The vacuolar ATPase: sulfite stabilization and the mechanism of nitrate inactivation

The V-type H+-ATPase is targeted in antidiuretic hormone control of the Malpighian "renal" tubules

Like other insects, secretion by mosquito Malpighian tubules (MTs) is driven by the V-type H+-ATPase (VA) localized in the apical membrane of principal cells. In Aedes aegypti, the antidiuretic neurohormone CAPA inhibits secretion by MTs stimulated by select diuretic hormones; however, the cellular effectors of this inhibitory signaling cascade remain unclear. Herein, we demonstrate that the VA inhibitor bafilomycin selectively inhibits serotonin (5HT)- and calcitonin-related diuretic hormone (DH31)-stimulated secretion. VA activity increases in DH31-treated MTs, whereas CAPA abolishes this increase through a NOS/cGMP/PKG signaling pathway. A critical feature of VA activation involves the reversible association of the cytosolic (V1) and membrane (Vo) complexes. Indeed, higher V1 protein abundance was found in membrane fractions of DH31-treated MTs, whereas CAPA significantly decreased V1 abundance in membrane fractions while increasing it in cytosolic fractions. V1 immunolocalization was observed strictly in the apical membrane of DH31-treated MTs, whereas immunoreactivity was dispersed following CAPA treatment. VA complexes colocalized apically in female MTs shortly after a blood meal consistent with the peak and postpeak phases of diuresis. Comparatively, V1 immunoreactivity in MTs was more dispersed and did not colocalize with the Vo complex in the apical membrane at 3 h post blood meal, representing a time point after the late phase of diuresis has concluded. Therefore, CAPA inhibition of MTs involves reducing VA activity and promotes complex dissociation hindering secretion. Collectively, these findings reveal a key target in hormone-mediated inhibition of MTs countering diuresis that provides a deeper understanding of this critical physiological process necessary for hydromineral balance.

The Plant V-ATPase

V-ATPase is the dominant proton pump in plant cells. It contributes to cytosolic pH homeostasis and energizes transport processes across endomembranes of the secretory pathway. Its localization in the trans Golgi network/early endosomes is essential for vesicle transport, for instance for the delivery of cell wall components. Furthermore, it is crucial for response to abiotic and biotic stresses. The V-ATPase's rather complex structure and multiple subunit isoforms enable high structural flexibility with respect to requirements for different organs, developmental stages, and organelles. This complexity further demands a sophisticated assembly machinery and transport routes in cells, a process that is still not fully understood. Regulation of V-ATPase is a target of phosphorylation and redox-modifications but also involves interactions with regulatory proteins like 14-3-3 proteins and the lipid environment. Regulation by reversible assembly, as reported for yeast and the mammalian enzyme, has not be proven in plants but seems to be absent in autotrophic cells. Addressing the regulation of V-ATPase is a promising approach to adjust its activity for improved stress resistance or higher crop yield.

Rotary Ion-Translocating ATPases/ATP Synthases: Diversity, Similarities, and Differences

Ion-translocating ATPases and ATP synthases (F-, V-, A-type ATPases, and several P-type ATPases and ABC-transporters) catalyze ATP hydrolysis or ATP synthesis coupled with the ion transport across the membrane. F-, V-, and A-ATPases are protein nanomachines that combine transmembrane transport of protons or sodium ions with ATP synthesis/hydrolysis by means of a rotary mechanism. These enzymes are composed of two multisubunit subcomplexes that rotate relative to each other during catalysis. Rotary ATPases phosphorylate/dephosphorylate nucleotides directly, without the generation of phosphorylated protein intermediates. F-type ATPases are found in chloroplasts, mitochondria, most eubacteria, and in few archaea. V-type ATPases are eukaryotic enzymes present in a variety of cellular membranes, including the plasma membrane, vacuoles, late endosomes, and trans-Golgi cisternae. A-type ATPases are found in archaea and some eubacteria. F- and A-ATPases have two main functions: ATP synthesis powered by the proton motive force (pmf) or, in some prokaryotes, sodium-motive force (smf) and generation of the pmf or smf at the expense of ATP hydrolysis. In prokaryotes, both functions may be vitally important, depending on the environment and the presence of other enzymes capable of pmf or smf generation. In eukaryotes, the primary and the most crucial function of F-ATPases is ATP synthesis. Eukaryotic V-ATPases function exclusively as ATP-dependent proton pumps that generate pmf necessary for the transmembrane transport of ions and metabolites and are vitally important for pH regulation. This review describes the diversity of rotary ion-translocating ATPases from different organisms and compares the structural, functional, and regulatory features of these enzymes.

Acrylamide-induced peripheral neuropathy: manifestations, mechanisms, and potential treatment modalities

Acrylamide is a chemical monomer; its polymer compounds are used in the manufacture of plastic, papers, adhesive tapes, dyes, and food packaging. Lately, scientists found that cooking (mainly roasting, baking, and frying) yields acrylamide. In addition to fried/baked potatoes, coffee and bakery products still contain substantial amounts of acrylamide. Acrylamide has toxic effects on different body systems include genitourinary, reproductive, nervous system, along with being a carcinogenic substance. The neurotoxicity of acrylamide includes central and peripheral neuropathy. In humans, the clinical manifestations include sensory or motor peripheral neuropathy, drowsiness, or cerebellar ataxia. Likewise, it presents with skeletal muscle weakness, hindlimb dysfunction, ataxia, and weight loss in animals. The suggested mechanisms for acrylamide neurotoxicity include direct inhibition of neurotransmission, cellular changes, inhibition of key cellular enzymes, and bonding of kinesin-based fast axonal transport. Moreover, it is suggested that acrylamide's molecular effect on SNARE core kinetics is carried out through the adduction of NSF and/or SNARE proteins. Lately, scientists showed disruption of focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) cell signaling pathways in human differentiating neuroblastoma SH-SY5Y cells, exposed to acrylamide. Different treatment modalities have been revealed to shield against or hasten recovery from acrylamide-induced neuropathy in preclinical studies, including phytochemical, biological, and vitamin-based compounds. Still, additional studies are needed to elucidate the pathogenesis and to identify the best treatment modality.

Arabidopsis V-ATPase d2 Subunit Plays a Role in Plant Responses to Oxidative Stress

Vacuolar-type H⁺-ATPase (V-ATPase), a multisubunit proton pump located on the endomembrane, plays an important role in plant growth. The Arabidopsis thaliana V-ATPase d subunit (VHA-d) consists of two isoforms; AtVHA-d1 and AtVHA-d2. In this study, the function of AtVHA-d2 was investigated. Histochemical analysis revealed that the expression of AtVHA-d1 and AtVHA-d2 was generally highly overlapping in multiple tissues at different developmental stages of Arabidopsis. Subcellular localization revealed that AtVHA-d2 was mainly localized to the vacuole. AtVHA-d2 expression was significantly induced by oxidative stress. Analysis of phenotypic and H₂O₂ content showed that the atvha-d2 mutant was sensitive to oxidative stress. The noninvasive microtest monitoring demonstrated that the net H⁺ influx in the atvha-d2 roots was weaker than that in the wild-type under normal conditions. However, oxidative stress resulted in the H⁺ efflux in atvha-d2 roots, which was significantly different from that in the wild-type. RNA-seq combined with qPCR analysis showed that the expression of several members of the plasma membrane H⁺-ATPase gene (AtAHA) family in atvha-d2 was significantly different from that in the wild-type. Overall, our results indicate that AtVHA-d2 plays a role in Arabidopsis in response to oxidative stress by affecting H⁺ flux and AtAHA gene expression.

