Functional comparisons between plant plasma membrane H(+)-ATPase isoforms expressed in yeast

The mechanism behind tenuazonic acid-mediated inhibition of plant plasma membrane H+-ATPase and plant growth

The increasing prevalence of herbicide-resistant weeds has led to a search for new herbicides that target plant growth processes differing from those targeted by current herbicides. In recent years, some studies have explored the use of natural compounds from microorganisms as potential new herbicides. We previously demonstrated that tenuazonic acid (TeA) from the phytopathogenic fungus Stemphylium loti inhibits the plant plasma membrane (PM) H+-ATPase, representing a new target for herbicides. In this study, we further investigated the mechanism by which TeA inhibits PM H+-ATPase and the effect of the toxin on plant growth using Arabidopsis thaliana. We also studied the biochemical effects of TeA on the PM H+-ATPases from spinach (Spinacia oleracea) and A. thaliana (AHA2) by examining PM H+-ATPase activity under different conditions and in different mutants. Treatment with 200 μM TeA-induced cell necrosis in larger plants and treatment with 10 μM TeA almost completely inhibited cell elongation and root growth in seedlings. We show that the isoleucine backbone of TeA is essential for inhibiting the ATPase activity of the PM H+-ATPase. Additionally, this inhibition depends on the C-terminal domain of AHA2, and TeA binding to PM H+-ATPase requires the Regulatory Region I of the C-terminal domain in AHA2. TeA likely has a higher binding affinity toward PM H+-ATPase than the phytotoxin fusicoccin. Finally, our findings show that TeA retains the H+-ATPase in an inhibited state, suggesting that it could act as a lead compound for creating new herbicides targeting the PM H+-ATPase.

Identification of Aly1 and Aly2 as Modulators of Cytoplasmic pH in Saccharomyces cerevisiae

The regulation of intracellular pH in yeast (Saccharomyces cerevisiae) cells is critical for cell function and viability. In yeast, protons (H+) can be excreted from the cell by plasma membrane ATPase PMA1 and pumped into vacuoles by vacuolar H+-ATPase. Because PMA1 is critical to the survival of yeast cells, it is unknown whether other compensatory components are involved in pH homeostasis in the absence of PMA1. To elucidate how intracellular pH is regulated independently of PMA1, we employed a screening approach by exposing the yeast haploid deletion mutant library (ver 4.0) to the selective plant plasma membrane H+-ATPase inhibitor PS-1, which we previously reported. After repeated screenings and verification, we identified two proteins, Aly1 and Aly2, that play a role in the regulation of intracellular pH when PMA1 is deficient. Our research uncovers a new perspective on the regulation of intracellular pH related to PMA1 and also preliminarily reveals a role for Aly1 and Aly2 in the regulation of intracellular pH.

Yeast Heterologous Expression Systems for the Study of Plant Membrane Proteins

Researchers are often interested in proteins that are present in cells in small ratios compared to the total amount of proteins. These proteins include transcription factors, hormones and specific membrane proteins. However, sufficient amounts of well-purified protein preparations are required for functional and structural studies of these proteins, including the creation of artificial proteoliposomes and the growth of protein 2D and 3D crystals. This aim can be achieved by the expression of the target protein in a heterologous system. This review describes the applications of yeast heterologous expression systems in studies of plant membrane proteins. An initial brief description introduces the widely used heterologous expression systems of the baker's yeast Saccharomyces cerevisiae and the methylotrophic yeast Pichia pastoris. S. cerevisiae is further considered a convenient model system for functional studies of heterologously expressed proteins, while P. pastoris has the advantage of using these yeast cells as factories for producing large quantities of proteins of interest. The application of both expression systems is described for functional and structural studies of membrane proteins from plants, namely, K⁺- and Na⁺-transporters, various ATPases and anion transporters, and other transport proteins.

Silicon-mediated growth promotion in maize (Zea mays L.) occurs via a mechanism that does not involve activation of the plasma membrane H+-ATPase

Silicon (Si)-mediated growth promotion of various grasses is well documented. In the present study, Si-induced changes in maize shoot growth and its underlying mechanisms were studied. Maize plants were grown with various concentrations of Si (0-3 mM) in the nutrient solution. Silicon nutrition improved plant expansion growth. Silicon-supplied maize plants (0.8 and 1.2 mM) showed higher plant height and leaf area compared to no-Si amended plants. It was assumed that Si-induced expansion growth was due to positive Si effects on plasma membrane (PM) H⁺-ATPase. In this context, western blot analysis revealed an increase in PM H⁺-ATPase abundance by 77% under Si nutrition. However, in vitro measurements of enzyme activities showed no significant effect on apoplast pH, proton pumping, passive H⁺ efflux and enzyme kinetics such as K_m, V_max, and activation energy. Further, these results were confirmed by in vivo ratiometric analysis of apoplastic pH, which showed non-significant changes upon Si supply. In contrast, 1 mM Si altered the relative transcripts of specific PM H⁺-ATPase isoforms. Silicon application resulted in a significant decrease of MHA3, and this decrease in transcription seems to be compensated by an increased concentration of H⁺-ATPase protein. From these results, it can be concluded that changes in cell wall composition and PM H⁺-ATPase may be responsible for Si-mediated growth improvement in maize.

MPK6 Kinase Regulates Plasma Membrane H+-ATPase Activity in Cold Acclimation

Cold and freezing stresses severely affect plant growth, development, and survival rate. Some plant species have evolved a process known as cold acclimation, in which plants exposed to temperatures above 0 °C trigger biochemical and physiological changes to survive freezing. During this response, several signaling events are mediated by transducers, such as mitogen activated protein kinase (MAPK) cascades. Plasma membrane H+-ATPase is a key enzyme for the plant cell life under regular and stress conditions. Using wild type and mpk3 and mpk6 knock out mutants in Arabidopsis thaliana, we explored the transcriptional, translational, and 14-3-3 protein regulation of the plasma membrane H+-ATPase activity under the acclimation process. The kinetic analysis revealed a differential profiling of the H+-ATPase activity depending on the presence or absence of MPK3 or MPK6 under non-acclimated or acclimated conditions. Negative regulation of the plasma membrane H+-ATPase activity was found to be exerted by MPK3 in non-acclimated conditions and by MPK6 in acclimated conditions, describing a novel form of regulation of this master ATPase. The MPK6 regulation involved changes in plasma membrane fluidity. Moreover, our results indicated that MPK6 is a critical regulator in the process of cold acclimation that leads to freezing tolerance and further survival.

Saccharomyces cerevisiae as a Tool to Investigate Plant Potassium and Sodium Transporters

Sodium and potassium are two alkali cations abundant in the biosphere. Potassium is essential for plants and its concentration must be maintained at approximately 150 mM in the plant cell cytoplasm including under circumstances where its concentration is much lower in soil. On the other hand, sodium must be extruded from the plant or accumulated either in the vacuole or in specific plant structures. Maintaining a high intracellular K+/Na+ ratio under adverse environmental conditions or in the presence of salt is essential to maintain cellular homeostasis and to avoid toxicity. The baker's yeast, Saccharomyces cerevisiae, has been used to identify and characterize participants in potassium and sodium homeostasis in plants for many years. Its utility resides in the fact that the electric gradient across the membrane and the vacuoles is similar to plants. Most plant proteins can be expressed in yeast and are functional in this unicellular model system, which allows for productive structure-function studies for ion transporting proteins. Moreover, yeast can also be used as a high-throughput platform for the identification of genes that confer stress tolerance and for the study of protein-protein interactions. In this review, we summarize advances regarding potassium and sodium transport that have been discovered using the yeast model system, the state-of-the-art of the available techniques and the future directions and opportunities in this field.

