cDNA sequence encoding the 16-kDa proteolipid of chromaffin granules implies gene duplication in the evolution of H+-ATPases

Cloning and expression of a glycine transporter from mouse brain

We have isolated a cDNA clone from a mouse brain library encoding the glycine transporter (GLYT). Xenopus oocytes injected with a synthetic mRNA accumulated [3h]glycine to levels of up to 80-fold above control values. The uptake was specific for glycine and dependent on the presence of Na+ and Cl- in the medium. The cDNA sequence predicts a highly hydrophobic protein of 633 amino acids with 12 potential transmembrane helices. The predicted amino acid sequence has 40-45% identity to the GABA, noradrenaline, serotonin and dopamine transporters. This implies that all of these neurotransmitter transporters may have evolved from a common ancestral gene that diverged into the GABA, glycine and catecholamine subfamilies at nearly the same time.

Mitochondrial ATP synthase subunit c storage in the ceroid-lipofuscinoses (Batten disease)

The ceroid-lipofuscinoses (Batten disease) are neurodegenerative inherited lysosomal storage diseases of children and animals. A common finding is the occurrence of fluorescent storage bodies (lipopigment) in cells. These have been isolated from tissues of affected sheep. Direct protein sequencing established that the major component is identical to the dicyclohexylcarbodiimide (DCCD) reactive proteolipid, subunit c, of mitochondrial ATP synthase and that this protein accounts for at least 50% of the storage body mass. No other mitochondrial components are stored. Direct sequencing of storage bodies isolated from tissues of children with juvenile and late infantile ceroid-lipofuscinosis established that they also contain large amounts of complete and normal subunit c. It is also stored in the disease in cattle and dogs but is not present in storage bodies from the human infantile form. Subunit c is normally found as part of the mitochondrial ATP synthase complex and accounts for 2-4% of the inner mitochondrial membrane protein. Mitochondria from affected sheep contain normal amounts of this protein. The P1 and P2 genes that code for it are normal as are mRNA levels. Oxidative phosphorylation is also normal. These findings suggest that ovine ceroid-lipofuscinosis is caused by a specific failure in the degradation of subunit c after its normal inclusion into mitochondria, and its consequent abnormal accumulation in lysosomes. This implies a unique pathway for subunit c degradation. It is probable that the human late infantile and juvenile diseases and the disease in cattle and dogs involve lesions in the same pathway.

Synaptic vesicle membrane proteins interact to form a multimeric complex

Potential interactions between membrane components of rat brain synaptic vesicles were analyzed by detergent solubilization followed by size fractionation or immunoprecipitation. The behavior of six synaptic vesicle membrane proteins as well as a plasma membrane protein was monitored by Western blotting. Solubilization of synaptic vesicle membranes in CHAPS resulted in the recovery of a large protein complex that included SV2, p65, p38, vesicle-associated membrane protein, and the vacuolar proton pump. Solubilization in octylglucoside resulted in the preservation of interactions between SV2, p38, and rab3A, while solubilization of synaptic vesicles with Triton X-100 resulted in two predominant interactions, one involving p65 and SV2, and the other involving p38 and vesicle-associated membrane protein. The multicomponent complex preserved with CHAPS solubilization was partially reconstituted following octylglucoside solubilization and subsequent dialysis against CHAPS. Reduction of the CHAPS concentration by gel filtration chromatography resulted in increased recovery of the multicomponent complex. Examination of the large complex isolated from CHAPS-solubilized vesicles by negative stain EM revealed structures with multiple globular domains, some of which were specifically labeled with gold-conjugated antibodies directed against p65 and SV2. The protein interactions defined in this report are likely to underlie aspects of neurotransmitter secretion, membrane traffic, and the spatial organization of vesicles within the nerve terminal.

Evidence for an association of the 15-kDa proteolipid of mediatophore with a 14-kDa polypeptide

The present report shows that mediatophore, a nerve terminal membrane protein that translocates acetylcholine on calcium action, forms a complex with a 14-kDa polypeptide. The complex was identified based on the following results. (a) A polyclonal antimediatophore antiserum that immunoprecipitates activity precipitates both the 15- and 14-kDa polypeptides. (b) After HPLC purification of mediatophore, both antigens were found in the same peak. (c) After 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate solubilization of presynaptic membranes or of the purified mediatophore, an immunoaffinity column made with the anti-14-kDa antigen monoclonal antibody retained both the 14-kDa and the 15-kDa polypeptide. Similarly, immunoprecipitation experiments using protein A-coated beads sedimented an immunocomplex in which both antigens were found. (d) The 14-kDa antigen could be localized in the synaptosomal membrane where mediatophore and its 15-kDa component are found.

