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

Regulation of macronutrient transport

In addition to light, water and CO(2), plants require a number of mineral nutrients, in particular the macronutrients nitrogen, sulphur, phosphorus, magnesium, calcium and potassium. After uptake from the soil by the root system they are either immediately assimilated into organic compounds or distributed within the plant for usage in different tissues. A good understanding of how the transport of macronutrients into and between plant cells is adjusted to different environmental conditions is essential to achieve an increase of nutrient usage efficiency and nutritional value in crops. Here, we review the current state of knowledge regarding the regulation of macronutrient transport, taking both a physiological and a mechanistic approach. We first describe how nutrient transport is linked to environmental and internal cues such as nutrient, carbon and water availability via hormonal, metabolic and physical signals. We then present information on the molecular mechanisms for regulation of transport proteins, including voltage gating, auto-inhibition, interaction with other proteins, oligomerization and trafficking. Combining of evidence for different nutrients, signals and regulatory levels creates an opportunity for making new connections within a large body of data, and thus contributes to an integrative understanding of nutrient transport.

Salt-stress-induced association of phosphatidylinositol 4,5-bisphosphate with clathrin-coated vesicles in plants

Plants exposed to hyperosmotic stress undergo changes in membrane dynamics and lipid composition to maintain cellular integrity and avoid membrane leakage. Various plant species respond to hyperosmotic stress with transient increases in PtdIns(4,5)P(2); however, the physiological role of such increases is unresolved. The plasma membrane represents the outermost barrier between the symplast of plant cells and its apoplastic surroundings. In the present study, the spatio-temporal dynamics of stress-induced changes in phosphoinositides were analysed in subcellular fractions of Arabidopsis leaves to delineate possible physiological roles. Unlabelled lipids were separated by TLC and quantified by gas-chromatographic detection of associated fatty acids. Transient PtdIns(4,5)P(2) increases upon exposure to hyperosmotic stress were detected first in enriched plasmamembrane fractions, however, at later time points, PtdIns(4,5)P(2) was increased in the endomembrane fractions of the corresponding two-phase systems. When major endomembranes were enriched from rosette leaves prior to hyperosmotic stress and during stimulation for 60 min, no stress-induced increases in the levels of PtdIns(4,5)P(2) were found in fractions enriched for endoplasmic reticulum, nuclei or plastidial membranes. Instead, increased PtdIns(4,5)P(2) was found in CCVs (clathrin-coated vesicles), which proliferated several-fold in mass within 60 min of hyperosmotic stress, according to the abundance of CCV-associated proteins and lipids. Monitoring the subcellular distribution of fluorescence-tagged reporters for clathrin and PtdIns(4,5)P(2) during transient co-expression in onion epidermal cells indicates rapid stress-induced co-localization of clathrin with PtdIns(4,5)P(2) at the plasma membrane. The results indicate that PtdIns(4,5)P(2) may act in stress-induced formation of CCVs in plant cells, highlighting the evolutionary conservation of the phosphoinositide system between organismic kingdoms.

AtCSLD2 is an integral Golgi membrane protein with its N-terminus facing the cytosol

Cellulose synthase-like proteins in the D family share high levels of sequence identity with the cellulose synthase proteins and also contain the processive beta-glycosyltransferase motifs conserved among all members of the cellulose synthase superfamily. Consequently, it has been hypothesized that members of the D family function as either cellulose synthases or glycan synthases involved in the formation of matrix polysaccharides. As a prelude to understanding the function of proteins in the D family, we sought to determine where they are located in the cell. A polyclonal antibody against a peptide located at the N-terminus of the Arabidopsis D2 cellulose synthase-like protein was generated and purified. After resolving Golgi vesicles from plasma membranes using endomembrane purification techniques including two-phase partitioning and sucrose density gradient centrifugation, we used antibodies against known proteins and marker enzyme assays to characterize the various membrane preparations. The Arabidopsis cellulose synthase-like D2 protein was found mostly in a fraction that was enriched with Golgi membranes. In addition, versions of the Arabidopsis cellulose synthase-like D2 proteins tagged with a green fluorescent protein was observed to co-localize with a DsRed-tagged Golgi marker protein, the rat alpha-2,6-sialyltransferase. Therefore, we postulate that the majority of Arabidopsis cellulose synthase-like D proteins, under our experimental conditions, are likely located at the Golgi membranes. Furthermore, protease digestion of Golgi-rich vesicles revealed almost complete loss of reaction with the antibodies, even without detergent treatment of the Golgi vesicles. Therefore, the N-terminus of the Arabidopsis cellulose synthase-like D2 protein likely faces the cytosol. Combining this observation with the transmembrane domain predictions, we postulate that the large hydrophilic domain of this protein also faces the cytosol.

Arabidopsis TONNEAU1 proteins are essential for preprophase band formation and interact with centrin

Plant cells have specific microtubule structures involved in cell division and elongation. The tonneau1 (ton1) mutant of Arabidopsis thaliana displays drastic defects in morphogenesis, positioning of division planes, and cellular organization. These are primarily caused by dysfunction of the cortical cytoskeleton and absence of the preprophase band of microtubules. Characterization of the ton1 insertional mutant reveals complex chromosomal rearrangements leading to simultaneous disruption of two highly similar genes in tandem, TON1a and TON1b. TON1 proteins are conserved in land plants and share sequence motifs with human centrosomal proteins. The TON1 protein associates with soluble and microsomal fractions of Arabidopsis cells, and a green fluorescent protein-TON1 fusion labels cortical cytoskeletal structures, including the preprophase band and the interphase cortical array. A yeast two-hybrid screen identified Arabidopsis centrin as a potential TON1 partner. This interaction was confirmed both in vitro and in plant cells. The similarity of TON1 with centrosomal proteins and its interaction with centrin, another key component of microtubule organizing centers, suggests that functions involved in the organization of microtubule arrays by the centrosome were conserved across the evolutionary divergence between plants and animals.

