Acidic residues necessary for pyrophosphate-energized pumping and inhibition of the vacuolar H+-pyrophosphatase by N,N'-dicyclohexylcarbodiimide

Systematic identification of sugarcane vacuolar H+-translocating pyrophosphatase (VPP) gene family and the role of ScVPP1 in salt resistance

KEY MESSAGE

A total of 24 genes of vacuolar H+-translocating pyrophosphatases H+-PPases (VPP) genes were identified in Saccharum spontaneum AP85-441 and the ScVPP1-overexpressed Arabidopsis plants conferred salt tolerance. The vital role of vacuolar H+-translocating pyrophosphatases H+-PPases (VPP) genes involved in plants in response to abiotic stresses. However, the understanding of VPP functions in sugarcane remained unclear. In this study, a total of 24 VPP genes (SsaVPP1-SsaVPP24) were identified in the Saccharum spontaneum genome of haploid clone AP85-441. These genes were distributed in two phylogenetic groups. The SsaVPPs displayed diverse physio-chemical and gene structure attributes. The SsaVPP family genes have expanded through segmental duplication (20 gene pairs) rather than tandem duplication. A full-length cDNA of ScVPP1 was cloned from the sugarcane cultivar ROC22 and shared 99.48% sequence identity (amino acid) with homologous gene SsaVPP21 from AP85-441. In ROC22, the ScVPP1 gene was considerably upregulated by NaCl and ABA treatments among leaf, root, and stem tissues, while this gene was exclusively upregulated in the root with PEG treatment. Under NaCl and ABA stresses, yeast cells transfected by the ScVPP1 plasmid showed distinct growth rates compared to control yeast cells transfected by the empty vector. In transgenic Arabidopsis lines overexpressing ScVPP1, the seed gemination and survival rate were enhanced under NaCl treatment but not under ABA stress as compared to wild-type plants. These results suggested that the ScVPP1 gene conferred tolerance to slat and may be used as a salt resistance gene source for sugarcane breeding.

Nitrate supply decreases uptake and accumulation of ciprofloxacin in Brassica parachinensis

How nitrate (NO₃^-) fertilization influences ciprofloxacin (CIP) uptake by crops remains unsolved. Here, two Brassica parachinensis cultivars differing in CIP accumulation were cultivated to investigate the effects of NO₃^- supply on CIP uptake and the underlying mechanism. The results showed that NO₃^- supply effectively reduced CIP toxicity and accumulation in the two cultivars, especially in the low CIP cultivar. Moreover, NO₃^- supply induced different mechanisms of coping with CIP stress in the two cultivars through influencing subcellular distribution of CIP. The uptake of CIP by root was demonstrated to be a carrier-mediated, energy-consuming, and proton motive force-dependent influx process. Consequently, a mechanism of nitrate supply decreasing CIP uptake was proposed that uptake of CIP and NO₃- into root cell would compete for the proton motive force and share a common energy source provided by plasma membrane H⁺-ATPase. Besides, regulating the concentration balances of cytoplasmic NO₃- and proton by inhibiting the activities of NRase and two tonoplast proton pumps (V-ATPase and V-PPase) led to opposite effect on CIP uptake, further supporting this inference. Our results provide a novel insight into CIP uptake by plant roots, and improve the strategy of minimizing CIP accumulation in crops for food safety by fertilization management.

IMPROvER: the Integral Membrane Protein Stability Selector

Identifying stabilising variants of membrane protein targets is often required for structure determination. Our new computational pipeline, the Integral Membrane Protein Stability Selector (IMPROvER) provides a rational approach to variant selection by employing three independent approaches: deep-sequence, model-based and data-driven. In silico tests using known stability data, and in vitro tests using three membrane protein targets with 7, 11 and 16 transmembrane helices provided measures of success. In vitro, individual approaches alone all identified stabilising variants at a rate better than expected by random selection. Low numbers of overlapping predictions between approaches meant a greater success rate was achieved (fourfold better than random) when approaches were combined and selections restricted to the highest ranked sites. The mix of information IMPROvER uses can be extracted for any helical membrane protein. We have developed the first general-purpose tool for selecting stabilising variants of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upalpha$$\end{document}α-helical membrane proteins, increasing efficiency and reducing workload. IMPROvER can be accessed at http://improver.ddns.net/IMPROvER/.

The H+-Translocating Inorganic Pyrophosphatase From Arabidopsis thaliana Is More Sensitive to Sodium Than Its Na+-Translocating Counterpart From Methanosarcina mazei

Overexpression of membrane-bound K+-dependent H+-translocating inorganic pyrophosphatases (H+-PPases) from higher plants has been widely used to alleviate the sensitivity toward NaCl in these organisms, a strategy that had been previously tested in Saccharomyces cerevisiae. On the other hand, H+-PPases have been reported to functionally complement the yeast cytosolic soluble pyrophosphatase (IPP1). Here, the efficiency of the K+-dependent Na+-PPase from the archaeon Methanosarcina mazei (MVP) to functionally complement IPP1 has been compared to that of its H+-pumping counterpart from Arabidopsis thaliana (AVP1). Both membrane-bound integral PPases (mPPases) supported yeast growth equally well under normal conditions, however, cells expressing MVP grew significantly better than those expressing AVP1 under salt stress. The subcellular distribution of the heterologously-expressed mPPases was crucial in order to observe the phenotypes associated with the complementation. In vitro studies showed that the PPase activity of MVP was less sensitive to Na+ than that of AVP1. Consistently, when yeast cells expressing MVP were grown in the presence of NaCl only a marginal increase in their internal PPi levels was observed with respect to control cells. By contrast, yeast cells that expressed AVP1 had significantly higher levels of this metabolite under the same conditions. The H+-pumping activity of AVP1 was also markedly inhibited by Na+. Our results suggest that mPPases primarily act by hydrolysing the PPi generated in the cytosol when expressed in yeast, and that AVP1 is more susceptible to Na+ inhibition than MVP both in vivo and in vitro. Based on this experimental evidence, we propose Na+-PPases as biotechnological tools to generate salt-tolerant plants.

Chinese wheat mosaic virus-derived vsiRNA-20 can regulate virus infection in wheat through inhibition of vacuolar- (H+ )-PPase induced cell death

Vacuolar (H⁺ )-PPases (VPs), are key regulators of active proton (H⁺ ) transport across membranes using the energy generated from PPi hydrolysis. The VPs also play vital roles in plant responses to various abiotic stresses. Their functions in plant responses to pathogen infections are unknown. Here, we show that TaVP, a VP of wheat (Triticum aestivum) is important for wheat resistance to Chinese wheat mosaic virus (CWMV) infection. Furthermore, overexpression of TaVP in plants induces the activity of PPi hydrolysis, leading to plants cell death. A virus-derived small interfering RNA (vsiRNA-20) generated from CWMV RNA1 can regulate the mRNA accumulation of TaVP in wheat. The accumulation of vsiRNA-20 can suppress cell death induced by TaVP in a dosage-dependent manner. Moreover, we show that the accumulation of vsiRNA-20 can affect PPi hydrolysis and the concentration of H⁺ in CWMV-infected wheat cells to create a more favorable cellular environment for CWMV replication. We propose that vsiRNA-20 regulates TaVP expression to prevent cell death and to maintain a weak alkaline environment in cytoplasm to enhance CWMV infection in wheat. This finding may be used as a novel strategy to minimize virus pathogenicity and to develop new antiviral stratagems.

