Kinetic analysis of proton transport by the vanadate-sensitive ATPase from maize root microsomes

Time-Resolved Investigation of Molecular Components Involved in the Induction of [Formula: see text] High Affinity Transport System in Maize Roots

The induction, i.e., the rapid increase of nitrate ([Formula: see text]) uptake following the exposure of roots to the anion, was studied integrating physiological and molecular levels in maize roots. Responses to [Formula: see text] treatment were characterized in terms of changes in [Formula: see text] uptake rate and plasma membrane (PM) H+-ATPase activity and related to transcriptional and protein profiles of NRT2, NRT3, and PM H+-ATPase gene families. The behavior of transcripts and proteins of ZmNRT2s and ZmNRT3s suggested that the regulation of the activity of inducible high-affinity transport system (iHATS) is mainly based on the transcriptional/translational modulation of the accessory protein ZmNRT3.1A. Furthermore, ZmNRT2.1 and ZmNRT3.1A appear to be associated in a ∼150 kDa oligomer. The expression trend during the induction of the 11 identified PM H+-ATPase transcripts indicates that those mainly involved in the response to [Formula: see text] treatment are ZmHA2 and ZmHA4. Yet, partial correlation between the gene expression, protein levels and enzyme activity suggests an involvement of post-transcriptional and post-translational mechanisms of regulation. A non-denaturing Deriphat-PAGE approach allowed demonstrating for the first time that PM H+-ATPase can occur in vivo as hexameric complex together with the already described monomeric and dimeric forms.

A thermodynamic analysis of the feasibility of water secretion into xylem vessels against a water potential gradient

A series of recent publications has launched a debate on trans-membrane water secretion into root xylem vessels against a water potential gradient, energised by a cotransport with salts (e.g. KCl) that follow their chemical potential gradient. Cation-chloride-cotransporter -type transporters that function in this way in mammalian epithelia were detected in root stelar cells bordering on xylem vessels. Using literature data on barley (Hordeum vulgare L.) seedlings, one study confirmed that K+ and Cl- gradients across stelar cell membranes favour salt efflux. Moreover, the energetic costs of putative water secretion into the xylem (required for maintaining ionic gradients) would amount to just 0.12% of the energy captured by photosynthetic C assimilation if transpirational water flow relied exclusively on this mechanism. Here, a detailed thermodynamic analysis of water secretion into xylem vessels is undertaken, including an approach that exploits its analogy to a desalinisation process. Water backflow due to the passive hydraulic conductivity of stelar cell membranes is also considered. By comparing free energy consumption by putative water secretion with (i) the free energy pool provided by root respiration and (ii) stelar ATPase activity, the feasibility of this mechanism is confirmed but is shown to depend critically on the plant's energy status.

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

Photosystem I (PSI) complexes can support a light-driven electrochemical gradient for protons, which is the driving force for energy-conserving reactions across biological membranes. In this work, a computational model that enables a quantitative description of the light-induced proton gradients across the membrane of PSI proteoliposomes is presented. Using a set of electrodiffusion equations, a compartmental model of a vesicle suspended in aqueous medium was studied. The light-mediated proton movement was modeled as a single proton pumping step with backpressure of the electric potential. The model fits determinations of pH obtained from PSI proteoliposomes illuminated in the presence of mediators of cyclic electron transport. The model also allows analysis of the proton gradients in relation to the transmembrane ion fluxes and electric potential. Sensitivity analysis enabled a determination of the parameters that have greater influence on steady-state levels and onset/decay rates of transmembrane pH and electric potential. This model could be used as a tool for optimizing PSI proteoliposomes for photo-electrochemical applications.

