Localization of subunit C (Vma5p) in the yeast vacuolar ATPase by immuno electron microscopy

Human V-ATPase function is positively and negatively regulated by TLDc proteins

Proteins that contain a highly conserved TLDc domain (Tre2/Bub2/Cdc16 LysM domain catalytic) offer protection against oxidative stress and are widely implicated in neurological health and disease. How this family of proteins exerts their function, however, is poorly understood. We have recently found that the yeast TLDc protein, Oxr1p, inhibits the proton pumping vacuolar ATPase (V-ATPase) by inducing disassembly of the pump. While loss of TLDc protein function in mammals shares disease phenotypes with V-ATPase defects, whether TLDc proteins impact human V-ATPase activity directly is unclear. Here we examine the effects of five human TLDc proteins, TLDC2, NCOA7, OXR1, TBC1D24, and mEAK7 on the activity of the human V-ATPase. We find that while TLDC2, TBC1D24, and the TLDc domains of OXR1 and NCOA7 inhibit V-ATPase by inducing enzyme disassembly, mEAK7 activates the pump. The data thus shed new light both on mammalian TLDc protein function and V-ATPase regulation.

The V-ATPases in cancer and cell death

Transmembrane ATPases are membrane-bound enzyme complexes and ion transporters that can be divided into F-, V-, and A-ATPases according to their structure. The V-ATPases, also known as H⁺-ATPases, are large multi-subunit protein complexes composed of a peripheral domain (V1) responsible for the hydrolysis of ATP and a membrane-integrated domain (V0) that transports protons across plasma membrane or organelle membrane. V-ATPases play a fundamental role in maintaining pH homeostasis through lysosomal acidification and are involved in modulating various physiological and pathological processes, such as macropinocytosis, autophagy, cell invasion, and cell death (e.g., apoptosis, anoikis, alkaliptosis, ferroptosis, and lysosome-dependent cell death). In addition to participating in embryonic development, V-ATPase pathways, when dysfunctional, are implicated in human diseases, such as neurodegenerative diseases, osteopetrosis, distal renal tubular acidosis, and cancer. In this review, we summarize the structure and regulation of isoforms of V-ATPase subunits and discuss their context-dependent roles in cancer biology and cell death. Updated knowledge about V-ATPases may enable us to design new anticancer drugs or strategies.

Regulation and function of V-ATPases in physiology and disease

The vacuolar H⁺-ATPases (V-ATPases) are essential, ATP-dependent proton pumps present in a variety of eukaryotic cellular membranes. Intracellularly, V-ATPase-dependent acidification functions in such processes as membrane traffic, protein degradation, autophagy and the coupled transport of small molecules. V-ATPases at the plasma membrane of certain specialized cells function in such processes as bone resorption, sperm maturation and urinary acidification. V-ATPases also function in disease processes such as pathogen entry and cancer cell invasiveness, while defects in V-ATPase genes are associated with disorders such as osteopetrosis, renal tubular acidosis and neurodegenerative diseases. This review highlights recent advances in our understanding of V-ATPase structure, mechanism, function and regulation, with an emphasis on the signaling pathways controlling V-ATPase assembly in mammalian cells. The role of V-ATPases in cancer and other human pathologies, and the prospects for therapeutic intervention, are also discussed.

Breaking up and making up: The secret life of the vacuolar H+ -ATPase

The vacuolar ATPase (V-ATPase; V₁ V_o -ATPase) is a large multisubunit proton pump found in the endomembrane system of all eukaryotic cells where it acidifies the lumen of subcellular organelles including lysosomes, endosomes, the Golgi apparatus, and clathrin-coated vesicles. V-ATPase function is essential for pH and ion homeostasis, protein trafficking, endocytosis, mechanistic target of rapamycin (mTOR), and Notch signaling, as well as hormone secretion and neurotransmitter release. V-ATPase can also be found in the plasma membrane of polarized animal cells where its proton pumping function is involved in bone remodeling, urine acidification, and sperm maturation. Aberrant (hypo or hyper) activity has been associated with numerous human diseases and the V-ATPase has therefore been recognized as a potential drug target. Recent progress with moderate to high-resolution structure determination by cryo electron microscopy and X-ray crystallography together with sophisticated single-molecule and biochemical experiments have provided a detailed picture of the structure and unique mode of regulation of the V-ATPase. This review summarizes the recent advances, focusing on the structural and biophysical aspects of the field.

