Origin of the gamma polypeptide of the Na+/K+-ATPase

The Na+/K+-ATPase purified from lamb kidney contains a gamma polypeptide fraction which is a collection of fragments derived from the alpha and beta polypeptides of the enzyme. This fraction has the solubility characteristics of a proteolipid and was isolated either by high performance liquid chromatography (size exclusion chromatography) in 1% sodium dodecyl sulfate or by sequential organic extraction of purified lamb kidney Na+/K+-ATPase. Formation of gamma polypeptide(s) from detergent solubilized holoenzyme was accelerated by sulfhydryl containing reagents and was unaffected by addition of inhibitors of proteolytic enzymes. Treatment of the holoenzyme with the photoaffinity reagent N-(2-nitro-4-azidophenyl)[3H]ouabain ([3H]NAP-ouabain) labeled the alpha polypeptide and the gamma polypeptide fraction but not the beta polypeptide. Amino acid sequence analysis of one gamma polypeptide preparation revealed homology of one component of this fraction with the N-terminus of the beta subunit of the Na+/K+-ATPase. Amino acid analysis of two preparations of proteolipid showed similar amino acid compositions with a peptide derived from the alpha subunit. The insolubility and complexity of the gamma polypeptide(s)/proteolipid fraction appears to preclude a conclusive sequence analysis of all components of this fraction.     

Purification and characterization of an (Na+ + K+)-ATPase proteolipid labeled with a photoaffinity derivative of ouabain

Highly purified lamb kidney (Na+ + K+)-ATPase was photoaffinity labeled with the tritiated 2-nitro-5-azidobenzoyl derivative of ouabain (NAB-ouabain). The labeled (Na+ + K+)-ATPase was mixed with unlabeled carrier enzyme. Two proteolipid (gamma 1 and gamma 2) fractions were then isolated by chromatography on columns of Sepharose CL-6B and Sephadex LH-60. The two fractions were interchangeable when rechromatographed on the LH-60 column, suggesting that gamma 1 is an aggregated form of gamma 2. The total yield was 0.8-1.5 mol of gamma component per mol of catalytic subunit recovered. This indicates that the gamma component is present in stoichiometric amounts in the Na+ + K+)-ATPase. The proteolipids that were labeled with NAB-ouabain copurified with the unlabeled proteolipids.     

Gas phase sequencing of the proteolipid subunit 9 of the human H+-ATPase in the presence of cetyltrimethylammonium bromide

The compatibility of cetyltrimethylammonium bromide (CTAB), a quaternary ammonium compound with detergent properties, with gas phase protein sequencing has been examined. Two proteins, one hydrophilic (sperm whale apomyoglobin) and one hydrophobic (the proteolipid subunit 9 of the human mitochondrial H+-ATPase), were successfully sequenced in the presence of this detergent. The presence of CTAB did not affect the repetitive yield during sequencing when compared with polybrene although at high detergent concentrations the initial yield was apparently lower. The sequence of the first forty amino acid residues of the human H+-ATPase proteolipid subunit 9 shows complete homology to the bovine sequence.     

Characterization of epitopes of the yeast mitochondrial H+-ATPase complex recognized by monoclonal antibodies

Nine monoclonal antibodies which react with the beta subunit of the yeast mitochondrial H+-ATPase and three which react with a 25 kDa subunit of the enzyme complex (P25) have been characterized. Competitive binding studies indicated the presence of at least four antigenic regions on the beta subunit of the enzyme complex. One antigenic region of the beta subunit is recognized by two monoclonal antibodies RH 57.1 and RH 45.5 which inhibit the ATPase activity to different degrees. Antibody RH 48.6 appears to bind to a second region on the beta subunit and has no effect on the ATPase activity. A third region of the beta subunit is recognized by antibodies RH 51.4 and RH 72.1. RH 51.4 has no effect on the ATPase activity, whereas RH 72.1 stimulates ATPase activity. Antibody RH 32.4 which has no effect on the ATPase activity appears to bind to the fourth epitope of the beta subunit. All three monoclonal anti-P25 antibodies, RH 66.3, RH 41.2 and RH 37.0, apparently bind to the same antigenic region on this subunit. Two of the monoclonal anti-beta antibodies RH 48.6 and RH 51.4 were found to be very effective in immunoprecipitating the whole H+-ATPase complex in a solid phase system. However, the other monoclonal antibodies (and also a polyclonal antiserum) appear to induce the dissociation of one or more of the H+-ATPase subunits by their binding to the epitopes on the beta or the P25 subunits.     