The plant plasma membrane proton pump ATPase: a highly regulated P-type ATPase with multiple physiological roles

Around 40 P-type ATPases have been identified in Arabidopsis and rice, for which the genomes are known. None seems to exchange sodium and potassium, as does the animal Na(+)/K(+)-ATPase. Instead, plants, together with fungi, possess a proton pumping ATPase (H(+)-ATPase), which couples ATP hydrolysis to proton transport out of the cell, and so establishes an electrochemical gradient across the plasma membrane, which is dissipated by secondary transporters using protons in symport or antiport, as sodium is used in animal cells. Additional functions, such as stomata opening, cell growth, and intracellular pH homeostasis, have been proposed. Crystallographic data and homology modeling suggest that the H(+)-ATPase has a broadly similar structure to the other P-type ATPases but has an extended C-terminal region, which is involved in enzyme regulation. Phosphorylation of the penultimate residue, a Thr, and the subsequent binding of regulatory 14-3-3 proteins result in the formation of a dodecamer (six H(+)-ATPase and six 14-3-3 molecules) and enzyme activation. This type of regulation is unique to the P-type ATPase family. However, the recent identification of additional phosphorylated residues suggests further regulatory features.

Assessment of organelle purity using antibodies and specific assays : the example of the chloroplast envelope

Proteomics provides a powerful tool to characterize the protein content of an organelle. However, identifications obtained through mass spectrometry and database searching only make sense if the organelle sample is not heavily cross-contaminated. Besides the proteomic analysis, which gives an overview of possible cross-contamination, biochemical methods can be used to assess sample purity. These methods use specific markers that are detected and measured. Here, we describe the use of immunological, enzymatic, lipid, and pigment markers that allow the purity of chloroplast envelope fractions to be estimated.

Isolation and preparation of chloroplasts from Arabidopsis thaliana plants

A major area of research in the postgenomic era has been the proteomic analysis of various subcellular and suborganellar compartments. The success of these studies is to a large extent dependent upon efficient protocols for the preparation of highly pure organelles or suborganellar components. Here we describe a simple, rapid, and low-cost method for isolating a high yield of Arabidopsis chloroplasts. The method can readily be applied to wild-type plants and different mutants, and at different developmental stages ranging from 10-day-old seedlings to rosette leaves from older plants. The isolated chloroplast fraction is highly pure, with immunologically undetectable contamination from other cellular organelles. Chloroplasts isolated using the method described here have been successfully used for proteomic analysis, as well as in studies on chloroplast protein import and other aspects of chloroplast biology.

Allele-specific suppression of a defective brassinosteroid receptor reveals a physiological role of UGGT in ER quality control

UDP-glucose:glycoprotein glucosyltransferase (UGGT) is a presumed folding sensor of protein quality control in the endoplasmic reticulum (ER). Previous biochemical studies with nonphysiological substrates revealed that UGGT can glucosylate nonnative glycoproteins by recognizing subtle folding defects; however, its physiological function remains undefined. Here, we show that mutations in the Arabidopsis EBS1 gene suppressed the growth defects of a brassinosteroid (BR) receptor mutant, bri1-9, in an allele-specific manner by restoring its BR sensitivity. Using a map-based cloning strategy, we discovered that EBS1 encodes the Arabidopsis homolog of UGGT. We demonstrated that bri1-9 is retained in the ER through interactions with several ER chaperones and that ebs1 mutations significantly reduce the stringency of the retention-based ER quality control, allowing export of the structurally imperfect yet biochemically competent bri1-9 to the cell surface for BR perception. Thus, our discovery provides genetic support for a physiological role of UGGT in high-fidelity ER quality control.

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.

Effects of brefeldin A on the localization of Tobamovirus movement protein and cell-to-cell movement of the virus

It has been demonstrated that the subcellular location of Tobamovirus movement protein (MP) which was fused with green fluorescent protein (MP:GFP) changed during the infection process. However, the intracellular route through which MP is transported and its biological meaning are still obscure. Treatment with brefeldin A (BFA), which disrupts ER-to-Golgi transport, inhibited the formation of irregularly shaped and filamentous structures of MP. In this condition, MP was still targeted to plasmodesmata in leaf cells. Furthermore, the viral cell-to-cell movement was not inhibited by BFA treatment. These data indicated that the targeting of viral replication complexes (VRCs) to plasmodesmata is mediated by a BFA-insensitive pathway and that the ER-to-Golgi transport pathway is not involved in viral intercellular movement.

Structure of a 14-3-3 coordinated hexamer of the plant plasma membrane H+ -ATPase by combining X-ray crystallography and electron cryomicroscopy

Regulatory 14-3-3 proteins activate the plant plasma membrane H(+)-ATPase by binding to its C-terminal autoinhibitory domain. This interaction requires phosphorylation of a C-terminal, mode III, recognition motif as well as an adjacent span of approximately 50 amino acids. Here we report the X-ray crystal structure of 14-3-3 in complex with the entire binding motif, revealing a previously unidentified mode of interaction. A 14-3-3 dimer simultaneously binds two H(+)-ATPase peptides, each of which forms a loop within the typical 14-3-3 binding groove and therefore exits from the center of the dimer. Several H(+)-ATPase mutants support this structure determination. Accordingly, 14-3-3 binding could result in H(+)-ATPase oligomerization. Indeed, by using single-particle electron cryomicroscopy, the 3D reconstruction of the purified H(+)-ATPase/14-3-3 complex demonstrates a hexameric arrangement. Fitting of 14-3-3 and H(+)-ATPase atomic structures into the 3D reconstruction map suggests the spatial arrangement of the holocomplex.