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.

Biochemical and biophysical properties of interactions between subunits of the peripheral stalk region of human V-ATPase

Peripheral stalk subunits of eukaryotic or mammalian vacuolar ATPases (V-ATPases) play key roles in regulating its assembly and disassembly. In a previous study, we purified several subunits and their isoforms of the peripheral stalk region of Homo sapiens (human) V-ATPase; such as C1, E1G1, H, and the N-terminal cytoplasmic region of V(o), a1. Here, we investigated the in vitro binding interactions of the subunits at the stalk region and measured their specific affinities. Surface plasmon resonance experiments revealed that the subunit C1 binds the E1G1 heterodimer with both high and low affinities (2.8 nM and 1.9 µM, respectively). In addition, an E1G1-H complex can be formed with high affinity (48 nM), whereas affinities of other subunit pairs appeared to be low (∼0.21-3.0 µM). The putative ternary complex of C1-H-E1G1 was not much strong on co-incubation of these subunits, indicating that the two strong complexes of C1-E1G1 and H-E1G1 in cooperation with many other weak interactions may be sufficiently strong enough to withstand the torque of rotation during catalysis. We observed a partially stable quaternary complex (consisting of E1G1, C1, a1(NT), and H subunits) resulting from discrete peripheral subunit interactions stabilizing the complex through their intrinsic affinities. No binding was observed in the absence of E1G1 (using only H, C1, and a1(NT)); therefore, it is likely that, in vivo, the E1G1 heterodimer has a significant role in the initiation of subunit assembly. Multiple interactions of variable affinity in the stalk region may be important to the mechanism of reversible dissociation of the intact V-ATPase.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

An update in the structure, function, and regulation of V-ATPases: the role of the C subunit

Vacuolar ATPases (V-ATPases) are present in specialized proton secretory cells in which they pump protons across the membranes of various intracellular organelles and across the plasma membrane. The proton transport mechanism is electrogenic and establishes an acidic pH and a positive transmembrane potential in these intracellular and extracellular compartments. V-ATPases have been found to be practically identical in terms of the composition of their subunits in all eukaryotic cells. They have two distinct structures: a peripheral catalytic sector (V1) and a hydrophobic membrane sector (V0) responsible for driving protons. V-ATPase activity is regulated by three different mechanisms, which control pump density, association/dissociation of the V1 and V0 domains, and secretory activity. The C subunit is a 40-kDa protein located in the V1 domain of V-ATPase. The protein is encoded by the ATP6V1C gene and is located at position 22 of the long arm of chromosome 8 (8q22.3). The C subunit has very important functions in terms of controlling the regulation of the reversible dissociation of V-ATPases.

Crystal structure of the yeast vacuolar ATPase heterotrimeric EGC(head) peripheral stalk complex

Vacuolar ATPases (V-ATPases) are multisubunit rotary motor proton pumps that function to acidify subcellular organelles in all eukaryotic organisms. V-ATPase is regulated by a unique mechanism that involves reversible dissociation into V₁-ATPase and V₀ proton channel, a process that involves breaking of protein interactions mediated by subunit C, the cytoplasmic domain of subunit "a" and three "peripheral stalks," each made of a heterodimer of E and G subunits. Here, we present crystal structures of a yeast V-ATPase heterotrimeric complex composed of EG heterodimer and the head domain of subunit C (C(head)). The structures show EG heterodimer folded in a noncanonical coiled coil that is stabilized at its N-terminal ends by binding to C(head). The coiled coil is disrupted by a bulge of partially unfolded secondary structure in subunit G and we speculate that this unique feature in the eukaryotic V-ATPase peripheral stalk may play an important role in enzyme structure and regulation by reversible dissociation.