Monoclonal antibodies against subunits of yeast mitochondrial H+-ATPase

Fourteen stable lines of myeloma-spleen cell hybrids producing antibodies against the mitochondrial H+-ATPase have been isolated. One reacted with the alpha-subunit of the enzyme complex (Mr 56000), nine with the beta-subunit (Mr 54000), and four with a 25 kDa subunit which has not been previously characterized. These antibodies are inhibitory or stimulatory or have no effect upon the enzyme activity. Two of the monoclonal anti-beta-subunit antibodies were found to be particularly effective in immunoprecipitating intact H+-ATPase complex.     

Sequence analysis and characterization of vacuolar-type H+ -ATPase proteolipid transcript from Acanthus ebracteatus Vah1.

The vacuolar-type H+ -ATPase (V-ATPase) is a multimeric enzyme with diverse functions in plants such as nutrient transport, flowering, stress tolerance, guard cell movement and development. A partial sequence of V-ATPase proteolipid was identified among the expressed sequence tags (ESTs) generated from Acanthus ebracteatus, and selected for full-length sequencing. The 876-nucleotide cDNA consists of an open reading frame of 165 amino acids. The deduced amino acid sequence displays high similarity (81%) with its homologs from Arabidopsis thaliana, Avecinnia marina and Gossypium hirsutum with the four transmembrane domains characteristics of the 16 kDa proteolipid subunit c of V-ATPase well conserved in this protein. Southern analysis revealed the existence of several members of proteolipid subunit c of V-ATPase in A. ebracteatus. The mRNA of this gene was detected in leaf, floral, stem and root tissues, however, the expression level was lower in stem and root tissues.     

Lysosomal H+-translocating ATPase has a similar subunit structure to chromaffin granule H+-ATPase complex

Subunit structure of the lysosomal H+-ATPase was investigated using cold inactivation, immunological cross-reactivity with antibodies against individual subunits of the H+-ATPase from chromaffin granules and chemical modification with N,N'-dicyclohexyl[14C]carbodiimide. The lysosomal H+-ATPase was irreversibly inhibited when incubated at 0 degrees C in the presence of chloride or nitrate and MgATP. Inactivation in the cold resulted in the release of several polypeptides (72, 57, 41, 34 and 33 kDa) from the membrane, which had the same electrophoretic mobility as the corresponding subunits of chromaffin granule H+-ATPase. Cross-reactivity of antibodies revealed that the 72, 57 and 34 kDa polypeptides were immunologically identical to the corresponding subunits of chromaffin granule H+-ATPase. Dicyclohexylcarbodiimide, which inhibits proton translocation in the vacuolar ATPase, predominantly labeled two polypeptides of 18 and 15 kDa, which compose the membrane sector of the enzyme. These results suggest that the lysosomal H+-ATPase is a multimeric enzyme, whose subunit structure is similar to the chromaffin granule H+-ATPase. The subunit structure of other vacuolar H+-ATPases, revealed by cold inactivation and immunological cross-reactivity, is also presented.     

Initial Steps in the Assembly of the Vacuole-Type H+-ATPase

The plant vacuole is acidified by a complex multimeric enzyme, the vacuole-type H⁺-ATPase (V-ATPase). The initial association of ATPase subunits on membranes was studied using an in vitro assembly assay. The V-ATPase assembled onto microsomes when V-ATPase subunits were supplied. However, when the A or B subunit or the proteolipid were supplied individually, only the proteolipid associated with membranes. By using poly(A⁺) RNA depleted in the B subunit and proteolipid subunit mRNA, we demonstrated A subunit association with membranes at substoichiometric amounts of the B subunit or the 16-kD proteolipid. These data suggest that poly(A⁺) RNA-encoded proteins are required to catalyze the A subunit membrane assembly. Initial events were further studied by in vivo protein labeling. Consistent with a temporal ordering of V-ATPase assembly, membranes contained only the A subunit at early times; at later times both the A and B subunits were found on the membranes. A large-mass ATPase complex was not efficiently formed in the absence of membranes. Together, these data support a model whereby the A subunit is first assembled onto the membrane, followed by the B subunit.     

