Three-dimensional structure and subunit topology of the V(1) ATPase from Manduca sexta midgut

Inhibition of Sodium-Hydrogen Antiport by Antibodies to NHA1 in Brush Border Membrane Vesicles from Whole Aedes aegypti Larvae

The present research report describes Na⁺/H⁺ antiport by brush border membrane vesicles isolated from whole larvae of Aedes aegypti (AeBBMVw). Our hypothesis is that acid quenching of acridine orange by AeBBMVw is predominantly mediated by Na⁺/H⁺ antiport via the NHA1 component of the AeBBMVw in the absence of amino acids and ATP. AeNHA1 is a Na⁺/H⁺ antiporter that has been postulated to exchange Na⁺ and H⁺ across the apical plasma membrane in posterior midgut of A. aegypti larvae. Its principal function is to recycle the H⁺ and Na⁺ that are transported during amino acid uptake, e.g., phenylalanine. This uptake is mediated, in part, by a voltage-driven, Na⁺-coupled, nutrient amino acid transporter (AeNAT8). The voltage is generated by an H⁺ V-ATPase. All three components, V-ATPase, antiporter, and nutrient amino acid transporter (VAN), are present in brush border membrane vesicles isolated from whole larvae of A. aegypti. By omitting ATP and amino acids, Na⁺/H⁺ antiport was measured by fluorescence quenching of acridine orange (AO) caused by acidification of either the internal vesicle medium (Na⁺_in > Na⁺_out) or the external fluid-membrane interface (Na⁺_in < Na⁺_out). Vesicles with 100 micromolar Na⁺ inside and 10 micromolar Na⁺ outside or with 0.01 micromolar Na⁺ inside and 100 micromolar Na⁺ outside quenched fluorescence of AO by as much as 30%. Acidification did not occur in the absence of AeBBMVw. Preincubation of AeBBMVw with antibodies to NHA1 inhibit Na⁺/H⁺ antiport dependent fluorescence quenching, indicating that AeNHA1 has a significant role in Na⁺/H⁺ exchange.

Crystal structure of subunits D and F in complex gives insight into energy transmission of the eukaryotic V-ATPase from Saccharomyces cerevisiae

Eukaryotic V1VO-ATPases hydrolyze ATP in the V1 domain coupled to ion pumping in VO. A unique mode of regulation of V-ATPases is the reversible disassembly of V1 and VO, which reduces ATPase activity and causes silencing of ion conduction. The subunits D and F are proposed to be key in these enzymatic processes. Here, we describe the structures of two conformations of the subunit DF assembly of Saccharomyces cerevisiae (ScDF) V-ATPase at 3.1 Å resolution. Subunit D (ScD) consists of a long pair of α-helices connected by a short helix ((79)IGYQVQE(85)) as well as a β-hairpin region, which is flanked by two flexible loops. The long pair of helices is composed of the N-terminal α-helix and the C-terminal helix, showing structural alterations in the two ScDF structures. The entire subunit F (ScF) consists of an N-terminal domain of four β-strands (β1-β4) connected by four α-helices (α1-α4). α1 and β2 are connected via the loop (26)GQITPETQEK(35), which is unique in eukaryotic V-ATPases. Adjacent to the N-terminal domain is a flexible loop, followed by a C-terminal α-helix (α5). A perpendicular and extended conformation of helix α5 was observed in the two crystal structures and in solution x-ray scattering experiments, respectively. Fitted into the nucleotide-bound A3B3 structure of the related A-ATP synthase from Enterococcus hirae, the arrangements of the ScDF molecules reflect their central function in ATPase-coupled ion conduction. Furthermore, the flexibility of the terminal helices of both subunits as well as the loop (26)GQITPETQEK(35) provides information about the regulatory step of reversible V1VO disassembly.

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.

Crystal and NMR structures give insights into the role and dynamics of subunit F of the eukaryotic V-ATPase from Saccharomyces cerevisiae

Subunit F of V-ATPases is proposed to undergo structural alterations during catalysis and reversible dissociation from the V1VO complex. Recently, we determined the low resolution structure of F from Saccharomyces cerevisiae V-ATPase, showing an N-terminal egg shape, connected to a C-terminal hook-like segment via a linker region. To understand the mechanistic role of subunit F of S. cerevisiae V-ATPase, composed of 118 amino acids, the crystal structure of the major part of F, F(1-94), was solved at 2.3 Å resolution. The structural features were confirmed by solution NMR spectroscopy using the entire F subunit. The eukaryotic F subunit consists of the N-terminal F(1-94) domain with four-parallel β-strands, which are intermittently surrounded by four α-helices, and the C terminus, including the α5-helix encompassing residues 103 to 113. Two loops (26)GQITPETQEK(35) and (60)ERDDI(64) are described to be essential in mechanistic processes of the V-ATPase enzyme. The (26)GQITPETQEK(35) loop becomes exposed when fitted into the recently determined EM structure of the yeast V1VO-ATPase. A mechanism is proposed in which the (26)GQITPETQEK(35) loop of subunit F and the flexible C-terminal domain of subunit H move in proximity, leading to an inhibitory effect of ATPase activity in V1. Subunits D and F are demonstrated to interact with subunit d. Together with NMR dynamics, the role of subunit F has been discussed in the light of its interactions in the processes of reversible disassembly and ATP hydrolysis of V-ATPases by transmitting movements of subunit d and H of the VO and V1 sector, respectively.

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.

Crystallization and preliminary X-ray crystallographic analysis of subunit F (F(1-94)), an essential coupling subunit of the eukaryotic V(1)V(O)-ATPase from Saccharomyces cerevisiae

V-ATPases are very complex multi-subunit enzymes which function as proton-pumping rotary nanomotors. The rotary and coupling subunit F (F(1-94)) was crystallized by the hanging-drop vapour-diffusion method. The native crystals diffracted to a resolution of 2.64 Å and belonged to space group C222(1), with unit-cell parameters a = 47.21, b = 160.26, c = 102.49 Å. The selenomethionyl form of the F(1-94) I69M mutant diffracted to a resolution of 2.3 Å and belonged to space group C222(1), with unit-cell parameters a = 47.22, b = 160.83, c = 102.74 Å. Initial phasing and model building suggested the presence of four molecules in the asymmetric unit.

Localization of two Na+- or K+-H+ antiporters, AgNHA1 and AgNHA2, in Anopheles gambiae larval Malpighian tubules and the functional expression of AgNHA2 in yeast

The newly identified metazoan Na(+)/H(+) antiporter (NHA) family is represented by two paralogues, AgNHA1 and AgNHA2, in the genome of the African malaria mosquito, Anopheles gambiae. Both antiporters are postulated to be electrophoretic i.e. voltage-driven. AgNHA1 was first cloned from An. gambiae larvae and immunolocalized with respect to the H(+) V-ATPase by the Harvey laboratory. Little is known about the properties of NHA1s; attempts to characterize AgNHA1 in Na(+)/H(+) exchanger (NHE)-lacking Chinese hamster ovary cells and in yeast cells or frog oocytes were unsuccessful. Even less is known about AgNHA2. It is predicted to have a relative molecular mass of ∼60 kDa and shares 30.5% amino acid identity with AgNHA1. Immunolocalization images show AgNHA2 on the apical plasma membrane of stellate cells in Malpighian tubules of An. gambiae larvae and adults. When heterologously expressed in a mutant strain of the yeast, Saccharomyces cerevisiae, which lacks endogenous cation/proton antiporters and pumps, AgNHA2 enhanced repression of growth by the alkali metal cations, Li(+), Na(+), or K(+) and enhanced Li(+) accumulation. The yeast growth studies invite the speculation that AgNHA2 is an electrophoretic antiporter with a stoichiometry of nNa(+) to 1H(+) with n > 1. Immunolocalization images provide direct evidence that H(+) V-ATPase is co-localized with AgNHA1 on the apical membrane of principal cells but it is not present in the stellate cells where AgNHA2 is localized apically. These results are consistent with the notion that the outside positive voltage that the H(+) V-ATPase generates across the apical membrane of principal cells appears with but little attenuation across the apical membrane of stellate cells. This immunolocalization pattern is consistent with the hypothesis that the voltage acts via AgNHA1 to drive nH(+) into the principal cells and Na(+) out to the lumen and acts via AgNHA2 to drive nNa(+) into the stellate cells and H(+) out to the lumen. Precious Na(+) is then retained by ejection into the blood via a basal Na(+)/K(+)-ATPase. Localizations of anion transporters and their functions in stellate and principal cells are described by Linser, Romero and associates in this volume. The role that the electrogenic H(+) V-ATPase and the electrophoretic cationic and anionic transporters play in ion homeostasis is incorporated into a model for Malpighian tubule cells of larval mosquitoes.