Subunit interactions at the V1-Vo interface in yeast vacuolar ATPase

Eukaryotic vacuolar ATPase (V-ATPase) is regulated by a reversible dissociation mechanism that involves breaking and reforming of protein-protein interactions at the interface of the V(1)-ATPase and V(o)-proton channel domains. We found previously that the head domain of the single copy C subunit (C(head)) binds one subunit EG heterodimer with high affinity (Oot, R.A. and Wilkens, S. (2010) J. Biol. Chem. 285, 24654-24664). Here we generated a water-soluble construct of the N-terminal domain of the V(o) "a" subunit composed of amino acid residues 104-372 (a(NT(104-372))). Analytical gel filtration chromatography and sedimentation velocity analysis revealed that a(NT(104-372)) undergoes reversible dimerization in a concentration-dependent manner. A low-resolution molecular envelope was calculated for the a(NT(104-372)) dimer using small angle x-ray scattering data. Isothermal titration calorimetry experiments revealed that a(NT(104-372)) binds the C(foot) and EG heterodimer with dissociation constants of 22 and 33 μM, respectively. We speculate that the spatial closeness of the a(NT), C(foot), and EG binding sites in the intact V-ATPase results in a high-avidity interaction that is able to resist the torque of rotational catalysis, and that reversible enzyme dissociation is initiated by breaking either the a(NT(104-372))-C(foot) or a(NT(104-372))-EG interaction by an as-yet unknown signaling mechanism.

Dopamine release via the vacuolar ATPase V0 sector c-subunit, confirmed in N18 neuroblastoma cells, results in behavioral recovery in hemiparkinsonian mice

A 16-kDa proteolipid, mediatophore, in Torpedo electric organs mediates Ca(2+)-dependent acetylcholine release. Mediatophore is identical to the pore-forming stalk c-subunit of the V0 sector of vacuolar proton ATPase (ATP6V0C). The function of ATP6V0C in the mammalian central nervous system is not clear. Here, we report transfection of adeno-associated viral vectors harboring rat ATP6V0C into the mouse substantia nigra, in which high potassium stimulation increased overflow of endogenous dopamine (DA) measured in the striatum by in vivo microdialysis. Next, in the striatum of 6-hydroxydopamine-lesioned mice, a model of Parkinson's disease (PD), human tyrosine hydroxylase, aromatic l-amino-acid decarboxylase and guanosine triphosphate cyclohydrolase 1, together with or without ATP6V0C, were expressed in the caudoputamen for rescue. Motor performance on the accelerating rotarod test and amphetamine-induced ipsilateral rotation were improved in the rescued mice coexpressing ATP6V0C. [(3)H]DA, taken up into cultured N18 neuronal tumor cells transformed to express ATP6V0C, was released by potassium stimulation. These results indicated that ATP6V0C mediates DA release from nerve terminals in the striatum of DA neurons of normal mice and from gene-transferred striatal cells of parkinsonian mice. The results suggested that ATP6V0C may be useful as a rescue molecule in addition to DA-synthetic enzymes in the gene therapy of PD.

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.

Domain characterization and interaction of the yeast vacuolar ATPase subunit C with the peripheral stator stalk subunits E and G

The proton pumping activity of the eukaryotic vacuolar ATPase (V-ATPase) is regulated by a unique mechanism that involves reversible enzyme dissociation. In yeast, under conditions of nutrient depletion, the soluble catalytic V(1) sector disengages from the membrane integral V(o), and at the same time, both functional units are silenced. Notably, during enzyme dissociation, a single V(1) subunit, C, is released into the cytosol. The affinities of the other V(1) and V(o) subunits for subunit C are therefore of particular interest. The C subunit crystal structure shows that the subunit is elongated and dumbbell-shaped with two globular domains (C(head) and C(foot)) separated by a flexible helical neck region (Drory, O., Frolow, F., and Nelson, N. (2004) EMBO Rep. 5, 1148-1152). We have recently shown that subunit C is bound in the V(1)-V(o) interface where the subunit is in contact with two of the three peripheral stators (subunit EG heterodimers): one via C(head) and one via C(foot) (Zhang, Z., Zheng, Y., Mazon, H., Milgrom, E., Kitagawa, N., Kish-Trier, E., Heck, A. J., Kane, P. M., and Wilkens, S. (2008) J. Biol. Chem. 283, 35983-35995). In vitro, however, subunit C binds only one EG heterodimer (Féthière, J., Venzke, D., Madden, D. R., and Böttcher, B. (2005) Biochemistry 44, 15906-15914), implying that EG has different affinities for the two domains of the C subunit. To determine which subunit C domain binds EG with high affinity, we have generated C(head) and C(foot) and characterized their interaction with subunit EG heterodimer. Our findings indicate that the high affinity site for EGC interaction is C(head). In addition, we provide evidence that the EGC(head) interaction greatly stabilizes EG heterodimer.