Gene Expression and Silencing Studies in Phytophthora infestans Reveal Infection-Specific Nutrient Transporters and a Role for the Nitrate Reductase Pathway in Plant Pathogenesis

To help learn how phytopathogens feed from their hosts, genes for nutrient transporters from the hemibiotrophic potato and tomato pest Phytophthora infestans were annotated. This identified 453 genes from 19 families. Comparisons with a necrotrophic oomycete, Pythium ultimum var. ultimum, and a hemibiotrophic fungus, Magnaporthe oryzae, revealed diversity in the size of some families although a similar fraction of genes encoded transporters. RNA-seq of infected potato tubers, tomato leaves, and several artificial media revealed that 56 and 207 transporters from P. infestans were significantly up- or down-regulated, respectively, during early infection timepoints of leaves or tubers versus media. About 17 were up-regulated >4-fold in both leaves and tubers compared to media and expressed primarily in the biotrophic stage. The transcription pattern of many genes was host-organ specific. For example, the mRNA level of a nitrate transporter (NRT) was about 100-fold higher during mid-infection in leaves, which are nitrate-rich, than in tubers and three types of artificial media, which are nitrate-poor. The NRT gene is physically linked with genes encoding nitrate reductase (NR) and nitrite reductase (NiR), which mobilize nitrate into ammonium and amino acids. All three genes were coregulated. For example, the three genes were expressed primarily at mid-stage infection timepoints in both potato and tomato leaves, but showed little expression in potato tubers. Transformants down-regulated for all three genes were generated by DNA-directed RNAi, with silencing spreading from the NR target to the flanking NRT and NiR genes. The silenced strains were nonpathogenic on leaves but colonized tubers. We propose that the nitrate assimilation genes play roles both in obtaining nitrogen for amino acid biosynthesis and protecting P. infestans from natural or fertilization-induced nitrate and nitrite toxicity.

Little is known of how plant pathogens adapt to different growth conditions and host tissues. To understand the interaction between the filamentous eukaryotic microbe Phytophthora infestans and its potato and tomato hosts, we mined the genome for genes encoding proteins involved in nutrient uptake and measured their expression in leaves, tubers, and three artificial media. We observed dynamic changes between the growth conditions, and identified transporters expressed mainly in the biotrophic stage, leaves, tubers, or artificial media. When we blocked the expression of a nitrate transporter and two other genes involved in assimilating nitrate, we observed that those genes were required for successful colonization of nitrate-rich leaves but not nitrate-poor tissues, and that nitrate had become toxic to the silenced strains. We therefore hypothesize that the nitrate assimilation pathway may help the pathogen use inorganic nitrogen for nutrition and/or detoxify nitrate when its levels may become damaging.

Loss of pH control in Plasmodium falciparum parasites subjected to oxidative stress

The intraerythrocytic malaria parasite is susceptible to oxidative stress and this may play a role in the mechanism of action of some antimalarial agents. Here we show that exposure of the intraerythrocytic malaria parasite to the oxidising agent hydrogen peroxide results in a fall in the intracellular ATP level and inhibition of the parasite's V-type H⁺-ATPase, causing a loss of pH control in both the parasite cytosol and the internal digestive vacuole. In contrast to the V-type H⁺-ATPase, the parasite's digestive vacuole H⁺-pyrophosphatase is insensitive to hydrogen peroxide-induced oxidative stress. This work provides insights into the effects of oxidative stress on the intraerythrocytic parasite, as well as providing an alternative possible explanation for a previous report that light-induced oxidative stress causes selective lysis of the parasite's digestive vacuole.

Potent nematicidal activity of phthalaldehyde, salicylaldehyde, and cinnamic aldehyde against Meloidogyne incognita

The nematicidal activity of selected aromatic aldehydes was tested against the root knot nematode Meloidogyne incognita. The most active aldehyde was phthalaldehyde (1) with an EC(50) value of 11 ± 6 mg/L followed by salicylaldehyde (2) and cinnamic aldehyde (3) with EC(50) values of 11 ± 1 and 12 ± 5 mg/L, respectively. On the other hand, structurally related aldehydes such as 2-methoxybenzaldehyde (21), 3,4-dimethoxybenzaldehyde, and vanillin (23) were not active at the concentration of 1000 mg/L. By liquid chromatography-mass spectrometry the reactivity of tested aldehydes against a synthetic peptide resembling the nematode cuticle was characterized. At the test concentration of 1 mM, the main adduct formation was observed for 3,4-dihydroxybenzaldehyde (22), 2-methoxybenzaldehyde (21), and 3,4-dimethoxybenzaldehyde. Considering that 2-methoxybenzaldehyde (21) and 3,4-dimethoxybenzaldehyde were not active against M. incognita in in vitro experiments led us to hypothesize a different mechanism of action rather than an effect on the external cuticle modification of nematodes. When the toxicity of the V-ATPase inhibitor pyocyanin (10) was tested against M. incognita J2 nematodes, an EC(50) at 24 h of 72 ± 25 mg/L was found. The redox-active compounds such as phthalaldehyde (1) and salicylaldehyde (2) may share a common mode of action inhibiting nematode V-ATPase enzyme. The results of this investigation reveal that aromatic redox-active aldehydes can be considered as potent nematicides, and further investigation is needed to completely clarify their mode of action.

Regulation of the V-type ATPase by redox modulation

ATP-hydrolysis and proton pumping by the V-ATPase (vacuolar proton-translocating ATPase) are subject to redox regulation in mammals, yeast and plants. Oxidative inhibition of the V-ATPase is ascribed to disulfide-bond formation between conserved cysteine residues at the catalytic site of subunit A. Subunits containing amino acid substitutions of one of three conserved cysteine residues of VHA-A were expressed in a vha-A null mutant background in Arabidopsis. In vitro activity measurements revealed a complete absence of oxidative inhibition in the transgenic line expressing VHA-A C256S, confirming that Cys256 is necessary for redox regulation. In contrast, oxidative inhibition was unaffected in plants expressing VHA-A C279S and VHA-A C535S, indicating that disulfide bridges involving these cysteine residues are not essential for oxidative inhibition. In vivo data suggest that oxidative inhibition might not represent a general regulatory mechanism in plants.