K+ pump: from caterpillar midgut to human cochlea

Deafness is a serious condition that affects millions of people and can also lead to dementia. Moreover, Karet and associates reported in 1999 that mutations in the gene encoding H(+) V-ATPase subunit B(1) lead to deafness. Yet ionic flows that enable humans to hear high-pitched sounds at 20,000 cycles/sec (20 kHz) are not well understood. Sound is transduced to electrical signals by stereocilia of hair cells by influx of Ca(2+) and K(+) as the "transducer channel" opens transiently and reduces the ∼90 mV (endolymph positive) endocochlear potential (EP) by ∼20 mV as the receptor potential. The EP as well as concentrations of Ca(2+), H(+) and K(+) must remain constant to produce reliable signals. Ca(2+) entry is balanced by Ca(2+) exit via a plasma membrane Ca(2+) ATPase (PMCA2a) but the Ca(2+) exit is coupled to H(+) entry. Moreover, K(+) entry is balanced by K(+) exit via a long diffusion route through several channels which is too slow to account for 20 kHz signaling. The problem is solved by a new hypothesis in which an H(+) V-ATPase generates the EP and removes the H(+) while a new K(+)/H(+) antiporter uses the voltage to drive H(+) back in and the K(+) back out. In the new model, Ca(2+), H(+) and K(+) cycle between unstirred layers on the endolymph- and cytoplasmic- borders of the stereocilial membrane through distances of ∼20 nanometers with travel time of ∼10 μs, which is fast enough to account for the 50 μs open/close time for 20 kHz signaling. Central to this model is the hypothesis that a K(+) pump which secretes K(+) into a K(+)-rich compartment is composed of a voltage producing (electrogenic) H(+) V-ATPase that is electrically coupled to a voltage-driven (electrophoretic) K(+)/nH(+) antiporter (KHA). Conversely, for an H(+) V-ATPase to secrete K(+) into a K(+) rich compartment, it must be coupled to a KHA. Richard Keynes reviewed evidence in 1969 that such a K(+) pump, which he called a Type V pump, is present in the stria vascularis of cochlea and the goblet cell apical membrane of caterpillars. Its signature is a large outside positive potential of ∼100 mV, K(+) secretion into a K(+) rich compartment and reversible inhibition by anoxia. The key role of the Type V K(+) pump in generating the EP was recognized by Sellick and Bock in 1974 and others but has disappeared from the hearing literature during the past decades. Its revival here is based on immunolocalization of KHA2 in the stereocilial membrane and Gillespie's generously shared mass spectroscopy evidence that all but one of the V(1) ATPase subunits are detected in isolated chicken stereocilia but V(o) and KHAs are not detected (implying that KHAs must be in the membrane). The new model proposed in the present paper could lead to important changes in our understanding of sensory physiology.

Structural elements of the C-terminal domain of subunit E (E₁₃₃₋₂₂₂) from the Saccharomyces cerevisiae V₁V₀ ATPase determined by solution NMR spectroscopy

Subunit E of the vacuolar ATPase (V-ATPase) contains an N-terminal extended α helix (Rishikesan et al. J Bioenerg Biomembr 43:187-193, 2011) and a globular C-terminal part that is predicted to consist of a mixture of α-helices and β-sheets (Grüber et al. Biochem Biophys Res Comm 298:383-391, 2002). Here we describe the production, purification and 2D structure of the C-terminal segment E₁₃₃₋₂₂₂ of subunit E from Saccharamyces cerevisiae V-ATPase in solution based on the secondary structure calculation from NMR spectroscopy studies. E₁₃₃₋₂₂₂ consists of four β-strands, formed by the amino acids from K136-V139, E170-V173, G186-V189, D195-E198 and two α-helices, composed of the residues from R144-A164 and T202-I218. The sheets and helices are arranged as β1:α1:β2:β3:β4:α2, which are connected by flexible loop regions. These new structural details of subunit E are discussed in the light of the structural arrangements of this subunit inside the V₁- and V₁V₀ ATPase.

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.

H(+) V-ATPase-energized transporters in brush border membrane vesicles from whole larvae of Aedes aegypti

Brush border membrane vesicles (BBMVs) from Whole larvae of Aedes aegypti (AeBBMVWs) contain an H(+) V-ATPase (V), a Na(+)/H(+) antiporter, NHA1 (A) and a Na(+)-coupled, nutrient amino acid transporter, NAT8 (N), VAN for short. All V-ATPase subunits are present in the Ae. aegypti genome and in the vesicles. AgNAT8 was cloned from Anopheles gambiae, localized in BBMs and characterized in Xenopus laevis oocytes. AgNHA1 was cloned and localized in BBMs but characterization in oocytes was compromised by an endogenous cation conductance. AeBBMVWs complement Xenopus oocytes for characterizing membrane proteins, can be energized by voltage from the V-ATPase and are in their natural lipid environment. BBMVs from caterpillars were used in radio-labeled solute uptake experiments but approximately 10,000 mosquito larvae are needed to equal 10 caterpillars. By contrast, functional AeBBMVWs can be prepared from 10,000 whole larvae in 4h. Na(+)-coupled (3H)phenylalanine uptake mediated by AeNAT8 in AeBBMVs can be compared to the Phe-induced inward Na(+) currents mediated by AgNAT8 in oocytes. Western blots and light micrographs of samples taken during AeBBMVW isolation are labeled with antibodies against all of the VAN components. The use of AeBBMVWs to study coupling between electrogenic V-ATPases and the electrophoretic transporters is discussed.

Solution structure of subunit F (Vma7p) of the eukaryotic V(1)V(O) ATPase from Saccharomyces cerevisiae derived from SAXS and NMR spectroscopy

Vacuolar ATPases use the energy derived from ATP hydrolysis, catalyzed in the A(3)B(3) sector of the V(1) ATPase to pump protons via the membrane-embedded V(O) sector. The energy coupling between the two sectors occurs via the so-called central stalk, to which subunit F does belong. Here we present the first low resolution structure of recombinant subunit F (Vma7p) of a eukaryotic V-ATPase from Saccharomyces cerevisiae, analyzed by small angle X-ray scattering (SAXS). The protein is divided into a 5.5nm long egg-like shaped region, connected via a 1.5nm linker to a hook-like segment at one end. Circular dichroism spectroscopy revealed that subunit F comprises of 43% α-helix, 32% β-sheet and a 25% random coil arrangement. To determine the localization of the N- and C-termini in the protein, the C-terminal truncated form of F, F(1-94) was produced and analyzed by SAXS. Comparison of the F(1-94) shape with the one of subunit F showed the missing hook-like region in F(1-94), supported by the decreased D(max) value of F(1-94) (7.0nm), and indicating that the hook-like region consists of the C-terminal residues. The NMR solution structure of the C-terminal peptide, F(90-116), was solved, displaying an α-helical region between residues 103 and 113. The F(90-116) solution structure fitted well in the hook-like region of subunit F. Finally, the arrangement of subunit F within the V(1) ATPase is discussed.

