also noted when digitonin-solubilized intact enzyme, and the trypsin-digested enzyme solubilized with digitonin or polyoxyethylene 10-laurylether were subjected to cross-linking, indicating that the detected beta,beta association was not due to random collisions. In the digested enzyme, K(+) but not Na(+) enhanced beta,beta dimer formation. The alternative cross-linking of beta-Cys(44) to a Cys residue of a transmembrane alpha-helix was antagonized specifically by K(+) or Na(+). The findings (i) indicate the role of beta,beta association in maintaining the minimum oligomeric structure of (alpha,beta)(2), (ii) provide further support for conformation-dependent flexibilities of the spatial relations of the transmembrane helices of alpha and beta and (iii) suggest the possibility of significant differences between the quaternary structures of the P-type ATPases that do and do not contain a beta subunit.

Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

This chapter describes contributions of transition metal-catalyzed oxidative cleavage of Na+,K+-ATPase to our understanding of structure-function relations. In the presence of ascorbate/H2O2, specific cleavages are catalyzed by the bound metal and because more than one peptide bond close to the metal can be cleaved, this technique reveals proximity of the different cleavage positions within the native structure. Specific cleavages are catalyzed by Fe2+ bound at the cytoplasmic surface or by complexes of ATP-Fe2+, which directs the Fe2+ to the normal ATP-Mg2+ site. Fe2+- and ATP-Fe2+-catalyzed cleavages reveal large conformation-dependent changes in interactions between cytoplasmic domains, involving conserved cytoplasmic sequences, and a change of ligation of Mg2+ ions between E1P and E2P, which may be crucial in facilitating hydrolysis of E2P. The pattern of domain interactions in E1 and E2 conformations, and role of Mg2+ ions, may be common to all P-type pumps. Specific cleavages can also be catalyzed by Cu2+ ions, bound at the extracellular surfaces, or a hydrophobic Cu2+-diphenyl phenanthroline (DPP) complex, which directs the Cu2+ to the membrane-water interface. Cu2+ or Cu2+-DPP-catalyzed cleavages are providing information on alpha/beta subunit interactions and spatial organization of transmembrane segments. Transition metal-catalyzed cleavage could be widely used to investigate other P-type pumps and membrane proteins and, especially, ATP binding proteins.

The functional role of beta subunits in oligomeric P-type ATPases

Na,K-ATPase and gastric and nongastric H,K-ATPases are the only P-type ATPases of higher organisms that are oligomeric and are associated with a beta subunit, which is obligatory for expression and function of enzymes. Topogenesis studies suggest that beta subunits have a fundamental and unique role in K+-transporting P-type ATPases in that they facilitate the correct membrane integration and packing of the catalytic a subunit of these P-type ATPases, which is necessary for their resistance to cellular degradation, their acquisition of functional properties, and their routing to the cell surface. In addition to this chaperone function, beta subunits also participate in the determination of intrinsic transport properties of Na,K- and H,K-ATPases. Increasing experimental evidence suggests that beta assembly is a highly ordered, beta isoform-specific process, which is mediated by multiple interaction sites that contribute in a coordinate, multistep process to the structural and functional maturation of Na,K- and H,K-ATPases.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Structural and functional features of the transmembrane domain of the Na,K-ATPase beta subunit revealed by tryptophan scanning

In oligomeric P2-ATPases such as Na,K- and H,K-ATPases, beta subunits play a fundamental role in the structural and functional maturation of the catalytic alpha subunit. In the present study we performed a tryptophan scanning analysis on the transmembrane alpha-helix of the Na,K-ATPase beta1 subunit to investigate its role in the stabilization of the alpha subunit, the endoplasmic reticulum exit of alpha-beta complexes, and the acquisition of functional properties of the Na,K-ATPase. Single or multiple tryptophan substitutions in the beta subunits transmembrane domain had no significant effect on the structural maturation of alpha subunits expressed in Xenopus oocytes nor on the level of expression of functional Na,K pumps at the cell surface. Furthermore, tryptophan substitutions in regions of the transmembrane alpha-helix containing two GXXXG transmembrane helix interaction motifs or a cysteine residue, which can be cross-linked to transmembrane helix M8 of the alpha subunit, had no effect on the apparent K(+) affinity of Na,K-ATPase. On the other hand, substitutions by tryptophan, serine, alanine, or cysteine, but not by phenylalanine of two highly conserved tyrosine residues, Tyr(40) and Tyr(44), on another face of the transmembrane helix, perturb the transport kinetics of Na,K pumps in an additive way. These results indicate that at least two faces of the beta subunits transmembrane helix contribute to inter- or intrasubunit interactions and that two tyrosine residues aligned in the beta subunits transmembrane alpha-helix are determinants of intrinsic transport characteristics of Na,K-ATPase.