The plant host Brassica napus induces in the pathogen Verticillium longisporum the expression of functional catalase peroxidase which is required for the late phase of disease

The devastating soilborne fungal pathogen Verticillium longisporum is host specific to members of the family Brassicaceae, including oilseed rape (Brassica napus) as the economically most important crop. The fungus infects through the roots and causes stunting and early senescence of susceptible host plants and a marked decrease in crop yield. We show here that V. longisporum reacts to the presence of B. napus xylem sap with the production of six distinct upregulated and eight downregulated proteins visualized by two-dimensional gel electrophoresis. Identification of 10 proteins by mass spectrometry revealed that all upregulated proteins are involved in oxidative stress response. The V. longisporum catalase peroxidase (VlCPEA) was the most upregulated protein and is encoded by two isogenes, VlcpeA-1 and VlcpeA-2. Both genes are 98% identical, corroborating the diploid or "amphihaploid" status of the fungus. Knock downs of both VlcpeA genes reduced protein expression by 80% and resulted in sensitivity against reactive oxygen species. Whereas saprophytic growth and the initial phase of the plant infection were phenotypically unaffected, the mutants were not able to perform the late phases of disease. We propose that the catalase peroxidase plays a role in protecting the fungus from the oxidative stress generated by the host plant at an advanced phase of the disease.

Reduced V-ATPase activity in the trans-Golgi network causes oxylipin-dependent hypocotyl growth Inhibition in Arabidopsis

Regulated cell expansion allows plants to adapt their morphogenesis to prevailing environmental conditions. Cell expansion is driven by turgor pressure created by osmotic water uptake and is restricted by the extensibility of the cell wall, which in turn is regulated by the synthesis, incorporation, and cross-linking of new cell wall components. The vacuolar H(+)-ATPase (V-ATPase) could provide a way to coordinately regulate turgor pressure and cell wall synthesis, as it energizes the secondary active transport of solutes across the tonoplast and also has an important function in the trans-Golgi network (TGN), which affects synthesis and trafficking of cell wall components. We have previously shown that det3, a mutant with reduced V-ATPase activity, has a severe defect in cell expansion. However, it was not clear if this is caused by a defect in turgor pressure or in cell wall synthesis. Here, we show that inhibition of the tonoplast-localized V-ATPase subunit isoform VHA-a3 does not impair cell expansion. By contrast, inhibition of the TGN-localized isoform VHA-a1 is sufficient to restrict cell expansion. Furthermore, we provide evidence that the reduced hypocotyl cell expansion in det3 is conditional and due to active, hormone-mediated growth inhibition caused by a cell wall defect.

Mechanisms of the ifosfamide-induced inhibition of endocytosis in the rat proximal kidney tubule

The Fanconi syndrome is a common side effect of the chemotherapeutic agent ifosfamide. Current evidences suggest that chloroacetaldehyde (CAA), one of the main metabolites of ifosfamide activation, contributes to its nephrotoxicity. However, the pathophysiology of CAA-induced Fanconi syndrome is not fully understood. The present work examined the adverse effects of CAA on precision-cut rat renal cortical slices, which allowed studying the toxic effect of CAA on proximal endocytosis. We demonstrated that clinically relevant concentrations of CAA (< or =200 microM) are able to inhibit the uptake of horseradish peroxidase, a marker of proximal tubular cell endocytosis in renal tubular proximal cells. CAA > or =75 microM has adverse effects, both on viability parameters and on energy metabolism, as shown by the great decrease in total glutathione and ATP levels. In addition, the V-ATPase, which plays a crucial role in intracellular vesicle trafficking, was inhibited by 100 microM of CAA. By contrast, the slight decrease in Na-K-ATPase activity observed for CAA> or = 125 microM (maximum inhibition: 33%) could not totally explain the inhibition of the reabsorption processes. In conclusion, the addition of the two main adverse effects of CAA (decrease in ATP levels and inhibition of the V-ATPase) could explain the inhibition of endocytosis and the Fanconi syndrome observed during ifosfamide treatments.

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

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

Thioredoxin reductase is required for growth and regulates entry into culmination of Dictyostelium discoideum

The thioredoxin system, consisting of thioredoxin, thioredoxin reductase and NADPH, has been well established to be critical for the redox regulation of protein function and signalling. To investigate the role of thioredoxin reductase (Trr) in Dictyostelium discoideum, we generated mutant cells that underexpress or overexpress Trr. Trr-underexpressing cells exhibited severe defects in axenic growth and development. Trr-overexpressing (TrrOE) cells formed very tiny plaques on a bacterial lawn and had a lower rate of bacterial uptake. When developed in the dark, TrrOE cells exhibited a slugger phenotype, defined by a prolonged migrating slug stage. Like other slugger mutants, they were hypersensitive to ammonia, which has been known to inhibit culmination by raising the pH of intracellular acidic compartments. Interestingly, TrrOE cells showed defective acidification of intracellular compartments and decreased activity of vacuolar H+-ATPase which functions in the acidification of intracellular compartments. Moreover, biochemical studies revealed that the thioredoxin system can directly reduce the catalytic subunit of vacuolar H+-ATPase whose activity is regulated by reversible disulphide bond formation. Taken together, these results suggest that Dictyostelium Trr may be essential for growth and play a role in regulation of phagocytosis and culmination, possibly through the modulation of vacuolar H+-ATPase activity.

Acrylamide Neurotoxicity: Neurological, Morhological and Molecular Endpoints in Animal Models

Acrylamide (AA) monomer is used in numerous chemical industries and is a contaminant in potato- and grain-based foods prepared at high temperatures. Although experimental animal studies have implicated carcinogenicity and reproductive toxicity as possible consequences of exposure, neurotoxicity is the only outcome identified by epidemiological studies of occupationally exposed human populations. Neurotoxicity in both humans and laboratory animals is characterized by ataxia and distal skeletal muscle weakness. Early neuropathological studies suggested that AA neurotoxicity was mediated by distal axon degeneration. However, more recent electrophysiological and quantitative morphometric analyses have identified nerve terminals as primary sites of AA action. A resulting defect in neurotransmitter release appears to be the pathophysiological basis of the developing neurotoxicity. Corresponding mechanistic research suggests that AA impairs release by adducting cysteine residues on functionally important presynaptic proteins. In this publication we provide an overview of recent advances in AA research. This includes a discussion of the cumulative nature of AA neurotoxicity and the putative sites and molecular mechanisms of action.

Differential response of vacuolar proton pumps to osmotica

The vacuole is a fundamental and dominant organelle and occupies a large part of the total cell volume in most mature plant cells. The higher-plant vacuole contains two types of proton-translocating pumps, H⁺-ATPase (EC 3.6.1.3) and H⁺-pyrophosphatase (EC 3.6.1.1), residing on the same membrane. These two enzymes generate roughly equal proton gradients across the vacuolar membrane for the secondary transport of ions and metabolites. However, the pumps respond differentially to stress in order to maintain critical functions of the vacuole. In this work, tonoplasts from etiolated mung bean seedlings (Vigna radiata L.) were used to investigate the function of these two enzymes under high osmotic pressure. At high concentrations of sucrose or sorbitol, the light scattering and volume of isolated vesicles were progressively changed. Concomitantly, enzymatic activities, proton translocation, and coupling efficiencies of these two proton-pumping enzymes were inhibited to various extents under high osmotic pressure. No significant change in enzymatic activities of purified vacuolar H⁺-PPase and H⁺-ATPase under similar conditions was observed. We thus believe that the membrane structure is an important determinant for proper function of proton pumping systems of plant vacuoles. Furthermore, kinetic analysis shows different variation in apparent V_max but not in K_M values of vacuolar H⁺-PPase and H⁺-ATPase at high osmolarity of sucrose and sorbitol, respectively, suggesting probable alterations in substrate hydrolysis reactions but not substrate-binding affinity of the enzymes. A working model is proposed to interpret supplemental roles of vacuolar H⁺-PPase and H⁺-ATPase to maintain appropriate functions of plant tonoplasts.