Voltage coupling of primary H+ V-ATPases to secondary Na+- or K+-dependent transporters

This review provides alternatives to two well established theories regarding membrane energization by H(+) V-ATPases. Firstly, we offer an alternative to the notion that the H(+) V-ATPase establishes a protonmotive force (pmf) across the membrane into which it is inserted. The term pmf, which was introduced by Peter Mitchell in 1961 in his chemiosmotic hypothesis for the synthesis of ATP by H(+) F-ATP synthases, has two parts, the electrical potential difference across the phosphorylating membrane, Deltapsi, and the pH difference between the bulk solutions on either side of the membrane, DeltapH. The DeltapH term implies three phases - a bulk fluid phase on the H(+) input side, the membrane phase and a bulk fluid phase on the H(+) output side. The Mitchell theory was applied to H(+) V-ATPases largely by analogy with H(+) F-ATP synthases operating in reverse as H(+) F-ATPases. We suggest an alternative, voltage coupling model. Our model for V-ATPases is based on Douglas B. Kell's 1979 'electrodic view' of ATP synthases in which two phases are added to the Mitchell model - an unstirred layer on the input side and another one on the output side of the membrane. In addition, we replace the notion that H(+) V-ATPases normally acidify the output bulk solution with the hypothesis, which we introduced in 1992, that the primary action of a H(+) V-ATPase is to charge the membrane capacitance and impose a Deltapsi across the membrane; the translocated hydrogen ions (H(+)s) are retained at the outer fluid-membrane interface by electrostatic attraction to the anions that were left behind. All subsequent events, including establishing pH differences in the outside bulk solution, are secondary. Using the surface of an electrode as a model, Kell's 'electrodic view' has five phases - the outer bulk fluid phase, an outer fluid-membrane interface, the membrane phase, an inner fluid-membrane interface and the inner bulk fluid phase. Light flash, H(+) releasing and binding experiments and other evidence provide convincing support for Kell's electrodic view yet Mitchell's chemiosmotic theory is the one that is accepted by most bioenergetics experts today. First we discuss the interaction between H(+) V-ATPase and the K(+)/2H(+) antiporter that forms the caterpillar K(+) pump, and use the Kell electrodic view to explain how the H(+)s at the outer fluid-membrane interface can drive two H(+) from lumen to cell and one K(+) from cell to lumen via the antiporter even though the pH in the bulk fluid of the lumen is highly alkaline. Exchange of outer bulk fluid K(+) (or Na(+)) with outer interface H(+) in conjunction with (K(+) or Na(+))/2H(+) antiport, transforms the hydrogen ion electrochemical potential difference, mu(H), to a K(+) electrochemical potential difference, mu(K) or a Na(+) electrochemical potential difference, mu(Na). The mu(K) or mu(Na) drives K(+)- or Na(+)-coupled nutrient amino acid transporters (NATs), such as KAAT1 (K(+) amino acid transporter 1), which moves Na(+) and an amino acid into the cell with no H(+)s involved. Examples in which the voltage coupling model is used to interpret ion and amino acid transport in caterpillar and larval mosquito midgut are discussed.

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.

New insights into structure-function relationships between archeal ATP synthase (A1A0) and vacuolar type ATPase (V1V0)

Adenosine triphosphate, ATP, is the energy currency of living cells. While ATP synthases of archae and ATP synthases of pro- and eukaryotic organisms operate as energy producers by synthesizing ATP, the eukaryotic V-ATPase hydrolyzes ATP and thus functions as energy transducer. These enzymes share features like the hydrophilic catalytic- and the membrane-embedded ion-translocating sector, allowing them to operate as nano-motors and to transform the transmembrane electrochemical ion gradient into ATP or vice versa. Since archaea are rooted close to the origin of life, the A-ATP synthase is probably more similar in its composition and function to the "original" enzyme, invented by Nature billion years ago. On the contrary, the V-ATPases have acquired specific structural, functional and regulatory features during evolution. This review will summarize the current knowledge on the structure, mechanism and regulation of A-ATP synthases and V-ATPases. The importance of V-ATPase in pathophysiology of diseases will be discussed.

Cationic pathway of pH regulation in larvae of Anopheles gambiae

Anopheles gambiae larvae (Diptera: Culicidae) live in freshwater with low Na(+) concentrations yet they use Na(+) for alkalinization of the alimentary canal, for electrophoretic amino acid uptake and for nerve function. The metabolic pathway by which larvae accomplish these functions has anionic and cationic components that interact and allow the larva to conserve Na(+) while excreting H(+) and HCO(3)(-). The anionic pathway consists of a metabolic CO(2) diffusion process, carbonic anhydrase and Cl(-)/HCO(3)(-) exchangers; it provides weak HCO(3)(-) and weaker CO(3)(2-) anions to the lumen. The cationic pathway consists of H(+) V-ATPases and Na(+)/H(+) antiporters (NHAs), Na(+)/K(+) P-ATPases and Na(+)/H(+) exchangers (NHEs) along with several (Na(+) or K(+)):amino acid(+/-) symporters, a.k.a. nutrient amino acid transporters (NATs). This paper considers the cationic pathway, which provides the strong Na(+) or K(+) cations that alkalinize the lumen in anterior midgut then removes them and restores a lower pH in posterior midgut. A key member of the cationic pathway is a Na(+)/H(+) antiporter, which was cloned recently from Anopheles gambiae larvae, localized strategically in plasma membranes of the alimentary canal and named AgNHA1 based upon its phylogeny. A phylogenetic comparison of all cloned NHAs and NHEs revealed that AgNHA1 is the first metazoan NHA to be cloned and localized and that it is in the same clade as electrophoretic prokaryotic NHAs that are driven by the electrogenic H(+) F-ATPase. Like prokaryotic NHAs, AgNHA1 is thought to be electrophoretic and to be driven by the electrogenic H(+) V-ATPase. Both AgNHA1 and alkalophilic bacterial NHAs face highly alkaline environments; to alkalinize the larva mosquito midgut lumen, AgNHA1, like the bacterial NHAs, would have to move nH(+) inwardly and Na(+) outwardly. Perhaps the alkaline environment that led to the evolution of electrophoretic prokaryotic NHAs also led to the evolution of an electrophoretic AgNHA1 in mosquito larvae. In support of this hypothesis, antibodies to both AgNHA1 and H(+) V-ATPase label the same membranes in An. gambiae larvae. The localization of H(+) V-ATPase together with (Na(+) or K(+)):amino acid(+/-) symporter, AgNAT8, on the same apical membrane in posterior midgut cells constitutes the functional equivalent of an NHE that lowers the pH in the posterior midgut lumen. All NATs characterized to date are Na(+) or K(+) symporters so the deduction is likely to have wide application. The deduced colocalization of H(+) V-ATPase, AgNHA1 and AgNAT8, on this membrane forms a pathway for local cycling of H(+) and Na(+) in posterior midgut. The local H(+) cycle would prevent unchecked acidification of the lumen while the local Na(+) cycle would regulate pH and support Na(+):amino acid(+/-) symport. Meanwhile, a long-range Na(+) cycle first transfers Na(+) from the blood to gastric caeca and anterior midgut lumen where it initiates alkalinization and then returns Na(+) from the rectal lumen to the blood, where it prevents loss of Na(+) during H(+) and HCO(3)(-) excretion. The localization of H(+) V-ATPase and Na(+)/K(+)-ATPase in An. gambiae larvae parallels that reported for Aedes aegypti larvae. The deduced colocalization of the two ATPases along with NHA and NAT in the alimentary canal constitutes a cationic pathway for Na(+)-conserving midgut alkalinization and de-alkalinization which has never been reported before.

