V-ATPase in plants: an overview V-ATPase: structure and role in plants.
The centrally located vacuole occupies about 90% of the total volume of a mature plant cell and represents a cellular compartment that is of the major importance. In the past, the vacuole was wrongly assumed to be a sump for waste and toxic molecules . However, recent intensive research has established that the vacuole is an essential compartment for maintaining cytosolic pH and ion homeostasis, nutrient acquisition [2,3] as well as for the short and long term storage of metabolites [4,5]. Molecules stored in the vacuole include various inorganic ions such as sodium, potassium, calcium, magnesium, heavy metals, chloride and nitrate , organic molecules like carbohydrates, organic and amino acids, polyamines, peptides , and products of secondary plant metabolism . The vacuoles may also possess limited biosynthetic capacity as has been demonstrated for the enzymes of hydroxycinnamic acid esters in red-radish seedlings [9,10]. The pH of the isolated vacuoles was found to be ca. 6.3 .
Various substances have to be transported into and out of the vacuole. Transport of these ions and metabolites across the vacuolar membrane is mediated at the expense of electrochemical gradient generated by energy-driven [H.sup.+]- translocating enzymes. These enzymes can convert the chemical free energy released by the hydrolysis of an energy rich biomolecule, often ATP, into vectorial proton transport and may also utilize the proton motive force (PMF) for ATP formation . According to chemiosmotic concept , the resulting pH gradient and membrane potential, i.e. primary active transport may be used as the driving force (Fig. 1) to transport solutes and ions across the membrane (secondary active transport) by specific ion channels and carriers [7,14-15].
[FIGURE 1 OMITTED]
There are three distinct classes of [H.sup.+] ATPases in plants viz. V-ATPases, P-ATPases and F-ATPases . The P-ATPases (or [E.sub.1]-[E.sub.2] type) (plasma membrane type) operates via phosphoenzyme intermediate ; the F-type catalyse the synthesis of ATP during oxidative (photo) phosphorylation in mitochondria and chloroplasts ; and the V-ATPases (the vacuolar type) (EC 188.8.131.52) couple ATP hydrolysis with H+-transport in the vacuoles as well as a variety of other vesicles . The V-ATPase creates an electrochemical proton gradient across the tonoplast, which is used for secondary-active solute uptake mediated by specific transporters, as well as is pivotal to pH homeostasis of the cytoplasm.
Recent studies, including proteomic analysis for several plant species have revealed the involvement of vacuole and vacuolar transporters in a variety of functions e.g. nitrogen storage, salinity tolerance, metal homeostasis, calcium signalling, guard cell movements and the cellular pH homeostasis . It is evident that vacuolar transporters are an integrated part of a complex cellular network that enables a plant to react properly to changing environmental conditions, to save nutrients and energy and to maintain optimal metabolic conditions in the cytosol. Of the two known vacuolar transporters viz. V-ATPase and [H.sup.+]-translocating inorganic pyrophosphatase (V-PPase) , the former is dominant and indispensable for the plant growth under normal conditions. Besides, the V-ATPase is of prime importance for plant cell expansion and stress adaptation . In 2000 the discovery of sequence information for all subunits of V-ATPase for two plant species; Arabidopsis thaliana  and Mesembryanthemum crystallium  has aroused a great deal of concern of plant biologists on advance molecular aspects of V-ATPase. Keeping both classical as well as advanced information in mind, the present review describes various aspects of the V-ATPase in plants.
Distribution and Characteristics of V-ATPase in Plants
ATPases are found in all eukaryotic cells and are highly conserved among species. The presence of V-ATPases is not exclusively restricted to the tonoplast but they have also been detected in other subcellular fractions in the plant cell. The biochemical data supported by immunocytochemical investigations provided evidence that the V-ATPases are also found in the membranes of secretary pathways e.g. endoplasmic reticulum, Golgi apparatus and clathrin coated vesicles [24-26].