H+-ATPase of crude homogenate of the outer mantle epithelium of Anodonta cygnea

The outer mantle epithelium of the freshwater bivalve, Anodonta cygnea, is responsible for the mineralisation of the shell. Under short circuit conditions, it generates a current that is due to the operation of an electrogenic proton pump located in the apical barrier of that epithelium. Bafilomycin A1 and Concanamycin A inhibited the short circuit current. The IC50 and maximum inhibition dose were 0.17 and 0.5 microM for Bafilomycin A1, and 0.7 and 5 microM for Concanamycin A. The present work was undertaken to further characterise V-type ATPase of the outer mantle cells. The V-ATPase enzymatic activity of crude homogenate, measured as the amount of inorganic phosphorous released, due to ATP hydrolysis, in the presence of Na2SO3 (200 mM) was found to be 4.6+/-1.1 micromol Pi/mg protein/h, at 27 degrees C, pH 7.0-7.4 and ATP 4.5-6.0 microM. Bafilomycin A1 and Concanamycin A inhibit the V-ATPase activity with an IC50 of 14 and 8 nmol mg(-1), respectively. Dicyclohexylcarbodiimide (DCCD; 100 mM) and NaNO3 (100 microM) inhibited the V-type ATPase in what it seems a non-specific manner and NEM (100 mM) was unable to do it. Bafilomycin A1 (10 microM) and Concanamycin A (10 microM), inhibited 50-60% of the total activity.

The Changing View of Acrylamide Neurotoxicity

Acrylamide (ACR) is a water-soluble, vinyl monomer that has multiple chemical and industrial applications: e.g., waste water management, ore processing. In addition, ACR is used extensively in molecular laboratories for gel chromatography and is present in certain foods that have been prepared at very high temperatures. Extensive studies in rodents and other laboratory animals have provided evidence that exposure to monomeric ACR causes cellular damage in both the nervous and reproductive systems, and produces tumors in certain hormonally responsive tissues. Whereas human epidemiological studies have demonstrated a significantly elevated incidence of neurotoxicity in occupationally exposed populations, such research has not, to date, revealed a corresponding increase in cancer risk. Since the announcement by a Swedish research group in April 2002 [J. Ag. Food Chem. 50 (2002) 4998] regarding the presence of ACR in potato and grain-based foods, there has been a renewed interest in the toxic actions of this chemical. Therefore, in this review, we consider the different toxic effects of ACR. The neurotoxic actions of ACR will be the focal point since neurotoxicity is a consequence of both human and laboratory animal exposure and since this area of investigation has received considerable attention over the past 30 years. As will be discussed, a growing body of evidence now indicates that the nerve terminal is a primary site of ACR action and that inhibition of corresponding membrane-fusion processes impairs neurotransmitter release and promotes eventual degeneration. The electrophilic nature of ACR suggests that this neurotoxicant adducts nucleophilic sulfhydryl groups on certain proteins that are critically involved in membrane fusion. Adduction of thiol groups also might be common to the reproductive and carcinogenic effects of ACR. A final goal of this review is to identify data gaps that retard a comprehensive understanding of ACR pathophysiological processes.

The V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti: localization and activity

The V-type H(+)-ATPase is thought to provide the driving force for transepithelial electrolyte and fluid secretion in Malpighian tubules. To confirm the presence of this proton pump in Malpighian tubules of the yellow fever mosquito Aedes aegypti, we used several antibodies raised against the V-type H(+)-ATPase of Manduca sexta. Western blot analysis confirmed the presence of the V-type H(+)-ATPase in Malpighian tubules of Aedes aegypti. In situ immunostaining identified the V-type H(+)-ATPase at the apical membrane of the mitochondrion-rich brush border of principal cells. The V-type H(+)-ATPase was not found in stellate cells. Measurements of ATPase activity revealed that bafilomycin-sensitive and NO(3)(-)-sensitive ATPase activity accounted for 50-60% of total ATPase activity in crude extracts of Malpighian tubules. No significant ouabain- or vanadate-sensitive Na(+)/K(+)-ATPase activity was detected. These results support the conclusion reached previously in electrophysiological studies that the mechanisms for transepithelial electrolyte secretion in the Aedes Malpighian tubules rely on the V-type H(+)-ATPase as the principal energizer of epithelial transport. Measures of transepithelial Na(+) and K(+) secretion and estimates of the H(+) flux mediated by the V-type H(+)-ATPase suggest a 1:1 stoichiometry for Na(+)/H(+) and K(+)/H(+) exchange transport across the apical membrane.

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).

Redox regulation of cytosolic glycerol-3-phosphate dehydrogenase: Cys(102) is the target of the redox control and essential for the catalytic activity

Cytosolic glycerol-3-phosphate dehydrogenase (cG3PDH) occupies the branch point between the glycolytic pathway and triglyceride biosynthesis. However, the regulatory mechanism of the cG3PDH activity has remained obscure. Here we report that cG3PDH is efficiently inhibited by modification of the thiol group through a redox mechanism. In this study, we found that sodium selenite and nitric oxide (NO) donors such as S-nitroso-N-acetylpenicillamine and 3-morpholinosydnonimine inhibited cG3PDH activity, and that similar effects could be achieved with selenium metabolites such as selenocysteine and selenomethionine. Furthermore, we found that reducing agents, such as dithiothreitol and beta-mercaptoethanol, restored the cG3PDH activity suppressed by selenite and NO both in vitro and in cultured cells. Buthionine sulfoximine depleted levels of both reduced glutathione and the oxidized form but had no effect on the suppression of cG3PDH activity by selenite in cultured cells. Moreover, thiol-reactive agents, such as N-ethylmaleimide and o-iodosobenzoic acid, blocked the enzyme activity of cG3PDH through the modification of redox-sensitive cysteine residues in cG3PDH. The inhibitor of NO synthase, L-N(G)-nitro-arginine, restored the cG3PDH activity inhibited by NO in cultured cells, whereas the inhibitor of guanylyl cyclase, 1H-[1,2,4] oxadiazole[4,3-alpha] quinoxalin-1-one (ODQ), has no effect. NO directly inhibits cG3PDH activity not via a cGMP-dependent mechanism. Finally, using site-directed mutagenesis, we found that Cys(102) of cG3PDH was sensitive to both selenite and NO. From the results, we suggest that cG3PDH is a target of cellular redox regulation.