A proteomics dissection of Arabidopsis thaliana vacuoles isolated from cell culture

To better understand the mechanisms governing cellular traffic, storage of various metabolites, and their ultimate degradation, Arabidopsis thaliana vacuole proteomes were established. To this aim, a procedure was developed to prepare highly purified vacuoles from protoplasts isolated from Arabidopsis cell cultures using Ficoll density gradients. Based on the specific activity of the vacuolar marker alpha-mannosidase, the enrichment factor of the vacuoles was estimated at approximately 42-fold with an average yield of 2.1%. Absence of significant contamination by other cellular compartments was validated by Western blot using antibodies raised against specific markers of chloroplasts, mitochondria, plasma membrane, and endoplasmic reticulum. Based on these results, vacuole preparations showed the necessary degree of purity for proteomics study. Therefore, a proteomics approach was developed to identify the protein components present in both the membrane and soluble fractions of the Arabidopsis cell vacuoles. This approach includes the following: (i) a mild oxidation step leading to the transformation of cysteine residues into cysteic acid and methionine to methionine sulfoxide, (ii) an in-solution proteolytic digestion of very hydrophobic proteins, and (iii) a prefractionation of proteins by short migration by SDS-PAGE followed by analysis by liquid chromatography coupled to tandem mass spectrometry. This procedure allowed the identification of more than 650 proteins, two-thirds of which copurify with the membrane hydrophobic fraction and one-third of which copurifies with the soluble fraction. Among the 416 proteins identified from the membrane fraction, 195 were considered integral membrane proteins based on the presence of one or more predicted transmembrane domains, and 110 transporters and related proteins were identified (91 putative transporters and 19 proteins related to the V-ATPase pump). With regard to function, about 20% of the proteins identified were known previously to be associated with vacuolar activities. The proteins identified are involved in ion and metabolite transport (26%), stress response (9%), signal transduction (7%), and metabolism (6%) or have been described to be involved in typical vacuolar activities, such as protein and sugar hydrolysis. The subcellular localization of several putative vacuolar proteins was confirmed by transient expression of green fluorescent protein fusion constructs.

Proteomics identification of differentially expressed proteins associated with pollen germination and tube growth reveals characteristics of germinated Oryza sativa pollen

Mature pollen from most plant species is metabolically quiescent; however, after pollination, it germinates quickly and gives rise to a pollen tube to transport sperms into the embryo sac. Because methods for collecting a large amount of in vitro germinated pollen grains for transcriptomics and proteomics studies from model plants of Arabidopsis and rice are not available, molecular information about the germination developmental process is lacking. Here we describe a method for obtaining a large quantity of in vitro germinating rice pollen for proteomics study. Two-dimensional electrophoresis of approximately 2300 protein spots revealed 186 that were differentially expressed in mature and germinated pollen. Most showed a changed level of expression, and only 66 appeared to be specific to developmental stages. Furthermore 160 differentially expressed protein spots were identified on mass spectrometry to match 120 diverse protein species. These proteins involve different cellular and metabolic processes with obvious functional skew toward wall metabolism, protein synthesis and degradation, cytoskeleton dynamics, and carbohydrate/energy metabolism. Wall metabolism-related proteins are prominently featured in the differentially expressed proteins and the pollen proteome as compared with rice sporophytic proteomes. Our study also revealed multiple isoforms and differential expression patterns between isoforms of a protein. These results provide novel insights into pollen function specialization.

Regulation of plant plasma membrane H+- and Ca2+-ATPases by terminal domains

In the last few years, major progress has been made to elucidate the structure, function, and regulation of P-type plasma membrane H(+)-and Ca(2+)-ATPases. Even though a number of regulatory proteins have been identified, many pieces are still lacking in order to understand the complete regulatory mechanisms of these pumps. In plant plasma membrane H(+)- and Ca(2+)-ATPases, autoinhibitory domains are situated in the C- and N-terminal domains, respectively. A model for a common mechanism of autoinhibition is discussed.

Heterologous expression of a mammalian ABC transporter in plant and its application to phytoremediation

Mammalian ATP-binding cassette (ABC) transporters involved in the multidrug-resistance of cancer cells can efflux cytotoxic compounds that show a wide variety of chemical structures and biological activities. Human multidrug resistance-associated protein (hMRP1) is one of the most intensively studied ABC transporters and many substrates have been identified, including both organic and inorganic compounds. In an attempt at novel 'transport engineering' using hMRP1 as a molecular pump, we established transgenic tobacco plants that showed clear resistance to cadmium and daunorubicin, although they were not resistant to etoposide, another known substrate of hMRP1. When expressed in tobacco cells, hMRP1 protein was localized at vacuolar membrane, while members of the MRP family are localized at plasma membrane in mammalian cells to reduce the cellular accumulation of various drugs. Thus, the hMRP1-expressing tobacco cells were able to take up these substrates across the tonoplast and sequestrate them in the vacuolar matrix. These results suggest that it may be possible to use the transgenic tobacco in phytoremediation, where a single transformation with an ABC transporter with broad substrate specificity should be effective for extracting various environmental pollutants including both organic and inorganic compounds, and accumulate them in the plant body. This should be advantageous for the remediation of a complex polluted environment, which is commonly found in the real world.

Proteomic analyses ofOryza sativa mature pollen reveal novel proteins associated with pollen germination and tube growth

As a highly reduced organism, pollen performs specialized functions to generate and carry sperm into the ovule by its polarily growing pollen tube. Yet the molecular genetic basis of these functions is poorly understood. Here, we identified 322 unique proteins, most of which were not reported previously to be in pollen, from mature pollen of Oryza sativa L. ssp japonica using a proteomic approach, 23% of them having more than one isoform. Functional classification reveals that an overrepresentation of the proteins was related to signal transduction (10%), wall remodeling and metabolism (11%), and protein synthesis, assembly and degradation (14%), as well as carbohydrate and energy metabolism (25%). Further, 11% of the identified proteins are functionally unknown and do not contain any conserved domain associated with known activities. These analyses also identified 5 novel proteins by de novo sequencing and revealed several important proteins, mainly involved in signal transduction (such as protein kinases, receptor kinase-interacting proteins, guanosine 5'-diphosphate dissociation inhibitors, C2 domain-containing proteins, cyclophilins), protein synthesis, assembly and degradation (such as prohibitin, mitochondrial processing peptidase, putative UFD1, AAA+ ATPase), and wall remodeling and metabolism (such as reversibly glycosylated polypeptides, cellulose synthase-like OsCsLF7). The study is the first close investigation, to our knowledge, of protein complement in mature pollen, and presents useful molecular information at the protein level to further understand the mechanisms underlying pollen germination and tube growth.