Chemiosmotic systems in bioenergetics: H(+)-cycles and Na(+)-cycles

The development of membrane bioenergetic studies during the last 25 years has clearly demonstrated the validity of the Mitchellian chemiosmotic H+ cycle concept. The circulation of H+ ions was shown to couple respiration-dependent or light-dependent energy-releasing reactions to ATP formation and performance of other types of membrane-linked work in mitochondria, chloroplasts, some bacteria, tonoplasts, secretory granules and plant and fungal outer cell membranes. A concrete version of the direct chemiosmotic mechanism, in which H+ potential formation is a simple consequence of the chemistry of the energy-releasing reaction, is already proved for the photosynthetic reaction centre complexes. Recent progress in the studies on chemiosmotic systems has made it possible to extend the coupling-ion principle to an ion other than H+. It was found that, in certain bacteria, as well as in the outer membrane of the animal cell, Na+ effectively substitutes for H+ as the coupling ion (the chemiosmotic Na+ cycle). A precedent is set when the Na+ cycle appears to be the only mechanism of energy production in the bacterial cell. In the more typical case, however, the H+ and Na+ cycles coexist in one and the same membrane (bacteria) or in two different membranes of one and the same cell (animals). The sets of delta mu H+ and delta mu Na+ generators as well as delta mu H+ and delta mu Na+ consumers found in different types of biomembranes, are listed and discussed.

A genomic sequence of the Schizosaccharomyces pombe 16 kDa vacuolar H(+)-ATPase

We have isolated the gene encoding the 16 kDa vacuolar H(+)-ATPase from Schizosaccharomyces pombe. On the basis of RNA splicing signals and amino acid sequence homology with other 16 kDa H(+)-ATPases, the genomic DNA sequence indicated the 16 kDa protein is encoded by five exons. The C-terminal 50 amino acids has more than 90% homology with vacuolar H(+)-ATPases of mammalian cells.

Chromaffin granule H(+)-ATPase has F1-like structure

Isolated H(+)-ATPase from chromaffin granules was reconstituted into liposomes and the resultant proteoliposomes were further purified by Ficoll density gradient centrifugation. Studies by electron microscopy showed that proteoliposomes had particle structures (average diameter, about 10 nm) on their outer surface. These particles could be removed from the proteoliposomes by cold treatment. Immuno-electron microscopy showed that these particles were recognized by antibodies against the hydrophilic sector of the enzyme. These results indicate that the H(+)-ATPase has a peripheral membrane structure similar to that of F1-ATPase.

Bovine papillomavirus E5 oncoprotein binds to the 16K component of vacuolar H(+)-ATPases

The major transforming protein of bovine papillomavirus type 1, E5, is mainly associated with endomembranes, specifically binding to a cellular protein of relative molecular mass 16,000 (16K). At the same time as transformation, E5 causes the phosphorylation of tyrosine residues in epidermal and platelet-derived growth factor receptors. We show here that the 16K protein associated with E5 is the 16K component of vacuolar ATPases. This protein is known to be an integral membrane protein in endosomes, bovine chromaffin granules, synaptic vesicles, fungal and plant vacuoles and clathrin-coated vesicles, as well as a component of gap-junction-like membrane complexes. Because proton pumps are critical for the function of cellular compartments that process growth-factor receptors, the interaction of E5 with the 16K protein could explain the pleiomorphic features of cells transformed by E5.

CpG island in the region of an autosomal dominant polycystic kidney disease locus defines the 5' end of a gene encoding a putative proton channel

In an attempt to isolate candidate genes for autosomal dominant polycystic kidney disease, a number of CpG-rich islands have been identified from a region defined genetically as the site of disease mutations. Genomic fragments adjacent to one of these islands were used to isolate cDNAs from both HeLa cells and cultured cystic epithelium that encode a 155-amino acid peptide having four putative transmembrane domains. The corresponding transcript was found in all tissues tested but was most abundant in brain and kidney. Potential control response elements were identified in the genomic region 5' to the initiation codon. The deduced amino acid sequence has 93% similarity to the 16-kDa proteolipid component that is believed to be part of the proton channel of the vacuolar H(+)-ATPase. Possible roles for a mutated proton channel in the pathogenesis of cystic disease were considered. However, sequencing of cDNAs corresponding to both alleles of an affected individual revealed no differences in the deduced amino acid sequence. Moreover, transcript size and abundance were not altered in cystic kidney.