ATP is the substrate of V-ATPase, the nucleotide specificity is in the order as ATP [much greater than] GTP > NTP [27,28]. The enzyme exhibits the Km value in the range from 0.25-0.8 mM [4,28-30]. The ATPase activity is stimulated with increasing ATP in the concentration range of 0.5-2 mM . The ATP divalent cation complexes [MgATP.sup.2-] > [MnATP.sup.2-] > [ZnATP.sup.2-] > [CaATP.sup.2-], [CoATP.sup.2-] function as substrates [28,32]. The pH optimum of the enzyme is in the range of 7.5-8.0 and the activity is stimulated by anions ([Cl.sub.-], [Br.sub.-] > HC[O.sub.3.sup.-]) [28,33]. The activity of V-ATPase can be distinguished from other [H.sup.+] ATPases by its specific characteristics. It is stimulated by anions , insensitive to vanadate, unlike plasma membrane ATPase that is inhibited by orthovanadate by formation of phosphorylated intermediate . Both V- and F-ATPases are inhibited by N[O.sub.3-], but azide, a specific inhibitor of mitochondrial ATPase  has no effect on the former. Further, bafilomycin [A.sub.1] inhibits the V-ATPase in nanomolar concentrations  and concanamycin A is even more effective by a factor of ten . Thus anion stimulated, nitrate, bafilomycin and concanamycin sensitive, azide resistant and vanadate insensitive activity is characterized as V-ATPase.
Amongst various phytohormones, the V-ATPase is stimulated only by phenylacetic acid, the other auxins have no effect, whereas [GA.sub.3] and kinetin acted as inhibitory to various degrees . Of the various phenolics, hydroxyquinone and hydroxybenzoic acid showed inhibition of ATPase activity; chlorogenic acid was completely inhibitory whereas cinnamic acid was stimulatory .
V-ATPase pumps the protons into membrane surrounded intercellular compartments at the expense of hydrolysis of ATP. The twin activities of V-ATPase, i.e. proton translocation and ATP hydrolysis (measured by the release of Pi) can be measured simultaneously . On the basis of investigations in CAM plants, a stoichiometry of [2H.sup.+] transported/ ATP hydrolysed was frequently measured, but values ranged from 1.7-3.3 [H.sup.+]/ATP [4,41]. It is suggested that the values may vary depending on the transtonoplast pH gradient , from considerations of the energetics, a vacuolar [H.sup.+]-accumulation to pH 3 or lower depends on a [H.sup.+]/ATP-ratio of <2.
Structure of V-ATPase: Subunit Composition and their Isoforms
The gross architecture of V-ATPases is similar to that of F-ATPases found at the mitochondrial and chloroplastidic membranes. The V-ATPase has a molecular mass of ca. > 700 kD in higher plants, a highly abundant protein making up to 6.5-35% of the total tonoplast protein in different species [4,26,43]. V-ATPase share a common lollipop shaped structure composed of a ball like head ([V.sub.1]) (9.4-11.9 nm diameter), a membrane-intrinsic part ([V.sub.o]) and connecting stalks (2.3-3.1 nm and 4.5-6.6 nm in diameter and length, respectively) similar to ATP-producing F-ATP synthases [30,44-45]. It is a multisubunit enzyme with up to 14 subunits in higher plants [26,46]. It is composed of two functional domains interconnected by peripheral and central stalks (Fig. 2). The 640 kD catalytic [V.sub.1] domain contains eight different subunits designated as VHA-A, B, C, D, E, F, G and H, and the 260 kD membranous [V.sub.o] domain contains up to six different subunits (a, c, c', c", d and e) [21,47-48]. On a stoichiometric basis, the most abundant peptides are subunit A (63-72 kD), subunit B (52-60 kD) and subunit c (16-20 kD). The V-ATPase holoenzyme contains 3 copies of subunit A and B, 6 copies of subunit c and one copy each of all other subunits [5,49-50].