V-ATPase deactivation in blowfly salivary glands is mediated by protein phosphatase 2C

The activity of vacuolar H(+)-ATPase (V-ATPase) in the apical membrane of blowfly (Calliphora vicina) salivary glands is regulated by the neurohormone serotonin (5-HT). 5-HT induces, via protein kinase A, the phosphorylation of V-ATPase subunit C and the assembly of V-ATPase holoenzymes. The protein phosphatase responsible for the dephosphorylation of subunit C and V-ATPase inactivation is not as yet known. We show here that inhibitors of protein phosphatases PP1 and PP2A (tautomycin, ocadaic acid) and PP2B (cyclosporin A, FK-506) do not prevent V-ATPase deactivation and dephosphorylation of subunit C. A decrease in the intracellular Mg(2+) level caused by loading secretory cells with EDTA-AM leads to the activation of proton pumping in the absence of 5-HT, prolongs the 5-HT-induced response in proton pumping, and inhibits the dephosphorylation of subunit C. Thus, the deactivation of V-ATPase is most probably mediated by a protein phosphatase that is insensitive to okadaic acid and that requires Mg(2+), namely, a member of the PP2C protein family. By molecular biological techniques, we demonstrate the expression of at least two PP2C protein family members in blowfly salivary glands.

Subunit interactions and requirements for inhibition of the yeast V1-ATPase

Disassembly of the yeast V-ATPase into cytosolic V(1) and membrane V(0) sectors inactivates MgATPase activity of the V(1)-ATPase. This inactivation requires the V(1) H subunit (Parra, K. J., Keenan, K. L., and Kane, P. M. (2000) J. Biol. Chem. 275, 21761-21767), but its mechanism is not fully understood. The H subunit has two domains. Interactions of each domain with V(1) and V(0) subunits were identified by two-hybrid assay. The B subunit of the V(1) catalytic headgroup interacted with the H subunit N-terminal domain (H-NT), and the C-terminal domain (H-CT) interacted with V(1) subunits B, E (peripheral stalk), and D (central stalk), and the cytosolic N-terminal domain of V(0) subunit Vph1p. V(1)-ATPase complexes from yeast expressing H-NT are partially inhibited, exhibiting 26% the MgATPase activity of complexes with no H subunit. The H-CT domain does not copurify with V(1) when expressed in yeast, but the bacterially expressed and purified H-CT domain inhibits MgATPase activity in V(1) lacking H almost as well as the full-length H subunit. Binding of full-length H subunit to V(1) was more stable than binding of either H-NT or H-CT, suggesting that both domains contribute to binding and inhibition. Intact H and H-CT can bind to the expressed N-terminal domain of Vph1p, but this fragment of Vph1p does not bind to V(1) complexes containing subunit H. We propose that upon disassembly, the H subunit undergoes a conformational change that inhibits V(1)-ATPase activity and precludes V(0) interactions.

Structure of the yeast vacuolar ATPase

The subunit architecture of the yeast vacuolar ATPase (V-ATPase) was analyzed by single particle transmission electron microscopy and electrospray ionization (ESI) tandem mass spectrometry. A three-dimensional model of the intact V-ATPase was calculated from two-dimensional projections of the complex at a resolution of 25 angstroms. Images of yeast V-ATPase decorated with monoclonal antibodies against subunits A, E, and G position subunit A within the pseudo-hexagonal arrangement in the V1, the N terminus of subunit G in the V1-V0 interface, and the C terminus of subunit E at the top of the V1 domain. ESI tandem mass spectrometry of yeast V1-ATPase showed that subunits E and G are most easily lost in collision-induced dissociation, consistent with a peripheral location of the subunits. An atomic model of the yeast V-ATPase was generated by fitting of the available x-ray crystal structures into the electron microscopy-derived electron density map. The resulting atomic model of the yeast vacuolar ATPase serves as a framework to help understand the role the peripheral stalk subunits are playing in the regulation of the ATP hydrolysis driven proton pumping activity of the vacuolar ATPase.