The functions of inter- and intracellular glutathione transport systems in plants

Glutathione is one of the major redox buffers in most aerobic cells, and it has a broad spectrum of functions in plants. Recent discoveries implicate this thiol peptide in signalling and cellular homeostasis. Glutathione can sense intracellular redox status: perturbations of glutathione reduction state are transduced into changes in gene expression. This central role demands precise control of both the concentration and the reduction state of glutathione in different compartments. In addition to the regulation of glutathione biosynthesis and redox state, attention is now turning to the role of glutathione transporters.

ATP analogue binding to the A subunit induces conformational changes in the E subunit that involves a disulfide bond formation in plant V-ATPase

Vacuolar H+-ATPase (V-ATPase) consists of a catalytic head, a stalk part and a membrane domain. We indirectly investigated the interaction between the A subunit (catalytic head) and the E subunit (stalk part) using an ATP analogue, adenosine 5'-[beta,gamma-imino]triphosphate (AMP-PNP), which holds the enzyme in the substrate-binding state. AMP-PNP treatment caused a mobility shift of the E subunit with a faster migration in SDS/polyacrylamide gel electrophoresis without a reductant, while ATP treatment did not. A mobility shift of the E subunit has been detected in several plants. As polypeptides with intramolecular disulfide bonds migrate faster than those without disulfide bonds, the mobility shift may be due to the formation of an intramolecular disulfide bond by two cysteine residues conserved among several plant species. The mobility shift may be involved in the binding of AMP-PNP to the ATP-binding site, which exists in the A and B subunits, as it was inhibited by the addition of ATP. Pretreatment with 2'-3'-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), which modifies the ATP-binding site of the B subunit under UV illumination, did not inhibit the mobility shift of the E subunit caused by AMP-PNP treatment. The response of V-ATPase following the AMP-PNP binding may cause a conformational change in the E subunit into a form that is susceptible to oxidation of cysteine residues. This is the first demonstration of interaction between the A and E subunits in the substrate-binding state of a plant V-ATPase.

Structure–function relationships of A-, F- and V-ATPases

Ion-translocating ATPases, such as the F1Fo-, V1Vo- and archaeal A1Ao enzymes, are essential cellular energy converters which transduce the chemical energy of ATP hydrolysis into transmembrane ionic electrochemical potential differences. Based on subunit composition and primary structures of the subunits, these types of ATPases are related through evolution; however, they differ with respect to function. Recent work has focused on the three-dimensional structural relationships of the major, nucleotide-binding subunits A and B of the A1/V1-ATPases and the corresponding β and α subunits of the F1-ATPase, and the location of the coupling subunits within the stalk that provide the physical linkage between the regions of ATP hydrolysis and ion transduction. This review focuses on the structural homologies and diversities of A1-, F1- and V1-ATPases, in particular on significant differences between the stalk regions of these families of enzymes.

Hex-1, a gene unique to filamentous fungi, encodes the major protein of the Woronin body and functions as a plug for septal pores

We have identified a gene, named hex-1, that encodes the major protein in the hexagonal crystals, or Woronin bodies, of Neurospora crassa. Analysis of a strain with a null mutation in the hex-1 gene showed that the septal pores in this organism were not plugged when hyphae were damaged, leading to extensive loss of cytoplasm. When grown on agar plates containing sorbose, the hex-1(-) strain showed extensive lysis of hyphal tips. The HEX-1 protein was predicted to be 19,125 Da. Analysis of the N-terminus of the purified protein indicated that 16 residues are cleaved, yielding a protein of 17,377 Da. A polyclonal antibody raised to the HEX-1 protein recognized multiple forms of the protein, apparently dimers and tetramers that were resistant to solubilization by sodium dodecyl sulfate and reducing reagents. Treatment of the protein with phosphatase caused dissociation of these oligomers. Preparations enriched in Woronin bodies contained catalase activity, which was not detected in comparable fractions from the hex-1(-) mutant strain. These results support the hypothesis that the Woronin body is a specialized peroxisome that functions as a plug for septal pores.

Evidence for major structural changes in the Manduca sexta midgut V1 ATPase due to redox modulation. A small angle X-ray scattering study

The shape and overall dimensions of the oxidized and reduced form of the V(1) ATPase from Manduca sexta were investigated by synchrotron radiation x-ray solution scattering. The radius of gyration of the oxidized and reduced complex differ noticeably, with dimensions of 6. 20 +/- 0.06 and 5.84 +/- 0.06 nm, respectively, whereas the maximum dimensions remain constant at 22.0 +/- 0.1 nm. Comparison of the low resolution shapes of both forms, determined ab initio, indicates that the main structural alteration occurs in the head piece, where the major subunits A and B are located, and at the bottom of the stalk. In conjunction with the solution scattering data, decreased susceptibility to tryptic digestion and tryptophan fluorescence of the reduced V(1) molecule provide the first strong evidence for major structural changes in the V(1) ATPase because of redox modulation.

Characterization of isolated acidocalcisomes of Trypanosoma cruzi

The acidocalcisome is an acidic calcium store in trypanosomatids with a vacuolar-type proton-pumping pyrophosphatase (V-H(+)-PPase) located in its membrane. In this paper, we describe a new method using iodixanol density gradients for purification of the acidocalcisome from Trypanosoma cruzi epimastigotes. Pyrophosphatase assays indicated that the isolated organelle was at least 60-fold purified compared with the large organelle (10,000 x g) fraction. Assays for other organelles generally indicated no enrichment in the acidocalcisome fraction; glycosomes were concentrated 5-fold. Vanadate-sensitive ATP-driven Ca(2+) uptake (Ca(2+)-ATPase) activity was detectable in the isolated acidocalcisome, but ionophore experiments indicated that it was not acidic. However, when pyrophosphate was added, the organelle acidified, and the rate of Ca(2+) uptake increased. Use of the indicator Oxonol VI showed that V-H(+)-PPase activity generated a membrane potential. Use of sulfate or nitrate in place of chloride in the assay buffer did not affect V-H(+)-PPase activity, but there was less activity with gluconate. Organelle acidification was countered by the chloride/proton symport cycloprogidiosin. No vacuolar H(+)-ATPase activity was detectable in isolated acidocalcisomes. However, immunoblots showed the presence of at least a membrane-bound V-H(+)-ATPase subunit, while experiments employing permeabilized epimastigotes suggested that vacuolar H(+)-ATPase and V-H(+)-PPase activities are present in the same Ca(2+)-containing compartment.

NaCl uptake by the branchial epithelium in freshwater teleost fish: an immunological approach to ion-transport protein localization

Teleost fishes, living in fresh water, engage in active ion uptake to maintain ion homeostasis. Current models for NaCl uptake involve Na(+) uptake via an apical amiloride-sensitive epithelial Na(+) channel (ENaC), energized by an apical vacuolar-type proton pump (V-ATPase) or alternatively by an amiloride-sensitive Na(+)/H(+) exchange (NHE) protein, and apical Cl(−) uptake mediated by an electroneutral, SITS-sensitive Cl(−)/HCO(3-) anion-exchange protein. Using non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models. Na(+)/K(+)-ATPase was used as a cellular marker for differentiating branchial epithelium mitochondria-rich (MR) cells from pavement cells. In both the freshwater tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-ATPase and ENaC-like immunoreactivity co-localized to pavement cells, although apical labelling was also found in MR cells in the trout. In the freshwater tilapia, apical anion-exchanger-like immunoreactivity is found in the MR cells. Thus, a freshwater-type MR chloride cell exists in teleost fishes. The NHE-like immunoreactivity is associated with the accessory cell type and with a small population of pavement cells in tilapia.

