Arrangement of subunits in the proteolipid ring of the V-ATPase

2, 4, 5-Trideoxyhexopyranosides derivatives of diphyllin: Synthesis and anticancer activity

Diphyllin and its natural derivatives were identified as potent vacuolar H⁺ -ATPase (V-ATPase) inhibitors. In this study, twelve 2, 4, 5-trideoxyhexopyranosides derivatives of diphyllin were synthesized. Most of these compounds showed potent abilities to inhibit the growth of HT-29, MCF-7, HepG2 cancer cells with IC₅₀ values at submicromolar concentration. The compounds 5c3 and 5c4 showed the best inhibitory activity on breast cancer cell lines MCF-7 with IC₅₀ values of 0.09 and 0.10 μM. Compounds 5c3 and 5c4 showed similar V-ATPase inhibitory potency to diphyllin. Molecular docking showed that a hydrogen bond was found between the hydroxyl of 5c3 and SerA534 in the pocket of the V-ATPase receptor.

The V-ATPase a3 Subunit: Structure, Function and Therapeutic Potential of an Essential Biomolecule in Osteoclastic Bone Resorption

This review focuses on one of the 16 proteins composing the V-ATPase complex responsible for resorbing bone: the a3 subunit. The rationale for focusing on this biomolecule is that mutations in this one protein account for over 50% of osteopetrosis cases, highlighting its critical role in bone physiology. Despite its essential role in bone remodeling and its involvement in bone diseases, little is known about the way in which this subunit is targeted and regulated within osteoclasts. To this end, this review is broadened to include the three other mammalian paralogues (a1, a2 and a4) and the two yeast orthologs (Vph1p and Stv1p). By examining the literature on all of the paralogues/orthologs of the V-ATPase a subunit, we hope to provide insight into the molecular mechanisms and future research directions specific to a3. This review starts with an overview on bone, highlighting the role of V-ATPases in osteoclastic bone resorption. We then cover V-ATPases in other location/functions, highlighting the roles which the four mammalian a subunit paralogues might play in differential targeting and/or regulation. We review the ways in which the energy of ATP hydrolysis is converted into proton translocation, and go in depth into the diverse role of the a subunit, not only in proton translocation but also in lipid binding, cell signaling and human diseases. Finally, the therapeutic implication of targeting a3 specifically for bone diseases and cancer is discussed, with concluding remarks on future directions.

V-ATPases and osteoclasts: ambiguous future of V-ATPases inhibitors in osteoporosis

Vacuolar ATPases (V-ATPases) play a critical role in regulating extracellular acidification of osteoclasts and bone resorption. The deficiencies of subunit a3 and d2 of V-ATPases result in increased bone density in humans and mice. One of the traditional drug design strategies in treating osteoporosis is the use of subunit a3 inhibitor. Recent findings connect subunits H and G1 with decreased bone density. Given the controversial effects of ATPase subunits on bone density, there is a critical need to review the subunits of V-ATPase in osteoclasts and their functions in regulating osteoclasts and bone remodeling. In this review, we comprehensively address the following areas: information about all V-ATPase subunits and their isoforms; summary of V-ATPase subunits associated with human genetic diseases; V-ATPase subunits and osteopetrosis/osteoporosis; screening of all V-ATPase subunits variants in GEFOS data and in-house data; spectrum of V-ATPase subunits during osteoclastogenesis; direct and indirect roles of subunits of V-ATPases in osteoclasts; V-ATPase-associated signaling pathways in osteoclasts; interactions among V-ATPase subunits in osteoclasts; osteoclast-specific V-ATPase inhibitors; perspective of future inhibitors or activators targeting V-ATPase subunits in the treatment of osteoporosis.

The 3.5-Å CryoEM Structure of Nanodisc-Reconstituted Yeast Vacuolar ATPase Vo Proton Channel

The molecular mechanism of transmembrane proton translocation in rotary motor ATPases is not fully understood. Here, we report the 3.5-Å resolution cryoEM structure of the lipid nanodisc-reconstituted V_o proton channel of the yeast vacuolar H⁺-ATPase, captured in a physiologically relevant, autoinhibited state. The resulting atomic model provides structural detail for the amino acids that constitute the proton pathway at the interface of the proteolipid ring and subunit a. Based on the structure and previous mutagenesis studies, we propose the chemical basis of transmembrane proton transport. Moreover, we discovered that the C terminus of the assembly factor Voa1 is an integral component of mature V_o. Voa1's C-terminal transmembrane α helix is bound inside the proteolipid ring, where it contributes to the stability of the complex. Our structure rationalizes possible mechanisms by which mutations in human V_o can result in disease phenotypes and may thus provide new avenues for therapeutic interventions.

