The vacuolar H+-ATPase: a universal proton pump of eukaryotes

Genomic organization of the gene coding for TIRC7, a novel membrane protein essential for T cell activation

A novel human membrane protein, TIRC7, was recently identified and demonstrated to be essential in T cell activation. Here we report on the genomic organization of the TIRC7 gene, which is composed of 15 exons and spans 7.9 kb. The seven predicted transmembrane-spanning domains of the TIRC7 protein coincide well with exon-intron boundaries. TIRC7 and OC116, a recently described putative subunit of the vacuolar proton pump that was demonstrated to be expressed in an osteoclastoma tumor as well as in a human pancreatic adenocarcinoma cell line, are demonstrated to be alternative transcripts of the same gene. OC116 consists of 20 exons with the last 14 introns and exons being identical with those of TIRC7. The chromosomal locus for both transcripts was identified on chromosome 11q13.4-q13.5. In human alloactivated T lymphocytes, mRNA expression of TIRC7, but not OC116, is demonstrated, indicating that OC116 is not involved in regular T cell proliferation.

Vesicular ATPase-overexpressing cells determine the distribution of malaria parasite oocysts on the midguts of mosquitoes

In Plasmodium-infected mosquitoes, oocysts are preferentially located at the posterior half of the posterior midgut. Because mosquitoes rest vertically after feeding, the effect of gravity on the ingested blood has been proposed as the cause of such a biased distribution. In this paper, we examined the oocyst distribution on the midguts of mosquitoes that were continuously rotated to nullify the effect of gravity and found that the typical pattern of oocyst distribution did not change. Invasion of the midgut epithelium by ookinetes was similarly found to be biased toward the posterior part of the posterior midgut. We examined whether the distribution of oocysts depends on the distribution of vesicular ATPase (V-ATPase)-overexpressing cells that Plasmodium ookinetes preferentially use to cross the midgut epithelium. An antiserum raised against recombinant Aedes aegypti V-ATPase B subunit indicated that the majority of V-ATPase-overexpressing cells in Ae. aegypti and Anopheles gambiae are localized at the posterior part of the posterior midgut. We propose that the typical distribution of oocysts on the mosquito midgut is attributable to the presence and the spatial distribution of the V-ATPase-overexpressing cells in the midgut epithelium.

A 29 kDa intracellular chloride channel p64H1 is associated with large dense-core vesicles in rat hippocampal neurons

A novel class of intracellular chloride channels, the p64 family, has been found on several types of vesicles. These channels, acting in concert with the electrogenic proton pump, regulate the pH of the vesicle interior, which is critical for vesicular function. Here we describe the molecular cloning of p64H1, a p64 homolog, from both human and cow. Northern blot analysis showed that p64H1 is expressed abundantly in brain and retina, whereas the other members of this family (e.g., p64 and NCC27) are expressed only at low levels in these tissues. Immunohistochemical analysis of p64H1 in rat brain, using an affinity-purified antibody, revealed a high level of expression in the limbic system-the hippocampal formation, the amygdala, the hypothalamus, and the septum. Immunoelectron microscopic analysis of p64H1 in hippocampal neurons demonstrated a striking association between p64H1 and large dense-core vesicles (LDCVs) and microtubules. In contrast, very low p64H1 labeling was found in perikarya or associated with small synaptic vesicles (SSVs) in axonal profiles. Immunoblot analysis confirmed that p64H1 is colocalized with heavy membrane fractions containing LDCVs rather than the fractions containing SSVs. These results suggest that p64H1-mediated Cl- permeability may be involved in the maintenance of low internal pH in LDCVs and in the maturation of LDCVs and the biogenesis of functional neuropeptides.