The yeast H+-ATPase Pma1 promotes Rag/Gtr-dependent TORC1 activation in response to H+-coupled nutrient uptake

The yeast Target of Rapamycin Complex 1 (TORC1) plays a central role in controlling growth. How amino acids and other nutrients stimulate its activity via the Rag/Gtr GTPases remains poorly understood. We here report that the signal triggering Rag/Gtr-dependent TORC1 activation upon amino-acid uptake is the coupled H⁺ influx catalyzed by amino-acid/H⁺ symporters. H⁺-dependent uptake of other nutrients, ionophore-mediated H⁺ diffusion, and inhibition of the vacuolar V-ATPase also activate TORC1. As the increase in cytosolic H⁺ elicited by these processes stimulates the compensating H⁺-export activity of the plasma membrane H⁺-ATPase (Pma1), we have examined whether this major ATP-consuming enzyme might be involved in TORC1 control. We find that when the endogenous Pma1 is replaced with a plant H⁺-ATPase, H⁺ influx or increase fails to activate TORC1. Our results show that H⁺ influx coupled to nutrient uptake stimulates TORC1 activity and that Pma1 is a key actor in this mechanism.

Cells adapt their growth rate depending on the amount of nutrients available. The protein complex called TORC1 plays a central role in this. When nutrients are abundant, TORC1 is very active and stimulates the production of proteins and other molecules needed for the cell to grow. However, when nutrients such as amino acids become scarce, TORC1 reduces its activity and allows the cells to adapt to starvation. This TORC1-mediated control of the metabolism is crucial for the cell to survive, and faulty TORC1 proteins have been associated with several diseases including cancers.

TORC1 was originally discovered in yeast, which provides a powerful model to study this control system. However, until now, it was not known how TORC1 is reactivated when amino acids are added to cells that have been starved of these molecules. Knowing the answer to this question would allow us to better understand how the availability of nutrients controls the activity of TORC1.

Now, Saliba et al. have discovered that TORC1 is not reactivated by the amino acids themselves, but by protons, which are positively charged hydrogen ions that travel into the cell together with the amino acids. This influx of protons is the driving force behind the active transport of amino acids and other nutrients into the cell, and potentially serves as a general signal to activate TORC1 in response to the uptake of nutrients, especially when cells have been starved.

Furthermore, the results showed that a specific enzyme in the cell membrane plays an essential role in activating TORC1. This enzyme pumps the protons out of the cell to compensate for their influx and to maintain the proton gradient in the membrane that drives the absorption of nutrients. When this enzyme was replaced with an equivalent plant enzyme, the proton-coupled nutrient uptake did not activate TORC1 in the yeast cells.

These findings may help scientists who are interested in how TORC1 is regulated in organisms other than mammals, such as plants or fungi. A next step will be to find out how exactly the proton pump in the cell membrane helps to activate TORC1.

Type IV P-type ATPases distinguish mono- versus diacyl phosphatidylserine using a cytofacial exit gate in the membrane domain

Type IV P-type ATPases (P4-ATPases) use the energy from ATP to "flip" phospholipid across a lipid bilayer, facilitating membrane trafficking events and maintaining the characteristic plasma membrane phospholipid asymmetry. Preferred translocation substrates for the budding yeast P4-ATPases Dnf1 and Dnf2 include lysophosphatidylcholine, lysophosphatidylethanolamine, derivatives of phosphatidylcholine and phosphatidylethanolamine containing a 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) group on the sn-2 C6 position, and were presumed to include phosphatidylcholine and phosphatidylethanolamine species with two intact acyl chains. We previously identified several mutations in Dnf1 transmembrane (TM) segments 1 through 4 that greatly enhance recognition and transport of NBD phosphatidylserine (NBD-PS). Here we show that most of these Dnf1 mutants cannot flip diacylated PS to the cytosolic leaflet to establish PS asymmetry. However, mutation of a highly conserved asparagine (Asn-550) in TM3 allowed Dnf1 to restore plasma membrane PS asymmetry in a strain deficient for the P4-ATPase Drs2, the primary PS flippase. Moreover, Dnf1 N550 mutants could replace the Drs2 requirement for growth at low temperature. A screen for additional Dnf1 mutants capable of replacing Drs2 function identified substitutions of TM1 and 2 residues, within a region called the exit gate, that permit recognition of dually acylated PS. These TM1, 2, and 3 residues coordinate with the "proline + 4" residue within TM4 to determine substrate preference at the exit gate. Moreover, residues from Atp8a1, a mammalian ortholog of Drs2, in these positions allow PS recognition by Dnf1. These studies indicate that Dnf1 poorly recognizes diacylated phospholipid and define key substitutions enabling recognition of endogenous PS.

Poplar maintains zinc homeostasis with heavy metal genes HMA4 and PCS1

Perennial woody species, such as poplar (Populus spp.) must acquire necessary heavy metals like zinc (Zn) while avoiding potential toxicity. Poplar contains genes with sequence homology to genes HMA4 and PCS1 from other species which are involved in heavy metal regulation. While basic genomic conservation exists, poplar does not have a hyperaccumulating phenotype. Poplar has a common indicator phenotype in which heavy metal accumulation is proportional to environmental concentrations but excesses are prevented. Phenotype is partly affected by regulation of HMA4 and PCS1 transcriptional abundance. Wild-type poplar down-regulates several transcripts in its Zn-interacting pathway at high Zn levels. Also, overexpressed PtHMA4 and PtPCS1 genes result in varying Zn phenotypes in poplar; specifically, there is a doubling of Zn accumulation in leaf tissues in an overexpressed PtPCS1 line. The genomic complement and regulation of poplar highlighted in this study supports a role of HMA4 and PCS1 in Zn regulation dictating its phenotype. These genes can be altered in poplar to change its interaction with Zn. However, other poplar genes in the surrounding pathway may maintain the phenotype by inhibiting drastic changes in heavy metal accumulation with a single gene transformation.

Kinetics of the H+-ATPase from dry and 5-hours-imbibed maize embryos in its native, solubilized, and reconstituted forms

Membranes undergo recovery upon rehydration in seed germination. Previous work has described that the plasma membrane H+-ATPase from maize embryos adopts two different forms at 0 and 5 h of imbibition. We investigated how the kinetics of these two forms could be affected by alterations in the plasma membrane (PM). In comparison to the 0-h, PMs from the 5-h imbibed embryos showed changes in glycerophospholipid composition, decrease in leakage, and increase in fluidity. Kinetics of the PM H+-ATPase from 0 and 5-h imbibed embryos showed negative cooperativity. With the removal of the membrane environment, the activity of the enzymes shifted to a more complex kinetics, displaying two enzyme components. Lipid reconstitution produced one component with positive cooperativity. In all cases, enzymes from 0 and 5-h imbibed embryos presented similar kinetics with some quantitative differences. These results indicate that the two enzyme forms have the potential ability to respond to changes in the membrane environment, but the fact that they do not show differences in the native membranes at 0 or 5 h implies that modifications in the membrane are not drastic enough to alter their kinetics, or that they are able to preserve their boundary lipids or associated proteins and thus retain the same kinetic behavior.