Tumor cell cholesterol depletion and V-ATPase inhibition as an inhibitory mechanism to prevent cell migration and invasiveness in melanoma

BACKGROUND

V-ATPase interactions with cholesterol enriched membrane microdomains have been related to metastasis in a variety of cancers, but the underlying mechanism remains at its beginnings. It has recently been reported that the inhibition of this H⁺ pump affects cholesterol mobilization to the plasma membrane.

METHODS

Inhibition of melanoma cell migration and invasiveness was assessed by wound healing and Transwell assays in murine cell lines (B16F10 and Melan-A). V-ATPase activity was measured in vitro by ATP hydrolysis and H⁺ transport in membrane vesicles, and intact cell H⁺ fluxes were measured by using a non-invasive Scanning Ion-selective Electrode Technique (SIET).

RESULTS

Cholesterol depletion by 5mM MβCD was found to be inhibitory to the hydrolytic and H⁺ pumping activities of the V-ATPase of melanoma cell lines, as well as to the migration and invasiveness capacities of these cells. Nearly the same effects were obtained using concanamycin A, a specific inhibitor of V-ATPase, which also promoted a decrease of the H⁺ efflux in live cells at the same extent of MβCD.

CONCLUSIONS

We found that cholesterol depletion significantly affects the V-ATPase activity and the initial metastatic processes following a profile similar to those observed in the presence of the V-ATPase specific inhibitor, concanamycin.

GENERAL SIGNIFICANCE

The results shed new light on the functional role of the interactions between V-ATPases and cholesterol-enriched microdomains of cell membranes that contribute with malignant phenotypes in melanoma.

Identification of Novel Bisbenzimidazole Derivatives as Anticancer Vacuolar (H⁺)-ATPase Inhibitors

The vacuolar (H⁺)-ATPases (V-ATPases) are a family of ATP-driven proton pumps and they have been associated with cancer invasion, metastasis, and drug resistance. Despite the clear involvement of V-ATPases in cancer, the therapeutic use of V-ATPase-targeting small molecules has not reached human clinical trials to date. Thus, V-ATPases are emerging as important targets for the identification of potential novel therapeutic agents. We identified a bisbenzimidazole derivative (V) as an initial hit from a similarity search using four known V-ATPase inhibitors (I–IV). Based on the initial hit (V), we designed and synthesized a focused set of novel bisbenzimidazole analogs (2a–e). All newly prepared compounds have been screened for selected human breast cancer (MDA-MB-468, MDA-MB-231, and MCF7) and ovarian cancer (A2780, Cis-A2780, and PA-1) cell lines, along with the normal breast epithelial cell line, MCF10A. The bisbenzimidazole derivative (2e) is active against all cell lines tested. Remarkably, it demonstrated high cytotoxicity against the triple-negative breast cancer (TNBC) cell line, MDA-MB-468 (IC₅₀ = 0.04 ± 0.02 μM). Additionally, it has been shown to inhibit the V-ATPase pump that is mainly responsible for acidification. To the best of our knowledge the bisbenzimidazole pharmacophore has been identified as the first V-ATPase inhibitor in its class. These results strongly suggest that the compound 2e could be further developed as a potential anticancer V-ATPase inhibitor for breast cancer treatment.

Organelle acidification negatively regulates vacuole membrane fusion in vivo

The V-ATPase is a proton pump consisting of a membrane-integral V0 sector and a peripheral V1 sector, which carries the ATPase activity. In vitro studies of yeast vacuole fusion and evidence from worms, flies, zebrafish and mice suggested that V0 interacts with the SNARE machinery for membrane fusion, that it promotes the induction of hemifusion and that this activity requires physical presence of V0 rather than its proton pump activity. A recent in vivo study in yeast has challenged these interpretations, concluding that fusion required solely lumenal acidification but not the V0 sector itself. Here, we identify the reasons for this discrepancy and reconcile it. We find that acute pharmacological or physiological inhibition of V-ATPase pump activity de-acidifies the vacuole lumen in living yeast cells within minutes. Time-lapse microscopy revealed that de-acidification induces vacuole fusion rather than inhibiting it. Cells expressing mutated V0 subunits that maintain vacuolar acidity were blocked in this fusion. Thus, proton pump activity of the V-ATPase negatively regulates vacuole fusion in vivo. Vacuole fusion in vivo does, however, require physical presence of a fusion-competent V0 sector.