The vacuolar H+-ATPase of lemon fruits is regulated by variable H+/ATP coupling and slip

Lemon fruit tonoplasts, unlike those of seedling epicotyls, contain nitrate-insensitive H+-ATPase activity (Müller, M. L., Irkens-Kiesecker, U., Rubinstein, B., and Taiz, L. (1996) J. Biol. Chem. 271, 1916-1924). However, the degree of nitrate-insensitivity fluctuates during the course of the year with a seasonal frequency. Nitrate uncouples H+ pumping from ATP hydrolysis both in epicotyls and in nitrate-sensitive fruit V-ATPases. Neither bafilomycin nor oxidation cause uncoupling. The initial rate H+/ATP coupling ratios of epicotyl and the nitrate-sensitive fruit proton pumping activities are the same. However, the H+/ATP coupling ratio of the nitrate-insensitive fruit H+ pumping activity is lower than that of nitrate-sensitive and epicotyl V-ATPases. Several properties of the nitrate-insensitive H+-ATPase of the fruit indicate that it is a modified V-ATPase rather than a P-ATPase: 1) insensitivity to low concentrations of vanadate; 2) it is initially strongly uncoupled by nitrate, but regains coupling as catalysis proceeds; 3) both the nitrate-sensitive and nitrate-insensitive fruit H+-pumps have identical Km values for MgATP, and show similar pH-dependent slip and proton leakage rates. We conclude that the ability of the juice sac V-ATPase to build up steep pH gradients involves three factors: variable coupling, i.e. the ability to regain coupling under conditions that initially induce uncoupling; a low pH-dependent slip rate; the low proton permeability of the membrane.

A 29 kDa intracellular chloride channel p64H1 is associated with large dense-core vesicles in rat hippocampal neurons

A novel class of intracellular chloride channels, the p64 family, has been found on several types of vesicles. These channels, acting in concert with the electrogenic proton pump, regulate the pH of the vesicle interior, which is critical for vesicular function. Here we describe the molecular cloning of p64H1, a p64 homolog, from both human and cow. Northern blot analysis showed that p64H1 is expressed abundantly in brain and retina, whereas the other members of this family (e.g., p64 and NCC27) are expressed only at low levels in these tissues. Immunohistochemical analysis of p64H1 in rat brain, using an affinity-purified antibody, revealed a high level of expression in the limbic system-the hippocampal formation, the amygdala, the hypothalamus, and the septum. Immunoelectron microscopic analysis of p64H1 in hippocampal neurons demonstrated a striking association between p64H1 and large dense-core vesicles (LDCVs) and microtubules. In contrast, very low p64H1 labeling was found in perikarya or associated with small synaptic vesicles (SSVs) in axonal profiles. Immunoblot analysis confirmed that p64H1 is colocalized with heavy membrane fractions containing LDCVs rather than the fractions containing SSVs. These results suggest that p64H1-mediated Cl- permeability may be involved in the maintenance of low internal pH in LDCVs and in the maturation of LDCVs and the biogenesis of functional neuropeptides.

Physiological consequence of disruption of the VMA1 gene in the riboflavin overproducer Ashbya gossypii

The vacuolar ATPase subunit A structural gene VMA1 of the biotechnologically important riboflavin overproducer Ashbya gossypii was cloned and disrupted to prevent riboflavin retention in the vacuolar compartment and to redirect the riboflavin flux into the medium. Cloning was achieved by polymerase chain reaction using oligonucleotide primers derived form conserved sequences of the Vma1 proteins from yeast and filamentous fungi. The deduced polypeptide comprises 617 amino acids with a calculated molecular mass of 67.8 kDa. The deduced amino acid sequence is highly similar to that of the catalytic subunits of Saccharomyces cerevisiae (67 kDa), Candida tropicalis (67 kDa), and Neurospora crassa (67 kDa) with 89, 87, and 60% identity, respectively, and shows about 25% identity to the beta-subunit of the FoF1-ATPase of S. cerevisiae and Schizosaccharomyces pombe. In contrast to S. cerevisiae, however, where disruption of the VMA1 gene was conditionally lethal, and to N. crassa, where viable disruptants could not be isolated, disruption of the VMA1 gene in A. gossypii did not cause a lethal phenotype. Disruption of the AgVMA1 gene led to complete excretion of riboflavin into the medium instead of retention in the vacuolar compartment, as observed in the wild type.

Selective inhibitors of the osteoclast vacuolar proton ATPase as novel bone antiresorptive agents

The proton ATPase located on the apical membrane of the osteoclast is essential to the bone resorption process. This proton pump is, therefore, an attractive molecular target for the design of novel inhibitors of bone resorption, and potentially useful for the treatment of osteoporosis and related metabolic diseases of bone. Recently, several inhibitors with different degrees of selectivity for the osteoclast V-ATPase have been reported. In particular, systematic chemical modifications of the macrolide antibiotic bafilomycin A1 have identified the minimal structural requirements for activity and allowed the design of simplified analogues that demonstrate high potency and selectivity for the osteoclast enzyme.