Allosteric modulation of the human P-glycoprotein involves conformational changes mimicking catalytic transition intermediates

The drug transport function of human P-glycoprotein (Pgp, ABCB1) can be inhibited by a number of pharmacological agents collectively referred to as modulators or reversing agents. In this study, we demonstrate that certain thioxanthene-based Pgp modulators with an allosteric mode of action induce a distinct conformational change in the cytosolic domain of Pgp, which alters susceptibility to proteolytic digestion. Both cis and trans-isomers of the Pgp modulator flupentixol confer considerable protection of an 80 kDa Pgp fragment against trypsin digestion, that is recognized by a polyclonal antibody specific for the NH(2)-terminal half to Pgp. The protection by flupentixol is abolished in the Pgp F983A mutant that is impaired in modulation by flupentixols, indicating involvement of the allosteric site in generating the conformational change. A similar protection to an 80 kDa fragment is conferred by ATP, its nonhydrolyzable analog ATPgammaS, and by trapping of ADP-vanadate at the catalytic domain, but not by transport substrate vinblastine or by the competitive modulator cyclosporin A, suggesting different outcomes from modulator interaction at the allosteric site and at the substrate site. In summary, we demonstrate that allosteric interaction of flupentixols with Pgp generates conformational changes that mimic catalytic transition intermediates induced by nucleotide binding and hydrolysis, which may play a crucial role in allosteric inhibition of Pgp-mediated drug transport.

Structure-function analysis of the plasma membrane- localized Arabidopsis defense component ACD6

The ACCELERATED CELL DEATH 6 (ACD6) protein, composed of an ankyrin-repeat domain and a predicted transmembrane region, is a necessary positive regulator of Arabidopsis defenses. ACD6 overexpression confers enhanced disease resistance by priming stronger and quicker defense responses during pathogen infection, plant development or treatment with an agonist of the key defense regulator salicylic acid (SA). Modulation of ACD6 affects both SA-dependent and SA-independent defenses. ACD6 localizes to the plasma membrane and is an integral membrane protein with a cytoplasmic ankyrin domain. An activated version of ACD6 with a predicted transmembrane helix mutation called ACD6-1 has the same localization and overall topology as the wild-type protein. A genetic screen for mutants that suppress acd6-1-conferred phenotypes identified 17 intragenic mutations of ACD6. The majority of these mutations reside in the ankyrin domain and in predicted transmembrane helices, suggesting that both ankyrin and transmembrane domains are important for ACD6 function. One mutation (S638F) also identified a key residue in a putative loop between two transmembrane helices. This mutation did not alter the stability or localization of ACD6, suggesting that S635 is a critical residue for ACD6 function. Based on structural modeling, two ankyrin domain mutations are predicted to be in surface-accessible residues. As ankyrin repeats are protein interaction modules, these mutations may disrupt protein-protein interactions. A plausible scenario is that information exchange between the ankyrin and transmembrane domains is involved in activating defense signaling.

HMA1, a new Cu-ATPase of the chloroplast envelope, is essential for growth under adverse light conditions

Although ions play important roles in the cell and chloroplast metabolism, little is known about ion transport across the chloroplast envelope. Using a proteomic approach specifically targeted to the Arabidopsis chloroplast envelope, we have identified HMA1, which belongs to the metal-transporting P1B-type ATPases family. HMA1 is mainly expressed in green tissues, and we validated its chloroplast envelope localization. Yeast expression experiments demonstrated that HMA1 is involved in copper homeostasis and that deletion of its N-terminal His-domain partially affects the metal transport. Characterization of hma1 Arabidopsis mutants revealed a lower chloroplast copper content and a diminution of the total chloroplast superoxide dismutase activity. No effect was observed on the plastocyanin content in these lines. The hma1 insertional mutants grew like WT plants in standard condition but presented a photosensitivity phenotype under high light. Finally, direct biochemical ATPase assays performed on purified chloroplast envelope membranes showed that the ATPase activity of HMA1 is specifically stimulated by copper. Our results demonstrate that HMA1 offers an additional way to the previously characterized chloroplast envelope Cu-ATPase PAA1 to import copper in the chloroplast.

NpPDR1, a pleiotropic drug resistance-type ATP-binding cassette transporter from Nicotiana plumbaginifolia, plays a major role in plant pathogen defense

Nicotiana plumbaginifolia NpPDR1, a plasma membrane pleiotropic drug resistance-type ATP-binding cassette transporter formerly named NpABC1, has been suggested to transport the diterpene sclareol, an antifungal compound. However, direct evidence for a role of pleiotropic drug resistance transporters in the plant defense is still lacking. In situ immunolocalization and histochemical analysis using the gusA reporter gene showed that NpPDR1 was constitutively expressed in the whole root, in the leaf glandular trichomes, and in the flower petals. However, NpPDR1 expression was induced in the whole leaf following infection with the fungus Botrytis cinerea, and the bacteria Pseudomonas syringae pv tabaci, Pseudomonas fluorescens, and Pseudomonas marginalis pv marginalis, which do not induce a hypersensitive response in N. plumbaginifolia, whereas a weaker response was observed using P. syringae pv syringae, which does induce a hypersensitive response. Induced NpPDR1 expression was more associated with the jasmonic acid than the salicylic acid signaling pathway. These data suggest that NpPDR1 is involved in both constitutive and jasmonic acid-dependent induced defense. Transgenic plants in which NpPDR1 expression was prevented by RNA interference showed increased sensitivity to sclareol and reduced resistance to B. cinerea. These data show that NpPDR1 is involved in pathogen resistance and thus demonstrate a new role for the ATP-binding cassette transporter family.

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.