Detection with monoclonal antibodies of a 15-kDa proteolipid in both presynaptic plasma membranes and synaptic vesicles in Torpedo electric organ

A protein, the mediatophore, has been purified from Torpedo electric organ presynaptic plasma membranes. This protein mediates the release of acetylcholine through artificial membranes when activated by calcium and is made up of 15-kDa proteolipid subunits. After immunization with purified delipidated mediatophore, monoclonal antibodies binding to the 15-kDa proteolipid band on Western blots of purified mediatophore were selected. A 15-kDa proteolipid antigen was also detected in cholinergic synaptic vesicles. Using an immunological assay, it was estimated that presynaptic plasma membranes and synaptic vesicles contain similar proportions of 15-kDa proteolipid antigen. Detection by immunofluorescence in the electric organ showed that only nerve endings were labeled. In electric lobes, the staining was associated with intracellular membranes of the electroneuron cell bodies and in axons. Nerve endings at Torpedo neuromuscular junctions were also labeled with anti-15-kDa proteolipid monoclonal antibodies.

Mutational analysis of yeast vacuolar H(+)-ATPase

Yeast mutants in which genes encoding subunits of the vacuolar H(+)-ATPase were interrupted were assayed for their vacuolar ATPase and proton-uptake activities. The vacuoles from the mutants lacking subunits A (72 kDa), B (57 kDa), or c (proteolipid, 16 kDa) were completely inactive in these reactions. Immunological studies revealed that in the absence of each one of those subunits the catalytic sector was not assembled. Labeling with N,N'-[14C]dicyclohexylcarbodiimide showed the presence of the proteolipid in vacuoles of mutants in which genes encoding subunits of the catalytic sectors were interrupted. No labeling was detected in the mutant in which the gene encoding the proteolipid was interrupted. We conclude that of all the ATPase subunits only the proteolipid is assembled independently and it serves as a template for the assembly of the other subunits. Site-specific mutations were generated in the gene encoding the proteolipid. All of the drastic changes and replacements gave inactive proteins. About half of the single amino acid replacements gave active proteins. Replacing glutamic acid-137 by any of several amino acids, except for aspartic acid, abolished the activity of the enzyme. Other amino acids that may function in proton conductance were changed. It was found that glycine residues may replace amino acids with exchangeable protons.

Transcriptional and post-transcriptional regulation of tyrosine hydroxylase gene by protein kinase C

The role played by protein kinase C (PKC) in TH gene regulation was investigated at transcriptional and post-transcriptional levels using PC12 cells. The cells were treated with the phorbol ester TPA, which not only activates PKC but also causes down-regulation. PKC levels were monitored by [3H]PDBU binding assay and by using an anti-PKC antibody that detected intact PKC (79 kd) as well as its catalytic and regulatory domains. The [3H]PDBU binding to the membrane-associated PKC increased within 15-30 min of TPA treatment; thereafter total cellular [3H]PDBU binding decreased to a minimum of 20% of the control at 8 h. The rate of decrease in binding was greater than the decrease in the intensity of the staining of PKC holo enzyme visualized by anti-PKC antibody. TH mRNA levels, measured over the same time period, rose within 15 min of TPA treatment to peak at 4 h and subsequently declined below control level, paralleling the depletion of PKC. If cells depleted of PKC were reincubated in the normal medium, a recovery in PKC level was seen and, in parallel, TH mRNA levels increased to above control level. Furthermore, if down-regulation of PKC was prevented by incubating the cells with the protease inhibitor leupeptin, a decrease beyond control level in TH mRNA was not observed. TPA rapidly induced TH gene transcription; a maximal increase of two-fold was observed at 15 min, but the transcriptional rate then declined although it did not decrease beyond control values after 8 and 24 h of TPA treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid transport by intracellular vesicles

Many intracellular organelles contain a unique primary, electrogenic proton pump termed the vacuolar H(+)-ATPase. This pump, found in many endocytic, secretory, and storage vesicles in fungal, plant and animal cells, functions, in conjunction with a chloride conductance, to acidify the vesicle interior. Although remotely related to the mitochondrial ATP synthase, the vacuolar H(+)-ATPase is a distinct pump which differs in inhibitor sensitivity, subunit composition and function. The vacuolar H(+)-ATPase transports only protons, and permeable anions (chloride) are required for optimal vesicle acidification. Allosteric and regulatory effects are not yet fully understood. Vesicle acidification appears to be essential for receptor-mediated endocytosis, protein synthesis, and secretion and storage of small solutes such as neurotransmitters. A similar plasma membrane-located H(+)-ATPase may contribute to urinary acidification and cell pH regulation.