Molecular cloning, phylogeny and localization of AgNHA1: the first Na+/H+ antiporter (NHA) from a metazoan,Anopheles gambiae

We have cloned a cDNA encoding a new ion transporter from the alimentary canal of larval African malaria mosquito, Anopheles gambiae Giles sensu stricto. Phylogenetic analysis revealed that the corresponding gene is in a group that has been designated NHA, and which includes(Na+ or K+)/H+ antiporters; so the novel transporter is called AgNHA1. The annotation of current insect genomes shows that both AgNHA1 and a close relative, AgNHA2, belong to the cation proton antiporter 2 (CPA2) subfamily and cluster in an exclusive clade of genes with high identity from Aedes aegypti, Drosophila melanogaster, D. pseudoobscura, Apis mellifera and Tribolium castaneum. Although NHA genes have been identified in all phyla for which genomes are available, no NHA other than AgNHA1 has previously been cloned,nor have the encoded proteins been localized or characterized.

The AgNHA1 transcript was localized in An. gambiae larvae by quantitative real-time PCR (qPCR) and in situ hybridization. AgNHA1 message was detected in gastric caeca and rectum, with much weaker transcription in other parts of the alimentary canal. Immunolabeling of whole mounts and longitudinal sections of isolated alimentary canal showed that AgNHA1 is expressed in the cardia, gastric caeca, anterior midgut, posterior midgut, proximal Malpighian tubules and rectum, as well as in the subesophageal and abdominal ganglia.

A phylogenetic analysis of NHAs and KHAs indicates that they are ubiquitous. A comparative molecular analysis of these antiporters suggests that they catalyze electrophoretic alkali metal ion/hydrogen ion exchanges that are driven by the voltage from electrogenic H+ V-ATPases. The tissue localization of AgNHA1 suggests that it plays a key role in maintaining the characteristic longitudinal pH gradient in the lumen of the alimentary canal of An. gambiae larvae.

Three-dimensional reconstruction of bovine brain V-ATPase by cryo-electron microscopy and single particle analysis

Bovine V-ATPase from brain clathrin-coated vesicles was investigated by cryo-electron microscopy and single particle analysis. Our studies revealed great flexibility of the central linker region connecting V1 and V0. As a consequence, the two sub-complexes were processed separately and the resulting volumes were merged computationally. We present the first three-dimensional (3D) map of a V-ATPase obtained from cryo-electron micrographs. The overall resolution was estimated 34A by Fourier shell correlation (0.5 cutoff). Our 3D reconstruction shows a large peripheral stalk and a smaller, isolated peripheral density, suggesting a second, less well-resolved peripheral connection. The 3D map reveals new features of the large peripheral stator and of the collar-like density attached to the membrane domain. Our analyses of the membrane domain indicate the presence of six proteolipid subunits. In addition, we could localize the V0 subunit a flanking the large peripheral stalk.

The emerging structure of vacuolar ATPases

Bioenergetics and physiology of primary pumps have been revitalized by new insights into the mechanism of energizing biomembranes. Structural information is becoming available, and the three-dimensional structure of F-ATPase is being resolved. The growing understanding of the fundamental mechanism of energy coupling may revolutionize our view of biological processes. The F- and V-ATPases (vacuolar-type ATPase) exhibit a common mechanical design in which nucleotide-binding on the catalytic sector, through a cycle of conformation changes, drives the transmembrane passage of protons by turning a membrane-embedded rotor. This motor can run in forward or reverse directions, hydrolyzing ATP as it pumps protons uphill or creating ATP as protons flow downhill. 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 an ATP-dependent proton pump. The pmf generated by V-ATPases in organelles and membranes of eukaryotic cells is utilized as a driving force for numerous secondary transport processes. V- and F-ATPases have similar structure and mechanism of action, and several of their subunits evolved from common ancestors. Electron microscopy studies of V-ATPase revealed its general structure at low resolution. Recently, several structures of V-ATPase subunits, solved by X-ray crystallography with atomic resolution, were published. This, together with electron microscopy low-resolution maps of the whole complex, and biochemistry cross-linking experiments, allows construction of a structural model for a part of the complex that may be used as a working hypothesis for future research.

Bioenergetics of archaea: ATP synthesis under harsh environmental conditions

Archaea are a heterogeneous group of microorganisms that often thrive under harsh environmental conditions such as high temperatures, extreme pHs and high salinity. As other living cells, they use chemiosmotic mechanisms along with substrate level phosphorylation to conserve energy in form of ATP. Because some archaea are rooted close to the origin in the tree of life, these unusual mechanisms are considered to have developed very early in the history of life and, therefore, may represent first energy-conserving mechanisms. A key component in cellular bioenergetics is the ATP synthase. The enzyme from archaea represents a new class of ATPases, the A1A0 ATP synthases. They are composed of two domains that function as a pair of rotary motors connected by a central and peripheral stalk(s). The structure of the chemically-driven motor (A1) was solved by small-angle X-ray scattering in solution, and the structure of the first A1A0 ATP synthases was obtained recently by single particle analyses. These studies revealed novel structural features such as a second peripheral stalk and a collar-like structure. In addition, the membrane-embedded electrically-driven motor (A0) is very different in archaea with sometimes novel, exceptional subunit composition and coupling stoichiometries that may reflect the differences in energy-conserving mechanisms as well as adaptation to temperatures at or above 100 degrees C.

The where, when, and how of organelle acidification by the yeast vacuolar H+-ATPase

All eukaryotic cells contain multiple acidic organelles, and V-ATPases are central players in organelle acidification. Not only is the structure of V-ATPases highly conserved among eukaryotes, but there are also many regulatory mechanisms that are similar between fungi and higher eukaryotes. These mechanisms allow cells both to regulate the pHs of different compartments and to respond to changing extracellular conditions. The Saccharomyces cerevisiae V-ATPase has emerged as an important model for V-ATPase structure and function in all eukaryotic cells. This review discusses current knowledge of the structure, function, and regulation of the V-ATPase in S. cerevisiae and also examines the relationship between biosynthesis and transport of V-ATPase and compartment-specific regulation of acidification.

Structural and functional separation of the N- and C-terminal domains of the yeast V-ATPase subunit H

The H subunit of the yeast V-ATPase is an extended structure with two relatively independent domains, an N-terminal domain consisting of amino acids 1-348 and a C-terminal domain consisting of amino acids 352-478. We have expressed these two domains independently and together in a yeast strain lacking the H subunit (vma13Delta mutant). The N-terminal domain partially complements the growth defects of the mutant and supports approximately 25% of the wild-type Mg(2+)-dependent ATPase activity in isolated vacuolar vesicles, but surprisingly, this activity is both largely concanamycin-insensitive and uncoupled from proton transport. The C-terminal domain does not complement the growth defects, and supports no ATP hydrolysis or proton transport, even though it is recruited to the vacuolar membrane. Expression of both domains in a vma13Delta strain gives better complementation than either fragment alone and results in higher concanamycin-sensitive ATPase activity and ATP-driven proton pumping than the N-terminal domain alone. Thus, the two domains make complementary contributions to structural and functional coupling of the peripheral V(1) and membrane V(o) sectors of the V-ATPase, but this coupling does not require that they be joined covalently. The N-terminal domain alone is sufficient for activation of ATP hydrolysis in V(1), but the C-terminal domain is essential for proper communication between the V(1) and V(o) sectors.

Mass spectrometric analysis of protein interactions

Mass spectrometry is a powerful tool for identification of interaction partners and structural characterization of protein interactions because of its high sensitivity, mass accuracy and tolerance towards sample heterogeneity. Several tools that allow studies of protein interaction are now available and recent developments that increase the confidence of studies of protein interaction by mass spectrometry include quantification of affinity-purified proteins by stable isotope labeling and reagents for surface topology studies that can be identified by mass-contributing reporters (e.g. isotope labels, cleavable cross-linkers or fragment ions. The use of mass spectrometers to study protein interactions using deuterium exchange and for analysis of intact protein complexes recently has progressed considerably.