Improved mathematical model for estimating H+ influx and H+ efflux in plant vacuolar vesicles acidified by ATPase or pyrophosphatase

To adapt to environmental changes, plant cells very likely possess a biochemical system, using vacuoles, for maintaining cytoplasmic pH homeostasis. A simple approach is to estimate the active H(+) influx and H(+) efflux of isolated vacuolar vesicles, although there is no good mathematical model to describe H(+) flux. To establish a new quantitative model, vacuolar vesicles were isolated from hypocotyls of mung bean (Vigna radiata L.), and pyrophosphate (PPi)- or ATP-dependent acidification was monitored using acridine orange. The change of pH inside the vesicles (pH(in)) was calculated using a pH calibration curve relating fluorescence quenching with DeltapH. After formation of a steady state DeltapH, passive H(+) efflux was monitored after terminating pumping with ethylenediaminetetraacetate, and the relative H(+) permeability coefficient (p(H+)) was calculated. The H(+) efflux simulated using the p(H+) corresponded to the H(+) efflux determined experimentally. H(+) influx was then calculated by subtracting the predicted H(+) efflux from the experimental net H(+) influx. H(+) influx into vesicles driven by H(+)-PPase or H(+)-ATPase decreased exponentially as the intravesicular pH(in) decreased, suggesting modulation of pumping by DeltapH, pH(in), or both. Finally, the PPi- or ATP-dependent H(+) accumulation determined experimentally was closely simulated by the predicted H(+) influx and H(+) efflux. The ability to predict H(+) flux under different conditions provides a powerful tool for studying pH homeostasis.

The plant plasma membrane H(+)-ATPase: structure, function and regulation

The proton-pumping ATPase (H(+)-ATPase) of the plant plasma membrane generates the proton motive force across the plasma membrane that is necessary to activate most of the ion and metabolite transport. In recent years, important progress has been made concerning the identification and organization of H(+)-ATPase genes, their expression, and also the kinetics and regulation of individual H(+)-ATPase isoforms. At the gene level, it is now clear that H(+)-ATPase is encoded by a family of approximately 10 genes. Expression, monitored by in situ techniques, has revealed a specific distribution pattern for each gene; however, this seems to differ between species. In the near future, we can expect regulatory aspects of gene expression to be elucidated. Already the expression of individual plant H(+)-ATPases in yeast has shown them to have distinct enzymatic properties. It has also allowed regulatory aspects of this enzyme to be studied through random and site-directed mutagenesis, notably its carboxy-terminal region. Studies performed with both plant and yeast material have converged towards deciphering the way phosphorylation and binding of regulatory 14-3-3 proteins intervene in the modification of H(+)-ATPase activity. The production of high quantities of individual functional H(+)-ATPases in yeast constitutes an important step towards crystallization studies to derive structural information. Understanding the specific roles of H(+)-ATPase isoforms in whole plant physiology is another challenge that has been approached recently through the phenotypic analysis of the first transgenic plants in which the expression of single H(+)-ATPases has been up- or down-regulated. In conclusion, the progress made recently concerning the H(+)-ATPase family, at both the gene and protein level, has come to a point where we can now expect a more integrated investigation of the expression, function and regulation of individual H(+)-ATPases in the whole plant context.

Role of the Plasma Membrane H+-ATPase in K+ Transport

The role of the plant plasma membrane H+-ATPase in K+ uptake was examined using red beet (Beta vulgaris L.) plasma membrane vesicles and a partially purified preparation of the red beet plasma membrane H+-ATPase reconstituted in proteoliposomes and planar bilayers. For plasma membrane vesicles, ATP-dependent K+ efflux was only partially inhibited by 100 [mu]M vanadate or 10 [mu]M carbonyl cyanide-p-trifluoromethoxyphenylhydrazone. However, full inhibition of ATP-dependent K+ efflux by these reagents occurred when the red beet plasma membrane H+-ATPase was partially purified and reconstituted in proteoliposomes. When reconstituted in a planar bilayer membrane, the current/voltage relationship for the plasma membrane H+-ATPase showed little effect of K+ gradients imposed across the bilayer membrane. When taken together, the results of this study demonstrate that the plant plasma membrane H+-ATPase does not mediate direct K+ transport chemically linked to ATP hydrolysis. Rather, this enzyme provides a driving force for cellular K+ uptake by secondary mechanisms, such as K+ channels or H+/K+ symporters. Although the presence of a small, protonophore-insensitive component of ATP-dependent K+ transport in a plasma membrane fraction might be mediated by an ATP-activated K+ channel, the possibility of direct K+ transport by other ATPases (i.e. K+-ATPases) associated with either the plasma membrane or other cellular membranes cannot be ruled out.