Plant proton pumping pyrophosphatase: the potential for its pyrophosphate synthesis activity to modulate plant growth

Cellular pyrophosphate (PPi) homeostasis is vital for normal plant growth and development. Plant proton-pumping pyrophosphatases (H⁺ -PPases) are enzymes with different tissue-specific functions related to the regulation of PPi homeostasis. Enhanced expression of plant H⁺ -PPases increases biomass and yield in different crop species. Here, we emphasise emerging studies utilising heterologous expression in yeast and plant vacuole electrophysiology approaches, as well as phylogenetic relationships and structural analysis, to showcase that the H⁺ -PPases possess a PPi synthesis function. We postulate this synthase activity contributes to modulating and promoting plant growth both in H⁺ -PPase-engineered crops and in wild-type plants. We propose a model where the PPi synthase activity of H⁺ -PPases maintains the PPi pool when cells adopt PPi-dependent glycolysis during high energy demands and/or low oxygen environments. We conclude by proposing experiments to further investigate the H⁺ -PPase-mediated PPi synthase role in plant growth.

Roles of the Hydrophobic Gate and Exit Channel in Vigna radiata Pyrophosphatase Ion Translocation

Membrane-embedded pyrophosphatase (M-PPase) hydrolyzes pyrophosphate to drive ion (H⁺ and/or Na⁺) translocation. We determined crystal structures and functions of Vigna radiata M-PPase (VrH⁺-PPase), the VrH⁺-PPase-2P_i complex and mutants at hydrophobic gate (residue L555) and exit channel (residues T228 and E225). Ion pore diameters along the translocation pathway of three VrH⁺-PPases complexes (P_i-, 2P_i- and imidodiphosphate-bound states) present a unique wave-like profile, with different pore diameters at the hydrophobic gate and exit channel, indicating that the ligands induced pore size alterations. The 2P_i-bound state with the largest pore diameter might mimic the hydrophobic gate open. In mutant structures, ordered waters detected at the hydrophobic gate among VrH⁺-PPase imply the possibility of solvation, and numerous waters at the exit channel might signify an open channel. A salt-bridge, E225-R562 is at the way out of the exit channel of VrH⁺-PPase; E225A mutant makes the interaction eliminated and reveals a decreased pumping ability. E225-R562 might act as a latch to regulate proton release. A water wire from the ion gate (R-D-K-E) through the hydrophobic gate and into the exit channel may reflect the path of proton transfer.

A Simple Strategy to Determine the Dependence of Membrane-Bound Pyrophosphatases on K+ as a Cofactor

Membrane-bound pyrophosphatases (mPPases) couple pyrophosphate hydrolysis to H⁺ and/or Na⁺ pumping across membranes and are found in all domains of life except for multicellular animals including humans. They are important for development and stress resistance in plants. Furthermore, mPPases play a role in virulence of human pathogens that cause severe diseases such as malaria and African sleeping sickness. Sequence analysis, functional studies, and recently solved crystal structures have contributed to the understanding of the mPPase catalytic cycle. However, several key mechanistic features remain unknown. During evolution, several subgroups of mPPases differing in their pumping specificity and cofactor dependency arose. mPPases are classified into one of five subgroups, usually by sequence analysis. However, classification based solely on sequence has been inaccurate in several instances due to our limited understanding of the molecular mechanism of mPPases. Thus, pumping specificity and cofactor dependency of mPPases require experimental confirmation. Here, we describe a simple method for the determination of K⁺ dependency in mPPases using a hydrolytic activity assay. By coupling these dependency studies with site-directed mutagenesis, we have begun to build a better understanding of the molecular mechanisms of mPPases. We optimized the assay for thermostable mPPases that are commonly used as model systems in our lab, but the method is equally applicable to mesophilic mPPases with minor modifications.

Regulation of H+-pyrophosphatase by 14-3-3 Proteins from Arabidopsis thaliana

Plant vacuolar H⁺-transporting inorganic pyrophosphatase (V-PPase; EC 3.6.1.1) is a crucial enzyme that exists on the tonoplast to maintain pH homeostasis across the vacuolar membrane. This enzyme generates proton gradient between cytosol and vacuolar lumen by hydrolysis of a metabolic byproduct, pyrophosphate (PP _i ). The regulation of V-PPase at protein level has drawn attentions of many workers for decades, but its mechanism is still unclear. In this work, we show that AVP1, the V-PPase from Arabidopsis thaliana, is a target protein for regulatory 14-3-3 proteins at the vacuolar membrane, and all twelve 14-3-3 isoforms were analyzed for their association with AVP1. In the presence of 14-3-3ν, -µ, -ο, and -ι, both enzymatic activities and its associated proton pumping of AVP1 were increased. Among these 14-3-3 proteins, 14-3-3 µ shows the highest stimulation on coupling efficiency. Furthermore, 14-3-3ν, -µ, -ο, and -ι exerted protection of AVP1 against the inhibition of suicidal substrate PP _i at high concentration. Moreover, the thermal profile revealed the presence of 14-3-3ο improves the structural stability of AVP1 against high temperature deterioration. Additionally, the 14-3-3 proteins mitigate the inhibition of Na⁺ to AVP1. Besides, the binding sites/motifs of AVP1 were identified for each 14-3-3 protein. Taken together, a working model was proposed to elucidate the association of 14-3-3 proteins with AVP1 for stimulation of its enzymatic activity.

Structural basis for the reversibility of proton pyrophosphatase

Proton Pyrophosphatase (H⁺-PPase) is an evolutionarily conserved enzyme regarded as a bona fide vacuolar marker. However, H⁺-PPase also localizes at the plasma membrane of the phloem, where, evidence suggests that it functions as a Pyrophosphate Synthase and participates in phloem loading and photosynthate partitioning. We believe that this pyrophosphate synthesising function of H⁺-PPase is fundamentally rooted to its molecular structure, and here we postulate, on the basis of published crystal structures of membrane-bound pyrophosphatases, a plausible mechanism of pyrophosphate synthesis.

Overexpression of a Populus trichocarpa H+-pyrophosphatase gene PtVP1.1 confers salt tolerance on transgenic poplar

The Arabidopsis vacuolar H(+)-pyrophosphatase (AVP1) has been well studied and subsequently employed to improve salt and/or drought resistance in herbaceous plants. However, the exact function of H(+)-pyrophosphatase in woody plants still remains unknown. In this work, we cloned a homolog of type I H(+)-pyrophosphatase gene, designated as PtVP1.1, from Populus trichocarpa, and investigated its function in both Arabidopsis and poplar. The deduced translation product PtVP1.1 shares 89.74% identity with AVP1. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and quantitative real-time PCR analyses revealed a ubiquitous expression pattern of PtVP1.1 in various tissues, including roots, stems, leaves and shoot tips. Heterologous expression of PtVP1.1 rescued the retarded-root-growth phenotype of avp1, an Arabidopsis knock out mutant of AVP1, on low carbohydrate medium. Overexpression of PtVP1.1 in poplar (P. davidiana × P. bolleana) led to more vigorous growth of transgenic plants in the presence of 150 mM NaCl. Microsomal membrane vesicles derived from PtVP1.1 transgenic plants exhibited higher H(+)-pyrophosphatase hydrolytic activity than those from wild type (WT). Further studies indicated that the improved salt tolerance was associated with a decreased Na(+) and increased K(+) accumulation in the leaves of transgenic plants. Na(+) efflux and H(+) influx in the roots of transgenic plants were also significantly higher than those in the WT plants. All these results suggest that PtVP1.1 is a functional counterpart of AVP1 and can be genetically engineered for salt tolerance improvement in trees.