Partial purification and characterization of the vacuolar H(+)-ATPase of mammalian synaptic vesicles

Several major proteins of synaptic vesicles from rat or cow brain sediment as a large complex on sucrose density gradients when solubilized in nonionic detergents. A vacuolar H(+)-ATPase identified by sensitivity to bafilomycin A1 appears to be associated with this oligomeric protein complex. Two subunits of this complex, synaptic vesicle proteins S and U, correspond to the 57-kDa (B) and 39-kDa accessory (Ac39) subunits, respectively, of bovine chromaffin granule vacuolar H(+)-ATPase as shown by Western immunoblot analysis. The five subunits of the oligomeric complex constitute approximately 20% of the total protein of rat brain synaptic vesicles. Taken together, these results strongly suggest that the abundant, multisubunit complex partially purified from brain synaptic vesicles by density gradient centrifugation is a vacuolar H(+)-ATPase. Bafilomycin A1 completely blocks proton pumping in rat brain synaptic vesicles as measured by [14C]methylamine uptake and also blocks catecholamine accumulation measured by [3H]dopamine uptake. Moreover, ATPase activity, [14C]methylamine uptake, and [3H]dopamine uptake are inhibited by bafilomycin A1 at similar I50 values of approximately 1.7 nmol/mg of protein. These findings indicate that the vacuolar H(+)-ATPase is essential for proton pumping as well as catecholamine uptake by mammalian synaptic vesicles.

Enhanced H Transport Capacity and ATP Hydrolysis Activity of the Tonoplast H-ATPase after NaCl Adaptation

Tonoplast enriched membrane vesicle fractions were isolated from unadapted and NaCl (428 millimolar) adapted tobacco cells (Nicotiana tabacum L. var Wisconsin 38). Polypeptides from the tonoplast enriched vesicle fractions were separated by SDS-PAGE and analyzed by Western blots using polyclonal antibodies to the 70 kilodalton subunit of the red beet tonoplast H(+)-ATPase. These antibodies cross-reacted exclusively to a tobacco polypeptide of an apparent molecular weight of 69 kilodaltons. The antibodies inhibited ATP-dependent, NO(3) (-) sensitive H(+) transport into vesicles in tonoplast enriched membrane fractions from both unadapted and NaCl adapted cells. The relative H(+) transport capacity per unit of 69 kilodalton subunit of the tonoplast ATPase of vesicles from NaCl adapted cells was fourfold greater than that observed for vesicles from unadapted cells. The increase in specific H(+) transport capacity after adaptation was also observed for ATP hydrolysis.

Cloning and expression of a rat brain GABA transporter

A complementary DNA clone (designated GAT-1) encoding a transporter for the neurotransmitter gamma-aminobutyric acid (GABA) has been isolated from rat brain, and its functional properties have been examined in Xenopus oocytes. Oocytes injected with GAT-1 synthetic messenger RNA accumulated [3H]GABA to levels above control values. The transporter encoded by GAT-1 has a high affinity for GABA, is sodium-and chloride-dependent, and is pharmacologically similar to neuronal GABA transporters. The GAT-1 protein shares antigenic determinants with a native rat brain GABA transporter. The nucleotide sequence of GAT-1 predicts a protein of 599 amino acids with a molecular weight of 67 kilodaltons. Hydropathy analysis of the deduced protein suggests multiple transmembrane regions, a feature shared by several cloned transporters; however, database searches indicate that GAT-1 is not homologous to any previously identified proteins. Therefore, GAT-1 appears to be a member of a previously uncharacterized family of transport molecules.

Cloning of the human brain GABA transporter

A cDNA clone encoding a transporter for the neurotransmitter gamma-aminobutyric acid in human brain was cloned and sequenced. The cDNA contains an open reading frame encoding a hydrophobic protein of 599 amino acids with a calculated molecular weight of 67022 Da. Hydropathy analysis revealed twelve potential transmembrane segments. The human protein is highly homologous to the protein from rat brain. Northern hybridization demonstrated a ubiquitous distribution of the transporter in various parts of the brain.