Involvement of plasma membrane H+-ATPase in anoxic elongation of stems in pondweed (Potamogeton distinctus) turions

• Pondweed (Potamogeton distinctus) turions can elongate in the absence of O(2). Alcoholic fermentation serves to produce energy for anoxic elongation via the breakdown of starch stored in cells. However, the mechanism of cell growth during anoxic elongation is not fully understood. • Changes in pH, H(+) equivalent and lactate content of the incubation medium were measured during anoxic elongation. The effects of fusicoccin (FC), indole-3-acetic acid (IAA), vanadate, erythrosine B and K(+) channel blockers on anoxic elongation were examined. Cytoplasmic pH and vacuolar pH were measured by (31)P nuclear magnetic resonance (NMR) spectroscopy. • Acidification of the incubation medium occurred during anoxic elongation. The contribution of CO(2) and lactic acid was not sufficient to explain the acidification. FC and IAA enhanced the elongation of stem segments. Vanadate and erythrosine B inhibited anoxic elongation. Acid growth of notched segments was observed. The activity of plasma membrane H(+)-ATPase extracted from pondweed turions was increased slightly in anoxic conditions, but that from pea epicotyls sensitive to anoxic conditions was decreased by incubation in anoxic conditions. Both the cytoplasmic pH and vacuolar pH of pondweed turion cells chased by (32)P NMR spectroscopy were stabilized during a short period < 3 h after anoxic conditions. • We propose that the enhancement of H(+) extrusion by anoxic conditions induces acidification in the apoplast and may contribute to the stabilization of pH in the cytoplasm.

In vitro effect of different Na+/K+ ratios on plasma membrane H+ -ATPase activity in maize and sugar beet shoot

Plant growth is impaired primarily by osmotic stress in the first phase of salt stress, whereas Na+ toxicity affects the plant growth mainly in the second phase. Salinity leads to increased Na+/K+ ratio and thus displacement of K+ by Na+ in the plant cell. Relatively higher cytosolic Na+ concentrations may have an effect on the activity of plasma membrane (PM) H+ -ATPase. A decreased PM-H+ -ATPase activity could increase the apoplastic pH. This process could limit the cell-wall extensibility and thus reduce growth according to the acid growth theory. To compare the effect of Na+ on PM H+ -ATPase activity in salt-sensitive maize (Zea mays L.) and salt-resistant sugar beet (Beta vulgaris L.) shoot, PM vesicles were isolated from growing shoots of both species and ATPase activity was determined by assaying the P(i) released by hydrolysis of ATP. The H+ pumping activity was measured as the quenching of acridine-orange absorbance. An increased Na+/K+ ratio decreased the PM H+ -ATPase activity in vesicles of maize as well as of sugar beet shoots. Nevertheless, the detrimental effect of increased Na+/K+ ratio was more severe in salt-sensitive maize compared to salt-resistant sugar beet. At 25 mM Na+ concentration, hydrolytic activity was not affected in sugar beet. However, a significant decrease in hydrolytic activity was observed in maize at pH 7. In maize and sugar beet, reduction in active H+ flux was 20% and 5% at 25 mM Na+ concentration in the assay, respectively. The active H+ flux was decreased to 80% and 60%, when 100 mM K+ were substituted by 100mM Na+. We conclude that PM H+ -ATPases of salt-resistant sugar beet and maize shoot are sensitive to higher concentration of Na+. However, sugar beet PM-H+ -ATPases are relatively efficient and may have constitutive resistance against lower concentration (25 mM) of Na+ as compared to that of salt-sensitive maize.

Hydrolytic and pumping activity of H+-ATPase from leaves of sugar beet (Beta vulgaris L.) as affected by salt stress

Cell wall extensibility plays an important role in plant growth. According to the acid-growth theory, lower apoplastic pH allows extension growth by affecting cell wall extensibility. A lowered apoplastic pH is presumed to activate wall-loosening enzymes that control plant growth. Plasma membrane (PM) H(+)-ATPases play a major role in the apoplastic acidification by H(+) transport from cytosol to the apoplast. A salt-induced decrease in H(+)-pumping activity of plasma membrane H(+)-ATPases in salt-sensitive maize plants has previously been found. This led us to formulate the hypothesis that salt-resistant plant species such as sugar beet (Beta vulgaris L.) may have a mechanism to eliminate the effect of higher salt concentrations on plasma membrane H(+)-ATPase activity. In the present study, sugar beet plants were grown in 1mM NaCl (control) or 150 mM NaCl in hydroponics. H(+)-ATPase hydrolytic and pumping activities were measured in plasma membrane vesicles isolated from sugar beet shoots. We found that plasma membrane H(+)-ATPase hydrolytic and pumping activities were not affected by application of 150 mM NaCl. Moreover, apoplastic pH was also not affected under salt stress. However, a decrease in plant growth was observed. We assume that growth reduction was not due to a decrease in PM-H(+)-ATPase activity, but that other factors may be responsible for growth inhibition of sugar beet plants under salt stress.

Salt-inducible isoform of plasma membrane H+ATPase gene in rice remains constitutively expressed in natural halophyte, Suaeda maritima

To look into a possible involvement of plasma membrane H+ATPase (PM-H+ATPase, EC 3.6.3.6) in mitigation of physiological disturbances imposed by salt stress, response of the enzyme was studied in two Oryza sativa Indica cultivars, salt-tolerant Lunishri and non-tolerant Badami, and a natural halophyte Suaeda maritima after challenge of the young plants with NaCl. Significant increase in activity of the enzyme was observed in response to NaCl in all the test plants with S. maritima showing maximum increase. Protein blot analysis, however, did not show any increase in the amount of the enzyme (protein). RNA blot analysis, on the other hand, revealed significant increase in transcript level of the enzyme upon NaCl treatment. In the rice cultivars, salt treatment also induced expression of a new isoform of PM-H+ATPase gene, not reported so far. The induced transcript showed maximum homology to OSA7 (O. sativa PM-H+ATPase isoform 7). Similar transcript message, however, remained constitutively present in S. maritima, along with the transcript of another isoform of PM-H+ATPase showing resemblance to OSA3 (O. sativa PM-H+ATPase isoform 3). The latter was the only PM-H+ATPase isoform expressed in both the rice cultivars not exposed to NaCl. In the salt-treated test plants, both rice and S. maritima, the salt-inducible PM-H+ATPase isoform resembling OSA7 was expressed in much greater amount than that resembling OSA3. Appearance of a new PM-H+ATPase transcript, besides increase in the enzyme activity, indicates the important role of the enzyme in maintaining ion-homeostasis in plants under salt stress, enabling them to survive under saline conditions.

The effects of the loss of TIP1;1 and TIP1;2 aquaporins in Arabidopsis thaliana

Loss of aquaporin TIP1;1 in Arabidopsis has been suggested to result in early senescence and plant death. This was based on the fact that a partial reduction of TIP1;1 by RNA interference (RNAi) led to gradual phenotypes, ranging from indistinguishable from wild type to lethality, depending on the degree of downregulation of the target messenger, and displaying pleiotropic effects in primary metabolism and cell signalling. A hypothesis was put forward to suggest that TIP1;1, apart from its transport function, may play an essential role in vesicle routing. Here we identify an Arabidopsis transposon insertion line tip1;1-1 that is completely devoid of TIP1;1 protein, as demonstrated by western blotting and immunolocalization using an isoform-specific antibody. Strikingly, the complete absence of the protein did not result in any significant effect on metabolism or elemental composition of the plants. Microarray analysis did not indicate increased expression of other aquaporins to compensate for the lack of TIP1;1 in tip1;1-1. We further developed a double mutant of TIPs in Arabidopsis, lacking both TIP1;1 and its closest paralog TIP1;2. Arabidopsis mutants lacking both TIP1;1 and TIP1;2 showed a minor increase in anthocyanin content, and a reduction in catalase activity, but showed no changes in water status. In contrast to earlier reports, plants lacking TIP1;1 and TIP1;2 aquaporins are alive and thriving. We suggest that RNAi directed towards TIP1;1 may have resulted in off-target gene silencing, a notion that is potentially interesting for various studies analysing gene function by RNAi.