Predicted location and limited accessibility of protein kinase A phosphorylation site on Na-K-ATPase

Regulation of Na-K-ATPase by cAMP-dependent protein kinase occurs in a variety of tissues. Phosphorylation of the enzyme's catalytic subunit at a classical phosphorylation consensus motif has been observed with purified enzyme. Demonstration of phosphorylation at the same site in normal living cells or tissues has been more difficult, however, making it uncertain that the Na-K-ATPase is a direct physiological substrate of the kinase. Recently, the structure of the homologous sarco(endo)plasmic reticulum Ca-ATPase (SERCA1a) has been determined at 2.6 A resolution (Toyoshima C, Nakasako M, Nomura H, and Ogawa H. Nature 405: 647-655, 2000.), and the Na-K- ATPase should have the same fold. Here, the Na-K-ATPase sequence has been aligned with the Ca-ATPase structure to examine the predicted disposition of the phosphorylation site. The location is close to the membrane and partially buried by adjacent loops, and the site is unlikely to be accessible to the kinase in this conformation. Conditions that may expose the site or further bury it are discussed to highlight the issues facing future research on regulation of Na-K-ATPase by cAMP-dependent pathways.

Thermal denaturation of the Na,K-ATPase provides evidence for alpha-alpha oligomeric interaction and gamma subunit association with the C-terminal domain

Thermal denaturation can help elucidate protein domain substructure. We previously showed that the Na,K-ATPase partially unfolded when heated to 55 degrees C (Arystarkhova, E., Gibbons, D. L., and Sweadner, K. J. (1995) J. Biol. Chem. 270, 8785-8796). The beta subunit unfolded without leaving the membrane, but three transmembrane spans (M8-M10) and the C terminus of the alpha subunit were extruded, while the rest of alpha retained its normal topology with respect to the lipid bilayer. Here we investigated thermal denaturation further, with several salient results. First, trypsin sensitivity at both surfaces of alpha was increased, but not sensitivity to V8 protease, suggesting that the cytoplasmic domains and extruded domain were less tightly packed but still retained secondary structure. Second, thermal denaturation was accompanied by SDS-resistant aggregation of alpha subunits as dimers, trimers, and tetramers without beta or gamma subunits. This implies specific alpha-alpha contact. Third, the gamma subunit, like the C-terminal spans of alpha, was selectively lost from the membrane. This suggests its association with M8-M10 rather than the more firmly anchored transmembrane spans. The picture that emerges is of a Na,K-ATPase complex of alpha, beta, and gamma subunits in which alpha can associate in assemblies as large as tetramers via its cytoplasmic domain, while beta and gamma subunits associate with alpha primarily in its C-terminal portion, which has a unique structure and thermal instability.

Determinants of topogenesis and glycosylation of type II membrane proteins. Analysis of Na,K-ATPase beta 1 AND beta 3 subunits by glycosylation mapping

The structural and molecular determinants that govern the correct membrane insertion and folding of membrane proteins are still ill-defined. By following the addition of sugar chains to engineered glycosylation sites (glycosylation mapping) in Na,K-ATPase beta isoforms expressed in vitro and in Xenopus oocytes, in combination with biochemical techniques, we have defined the C-terminal end of the transmembrane domain of these type II proteins. N-terminal truncation and the removal of a single charged residue at the N-terminal start of the putative transmembrane domain influence the proper positioning of the transmembrane domain in the membrane as reflected by a repositioning of the transmembrane domain, the exposure of a putative cryptic signal peptidase cleavage site, and the production of protein species unable to insert into the membrane. Glycosylation mapping in vivo revealed that the degree of glycosylation at acceptor sites located close to the membrane increases with the time proteins spend in the endoplasmic reticulum. Furthermore, core sugars added to such acceptor sites cannot be processed to fully glycosylated species even when the protein is transported to the cell surface. Thus, the glycosylation mapping strategy applied in intact cells is a useful tool for the study of determinants for the correct membrane insertion of type II and probably other membrane proteins, as well as for the processing of sugar chains in glycoproteins.