Yeast V-ATPase Proteolipid Ring Acts as a Large-conductance Transmembrane Protein Pore

The vacuolar H⁺ -ATPase (V-ATPase) is a rotary motor enzyme that acidifies intracellular organelles and the extracellular milieu in some tissues. Besides its canonical proton-pumping function, V-ATPase’s membrane sector, V_o, has been implicated in non-canonical functions including membrane fusion and neurotransmitter release. Here, we report purification and biophysical characterization of yeast V-ATPase c subunit ring (c-ring) using electron microscopy and single-molecule electrophysiology. We find that yeast c-ring forms dimers mediated by the c subunits’ cytoplasmic loops. Electrophysiology measurements of the c-ring reconstituted into a planar lipid bilayer revealed a large unitary conductance of ~8.3 nS. Thus, the data support a role of V-ATPase c-ring in membrane fusion and neuronal communication.

Affinity Purification and Structural Features of the Yeast Vacuolar ATPase Vo Membrane Sector

The membrane sector (Vo) of the proton pumping vacuolar ATPase (V-ATPase, V1Vo-ATPase) from Saccharomyces cerevisiae was purified to homogeneity, and its structure was characterized by EM of single molecules and two-dimensional crystals. Projection images of negatively stained Vo two-dimensional crystals showed a ring-like structure with a large asymmetric mass at the periphery of the ring. A cryo-EM reconstruction of Vo from single-particle images showed subunits a and d in close contact on the cytoplasmic side of the proton channel. A comparison of three-dimensional reconstructions of free Vo and Vo as part of holo V1Vo revealed that the cytoplasmic N-terminal domain of subunit a (aNT) must undergo a large conformational change upon enzyme disassembly or (re)assembly from Vo, V1, and subunit C. Isothermal titration calorimetry using recombinant subunit d and aNT revealed that the two proteins bind each other with a Kd of ~5 μm. Treatment of the purified Vo sector with 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-rac-(1-glycerol)] resulted in selective release of subunit d, allowing purification of a VoΔd complex. Passive proton translocation assays revealed that both Vo and VoΔd are impermeable to protons. We speculate that the structural change in subunit a upon release of V1 from Vo during reversible enzyme dissociation plays a role in blocking passive proton translocation across free Vo and that the interaction between aNT and d seen in free Vo functions to stabilize the Vo sector for efficient reassembly of V1Vo.

Eukaryotic V-ATPase: novel structural findings and functional insights

The eukaryotic V-type adenosine triphosphatase (V-ATPase) is a multi-subunit membrane protein complex that is evolutionarily related to F-type adenosine triphosphate (ATP) synthases and A-ATP synthases. These ATPases/ATP synthases are functionally conserved and operate as rotary proton-pumping nano-motors, invented by Nature billions of years ago. In the first part of this review we will focus on recent structural findings of eukaryotic V-ATPases and discuss the role of different subunits in the function of the V-ATPase holocomplex. Despite structural and functional similarities between rotary ATPases, the eukaryotic V-ATPases are the most complex enzymes that have acquired some unconventional cellular functions during evolution. In particular, the novel roles of V-ATPases in the regulation of cellular receptors and their trafficking via endocytotic and exocytotic pathways were recently uncovered. In the second part of this review we will discuss these unique roles of V-ATPases in modulation of function of cellular receptors, involved in the development and progression of diseases such as cancer and diabetes as well as neurodegenerative and kidney disorders. Moreover, it was recently revealed that the V-ATPase itself functions as an evolutionarily conserved pH sensor and receptor for cytohesin-2/Arf-family GTP-binding proteins. Thus, in the third part of the review we will evaluate the structural basis for and functional insights into this novel concept, followed by the analysis of the potentially essential role of V-ATPase in the regulation of this signaling pathway in health and disease. Finally, future prospects for structural and functional studies of the eukaryotic V-ATPase will be discussed.