Metabolic modulation of transport coupling ratio in yeast plasma membrane H(+)-ATPase

The plasma membrane proton pump (H(+)-ATPase) of yeast energizes solute uptake by secondary transporters and regulates cytoplasmic pH. The addition of glucose to yeast cells stimulates proton efflux mediated by the H(+)- ATPase. A > 50-fold increase in proton extrusion from yeast cells is observed in vivo, whereas the ATPase activity of purified plasma membranes is increased maximally 8-fold after glucose treatment (Serrano, R. (1983) FEBS Lett. 156, 11-14). The low capacity of yeast cells for proton extrusion in the absence of glucose can be explained by the finding that, in H(+)-ATPase isolated from glucose-starved cells, ATP hydrolysis is essentially uncoupled from proton pumping. The number of protons transported per ATP hydrolyzed is significantly increased after glucose activation. We suggest that intrinsic uncoupling is an important mechanism for regulation of pump activity.

The fungal H(+)-ATPase from Neurospora crassa reconstituted with fusicoccin receptors senses fusicoccin signal

Fusicoccin affects several physiological processes regulated by the plasma membrane H(+)-ATPase in higher plants while other organisms having P-type H(+)-ATPases (e.g., fungi) are fusicoccin-insensitive. We have previously shown that fusicoccin binding to its receptor is necessary for H(+)-ATPase stimulation and have achieved the functional reconstitution into liposomes of fusicoccin receptors and the H(+)-ATPase from maize. In this paper we show that fusicoccin sensitivity can be conferred on the H(+)-ATPase from Neurospora crassa, a fungus insensitive to fusicoccin. In fact, H+ pumping by purified H(+)-ATPase from Neurospora crassa reconstituted into liposomes containing crude or partially purified fusicoccin receptors from maize was markedly enhanced by fusicoccin. The stimulation of H+ pumping by fusicoccin is dependent upon pH, fusicoccin, and protein concentration, as was reported for the system reconstituted with both proteins from maize.

Effects of solubilization on the inhibition of the p-type ATPase from maize roots by N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline

The biochemical events utilized by transport proteins to convert the chemical energy from the hydrolysis of ATP into an electro-chemical gradient are poorly understood. The inhibition of the plasma membrane ATPase from corn (Zea mays L.) roots by N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ) was compared to that of ATPase solubilized with N-tetradecyl-N,N-dimethyl-3-ammonio-1-propane-sulfonate (3-14) to provide insight into the minimal functional unit. The chromatographic behavior of the 3-14-solubilized ATPase activity during size exclusion chromatography and glycerol gradient centrifugation indicated that the solubilized enzyme was in a monomeric form. Both plasma membrane-bound and solubilized ATPase were inhibited by EEDQ in a time- and concentration-dependent manner consistent with a first-order reaction. When the log of the reciprocal of the half-time for inhibition was plotted as a function of the log of the EEDQ concentration, straight lines were obtained with slopes of approximately 0.5 and 1.0 for membrane-bound and 3-14-solubilized ATPase, respectively, indicating a change in the number of polypeptides per functional ATPase complex induced by solubilization with 3-14.