Constitutive activation of a plasma membrane H(+)-ATPase prevents abscisic acid-mediated stomatal closure

Light activates proton (H(+))-ATPases in guard cells, to drive hyperpolarization of the plasma membrane to initiate stomatal opening, allowing diffusion of ambient CO(2) to photosynthetic tissues. Light to darkness transition, high CO(2) levels and the stress hormone abscisic acid (ABA) promote stomatal closing. The overall H(+)-ATPase activity is diminished by ABA treatments, but the significance of this phenomenon in relationship to stomatal closure is still debated. We report two dominant mutations in the OPEN STOMATA2 (OST2) locus of Arabidopsis that completely abolish stomatal response to ABA, but importantly, to a much lesser extent the responses to CO(2) and darkness. The OST2 gene encodes the major plasma membrane H(+)-ATPase AHA1, and both mutations cause constitutive activity of this pump, leading to necrotic lesions. H(+)-ATPases have been traditionally assumed to be general endpoints of all signaling pathways affecting membrane polarization and transport. Our results provide evidence that AHA1 is a distinct component of an ABA-directed signaling pathway, and that dynamic downregulation of this pump during drought is an essential step in membrane depolarization to initiate stomatal closure.

Plant proton pumps

Chemiosmotic circuits of plant cells are driven by proton (H(+)) gradients that mediate secondary active transport of compounds across plasma and endosomal membranes. Furthermore, regulation of endosomal acidification is critical for endocytic and secretory pathways. For plants to react to their constantly changing environments and at the same time maintain optimal metabolic conditions, the expression, activity and interplay of the pumps generating these H(+) gradients have to be tightly regulated. In this review, we will highlight results on the regulation, localization and physiological roles of these H(+)- pumps, namely the plasma membrane H(+)-ATPase, the vacuolar H(+)-ATPase and the vacuolar H(+)-PPase.

Effect of transporters on the secretion of phytochemicals by the roots of Arabidopsis thaliana

Root exudation, the process by which plants secrete compounds into the soil, is becoming accepted as a communicative process that determines organismal interactions in the rhizosphere. However, the mechanistic processes involved in the root exudation of phytochemicals have not been elucidated; traditionally, exudation has been regarded as a passive process. There is evidence that transporters in plants (and other organisms) have been involved in the movement of chemicals across different membranes. Here, we describe the involvement of different transporters in root exudation of phytochemicals by employing a pharmacological approach. We used a range of concentrations of several compounds known to inhibit different transporters, including potassium cyanide, orthovanadate, quinidine, glibenclamide, nifedipine and verapamil, to examine the effects of transporter inhibition on root exudation profiles in Arabidopsis. Generally, the exudation profile of phenolic compounds in 18-day-old plants shows more than 15 major phytochemicals. In contrast, the inhibitors listed above caused differences in the secretion of specific compounds. For instance, nifedipine and verapamil completely inhibited the exudation of the phytochemicals with molecular masses of 142 and 294, respectively. These results highlight that root exudation of phytochemicals is an active process controlled at the biochemical level and that different transporters may be involved in this root-specific mechanism.

A COOH-terminal domain regulates the activity of Leishmania proton pumps LDH1A and LDH1B

Leishmania donovani requires actively transporting proton efflux pumps to survive the acidic environment of macrophage phagolysosomal vacuoles and to maintain an electrogenic H(+) gradient for nutrient uptake. The L. donovani genome contains a differentially expressed pair of genes, LDH1A and LDH1B, with homology to yeast H(+)-ATPases that are 98% identical in sequence with amino acid differences concentrated at the COOH-terminus (15 of last 37 differ), a region implicated in regulation of yeast and plant proton pumps. Functional complementation of a Saccharomyces cerevisiae strain deficient in endogenous H(+)-ATPase activity, support of yeast growth at low pH, and ability to acidify media demonstrate that LDH1A and LDH1B encode proton pumps. LDH1A and LDH1B encode a COOH-terminal autoinhibitory domain as COOH-truncated peptides support increased rates of growth in yeast, enhanced media acidification, increased enzyme activity (V(max)) and decreased K(m). This regulatory domain mediates differing function properties; LDH1A, but not LDH1B, supports yeast growth at pH 3 and LDH1A shows a greater ability to acidify media. Deletion of the last eight amino acids from LDH1B permits growth at pH 3 and increases media acidification, swapping of the COOH-tails between LDH1A and LDH1B results in LDH1A (with LDH1B tail) unable to support yeast growth at pH 3 and LDH1B (with LDH1A tail) now able to support growth at pH 3. Replacement of the COOH-terminal eight amino acids of LDH1B with those from LDH1A also confers the ability to support growth at pH 3. The complementation system for the Leishmania proton pumps in yeast described here provides a means to dissect the functional properties of the two isoforms, a convenient supply of protein for structural analysis and a model amenable to screening proton pump inhibitors for potential anti-leishmanial therapeutics.

Heterologous expression of a plant uracil transporter in yeast: Improvement of plasma membrane targeting in mutants of the Rsp5p ubiquitin protein ligase

Plasma membrane proteins involved in transport processes play a crucial role in cell physiology. On account of these properties, these molecules are ideal targets for development of new therapeutic and agronomic agents. However, these proteins are of low abundance, which limits their study. Although yeast seems ideal for expressing heterologous transporters, plasma membrane proteins are often retained in intracellular compartments. We tried to find yeast mutants potentially able to improve functional expression of a whole set of heterologous transporters. We focused on Arabidopsis thaliana ureide transporter 1 (AtUPS1), previously cloned by functional complementation in yeast. Tagged versions of AtUPS1 remain mostly trapped in the endoplasmic reticulum and were able to reach slowly the plasma membrane. In contrast, untagged AtUPS1 is rapidly delivered to plasma membrane, where it remains in stable form. Tagged and untagged versions of AtUPS1 were expressed in cells deficient in the ubiquitin ligase Rsp5p, involved in various stages of the intracellular trafficking of membrane-bound proteins. rsp5 mutants displayed improved steady state amounts of untagged and tagged versions of AtUPS1. rsp5 cells are thus powerful tools to solve the many problems inherent to heterologous expression of membrane proteins in yeast, including ER retention.

Relationship between expression of the PM H+-ATPase, growth and ion partitioning in the leaves of salt-treated Medicago species

The role of the plasma membrane (PM) H(+)-ATPase (E.C. 3.6.1.3) in the plant's response to salt stress was studied in the perennial leguminosae forage Medicago arborea L. and its close relative Medicago citrina (Font-Quer) Greuter, a species exposed to saline conditions in its original habitat. Plants were solution cultured for 8 days in 1 or 100 mM NaCl. Leaf growth and CO(2) assimilation were more inhibited by salt in M. arborea than in M. citrina. Both species were able to osmoregulate, and salt-treated plants maintained turgor potentials, with no differences between species. Contrasting ion distribution patterns showed that M. citrina was able to exclude Na(+) from the leaves more selectively, while M. arborea had a greater buildup of leaf blade Na(+). Isolation of purified PM and quantification of H(+)-ATPase protein by Western blot analysis against the 46E5B11D5 or AHA3 antibodies showed an increase in response to salt stress in the expanding (92%) and expanded leaves (87%) of M. citrina, while no differences were found in the corresponding leaves of M. arborea. The assay of H(+)-ATPase specific activity of the two leaf types in salinized M. citrina confirmed this increase, as activities increased with 55% and 104% for the expanded and expanding leaves, respectively, while no significant differences were found for either leaf type of salinized M. arborea. A possible role of the increased expression of the PM H(+)-ATPase for leaf expansion and ion exclusion in salt-stressed plants is discussed.