V-ATPases in osteoclasts: structure, function and potential inhibitors of bone resorption

The vacuolar-type H(+)-ATPase (V-ATPase) proton pump is a macromolecular complex composed of at least 14 subunits organized into two functional domains, V(1) and V(0). The complex is located on the ruffled border plasma membrane of bone-resorbing osteoclasts, mediating extracellular acidification for bone demineralization during bone resorption. Genetic studies from mice to man implicate a critical role for V-ATPase subunits in osteoclast-related diseases including osteopetrosis and osteoporosis. Thus, the V-ATPase complex is a potential molecular target for the development of novel anti-resorptive agents useful for the treatment of osteolytic diseases. Here, we review the current structure and function of V-ATPase subunits, emphasizing their exquisite roles in osteoclastic function. In addition, we compare several distinct classes of V-ATPase inhibitors with specific inhibitory effects on osteoclasts. Understanding the structure-function relationship of the osteoclast V-ATPase may lead to the development of osteoclast-specific V-ATPase inhibitors that may serve as alternative therapies for the treatment of osteolytic diseases.

The proton translocation domain of cellular vacuolar ATPase provides a target for the treatment of influenza A virus infections

BACKGROUND AND PURPOSE

Cellular vacuolar ATPases (v-ATPase) play an important role in endosomal acidification, a critical step in influenza A virus (IAV) host cell infection. We investigated the antiviral activity of the v-ATPase inhibitor saliphenylhalamide (SaliPhe) and compared it with several older v-ATPase inhibitors concanamycin A, bafilomycin A1, (BafA) and archazolid B targeting the subunit c of the V(0) sector.

EXPERIMENTAL APPROACH

An in vitro assay was devised to quantify the anti-influenza effect of v-ATPase inhibitors by measuring green fluorescent protein fluorescence of a reporter IAV. These data were combined with cytotoxicity testing to calculate selectivity indices. Data were validated by testing v-ATPase inhibitors against wild-type IAV in vitro and in vivo in mice.

KEY RESULTS

In vitro SaliPhe blocked the proliferation of pandemic and multidrug resistant viruses at concentrations up to 51-fold below its cytotoxic concentrations. At essentially non-toxic concentrations, SaliPhe protected 62.5% of mice against a lethal challenge of a mouse-adapted influenza strain, while BafA at cytotoxic concentrations showed essentially no protection against infection with IAV (SaliPhe vs. BafA P < 0.001).

CONCLUSIONS AND IMPLICATIONS

Our results show that a distinct binding site of the proton translocation domain of cellular v-ATPase can be selectively targeted by a new generation v-ATPase inhibitor with reduced toxicity to treat influenza virus infections, including multi-resistant strains. Treatment strategies against influenza that target host cellular proteins are expected to be more resistant to virus mutations than drugs blocking viral proteins.

Evolution of increased complexity in a molecular machine

Many cellular processes are carried out by molecular 'machines'-assemblies of multiple differentiated proteins that physically interact to execute biological functions. Despite much speculation, strong evidence of the mechanisms by which these assemblies evolved is lacking. Here we use ancestral gene resurrection and manipulative genetic experiments to determine how the complexity of an essential molecular machine--the hexameric transmembrane ring of the eukaryotic V-ATPase proton pump--increased hundreds of millions of years ago. We show that the ring of Fungi, which is composed of three paralogous proteins, evolved from a more ancient two-paralogue complex because of a gene duplication that was followed by loss in each daughter copy of specific interfaces by which it interacts with other ring proteins. These losses were complementary, so both copies became obligate components with restricted spatial roles in the complex. Reintroducing a single historical mutation from each paralogue lineage into the resurrected ancestral proteins is sufficient to recapitulate their asymmetric degeneration and trigger the requirement for the more elaborate three-component ring. Our experiments show that increased complexity in an essential molecular machine evolved because of simple, high-probability evolutionary processes, without the apparent evolution of novel functions. They point to a plausible mechanism for the evolution of complexity in other multi-paralogue protein complexes.