Expression of a proteolipid gene from a high-copy-number plasmid confers trifluoperazine resistance to Saccharomyces cerevisiae

A wild-type haploid yeast strain was transformed with a library of wild-type yeast DNA fragments ligated into a high-copy-number plasmid vector (YEp24). The pooled URA+ transformants were plated on rich medium containing a lethal concentration of trifluoperazine (TFP). Plasmids rescued into Escherichia coli from TFP-resistant yeast colonies contained overlapping DNA fragments from a unique region of yeast chromosome XVI. Deletion and disruption experiments, mini-Tn10 LUK hop analysis, and DNA sequencing defined a novel gene with significant amino acid identity to bovine and yeast vacuoletype proteolipid subunits. This is the second locus identified that can be altered to confer TFP resistance to Saccharomyces cerevisiae and that has significant amino acid identity to a vacuolar ATPase subunit. This suggests that a target for TFP in S. cerevisiae is the electrogenic membranes of the vacuolar network and that alteration of expression or activity of vacuolar proton ATPase subunits is a general mechanism for TFP resistance in this yeast.

Expression of a proteolipid gene from a high-copy-number plasmid confers trifluoperazine resistance to Saccharomyces cerevisiae

A wild-type haploid yeast strain was transformed with a library of wild-type yeast DNA fragments ligated into a high-copy-number plasmid vector (YEp24). The pooled URA+ transformants were plated on rich medium containing a lethal concentration of trifluoperazine (TFP). Plasmids rescued into Escherichia coli from TFP-resistant yeast colonies contained overlapping DNA fragments from a unique region of yeast chromosome XVI. Deletion and disruption experiments, mini-Tn10 LUK hop analysis, and DNA sequencing defined a novel gene with significant amino acid identity to bovine and yeast vacuoletype proteolipid subunits. This is the second locus identified that can be altered to confer TFP resistance to Saccharomyces cerevisiae and that has significant amino acid identity to a vacuolar ATPase subunit. This suggests that a target for TFP in S. cerevisiae is the electrogenic membranes of the vacuolar network and that alteration of expression or activity of vacuolar proton ATPase subunits is a general mechanism for TFP resistance in this yeast.

The sequence of the major protein stored in ovine ceroid lipofuscinosis is identical with that of the dicyclohexylcarbodiimide-reactive proteolipid of mitochondrial ATP synthase

The ceroid lipofuscinoses are a group of neurodegenerative lysosomal storage diseases of children and animals that are recessively inherited. In diseased individuals fluorescent storage bodies accumulate in a wide variety of cells, including neurons. Previous studies of these bodies isolated from tissues of affected sheep confirmed that the storage occurs in lysosomes, and showed that the storage body is mostly made of a single protein with an apparent molecular mass of 3500 Da with an N-terminal amino acid sequence that is the same as residues 1-40 of the c-subunit (or dicyclohexylcarbodi-imide-reactive proteolipid) of mitochondrial ATP synthase. In the present work we have shown by direct analysis that the stored protein is identical in sequence with the entire c-subunit of mitochondrial ATP synthase, a very hydrophobic protein of 75 amino acid residues. As far as can be detected by the Edman degradation, the stored protein appears not to have been subject to any post-translational modification other than the correct removal of the mitochondrial import sequences that have been shown in other experiments to be present at the N-terminal of its two different precursors. No other protein accumulates in the storage bodies to any significant extent. Taken with studies of the cDNAs for the c-subunit in normal and diseased sheep, these results indicate that the material that is stored in lysosomes of diseased animals has probably entered mitochondria and has been subjected to the proteolytic processing that is associated with mitochondrial import. This implies that the defect that leads to the lysosomal accumulation concerns the degradative pathway of the c-subunit of ATP synthase. An alternative, but less likely, hypothesis is that for some unknown reason the precursors of subunit c are being directly mis-targeted to lysosomes, where they become processed to yield a protein identical with the protein that is normally found in the mitochondrial ATP synthase assembly, and which then accumulates.

Anion-exchange mechanisms in bacteria

This article discusses the physiological, biochemical, and molecular properties of bacterial anion-exchange reactions, with a particular focus on a family of phosphate (Pi)-linked antiporters that accept as their primary substrates sugar phosphates such as glucose 6-phosphate (G6P), mannose 6-phosphate, or glycerol 3-phosphate. Pi-linked antiporters may be found in both gram-positive and gram-negative cells. As their name suggests, these exchange proteins accept both inorganic and organic phosphates, but the two classes of substrate interact very differently with the protein. Thus, Pi is always accepted with a relatively low affinity, and when it participates in exchange, it is always taken as the monovalent anion. By contrast, when the high-affinity organic phosphates are used, these same systems fail to discriminate between monovalent and divalent forms. Tests of heterologous exchange (e.g., Pi: G6P) indicate that these proteins have a bifunctional active site that accepts a pair of negative charges, whether as two monovalent anions or as a single divalent anion. For this reason, exchange stoichiometry moves between limits of 2:1 and 2:2, according to the ratio of mono- and divalent substrates at either membrane surface. Since G6P has a pK2 within the physiological range (pK of 6.1), this predicts a novel reaction sequence in vivo because internal pH is more alkaline than external pH. Accordingly, one expects an asymmetric exchange as two monovalent G6P anions from the relatively acidic exterior move against a single divalent G6P from the alkaline interior. In this way an otherwise futile self-exchange of G6P can be biased towards a net inward flux driven (indirectly) by the pH gradient. Despite the biochemical complexity exhibited by Pi-linked antiporters, they resemble all other secondary carriers at a molecular level and show a likely topology in which two sets of six transmembrane alpha-helices are connected by a central hydrophilic loop. Speculations on the derivation of this common form suggest a limited number of structural models to accommodate such proteins. Three such models are presented.