Mapping of C-termini of V-ATPase subunits by in vivo-FRET measurements

The plant V-ATPase is a protein complex of 13 different VHA-subunits and functions as ATP driven motor that electrogenically translocates H+ into endomembrane compartments. The central rotor extends into the hexameric head that is fixed by peripheral stators to an eccentric membrane domain. The localization and orientation of VHA-subunits of the head and peripheral stalk region were investigated by in vivo fluorescence resonance energy transfer (FRET). To this end, VHA-E, VHA-G, VHA-H of the peripheral stalks as well as subunits VHA-A and VHA-B were C-terminally fused to cyan (CFP) and yellow fluorescent protein (YFP). Protoplasts transfected with FRET-pairs of CFP-donor and YFP-acceptor fluorophores fused to VHA-subunits were analysed for FRET by laser scanning microscopy. The result of the C-termini mapping allows to refine the arrangement and interaction of the subunits within the V-ATPase complex in vivo. Furthermore, expression of fused VHA-E and VHA-H stimulated acidification of protoplast vacuoles, while other constructs had no major effect on vacuolar pH tentatively indicating a regulatory role of these subunits in plants.

A structural model of the vacuolar ATPase from transmission electron microscopy

Vacuolar ATPases (V-ATPases) are large, membrane bound, multisubunit protein complexes which function as ATP hydrolysis driven proton pumps. V-ATPases and related enzymes are found in the endomembrane system of eukaryotic organsims, the plasma membrane of specialized cells in higher eukaryotes, and the plasma membrane of prokaryotes. The proton pumping action of the vacuolar ATPase is involved in a variety of vital intra- and inter-cellular processes such as receptor mediated endocytosis, protein trafficking, active transport of metabolites, homeostasis and neurotransmitter release. This review summarizes recent progress in the structure determination of the vacuolar ATPase focusing on studies by transmission electron microscopy. A model of the subunit architecture of the vacuolar ATPase is presented which is based on the electron microscopic images and the available information from genetic, biochemical and biophysical experiments.

Ligand-dependent structural changes in the V(1) ATPase from Manduca sexta

The response of V(1) ATPase of the tobacco hornworm Manduca sexta to Mg(2+) and nucleotide binding in the presence of the enhancer methanol has been studied by CuCl(2)-induced disulfide formation, fluorescence spectroscopy, and small-angle X-ray scattering. When the V(1) complex was supplemented with CuCl(2) nucleotide-dependence of A-B-E and A-B-E-D cross-linking products was observed in absence of nucleotides and presence of MgADP+Pi but not when MgAMP.PNP or MgADP were added. A zero-length cross-linking product of subunits D and E was formed, supporting their close proximity in the V(1) complex. The catalytic subunit A was reacted with N-4[4-[7-(dimethylamino)-4-methyl]coumarin-3-yl]maleimide (CM) and spectral shifts and changes in fluorescence intensity were detected upon addition of MgAMP.PNP, -ATP, -ADP+Pi, or -ADP. Differences in the fluorescence emission of these nucleotide-binding states were monitored using the intrinsic tryptophan fluorescence. The structural composition of the V(1) ATPase from M. sexta and conformational alterations in this enzyme due to Mg(2+) and nucleotide binding are discussed on the basis of these and previous observations.

Rotary molecular motors

The F-, V-, and A-adenosine triphosphatases (ATPases) represent a family of evolutionarily related ion pumps found in every living cell. They either function to synthesize adenosine triphosphate (ATP) at the expense of an ion gradient or they act as primary ion pumps establishing transmembrane ion motive force at the expense of ATP hydrolysis. The A-, F-, and V-ATPases are rotary motor enzymes. Synthesis or hydrolysis of ATP taking place in the three catalytic sites of the membrane extrinsic domain is coupled to ion translocation across the single ion channel in the membrane-bound domain via rotation of a central part of the complex with respect to a static portion of the enzyme. This chapter reviews recent progress in the structure determination of several members of the family of F-, A-, and V-ATPases and our current understanding of the rotary mechanism of energy coupling.

Structural Characterization of an ATPase Active F1-/V1 -ATPase (α3β3EG) Hybrid Complex

Co-reconstitution of subunits E and G of the yeast V-ATPase and the alpha and beta subunits of the F(1)-ATPase from the thermophilic Bacillus PS3 (TF(1)) resulted in an alpha(3)beta(3)EG hybrid complex showing 53% of the ATPase activity of TF(1). The alpha(3)beta(3)EG oligomer was characterized by electron microscopy. By processing 40,000 single particle projections, averaged two-dimensional projections at 1.2-2.4-nm resolution were obtained showing the hybrid complex in various positions. Difference mapping of top and side views of this complex with projections of the atomic model of the alpha(3)beta(3) subcomplex from TF(1) (Shirakihara, Y., Leslie, A. G., Abrahams, J. P., Walker, J. E., Ueda, T., Sekimoto, Y., Kambara, M., Saika, K., Kagawa, Y., and Yoshida, M. (1997) Structure 5, 825-836) demonstrates that a seventh mass is located inside the shaft of the alpha(3)beta(3) barrel and extends out from the hexamer. Furthermore, difference mapping of the alpha(3)beta(3)EG oligomer with projections of the A(3)B(3)E and A(3)B(3)EC subcomplexes of the V(1) from Caloramator fervidus (Chaban, Y., Ubbink-Kok, T., Keegstra, W., Lolkema, J. S., and Boekema, E. J. (2002) EMBO Rep. 3, 982-987) shows that the mass inside the shaft is made up of subunit E, whereby subunit G was assigned to belong at least in part to the density of the protruding stalk. The formation of an active alpha(3)beta(3)EG hybrid complex indicates that the coupling subunit gamma inside the alpha(3)beta(3) oligomer of F(1) can be effectively replaced by subunit E of the V-ATPase. Our results have also demonstrated that the E and gamma subunits are structurally similar, despite the fact that their genes do not show significant homology.

Electron-microscopic structure of the V-ATPase from mung bean

The vacuolar H(+)-ATPase from mung bean (Vigna radiata L. cv. Wilczek) was purified to homogeneity. The purified complex contained all the reported subunits from mung bean, but also included a 40-kDa subunit, corresponding to the membrane-associated subunit d, which has not previously been observed. The structure of the V-ATPase from mung bean was studied by electron microscopy of negatively stained samples. An analysis of over 6,000 single-particle images obtained by electron microscopy of the purified complex revealed that the complex, similar to other V-ATPases, is organized into two major domains V1 and Vo with overall dimensions of 25 nm x 13.7 nm and a stalk region connecting the V1 and Vo domains. Several individual areas of protein density were observed in the stalk region, indicating its complexity. The projections clearly showed that the complex contained one central stalk and at least two peripheral stalks. Subcomplexes containing subunits A, B and E, dissociated from the tonoplast membrane by KI, were purified. The structure of the subcomplex was also studied by electron microscopy followed by single-molecule analysis of 13,000 projections. Our preliminary results reveal an area of high protein density at the bottom of the subcomplex immediately below the cavity formed by the A and B subunits, indicating the position of subunit E.

Three-Dimensional Structure of the Vacuolar ATPase

The structure of the proton-pumping vacuolar ATPase (V-ATPase) from bovine brain clathrin coated vesicles was analyzed by electron microscopy and single molecule image analysis. A three-dimensional structural model of the complex was calculated by the angular reconstitution method at a resolution of 27 A. Overall, the appearance of the V(0) and V(1) domains in the three-dimensional model of the intact bovine V-ATPase resembles the models of the isolated bovine V(0) and yeast V(1) domains determined previously. To determine the binding position of subunit H in the V-ATPase, electron microscopy and cysteine-mediated photochemical cross-linking were used. Difference maps calculated from projection images of intact bovine V-ATPase and a V-ATPase preparation in which the two H subunit isoforms were removed by treatment with cystine revealed less protein density at the bottom of the V(1) in the subunit H-depleted enzyme, suggesting that subunit H isoforms bind at the interface of the V(1) and V(0) domains. A comparison of three-dimensional models calculated for intact and subunit H-depleted enzyme indicated that at least one of the subunit H isoforms, although poorly resolved in the three-dimensional electron density, binds near the putative N-terminal domain of the a subunit of the V(0). For photochemical cross-linking, unique cysteine residues were introduced into the yeast V-ATPase B subunit at sites that were localized based on molecular modeling using the crystal structure of the mitochondrial F(1) domain. Cross-linking was performed using the photoactivatable sulfhydryl reagent 4-(N-maleimido)benzophenone. Cross-linking to subunit H was observed from two sites on subunit B (E494 and T501) predicted to be located on the outer surface of the subunit closest to the membrane. Results from both electron microscopy and cross-linking analysis thus place subunit H near the interface of the V(1) and V(0) domains and suggest a close structural similarity between the V-ATPases of yeast and mammals.