Plasma membrane H+-ATPase is involved in auxin-mediated cell elongation during wheat embryo development

Previous investigations suggested that specific auxin spatial distribution due to auxin movements to particular embryonic regions was important for normal embryonic pattern formation. To gain information on the molecular mechanism(s) by which auxin acts to direct pattern formation in specific embryonic regions, the role of a plasma membrane (PM) ATPase was evaluated as downstream target of auxin in the present study. Western-blot analysis revealed that the PM H(+)-ATPase expression level was significantly increased by auxin in wheat (Triticum aestivum) embryos (two-three times increase). In bilaterally symmetrical embryos, the spatial expression pattern of the PM H(+)-ATPase correlates with the distribution pattern of the auxin analog, tritiated 5-azidoindole-3-acetic acid. A strong immunosignal was observed in the abaxial epidermis of the scutellum and in the epidermal cells at the distal tip of this organ. Pseudoratiometric analysis using a fluorescent pH indicator showed that the pH in the apoplast of the cells expressing the PM H(+)-ATPase was in average more acidic than the apoplastic pH of nonexpressing cells. Cellulose staining of living embryos revealed that cells of the scutellum abaxial epidermis expressing the ATPase were longer than the scutellum adaxial epidermal cells, where the protein was not expressed. Our data indicate that auxin activates the proton pump resulting in apoplastic acidification, a process contributing to cell wall loosening and elongation of the scutellum. Therefore, we suggest that the PM H(+)-ATPase is a component of the auxin-signaling cascade that may direct pattern formation in embryos.

Trypanosoma cruzi H+-ATPase 1 (TcHA1) and 2 (TcHA2) genes complement yeast mutants defective in H+ pumps and encode plasma membrane P-type H+-ATPases with different enzymatic properties

Previous studies in Trypanosoma cruzi have shown that intracellular pH homeostasis requires ATP and is affected by H(+)-ATPase inhibitors, indicating a major role for ATP-driven proton pumps in intracellular pH control. In the present study, we report the cloning and sequencing of a pair of genes linked in tandem (TcHA1 and TcHA2) in T. cruzi which encode proteins with homology to fungal and plant P-type proton-pumping ATPases. The genes are expressed at the mRNA level in different developmental stages of T. cruzi: TcHA1 is expressed maximally in epimastigotes, whereas TcHA2 is expressed predominantly in trypomastigotes. The proteins predicted from the nucleotide sequence of the genes have 875 and 917 amino acids and molecular masses of 96.3 and 101.2 kDa, respectively. Full-length TcHA1 and an N-terminal truncated version of TcHA2 complemented a Saccharomyces cerevisiae strain deficient in P-type H(+)-ATPase activity, the proteins localized to the yeast plasma membrane, and ATP-driven proton pumping could be detected in proteoliposomes reconstituted from plasma membrane purified from transfected yeast. The reconstituted proton transport activity was reduced by inhibitors of P-type H(+)-ATPases. C-terminal truncation did not affect complementation of mutant yeast, suggesting the lack of C-terminal autoinhibitory domains in these proteins. ATPase activity in plasma membrane from TcHA1- and (N-terminal truncated) TcHA2-transfected yeast was inhibited to different extents by vanadate, whereas the latter yeast strain was more resistant to extremes of pH, suggesting that the native proteins may serve different functions at different stages in the T. cruzi life cycle.

Salt-tolerant ATPase activity in the plasma membrane of the marine angiosperm Zostera marina L

Plasma membrane (PM) H(+)-ATPase and H(+) transport activity were detected in PM fractions prepared from Zostera marina (a seagrass), Vallisneria gigantea (a freshwater grass) and Oryza sativa (rice, a terrestrial plant). The properties of Z. marina PM H(+)-ATPase, specifically, the optimal pH for ATPase activity and the result of trypsin treatment, were similar to those of authentic PM H(+)-ATPases in higher plants. In V. gigantea and O. sativa PM fractions, vanadate-sensitive (P-type) ATPase activities were inhibited by the addition of NaCl. In contrast, activity in the Z. marina PM fraction was not inhibited. The nitrate-sensitive (V-type) and azide-sensitive (F-type) ATPase activities in the Z. marina crude microsomal fraction and the cytoplasmic phosphoenolpyruvate carboxylase activity, however, were inhibited by NaCl, indicating that not all enzyme activities in Z. marina are insensitive to salt. Although the ratio of Na(+) to K(+) (Na(+)/K(+)) in seawater is about 30, Na(+)/K(+) in the Z. marina cells was about 1.0. The salt-tolerant ATPase activity in the plasma membrane must play an important role in maintaining a low Na(+) concentration in the seagrass cells.

Adaptation of H+-pumping and plasma membrane H+ ATPase activity in proteoid roots of white lupin under phosphate deficiency

White lupin (Lupinus albus) is able to adapt to phosphorus deficiency by producing proteoid roots that release a huge amount of organic acids, resulting in mobilization of sparingly soluble soil phosphate in rhizosphere. The mechanisms responsible for the release of organic acids by proteoid root cells, especially the trans-membrane transport processes, have not been elucidated. Because of high cytosolic pH, the release of undissociated organic acids is not probable. In the present study, we focused on H+ export by plasma membrane H+ ATPase in active proteoid roots. In vivo, rhizosphere acidification of active proteoid roots was vanadate sensitive. Plasma membranes were isolated from proteoid roots and lateral roots from P-deficient and -sufficient plants. In vitro, in comparison with two types of lateral roots and proteoid roots of P-sufficient plants, the following increase of the various parameters was induced in active proteoid roots of P-deficient plants: (a) hydrolytic ATPase activity, (b) Vmax and Km, (c) H+ ATPase enzyme concentration of plasma membrane, (d) H+-pumping activity, (e) pH gradient across the membrane of plasmalemma vesicles, and (f) passive H+ permeability of plasma membrane. In addition, lower vanadate sensitivity and more acidic pH optimum were determined for plasma membrane ATPase of active proteoid roots. Our data support the hypothesis that in active proteoid root cells, H+ and organic anions are exported separately, and that modification of plasma membrane H+ ATPase is essential for enhanced rhizosphere acidification by active proteoid roots.

Post-translational modification of plant plasma membrane H(+)-ATPase as a requirement for functional complementation of a yeast transport mutant

Many heterologous membrane proteins expressed in the yeast Saccharomyces cerevisiae fail to reach their normal cellular location and instead accumulate in stacked internal membranes. Arabidopsis thaliana plasma membrane H(+)-ATPase isoform 2 (AHA2) is expressed predominantly in yeast internal membranes and fails to complement a yeast strain devoid of its endogenous H(+)-ATPase Pma1. We observed that phosphorylation of AHA2 in the heterologous host and subsequent binding of 14-3-3 protein is crucial for the ability of AHA2 to substitute for Pma1. Thus, mutants of AHA2, complementing pma1, showed increased phosphorylation at the penultimate residue (Thr(947)), which creates a binding site for endogenous 14-3-3 protein. Only a pool of ATPase in the plasma membrane is phosphorylated. Double mutants carrying in addition a T947A substitution lost their ability to complement pma1. However, mutants affected in both autoinhibitory regions of the C-terminal regulatory domain complemented pma1 irrespective of their ability to become phosphorylated at Thr(947). This demonstrates that it is the activity status of the mutant enzyme and neither redirection of trafficking nor 14-3-3 binding per se that determines the ability of H(+)-pumps to rescue pma1.