Purification of the N,N'-dicyclohexylcarbodiimide-binding proteolipid of a higher plant tonoplast H+-ATPase

The H+-ATPase of Beta vacuolar membrane (tonoplast) comprises at least three functionally distinct subunits of Mr = 67,000, 57,000, and 16,000, respectively (Manolson, M. F., Rea, P. A., and Poole, R. J. (1985) J. Biol. Chem. 260, 12273-12279). The hydrophobic carboxyl reagent N,N'-dicyclohexylcarbodiimide (DCCD) inactivates the enzyme with pseudo-first order kinetics, and the concentration dependence of the reaction indicates that DCCD interacts with a single site on the enzyme to exert its inhibitory effect. The apparent pseudo-first order rate constant (k0) is reciprocally dependent on membrane protein concentration, which is expected if a large fraction of the DCCD partitions into the lipid phase. k0 has a nominal value of 1000 M-1 min-1 at a protein concentration of 250 micrograms/ml, although when phase partitioning is taken into account, the true, protein concentration-independent value of k0 is calculated to be about an order of magnitude lower. [14C]DCCD primarily labels the Mr = 16,000 polypeptide of native tonoplast vesicles. Binding is venturicidin-insensitive and occurs at a rate similar to the rate of enzyme inactivation, implying that inhibition is a direct result of covalent modification of the Mr = 16,000 polypeptide. Labeling of the containing Mr = 8,000 subunit of mitochondrial F0F1-ATPase is, on the other hand, faster by a factor of 5 and totally abolished by venturicidin. These results confirm that the Mr = 16,000 polypeptide which copurifies with tonoplast H+-ATPase activity is a subunit of the enzyme. Most of the DCCD-reactive Mr = 16,000 subunit is extracted from acetone:ethanol-washed tonoplast vesicles by chloroform:methanol. [14C]DCCD bound to the Mr = 16,000 polypeptide is enriched in the chloroform:methanol extract by 5-fold compared with native tonoplast and the specific activity (nmol of [14C]DCCD/mg of protein) can be increased a further 37-fold by chromatography on DEAE-Sephadex. It is concluded that the Mr = 16,000 subunit of the tonoplast H+-ATPase is a proteolipid.     

Identification of subunits a, b, and c1 from Acetobacterium woodii Na+-F1F0-ATPase. Subunits c1, c2, AND c3 constitute a mixed c-oligomer

The Na(+)-F(1)F(0)-ATPase operon of Acetobacterium woodii was recently shown to contain, among eleven atp genes, those genes that encode subunit a and b, a gene encoding a 16-kDa proteolipid (subunit c(1)), and two genes encoding 8-kDa proteolipids (subunits c(2) and c(3)). Because subunits a, b, and c(1) were not found in previous enzyme preparations, we re-determined the subunit composition of the enzyme. The genes were overproduced, and specific antibodies were raised. Western blots revealed that subunits a, b, and c(1) are produced and localized in the cytoplasmic membrane. Membrane protein complexes were solubilized by dodecylmaltoside and separated by blue native-polyacrylamide gel electrophoresis, and the ATPase subunits were resolved by SDS-polyacrylamide gel electrophoresis. N-terminal sequence analyses revealed the presence of subunits a, c(2), c(3), b, delta, alpha, gamma, beta, and epsilon. Biochemical and immunological analyses revealed that subunits c(1), c(2), and c(3) are all part of the c-oligomer, the first of a F(1)F(0)-ATPase that contains 8- and 16-kDa proteolipids.     

Isoforms of Na+, K+-ATPase in human prostate; specificity of expression and apical membrane polarization

The cellular distribution of Na+, K+-ATPase subunit isoforms was mapped in the secretory epithelium of the human prostate gland by immunostaining with antibodies to the alpha and beta subunit isoforms of the enzyme. Immunolabeling of the alpha1, beta1 and beta2 isoforms was observed in the apical and lateral plasma membrane domains of prostatic epithelial cells in contrast to human kidney where the alpha1 and beta1 isoforms of Na+, K+-ATPase were localized in the basolateral membrane of both proximal and distal convoluted tubules. Using immunohistochemistry and PCR we found no evidence of Na+, K+-ATPase alpha2 and alpha3 isoform expression suggesting that prostatic Na+, K+-ATPase consists of alpha1/beta1 and alpha1/beta2 isozymes. Our immunohistochemical findings are consistent with previously proposed models placing prostatic Na+, K+-ATPase in the apical plasma membrane domain. Abundant expression of Na+, K+-ATPase in epithelial cells lining tubulo-alveoli in the human prostate gland confirms previous conclusions drawn from biochemical, pharmacological and physiological data and provides further evidence for the critical role of this enzyme in prostatic cell physiology and ion homeostasis. Na+, K+-ATPase most likely maintains an inwardly directed Na+ gradient essential for nutrient uptake and active citrate secretion by prostatic epithelial cells. Na+, K+-ATPase may also regulate lumenal Na+ and K+, major counter-ions for citrate.     