[FIGURE 2 OMITTED]
The V-ATPase is closely related to bacterial and mitochondrial F-ATPase, and they both share structural and functional similarities . As revealed by rotational image analysis  and image averaging techniques , the V-ATPase head contain 3 copies each of the subunit A and B, which are seen as a hexameric structure. Whereas both the subunits contain nucleotide-binding domains, subunit A seems to represent the catalytic subunit as revealed through extensive biochemical characterization . Subunit B is considered to have regulatory function. Subunit C, D, E, F, G and H represent the stalk subunits with D as the main subunit forming the central stalk [50,55]. The three-dimensional maps of V-ATPase based on electron microscopy revealed that a central stalk was surrounded by three peripheral stalks of different sizes and shapes  (Fig. 3). The thin central stalk had a diameter of 3.6 nm. Three peripheral stalks connected the [V.sub.1] head to the [V.sub.O] sector and were denominated as prominent (4.9 nm), intermediate (3.6 nm) and faint (2.4 nm) stalk, respectively. The enzyme showed hexagonal arrangement only in the upper part of [V.sub.1], where the three peripheral stalks did not interfere with the subunit arrangements of A and B subunits. VHA (Vacuolar-type [H.sup.+]-pumping ATP hydrolase) subunit H has been crystallized  and is considered to activate and regulate V-ATPase by functionally coupling ATP hydrolysis to proton flow through the [V.sub.O]-domain . The [V.sub.O] component has at least 56 subunits , i.e. a, d and the proteolipids c, c' and c" and e. The characteristic structural component of [V.sub.O] seems to be a ring of at least 6 proteolipid molecules of subunit c . Various copies of subunit c show high degree of cooperative binding amongst them as the binding of a single DCCD molecule abolishes the V-ATPase activity . VHA-a is the subunit with the highest molecular mass within the V-ATPase and reveals a bipartite structure. On the basis of sequence data VHA-a appears to be an essential structural and functional element of V-ATPase, although previously a sole function in assembly has been proposed . VHA-d and VHA-e (8 kDa) have not been analysed in plants yet .
[FIGURE 3 OMITTED]
In recent times, the existence of various isoforms of different subunits of V-ATPase has also been demonstrated. On the basis of immuno-localisation studies, it was shown that one distinct isoform of VHA-a subunit is exclusively located on the endoplasmic reticulum . This may imply that the different isoforms of VHA-a may localize on distinct endomembrane compartments. In Arabidopsis, there is differential expression of two VHA-c genes to support growth in expanding cells and to supply increased demand for V-ATPase in cells with active exocytosis . Two different isoforms [A.sub.1] and [A.sub.2] of VHA-A were shown in tomato; the subcellular localization indicated that the highest levels of both these isoforms were in the tonoplast . On the basis of MALDI mass spectrometry analysis and N-terminal amino acid sequencing of proteolytic fragments, molecular evidence was provided for existence of two different forms of subunit E in the leaves of the obligate CAM species, Kalanchoe daigremontiana .
Mechanism of Action of V-ATPases
There is sufficient evidence to suggest that V-ATPase shows functional analogy with the F-ATPase in its mechanism of action . The F-ATPase has directly been shown to work by a rotary mechanism in which conformational changes in the catalytic sector cause a continuous rotation of the [gamma] subunit within it, leading to a counter clockwise turning of the membrane sector c-ring against the membranous large a subunit . In the F-ATP synthase, the suggested rotary mechanism of catalysis couples proton conduction in the [V.sub.O]-sector to ATP synthesis in the [V.sub.1]-head . A similar approach was recently used to show the rotation mechanism of V-ATPase . Using a genetically engineered enzyme with a His-tagged proteolipid subunit and a biotin binding domain connected to the a subunit, it was concluded that the rotary mechanism is well conserved in both F- and V-ATPases, despite significant structural and functional differences between them.
Though V- and F-ATPases are structurally and functionally related, they are distinguished in eukaryotic cells by the distribution of their genes. The genes encoding all the subunits of V-ATPases are present in nucleus, while the genetic information for F- ATPases is shared by the nuclear and organellar DNA . The sequence information for the various subunits of V-ATPase is available from two plant species, Arabidopsis thaliana  and Mesembryanthemum crystallinum . This has allowed the identification of gene families for VHA subunits [21,47]. Undoubtedly the regulation of V-ATPases is a highly complex process as it requires the expression of genes encoding the various subunits, assembly and targeting of the V-ATPase holoenzyme to functionally diverse subcellular compartments, and modulation of activity in response to a host of developmental, physiological and environmental stimuli. The gene knockout studies of different subunits of ATPase result in lethal phenotypes clearly proving the essentiality of the enzyme for survival of plant cell . Much of the current understanding of V-ATPase structure, and the steps involved in the assembly of the [V.sub.O] [V.sub.1] holoenzyme is in fact, derived almost entirely from molecular, genetic and biochemical analysis of the VMA mutants [26,59]. Functional complementation of VMA1 and VMA10 mutants by plant genes encoding homologs to subunits A  and G , respectively restores the wild type phenotype and attests to the conservation of V-ATPase structure and function in eukaryotes during the course of evolution  especially in plant and animal species.