Over-expression of the Arabidopsis proton-pyrophosphatase AVP1 enhances transplant survival, root mass, and fruit development under limiting phosphorus conditions

Phosphorus (P), an element required for plant growth, fruit set, fruit development, and fruit ripening, can be deficient or unavailable in agricultural soils. Previously, it was shown that over-expression of a proton-pyrophosphatase gene AVP1/AVP1D (AVP1DOX) in Arabidopsis, rice, and tomato resulted in the enhancement of root branching and overall mass with the result of increased mineral P acquisition. However, although AVP1 over-expression also increased shoot biomass in Arabidopsis, this effect was not observed in tomato under phosphate-sufficient conditions. AVP1DOX tomato plants exhibited increased rootward auxin transport and root acidification compared with control plants. AVP1DOX tomato plants were analysed in detail under limiting P conditions in greenhouse and field trials. AVP1DOX plants produced 25% (P=0.001) more marketable ripened fruit per plant under P-deficient conditions compared with the controls. Further, under low phosphate conditions, AVP1DOX plants displayed increased phosphate transport from leaf (source) to fruit (sink) compared to controls. AVP1DOX plants also showed an 11% increase in transplant survival (P<0.01) in both greenhouse and field trials compared with the control plants. These results suggest that selection of tomato cultivars for increased proton pyrophosphatase gene expression could be useful when selecting for cultivars to be grown on marginal soils.

Synthesis of 3-(3-aryl-pyrrolidin-1-yl)-5-aryl-1,2,4-triazines that have antibacterial activity and also inhibit inorganic pyrophosphatase

Inorganic pyrophosphatases are potential targets for the development of novel antibacterial agents. A pyrophosphatase-coupled high-throughput screening assay intended to detect o-succinyl benzoic acid coenzyme A (OSB CoA) synthetase inhibitors led to the unexpected discovery of a new series of novel inorganic pyrophosphatase inhibitors. Lead optimization studies resulted in a series of 3-(3-aryl-pyrrolidin-1-yl)-5-aryl-1,2,4-triazine derivatives that were prepared by an efficient synthetic pathway. One of the tetracyclic triazine analogues 22h displayed promising antibiotic activity against a wide variety of drug-resistant Staphylococcus aureus strains, as well as activity versus Mycobacterium tuberculosis and Bacillus anthracis, at a concentration that was not cytotoxic to mammalian cells.

Isolation and characterization of a conserved domain in the eremophyte H+-PPase family

H⁺-translocating inorganic pyrophosphatases (H⁺-PPase) were recognized as the original energy donors in the development of plants. A large number of researchers have shown that H⁺-PPase could be an early-originated protein that participated in many important biochemical and physiological processes. In this study we cloned 14 novel sequences from 7 eremophytes: Sophora alopecuroid (Sa), Glycyrrhiza uralensis (Gu), Glycyrrhiza inflata (Gi), Suaeda salsa (Ss), Suaeda rigida (Sr), Halostachys caspica (Hc), and Karelinia caspia (Kc). These novel sequences included 6 ORFs and 8 fragments, and they were identified as H⁺-PPases based on the typical conserved domains. Besides the identified domains, sequence alignment showed that there still were two novel conserved motifs. A phylogenetic tree was constructed, including the 14 novel H⁺-PPase amino acid sequences and the other 34 identified H⁺-PPase protein sequences representing plants, algae, protozoans and bacteria. It was shown that these 48 H⁺-PPases were classified into two groups: type I and type II H⁺-PPase. The novel 14 eremophyte H⁺-PPases were classified into the type I H⁺-PPase. The 3D structures of these H⁺-PPase proteins were predicted, which suggested that all type I H⁺-PPases from higher plants and algae were homodimers, while other type I H⁺-PPases from bacteria and protozoans and all type II H⁺-PPases were monomers. The 3D structures of these novel H⁺-PPases were homodimers except for SaVP3, which was a monomer. This regular structure could provide important evidence for the evolutionary origin and study of the relationship between the structure and function among members of the H⁺-PPase family.

Squeezing at entrance of proton transport pathway in proton-translocating pyrophosphatase upon substrate binding

Homodimeric proton-translocating pyrophosphatase (H(+)-PPase; EC 3.6.1.1) is indispensable for many organisms in maintaining organellar pH homeostasis. This unique proton pump couples the hydrolysis of PPi to proton translocation across the membrane. H(+)-PPase consists of 14-16 relatively hydrophobic transmembrane domains presumably for proton translocation and hydrophilic loops primarily embedding a catalytic site. Several highly conserved polar residues located at or near the entrance of the transport pathway in H(+)-PPase are essential for proton pumping activity. In this investigation single molecule FRET was employed to dissect the action at the pathway entrance in homodimeric Clostridium tetani H(+)-PPase upon ligand binding. The presence of the substrate analog, imidodiphosphate mediated two sites at the pathway entrance moving toward each other. Moreover, single molecule FRET analyses after the mutation at the first proton-carrying residue (Arg-169) demonstrated that conformational changes at the entrance are conceivably essential for the initial step of H(+)-PPase proton translocation. A working model is accordingly proposed to illustrate the squeeze at the entrance of the transport pathway in H(+)-PPase upon substrate binding.

Enhanced proton translocating pyrophosphatase activity improves nitrogen use efficiency in Romaine lettuce

Plant nitrate (NO3(-)) acquisition depends on the combined activities of root high- and low-affinity NO3(-) transporters and the proton gradient generated by the plasma membrane H(+)-ATPase. These processes are coordinated with photosynthesis and the carbon status of the plant. Here, we present the characterization of romaine lettuce (Lactuca sativa 'Conquistador') plants engineered to overexpress an intragenic gain-of-function allele of the type I proton translocating pyrophosphatase (H(+)-PPase) of Arabidopsis (Arabidopsis thaliana). The proton-pumping and inorganic pyrophosphate hydrolytic activities of these plants are augmented compared with control plants. Immunohistochemical data show a conspicuous increase in H(+)-PPase protein abundance at the vasculature of the transgenic plants. Transgenic plants displayed an enhanced rhizosphere acidification capacity consistent with the augmented plasma membrane H(+)-ATPase proton transport values, and ATP hydrolytic capacities evaluated in vitro. These transgenic lines outperform control plants when challenged with NO3(-) limitations in laboratory, greenhouse, and field scenarios. Furthermore, we report the characterization of a lettuce LsNRT2.1 gene that is constitutive up-regulated in the transgenic plants. Of note, the expression of the LsNRT2.1 gene in control plants is regulated by NO3(-) and sugars. Enhanced accumulation of (15)N-labeled fertilizer by transgenic lettuce compared with control plants was observed in greenhouse experiments. A negative correlation between the level of root soluble sugars and biomass is consistent with the strong root growth that characterizes these transgenic plants.

Crystal structure of a membrane-embedded H+-translocating pyrophosphatase

H(+)-translocating pyrophosphatases (H(+)-PPases) are active proton transporters that establish a proton gradient across the endomembrane by means of pyrophosphate (PP(i)) hydrolysis. H(+)-PPases are found primarily as homodimers in the vacuolar membrane of plants and the plasma membrane of several protozoa and prokaryotes. The three-dimensional structure and detailed mechanisms underlying the enzymatic and proton translocation reactions of H(+)-PPases are unclear. Here we report the crystal structure of a Vigna radiata H(+)-PPase (VrH(+)-PPase) in complex with a non-hydrolysable substrate analogue, imidodiphosphate (IDP), at 2.35 Å resolution. Each VrH(+)-PPase subunit consists of an integral membrane domain formed by 16 transmembrane helices. IDP is bound in the cytosolic region of each subunit and trapped by numerous charged residues and five Mg(2+) ions. A previously undescribed proton translocation pathway is formed by six core transmembrane helices. Proton pumping can be initialized by PP(i) hydrolysis, and H(+) is then transported into the vacuolar lumen through a pathway consisting of Arg 242, Asp 294, Lys 742 and Glu 301. We propose a working model of the mechanism for the coupling between proton pumping and PP(i) hydrolysis by H(+)-PPases.