Vacuolar and plasma membrane proton-adenosinetriphosphatases

The vacuolar H+-ATPase (V-ATPase) is one of the most fundamental enzymes in nature. It functions in almost every eukaryotic cell and energizes a wide variety of organelles and membranes. V-ATPases have similar structure and mechanism of action with F-ATPase and several of their subunits evolved from common ancestors. In eukaryotic cells, F-ATPases are confined to the semi-autonomous organelles, chloroplasts, and mitochondria, which contain their own genes that encode some of the F-ATPase subunits. In contrast to F-ATPases, whose primary function in eukaryotic cells is to form ATP at the expense of the proton-motive force (pmf), V-ATPases function exclusively as ATP-dependent proton pumps. The pmf generated by V-ATPases in organelles and membranes of eukaryotic cells is utilized as a driving force for numerous secondary transport processes. The mechanistic and structural relations between the two enzymes prompted us to suggest similar functional units in V-ATPase as was proposed to F-ATPase and to assign some of the V-ATPase subunit to one of four parts of a mechanochemical machine: a catalytic unit, a shaft, a hook, and a proton turbine. It was the yeast genetics that allowed the identification of special properties of individual subunits and the discovery of factors that are involved in the enzyme biogenesis and assembly. The V-ATPases play a major role as energizers of animal plasma membranes, especially apical plasma membranes of epithelial cells. This role was first recognized in plasma membranes of lepidopteran midgut and vertebrate kidney. The list of animals with plasma membranes that are energized by V-ATPases now includes members of most, if not all, animal phyla. This includes the classical Na+ absorption by frog skin, male fertility through acidification of the sperm acrosome and the male reproductive tract, bone resorption by mammalian osteoclasts, and regulation of eye pressure. V-ATPase may function in Na+ uptake by trout gills and energizes water secretion by contractile vacuoles in Dictyostelium. V-ATPase was first detected in organelles connected with the vacuolar system. It is the main if not the only primary energy source for numerous transport systems in these organelles. The driving force for the accumulation of neurotransmitters into synaptic vesicles is pmf generated by V-ATPase. The acidification of lysosomes, which are required for the proper function of most of their enzymes, is provided by V-ATPase. The enzyme is also vital for the proper function of endosomes and the Golgi apparatus. In contrast to yeast vacuoles that maintain an internal pH of approximately 5.5, it is believed that the vacuoles of lemon fruit may have a pH as low as 2. Similarly, some brown and red alga maintain internal pH as low as 0.1 in their vacuoles. One of the outstanding questions in the field is how such a conserved enzyme as the V-ATPase can fulfill such diverse functions.

Functional complementation of yeast vma1 delta cells by a plant subunit A homolog rescues the mutant phenotype and partially restores vacuolar H(+)-ATPase activity

The ability of a vacuolar H(+)-ATPase (V-ATPase) subunit homolog (subunit A) from plants to rescue the vma mutant phenotype of yeast was investigated as a first step towards investigating the structure and function of plant subunits in molecular detail. Heterologous expression of cotton cDNAs encoding near-identical isoforms of subunit A in mutant vma1 delta yeast cells successfully rescued the mutant vma phenotype, indicating that subunit A of plants and yeast have retained elements essential to V-ATPases during the course of evolution. Although vacuoles become acidified, the plant-yeast hybrid holoenzyme only partially restored V-ATPase activity (approximately 60%) in mutant yeast cells. Domain substitution of divergent N- or C-termini only slightly enhanced V-ATPase activity, whereas swapping both domains acted synergistically, increasing coupled ATP hydrolysis and proton translocation by approximately 22% relative to the native plant subunit. Immunoblot analysis indicated that similar amounts of yeast, plant or plant-yeast chimeric subunits are membrane-bound. These results suggest that subunit A terminal domains contain structural information that impact V-ATPase structure and function.