Biochemical evidence for ATPase activity in CFTR-enriched apical membrane vesicles from tracheal epithelium

In apical membrane vesicles from beef tracheal epithelia expressing up to 30% of the proteins as functional cystic fibrosis transmembrane conductance regulator (CFTR)-- i.e. a voltage-independent and PKA-sensitive 36Cl- flux--an ATPase activity, different from P, F0F1 and V types, was reproducibly detected. Its specific activity averaged 20 micromol Pi h(-1) mg(-1) with an apparent affinity for ATP of 530 +/- 30 microM. Its possible involvement in CFTR functions was supported by (1) the linear relationship between the ATPase activity and the magnitude of 36Cl- fluxes (turnover rate: 3 ATP hydrolyzed per CFTR per second), (2) the same rank of potency of ATP, ITP, GTP, UTP and CTP to be hydrolyzed and to open CFTR chloride channels, (3) the similar and parallel inhibition of the ATPase and CFTR Cl- fluxes by NS004 (IC50: 60 microM) and (4) the potency of anti-R domain antibodies to increase by 18% the ATPase activity.

Structure, function and regulation of the plant vacuolar H(+)-translocating ATPase

The plant V-ATPase is a primary-active proton pump present at various components of the endomembrane system. It is assembled by different protein subunits which are located in two major domains, the membrane-integral V(o)-domain and the membrane peripheral V(1)-domain. At the plant vacuole the V-ATPase is responsible for energization of transport of ions and metabolites, and thus the V-ATPase is important as a 'house-keeping' and as a stress response enzyme. It has been shown that transcript and protein amount of the V-ATPase are regulated depending on metabolic conditions indicating that the expression of V-ATPase subunit is highly regulated. Moreover, there is increasing evidence that modulation of the holoenzyme structure might influence V-ATPase activity.

Sulfite and membrane energization induce two different active states of the Paracoccus denitrificans F0F1-ATPase

Activation of the latent ATPase activity of inside-out vesicles from plasma membranes of Paracoccus denitrificans was studied. Several factors were found to induce activation: heat, membrane energization by succinate oxidation, methanol, oxyanions (sulfite, phosphate, arsenate, bicarbonate) and limited proteolysis with trypsin. Among the oxyanions, sulfite induced the higher increase in ATPase activity. Sulfite functioned as a nonessential activator that slightly modified the affinity for ATP and increased notoriously the Vmax. There was a competitive effect between sulfite, bicarbonate and phosphate for ATPase activation; their similar chemical geometry suggests that these oxyanions have a common binding site on the enzyme. Dithiothreitol did not affect the ATPase activity. ATPase activation by sulfite was decreased by uncoupler, enhanced by trypsin and inhibited by ADP, oligomycin and venturicidin. In contrast, activation induced by succinate was less sensitive to ADP, oligomycin, venturicidin and trypsin. It is proposed that the active states induced by sulfite and succinate reflect two conformations of the enzyme, in which the inhibitory subunit epsilon is differently exposed to trypsin.

Substrate- and inhibitor-induced conformational changes in the yeast V-ATPase provide evidence for communication between the catalytic and proton-translocating sectors

The vacuolar-type H(+)-ATPases (V-ATPases) are composed of two distinct sectors, a catalytic complex (V(1)) involved in ATP hydrolysis and a membrane-associated complex (V(0)) mediating proton translocation across a lipid bilayer. To date, little is known about the mechanism by which these two functions are coupled. We sought to examine the impact of nucleotide and cation binding on the structure of the core components of the catalytic complex and to determine whether conformational changes within the catalytic complex impact subunits of the membrane-associated complex. Nucleotide- and cation- induced changes in the catalytic core of the V-ATPase were investigated by monitoring changes in the rate and pattern of tryptic digests. ATP.Mg-induced changes were detected in both the catalytic (Vma1p or 69 kDa) and the regulatory subunits (Vma2p or 60 kDa) of the V(1) sector. ATP alone increased the rate of trypsinization of the regulatory subunit, but did not have any effect on Vma1p. Surprisingly, ATP also had an impact on the 95-kDa subunit, a component of the V(0) sector of the V-ATPase. Although the presence of divalent cations had no impact on the V(1) sector, the rate of trypsinization of the 95-kDa subunit was greatly enhanced. The effect of divalent cations on the structure of the 95-kDa subunit was abrogated when trypsinization was performed in the absence of the catalytic sector. Addition of bafilomycin A(1), a V-ATPase inhibitor that putatively binds to the 95-kDa subunit, increased the rate of trypsinization of the catalytic subunit. These data suggest that structural alterations within the V(1) sector result in alterations within the V(0) sector and vice versa. Clearly, a structural link must exist to couple the two sectors. The 95-kDa subunit is ideally suited to fulfill this role. Hydropathy analysis suggests a bipartite structure, with the NH(2)-terminal portion predicted to lie in an aqueous environment and the C-terminal portion predicted to contain 6 transmembrane segments. Tryptic digests of sealed vacuolar vesicles and immunofluorescence studies revealed that the large hydrophilic NH(2)-terminal domain of the 95-kDa subunit is localized toward the cytosol. This region therefore is ideally positioned to interact with components of the V(1) complex, potentially functioning as the elusive link between the two sectors of the V-ATPase.

The vacuolar H+-ATPase of lemon fruits is regulated by variable H+/ATP coupling and slip

Lemon fruit tonoplasts, unlike those of seedling epicotyls, contain nitrate-insensitive H+-ATPase activity (Müller, M. L., Irkens-Kiesecker, U., Rubinstein, B., and Taiz, L. (1996) J. Biol. Chem. 271, 1916-1924). However, the degree of nitrate-insensitivity fluctuates during the course of the year with a seasonal frequency. Nitrate uncouples H+ pumping from ATP hydrolysis both in epicotyls and in nitrate-sensitive fruit V-ATPases. Neither bafilomycin nor oxidation cause uncoupling. The initial rate H+/ATP coupling ratios of epicotyl and the nitrate-sensitive fruit proton pumping activities are the same. However, the H+/ATP coupling ratio of the nitrate-insensitive fruit H+ pumping activity is lower than that of nitrate-sensitive and epicotyl V-ATPases. Several properties of the nitrate-insensitive H+-ATPase of the fruit indicate that it is a modified V-ATPase rather than a P-ATPase: 1) insensitivity to low concentrations of vanadate; 2) it is initially strongly uncoupled by nitrate, but regains coupling as catalysis proceeds; 3) both the nitrate-sensitive and nitrate-insensitive fruit H+-pumps have identical Km values for MgATP, and show similar pH-dependent slip and proton leakage rates. We conclude that the ability of the juice sac V-ATPase to build up steep pH gradients involves three factors: variable coupling, i.e. the ability to regain coupling under conditions that initially induce uncoupling; a low pH-dependent slip rate; the low proton permeability of the membrane.