The V-ATPase proteolipid cylinder promotes the lipid-mixing stage of SNARE-dependent fusion of yeast vacuoles

The V-ATPase V(0) sector associates with the peripheral V(1) sector to form a proton pump. V(0) alone has an additional function, facilitating membrane fusion in the endocytic and late exocytic pathways. V(0) contains a hexameric proteolipid cylinder, which might support fusion as proposed in proteinaceous pore models. To test this, we randomly mutagenized proteolipids. We recovered alleles that preserve proton translocation, normal SNARE activation and trans-SNARE pairing but that impair lipid and content mixing. Critical residues were found in all subunits of the proteolipid ring. They concentrate within the bilayer, close to the ring subunit interfaces. The fusion-impairing proteolipid substitutions stabilize the interaction of V(0) with V(1). Deletion of the vacuolar v-SNARE Nyv1 has the same effect, suggesting that both types of mutations similarly alter the conformation of V(0). Also covalent linkage of subunits in the proteolipid cylinder blocks vacuole fusion. We propose that a SNARE-dependent conformational change in V(0) proteolipids might stimulate fusion by creating a hydrophobic crevice that promotes lipid reorientation and formation of a lipidic fusion pore.

Vacuolar H(+)-ATPase subunits Voa1 and Voa2 cooperatively regulate secretory vesicle acidification, transmitter uptake, and storage

Voa1 and Voa2 cooperatively regulate the acidification and transmitter uptake/storage of dense-core vesicles, although they might not be as critical for exocytosis as recently proposed.

The Vo sector of the vacuolar H⁺-ATPase is a multisubunit complex that forms a proteolipid pore. Among the four isoforms (a1–a4) of subunit Voa, the isoform(s) critical for secretory vesicle acidification have yet to be identified. An independent function of Voa1 in exocytosis has been suggested. Here we investigate the function of Voa isoforms in secretory vesicle acidification and exocytosis by using neurosecretory PC12 cells. Fluorescence-tagged and endogenous Voa1 are primarily localized on secretory vesicles, whereas fluorescence-tagged Voa2 and Voa3 are enriched on the Golgi and early endosomes, respectively. To elucidate the functional roles of Voa1 and Voa2, we engineered PC12 cells in which Voa1, Voa2, or both are stably down-regulated. Our results reveal significant reductions in the acidification and transmitter uptake/storage of dense-core vesicles by knockdown of Voa1 and more dramatically of Voa1/Voa2 but not of Voa2. Overexpressing knockdown-resistant Voa1 suppresses the acidification defect caused by the Voa1/Voa2 knockdown. Unexpectedly, Ca²⁺-dependent peptide secretion is largely unaffected in Voa1 or Voa1/Voa2 knockdown cells. Our data demonstrate that Voa1 and Voa2 cooperatively regulate the acidification and transmitter uptake/storage of dense-core vesicles, whereas they might not be as critical for exocytosis as recently proposed.

Structural divergence of the rotary ATPases

The rotary ATPase family of membrane protein complexes may have only three members, but each one plays a fundamental role in biological energy conversion. The F1Fo-ATPase (F-ATPase) couples ATP synthesis to the electrochemical membrane potential in bacteria, mitochondria and chloroplasts, while the vacuolar H+-ATPase (V-ATPase) operates as an ATP-driven proton pump in eukaryotic membranes. In different species of archaea and bacteria, the A1Ao-ATPase (A-ATPase) can function as either an ATP synthase or an ion pump. All three of these multi-subunit complexes are rotary molecular motors, sharing a fundamentally similar mechanism in which rotational movement drives the energy conversion process. By analogy to macroscopic systems, individual subunits can be assigned to rotor, axle or stator functions. Recently, three-dimensional reconstructions from electron microscopy and single particle image processing have led to a significant step forward in understanding of the overall architecture of all three forms of these complexes and have allowed the organisation of subunits within the rotor and stator parts of the motors to be more clearly mapped out. This review describes the emerging consensus regarding the organisation of the rotor and stator components of V-, A- and F-ATPases, examining core similarities that point to a common evolutionary origin, and highlighting key differences. In particular, it discusses how newly revealed variation in the complexity of the inter-domain connections may impact on the mechanics and regulation of these molecular machines.

Structural bases of physiological functions and roles of the vacuolar H(+)-ATPase

Vacuolar-type H(+)-ATPases (V-ATPases) is a large multi-protein complex containing at least 14 different subunits, in which subunits A, B, C, D, E, F, G, and H compose the peripheral 500-kDa V(1) responsible for ATP hydrolysis, and subunits a, c, c', c″, and d assembly the 250-kDa membrane-integral V(0) harboring the rotary mechanism to transport protons across the membrane. The assembly of V-ATPases requires the presence of all V(1) and V(0) subunits, in which the V(1) must be completely assembled prior to association with the V(0), accordingly the V(0) failing to assemble cannot provide a membrane anchor for the V(1), thereby prohibiting membrane association of the V-ATPase subunits. The V-ATPase mediates acidification of intracellular compartments and regulates diverse critical physiological processes of cell for functions of its numerous functional subunits. The core catalytic mechanism of the V-ATPase is a rotational catalytic mechanism. The V-ATPase holoenzyme activity is regulated by the reversible assembly/disassembly of the V(1) and V(0), the targeting and recycling of V-ATPase-containing vesicles to and from the plasma membrane, the coupling ratio between ATP hydrolysis and proton pumping, ATP, Ca(2+), and its inhibitors and activators.