Use of a proteome strategy for tagging proteins present at the plasma membrane

A plasma membrane (PM) fraction was purified from Arabidopsis thaliana using a standard procedure and analyzed by two-dimensional (2D) gel electrophoresis. The proteins were classified according to their relative abundance in PM or cell membrane supernatant fractions. Eighty-two of the 700 spots detected on the PM 2D gels were microsequenced. More than half showed sequence similarity to proteins of known function. Of these, all the spots in the PM-specific and PM-enriched fractions, together with half of the spots with similar abundance in PM fraction and supernatant, have previously been found at the PM, supporting the validity of this approach. Extrapolation from this analysis indicates that (i) approximately 550 polypeptides found at the PM could be resolved on 2D gels; (ii) that numerous proteins with multiple locations are found at the PM; and (iii) that approximately 80% of PM-specific spots correspond to proteins with unknown function. Among the later, half are represented by ESTs or cDNAs in databases. In this way, several unknown gene products were potentially localized to the PM. These data are discussed with respect to the efficiency of organelle proteome approaches to link systematically genomic data to genome expression. It is concluded that generalized proteomes can constitute a powerful resource, with future completion of Arabidopsis genome sequencing, for genome-wide exploration of plant function.

Characterization of a red beet protein homologous to the essential 36-kilodalton subunit of the yeast V-type ATPase

V-type proton-translocating ATPases (V-ATPases) (EC 3.6.1.3) are electrogenic proton pumps involved in acidification of endomembrane compartments in all eukaryotic cells. V-ATPases from various species consist of 8 to 12 polypeptide subunits arranged into an integral membrane proton pore sector (Vo) and a peripherally associated catalytic sector (V1). Several V-ATPase subunits are functionally and structurally conserved among all species examined. In yeast, a 36-kD peripheral subunit encoded by the yeast (Saccharomyces cerevisiae) VMA6 gene (Vma6p) is required for stable assembly of the Vo sector as well as for V1 attachment. Vma6p has been characterized as a nonintegrally associated Vo subunit. A high degree of sequence similarity among Vma6p homologs from animal and fungal species suggest that this subunit has a conserved role in V-ATPase function. We have characterized a novel Vma6p homolog from red beet (Beta vulgaris) tonoplast membranes. A 44-kD polypeptide cofractionated with V-ATPases upon gel-filtration chromatography of detergent-solubilized tonoplast membranes and was specifically cross-reactive with anti-Vma6p polyclonal antibodies. The 44-kD polypeptide was dissociated from isolated tonoplast preparations by mild chaotropic agents and thus appeared to be nonintegrally associated with the membrane. The putative 44-kD homolog appears to be structurally similar to yeast Vma6p and occupies a similar position within the holoenzyme complex.

Synthesis and structure-activity relationships of bafilomycin A1 derivatives as inhibitors of vacuolar H+-ATPase

The macrolide antibiotic bafilomycin A1 is a highly potent and selective inhibitor of all the vacuolar ATPases (V-ATPases). With the aim of obtaining novel analogues specific for the osteoclast subclass of vacuolar ATPase, 31 derivatives of bafilomycin A1 were synthesized and tested for their ability to inhibit differentially the V-ATPase-driven proton transport in membrane vesicles derived from chicken osteoclasts (cOc) and bovine chromaffin granules (bCG). Although none of the new analogues were more potent than the parent compound, the obtained data provided a significant amount of information about the structural requirements for the inhibitory activity of bafilomycin A1. The different effects of a few analogues on the two enzymes could also suggest the possibility of a selective modulation of the V-ATPases in different tissues.

5-(5,6-Dichloro-2-indolyl)-2-methoxy-2,4-pentadienamides: novel and selective inhibitors of the vacuolar H+-ATPase of osteoclasts with bone antiresorptive activity

The vacuolar H+-ATPase (V-ATPase), located on the ruffled border of the osteoclast, is a proton pump which is responsible for secreting the massive amounts of protons that are required for the bone resorption process. With the aim to identify new agents which are able to prevent the excessive bone resorption associated with osteoporosis, we have designed a novel class of potent and selective inhibitors of the osteoclast proton pump, starting from the structure of the specific V-ATPase inhibitor bafilomycin A1. Compounds 3a-d potently inhibited the V-ATPase in chicken osteoclast membranes (IC50 = 60-180 nM) and were able to prevent bone resorption by human osteoclasts in vitro at low-nanomolar concentrations. Notably, the EC50 of compound 3c in this assay was 45-fold lower than the concentration required for half-maximal inhibition of the V-ATPase from human kidney cortex. These results support the validity of the osteoclast proton pump as a useful molecular target to produce novel inhibitors of bone resorption, potentially useful as antiosteporotic agents.