Effects of temperature on the coupled activities of the vanadate-sensitive proton pump from maize root microsomes

The mechanism by which proton transport is coupled to ATP hydrolysis by vanadate-sensitive pumps is poorly understood. The effects of temperature on the activities of the vanadate-sensitive ATPase from maize (Zea mays) roots were assessed to provide insight into the coupling mechanism. The initial rate of proton transport had a bell-shaped dependence on temperature with an optimal range between 20 and 30 degrees C. However, the rate of vanadate-sensitive ATP hydrolysis increased as the temperature was raised from 4 to 43 degrees C. The differential sensitivity of proton transport to temperatures above 30 degrees C was also observed when the ATPase was reconstituted into dioleoylphosphatidylcholine vesicles. Inhibition of proton transport with temperatures above 30 degrees C was associated with higher rates of proton leakage from the membranes. In addition, proton transport was more inhibited than ATP hydrolysis at temperatures below 10 degrees C. Reduced rates of proton transport at lower temperatures were not associated with higher rate of proton conductivity across the membranes. Therefore, the preferential inhibition of proton transport at temperatures below 10 degrees C may reflect an effect of temperature on the coupling between proton transport and ATP hydrolysis within the vanadate-sensitive ATPase.

N-Cyclo-N'-(4-Dimethylamino-alpha-Naphthyl)Carbodiimide Inhibits Membrane-Bound and Partially Purified Tonoplast ATPase from Maize Roots

Certain carboxylic acid groups within the primary structure of proton translocating proteins are thought to be involved in the proton pathway. In this report, the effects of a lipophilic carboxylic acid reactive reagent, N-cyclo-N'(4-dimethylamino-alpha-naphthyl)carbodiimide (NCD-4), on the two types of proton pumps in maize (Zea mays L.) root microsomes were investigated. NCD-4 was found to inhibit the vacuolar-type H(+)-ATPase in microsomal preparations; however, the plasma membrane-type H(+)-ATPase was unaffected. The H(+)-ATPase in highly purified tonoplast vesicles was also inhibited by NCD-4. Inhibition was dependent on the concentration and length of exposure to the reagent. However, there was little, if any, increase in the fluorescence of treated vesicles, indicating few carboxylic acid residues were reacting. Inhibition of the tonoplast H(+)-ATPase by NCD-4 was examined further with a partially purified preparation. The partially purified H(+)-ATPase also showed sensitivity to the NCD-4, supporting the hypothesis that this carboxylic acid reagent is an inhibitor of the tonoplast ATPase from maize roots.

Acridine orange as a probe for measuring pH gradients across membranes: mechanism and limitations

Acridine orange is an optical probe commonly used to monitor pH gradients across membranes. In the present study, the changes observed in the visible absorption spectrum of acridine orange during intravesicular acidification of oat root plasma membrane vesicles are shown to be identical with those obtained by increasing the free dye concentration, adding anions, or lowering the temperature, but different from those obtained on addition of biological membranes. It is therefore suggested that the absorbance changes observed during the formation of the pH gradient are simply due to accumulation of free dye inside the vesicles and subsequent dimerization, and not the result of dye-membrane interactions. The proportion of monomeric acridine orange that could undergo dimerization decreased with decreasing temperature. Furthermore, in a membrane-free system different anions induced the formation of dimer-excimer complexes to different degrees. During the formation of the pH gradient permeant anions present in the reaction medium follow the movement of protons into the vesicles, and the intravesicular accumulation of anions thereby amplifies acridine orange quenching, the degree of amplification being dependent on the anion species. Therefore, the use of acridine orange, and probably all metachromatic dyes, as probes for monitoring pH gradients is limited, since these probes neither reflect quantitatively the amount of H+ pumped nor the effect of anions and temperature on transmembrane H+ transport.