A 15 kDa proteolipid found in mediatophore preparations from Torpedo electric organ presents high sequence homology with the bovine chromaffin granule protonophore

Upon SDS PAGE of isolated mediatophore, an acetylcholine-translocating protein, a doublet at 15 kDa was identified. Amino acid sequencing after CNBr cleavage gave a 17 residue-long peptide completely homologous with a sequence of the proton-translocating proteolipid from bovine chromaffin granules. A 51-mer oligodeoxynucleotide corresponding to this sequence was used to screen a library of electric lobe cDNAs constructed in lambda Zap II. A positive recombinant clone was isolated and found to encode the complete sequence of a 15.5 kDa protein highly homologous to the bovine chromaffin or yeast vacuolar ATPase proteolipid. In vitro translation of sense RNA transcripts of the clone indeed yielded a single 15 kDa proteolipid. Northern blot analysis showed that the 1.3 kb mRNA encoding this protein is significantly expressed in nervous tissues but not in electric organ or liver of Torpedo marmorata.

Chemiosmotic energy conversion and the membrane ATPase of Methanolobus tindarius

Electron transport phosphorylation has been demonstrated to drive ATP synthesis for the methanogenic archaebacterium Methanolobus tindarius: Protonophores evoked uncoupler effects and lowered the membrane potential delta psi. Under the influence of N,N'-dicyclohexylcarbodiimide [(cHxN)2C] the membrane potential increased while methanol turnover was inhibited. 2-Bromoethanesulfonate, an inhibitor of methanogenesis, had no effect on the membrane potential but, like (cHxN)2C and protonophores, decreased the intracellular ATP concentration. Labeling experiments with (cHxN)2(14)C showed membranes to contain a proteolipid, with a molecular mass of 5.5 kDa, that resembles known (cHxN)2C-binding proteins of F0-F1 ATPases. The (cHxN)2-sensitive membrane ATPase hydrolysed Mg.ATP at a pH optimum of 5.0 with a Km (ATP) of 2.5 mM (V = 77 mU/mg). It was inhibited competitively by ADP; Ki (ADP) = 0.65 mM. Azide or vanadate caused no significant loss in ATPase activity, but millimolar concentrations of nitrate showed an inhibitory effect, suggesting a relationship to ATPases from vacuolar membranes. In contrast, no inhibition occurred in the presence of bafilomycin A1. The ATPase was extractable with EDTA at low salt concentrations. The purified enzyme consists of four different subunits, alpha (67 kDa), beta (52 kDa), gamma (20 kDa) and beta (less than 10 kDa), as determined from SDS gel electrophoresis.

Gene duplication as a means for altering H+/ATP ratios during the evolution of FoF1 ATPases and synthases

In the evolution of the FoF1 family of proton-translocating membrane complexes, two reversals in function appear to have occurred, first changing it from an ATPase to an ATP synthase and then back again to an ATPase. Here we suggest that with each change in function, the ratio of protons transported per ATP hydrolyzed or synthesized (H+/ATP) was altered in order for the complex to better adapt to its new role. We propose that this was accomplished by gene duplication with partial loss in the number of functional catalytic sites (to increase H+/ATP) or functional proton channels (to decrease H+/ATP). This method of changing the H+/ATP ratio preserved overall structural features of the complex essential to energy coupling.