Evidence for a role in growth and salt resistance of a plasma membrane H+-ATPase in the root endodermis

The plasma membrane of plant cells is energized by an electrochemical gradient produced by P-type H+-ATPases (proton pumps). These pumps are encoded by at least 12 genes in Arabidopsis. Here we provide evidence that isoform AHA4 contributes to solute transport through the root endodermis. AHA4 is expressed most strongly in the root endodermis and flowers, as suggested by promoter-GUS reporter assays. A disruption of this pump (aha4-1) was identified as a T-DNA insertion in the middle of the gene (after VFP(574)). Truncated aha4-1 transcripts accumulate to approximately 50% of the level observed for AHA4 mRNA in wild-type plants. Plants homozygous for aha4-1 (-/-) show a subtle reduction in root and shoot growth compared with wild-type plants when grown under normal conditions. However, a mutant phenotype is very clear in plants grown under salt stress (e.g., 75 or 110 mM NaCl). In leaves of mutant plants subjected to Na stress, the ratio of Na to K increased 4-5-fold. Interestingly, the aha4-1 mutation appears to be semidominant and was only partially complemented by the introduction of additional wild-type copies of AHA4. These results are consistent with the hypothesis that aha4-1 may produce a dominant negative protein or RNA that partially disrupts the activity of other pumps or functions in the root endodermal tissue, thereby compromising the function of this cell layer in controlling ion homeostasis and nutrient transport.

Large scale expression, purification and 2D crystallization of recombinant plant plasma membrane H+-ATPase

P-type ATPases convert chemical energy into electrochemical gradients that are used to energize secondary active transport. Analysis of the structure and function of P-type ATPases has been limited by the lack of active recombinant ATPases in quantities suitable for crystallographic studies aiming at solving their three-dimensional structure. We have expressed Arabidopsis thaliana plasma membrane H+-ATPase isoform AHA2, equipped with a His(6)-tag, in the yeast Saccharomyces cerevisiae. The H+-ATPase could be purified both in the presence and in the absence of regulatory 14-3-3 protein depending on the presence of the diterpene fusicoccin which specifically induces formation of the H+-ATPase/14-3-3 protein complex. Amino acid analysis of the purified complex suggested a stoichiometry of two 14-3-3 proteins per H+-ATPase polypeptide. The purified H(+)-ATPase readily formed two-dimensional crystals following reconstitution into lipid vesicles. Electron cryo-microscopy of the crystals yielded a projection map at approximately 8 A resolution, the p22(1)2(1) symmetry of which suggests a dimeric protein complex. Three distinct regions of density of approximately equal size are apparent and may reflect different domains in individual molecules of AHA2.

PLANT PLASMA MEMBRANE H+-ATPases: Powerhouses for Nutrient Uptake

Most transport proteins in plant cells are energized by electrochemical gradients of protons across the plasma membrane. The formation of these gradients is due to the action of plasma membrane H+ pumps fuelled by ATP. The plasma membrane H+-ATPases share a membrane topography and general mechanism of action with other P-type ATPases, but differ in regulatory properties. Recent advances in the field include the identification of the complete H+-ATPase gene family in Arabidopsis, analysis of H+-ATPase function by the methods of reverse genetics, an improved understanding of the posttranslational regulation of pump activity by 14-3-3 proteins, novel insights into the H+ transport mechanism, and progress in structural biology. Furthermore, the elucidation of the three-dimensional structure of a related Ca2+ pump has implications for understanding of structure-function relationships for the plant plasma membrane H+-ATPase.

Plasma membrane H-ATPase activity is involved in adaptation of tomato calli to NaCl

A tomato (Lycopersicon esculentum Mill. cv. Pera) callus culture tolerant to NaCl was obtained by successive subcultures of NaCl-sensitive calli in medium supplemented with 50 mM NaCl. NaCl-tolerant calli grew better than NaCl-sensitive calli in media supplemented with 50 and 100 mM NaCl. Analysis of callus ion content showed a strong increase in Na+ and Cl- both in NaCl-tolerant and -sensitive calli grown in media containing NaCl for one subculture. Cells from NaCl-tolerant calli showed a higher H+ extrusion activity than those from NaCl-sensitive calli grown for one subculture in the presence of NaCl. The inhibition of H+ extrusion by NaCl-sensitive cells was correlated with an inhibition of microsomal vanadate-sensitive H+-ATPase (EC 3.6.1.35) and ATP-dependent H+ transport, while the stimulation of H+ extrusion by cells tolerant to 50 mM NaCl was correlated with an increase in plasma membrane ATP-dependent H+ transport. The increase of ATP-dependent H+ extrusion in plasma membranes isolated from 50 mM NaCl-tolerant calli was not a result of stimulation of a vanadate-sensitive ATP hydrolytic activity or an increase in passive permeability to H+. Relative to NaCl-sensitive calli, plasma membrane H+-ATPase from calli tolerant to 50 mM NaCl showed a lower Km for Mg2+-ATP. Our results indicate that tolerance of tomato calli to 50 mM NaCl increases the affinity of plasma membrane H+-ATPase for the substrate ATP and stimulates the H+-pumping activity of this enzyme without modifying its phosphohydrolytic activity.

The two major plant plasma membrane H+-ATPases display different regulatory properties

The major plant plasma membrane H(+)-ATPases fall into two gene categories, subfamilies I and II. However, in many plant tissues, expression of the two subfamilies overlaps, thus precluding individual characterization. Yeast expression of PMA2 and PMA4, representatives of the two plasma membrane H(+)-ATPase subfamilies in Nicotiana plumbaginifolia, has previously shown that (i) the isoforms have distinct enzymatic properties and that (ii) PMA2 is regulated by phosphorylation of its penultimate residue (Thr) and binds regulatory 14-3-3 proteins, resulting in the displacement of the autoinhibitory C-terminal domain. To obtain insights into regulatory differences between the two subfamilies, we have constructed various chimeric proteins in which the 110-residue C-terminal-encoding region of PMA2 was progressively substituted by the corresponding sequence from PMA4. The PMA2 autoinhibitory domain was localized to a region between residues 851 and 915 and could not be substituted by the corresponding region of PMA4. In contrast to PMA2, PMA4 was poorly phosphorylated at its penultimate residue (Thr) and bound 14-3-3 proteins weakly. The only sequence difference around the phosphorylation site is located two residues upstream of the phosphorylated Thr. It is Ser in PMA2 (as in most members of subfamily I) and His in PMA4 (as in most members of subfamily II). Substitution of His by Ser in PMA4 resulted in an enzyme showing increased phosphorylation status, 14-13-3 binding, and ATPase activity, as well as improved yeast growth. The reverse substitution of Ser by His in PMA2 resulted in the failure of this enzyme to complement the absence of yeast H(+)-ATPases. These results show that the two plant H(+)-ATPase subfamilies differ functionally in their regulatory properties.

Sequencing and heterologous expression in Saccharomyces cerevisiae of a Cryptococcus neoformans cDNA encoding a plasma membrane H(+)-ATPase

A cDNA containing an open reading frame encoding a putative plasma membrane H(+)-ATPase in the human pathogenic basidiomycetous yeast Cryptococcus neoformans was cloned and sequenced by means of PCR and cDNA library hybridization. The cloned cDNA is 3475 bp in length, containing a 2994 bp open reading frame encoding a polypeptide of 997 amino acids. As in the case of another basidiomycetous fungus (Uromyces fabae), the deduced amino acid sequence of CnPMA1 was found to be more homologous to those of P-type H(+)-ATPases from higher plants than to those from ascomycetous fungi. In order to prove the sequenced cDNA corresponds to a H(+)-ATPase, it was expressed in Saccharomyces cerevisiae and found to functionally replace its own H(+)-ATPase. Kinetic studies of CnPMA1 compared to ScPMA1 show differences in V(max) values and H(+)-pumping in reconstituted vesicles. The pH optimum and K(m) values are similar in both enzymes.