Na+-translocating membrane pyrophosphatases are widespread in the microbial world and evolutionarily precede H+-translocating pyrophosphatases

Membrane pyrophosphatases (PPases), divided into K(+)-dependent and K(+)-independent subfamilies, were believed to pump H(+) across cell membranes until a recent demonstration that some K(+)-dependent PPases function as Na(+) pumps. Here, we have expressed seven evolutionarily important putative PPases in Escherichia coli and estimated their hydrolytic, Na(+) transport, and H(+) transport activities as well as their K(+) and Na(+) requirements in inner membrane vesicles. Four of these enzymes (from Anaerostipes caccae, Chlorobium limicola, Clostridium tetani, and Desulfuromonas acetoxidans) were identified as K(+)-dependent Na(+) transporters. Phylogenetic analysis led to the identification of a monophyletic clade comprising characterized and predicted Na(+)-transporting PPases (Na(+)-PPases) within the K(+)-dependent subfamily. H(+)-transporting PPases (H(+)-PPases) are more heterogeneous and form at least three independent clades in both subfamilies. These results suggest that rather than being a curious rarity, Na(+)-PPases predominantly constitute the K(+)-dependent subfamily. Furthermore, Na(+)-PPases possibly preceded H(+)-PPases in evolution, and transition from Na(+) to H(+) transport may have occurred in several independent enzyme lineages. Site-directed mutagenesis studies facilitated the identification of a specific Glu residue that appears to be central in the transport mechanism. This residue is located in the cytoplasm-membrane interface of transmembrane helix 6 in Na(+)-PPases but shifted to within the membrane or helix 5 in H(+)-PPases. These results contribute to the prediction of the transport specificity and K(+) dependence for a particular membrane PPase sequence based on its position in the phylogenetic tree, identity of residues in the K(+) dependence signature, and position of the membrane-located Glu residue.

Identification of essential lysines involved in substrate binding of vacuolar H+-pyrophosphatase

H+-translocating pyrophosphatase (H+-PPase; EC 3.6.1.1) drives proton transport against an electrochemical potential gradient by hydrolyzing pyrophosphate (PPi) and is found in various endomembranes of higher plants, bacteria, and some protists. H+-PPase contains seven highly conserved lysines. We examined the functional roles of these lysines, which are, for the most part, found in the cytosolic regions of mung bean H+-PPase by site-directed mutagenesis. Construction of mutants that each had a cytosolic and highly conserved lysine substituted with an alanine resulted in dramatic drops in the PPi hydrolytic activity. The effects caused by ions on the activities of WT and mutant H+-PPases suggest that Lys-730 may be in close proximity to the Mg2+-binding site, and the great resistance of the K694A and K695A mutants to fluoride inhibition suggests that these lysines are present in the active site. The modifier fluorescein 5'-isothiocyanate (FITC) labeled a lysine at the H+-PPase active site but did not inhibit the hydrolytic activities of K250A, K250N, K250T, and K250S, which suggested that Lys-250 is essential for substrate binding and may be involved in proton translocation. Analysis of tryptic digests indicated that Lys-711 and Lys-717 help maintain the conformation of the active site. Proteolytic evidence also demonstrated that Lys-250 is the primary target of trypsin and confirmed its crucial role in H+-PPase hydrolysis.

Molecular cloning and characterization of a vacuolar H+₋pyrophosphatase from Dunaliella viridis

The halotolerant alga Dunaliella adapts to exceptionally high salinity and possesses efficient mechanisms for regulating intracellular Na(+). In plants, sequestration of Na(+) into the vacuole is driven by the electrochemical H(+) gradient generated by H(+) pumps, and this Na(+) sequestration is one mechanism that confers salt tolerance to plants. To investigate the role of vacuolar H(+) pumps in the salt tolerance of Dunaliella, we isolated the cDNA of the vacuolar proton-translocating inorganic pyrophosphatase (V-H(+)-PPase) from Dunaliella viridis. The DvVP cDNA is 2,984 bp in length, codes for a polypeptide of 762 amino acids and has 15 transmembrane domains. The DvVP protein is highly similar to V-H(+)-PPases from other green algae and higher plant species, in terms of its amino acid sequence and its transmembrane model. A phylogenetic analysis of V-H(+)-PPases revealed the close relationship of Dunaliella to green algal species of Charophyceae and land plants. The heterologous expression of DvVP in the yeast mutant G19 (Δena1-4) suppressed Na(+) hypersensitivity, and a GFP-fusion of DvVP localized to the vacuole membranes in yeast, indicating that DvVP encodes a functional V-H(+)-PPase. A northern blot analysis showed a decrease in the transcript abundance of DvVP at higher salinity in D. viridis cells, which is in contrast to the salt-induced upregulation of V-H(+)-PPase in some plants, suggesting that the expression of DvVP under salt stress may be regulated by different mechanisms in Dunaliella. This study not only enriched our knowledge about the biological functions of V-H(+)-PPases in different organisms but also improved our understanding of the molecular mechanism of salt tolerance in Dunaliella.

The transmembrane domain 6 of vacuolar H(+)-pyrophosphatase mediates protein targeting and proton transport

Vacuolar H(+)-pyrophosphatase (V-PPase; EC 3.6.1.1) plays a significant role in the maintenance of the pH in cytoplasm and vacuoles via proton translocation from the cytosol to the vacuolar lumen at the expense of PP(i) hydrolysis. The topology of V-PPase as predicted by TopPred II suggests that the catalytic site is putatively located in loop e and exposed to the cytosol. The adjacent transmembrane domain 6 (TM6) is highly conserved and believed to participate in the catalytic function and conformational stability of V-PPase. In this study, alanine-scanning mutagenesis along TM6 of the mung bean V-PPase was carried out to identify its structural and functional role. Mutants Y299A, A306S and L317A exhibited gross impairment in both PP(i) hydrolysis and proton translocation. Meanwhile, mutations at L307 and N318 completely abolished the targeting of the enzyme, causing broad cytosolic localization and implicating a possible role of these residues in protein translocation. The location of these amino acid residues was on the same side of the helix wheel, suggesting their involvement in maintaining the stability of enzyme conformation. G297A, E301A and A305S mutants showed declines in proton translocation but not in PP(i) hydrolysis, consequently resulting in decreases in the coupling efficiency. These amino acid residues cluster at one face of the helix wheel, indicating their direct/indirect participation in proton translocation. Taken together, these data indicate that TM6 is crucial to vacuolar H(+)-pyrophosphatase, probably mediating protein targeting, proton transport, and the maintenance of enzyme structure.

Expression and responses to dehydration and salinity stresses of V-PPase gene members in wheat

Vacuolar H(+)-translocating pyrophosphatase (V-PPase) is a key enzyme related to plant growth as well as abiotic stress tolerance. In this work, wheat V-PPase genes TaVP1, TaVP2 and TaVP3 were identified. TaVP1 and TaVP2 are more similar to each other than to TaVP3. Their deduced polypeptide sequences preserve the topological structure and essential residues of V-PPases. Phylogenetic studies suggested that monocot plants, at least monocot grasses, have three VP paralogs. TaVP3 transcripts were only detected in developing seeds, and no TaVP2 transcripts were found in germinating seeds. TaVP2 was mainly expressed in shoot tissues and down-regulated in leaves under dehydration. Its expression was up-regulated in roots under high salinity. TaVP1 was relatively more ubiquitously and evenly expressed than TaVP2. Its expression level in roots was highest among the tissues examined, and was inducible by salinity stress. These results indicated that the V-PPase gene paralogs in wheat are differentially regulated spatially and in response to dehydration and salinity stresses.

Functional enhancement by single-residue substitution of Streptomyces coelicolor A3(2) H+-translocating pyrophosphatase

H(+)-translocating pyrophosphatase converts energy from hydrolysis of pyrophosphate to active H(+) transport across biomembranes. Mutational analysis of Streptomyces coelicolor A3(2) enzyme revealed that amino acid substitution of Phe-388 and Ala-514 altered the enzyme activity. Both residues are located at the interface between the transmembrane domains and cytosolic loops, in which the catalytic domain exists. Systematic amino acid substitution was carried out using the Escherichia coli heterologous expression system. Two of the 38 mutant enzymes, F388Y and A514S, showed a high ratio of H(+)-pump to substrate hydrolysis without decrease in the substrate hydrolysis activity, indicating high energy-coupling efficiency.