Structural and Functional Analysis of the C-terminal STAS (Sulfate Transporter and Anti-sigma Antagonist) Domain of the Arabidopsis thaliana Sulfate Transporter SULTR1.2

The C-terminal region of sulfate transporters from plants and animals belonging to the SLC26 family members shares a weak but significant similarity with the Bacillus sp. anti-anti-sigma protein SpoIIAA, thus defining the STAS domain (sulfate transporter and anti-sigma antagonist). The present study is a structure/function analysis of the STAS domain of SULTR1.2, an Arabidopsis thaliana sulfate transporter. A three-dimensional model of the SULTR1.2 STAS domain was built which indicated that it shares the SpoIIAA folds. Moreover, the phosphorylation site, which is necessary for SpoIIAA activity, is conserved in the SULTR1.2 STAS domain. The model was used to direct mutagenesis studies using a yeast mutant defective for sulfate transport. Truncation of the whole SULTR1.2 STAS domain resulted in the loss of sulfate transport function. Analyses of small deletions and mutations showed that the C-terminal tail of the SULTR1.2 STAS domain and particularly two cysteine residues plays an important role in sulfate transport by SULTR1.2. All the substitutions made at the putative phosphorylation site Thr-587 led to a complete loss of the sulfate transport function of SULTR1.2. The reduction or suppression of sulfate transport of the SULTR1.2 mutants in yeast was not due to an incorrect targeting to the plasma membrane. Both our three-dimensional modeling and mutational analyses strengthen the hypothesis that the SULTR1.2 STAS domain is involved in protein-protein interactions that could control sulfate transport.

Loss of autoinhibition of the plasma membrane Ca(2+) pump by substitution of aspartic 170 by asparagin. A ctivation of plasma membrane calcium ATPase 4 without disruption of the interaction between the catalytic core and the C-terminal regulatory domain

The plasma membrane calcium ATPase (PMCA) actively transports Ca(2+) from the cytosol to the extra cellular space. The C-terminal segment of the PMCA functions as an inhibitory domain by interacting with the catalytic core. Ca(2+)-calmodulin binds to the C-terminal segment and stops inhibition. Here we showed that residue Asp(170), in the putative "A" domain of human PMCA isoform 4xb, plays a critical role in autoinhibition. In the absence of calmodulin a PMCA containing a site-specific mutation of D170N had 80% of the maximum activity of the calmodulin-activated PMCA and a similar high affinity for Ca(2+). The mutation did not change the activation of the PMCA by ATP. Deletion of the C-terminal segment further downstream of the calmodulin-binding site led to an additional increase in the maximal activity of the mutant, which suggests that the mutation did not affect the inhibition because of this portion of the C-terminal segment. The calmodulin-activated PMCA was more sensitive to vanadate inhibition than the autoinhibited enzyme. In contrast, inhibition of the D170N mutant required higher concentrations of vanadate and was not affected by calmodulin. Despite its higher basal activity, the mutant had an apparent affinity for calmodulin similar to that of the wild type enzyme, and its rate of proteolysis at the C-terminal segment was still calmodulin-dependent. Altogether these results suggest that activation by mutation D170N does not involve the displacement of the calmodulin-binding autoinhibitory domain from the catalytic core and may arise directly from changes in the accessibility to the calcium-binding residues of the pump.

Cross-talk between the paired domain and the homeodomain of Pax3: DNA binding by each domain causes a structural change in the other domain, supporting interdependence for DNA Binding

The Pax3 protein has two DNA binding domains, a Paired domain (PD) and a paired-type Homeo domain (HD). Although the PD and HD can bind to cognate DNA sequences when expressed individually, genetic and biochemical data indicate that the two domains are functionally interdependent in intact Pax3. The mechanistic basis of this functional interdependence is unknown and was studied by protease sensitivity. Pax3 was modified by the creation of Factor Xa cleavage sites at discrete locations in the PD, the HD, and in the linker segment joining the PD and the HD (Xa172, Xa189, and Xa216) in individual Pax3 mutants. The effect of Factor Xa insertions on protein stability and on DNA binding by the PD and the HD was measured using specific target site sequences. Independent insertions at position 100 in the linker separating the first from the second helix-turn-helix motif of the PD and at position 216 immediately upstream of the HD were found to be readily accessible to Factor Xa cleavage. The effect of DNA binding by the PD or the HD on accessibility of Factor Xa sites inserted in the same or in the other domain was monitored and quantitated for multiple mutants bearing different numbers of Xa sites at each position. In general, DNA binding reduced accessibility of all sites, suggesting a more compact and less solvent-exposed structure of DNA-bound versus DNA-free Pax3. Results of dose response and time course experiments were consistent and showed that DNA binding by the PD not only caused a local structural change in the PD but also caused a conformational change in the HD (P3OPT binding to Xa216 mutants); similarly, DNA binding by the HD also caused a conformational change in the PD (P2 binding to Xa100 mutants). These results provide a structural basis for the functional interdependence of the two DNA binding domains of Pax3.

Restricted spatial expression of a high-affinity phosphate transporter in potato roots

Phosphorus deficiency limits plant growth, and high-affinity phosphate transporters, of the Pht1 family, facilitate phosphate uptake and translocation. The family is subdivided into root specific, phosphate deprivation induced members and those also expressed in leaves. An antibody to StPT2, a potato root specific transporter, detected two bands (52 kDa and 30 kDa) on western blots of root plasma membrane extracts that were most intense in whole extracts from the root tip and slightly increased throughout the root in response to phosphate depletion. RT-PCR, using StPT2 specific primers, confirmed these findings. Low power confocal immunofluorescent images showed StPT2 expression mainly in the elongation zone at the root tip. By contrast, a vacuolar pyrophosphatase and a plasma membrane ATPase antibody labelled the whole root. High power images showed, by comparison with alpha-tubulin, cell wall and plasma membrane ATPase labelling, that StPT2 was in the epidermal plasma membrane and restricted to the apical surface. This is the first evidence of polar plasma membrane localisation of a plant nutrient transporter and is consistent with a role for StPT2 in phosphate capture and uptake.