The insect plasma membrane H+ V-ATPase: intra-, inter-, and supramolecular aspects

The plasma membrane H+ V-ATPase from the midgut of larval Manduca sexta, commonly called the tobacco hornworm, is the sole energizer of epithelial ion transport in this tissue, being responsible for the alkalinization of the gut lumen up to a pH of more than 11 and for any active ion movement across the epithelium. This minireview deals with those topics of our recent research on this enzyme that may contribute novel aspects to the biochemistry and physiology of V-ATPases. Our research approaches include intramolecular aspects such as subunit topology and the inhibition by macrolide antibiotics, intermolecular aspects such as the hormonal regulation of V-ATPase biosynthesis and the interaction of the V-ATPase with the actin cytoskeleton, and supramolecular aspects such as the interactions of V-ATPase, K+/H+ antiporter, and ion channels, which all function as an ensemble in the transepithelial movement of potassium ions.

Subunit composition, structure, and distribution of bacterial V-type ATPases

The overall structure of V-ATPase complexes resembles that of F-type ATPases, but the stalk region is different and more complex. Database searches followed by sequence analysis of the five water-soluble stalk region subunits C-G revealed that (i) to date V-ATPases are found in 16 bacterial species, (ii) bacterial V-ATPases are closer to archaeal A-ATPases than to eukaryotic V-ATPases, and (iii) different groups of bacterial V-ATPases exist. Inconsistencies in the nomenclature of types and subunits are addressed. Attempts to assign subunit positions in V-ATPases based on biochemical experiments, chemical cross-linking, and electron microscopy are discussed. A structural model for prokaryotic and eukaryotic V-ATPases is proposed. The prokaryotic V-ATPase is considered to have a central stalk between headpiece and membrane flanked by two peripheral stalks. The eukaryotic V-ATPases have one additional peripheral stalk.

TM2 but not TM4 of subunit c'' interacts with TM7 of subunit a of the yeast V-ATPase as defined by disulfide-mediated cross-linking

The vacuolar (H+)-ATPase (or V-ATPase) is an ATP-dependent proton pump which couples the energy released upon ATP hydrolysis to rotational movement of a ring of proteolipid subunits (c, c', and c'') relative to the integral subunit a. The proteolipid subunits each contain a single buried acidic residue that is essential for proton transport, with this residue located in TM4 of subunits c and c' and TM2 of subunit c''. Subunit c'' contains an additional buried acidic residue in TM4 that is not required for proton transport. The buried acidic residues of the proteolipid subunits are believed to interact with an essential arginine residue (Arg735) in TM7 of subunit a during proton translocation. We have previously shown that the helical face of TM7 of subunit a containing Arg735 interacts with the helical face of TM4 of subunit c' bordered by Glu145 and Leu147 (Kawasaki-Nishi et al. (2003) J. Biol. Chem. 278, 41908-41913). We have now analyzed interaction of subunits a and c'' using disulfide-mediated cross-linking. The results indicate that the helical face of TM7 of subunit a containing Arg735 interacts with the helical face of TM2 of subunit c'' centered on Ile105, with the essential glutamic acid residue (Glu108) located near the opposite border of this face compared with TM4 of subunit c'. By contrast, TM4 of subunit c'' does not form strong cross-links with TM7 of subunit a, suggesting that these transmembrane segments are not normally in close proximity. These results are discussed in terms of a model involving rotation of interacting helices in subunit a and the proteolipid subunits relative to each other.

Building the stator of the yeast vacuolar-ATPase: specific interaction between subunits E and G

The vacuolar (H+)-ATPase (or V-ATPase) is a membrane protein complex that is structurally related to F1 and F0 ATP synthases. The V-ATPase is composed of an integral domain (V0) and a peripheral domain (V1) connected by a central stalk and up to three peripheral stalks. The number of peripheral stalks and the proteins that comprise them remain controversial. We have expressed subunits E and G in Escherichia coli as maltose binding protein fusion proteins and detected a specific interaction between these two subunits. This interaction was specific for subunits E and G and was confirmed by co-expression of the subunits from a bicistronic vector. The EG complex was characterized using size exclusion chromatography, cross-linking with short length chemical cross-linkers, circular dichroism spectroscopy, and electron microscopy. The results indicate a tight interaction between subunits E and G and revealed interacting helices in the EG complex with a length of about 220 angstroms. We propose that the V-ATPase EG complex forms one of the peripheral stators similar to the one formed by the two copies of subunit b in F-ATPase.

Colocalization and FRET-analysis of subunits c and a of the vacuolar H+-ATPase in living plant cells

The proton-translocating plant vacuolar H(+)-ATPase (VHA) is of prime importance for acidification of intracellular compartments and is essential for processes such as secondary activated transport, maintenance of ion homeostasis, and adaptation to environmental stress. Twelve genes have been identified that encode subunits of the functional V-ATPase complex. In this study, subunits c and a of the V-ATPase from the plant Mesembryanthemum crystallinum were fused to cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP), respectively, and were transiently coexpressed in protoplasts. Two-colour scanning confocal fluorescence microscopy demonstrates that the fusion proteins VHA-c-CFP and VHA-a-YFP are colocalized at the tonoplast, the plasmamembrane, and at endoplasmic membrane structures indicating expression in cytoplasmic vesicles. Furthermore, fluorescence resonance energy transfer (FRET) was used to visualize the interaction of VHA-c and VHA-a in vivo on the nanometer length scale. Excitation of CFP as donor fluorophore caused increased emission of YFP-fluorescence in protoplasts due to FRET. Our results give strong evidence for physical interaction of subunits c and a in living plant cells.

Structure and subunit arrangement of the A-type ATP synthase complex from the archaeon Methanococcus jannaschii visualized by electron microscopy

In Archaea, bacteria, and eukarya, ATP provides metabolic energy for energy-dependent processes. It is synthesized by enzymes known as A-type or F-type ATP synthase, which are the smallest rotatory engines in nature (Yoshida, M., Muneyuki, E., and Hisabori, T. (2001) Nat. Rev. Mol. Cell. Biol. 2, 669-677; Imamura, H., Nakano, M., Noji, H., Muneyuki, E., Ohkuma, S., Yoshida, M., and Yokoyama, K. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2312-2315). Here, we report the first projected structure of an intact A(1)A(0) ATP synthase from Methanococcus jannaschii as determined by electron microscopy and single particle analysis at a resolution of 1.8 nm. The enzyme with an overall length of 25.9 nm is organized in an A(1) headpiece (9.4 x 11.5 nm) and a membrane domain, A(0) (6.4 x 10.6 nm), which are linked by a central stalk with a length of approximately 8 nm. A part of the central stalk is surrounded by a horizontal-situated rodlike structure ("collar"), which interacts with a peripheral stalk extending from the A(0) domain up to the top of the A(1) portion, and a second structure connecting the collar structure with A(1). Superposition of the three-dimensional reconstruction and the solution structure of the A(1) complex from Methanosarcina mazei Gö1 have allowed the projections to be interpreted as the A(1) headpiece, a central and the peripheral stalk, and the integral A(0) domain. Finally, the structural organization of the A(1)A(0) complex is discussed in terms of the structural relationship to the related motors, F(1)F(0) ATP synthase and V(1)V(0) ATPases.