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.

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.

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.

Stoichiometry of the peripheral stalk subunits E and G of yeast V1-ATPase determined by mass spectrometry

The stoichiometry of yeast V(1)-ATPase peripheral stalk subunits E and G was determined by two independent approaches using mass spectrometry (MS). First, the subunit ratio was inferred from measuring the molecular mass of the intact V(1)-ATPase complex and each of the individual protein components, using native electrospray ionization-MS. The major observed intact complex had a mass of 593,600 Da, with minor components displaying masses of 553,550 and 428,300 Da, respectively. Second, defined amounts of V(1)-ATPase purified from yeast grown on (14)N-containing medium were titrated with defined amounts of (15)N-labeled E and G subunits as internal standards. Following protease digestion of subunit bands, (14)N- and (15)N-containing peptide pairs were used for quantification of subunit stoichiometry using matrix-assisted laser desorption/ionization-time of flight MS. Results from both approaches are in excellent agreement and reveal that the subunit composition of yeast V(1)-ATPase is A(3)B(3)DE(3)FG(3)H.

Stoichiometry and localization of the stator subunits E and G in Thermus thermophilus H+-ATPase/synthase

Proton-translocating ATPases are central to biological energy conversion. Although eukaryotes contain specialized F-ATPases for ATP synthesis and V-ATPases for proton pumping, eubacteria and archaea typically contain only one enzyme for both tasks. Although many eubacteria contain ATPases of the F-type, some eubacteria and all known archaea contain ATPases of the A-type. A-ATPases are closely related to V-ATPases but simpler in design. Although the nucleotide-binding and transmembrane rotor subunits share sequence homology between A-, V-, and F-ATPases, the peripheral stalk is strikingly different in sequence, composition, and stoichiometry. We have analyzed the peripheral stalk of Thermus thermophilus A-ATPase by using phage display-derived single-domain antibody fragments in combination with electron microscopy and tandem mass spectrometry. Our data provide the first direct evidence for the existence of two peripheral stalks in the A-ATPase, each one composed of heterodimers of subunits E and G arranged symmetrically around the soluble A(1) domain. To our knowledge, this is the first description of phage display-derived antibody selection against a multi-subunit membrane protein used for purification and single particle analysis by electron microscopy. It is also the first instance of the derivation of subunit stoichiometry by tandem mass spectrometry to an intact membrane protein complex. Both approaches could be applicable to the structural analysis of other membrane protein complexes.

Vacuolar ATPases: rotary proton pumps in physiology and pathophysiology

The acidity of intracellular compartments and the extracellular environment is crucial to various cellular processes, including membrane trafficking, protein degradation, bone resorption and sperm maturation. At the heart of regulating acidity are the vacuolar (V-)ATPases--large, multisubunit complexes that function as ATP-driven proton pumps. Their activity is controlled by regulating the assembly of the V-ATPase complex or by the dynamic regulation of V-ATPase expression on membrane surfaces. The V-ATPases have been implicated in a number of diseases and, coupled with their complex isoform composition, represent attractive and potentially highly specific drug targets.

Stimulus-induced phosphorylation of vacuolar H(+)-ATPase by protein kinase A

Eukaryotic vacuolar-type H(+)-ATPases (V-ATPases) are regulated by the reversible disassembly of the active V(1)V(0) holoenzyme into a cytosolic V(1) complex and a membrane-bound V(0) complex. The signaling cascades that trigger these events in response to changing cellular conditions are largely unknown. We report that the V(1) subunit C of the tobacco hornworm Manduca sexta interacts with protein kinase A and is the only V-ATPase subunit that is phosphorylated by protein kinase A. Subunit C can be phosphorylated as single polypeptide as well as a part of the V(1) complex but not as a part of the V(1)V(0) holoenzyme. Both the phosphorylated and the unphosphorylated form of subunit C are able to reassociate with the V(1) complex from which subunit C had been removed before. Using salivary glands of the blowfly Calliphora vicina in which V-ATPase reassembly and activity is regulated by the neurohormone serotonin via protein kinase A, we show that the membrane-permeable cAMP analog 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (8-CPT-cAMP) causes phosphorylation of subunit C in a tissue homogenate and that phosphorylation is reduced by incubation with antibodies against subunit C. Similarly, incubation of intact salivary glands with 8-CPT-cAMP or serotonin leads to the phosphorylation of subunit C, but this is abolished by H-89, an inhibitor of protein kinase A. These data suggest that subunit C binds to and serves as a substrate for protein kinase A and that this phosphorylation may be a regulatory switch for the formation of the active V(1)V(0) holoenzyme.