Identification of regulatory sequence elements within the transcription promoter region of NpABC1, a gene encoding a plant ABC transporter induced by diterpenes

Expression of NpABC1, a gene encoding a plasma membrane ATP binding cassette (ABC) transporter in Nicotiana plumbaginifolia, is induced by sclareol, an antifungal diterpene produced at the leaf surface, as well as by sclareolide, a close analog. A genomic fragment including the 1282-bp region upstream of the NpABC1 transcription start was fused to the reporter beta-glucuronidase (gus) gene and introduced into N. tabacum BY2 cells for stable transformation. A 25-fold increase in gus expression was observed when cells were treated with sclareolide and some other terpenes. The combined use of 5'-deletion promoter analysis, gel mobility shift assays, DNase I footprinting, and site-directed mutagenesis allowed us to identify three cis-elements (sclareol box 1 (SB1), SB2, and SB3) located, respectively, within nucleotides -827 to -802, -278 to -243, and -216 to -190 upstream of the NpABC1 transcription start. In vivo evaluation of these elements on sclareolide-induced expression showed that mutation of SB1 reduced expression by twofold, while that of SB2 had no effect. On the other hand, SB3 had a marked effect as it completely abolished sclareolide-mediated expression. NpABC1-gus expression was not induced by the stress signals, salicylic acid and ethylene, but was mediated, to some extent, by methyl jasmonate, which is known to promote diterpene synthesis.

Function and regulation of the two major plant plasma membrane H+-ATPases

Plant plasma membrane H(+)-ATPases are encoded by a family of about ten genes organized into five subfamilies. Subfamilies I and II contain the most widely and highly expressed genes. In Nicotiana plumbaginifolia, they are represented, respectively, by pma2 (plasma membrane H(+)-ATPase) and pma4. When expressed in the yeast Saccharomyces cerevisiae, the two isoforms show different kinetics and are differently regulated by phosphorylation of the penultimate threonine residue and binding of regulatory 14-3-3 proteins. To determine if these differences also occurred in plant tissues, we developed an experimental approach allowing the characterization of a single isoform in the plant. When PMA2 bearing a 6-His tag was expressed under a strong transcription promoter in Nicotiana tabacum BY2 cells, solubilized from microsomal membranes and purified, the penultimate threonine was found to be phosphorylated, thus validating the model.

A novel interaction partner for the C-terminus of Arabidopsis thaliana plasma membrane H+-ATPase (AHA1 isoform): site and mechanism of action on H+-ATPase activity differ from those of 14-3-3 proteins

Using the two-hybrid technique we identified a novel protein whose N-terminal 88 amino acids (aa) interact with the C-terminal regulatory domain of the plasma membrane (PM) H+-ATPase from Arabidopsis thaliana (aa 847-949 of isoform AHA1). The corresponding gene has been named Ppi1 for Proton pump interactor 1. The encoded protein is 612 aa long and rich in charged and polar residues, except for the extreme C-terminus, where it presents a hydrophobic stretch of 24 aa. Several genes in the A. thaliana genome and many ESTs from different plant species share significant similarity (50-70% at the aa level over stretches of 200-600 aa) to Ppi1. The PPI1 N-terminus, expressed in bacteria as a fusion protein with either GST or a His-tag, binds the PM H+-ATPase in overlay experiments. The same fusion proteins and the entire coding region fused to GST stimulate H+-ATPase activity. The effect of the His-tagged peptide is synergistic with that of fusicoccin (FC) and of tryptic removal of a C-terminal 10 kDa fragment. The His-tagged peptide binds also the trypsinised H+-ATPase. Altogether these results indicate that PPI1 N-terminus is able to modulate the PM H+-ATPase activity by binding to a site different from the 14-3-3 binding site and is located upstream of the trypsin cleavage site.

Seasonal changes of plasma membrane H(+)-ATPase and endogenous ion current during cambial growth in poplar plants

The plasma membrane H(+)-ATPase (PM H(+)-ATPase), potassium ions, and endogenous ion currents might play a fundamental role in the physiology of cambial growth. Seasonal changes of these parameters were studied in twigs of Populus nigra and Populus trichocarpa. Monoclonal and polyclonal antibodies against the PM H(+)-ATPase, x-ray analysis for K(+) localization and a vibrating electrode for measurement of endogenous ion currents were used as probes. In dormant plants during autumn and winter, only a slight immunoreactivity against the PM H(+)-ATPase was found in cross sections and tissue homogenates, K(+) was distributed evenly, and the density of endogenous current was low. In spring during cambial growth, strong immunoreactivity against a PM H(+)-ATPase was observed in cambial cells and expanding xylem cells using the monoclonal antibody 46 E5 B11 F6 for fluorescence microscopy and transmission electron microscopy. At the same time, K(+) accumulated in cells of the cambial region, and strong endogenous current was measured in the cambial and immature xylem zone. Addition of auxin to dormant twigs induced the formation of this PM H(+)-ATPase in the dormant cambial region within a few days and an increase in density of endogenous current in shoot cuttings within a few hours. The increase in PM H(+)-ATPase abundance and in current density by auxin indicates that auxin mediates a rise in number and activity of an H(+)-ATPase in the plasma membrane of cambial cells and their derivatives. This PM H(+)-ATPase generates the necessary H(+)-gradient (proton-motive force) for the uptake of K(+) and nutrients into cambial and expanding xylem cells.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

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.