Chemiosmotic energy conversion of the archaebacterial thermoacidophile Sulfolobus acidocaldarius: oxidative phosphorylation and the presence of an F0-related N,N'-dicyclohexylcarbodiimide-binding proteolipid

The energy-transducing mechanism of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius DSM 639 has been studied, addressing the question whether chemiosmotic proton gradients serve as an intermediate energy store driving an F0F1-analogous ATP synthase. At pH 3.5, respiring S. acidocaldarius cells developed an electrochemical potential of H+ ions, consisting mainly of a proton gradient and a small inside-negative membrane potential. The steady-state proton motive force of 140 to 160 mV was collapsed by protonophores, while N,N'-dicyclohexylcarbodiimide (DCCD) caused a hyperpolarization of the membrane, as expected for a reagent commonly used to inhibit the flux through proton channels of F0F1-type ATP synthases. Cellular ATP content was strongly related to the proton motive force generated by respiration and declined rapidly, either by uncoupling or by action of DCCD, which in turn induced a marked respiratory control effect. This observation strongly supports the operation of chemiosmotic ATP synthesis with H+ as the coupling ion. The inhibition of ATP synthesis by [14C]DCCD was correlated with covalent reactions with membrane proteins. The extraction of labeled membranes with organic solvents specifically yielded a readily aggregating proteolipid of 6 to 7 kilodaltons apparent molecular mass. Its amino acid composition revealed significant similarity to the proteolipid found in eubacteria, such as Escherichia coli, as an extremely hydrophobic constituent of the F0 proton channel. Moreover, the N-terminal amino acid sequence of the Sulfolobus proteolipid displays a high degree of homology to eubacterial sequences, as well as to one derived from nucleic acid sequencing of another Sulfolobus strain (K. Denda, J. Konishi, T. Oshima, T. Date, and M. Yoshida, J. Biol. Chem. 264:7119-7121, 1989). Despite certain structural similarities between eucaryotic vacuolar ATPases and the F1-analogous ATPase from Sulfolobus sp. described earlier, the results reported here promote the view that the archaebacterial ATP-synthesizing complex functionally belongs to the F0F1 class of ATPases. These may be considered as phylogenetically conserved catalysts of energy transduction present in all kingdoms of organisms.

Structure, molecular genetics, and evolution of vacuolar H+-ATPases

Proton-ATPases can be divided into three classes denoted as P-, F-, and V-ATPases. The P-ATPases are evolutionarily distinct from the F- and V-type ATPases which have been shown to be related, probably evolved from a common ancestral enzyme. Like F-ATPases, V-ATPases are composed of two distinct structures: a catalytic sector that is hydrophilic in nature and a hydrophobic membrane sector which functions in proton conduction. Recent studies on the molecular biology of vacuolar H+-ATPases revealed surprising findings about the evolution of pronon pumps as well as important clues for the evolution of eukaryotic cells.

Topography of a vacuolar-type H+-translocating ATPase: chromaffin-granule membrane ATPase I

Proteins exposed on the cytoplasmic face of isolated chromaffin granules were labelled by lactoperoxidase-catalysed radioiodination and by non-enzymic biotinylation. Granule membranes were then prepared, and the H+-translocating ATPase isolated by fractionation with Triton X-114. The labelling of individual ATPase subunits was assessed by polyacrylamide-gel electrophoresis, followed by autoradiography or by blotting and decoration with 125I-labelled streptavidin. Subunits of 72, 57 and kDa were strongly labelled, and could be removed from the membrane at pH 11: they are therefore extrinsic proteins. The 120 kDa subunit was also labelled, but it was not solubilized at pH 11. Photolabelling with a hydrophobic probe indicated that this subunit penetrates the bilayer, and enzymic degradation studies showed the presence of N-linked oligosaccharides; this subunit therefore spans the chromaffin-granule membrane. Labelling of the 17 kDa subunit occurred predominantly on the extracytoplasmic (matrix) face of the granule membrane. These results are consistent with this V-type ATPase having a structure that is generally similar to that of mitochondrial (F-type) ATPases, although the attachment of the 120 kDa subunit may be asymmetrical.

Vacuolar proton pumps

Recently a new class of proton-translocating ATPases has been localized to endomembrane compartments in plant, fungal, and mammalian cells. These proton pumps are large hetero-oligomers which have an ATP hydrolytic sector that is functionally and structurally distinct from a transmembranous proton pore. Enzymatic characteristics of these proton pumps are discussed as well as the current state of knowledge regarding subunit composition and function. In addition, recent primary sequence data are discussed which indicate that these proton pumps share a common ancestor with F1F0-type proton pumps of mitochondria.