RAVE is essential for the efficient assembly of the C subunit with the vacuolar H(+)-ATPase

The RAVE complex is required for stable assembly of the yeast vacuolar proton-translocating ATPase (V-ATPase) during both biosynthesis of the enzyme and regulated reassembly of disassembled V(1) and V(0) sectors. It is not yet known how RAVE effects V-ATPase assembly. Previous work has shown that V(1) peripheral or stator stalk subunits E and G are critical for binding of RAVE to cytosolic V(1) complexes, suggesting that RAVE may play a role in docking of the V(1) peripheral stalk to the V(0) complex at the membrane. Here we provide evidence for an interaction between the RAVE complex and V(1) subunit C, another subunit that has been assigned to the peripheral stalk. The C subunit is unique in that it is released from both V(1) and V(0) sectors during disassembly, suggesting that subunit C may control the regulated assembly of the V-ATPase. Mutants lacking subunit C have assembly phenotypes resembling that of RAVE mutants. Both are able to assemble V(1)/V(0) complexes in vivo, but these complexes are highly unstable in vitro, and V-ATPase activity is extremely low. We show that in the absence of the RAVE complex, subunit C is not able to stably assemble with the vacuolar ATPase. Our data support a model where RAVE, through its interaction with subunit C, is facilitating V(1) peripheral stalk subunit interactions with V(0) during V-ATPase assembly.

Intracellular pH regulation in oral squamous cell carcinoma is mediated by increased V-ATPase activity via over-expression of the ATP6V1C1 gene

Oral squamous cell carcinomas represent more than 90% of all head and neck cancers, and comprise about 4% of all malignancies in western countries. Tumor cell mobility related to increasing intracellular pH results in impaired proliferation and metastasis, suggesting an important role of pH regulation in solid cancer tumorigenesis. The mechanism of physiological pH regulation has been shown to be activated in several solid tumors through constitutive activation of the ATPase complex. How cells regulate this mechanism has not been elucidated in human cancer in detail. The present study, using expression profiling by cDNA array analysis of oral squamous cell carcinoma cells, identified the V-ATPase system as a significant regulatory mechanism. ATP6V1C1 was the most strongly over-expressed gene in oral squamous cell carcinoma at the mRNA level compared to other genes of the V-ATPase complex. These findings provide evidence that intracellular pH regulation is mainly controlled by expression of a single gene, ATP6V1C1, notwithstanding the possible action of other secondary regulatory factors.

Identification of a Domain in the Vo Subunit d That Is Critical for Coupling of the Yeast Vacuolar Proton-translocating ATPase

Vacuolar proton-translocating ATPase pumps consist of two domains, V(1) and V(o). Subunit d is a component of V(o) located in a central stalk that rotates during catalysis. By generating mutations, we showed that subunit d couples ATP hydrolysis and proton transport. The mutation F94A strongly uncoupled the enzyme, preventing proton transport but not ATPase activity. C-terminal mutations changed coupling as well; ATPase activity was decreased by 59-72%, whereas proton transport was not measurable (E328A) or was moderately reduced (E317A and C329A). Except for W325A, which had low levels of V(1)V(o), mutations allowed wild-type assembly regardless of the fact that subunits E and d were reduced at the membrane. N- and C-terminal deletions of various lengths were inhibitory and gradually destabilized subunit d, limiting V(1)V(o) formation. Both N and C terminus were required for V(o) assembly. The N-terminal truncation 2-19Delta prevented V(1)V(o) formation, although subunit d was available. The C terminus was required for retention of subunits E and d at the membrane. In addition, the C terminus of its bacterial homolog (subunit C from T. thermophilus) stabilized the yeast subunit d mutant 310-345Delta and allowed assembly of the rotor structure with subunits A and B. Structural features conserved between bacterial and eukaryotic subunit d and the significance of domain 3 for vacuolar proton-translocating ATPase function are discussed.