Effect of N,N-Dicyclohexylcarbodiimide on Acetylcholine Release from Torpedo Synaptosomes and Proteoliposomes Reconstituted with the Proteolipid Mediatophore

The mediatophore is a presynaptic membrane protein that has been shown to translocate acetylcholine (ACh) under calcium stimulation when reconstituted into artificial membranes. The mediatophore subunit, a 15-kDa proteolipid, presents a very high sequence homology with the N,N'-dicyclohexylcarbodiimide (DCCD)-binding proteolipid subunit of the vacuolar-type H(+)-ATPase. This prompted us to study the effect of DCCD, a potent blocker of proton translocation, on calcium-dependent ACh release. The present work shows that DCCD has no effect on ACh translocation either from Torpedo synaptosomes or from proteoliposomes reconstituted with purified mediatophore. However, using [14C]DCCD, we were able to demonstrate that the drug does bind to the 15-kDa proteolipid subunit of the mediatophore. These results suggest that although the 15-kDa proteolipid subunits of the mediatophore and the vacuolar H(+)-ATPase may be identical, different domains of these proteins are involved in proton translocation and calcium-dependent ACh release and that the two proteins have a different membrane organization.     

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.     

The (Na+ + K+)-ATPase of chick sensory neurons. Studies on biosynthesis and intracellular transport

The (Na+ + K+)-ATPase of cultured chick sensory neurons was studied with the aid of antibodies specific for this enzyme. Immunofluorescent labeling indicated the (Na+ + K+)-ATPase is evenly distributed on the neuronal cell surface; cell bodies, neurites, and growth cones were labeled with comparable intensity. Pulse-chase experiments with [35S]methionine, followed by immunoprecipitation, indicated concurrent synthesis and rapid association of the alpha (Mr = 105,000) and beta (Mr = 47,000) subunits. The alpha subunit is oligosaccharide-free while the beta subunit contains three Asn-linked oligosaccharide chains attached to a core peptide of 32,000 molecular weight. The time required for oligosaccharide processing of the newly synthesized beta subunit to endoglycosidase H-resistance suggests the (Na+ + K+)-ATPase takes 45-60 min to move from the site of polypeptide synthesis to the Golgi apparatus. Significantly less time was required for transport through the Golgi apparatus and insertion in the plasma membrane. From 30% to 55% of the newly synthesized (Na+ + K+)-ATPase did not appear on the cell surface but accumulated intracellularly. When tunicamycin was used to inhibit glycosylation of the beta subunit, there was no effect upon subunit assembly, intracellular transport, or degradation rate (t1/2 = 40 h).     

A gene encoding the proteolipid subunit of Sulfolobus acidocaldarius ATPase complex

An analysis of genes for the major two subunits of the membrane-associated ATPase from an acidothermophilic archaebacterium, Sulfolobus acidocaldarius, suggested that it belongs to a different ATPase family from the F1-ATPase (Denda, K., Konishi, J., Oshima, T., Date, T., and Yoshida, M. (1988) J. Biol. Chem. 263, 17251-17254). In the same operon of the above two genes we found a gene encoding a very hydrophobic protein of 101 amino acids (Mr = 10,362). A proteolipid was purified from the membranes of this bacteria in which partial amino acid sequences matched with the sequence deduced from the gene. Significant amino acid sequence homology and a similar hydropathy profile appeared when the sequence was compared with the 8-kDa proteolipid subunit of F0F1-ATPases. It is about 30 amino acids larger than the 8-kDa proteolipid and has a small (11-amino acid) repeat sequence. However, it is distinct from the 16-kDa proteolipid subunit of an eukaryotic vacuolar H+-ATPase (Mandel, M., Moriyama, Y., Hulmes, J.D., Pan, Y.-E., Nelson, H., and Nelson, N. (1988) Proc. Natl. Acad. Sci. U.S.A. 85,5521-5524).     

Helical interactions and membrane disposition of the 16-kDa proteolipid subunit of the vacuolar H(+)-ATPase analyzed by cysteine replacement mutagenesis.

Theoretical mechanisms of proton translocation by the vacuolar H(+)-ATPase require that a transmembrane acidic residue of the multicopy 16-kDa proteolipid subunit be exposed at the exterior surface of the membrane sector of the enzyme, contacting the lipid phase. However, structural support for this theoretical mechanism is lacking. To address this, we have used cysteine mutagenesis to produce a molecular model of the 16-kDa proteolipid complex. Transmembrane helical contacts were determined using oxidative cysteine cross-linking, and accessibility of cysteines to the lipid phase was determined by their reactivity to the lipid-soluble probe N-(1-pyrenyl)maleimide. A single model for organization of the four helices of each monomeric proteolipid was the best fit to the experimental data, with helix 1 lining a central pore and helix 2 and helix 3 immediately external to it and forming the principal intermolecular contacts. Helix 4, containing the crucial acidic residue, is peripheral to the complex. The model is consistent not only with theoretical proton transport mechanisms, but has structural similarity to the dodecameric ring complex formed by the related 8-kDa proteolipid of the F(1)F(0)-ATPase. This suggests some commonality between the proton translocating mechanisms of the vacuolar and F(1)F(0)-ATPases.     