Differential expression of vacuolar H+-ATPase subunit c genes in tissues active in membrane trafficking and their roles in plant growth as revealed by RNAi

Acidification of intracellular compartments by the vacuolar-type H(+)-ATPases (VHA) is known to energize ion and metabolite transport, though cellular processes influenced by this activity are poorly understood. At least 26 VHA genes encode 12 subunits of the V(1)V(o)-ATPase complex in Arabidopsis, and how the expression, assembly, and activity of the pump are integrated into signaling networks that govern growth and adaptation are largely unknown. The role of multiple VHA-c genes encoding the 16-kD subunit of the membrane V(o) sector was investigated. Expression of VHA-c1, monitored by promoter-driven beta-glucuronidase (GUS) activity was responsive to light or dark in an organ-specific manner. VHA-c1 expression in expanding cotyledons, hypocotyls of etiolated seedlings, and elongation zone of roots supported a role for V-ATPase in cell enlargement. Mutants reduced in VHA-c1 transcript using dsRNA-mediated interference showed reduction in root growth relative to wild-type seedlings. In contrast, VHA-c3 promoter::GUS expression was undetectable in most organs of seedlings, but strong in the root cap. Interestingly, dsRNA-mediated mutants of vha-c3 also showed reduced root length and decreased tolerance to moderate salt stress. The results suggest that V-ATPase functions in the root cap influenced root growth. Expression of VHA-c1 and VHA-c3 in tissues with active membrane flow, including root cap, vascular strands, and floral style would support a model for participation of the V(o) sector and V(1)V(o)-ATPase in membrane trafficking and fusion. Two VHA-c genes are thus differentially expressed to support growth in expanding cells and to supply increased demand for V-ATPase in cells with active exocytosis.

Three-dimensional organization of the archaeal A1-ATPase from Methanosarcina mazei Gö1

A modified isolation procedure provides a homogeneous A(1)-ATPase from the archaeon Methanosarcina mazei Gö1, containing the five subunits in stoichiometric amounts of A(3):B(3):C:D:F. A(1) obtained in this way was characterized by three-dimensional electron microscopy of single particles, resulting in the first three-dimensional reconstruction of an A(1)-ATPase at a resolution of 3.2 nm. The A(1) consists of a headpiece of 10.2 nm in diameter and 10.8 nm in height, formed by the six elongated subunits A(3) and B(3). At the bottom of the A(3)B(3) complex, a stalk of 3.0 nm in length can be seen. The A(3)B(3) domain surrounds a large cavity that extends throughout the length of the A(3)B(3) barrel. A part of the stalk penetrates inside this cavity and is displaced toward an A-B-A triplet. To investigate further the topology of the stalk subunits C-F in A(1), cross-linking has been carried out by using dithiobis[sulfosuccinimidylpropionate] (DSP) and 1-ethyl-3-(dimethylaminopropyl)-carbodiimide (EDC). In experiments where DSP was added the cross-linked products B-F, A(x)-D, A-B-D, and A(x)-B(x)-D were formed. Subunits B-F, A-D, A-B-D, and A-B-C-D could be cross-linked by EDC. The subunit-subunit interaction in the presence of DSP was also studied as a function of nucleotide binding, demonstrating movements of subunits C, D, and F during ATP cleavage. Finally, the three-dimensional organization of this A(1) complex is discussed in terms of the relationship to the F(1)- and V(1)-ATPases at a resolution of 3.2 nm.

Yeast V1-ATPase: affinity purification and structural features by electron microscopy

V1-ATPase from the yeast Saccharomyces cerevisiae was purified via a FLAG affinity tag introduced into the N terminus of the G subunit. The preparation migrated as a single band in native gel electrophoresis and contained subunits ABCDEFGH (with subunit C present at substoichiometric amounts) as determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The initial specific Ca-ATPase activity was approximately 6 micromol/min/mg. The structure of the yeast V1-ATPase was studied by electron microscopy of negatively stained and frozen hydrated samples. A 25-A resolution three-dimensional model of the complex was calculated from two-dimensional projections by the angular reconstitution technique. The model shows six elongated densities arranged in pseudo-3-fold symmetry around a large central cavity. At the top of the molecule, various protrusions can be seen. At the bottom of the complex, two large masses are visible that are connected to the main body of the molecule. Comparison of the yeast V1 structure with the structure of the intact V1V0-ATPase from bovine brain clathrin-coated vesicles (Wilkens, S., Vasilyeva, E., and Forgac, M. (1999) J. Biol. Chem. 274, 31804-31810) indicates that the structure of the isolated V1 from yeast is very similar to the structure of the V1 domain in the intact V-ATPase complex.

Subunit arrangement in V-ATPase from Thermus thermophilus

The V0V1-ATPase of Thermus thermophilus catalyzes ATP synthesis coupled with proton translocation. It consists of an ATPase-active V1 part (ABDF) and a proton channel V0 part (CLEGI), but the arrangement of each subunit is still largely unknown. Here we found that acid treatment of V0V1-ATPase induced its dissociation into two subcomplexes, one with subunit composition ABDFCL and the other with EGI. Exposure of the isolated V0 to acid or 8 m urea also produced two subcomplexes, EGI and CL. Thus, the C subunit (homologue of d subunit, yeast Vma6p) associates with the L subunit ring tightly, and I (homologue of 100-kDa subunit, yeast Vph1p), E, and G subunits constitute a stable complex. Based on these observations and our recent demonstration that D, F, and L subunits rotate relative to A3B3 (Imamura, H., Nakano, M., Noji, H., Muneyuki, E., Ohkuma, S., Yoshida, M., and Yokoyama, K. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2312-2315; Yokoyama, K., Nakano, M., Imamura, H., Yoshida, M., and Tamakoshi, M. (2003) J. Biol. Chem. 278, 24255-24258), we propose that C, D, F, and L subunits constitute the central rotor shaft and A, B, E, G, and I subunits comprise the surrounding stator apparatus in the V0V1-ATPase.

Biochemical support for the V-ATPase rotary mechanism: antibody against HA-tagged Vma7p or Vma16p but not Vma10p inhibits activity

V-ATPase null mutants in yeast have a distinct, conditionally lethal phenotype that can be obtained through disruption of any one of its subunits. This enables supplementation of this mutant with the relevant subunit tagged with an epitope against which an antibody is available. In this system, the effect of antibody on the activity of the enzyme can be analyzed. Towards this end we used HA to tag subunits Vma7p, Vma10p and Vma16p, which are assumed to represent, respectively, the shaft, stator and turbine of the enzyme, and used them to supplement the corresponding yeast V-ATPase null mutants. The anti-HA epitope antibody inhibited both the ATP-dependent proton uptake and the ATPase activities of the Vma16p-HA and Vma7p-HA containing complexes, in intact vacuoles and in the detergent-solubilized enzyme. Neither of these activities was inhibited by the antibody in Vma10p-HA containing enzyme. These results support the function of Vma10p as part of the stator, while the other tagged subunits are part of the rotor apparatus. The HA-tag was attached to the N terminus of Vma16p; thus the antibody inhibition points to its accessibility outside the vacuolar membrane. This assumption is supported by the supplementation of the yeast mutant by the homologues of Vma16p isolated from Arabidopsis thaliana and lemon fruit c-DNA. Contrary to yeast, which has five predicted helices, the plant subunit Vma16p has only four. Our results confirm a recent report that only four of the yeast Vma16p complexes are actually transmembrane helices.