Regulation and isoform function of the V-ATPases

The vacuolar (H(+))-ATPases are ATP-dependent proton pumps that acidify intracellular compartments and, in some cases, transport protons across the plasma membrane of eukaryotic cells. Intracellular V-ATPases play an important role in normal physiological processes such as receptor-mediated endocytosis, intracellular membrane trafficking, pro-hormone processing, protein degradation, and the coupled uptake of small molecules, such as neurotransmitters. They also function in the entry of various pathogenic agents, including many envelope viruses, like influenza virus, and toxins, like anthrax toxin. Plasma membrane V-ATPases function in renal pH homeostasis, bone resorption and sperm maturation, and various disease processes, including renal tubular acidosis, osteopetrosis, and tumor metastasis. V-ATPases are composed of a peripheral V(1) domain containing eight different subunits that is responsible for ATP hydrolysis and an integral V(0) domain containing six different subunits that translocates protons. In mammalian cells, most of the V-ATPase subunits exist in multiple isoforms which are often expressed in a tissue specific manner. Isoforms of one of the V(0) subunits (subunit a) have been shown to possess information that targets the V-ATPase to distinct cellular destinations. Mutations in isoforms of subunit a lead to the human diseases osteopetrosis and renal tubular acidosis. A number of mechanisms are employed to regulate V-ATPase activity in vivo, including reversible dissociation of the V(1) and V(0) domains, control of the tightness of coupling of proton transport and ATP hydrolysis, and selective targeting of V-ATPases to distinct cellular membranes. Isoforms of subunit a are involved in regulation both via the control of coupling and via selective targeting. This review will begin with a brief introduction to the function, structure, and mechanism of the V-ATPases followed by a discussion of the role of V-ATPase subunit isoforms and the mechanisms involved in regulation of V-ATPase activity.

Cellular physiology of the renal H+ATPase

PURPOSE OF REVIEW

Vacuolar-type H+ATPases are multisubunit macromolecules that play an essential role in renal acid-base homeostasis. Other cellular processes also rely on the proton pumping ability of H+ATPases to acidify organellar or lumenal spaces. Several diseases, including distal renal tubular acidosis, osteoporosis and wrinkly skin syndrome, are due to mutations in genes encoding alternate subunits that make up the H+ATPase. This review highlights recent key articles in this research area.

RECENT FINDINGS

Further insights into the structure, expression and regulation of H+ATPases have been elucidated, within the kidney and elsewhere. This knowledge may enhance the potential for future drug targeting.

SUMMARY

Novel findings concerning tissue-specific subunits of the H+ATPase that are important in the kidney and more general lessons of H+ATPase function and regulation are slowly emerging, though the paucity of cellular tools available has to date limited progress.

The little we know on the structure and machinery of V-ATPase

The life of every eukaryotic cell depends on the function of vacuolar H+-ATPase (V-ATPase). Today we know that V-ATPase is vital for many more physiological and biochemical processes than it was expected three decades ago when the enzyme was discovered. These range from a crucial role in the function of internal organelles such as vacuoles, lysosomes, synaptic vesicles, endosomes, secretory granules and the Golgi apparatus to the plasma membrane of several organisms and specific tissues, and specialized cells. The overall structure and mechanism of action of the V-ATPase is supposed to be similar to that of the well-characterized F-type ATP synthase (F-ATPase). Both consist of a soluble catalytic domain (V1 or F1) that is coupled to a membrane-spanning domain (Vo or Fo) by one or more `stalk' components. Owing to the complexity and challenging properties of V-ATPase its study is lagging behind that of its relative F-ATPase. Time will tell whether V-ATPase shares an identical mechanism of action with F-ATPase or its mode of operation is unique.