Enhanced phosphorus nutrition in monocots and dicots over-expressing a phosphorus-responsive type I H+-pyrophosphatase

Plants challenged by limited phosphorus undergo dramatic morphological and architectural changes in their root systems in order to increase their absorptive surface area. In this paper, it is shown that phosphorus deficiency results in increased expression of the type I H+-pyrophosphatase AVP1 (AVP, Arabidopsis vacuolar pyrophosphatase), subsequent increased P-type adenosine triphosphatase (P-ATPase)-mediated rhizosphere acidification and root proliferation. Molecular genetic manipulation of AVP1 expression in Arabidopsis, tomato and rice results in plants that outperform controls when challenged with limited phosphorus. However, AVP1 over-expression and the resulting rhizosphere acidification do not result in increased sensitivity to AlPO4, apparently because of the enhancement of potassium uptake and the release of organic acids. Thus, the over-expression of type I H+-pyrophosphatases appears to be a generally applicable technology to help alleviate agricultural losses in low-phosphorus tropical/subtropical soils and to reduce phosphorus runoff pollution of aquatic and marine environments resulting from fertilizer application.

Functional roles of arginine residues in mung bean vacuolar H+-pyrophosphatase

Plant vacuolar H+-translocating inorganic pyrophosphatase (V-PPase EC 3.6.1.1) utilizes inorganic pyrophosphate (PPi) as an energy source to generate a H+ gradient potential for the secondary transport of ions and metabolites across the vacuole membrane. In this study, functional roles of arginine residues in mung bean V-PPase were determined by site-directed mutagenesis. Alignment of amino-acid sequence of K+-dependent V-PPases from several organisms showed that 11 of all 15 arginine residues were highly conserved. Arginine residues were individually substituted by alanine residues to produce R-->A-substituted V-PPases, which were then heterologously expressed in yeast. The characteristics of mutant variants were subsequently scrutinized. As a result, most R-->A-substituted V-PPases exhibited similar enzymatic activities to the wild-type with exception that R242A, R523A, and R609A mutants markedly lost their abilities of PPi hydrolysis and associated H+-translocation. Moreover, mutation on these three arginines altered the optimal pH and significantly reduced K+-stimulation for enzymatic activities, implying a conformational change or a modification in enzymatic reaction upon substitution. In particular, R242A performed striking resistance to specific arginine-modifiers, 2,3-butanedione and phenylglyoxal, revealing that Arg242 is most likely the primary target residue for these two reagents. The mutation at Arg242 also removed F- inhibition that is presumably derived from the interfering in the formation of substrate complex Mg2+-PPi. Our results suggest accordingly that active pocket of V-PPase probably contains the essential Arg242 which is embedded in a more hydrophobic environment.

Arabidopsis–rice–wheat gene orthologues for Na+ transport and transcript analysis in wheat–L. elongatum aneuploids under salt stress

Lophopyrum elongatum is a wild relative of wheat that provides a source of novel genes for improvement of the salt tolerance of bread wheat. Improved Na(+) 'exclusion' is associated with salt tolerance in a wheat-L. elongatum amphiploid, in which a large proportion (ca. 50%) of the improved regulation of leaf Na(+) concentrations is controlled by chromosome 3E. In this study, genes that might control Na(+) accumulation, such as for transporters responsible for Na(+) entry (HKT1) and exit (SOS1) from cells, or compartmentalisation within vacuoles (NHX1, NHX5, AVP1, AVP2) in the model plant, Arabidopsis thaliana, were targeted for comparative analyses in wheat. Putative rice orthologues were identified and characterised as a means to bridge the large evolutionary distance between genomes from the model dicot and the more complex grass species. Wheat orthologues were identified through BLAST searching to identify either FL-cDNAs or ESTs and were subsequently used to design primers to amplify genomic DNA. The probable orthologous status of the wheat genes was confirmed through demonstration of similar intron-exon structure with their counterparts in Arabidopsis and rice. The majority of exons for Arabidopsis, rice and wheat orthologues of NHX1, NHX5 and SOS1 were conserved except for those at the amino and carboxy terminal ends. However, additional exons were identified in the predicted NHX1 and SOS1 genes of rice and wheat, as compared with Arabidopsis, indicating gene rearrangement events during evolution from a common ancestor. Nullisomic-tetrasomic, deletion and addition lines in wheat were used to assign gene sequences to chromosome regions in wheat and L. elongatum. Most sequences were assigned to homoeologous chromosomes, however, in some instances, such as for SOS1, genes were mapped to other unpredicted locations. Differential transcript abundance under salt stress indicated a complex pattern of expression for wheat orthologues that may regulate Na(+) accumulation in wheat lines containing chromosomes from L. elongatum. The identification of wheat orthologues to well characterized Arabidopsis genes, map locations and gene expression profiles increases our knowledge on the complex mechanisms regulating Na(+) transport in wheat and wheat-L. elongatum lines under salt stress.

Roles of basic residues and salt-bridge interaction in a vacuolar H+-pumping pyrophosphatase (AVP1) from Arabidopsis thaliana

To investigate the possible role of basic residues in H+ translocation through vacuolar-type H+-pumping pyrophosphatases (V-PPases), conserved arginine and lysine residues predicted to reside within or close to transmembrane domains of an Arabidopsis thaliana V-PPase (AVP1) were subjected to site-directed mutagenesis. One of these mutants (K461A) exhibited a "decoupled" phenotype in which proton-pumping but not hydrolysis was inhibited. Similar results were reported previously for an E427Q mutant, resulting in the proposal that E427 might be involved in proton translocation. However, the double mutant E427K/K461E has a wild type phenotype, suggesting that E427 and K461 form a stabilising salt bridge, but that neither residue plays a critical role in proton translocation.

Up-regulation of a H+-pyrophosphatase (H+-PPase) as a strategy to engineer drought-resistant crop plants

Engineering drought -resistant crop plants is a critically important objective. Overexpression of the vacuolar H(+)-pyrophosphatase (H(+)-PPase) AVP1 in the model plant Arabidopsis thaliana results in enhanced performance under soil water deficits. Recent work demonstrates that AVP1 plays an important role in root development through the facilitation of auxin fluxes. With the objective of improving crop performance, we expressed AVP1 in a commercial cultivar of tomato. This approach resulted in (i) greater pyrophosphate-driven cation transport into root vacuolar fractions, (ii) increased root biomass, and (iii) enhanced recovery of plants from an episode of soil water deficit stress. More robust root systems allowed transgenic tomato plants to take up greater amounts of water during the imposed water deficit stress, resulting in a more favorable plant water status and less injury. This study documents a general strategy for improving drought resistance of crops.

Role of transmembrane segment 5 of the plant vacuolar H+-pyrophosphatase

Vacuolar H+-translocating inorganic pyrophosphatase (V-PPase; EC 3.6.1.1) is a homodimeric proton translocase consisting of a single type of polypeptide with a molecular mass of approximately 81 kDa. Topological analysis tentatively predicts that mung bean V-PPase contains 14 transmembrane domains. Alignment analysis of V-PPase demonstrated that the transmembrane domain 5 (TM5) of the enzyme is highly conserved in plants and located at the N-terminal side of the putative substrate-binding loop. The hydropathic analysis of V-PPase showed a relatively lower degree of hydrophobicity in the TM5 region as compared to other domains. Accordingly, it appears that TM5 is probably involved in the proton translocation of V-PPase. In this study, we used site-directed mutagenesis to examine the functional role of amino acid residues in TM5 of V-PPase. A series of mutants singly replaced by alanine residues along TM5 were constructed and over-expressed in Saccharomyces cerevisiae; they were then used to determine their enzymatic activities and proton translocations. Our results indicate that several mutants displayed minor variations in enzymatic properties, while others including those mutated at E225, a GYG motif (residues from 229 to 231), A238, and R242, showed a serious decline in enzymatic activity, proton translocation, and coupling efficiency of V-PPase. Moreover, the mutation at Y230 relieved several cation effects on the V-PPase. The GYG motif presumably plays a significant role in maintaining structure and function of V-PPase.