Stalk segment 5 of the yeast plasma membrane H+-ATPase: mutational evidence for a role in glucose regulation

In P(2)-type ATPases, a stalk region connects the cytoplasmic part of the molecule, which binds and hydrolyzes ATP, to the membrane-embedded part through which cations are pumped. The present study has used cysteine scanning mutagenesis to examine structure-function relationships within stalk segment 5 (S5) of the yeast plasma-membrane H(+)-ATPase. Of 29 Cys mutants that were made and examined, two (G670C and R682C) were blocked in biogenesis, presumably due to protein misfolding. In addition, one mutant (S681C) had very low ATPase activity, and another (F685C) displayed a 40-fold decrease in sensitivity to orthovanadate, reflecting a shift in equilibrium from the E(2) conformational state toward E(1). By far the most striking group of mutants (F666C, L671C, I674C, A677C, I684C, R687C, and Y689C) were constitutively activated even in the absence of glucose, with rates of ATP hydrolysis and kinetic properties normally seen only in glucose-metabolizing cells. Previous work has suggested that activation of the wild-type H(+)-ATPase results from kinase-mediated phosphorylation in the auto-inhibitory C-terminal region of the 100-kDa polypeptide. The seven residues identified in the present study are located on one face of the S5 alpha-helix, consistent with the idea that mutations along this face serve to release the auto-inhibition.

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.

A putative proton binding site of plasma membrane H(+)-ATPase identified through homology modelling

We have used the 2.6 A structure of the rabbit sarcoplasmic reticulum Ca(2+)-ATPase isoform 1a, SERCA1a [Toyoshima, C., Nakasako, M., Nomura, H. and Ogawa, H. (2000) Nature 405, 647-655], to build models by homology modelling of two plasma membrane (PM) H(+)-ATPases, Arabidopsis thaliana AHA2 and Saccharomyces cerevisiae PMA1. We propose that in both yeast and plant PM H(+)-ATPases a strictly conserved aspartate in transmembrane segment (M)6 (D684(AHA2)/D730(PMA1)), and three backbone carbonyls in M4 (I282(AHA2)/I331(PMA1), G283(AHA2)/I332(PMA1) and I285(AHA2)/V334(PMA1)) comprise a binding site for H3O(+), suggesting a previously unknown mechanism for transport of protons. Comparison with the structure of the SERCA1a made it feasible to suggest a possible receptor region for the C-terminal auto-inhibitory domain extending from the phosphorylation and anchor domains into the transmembrane region.

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.

The Trypanosoma cruzi genome contains ion motive ATPase genes which closely resemble Leishmania proton pumps

DNA fragments homologous to members of the family of P-type ion-motive ATPases were identified in Trypanosoma cruzi by polymerase chain reaction (PCR) amplification. The sequence of one fragment, which closely resembled (87% identity) the tandemly linked proton pumps in Leishmania, was used to characterize the H(+)-ATPase genes in T. cruzi. The T. cruzi proton pump locus contains four tandemly repeated genes (TCH1-4) separated by 1.1 kb intergenic regions. The nucleotide sequence of one cloned gene of the tandem array contains a 2775 nt open reading frame encoding a predicted 101908-Da protein of 925 amino acids. The TCH genes are expressed as 3.8 and 4.9 kb polyadenylated transcripts in the epimastigote stage; expression of both transcripts is reduced in metacyclic trypomastigotes. Results of 5' and 3' RACE transcript mapping indicate that the 3.8 kb message is generated from within the tandemly repeated locus. The 3.8 kb TCH transcript has the T. cruzi mini-exon appended to a short (40 nt) 5' untranslated region (UTR) and has a 927 nt 3' UTR. The full peptide sequence of the T. cruzi proton pump is 80% identical to the Leishmania pump but lacks the extended carboxyl tail present in the Leishmania ATPase. An antibody that recognizes the 110-kDa Leishmania donovani proton pump cross-reacts with a 100-kDa protein in lysates of T. cruzi epimastigotes.

Localization and control of expression of Nt-Syr1, a tobacco SNARE protein

Syntaxins and other SNARE proteins are crucial for intracellular vesicle trafficking, fusion and secretion. Previously, we isolated the syntaxin-related protein Nt-Syr1 from Nicotiana in a screen for ABA-related signalling elements, and demonstrated its role in determining the ABA sensitivity of stomatal guard cells. Because the location and expression of SNAREs are often important clues to their functioning, we have examined the distribution and stimulus-dependent expression of Nt-Syr1 between tissues, as well as its location within the cell, using antisera raised against purified recombinant peptides corresponding to overlapping cytosolic domains of Nt-Syr1. The Nt-Syr1 epitope was strongly represented in roots and to lesser extents in stems, leaves and flowers of well-watered plants. Biochemical analysis and examination of immunogold labelling under the electron microscope indicated Nt-Syr1 to be located primarily at the plasma membrane. Expression of the protein in leaves and to a lesser extent in flowers and stems was transiently enhanced by ABA, but not by auxin, kinetin or gibberellic acid. Expression in leaves was promoted by salt stress and wounding, but not by cold. By contrast, Nt-Syr1 levels in the root were unaffected by ABA. In the leaves, enhanced expression of Nt-Syr1 by salt stress was not observed in aba1 mutant Nicotiana, which is deficient in ABA synthesis, and in plants carrying the Arabidopsis abi1 transgene that suppresses a number of ABA-evoked responses in these plants. However, an enhanced expression in response to wounding was observed, even in the mutant backgrounds. We conclude that Nt-Syr1 expression at the plasma membrane is important for its function and is subject to control by parallel, stress-related signalling pathways, both dependent on and independent of ABA. Nt-Syr1 may be associated with additional functions, especially in the roots, that are unrelated to ABA or stress responses in the plant.

Autoinhibition of a calmodulin-dependent calcium pump involves a structure in the stalk that connects the transmembrane domain to the ATPase catalytic domain

The regulation of Ca(2+)-pumps is important for controlling [Ca(2+)] in the cytosol and organelles of all eukaryotes. Here, we report a genetic strategy to identify residues that function in autoinhibition of a novel calmodulin-activated Ca(2+)-pump with an N-terminal regulatory domain (isoform ACA2 from Arabidopsis). Mutant pumps with constitutive activity were identified by complementation of a yeast (K616) deficient in two Ca(2+)-pumps. Fifteen mutations were found that disrupted a segment of the N-terminal autoinhibitor located between Lys(23) and Arg(54). Three mutations (E167K, D219N, and E341K) were found associated with the stalk that connects the ATPase catalytic domain (head) and with the transmembrane domain. Enzyme assays indicated that the stalk mutations resulted in calmodulin-independent activity, with V(max), K(mATP), and K(mCa(2+)) similar to that of a pump in which the N-terminal autoinhibitor had been deleted. A highly conservative substitution at Asp(219) (D219E) still produced a deregulated pump, indicating that the autoinhibitory structure in the stalk is highly sensitive to perturbation. In plasma membrane H(+)-ATPases from yeast and plants, similarly positioned mutations resulted in hyperactive pumps. Together, these results suggest that a structural feature of the stalk is of general importance in regulating diverse P-type ATPases.