The tether connecting cytosolic (N terminus) and membrane (C terminus) domains of yeast V-ATPase subunit a (Vph1) is required for assembly of V0 subunit d

V-ATPases are molecular motors that reversibly disassemble in vivo. Anchored in the membrane is subunit a. Subunit a has a movable N terminus that switches positions during disassembly and reassembly. Deletions were made at residues securing the N terminus of subunit a (yeast isoform Vph1) to its membrane-bound C-terminal domain in order to understand the role of this conserved region for V-ATPase function. Shrinking of the tether made cells pH-sensitive (vma phenotype) because assembly of V(0) subunit d was harmed. Subunit d did not co-immunoprecipitate with subunit a and the c-ring. Cells contained pools of V(1) and V(0)(-d) that failed to form V(1)V(0), and very low levels of V-ATPase subunits were found at the membrane. Although subunit d expression was stable and at wild-type levels, growth defects were rescued by exogenous VMA6 (subunit d). Stable V(1)V(0) assembled after yeast cells were co-transformed with VMA6 and mutant VPH1. Tether-less V(1)V(0) was delivered to the vacuole and active. It retained 63-71% of the wild-type activity and was responsive to glucose. Tether-less V(1)V(0) disassembled and reassembled after brief glucose depletion and readdition. The N terminus retained binding to V(1) subunits and the C terminus to phosphofructokinase. Thus, no major structural change was generated at the N and C termini of subunit a. We concluded that early steps of V(0) assembly and trafficking were likely impaired by shorter tethers and rescued by VMA6.

Structural organization of the V-ATPase and its implications for regulatory assembly and disassembly

V-ATPases (vacuolar ATPases) are membrane-bound multiprotein complexes that are localized in the endomembrane systems of eukaryotic cells and in the plasma membranes of some specialized cells. They couple ATP hydrolysis with the transport of protons across membranes. On nutrient shortage, V-ATPases disassemble into a membrane-embedded part (V0), which contains the proton translocation machinery, and an extrinsic part (V1), which carries the nucleotide-binding sites. Disassembly decouples ATP hydrolysis and proton translocation. Furthermore, the disassembled parts are inactive, leading to an efficient shutdown of ATP consumption. On restoring the nutrient levels, V1 and V0 reassemble and restore ATP-hydrolysis activity coupled with proton translocation. This reversible assembly/disassembly process has certain conformational constraints, which are best fulfilled by adopting a unique conformation before disassembly.

Voa1p functions in V-ATPase assembly in the yeast endoplasmic reticulum

The yeast Saccharomyces cerevisiae vacuolar ATPase (V-ATPase) is a multisubunit complex divided into two sectors: the V(1) sector catalyzes ATP hydrolysis and the V(0) sector translocates protons, resulting in acidification of its resident organelle. Four protein factors participate in V(0) assembly. We have discovered a fifth V(0) assembly factor, Voa1p (YGR106C); an endoplasmic reticulum (ER)-localized integral membrane glycoprotein. The role of Voa1p in V(0) assembly was revealed in cells expressing an ER retrieval-deficient form of the V-ATPase assembly factor Vma21p (Vma21pQQ). Loss of Voa1p in vma21QQ yeast cells resulted in loss of V-ATPase function; cells were unable to acidify their vacuoles and exhibited growth defects typical of cells lacking V-ATPase. V(0) assembly was severely compromised in voa1 vma21QQ double mutants. Isolation of V(0)-Vma21p complexes indicated that Voa1p associates most strongly with Vma21p and the core proteolipid ring of V(0) subunits c, c', and c''. On assembly of the remaining three V(0) subunits (a, d, and e) into the V(0) complex, Voa1p dissociates from the now fully assembled V(0)-Vma21p complex. Our results suggest Voa1p functions with Vma21p early in V(0) assembly in the ER, but then it dissociates before exit of the V(0)-Vma21p complex from the ER for transport to the Golgi compartment.