Lst1p and Sec24p cooperate in sorting of the plasma membrane ATPase into COPII vesicles in Saccharomyces cerevisiae

Formation of ER-derived protein transport vesicles requires three cytosolic components, a small GTPase, Sar1p, and two heterodimeric complexes, Sec23/24p and Sec13/31p, which comprise the COPII coat. We investigated the role of Lst1p, a Sec24p homologue, in cargo recruitment into COPII vesicles in Saccharomyces cerevisiae. A tagged version of Lst1p was purified and eluted as a heterodimer complexed with Sec23p comparable to the Sec23/24p heterodimer. We found that cytosol from an lst1-null strain supported the packaging of alpha-factor precursor into COPII vesicles but was deficient in the packaging of Pma1p, the essential plasma membrane ATPase. Supplementation of mutant cytosol with purified Sec23/Lst1p restored Pma1p packaging into the vesicles. When purified COPII components were used in the vesicle budding reaction, Pma1p packaging was optimal with a mixture of Sec23/24p and Sec23/Lst1p; Sec23/Lst1p did not replace Sec23/24p. Furthermore, Pma1p coimmunoprecipitated with Lst1p and Sec24p from vesicles. Vesicles formed with a mixture of Sec23/Lst1p and Sec23/24p were similar morphologically and in their buoyant density, but larger than normal COPII vesicles (87-nm vs. 75-nm diameter). Immunoelectronmicroscopic and biochemical studies revealed both Sec23/Lst1p and Sec23/24p on the membranes of the same vesicles. These results suggest that Lst1p and Sec24p cooperate in the packaging of Pma1p and support the view that biosynthetic precursors of plasma membrane proteins must be sorted into ER-derived transport vesicles. Sec24p homologues may comprise a more complex coat whose combinatorial subunit composition serves to expand the range of cargo to be packaged into COPII vesicles. By changing the geometry of COPII coat polymerization, Lst1p may allow the transport of bulky cargo molecules, polymers, or particles.

A plant plasma membrane H+-ATPase expressed in yeast is activated by phosphorylation at its penultimate residue and binding of 14-3-3 regulatory proteins in the absence of fusicoccin

The Nicotiana plumbaginifolia plasma membrane H(+)-ATPase isoform PMA2, equipped with a His(6) tag, was expressed in Saccharomyces cerevisiae and purified. Unexpectedly, a fraction of the purified tagged PMA2 associated with the two yeast 14-3-3 regulatory proteins, BMH1 and BMH2. This complex was formed in vivo without treatment with fusicoccin, a fungal toxin known to stabilize the equivalent complex in plants. When gel filtration chromatography was used to separate the free ATPase from the 14-3-3.H(+)-ATPase complex, the complexed ATPase was twice as active as the free form. Trypsin treatment of the complex released a smaller complex, composed of a 14-3-3 dimer and a fragment from the PMA2 C-terminal region. The latter was identified by Edman degradation and mass spectrometry as the PMA2 C-terminal 57 residues, whose penultimate residue (Thr-955) was phosphorylated. In vitro dephosphorylation of this C-terminal fragment prevented binding of 14-3-3 proteins, even in the presence of fusicoccin. Mutation of Thr-955 to alanine, aspartate, or a stop codon prevented PMA2 from complementing the yeast H(+)-ATPase. These mutations were also introduced in an activated PMA2 mutant (Gln-14 --> Asp) characterized by a higher H(+) pumping activity. Each mutation directly modifying Thr-955 prevented 14-3-3 binding, decreased ATPase specific activity, and reduced yeast growth. We conclude that the phosphorylation of Thr-955 is required for 14-3-3 binding and that formation of the complex activates the enzyme.

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.

Regulation of plasma membrane H(+)-ATPase in fungi and plants

The plasma membrane H+-ATPase from fungi and plants is a proton pump which plays a key role in the physiology of these organisms controlling essential functions such as nutrient uptake and intracellular pH regulation. In fungal and plant cells the activity of the proton pump is regulated by a large number of environmental factors at both transcriptional and post-translational levels. During the last years the powerful tools of molecular biology have been successfully used in fungi and plants allowing the cloning of a wide diversity of H+-ATPase genes and rapid progress on the molecular basis of reaction mechanism and regulation of the proton pump. This review focuses on recent results on regulation of plasma membrane H+-ATPase obtained by molecular approaches.

The two major types of plant plasma membrane H+-ATPases show different enzymatic properties and confer differential pH sensitivity of yeast growth

The proton-pumping ATPase (H+-ATPase) of the plant plasma membrane is encoded by two major gene subfamilies. To characterize individual H+-ATPases, PMA2, an H+-ATPase isoform of tobacco (Nicotiana plumbaginifolia), was expressed in Saccharomyces cerevisiae and found to functionally replace the yeast H+-ATPase if the external pH was kept above 5.0 (A. de Kerchove d'Exaerde, P. Supply, J.P. Dufour, P. Bogaerts, D. Thinès, A. Goffeau, M. Boutry [1995] J Biol Chem 270: 23828-23837). In the present study we replaced the yeast H+-ATPase with PMA4, an H+-ATPase isoform from the second subfamily. Yeast expressing PMA4 grew at a pH as low as 4.0. This was correlated with a higher acidification of the external medium and an approximately 50% increase of ATPase activity compared with PMA2. Although both PMA2 and PMA4 had a similar pH optimum (6.6-6.8), the profile was different on the alkaline side. At pH 7.2 PMA2 kept more than 80% of the maximal activity, whereas that of PMA4 decreased to less than 40%. Both enzymes were stimulated up to 3-fold by 100 microgram/mL lysophosphatidylcholine, but this stimulation vanished at a higher concentration in PMA4. These data demonstrate functional differences between two plant H+-ATPases expressed in the same heterologous host. Characterization of two PMA4 mutants selected to allow yeast growth at pH 3.0 revealed that mutations within the carboxy-terminal region of PMA4 could still improve the enzyme, resulting in better growth of yeast cells.

Single point mutations distributed in 10 soluble and membrane regions of the Nicotiana plumbaginifolia plasma membrane PMA2 H+-ATPase activate the enzyme and modify the structure of the C-terminal region

The Nicotiana plumbaginifolia pma2 (plasma membrane H+-ATPase) gene is capable of functionally replacing the H+-ATPase genes of the yeast Saccharomyces cerevisiae, provided that the external pH is kept above 5.0. Single point mutations within the pma2 gene were previously identified that improved H+-ATPase activity and allowed yeast growth at pH 4.0. The aim of the present study was to identify most of the PMA2 positions, the mutation of which would lead to improved growth and to determine whether all these mutations result in similar enzymatic and structural modifications. We selected additional mutants in total 42 distinct point mutations localized in 30 codons. They were distributed in 10 soluble and membrane regions of the enzyme. Most mutant PMA2 H+-ATPases were characterized by a higher specific activity, lower inhibition by ADP, and lower stimulation by lysophosphatidylcholine than wild-type PMA2. The mutants thus seem to be constitutively activated. Partial tryptic digestion and immunodetection showed that the PMA2 mutants had a conformational change making the C-terminal region more accessible. These data therefore support the hypothesis that point mutations in various H+-ATPase parts displace the inhibitory C-terminal region, resulting in enzyme activation. The high density of mutations within the first half of the C-terminal region suggests that this part is involved in the interaction between the inhibitory C-terminal region and the rest of the enzyme.