Determination of H/ATP Stoichiometry for the Plasma Membrane H-ATPase from Red Beet (Beta vulgaris L.) Storage Tissue

The H(+)/ATP stoichiometry was determined for the plasma membrane H(+)-ATPase from red beet (Beta vulgaris L., var Detroit Dark Red) storage tissue associated with native vesicles. The determination of H(+)/ATP stoichiometry utilized a kinetic approach where rates of H(+) influx, estimated by three different methods, were compared to rates of ATP hydrolysis measured by the coupled enzyme assay under identical conditions. These methods for estimating H(+) influx were based upon either determining the initial rate of alkalinization of the external medium from pH 6.13, measuring the rate of vesicle H(+) leakage from a steadystate pH gradient after stopping the H(+)-ATPase or utilizing a mathematical model which describes the net transport of H(+) at any given point in time. When the rate of H(+) influx estimated by each of these methods was compared to the rate of ATP hydrolysis, a H(+)/ATP stoichiometry of about 1 was observed. In consideration of the maximum free energy available from ATP hydrolysis (DeltaG(atp)), this value for H(+)/ATP stoichiometry is sufficient to account for the magnitude of the proton electrochemical gradient observed across the plasma membrane in vivo.

An H-ATPase Assay: Proton Pumping and ATPase Activity Determined Simultaneously in the Same Sample

A continuous spectrophotometric assay of H(+)-ATPase activity was developed by combining two well-known methods for measuring proton pumping and ATPase activity. Proton uptake into plasma membrane vesicles from Avena sativa L. (cv Rhiannon) was monitored as the absorbance decrease at 495 nm of the DeltapH probe acridine orange. Simultaneously, ATPase activity was measured by following the absorbance decrease at 340 nanometers by coupling ATP hydrolysis enzymatically to the oxidation of NADH. This H(+)-ATPase assay is convenient for determining the relative relationship between ATP hydrolysis and proton pumping.

Calcium and proton transport in membrane vesicles from barley roots

Ca(2+) uptake by membrane fractions from barley (Hordeum vulgare L. cv CM72) roots was characterized. Uptake of (45)Ca(2+) was measured in membrane vesicles obtained from continuous and discontinuous sucrose gradients. A single, large peak of Ca(2+) uptake coincided with the peak of proton transport by the tonoplast H(+)-ATPase. Depending on the concentration of Ca(2+) in the assay, Ca(2+) uptake was inhibited 50 to 75% by those combinations of ionophores and solutes that eliminated the pH gradient and membrane potential. However, 25 to 50% of the Ca(2+) uptake in the tonoplast-enriched fraction was not sensitive to ionophores but was inhibited by vanadate. The results suggest that (45)Ca uptake was driven by the low affinity, high capacity tonoplast Ca(2+)/nH(+) antiporter and also by a high affinity, lower capacity Ca(2+)-ATPase. The Ca(2+)-ATPase may be associated with tonoplast, Golgi or contaminating vesicles of unknown origin. No Ca(2+) transport was specifically associated with the distinct peak of endoplasmic reticulum that was identified by NADH cytochrome c reductase, choline phosphotransferase, and dolichol-P-man-nosyl synthase activities. A small shoulder of Ca(2+) uptake in the plasma membrane region of the gradient was inhibited by vanadate and erythrosin B and may represent the activity of a separate plasma membrane Ca(2+)-ATPase. Vesicle volumes were estimated using electron spin resonance techniques, and intravesicular Ca(2+) concentrations were estimated to be as high as 5 millimolar. ATP-driven uptake of Ca(2+) created 800- to 2000-fold concentration gradients within minutes. Problems in interpreting the effects of Ca(2+) on ATP-generated pH gradients are discussed and the suggestion is made that Ca(2+) dissipates pH gradients by a different mechanism than is responsible for Ca(2+) uptake into tonoplast vesicles.