Identification and characterization of a novel 9.2-kDa membrane sector-associated protein of vacuolar proton-ATPase from chromaffin granules

Vacuolar proton-translocating ATPase (holoATPase and free membrane sector) was isolated from bovine chromaffin granules by blue native polyacrylamide gel electrophoresis. A 5-fold excess of membrane sector over holoenzyme was determined in isolated chromaffin granule membranes. M9.2, a novel extremely hydrophobic 9.2-kDa protein comprising 80 amino acids, was detected in the membrane sector. It shows sequence and structural similarity to Vma21p, a yeast protein required for assembly of vacuolar ATPase. A second membrane sector-associated protein (M8-9) was identified and characterized by amino-terminal protein sequencing.

Two types of H+-ATPase are involved in the acidification of internal compartments in Trypanosoma cruzi

ATP-driven acidification of internal compartments of Trypanosoma cruzi epimastigotes was assayed spectrophotometrically with Acridine Orange and cells permeabilized with filipin. H+-ATPase activity was not inhibited fully by either 500 nM concanamycin A or 500 microM orthovanadate, but a combination of 5 nM concanamycin A and 25 microM vanadate completely inhibited activity, suggesting the operation of separate V-type (concanamycin-sensitive) and P-type (vanadate-sensitive) H+-ATPase activities in the permeabilized cells. This was supported by different kinetics of Acridine Orange uptake seen in the presence of the different inhibitors, and by different optimal protein (cell) concentrations for the two apparent activities. The use of different buffers further distinguished the ATPases. The V-H+-ATPase activity was stimulated by K+ and inhibited by a lack of anions or the replacement of Cl- with gluconate. The P-type H+-ATPase activity was not affected by a lack of Cl- or K+ but was substantially inhibited in a largely anion-free buffer. This inhibition could be annulled by the addition of the K+ ionophore valinomycin, which probably acted via the establishment of a countercurrent efflux of K+ from the compartment containing the P-type H+-ATPase and the relief of the potential difference generated by the electrogenic proton pump. Valinomycin showed some stimulation of P-type activity in all buffers tested, but its effects on V-H+-ATPase activity were at best transient except in a K+-free buffer, which suggested that the V-H+-ATPase was located in an organelle with relatively low [K+] that was different from that which accommodated the P-type activity. On the basis of acidity and K+ content, these organelles might correspond, in part at least, to the acidocalcisomes (V-H+-ATPase activity) and the reservosomes (P-type activity) previously identified in these cells. Both activities could also be found in the human-infective forms of the parasite, amastigotes and trypomastigotes, but the P-type activity was relatively weak in these cells types, which is correlated with a lack of reservosomes in these forms.

The putative 116 kDa osteoclast specific vacuolar proton pump subunit has ubiquitous tissue distribution

The pharmacological profile of the osteoclast proton pump has been demonstrated to be unique and to be the most active of all acid transport systems thus far studied. The recently reported putative 116 kDa osteoclast specific vacuolar proton pump subunit could possibly explain the unique nature of this proton pump. Here, we demonstrate however, that the osteoclast 116 kDa subunit is not osteoclast specific but has ubiquitous expression in human tissue.

Energy transduction in ATP synthase

Mitochondria, bacteria and chloroplasts use the free energy stored in transmembrane ion gradients to manufacture ATP by the action of ATP synthase. This enzyme consists of two principal domains. The asymmetric membrane-spanning F0 portion contains the proton channel, and the soluble F1 portion contains three catalytic sites which cooperate in the synthetic reactions. The flow of protons through F0 is thought to generate a torque which is transmitted to F1 by an asymmetric shaft, the coiled-coil gamma-subunit. This acts as a rotating 'cam' within F1, sequentially releasing ATPs from the three active sites. The free-energy difference across the inner membrane of mitochondria and bacteria is sufficient to produce three ATPs per twelve protons passing through the motor. It has been suggested that this proton motive force biases the rotor's diffusion so that F0 constitutes a rotary motor turning the gamma shaft. Here we show that biased diffusion, augmented by electrostatic forces, does indeed generate sufficient torque to account for ATP production. Moreover, the motor's reversibility-supplying torque from ATP hydrolysis in F1 converts the motor into an efficient proton pump-can also be explained by our model.