Binding domain of oligomycin on Na(+),K(+)-ATPase

Oligomycin inhibits Na(+),K(+)-ATPase activity by stabilizing the Na(+) occlusion but not the K(+) occlusion. To locate the binding domain of oligomycin on Na(+),K(+)-ATPase, the tryptic-digestion profile of Na(+),K(+)-ATPase was compared with the profile of Na(+) occlusion within the digested Na(+),K(+)-ATPase in the presence of oligomycin. The Na(+) occlusion profile is responsible for the digestion profile of the alpha-subunit, which is the catalytic subunit of the ATPase. The effect of oligomycin on chimeric Ca(2+)-ATPase activity was examined. The chimera used, in which the 163 N-terminal amino acids of chicken sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 1 were replaced with the 200 N-terminal amino acids of the chicken Na(+),K(+)-ATPase alpha1-subunit, partially retains the Na(+)-dependent characteristics of Na(+), K(+)-ATPase, because the chimeric Ca(2+)-ATPase activity is activated by Na(+) but inhibited by ouabain, a specific inhibitor of Na(+),K(+)-ATPase (Ishii, T., Lemas, M.V., Takeyasu, K., 1994, Proc. Natl. Acad. Sci. U. S. A., 91, 6103-6107). Oligomycin depressed the activation by Na(+) of the chimeric Ca(2+)-ATPase activity. These findings suggest that the 200 N-terminal amino acids of the Na(+), K(+)-ATPase alpha-subunit include a binding domain for oligomycin.

Entrance port for Na(+) and K(+) ions on Na(+),K(+)-ATPase in the cytoplasmic loop between trans-membrane segments M6 and M7 of the alpha subunit. Proximity Of the cytoplasmic segment of the beta subunit

Based on the following observations we propose that the cytoplasmic loop between trans-membrane segments M6 and M7 (L6/7) of the alpha subunit of Na(+),K(+)-ATPase acts as an entrance port for Na(+) and K(+) ions. 1) In defined conditions chymotrypsin specifically cleaves L6/7 in the M5/M6 fragment of 19-kDa membranes, produced by extensive proteolysis of Na(+),K(+)-ATPase, and in parallel inactivates Rb(+) occlusion. 2) Dissociation of the M5/M6 fragment from 19-kDa membranes is prevented either by occluded cations or by competitive antagonists such as Ca(2+), Mg(2+), La(3+), p-xylylene bisguanidinium and m-xylylene bisguanidinium, or 1-bromo-2,4, 6-tris(methylisothiouronium)benzene and 1,3-dibromo-2,4,6-tris (methylisothiouronium)benzene (Br(2)-TITU(3+)). 3) Ca(2+) ions raise electrophoretic mobility of the M5/M6 fragment but not that of the other fragments of the alpha subunit. It appears that negatively charged residues in L6/7 recognize either Na(+) or K(+) ions or the competitive cation antagonists. Na(+) and K(+) ions are then occluded within trans-membrane segments and can be transported, whereas the cation antagonists are not occluded and block transport at the entrance port. The cytoplasmic segment of the beta subunit appears to be close to or contributes to the entrance port, as inferred from the following observations. 1) Specific chymotryptic cleavage of the 16-kDa fragment of the beta subunit to 15-kDa at 20 degrees C (Shainskaya, A., and Karlish, S. J. D. (1996) J. Biol. Chem. 271, 10309-10316) markedly reduces affinity for Br(2)-TITU(3+) and for Na(+) ions, detected by Na(+) occlusion assays or electrogenic Na(+) binding, whereas Rb(+) occlusion is unchanged. 2) Na(+) ions specifically protect the 16-kDa fragment against this chymotryptic cleavage.