Dimer formation of subunit G of the yeast V-ATPase

The G subunit of the vacuolar ATPase (V-ATPase) is a component of the stalk connecting the V(1) and V(O) sectors of the enzyme and is essential for normal assembly and function. Subunit G (Vma10p) of the yeast V-ATPase was expressed in Escherichia coli as a soluble protein and was purified to homogeneity. The molecular mass of subunit G, determined by Native-polyacrylamide gel electrophoresis, gel filtration analysis and small-angle X-ray scattering, was approximately 28+/-2 kDa, indicating that this protein is dimeric. With a radius of gyration (R(g)) and a maximum size (D(max)) of 2.7+/-0.2 nm and 8.0+/-0.3 nm, respectively, the G-dimer is rather elongated. To understand which region of subunit G is required to mediate dimerization, a G(38-144) form (the carboxyl-terminus) was expressed and purified. G(38-144) is homogeneous, with a molecular mass of approximately 12+/-3 kDa, indicating a monomeric form in solution.

Proton translocation driven by ATP hydrolysis in V-ATPases

The vacuolar H(+)-ATPases (or V-ATPases) are a family of ATP-dependent proton pumps responsible for acidification of intracellular compartments and, in certain cases, proton transport across the plasma membrane of eukaryotic cells. They are multisubunit complexes composed of a peripheral domain (V(1)) responsible for ATP hydrolysis and an integral domain (V(0)) responsible for proton translocation. Based upon their structural similarity to the F(1)F(0) ATP synthases, the V-ATPases are thought to operate by a rotary mechanism in which ATP hydrolysis in V(1) drives rotation of a ring of proteolipid subunits in V(0). This review is focused on the current structural knowledge of the V-ATPases as it relates to the mechanism of ATP-driven proton translocation.

Resolution of the V1 ATPase from Manduca sexta into subcomplexes and visualization of an ATPase-active A3B3EG complex by electron microscopy

The effect of the ATPase activity of Manduca sexta V(1) ATPase by the amphipathic detergent lauryldimethylamine oxide (LDAO) and the relationship of these activities to the subunit composition of V(1) were studied. The V(1) was highly activated in the presence of 0.04-0.06% LDAO combined with release of the subunits H, C, and F from the enzyme. Increase of LDAO concentration to 0.1-0.2% caused the characterized subcomplexes A(3)B(3)HEGF and A(3)B(3)EG with a remaining ATPase activity of 52 and 65%, respectively. The hydrolytic-active A(3)B(3)EG subcomplex has been visualized by electron microscopy showing six major masses of density in a pseudo-hexagonal arrangement surrounding a seventh mass. The compositions of the various subcomplexes and fragments of V(1) provide an organization of the subunits in the enzyme in the framework of the known three-dimensional reconstruction of the V(1) ATPase from M. sexta (Radermacher, M., Ruiz, T., Wieczorek, H., and Grüber, G. (2001) J. Struct. Biol. 135, 26-37).

Expression, purification, and characterization of subunit E, an essential subunit of the vacuolar ATPase

A recombinant form of subunit E (Vma4p) from yeast vacuolar ATPases (V-ATPases) has been overexpressed in Escherichia coli, purified to homogeneity, and explored by mass spectrometry. Analysis of the secondary structure of Vma4p by circular dichroism spectroscopy indicated 32% alpha-helix and 23% beta-sheet content. Vma4p formed a hybrid-complex with the nucleotide-binding subunits alpha and beta of the closely related F(1) ATPase of the thermophilic bacterium PS3 (TF(1)). The alpha(3)beta(3)E-hybrid-complex had 56% of the ATPase activity of the native TF(1). By comparison, an alpha(3)beta(3)-formation without Vma4p showed about 24% of total TF(1) ATPase activity. This is the first demonstration of a hydrolytically active hybrid-complex consisting of F(1) and V(1) subunits. The arrangement of subunit E in V(1) has been probed using the recombinant Vma4p, the alpha(3)beta(3)E-hybrid-complex together with V(1) and an A(3)B(3)HEG-subcomplex of the V(1) ATPase from Manduca sexta, respectively, indicating that subunit E is shielded in V(1).

Cross-talk in the A1-ATPase from Methanosarcina mazei Go1 due to nucleotide binding

Changes in the A(3)B(3)CDF-complex of the Methanosarcina mazei Gö1 A(1)-ATPase in response to ligand binding have been studied by small-angle x-ray scattering, protease digestion, fluorescence spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, and CuCl(2)-induced disulfide formation. The value of the radius of gyration, R(g), increases slightly when MgATP, MgADP, or MgADP + P(i) (but not MgAMP-PNP) is present. The nucleotide-binding subunits A and B were reacted with N-4[4-[7-(dimethylamino)-4-methyl]coumarin-3-yl]maleimide, and spectral shifts and changes in fluorescence intensity were detected upon addition of MgAMP-PNP, MgATP, MgADP + P(i), or MgADP. Trypsin treatment of A(1) resulted in cleavage of the stalk subunits C and F, which was rapid in the presence of MgAMP-PNP but slow when MgATP or MgADP were added to the enzyme. When A(1) was supplemented with CuCl(2) a clear nucleotide dependence of an A-A-D cross-linking product was generated in the presence of MgADP and MgATP but not when MgAMP-PNP or MgADP + P(i) was added. The site of cross-link formation was located in the region of the N and C termini of subunit D. The data suggest that the stalk subunits C, D, and F in A(1) undergo conformational changes during ATP hydrolysis.

Expression ofManduca sextaV-ATPase genesmvB, mvGandmvdis regulated by ecdysteroids

V-ATPases are complex proteins consisting of a peripheral, ATP-hydrolysing V1 complex and a membrane-bound H+-translocating Vo complex. The plasma membrane V-ATPase from the tobacco hornworm(Manduca sexta) midgut is made up of eight different V1and four different Vo subunits. During starvation and moulting,V-ATPase activity decreases as a result of the dissociation of the V1 complex from the Vo complex. To determine whether subunit biosynthesis is reduced during periods of enzyme inactivity, we measured the transcript levels and transcriptional activities of V-ATPase genes. Northern blots revealed the downregulation of almost all V-ATPase transcripts during starvation. During moulting, transcript levels of the three V-ATPase genes examined, mvB, mvG and mvd, also decreased,and this decrease was negatively correlated with the titre of 20-hydroxyecdysone (20-HE) and positively correlated with the titre of juvenile hormone (JH). To test the biological significance of these correlations, we injected both hormones into feeding larvae and measured transcript levels several hours later. A short-term increase and a long-term decrease in levels of mRNA were observed after 20-HE injection, whereas JH injection had no significant effect. Immunohistochemical studies of the midgut epithelium revealed that 20-HE injection led to changes in goblet cell morphology and in the subcellular distribution of the V1 complex comparable with the situation during the moult and during starvation. Reporter gene assays in Sf21 cells using mvB, mvG and mvdpromoters to initiate transcription of firefly luciferase led, after incubation of the cells with 20-HE, to results comparable with those obtained in the injection experiments. These findings suggest that putative ecdysone-responsive elements are present in all three promoters. Taken together, our results suggest that the expression of V-ATPase genes is controlled in a coordinated manner by ecdysteroids.

Three-dimensional map of a plant V-ATPase based on electron microscopy

V-ATPases pump protons into the interior of various subcellular compartments at the expense of ATP. Previous studies have shown that these pumps comprise a membrane-integrated, proton-translocating (V(0)), and a soluble catalytic (V(1)) subcomplex connected to one another by a thin stalk region. We present two three-dimensional maps derived from electron microscopic images of the complete V-ATPase complex from the plant Kalanchoë daigremontiana at a resolution of 2.2 nm. In the presence of a non-hydrolyzable ATP analogue, the details of the stalk region between V(0) and V(1) were revealed for the first time in their three-dimensional organization. A central stalk was surrounded by three peripheral stalks of different sizes and shapes. In the absence of the ATP analogue, the tilt of V(0) changed with respect to V(1), and the stalk region was less clearly defined, perhaps due to increased flexibility and partial detachment of some of the peripheral stalks. These structural changes corresponded to decreased stability of the complex and might be the initial step in a controlled disassembly.