Comparison of plasma membrane H+-ATPase activity in vesicles obtained from dry and hydrated maize embryos

ATP hydrolysis from H+-ATPase of plasma membrane was measured in vesicles from maize embryos imbibed at times between 0 and 5 h. The activity had a maximum at 2 h of imbibition. In order to detect whether the enzyme had the same characteristics through the first 5 h of imbibition, vanadate and lysophophatydilcholine sensitivities, as well as trypsin, pH and temperature effects on the activity of the H+-ATPase from plasma membrane vesicles isolated from embryos imbibed at 0 or 5 h were studied. The results indicate that the activity expressed at 0 h is very different from the activity at 5 h. The activity from embryos imbibed for 5 h was less sensitive to vanadate, trypsin and lysophosphatidylcholine, more sensitive to denaturing temperatures and with a broader pH dependence, as compared to the activity from embryos that were not imbibed. When vanadate-sensitive ATPase activity was purified by anion exchange chromatography, the peaks obtained from the 0 and 5 h imbibed embryos were different and non-overlapping. These data could be interpreted in terms of different enzyme structures from dry and imbibed embryos due to either different primary structures or covalent modifications, or differences in membrane vicinities.

Complementation of the Saccharomyces cerevisiae plasma membrane H+-ATPase by a plant H+-ATPase generates a highly abundant fusicoccin binding site

Accumulating evidence suggests that the H+-ATPase of the plant plasma membrane is activated by a direct, reversible interaction with 14-3-3 proteins involving the displacement of the C-terminal autoinhibitory domain of the enzyme. The fungal phytotoxin fusicoccin (FC) appears to stabilize this H+-ATPase.14-3-3 complex, thus leading to a persistent activation of the H+-ATPase in vivo. In this study we show that functional replacement of the Saccharomyces cerevisiae H+-ATPase genes by a Nicotiana plumbaginifolia H+-ATPase (pma2) results in the generation of a high affinity fusicoccin binding site that is exceptionally abundant. Acquisition of FC binding capacity is accompanied by a significant increase in the amount of plasma membrane-associated yeast 14-3-3 homologs. The existence of a (plant) PMA2.(yeast)14-3-3 complex was demonstrated using two-dimensional gel systems (native/denaturing). After expression of PMA2 lacking most of its C-terminal region, neither H+-ATPase.14-3-3 complex formation nor FC binding activity could be observed. Furthermore, we obtained direct biochemical evidence for a minimal FC binding complex consisting of the C-terminal PMA2 domain and yeast 14-3-3 homologs. Thus we demonstrated unambiguously the relevance of this regulatory ATPase domain for 14-3-3 interaction as well as its requirement for FC binding.

Modulation of plant plasma membrane H+-ATPase by phytotoxic lipodepsipeptides produced by the plant pathogen Pseudomonas fuscovaginae

Pseudomonas fuscovaginae produces the lipodepsipeptides syringotoxin, fuscopeptin A and fuscopeptin B concurrently. These phytotoxins inhibit acidification of the external medium by fusicoccin-treated rice leaf sheath discs. When tested in vitro on H+-ATPase of rice shoot plasma membranes, syringotoxin and its structural analogue syringomycin, produced by P. syringae pv. syringae, displayed a double effect. At low concentrations they stimulated the ATPase activity of native right-side-out membrane vesicles in a detergent-like manner. At higher concentrations, however, this stimulation was reversed. With membranes treated with the detergent Brij 58, inhibition of ATPase activity was observed at low concentrations of the nonapeptides. The latter effect required the presence of an intact lactone ring formed by the nonapeptide head of these molecules. In contrast, fuscopeptins A and B inhibited enzyme activity regardless of the orientation of the vesicles. These observations were confirmed using plasma membranes from a yeast strain whose own H+-ATPase had been replaced by a single plant H+-ATPase isoform, PMA2, from Nicotiana plumbaginifolia. The kinetics of inhibition induced by the most active compound fuscopeptin B, showed a non-competitive pattern, with a Ki of about 1 microM. The combination of syringotoxin (or syringomycin) with the more hydrophobic fuscopeptins, in amounts with little or no effect, resulted in strong inhibition of the enzyme activity of rice membranes, suggesting a synergistic effect for the two types of toxins.

The plasma membrane H(+)-ATPase from the biotrophic rust fungus Uromyces fabae: molecular characterization of the gene (PMA1) and functional expression of the enzyme in yeast

To study the molecular basis of biotrophic nutrient uptake by plant parasitic rust fungi, the gene (Uf-PMA1) encoding the plasma membrane H(+)-ATPase from Uromyces fabae was isolated. Uf-PMA1 exists probably as a single gene. However, two nearly identical sequences were identified; the similarity apparently is due to two Uf-PMA1 alleles in the dikaryotic hyphae. Multiple Uf-PMA1 transcripts were observed during early rust development, and reduced amounts of a single Uf-PMA1 mRNA were observed in haustoria and rust-infected leaves. This is in contrast to elevated enzyme activity in haustoria compared to germinated spores (C. Struck, M. Hahn, and K. Mendgen. Fungal Genet. Biol. 20:30-35, 1996). Unexpectedly, the PMA1-encoded rust protein is more similar to H(+)-ATPases from plants (55% identity) than from ascomycetous fungi (36% identity). When the rust PMA1 cDNA was expressed in Saccharomyces cerevisiae, both the wild-type enzyme and a mutant derivative (delta 76) deleted for the 76 C-terminal amino acids were able to support growth of a yeast strain lacking its own H(+)-ATPases. Compared to the wild-type, the delta 76 mutant enzyme displayed increased affinity to ATP, a higher vanadate sensitivity, and a more alkaline pH optimum. These results indicate that the C-terminal region of the rust enzyme exhibits auto-regulatory properties.

A transgene encoding a plasma membrane H+-ATPase that confers acid resistance in Arabidopsis thaliana seedlings

Proton pumps (H+-ATPases) are the primary active transport systems in the plasma membrane of higher plant cells. These enzymes are encoded by a large gene family expressed throughout the plant, with specific isoforms directed to various specialized cells. While their involvement in membrane energetics has been suggested by a large body of biochemical and physiological studies, a genetic analysis of their role in plants has not yet been performed. We report here that mutant Arabidopsis thaliana plants containing a phloem-specific transgene encoding a plasma membrane H+-ATPase with an altered carboxy terminus show improved growth at low pH during seedling development. These observations provide the first genetic evidence for a role of the plasma membrane H+-ATPase in cytoplasmic pH homeostasis in plants.

Adaptation of active proton pumping and plasmalemma ATPase activity of corn roots to low root medium pH

Corn (Zea mays L.) root adaptation to pH 3.5 in comparison with pH 6. 0 (control) was investigated in long-term nutrient solution experiments. When pH was gradually reduced, comparable root growth was observed irrespective of whether the pH was 3.5 or 6.0. After low-pH adaptation, H+ release of corn roots in vivo at pH 5.6 was about 3 times higher than that of control. Plasmalemma of corn roots was isolated for investigation in vitro. At optimum assay pH, in comparison with control, the following increases of the various parameters were caused by low-pH treatment: (a) hydrolytic ATPase activity, (b) maximum initial velocity and Michaelis constant (c) activation energy of H+-ATPase, (d) H+-pumping activity, (e) H+ permeability of plasmalemma, and (f) pH gradient across the membranes of plasmalemma vesicles. In addition, vanadate sensitivity remained unchanged. It is concluded that plasmalemma H+-ATPase contributes significantly to the adaptation of corn roots to low pH. A restricted net H+ release at low pH in vivo may be attributed to the steeper pH gradient and enhanced H+ permeability of plasmalemma but not to deactivation of H+-ATPase. Possible mechanisms responsible for adaptation of plasmalemma H+-ATPase to low solution pH during plant cultivation are discussed.