The vacuolar H+-ATPase of clathrin-coated vesicles is reversibly inhibited by S-nitrosoglutathione

It has been previously demonstrated that the vacuolar H+-ATPase (V-ATPase) of clathrin-coated vesicles is reversibly inhibited by disulfide bond formation between conserved cysteine residues at the catalytic site on the A subunit (Feng, Y., and Forgac, M. (1994) J. Biol. Chem. 269, 13224-13230). Proton transport and ATPase activity of the purified, reconstituted V-ATPase are now shown to be inhibited by the nitric oxide-generating reagent S-nitrosoglutathione (SNG). The K0.5 for inhibition by SNG following incubation for 30 min at 37 degreesC is 200-400 microM. As with disulfide bond formation at the catalytic site, inhibition by SNG is reversed upon treatment with 100 mM dithiothreitol and is partially protected in the presence of ATP. Also as with disulfide bond formation, treatment of the V-ATPase with SNG protects activity from subsequent inactivation by N-ethylmaleimide, as demonstrated by restoration of activity by dithiothreitol following sequential treatment of the V-ATPase with SNG and N-ethylmaleimide. Moreover, inhibition by SNG is readily reversed by dithiothreitol but not by the reduced form of glutathione, suggesting that the disulfide bond formed at the catalytic site of the V-ATPase may not be immediately reduced under intracellular conditions. These results suggest that SNG inhibits the V-ATPase through disulfide bond formation between cysteine residues at the catalytic site and that nitric oxide (or nitrosothiols) might act as a negative regulator of V-ATPase activity in vivo.

Characterization of ATPases of apical membrane fractions from Locusta migratoria Malpighian tubules

Apical and basal membrane fractions from Locusta Malpighian tubules were prepared and were characterized by marker enzyme analysis. The apical membranes contained an azide- and orthovanadate-insensitive ATPase activity that was inhibited by bafilomycin A1 (IC50 = 0.44 nM) and NEM (IC50 = 2.15 microM), and thus was characterized as putative V-type ATPase. The enzyme was stimulated by a variety of monovalent cations (Tris > K = Na > choline > Li = Rb) maximal stimulation occurring at 30-40 mM. It was also stimulated by a variety of monovalent anions (maximal activation 30-40 mM), but was strongly inhibited by nitrate and thiocyanate. SDS-PAGE separation of proteins present in the various membrane fractions was carried out. The apical membrane fraction alone contained a 28 kDa protein band that bound a monoclonal antibody specific for a 28 kDa peptide which was a component of the V-type ATPase from midgut of Manduca sexta and, in native gels, possessed ATPase activity which was also sensitive to both bafilomycin and NEM but not to azide or orthovanadate. Binding of the fluorescent monoclonal antibody was located at the apical boundary of the tubule cells. It was concluded that a V-type ATPase is present at the apical surface of Locusta Malpighian tubule cells and that it is involved in their secretory functioning.

Structure, function and regulation of the vacuolar (H+)-ATPase

The vacuolar (H+)-ATPases (or V-ATPases) function in the acidification of intracellular compartments in eukaryotic cells. The V-ATPases are multisubunit complexes composed of two functional domains. The peripheral V1 domain, a 500-kDa complex responsible for ATP hydrolysis, contains at least eight different subunits of molecular weight 70-13 (subunits A-H). The integral V0 domain, a 250-kDa complex, functions in proton translocation and contains at least five different subunits of molecular weight 100-17 (subunits a-d). Biochemical and genetic analysis has been used to identify subunits and residues involved in nucleotide binding and hydrolysis, proton translocation, and coupling of these activities. Several mechanisms have been implicated in the regulation of vacuolar acidification in vivo, including control of pump density, regulation of assembly of V1 and V0 domains, disulfide bond formation, activator or inhibitor proteins, and regulation of counterion conductance. Recent information concerning targeting and regulation of V-ATPases has also been obtained.

The intriguing evolution of the "b" and "G" subunits in F-type and V-type ATPases: isolation of the vma-10 gene from Neurospora crassa

We have characterized the vma-10 gene which encodes the G subunit of the vacuolar ATPase in Neurospora crassa. The gene is somewhat unusual in filamentous fungi because it contains five introns, comprising 71% of the region between the translation start and stop codons. The 5' untranslated region of the gene contains several elements that have been identified in other genes that encode subunits of the vacuolar ATPase in N. crassa. A comparison of G subunits from N. crassa, S. cerevisiae, and animal cells showed that the N-terminal half of the polypeptide shows the highest degree of sequence conservation. Most striking is the observation that this region could form an alpha helix in which all of the conserved residues are clustered on one face. Subunit G appears to be homologous to the b subunit found in F-type ATPases. The major difference between the b and G subunits is the lack of a membrane-spanning region in the G subunit. We have also identified homologous subunits in the operons which encode V-type ATPases in a eubacterium, Enterrococcus hirae, and an archaebacterium, Methanococcus jannaschii. As in eukaryotic vacuolar ATPases the G subunit homologs lack a membrane-spanning region. Although the b and G subunits appear to be derived from a common ancestor, significant changes have evolved. In F-type and V-type ATPases these subunits can have zero, one, or two membrane-spanning regions and can also differ significantly in the number of copies per enzyme.

Mutations in the CYS4 gene provide evidence for regulation of the yeast vacuolar H+-ATPase by oxidation and reduction in vivo

The vma41-1 mutant was identified in a genetic screen designed to identify novel genes required for vacuolar H+-ATPase activity in Saccharomyces cerevisiae. The VMA41 gene was cloned and shown to be allelic to the CYS4 gene. The CYS4 gene encodes the first enzyme in cysteine biosynthesis, and in addition to cysteine auxotrophy, cys4 mutants have much lower levels of intracellular glutathione than wild-type cells. cys4 mutants display the pH-dependent growth phenotypes characteristic of vma mutants and are unable to accumulate quinacrine in the vacuole, indicating loss of vacuolar acidification in vivo. The vacuolar proton-translocating ATPases (V-ATPase) is synthesized at normal levels and assembled at the vacuolar membrane in cys4 mutants, but its specific activity is reduced (47% of wild type) and the activity is unstable. Addition of reduced glutathione to the growth medium complements the pH-dependent growth phenotype, partially restores vacuolar acidification, and restores wild type levels of ATPase activity. The CYS4 gene was deleted in a strain in which the catalytic site cysteine residue implicated in oxidative inhibition of the yeast V-ATPase has been mutagenized (Liu, Q., Leng, X.-H., Newman, P., Vasilyeva, E., Kane, P. M., and Forgac, M. (1997) J. Biol. Chem. 272, 11750-11756). This catalytic site point mutation suppresses the effects of the cys4 mutation. The data indicate that the acidification defect of cys4 mutants arises from inactivation of the vacuolar ATPase in the less reducing cytosol resulting from loss of Cys4p activity and provide the first evidence for the modulation of V-ATPase activity by the redox state of the environment in vivo.