V-ATPase Versus Stress
On the basis of the available literature there is sufficient evidence to conclude a significant relevance of functional V-ATPase adaptation of plants to unfavourable growth conditions under stress. The V-ATPase has been recognized as an enzyme both serving stress responses and undergoing stress-related modifications [69-71]. Due to these stress related responses, the V-ATPase has been appropriately named as 'ecoenzyme' .
The V-ATPase plays a major role in salt adaptation of the plants by efficient exclusion of excess sodium from the cytoplasmic compartment and its accumulation in the vacuoles. Detailed analyses of the regulation of the V-ATPase activity and of the transcription and translation of V-ATPase subunits in response to salinity stress have been carried out for the facultative halophyte Mesembryanthemum crystallinum and it has been shown that the activity of the V-ATPase increases by a factor of 2.5 in M. crystallinum under treatment with NaCl . There is transcriptional activation of the V-ATPase subunits A, B, E, F and c . This transport protein has been shown to undergo changes in enzyme activity, protein amount and molecular structure [74,75]. In other plants too, e.g. Beta vulgaris, the promoter activity of subunits A and c, measured as the expression of the luciferase reporter gene, increased under salt stress . However, in a few other plants including peanut seedlings , the response of V-ATPase was either poor or not detectable [78,79].
In plants, the [Na.sup.+]/[H.sup.+] antiporter in vacuolar membranes transports [Na.sup.+] from the cytoplasm to vacuoles using the electrochemical [H.sup.+] gradient generated by V-ATPase. There are many reports of increased expression of the antiporter genes by salt stress in the glycophytes, A. thaliana and Oryza sativa, and in the halophytes, M. crystallinum  including the overexpression of the antiporter gene resulting in enhanced salt tolerance . In M. crystallinum salt stress resulted in tissue-specific and age-dependent increases in subunit c mRNA levels [82,83] and in V-ATPase activity . Abscisic acid also caused an increase in subunit c mRNA abundance  and in V-ATPase activity  in M. crystallinum.
Besides salinity, the V-ATPase responds to a variety of other stresses, e.g. drought , sequestration of heavy metals through metal-proton antiport systems  and chilling stress . Based on low temperature treatment studies, it was concluded that there had to be two types of V-ATPases, i.e. 'mung-bean-type' and 'pea-type', the latter being a cold-tolerant enzyme which shows no [V.sub.1]-dissociation during treatment with chaotropic salts in contrast to the former where it dissociates easily and is a sensitive target of cold stress . Higher V-ATPase activity and amount in Butea monosperma plants growing under natural conditions pointed to an important role of this enzyme in water stress [46,87], an aspect that has been overlooked hitherto. Western blot analysis and immunological quantification of various subunits of V-ATPase, suggested the presence Di and Ei (though need further confirmation by N-terminal sequencing), not detected earlier in an obligate [C.sub.3] plant species growing under water stress. This may point to much broader role of these subunits in stress adaptation. So far, these subunits were known to be produced in Mesembryanthemum crystallinum during the transition of the plant from [C.sub.3] to CAM metabolism under salt stress [72,88].
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Dr. Vinay Sharma
Professor & Head, Department of Bioscience and Biotechnology
Banasthali University, P.O. Banasthali Vidyapith
Rajasthan- 304 022, INDIA
Vinay Sharma, Nilima Kumari and Bhumi Nath Tripathi
Department of Bioscience and Biotechnology
Banasthali University, P.O. Banasthali Vidyapith
Rajasthan- 304 022, INDIA
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|Title Annotation:||Vacuolar type adenosine triphosphatase|
|Author:||Sharma, Vinay; Kumari, Nilima; Tripathi, Bhumi Nath|
|Publication:||International Journal of Biotechnology & Biochemistry|
|Date:||Jan 1, 2009|
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