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.

Nucleotide-induced conformational changes in P-glycoprotein and in nucleotide binding site mutants monitored by trypsin sensitivity

Limited trypsin digestion was used to monitor nucleotide-induced conformational changes in wild-type P-glycoprotein (Pgp) as well as in nucleotide binding domain (NBD) Pgp mutants. Purified and reconstituted wild-type or mutant mouse Mdr3 Pgps were preincubated with different hydrolyzable or nonhydrolyzable nucleotides, followed by limited proteolytic cleavage at different trypsin:protein ratios. The Pgp tryptic digestion products were separated by SDS-PAGE followed by immunodetection with the mouse monoclonal anti-Pgp antibody C219, which recognizes a conserved epitope (VVQE/AALD) in each half of the protein. Different trypsin digestion patterns were observed for wild-type Pgp incubated with MgCl(2) alone, MgADP, MgAMP.PNP, MgATP, and MgATP + vanadate. A unique trypsin digestion profile suggestive of enhanced resistance to trypsin was observed under conditions of vanadate-induced trapping of nucleotides (MgATP + vanadate). The trypsin sensitivity profiles of Pgp mutants bearing either single or double mutations in Walker A (K429R, K1072R) and Walker B (D551N, D1196N) sequence signatures of NBD1 and NBD2 were analyzed under conditions of vanadate-induced trapping of nucleotides. The proteolytic cleavage pattern observed for the double mutants K429R/K1072R and D551N/D1196N, and for the single mutants K429R, K1072R, and D1196N were similar and clearly distinct from wild-type Pgp under the same conditions. This is consistent with the absence of ATP hydrolysis and of vanadate-induced trapping of 8-azido-ADP previously reported for these mutants [Urbatsch et al. (1998) Biochemistry 37, 4592-4602]. Interestingly, the trypsin digestion profiles observed under vanadate-induced trapping for the D551N and D1196N mutants were quite different, with the D551N mutant showing a profile resembling that seen for wild-type Pgp. The different sensitivity profiles of Pgp mutants bearing mutations at the homologous residue in NBD1 (D551N) and NBD2 (D1196N) suggest possible structural and functional differences between the two sites.

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 C-terminal tetrapeptide HWFW of the Chlorella HUP1 hexose/H(+)-symporter is essential for full activity and an alpha-helical structure of the C-terminus

C-terminal tails of plant hexose/H(+)-symporters of the major facilitator superfamily contain a highly conserved motif of four amino acids: HWFW. A deletion of these four amino acids in the Chlorella HUP1 protein leads to a decrease in transport activity by a factor of 3-4. The mutated tail is highly sensitive to trypsin; it does not show alpha-helical conformation in contrast to the wild type C-terminal peptide with an alpha-helical content of at least 15%. The production of monoclonal antibody 416B8 recognizing an epitope within the central loop of HUP1 protein has been a prerequisite for the experiments described.

Binding of 14-3-3 protein to the plasma membrane H(+)-ATPase AHA2 involves the three C-terminal residues Tyr(946)-Thr-Val and requires phosphorylation of Thr(947)

14-3-3 proteins play a regulatory role in a diverse array of cellular functions such as apoptosis, regulation of the cell cycle, and regulation of gene transcription. The phytotoxin fusicoccin specifically induces association of virtually any 14-3-3 protein to plant plasma membrane H(+)-ATPase. The 14-3-3 binding site in the Arabidopsis plasma membrane H(+)-ATPase AHA2 was localized to the three C-terminal residues of the enzyme (Tyr(946)-Thr-Val). Binding of 14-3-3 protein to this target was induced by phosphorylation of Thr(947) (K(D) = 88 nM) and was in practice irreversible in the presence of fusicoccin (K(D) = 7 nM). Mass spectrometry analysis demonstrated that AHA2 expressed in yeast was phosphorylated at Thr(947). We conclude that the extreme end of AHA2 contains an unusual high-affinity binding site for 14-3-3 protein.

Molecular dissection of the C-terminal regulatory domain of the plant plasma membrane H+-ATPase AHA2: mapping of residues that when altered give rise to an activated enzyme

The plasma membrane H+-ATPase is a proton pump belonging to the P-type ATPase superfamily and is important for nutrient acquisition in plants. The H+-ATPase is controlled by an autoinhibitory C-terminal regulatory domain and is activated by 14-3-3 proteins which bind to this part of the enzyme. Alanine-scanning mutagenesis through 87 consecutive amino acid residues was used to evaluate the role of the C-terminus in autoinhibition of the plasma membrane H+-ATPase AHA2 from Arabidopsis thaliana. Mutant enzymes were expressed in a strain of Saccharomyces cerevisiae with a defective endogenous H+-ATPase. The enzymes were characterized by their ability to promote growth in acidic conditions and to promote H+ extrusion from intact cells, both of which are measures of plasma membrane H+-ATPase activity, and were also characterized with respect to kinetic properties such as affinity for H+ and ATP. Residues that when altered lead to increased pump activity group together in two regions of the C-terminus. One region stretches from K863 to L885 and includes two residues (Q879 and R880) that are conserved between plant and fungal H+-ATPases. The other region, incorporating S904 to L919, is situated in an extension of the C-terminus unique to plant H+-ATPases. Alteration of residues in both regions led to increased binding of yeast 14-3-3 protein to the plasma membrane of transformed cells. Taken together, our data suggest that modification of residues in two regions of the C-terminal regulatory domain exposes a latent binding site for activatory 14-3-3 proteins.