Function, structure and regulation of the vacuolar (H+)-ATPases

The vacuolar ATPases (or V-ATPases) are ATP-driven proton pumps that function to both acidify intracellular compartments and to transport protons across the plasma membrane. Intracellular V-ATPases function in such normal cellular processes as receptor-mediated endocytosis, intracellular membrane traffic, prohormone processing, protein degradation and neurotransmitter uptake, as well as in disease processes, including infection by influenza and other viruses and killing of cells by anthrax and diphtheria toxin. Plasma membrane V-ATPases are important in such physiological processes as urinary acidification, bone resorption and sperm maturation as well as in human diseases, including osteopetrosis, renal tubular acidosis and tumor metastasis. V-ATPases are large multi-subunit complexes composed of a peripheral domain (V(1)) responsible for hydrolysis of ATP and an integral domain (V(0)) that carries out proton transport. Proton transport is coupled to ATP hydrolysis by a rotary mechanism. V-ATPase activity is regulated in vivo using a number of mechanisms, including reversible dissociation of the V(1) and V(0) domains, changes in coupling efficiency of proton transport and ATP hydrolysis and changes in pump density through reversible fusion of V-ATPase containing vesicles. V-ATPases are emerging as potential drug targets in treating a number of human diseases including osteoporosis and cancer.

Organelle-specific isoenzymes of plant V-ATPase as revealed by in vivo-FRET analysis

Background

The V-ATPase (VHA) is a protein complex of 13 different VHA-subunits. It functions as an ATP driven rotary-motor that electrogenically translocates H⁺ into endomembrane compartments. In Arabidopsis thaliana V-ATPase is encoded by 23 genes posing the question of specific versus redundant function of multigene encoded isoforms.

Results

The transmembrane topology and stoichiometry of the proteolipid VHA-c" as well as the stoichiometry of the membrane integral subunit VHA-e within the V-ATPase complex were investigated by in vivo fluorescence resonance energy transfer (FRET). VHA-c", VHA-e1 and VHA-e2, VHA-a, VHA-c3, truncated variants of VHA-c3 and a chimeric VHA-c/VHA-c" hybrid were fused to cyan (CFP) and yellow fluorescent protein (YFP), respectively. The constructs were employed for transfection experiments with Arabidopsis thaliana mesophyll protoplasts. Subcellular localization and FRET analysis by confocal laser scanning microscopy (CLSM) demonstrated that (i.) the N- and C-termini of VHA-c" are localised in the vacuolar lumen, (ii.) one copy of VHA-c" is present within the VHA-complex, and (iii.) VHA-c" is localised at the ER and associated Golgi bodies. (iv.) A similar localisation was observed for VHA-e2, whereas (v.) the subcellular localisation of VHA-e1 indicated the trans Golgi network (TGN)-specifity of this subunit.

Conclusion

The plant proteolipid ring is a highly flexible protein subcomplex, tolerating the incorporation of truncated and hybrid proteolipid subunits, respectively. Whereas the membrane integral subunit VHA-e is present in two copies within the complex, the proteolipid subunit VHA-c" takes part in complex formation with only one copy. However, neither VHA-c" isoform 1 nor any of the two VHA-e isoforms were identified at the tonoplast. This suggest a function in endomembrane specific VHA-assembly or targeting rather than proton transport.

Structure and regulation of the vacuolar ATPases

The vacuolar (H(+))-ATPases (V-ATPases) are ATP-dependent proton pumps responsible for both acidification of intracellular compartments and, for certain cell types, proton transport across the plasma membrane. Intracellular V-ATPases function in both endocytic and intracellular membrane traffic, processing and degradation of macromolecules in secretory and digestive compartments, coupled transport of small molecules such as neurotransmitters and ATP and in the entry of pathogenic agents, including envelope viruses and bacterial toxins. V-ATPases are present in the plasma membrane of renal cells, osteoclasts, macrophages, epididymal cells and certain tumor cells where they are important for urinary acidification, bone resorption, pH homeostasis, sperm maturation and tumor cell invasion, respectively. The V-ATPases are composed of a peripheral domain (V(1)) that carries out ATP hydrolysis and an integral domain (V(0)) responsible for proton transport. V(1) contains eight subunits (A-H) while V(0) contains six subunits (a, c, c', c'', d and e). V-ATPases operate by a rotary mechanism in which ATP hydrolysis within V(1) drives rotation of a central rotary domain, that includes a ring of proteolipid subunits (c, c' and c''), relative to the remainder of the complex. Rotation of the proteolipid ring relative to subunit a within V(0) drives active transport of protons across the membrane. Two important mechanisms of regulating V-ATPase activity in vivo are reversible dissociation of the V(1) and V(0) domains and changes in coupling efficiency of proton transport and ATP hydrolysis. This review focuses on recent advances in our lab in understanding the structure and regulation of the V-ATPases.