A monoclonal antibody against horse kidney (Na+ + K+)-ATPase inhibits sodium pump and E2K to E1 conversion of (Na+ + K+)-ATPase from outside of the cell membrane

Monoclonal antibodies against horse kidney outer medulla (Na+ + K+)-ATPase were prepared. One of these antibodies (M45-80), was identified as an IgM, recognized the alpha subunit of the enzyme. M45-80 had the following effects on horse kidney (Na+ + K+)-ATPase: (1) it inhibited the enzyme activity by 50% in 140 mM Na+ and by 80% in 8.3 mM Na+; (2) it increased the Na+ concentration necessary for half-maximal activation (K0.5 for Na+) from 12.0 to 57.6 mM, but did not affect K0.5 for K+; (3) it slightly increased the K+-dependent p-nitrophenylphosphatase (K-pNPPase) activity; (4) it inhibited phosphorylation of the enzyme with ATP by 30%, but did not affect the step of dephosphorylation; and (5) it enhanced the ouabain binding rate. These data are compatible with a stabilizing effect on the E2 form of (Na+ + K+)-ATPase. M45-80 was concluded to bind to the extracellular surface of the plasmamembrane, based on the following evidence: (1) M45-80 inhibited by 50% the ouabain-sensitive 86Rb+ uptake in human intact erythrocytes from outside of the cells; (2) the inhibition of (Na+ + K+)-ATPase activity in right-side-out vesicles of human erythrocytes was greater than that in inside-out vesicles; and (3) the fluorescence intensity due to FITC-labeled rabbit anti-mouse IgM that reacted with M45-80 bound to the right-side-out vesicles was much greater than that in the case of the inside-out vesicles.     

Purification and functional properties of the DCCD-reactive proteolipid subunit of the H+-translocating ATPase from Mycobacterium phlei

Interaction of N,N'-dicyclohexylcarbodiimide (DCCD) with ATPase of Mycobacterium phlei membranes results in inactivation of ATPase activity. The rate of inactivation of ATPase was pseudo-first order for the initial 30-65% inactivation over a concentration range of 5-50 microM DCCD. The second-order rate constant of the DCCD-ATPase interaction was k = 8.5 X 10(5) M-1 X min(-1). The correlation between the initial binding of [14C]DCCD and 100% inactivation of ATPase activity shows 1.57 nmol DCCD bound per mg membrane protein. The proteolipid subunit of the F0F1-ATPase complex in membranes of M. phlei with which DCCD covalently reacts to inhibit ATPase was isolated by labeling with [14C]DCCD. The proteolipid was purified from the membrane in free and DCCD-modified form by extraction with chloroform/methanol and subsequent chromatography on Sephadex LH-20. The polypeptide was homogeneous on SDS-acrylamide gel electrophoresis and has an apparent molecular weight of 8000. The purified proteolipid contains phosphatidylinositol (67%), phosphatidylethanolamine (18%) and cardiolipin (8%). Amino acid analysis indicates that glycine, alanine and leucine were present in elevated amounts, resulting in a polarity of 27%. Cysteine and tryptophan were lacking. Butanol-extracted proteolipid mediated the translocation of protons across the bilayer, in K+-loaded reconstituted liposomes, in response to a membrane potential difference induced by valinomycin. The proton translocation was inhibited by DCCD, as measured by the quenching of fluorescence of 9-aminoacridine. Studies show that vanadate inhibits the proton gradient driven by ATP hydrolysis in membrane vesicles of M. phlei by interacting with the proteolipid subunit sector of the F0F1-ATPase complex.     

Binding of the chloroplast coupling factor CF to lipid vesicles

The binding of the chloroplast coupling factor CF to lipid vesicles was analyzed by gel filtration. CF can be bound to vesicles made of chloroplast lipids but not of lecithin. The presence in the vesicle walls of a proteolipid subunit of the hydrophobic component of the coupling factor increases the binding of CF. The apparent binding constant and the maximum protein/lipid ratio are calculated. The Ca2+-ATPase activity of bound CF is markedly lower than that of dissolved CF. It is confirmed that the proteolipid is a N,N'-dicyclohexylcarbodiimide sensitive proton channel. The binding of CF on proteolipid vesicles decreases their proton permeability.