Alpha- and beta-subunits of a V-type membrane ATPase in a hyperthermophilic sulfur-dependent archaeum, Thermococcus sp. KI

The genes encoding alpha- and beta-subunits of a V-type ATPase in a sulfur-dependent hyperthermophilic archaeum, Thermococcus sp. KI, were cloned and sequenced. The deduced amino acid sequences were approximately 60, 50 and 25% identical to those of other archaeal, eukaryotic V-type and E. coli F-type ATPase, respectively. Phylogenetic analysis revealed that Thermococcus ATPase was closely related to that of Thermus, and those of Methanosarcina and Halobacterium.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

Reconstitution in vitro of the V1 complex from the yeast vacuolar proton-translocating ATPase. Assembly recapitulates mechanism

Oligomeric assembly is a fundamental aspect of many complex enzymes. Using our native gel technique for examining subcomplexes of the V-ATPase V1 sector, we have developed an in vitro reconstitution assay for assembly of this complex. Assembly of complex II, the soluble V1 complex observed in native gels, is dependent upon the presence of divalent cations and physiological temperatures. Assembly of soluble V1 can occur in a stepwise fashion from smaller subcomplexes found in some strains deleted for V-ATPase subunits. Specifically, V1 can be assembled directly from complex III (subunits E and G) with complex IV (subunits A, B, D, and F) without prior disassembly of complex IV. The formation of complex III in vivo is also shown to be essential and could not be achieved in vitro. Assembly from simpler precursors is possible and is enhanced by added ATP. Assembly can be blocked by N-ethylmaleimide in a Vma1p (subunit A)-specific manner. From these data, we extend our previous model to consider an assembly pathway whose steps reflect the catalytic mechanism of the Boyer binding-change model.

The vacuolar H+-ATPase: a universal proton pump of eukaryotes

The vacuolar H+-ATPase (V-ATPase) is a universal component of eukaryotic organisms. It is present in the membranes of many organelles, where its proton-pumping action creates the low intra-vacuolar pH found, for example, in lysosomes. In addition, there are a number of differentiated cell types that have V-ATPases on their surface that contribute to the physiological functions of these cells. The V-ATPase is a multi-subunit enzyme composed of a membrane sector and a cytosolic catalytic sector. It is related to the familiar FoF1 ATP synthase (F-ATPase), having the same basic architectural construction, and many of the subunits from the two display identity with one another. All the core subunits of the V-ATPase have now been identified and much is known about the assembly, regulation and pharmacology of the enzyme. Recent genetic analysis has shown the V-ATPase to be a vital component of higher eukaryotes. At least one of the subunits, i.e. subunit c (ductin), may have multifunctional roles in membrane transport, providing a possible pathway of communication between cells. The structure of the membrane sector is known in some detail, and it is possible to begin to suggest how proton pumping is coupled to ATP hydrolysis.

Glutathione is involved in the granular storage of dopamine in rat PC 12 pheochromocytoma cells: implications for the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is characterized by degeneration of dopamine (DA)-containing nigro-striatal neurons. Loss of the antioxidant glutathione (GSH) has been implicated in the pathogenesis of PD. Previously, we showed that the oxidant hydrogen peroxide inhibits vesicular uptake of DA in nigro-striatal neurons. Hydrogen peroxide is scavenged by GSH and, therefore, we investigated a possible link between the process of vesicular storage of DA and GSH metabolism. For this purpose, we used rat pheochromocytoma-derived PC12 cells, a model system applied extensively for studying monoamine storage mechanisms. We show that depletion of endogenous DA stores with reserpine was accompanied in PC12 cells by a long-lasting, significant increase in GSH content the extent of which appeared to be inversely related to the rate of GSH synthesis. A similar increase in GSH content was observed after depletion of DA stores with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine. In the presence of alpha-methyl-p-tyrosine, refilling of the DA stores by exogenous DA reduced GSH content back to control level. Lowering of PC12 GSH content, via blockade of its synthesis with buthionine sulfoximine, however, led to a significantly decreased accumulation of exogenous [3H]DA without affecting uptake of the acetylcholine precursor [14C]choline. These data suggest that GSH is involved in the granular storage of DA in PC12 cells and that, considering the molecular characteristics of the granular transport system, it is likely that GSH is used to protect susceptible parts of this system against (possibly DA-induced) oxidative damage.

Characterization of a vacuolar protease in Neurospora crassa and the use of gene RIPing to generate protease-deficient strains

We have isolated a gene from Neurospora crassa that appears to encode a pepstatin-sensitive protease found both in membranes and in soluble contents of vacuoles. The gene contains two introns and encodes a 396-residue protein with a molecular mass of 42,900 Da. Because of the similarity of the protein to proteinase A in Saccharomyces cerevisiae the gene has been named pep-4. Strains with mutations in the pep-4 gene were generated in vivo by the gene RIPing procedure described by Selker and Garrett (Selker, E. U., and Garrett, P. W. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 6870-6874). The mutant strains were deficient in pepstatin-sensitive protease activity and did not appear to produce a major 42-kDa polypeptide in the vacuole. The mutant strains grew at the same rate as the wild type and had no other observable phenotype. When compared with inactivation of the PEP4 gene of S. cerevisiae, inactivation of the pep-4 gene in N. crassa produced a phenotype that was different in several ways. In N. crassa the mutant strains did not exhibit reduced sporulation or reduced viability after nitrogen starvation, and they had elevated levels of proteinase B and carboxypeptidase activities. The pep-4 gene appears to encode the N. crassa, homolog of proteinase A, but the maturation of vacuolar hydrolases appeared to be less dependent on this protease than has been observed in S. cerevisiae.

Mutational analysis of the catalytic subunit of the yeast vacuolar proton-translocating ATPase

In order to generate a set of tools for probing structure-function relationships in the catalytic subunit of the yeast vacuolar H(+)-ATPase, the gene encoding this subunit (VMA1) was randomly mutagenized. Mutant plasmids unable to complement the growth defects of yeast cells lacking an intact VMA1 gene were isolated and sequenced. Eight different mutant alleles of VMA1 were examined for levels of the catalytic subunit and other subunits of the enzyme, assembly of the ATPase complex, targeting to the vacuolar membrane, and concanamycin A-sensitive ATPase activity. The mutations S811P and E740D resulted in mutant enzymes that assembled fully but were incapable of ATP hydrolysis, and the mutation E785G generated a similar but somewhat less severe phenotype (17% of the ATPase activity of wild-type vacuoles). When MgATP-dependent stripping of the peripheral subunits by 100 mM KNO3 was examined in these three mutants, only the E785G mutant exhibited significant stripping, suggesting that ATP hydrolysis, even at relatively low levels, generates a conformation susceptible to dissociation. Plasmids containing the mutations E751G and F752S partially complemented the growth defects and resulted in partial defects in ATPase activity that appear to reflect reduced catalytic efficiency. Partial defects in growth and ATPase activity were also seen in the Y797H mutant, but this mutation caused an assembly defect manifested as a preferential loss of two of the peripheral subunits of the enzyme. The phenotypes of these mutants are interpreted in the context of homologies with other V-type and F-type ATPases.