Deletion mutation analysis on C-terminal domain of plant vacuolar H+-pyrophosphatase

Vacuolar H(+)-translocating inorganic pyrophosphatase (V-PPase; EC 3.6.1.1) is a homodimeric proton-translocase; it contains a single type of polypeptide of approximately 81kDa. A line of evidence demonstrated that the carboxyl terminus of V-PPase is relatively conserved in various plant V-PPases and presumably locates in the vicinity of the catalytic site. In this study, we attempt to identify the roles of the C-terminus of V-PPase by generating a series of C-terminal deletion mutants over-expressed in Saccharomyces cerevisiae, and determining their enzymatic and proton translocating reactions. Our results showed that the deletion mutation at last 5 amino acids in the C-terminus (DeltaC5) induced a dramatic decline in enzymatic activity, proton translocation, and coupling efficiency of V-PPase; but the mutant lacking last 10 amino acids (DeltaC10) retained about 60-70% of the enzymatic activity of wild-type. Truncation of the C-terminus by more than 10 amino acids completely abolished the enzymatic activity and proton translocation of V-PPase. Furthermore, the DeltaC10 mutant displayed a shift in T(1/2) (pretreatment temperature at which half enzymatic activity is observed) but not the optimal pH for PP(i) hydrolytic activity. The deletion of the C-terminus substantially modified apparent K(+) binding constant, but exert no significant changes in the Na(+)-, F(-)-, and Ca(2+)-inhibition of the enzymatic activity of V-PPase. Taken together, we speculate that the C-terminus of V-PPase may play a crucial role in sustaining enzymatic activity and is likely involved in the K(+)-regulation of the enzyme in an indirect manner.

Oligomerization of H(+)-pyrophosphatase and its structural and functional consequences

The H(+)-pyrophosphatase (H(+)-PPase) consists of a single polypeptide, containing 16 or 17 transmembrane domains. To determine the higher order oligomeric state of Streptomyces coelicolor H(+)-PPase, we constructed a series of cysteine substitution mutants and expressed them in Escherichia coli. Firstly, we analyzed the formation of disulfide bonds, promoted by copper, in mutants with single cysteine substitutions. 28 of 39 mutants formed disulfide bonds, including S545C, a substitution at the periplasmic side. The formation of intermolecular disulfide bonds suppressed the enzyme activity of several, where the substituted residues were located in the cytosol. Creating disulfide links in the cytosol may interfere with the enzyme's catalytic function. Secondly, we prepared double mutants by introducing second cysteine substitutions into the S545C mutant. These double-cysteine mutants produced cross-linked complexes, estimated to be at least tetramers and possibly hexamers. Thirdly, we co-expressed epitope-tagged, wild type, and inactive mutant H(+)-PPases in E. coli and confirmed the formation of oligomers by co-purifying one subunit using the epitope tag used to label the other. The enzyme activity of these oligomers was markedly suppressed. We propose that H(+)-PPase is present as an oligomer made up of at least two or three sets of dimers.

Expression of Functional Streptomyces coelicolor H+-Pyrophosphatase and Characterization of Its Molecular Properties

H(+)-translocating pyrophosphatases (H(+)-PPases) are proton pumps that are found in many organisms, including plants, bacteria and protozoa. Streptomyces coelicolor is a soil bacterium that produces several useful antibiotics. Here we investigated the properties of the H(+)-PPase of S. coelicolor by expressing a synthetic DNA encoding the amino-acid sequence of the H(+)-PPase in Escherichia coli. The H(+)-PPase from E. coli membranes was active at a relatively high pH, stable up to 50 degrees C, and sensitive to N-ethylmaleimide, N,N'-dicyclohexylcarbodiimide and acylspermidine. Enzyme activity increased by 60% in the presence of 120 mM K(+), which was less than the stimulation observed with plant vacuolar H(+)-PPases (type I). Substitutions of Lys-507 in the Gly-Gln-x-x-(Ala/Lys)-Ala motif, which is thought to determine the K(+) requirement of H(+)-PPases, did not alter its K(+) dependence, suggesting that other residues control this feature of the S. coelicolor enzyme. The H(+)-PPase was detected during early growth and was present mainly on the plasma membrane and to a lesser extent on intracellular membranous structures.

Identification and analysis of proton-translocating pyrophosphatases in the methanogenic archaeon Methansarcina mazei

Analysis of genome sequence data from the methanogenic archaeon Methanosarcina mazei Gö1 revealed the existence of two open reading frames encoding proton-translocating pyrophosphatases (PPases). These open reading frames are linked by a 750-bp intergenic region containing TC-rich stretches and are transcribed in opposite directions. The corresponding polypeptides are referred to as Mvp1 and Mvp2 and consist of 671 and 676 amino acids, respectively. Both enzymes represent extremely hydrophobic, integral membrane proteins with 15 predicted transmembrane segments and an overall amino acid sequence similarity of 50.1%. Multiple sequence alignments revealed that Mvp1 is closely related to eukaryotic PPases, whereas Mvp2 shows highest homologies to bacterial PPases. Northern blot experiments with RNA from methanol-grown cells harvested in the mid-log growth phase indicated that only Mvp2 was produced under these conditions. Analysis of washed membranes showed that Mvp2 had a specific activity of 0.34 U mg (protein)(-1). Proton translocation experiments with inverted membrane vesicles prepared from methanol-grown cells showed that hydrolysis of 1 mol of pyrophosphate was coupled to the translocation of about 1 mol of protons across the cytoplasmic membrane. Appropriate conditions for mvp1 expression could not be determined yet. The pyrophosphatases of M. mazei Gö1 represent the first examples of this enzyme class in methanogenic archaea and may be part of their energy-conserving system.

Cloning and characterization of a wheat vacuolar cation/proton antiporter and pyrophosphatase proton pump

Sodium at high millimolar levels in the cytoplasm is toxic to plant and yeast cells. Sequestration of Na+ ions into the vacuole through the action of tonoplast proton pumps (an H+ -ATPase in the case of yeast, and either a H+ -pyrophosphatase (H+ -PPase) or H+ -ATPase in the case of plants) and a Na+/H+ antiporter is one mechanism that confers salt tolerance to these organisms. The cloning and characterization of genes encoding these tonoplast transport proteins from crop plants may contribute to our understanding of how to enhance crop plant response to saline stress. We cloned wheat orthologs of the Arabidopsis genes AtNHX1 and AVP1 using the polymerase chain reaction and primers corresponding to conserved regions of the respective coding sequences, and a wheat cDNA library as template. The wheat NHX cDNA cloned by this approach was a variant of the previously reported TNHX1 gene. The vacuolar H+ -PPase pump we cloned (TVP1) is the first member of this gene family cloned from wheat; it is deduced translation product is homologous to proteins encoded by genes in barley, rice, and Arabidopsis. Function of TNHX1 as a cation/proton antiporter was demonstrated using the nhx1 yeast mutant. TNHX1 was capable of suppressing the hyg sensitivity of nhx1. Functional characterization of the wheat H+ -PPase TVP1 was demonstrated using the yeast ena1 (plasma membrane Na+ -efflux transporter) mutant. Expression of TVP1 in ena1 suppressed its Na+ hypersensitivity. Expression analysis of salt-stressed wheat plants showed substantial up-regulation of TNHX1 transcript levels as compared to control plants, while transcript accumulation for TVP1 was not greatly affected by exposure of plants to salt stress.