Structure and function of the yeast vacuolar membrane proton ATPase

Our current work on a vacuolar membrane proton ATPase in the yeast Saccharomyces cerevisiae has revealed that it is a third type of H+-translocating ATPase in the organism. A three-subunit ATPase, which has been purified to near homogeneity from vacuolar membrane vesicles, shares with the native, membrane-bound enzyme common enzymological properties of substrate specificities and inhibitor sensitivities and are clearly distinct from two established types of proton ATPase, the mitochondrial F0F1-type ATP synthase and the plasma membrane E1E2-type H+-ATPase. The vacuolar membrane H+-ATPase is composed of three major subunits, subunit a (Mr = 67 kDa), b (57 kDa), and c (20 kDa). Subunit a is the catalytic site and subunit c functions as a channel for proton translocation in the enzyme complex. The function of subunit b has not yet been identified. The functional molecular masses of the H+-ATPase under two kinetic conditions have been determined to be 0.9-1.1 x 10(5) daltons for single-cycle hydrolysis of ATP and 4.1-5.3 x 10(5) daltons for multicycle hydrolysis of ATP, respectively. N,N'-Dicyclohexyl-carbodiimide2 does not inhibit the former reaction but strongly inhibits the latter reaction. The kinetics of single-cycle hydrolysis of ATP indicates the formation of an enzyme-ATP complex and subsequent hydrolysis of the bound ATP to ADP and Pi at a 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole-sensitive catalytic site. Cloning of structural genes for the three subunits of the H+-ATPase (VMA1, VMA2, and VMA3) and their nucleotide sequence determination have been accomplished, which provide greater advantages for molecular biological studies on the structure-function relationship and biogenesis of the enzyme complex. Bioenergetic aspects of the vacuole as a main, acidic compartment ensuring ionic homeostasis in the cytosol have been described.

A 16 kDa protein co-isolating with gap junctions from brain tissue belonging to the class of proteolipids of the vacuolar H+-ATPases

A 16 kDa protein from an enriched gap junction preparation was isolated from bovine brain tissues. N-terminal amino acid microsequencing of the first 20 amino acids showed a complete homology with a recently published sequence of a proteolipid from a vacuolar H+-ATPase from chromaffin granules. Incubation of the brain gap junction preparation with 14C-N,N'-dicyclohexylcarbodiimide showed a significant binding of this compound to the 16 kDa protein, indicating that a proton binding site also occurs within that particular protein. The data suggest that this 16 kDa protein, which has also been described in gap junction preparations from various other tissues, belongs to the proton transporting ATPase.

Human endomembrane H+ pump strongly resembles the ATP-synthetase of Archaebacteria

Preparations of mammalian H+ pumps that acidify intracellular vesicles contain eight or nine polypeptides, ranging in size from 116 to 17 kDa. Biochemical analysis indicates that the 70- and 58-kDa polypeptides are subunits critical for ATP hydrolysis. The amino acid sequences of the major catalytic subunits (58 and 70 kDa) of the endomembrane H+ pump are unknown from animal cells. We report here the complete sequence of the 58-kDa subunit derived from a human kidney cDNA clone and partial sequences of the 70- and 58-kDa subunits purified from clathrin-coated vesicles of bovine brain. The amino acid sequences of both proteins strongly resemble the sequences of the corresponding subunits of the vacuolar H+ pumps of Archaebacteria, plants, and fungi. The archaebacterial enzyme is believed to use a H+ gradient to synthesize ATP. Thus, a common ancestral protein has given rise to a H+ pump that synthesizes ATP in one organism and hydrolyzes it in another and is highly conserved from prokaryotes to humans. The same pump appears to mediate the acidification of intracellular organelles, including coated vesicles, lysosomes, and secretory granules, as well as extracellular fluids such as urine.

The progenitor of ATP synthases was closely related to the current vacuolar H+-ATPase

The gene encoding the proteolipid of the vacuolar H+-ATPase of yeast was cloned and sequenced. The deduced amino acid sequence of the yeast protein is highly homologous to that of the proteolipid from bovine chromaffin granules. In contrast to other membrane proteins the transmembrane segments of the bovine and yeast proteolipids were much more conserved than the hydrophilic parts. The fourth transmembrane segment, which contains the DCCD-binding site, was conserved 100%. Comparison of vacuolar and eubacterial proteolipids revealed a homology which pointed to a common ancestral gene that underwent gene duplication to form the vacuolar proteolipids. Additional support for this notion came from the amino acid sequences of subunits involved in the catalytic sectors of archaebacterial ATP synthase and plant and yeast vacuolar H+-ATPases, which reveal extensive sequence homology. Slight, but significant, homology between the archaebacterial and eubacterial ATP synthases was observed. These observations might suggest that the progenitor of ATP synthases was closely related to the present vacuolar H+-ATPases.