Differential Inhibition of Tonoplast H-ATPase Activities by Fluorescamine and Its Derivatives

Corn (Zea mays L.) root tonoplast vesicles were treated with the primary-amine specific reagent, fluorescamine (FL). Modification by FL caused a differential inhibition to the coupled activities of tonoplast H(+)-ATPase. Within the range of 0 to 5 micromoles of FL per milligram of protein, the proton pumping rate was significantly reduced but ATP hydrolysis was only slightly affected. Yet, the membrane H(+) leakage during the pumping stage increased only slightly. FL treatment resulted in (a) a decrease in amine containing phospholipids and (b) an insertion of multiple H-bonding moieties into the membrane. To test which of these two possible effects were responsible for inhibition, FL derivatives of benzylamine, butylamine, and phenylalanine were synthesized. It was found that the acyclic derivatives with high H-bonding potential at concentrations of 10 micromolar inhibited proton pumping by 50% without a significant effect on ATP hydrolysis. Cyclic derivatives were largely ineffectual. Proton leakage during pumping was not affected by these acyclic modifiers. Membrane fluidity, as measured by the polarization of diphenyl hexatriene, decreased upon treatment with either FL or its derivatives. The results suggest that the proton pumping is indirectly linked to ATP hydrolysis in the tonoplast vesicles, and the link between these processes is apparently weakened by the presence of acyclic fluorescamine derivatives in the membrane.

Effect of temperature on the plasma membrane and tonoplast ATPases of barley roots : comparison of results obtained with acridine orange and quinacrine

The effect of temperature on the rate of proton transport and ATP hydrolysis by plasma membrane (PM) and tonoplast (TN) ATPases from barley (Hordeum vulgare L. cv CM 72) roots were compared. Rates of proton transport were estimated using the fluorescent amine dyes quinacrine and acridine orange. The ratio between rate of transport and ATP hydrolysis was found to depend on the dye, the temperature, and the type of membrane. The PM ATPase had an estimated Arrhenius energy of activation (Ea) of approximately 18 kilocalories per mole for ATP hydrolysis, and the Ea for proton transport was best estimated with acridine orange, which gave an Ea of 19 kilocalories per mole. The TN ATPase had an Ea for ATP hydrolysis of approximately 10 kilocalories per mole and the Ea for proton transport was best estimated with quinacrine, which gave an Ea of 10 kilocalories per mole. Acridine orange did not give an accurate estimate of Ea for the TN ATPase, nor did quinacrine for the PM ATPase. Reasons for the differences are discussed. Because it was suggested (AJ Pope, RA Leigh [1988] Plant Physiol 86: 1315-1322) that acridine orange interacts with anions to dissipate the pH gradient in TN vesicles, the complex effects of NO(3) (-) on the TN ATPase were also examined using acridine orange and quinacrine and membranes from oats and barley. Fluorescent amine dyes can be used to evaluate the effects of ions, substrates, inhibitors, and temperature on transport but caution is required in using rates of quench to make quantitative estimates of proton fluxes.

Phospholipid requirement of the vanadate-sensitive ATPase from maize roots evaluated by two methods

The activation of the vanadate-sensitive ATPase from maize (Zea mays L.) root microsomes by phospholipids was assessed by two different methods. First, the vanadate-sensitive ATPase was partially purified and substantially delipidated by treating microsomes with 0.6% deoxycholate (DOC) at a protein concentration of 1 milligram per milliliter. Vanadate-sensitive ATP hydrolysis by the DOC-extracted microsomes was stimulated up to 100% by the addition of asolectin. Of the individual phospholipids tested, phosphatidylserine and phosphatidylglycerol stimulated activity as much as asolectin, whereas phosphatidylcholine did not. Second, phospholipid dependence of the ATPase was also assessed by reconstituting the enzyme into proteoliposomes of differing phospholipid composition. In these experiments, the rate of proton transport and ATP hydrolysis was only slightly affected by phospholipid composition. DOC-extracted microsomes reconstituted with dioleoylphosphatidylcholine had rates of proton transport similar to those found with microsomes reconstituted with asolectin. The difference between the two types of assays is discussed in terms of factors contributing to the interaction between proteins and lipids.