The H+-pyrophosphatase of Rhodospirillum rubrum Is Predominantly Located in Polyphosphate-rich Acidocalcisomes

Acidocalcisomes are acidic, calcium storage compartments with a H(+) pump located in their membrane that have been described in several unicellular eukaryotes, including trypanosomatid and apicomplexan parasites, algae, and slime molds, and have also been found in the bacterium Agrobacterium tumefaciens. In this work, we report that the H(+)-pyrophosphatase (H(+)-PPase) of Rhodospirillum rubrum, the first enzyme of this type that was identified and thought to be localized only to chromatophore membranes, is predominantly located in acidocalcisomes. The identification of the acidocalcisomes of R. rubrum was carried out by using transmission electron microscopy, x-ray microanalysis, and immunofluorescence microscopy. Purification of acidocalcisomes using iodixanol gradients indicated co-localization of the H(+)-PPase with pyrophosphate (PPi) and short and long chain polyphosphates (polyPs) but a lack of markers of the plasma membrane. polyP was also localized to the acidocalcisomes by using 4',6'-diamino-2-phenylindole staining and identified by using 31P NMR and biochemical methods. Calcium in the acidocalcisomes increased when the bacteria were incubated at high extracellular calcium concentrations. The number of acidocalcisomes and chromatophore membranes as well as the amounts of PPi and polyP increased when bacteria were grown in the light. Taken together, these results suggest that the H(+)-PPase of R. rubrum has two distinct roles depending on its location acting as an intracellular proton pump in acidocalcisomes but in PPi synthesis in the chromatophore membranes.

Overexpression of the plasma membrane-localized NDR1 protein results in enhanced bacterial disease resistance in Arabidopsis thaliana

Previous studies have established that mutations in the NDR1 gene in Arabidopsis thaliana suppress the resistance response of three resistance proteins, RPS2, RPM1, and RPS5, to Pseudomonas syringae pv. tomato (Pst) strain DC3000 containing the cognate effector genes, avrRpt2, avrRpm1, and avrpPhB, respectively. NDR1 is a plasma membrane (PM)-localized protein, and undergoes several post-translational modifications including carboxy-terminal processing and N-linked glycosylation. Expression of NDR1 under the NDR1 native promoter complements the ndr1-1 mutation, while overexpression of NDR1 results in enhanced resistance to virulent Pst. Sequence analysis and mass spectrometry suggest that NDR1 is localized to the PM via a C-terminal glycosylphosphatidyl-inositol (GPI) anchor. GPI modification would potentially place NDR1 on the outer surface of the PM, perhaps allowing NDR1 to act as a transducer of pathogen signals and/or interact directly with the pathogen.

Acidocalcisomes of Phytomonas françai possess distinct morphological characteristics and contain iron

Acidocalcisomes are acidic calcium storage compartments described initially in trypanosomatid and apicomplexan parasites, and recently found in other unicellular eukaryotes. The aim of this study was to identify the presence of acidocalcisomes in the plant trypanosomatid Phytomonas françai. Electron-dense organelles of P. françai were shown to contain large amounts of oxygen, sodium, magnesium, phosphorus, potassium, calcium, iron, and zinc as determined by X-ray microanalysis, either in situ or when purified using iodixanol gradient centrifugation or by elemental mapping. The presence of iron is not common in other acidocalcisomes. In situ, but not when purified, these organelles showed an elongated shape differing from previously described acidocalcisomes. However, these organelles also possessed a vacuolar H+-pyrophosphatase (V-H+-PPase) as determined by biochemical methods and by immunofluorescence microscopy using antibodies against the enzyme. Together, these results suggest that the electron-dense organelles of P. françai are homologous to the acidocalcisomes described in other trypanosomatids, although with distinct morphology and elemental content.

Membrane Topology of the H+-pyrophosphatase of Streptomyces coelicolor Determined by Cysteine-scanning Mutagenesis

The H+-translocating pyrophosphatase (H+-PPase) is a proton pump that is found in a wide variety of organisms. It consists of a single polypeptide chain that is thought to possess between 14 and 17 transmembrane domains. To determine the topological arrangement of its conserved motifs and transmembrane domains, we carried out a cysteine-scanning analysis by determining the membrane topology of cysteine substitution mutants of Streptomyces coelicolor H+-PPase expressed in Escherichia coli using chemical reagents. First, we prepared a synthetic DNA that encoded the enzyme and constructed a functional cysteine-less mutant by substituting the four cysteine residues. We then introduced cysteine residues individually into 42 sites in its hydrophilic regions and N- and C-terminal segments. Thirty-six of the mutant enzymes retained both pyrophosphatase and H+-translocating activities. Analysis of 29 of these mutant forms using membrane-permeable and -impermeable sulfhydryl reagents revealed that S. coelicolor H+-PPase contains 17 transmembrane domains and that several conserved segments, such as the substrate-binding domains, are exposed to the cytoplasm. Four essential serine residues that were located on the cytoplasmic side were also identified. A marked characteristic of the S. coelicolor enzyme is a long additional sequence that includes a transmembrane domain at the C terminus. We propose that the basic structure of H+-PPases has 16 transmembrane domains with several large cytoplasmic loops containing functional motifs.

Thermoinactivaion analysis of vacuolar H+-pyrophosphatase

Vacuolar H(+)-translocating pyrophosphatase (H(+)-PPase; EC 3.6.1.1) catalyzes both the hydrolysis of PP(i) and the electrogenic translocation of proton from the cytosol to the lumen of the vacuole. Vacuolar H(+)-PPase, purified from etiolated hypocotyls of mung bean (Vigna radiata L.), is a homodimer with a molecular mass of 145 kDa. To investigate the relationship between structure and function of this H(+)-translocating enzyme, thermoinactivation analysis was employed. Thermoinactivation studies suggested that vacuolar H(+)-PPase consists of two distinct states upon heat treatment and exhibited different transition temperatures in the presence and absence of ligands (substrate and inhibitors). Substrate protection of H(+)-PPase stabilizes enzyme structure by increasing activation energy from 54.9 to 70.2 kJ/mol. We believe that the conformation of this enzyme was altered in the presence of substrate to protect against the thermoinactivation. In contrast, the modification of H(+)-PPase by inhibitor (fluorescein 5'-isothiocyanate; FITC) augmented the inactivation by heat treatment. The native, substrate-bound, and FITC-labeled vacuolar H(+)-PPases possess probably distinct conformation and show different modes of susceptibility to thermoinactivation. Our results also indicate that the structure of one subunit of this homodimer exerts long distance effect on the other, suggesting a specific subunit-subunit interaction in vacuolar H(+)-PPase. A working model was proposed to interpret the relationship of the structure and function of vacuolar H(+)-PPase.

Elucidating the Role of Conserved Glutamates in H+-pyrophosphatase of Rhodospirillum rubrum

H(+)-pyrophosphatase (H(+)-PPase) catalyzes pyrophosphate-driven proton transport against the electrochemical potential gradient in various biological membranes. All 50 of the known H(+)-PPase amino acid sequences contain four invariant glutamate residues. In this study, we use site-directed mutagenesis in conjunction with functional studies to determine the roles of the glutamate residues Glu(197), Glu(202), Glu(550), and Glu(649) in the H(+)-PPase of Rhodospirillum rubrum (R-PPase). All residues were replaced with Asp and Ala. The resulting eight variant R-PPases were expressed in Escherichia coli and isolated as inner membrane vesicles. All substitutions, except E202A, generated enzymes capable of PP(i) hydrolysis and PP(i)-energized proton translocation, indicating that the negative charge of Glu(202) is essential for R-PPase function. The hydrolytic activities of all other PPase variants were impaired at low Mg(2+) concentrations but were only slightly affected at high Mg(2+) concentrations, signifying that catalysis proceeds through a three-metal pathway in contrast to wild-type R-PPase, which employs both two- and three-metal pathways. Substitution of Glu(197), Glu(202), and Glu(649) resulted in decreased binding affinity for the substrate analogues aminomethylenediphosphonate and methylenediphosphonate, indicating that these residues are involved in substrate binding as ligands for bridging metal ions. Following the substitutions of Glu(550) and Glu(649), R-PPase was more susceptible to inactivation by the sulfhydryl reagent mersalyl, highlighting a role of these residues in maintaining enzyme tertiary structure. None of the substitutions affected the coupling of PP(i) hydrolysis to proton transport.

Inhibition studies on Rhodospirillum rubrum H+-pyrophosphatase expressed in Escherichia coli

The membrane-bound proton-pumping inorganic pyrophosphatase from Rhodospirillum rubrum was heterologously expressed in Escherichia coli C43(DE3) cells and was inhibited by 4-bromophenacyl bromide (BPB), N,N'-dicyclohexylcarbodiimid (DCCD), diethyl pyrocarbonate (DEPC) and fluorescein 5'-isothiocyanate (FITC). In each case, the enzyme activity was rather well protected against inhibitory action by the substrate Mg(2)PPi. Site-directed mutagenesis was employed in attempts to identify target residues for these inhibitors. D217 and K469 appear to be the prime targets for DCCD and FITC, respectively, and may thus be involved in substrate binding. No major effect on enzyme activities was seen when any one of the four histidine residues present in the enzyme were substituted. Nevertheless, a mutant with all of the four charged histidine residues replaced retained only less than 10% of the hydrolysis and proton-pumping activities. Substitution of D217 with A or H yielded an enzyme with at least an order of magnitude lower hydrolysis activity. In contrast with the wild-type, these variants showed higher hydrolysis rates at lower concentrations of Mg(2+), possibly reflecting a change in substrate preference from Mg(2)PPi to MgPPi. BPB is a H(+)-pyrophosphatase inhibitor that apparently has not been used previously as an inhibitor of these enzymes.

Roles of histidine residues in plant vacuolar H+-pyrophosphatase

Vacuolar proton pumping pyrophosphatase (H(+)-PPase; EC 3.6.1.1) plays a pivotal role in electrogenic translocation of protons from cytosol to the vacuolar lumen at the expense of PP(i) hydrolysis. Alignment analysis on amino acid sequence demonstrates that vacuolar H(+)-PPase of mung bean contains six highly conserved histidine residues. Previous evidence indicated possible involvement of histidine residue(s) in enzymatic activity and H(+)-translocation of vacuolar H(+)-PPase as determined by using histidine specific modifier, diethylpyrocarbonate [J. Protein Chem. 21 (2002) 51]. In this study, we further attempted to identify the roles of histidine residues in mung bean vacuolar H(+)-PPase by site-directed mutagenesis. A line of mutants with histidine residues singly replaced by alanine was constructed, over-expressed in Saccharomyces cerevisiae, and then used to determine their enzymatic activities and proton translocations. Among the mutants scrutinized, only the mutation of H716 significantly decreased the enzymatic activity, the proton transport, and the coupling ratio of vacuolar H(+)-PPase. The enzymatic activity of H716A is relatively resistant to inhibition by diethylpyrocarbonate as compared to wild-type and other mutants, indicating that H716 is probably the target residue for the attack by this modifier. The mutation at H716 of V-PPase shifted the optimum pH value but not the T(1/2) (pretreatment temperature at which half enzymatic activity is observed) for PP(i) hydrolytic activity. Mutation of histidine residues obviously induced conformational changes of vacuolar H(+)-PPase as determined by immunoblotting analysis after limited trypsin digestion. Furthermore, mutation of these histidine residues modified the inhibitory effects of F(-) and Na(+), but not that of Ca(2+). Single substitution of H704, H716 and H758 by alanine partially released the effect of K(+) stimulation, indicating possible location of K(+) binding in the vicinity of domains surrounding these residues.

A new model for proton pumping in animal cells: the role of pyrophosphate

The H+-PPase activity was characterized in membrane fractions of ovary and eggs of Rhodnius prolixus. This activity is totally dependent on Mg2+, independent of K+ and strongly inhibited by NaF, IDP and Ca2+. The membrane proteins of eggs were analyzed by western blot using antibodies to the H+-PPase from Arabidopsis thaliana. The immunostain was associated with a single 65-kDa polypeptide. This polypeptide was immunolocalized in yolk granule membranes by optical and transmission electron microscopy. We describe the acidification of yolk granules in the presence of PPi and ATP. This acidification is inhibited in the presence of NAF, Ca2+ and antibodies against H+-PPase. These data show for the first time in animal cells that acidification of yolk granules involves an H+-PPase as well as H+-ATPase.

Properties of mutated Rhodospirillum rubrum H+-pyrophosphatase expressed in Escherichia coli

The membrane-bound proton pumping inorganic pyrophosphate synthase/pyrophosphatase (H(+)-PPi synthase/H(+)-PPase) from the photosynthetic bacterium Rhodospirillum rubrum was functionally expressed in Escherichia coli C43(DE3) cells. Based on a new topology model of the enzyme, charged residues predicted to be located near or within the membrane were selected for site-directed mutagenesis. Several of these mutations resulted in an almost complete inactivation of the enzyme. Four mutated residues appear to show a selective impairment of proton translocation and are thus likely to be involved in coupling pyrophosphate hydrolysis with electrogenic proton pumping. Two of these mutations, R176K and E584D, caused increased tolerance to salt. In addition, the former mutation caused an increased K(m) of one order of magnitude for the hydrolysis reaction. These results and their possible implications for the enzyme function are discussed.

Isolation and characterization of TgVP1, a type I vacuolar H+-translocating pyrophosphatase from Toxoplasma gondii. The dynamics of its subcellular localization and the cellular effects of a diphosphonate inhibitor

Here we report the isolation and characterization of a type I vacuolar-type H(+)-pyrophosphatase (V-PPase), TgVP1, from an apicomplexan, Toxoplasma gondii, a parasitic protist that is particularly amenable to molecular and genetic manipulation. The 816-amino acid TgVP1 polypeptide is 50% sequence-identical (65% similar) to the prototypical type I V-PPase from Arabidopsis thaliana, AVP1, and contains all the sequence motifs characteristic of this pump category. Unlike AVP1 and other known type I enzymes, however, TgVP1 contains a 74-residue N-terminal extension encompassing a 42-residue N-terminal signal peptide sequence, sufficient for targeting proteins to the secretory pathway of T. gondii. Providing that the coding sequence for the entire N-terminal extension is omitted from the plasmid, transformation of Saccharomyces cerevisiae with plasmid-borne TgVP1 yields a stable and functional translation product that is competent in aminomethylenediphosphonate (AMDP)-inhibitable K(+)-activated pyrophosphate (PP(i)) hydrolysis and PP(i)-energized H(+) translocation. Immunofluorescence microscopy of both free and intracellular T. gondii tachyzoites using purified universal V-PPase polyclonal antibodies reveals a punctate apical distribution for the enzyme. Equivalent studies of the tachyzoites during host cell invasion, by contrast, disclose a transverse radial distribution in which the V-PPase is associated with a collar-like structure that migrates along the length of the parasite in synchrony with and in close apposition to the penetration furrow. Although treatment of T. gondii with AMDP concentrations as high as 100 microm had no discernible effect on the efficiency of host cell invasion and integration, concentrations commensurate with the I(50) for the inhibition of TgVP1 activity in vitro (0.9 microm) do inhibit cell division and elicit nuclear enlargement concomitant with the inflation and eventual disintegration of acidocalcisome-like vesicular structures. A dynamic association of TgVP1 with the host cell invasion apparatus is invoked, one in which the effects of inhibitory V-PPase substrate analogs are exerted after rather than during host cell invasion.