Calcium and proton transport in membrane vesicles from barley roots

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Role of dynamics of intracellular calcium in aluminium-toxicity syndrome

This review is concentrating on the role of aluminium (Al)-calcium (Ca) interactions in Al toxicity syndrome in plants. Disruption of cytoplasmic Ca²⁺ homeostasis has been suggested as a primary trigger of Al toxicity. Aluminium causes an increase in cytosolic Ca²⁺ activity, potentially disrupting numerous biochemical and physiological processes, including those involved in the root growth. The source of Ca²⁺ for the increase in cytosolic Ca²⁺ activity under Al exposure is partly extracellular (likely to be due to the Al-resistant portion of the flux through depolarization-activated Ca²⁺ channels and fluxes through Ca²⁺ -permeable nonselective cation channels in the plasma membrane) as well as intracellular (increased cytosolic Ca²⁺ activity enhances the activity of Ca²⁺ release channels in the tonoplast and the endoplasmic reticulum membrane). The effect on increased cytosolic Ca²⁺ activity of possible Al-related inhibition of the plasma membrane and endo-membrane Ca²⁺ -ATPases and Ca²⁺ exchangers (CaX) that sequester Ca²⁺ out of the cytosol is insufficiently documented at present. The relationship between Al toxicity, cytoplasmic Ca²⁺ homeostasis and cytoplasmic pH needs to be elucidated. Technical improvements that would allow measurements of cytosolic Ca²⁺ activity within the short time after exposure to Al (seconds or shorter) are eagerly awaited. Contents I. Introduction 296 II. Symptoms of aluminium toxicity 296 III. Calcium - aluminium interactions 297 IV. The role of electrical properties of the plasma membrane in calcium-aluminium interactions 306 V. Oxidative stress 307 VI. Callose 308 VII. Cytoskeleton 308 VIII. Conclusions 309 Acknowledgements 309 References 309.

Variation between two near isogenic barley (Hordeum vulgare) cultivars in expression of the B subunit of the vacuolar ATPase in response to salinity

A gene encoding the barley vacuolar ATPase subunit B (BSVAP) was differentially expressed between two near isogenic barley cultivars, Golden Promise and Maythorpe. This gene (BSVAP) was isolated by the mRNA differential display technique (DDRT-PCR). BSVAP was salt inducible under long-term salinity stress in the salt sensitive cultivar Maythorpe but less so in the relatively salt tolerant Golden Promise and was more highly expressed under control conditions in Maythorpe. The physiological consequences of altered vacuolar ATPase expression are discussed in relation to the salt sensitivity of Maythorpe.

Diversity and regulation of plant Ca2+ pumps: insights from expression in yeast

The spatial and temporal regulation of calcium concentration in plant cells depends on the coordinate activities of channels and active transporters located on different organelles and membranes. Several Ca2+ pumps have been identified and characterized by functional expression of plant genes in a yeast mutant (K616). This expression system has opened the way to a genetic and biochemical characterization of the regulatory and catalytic features of diverse Ca2+ pumps. Plant Ca(2+)-ATPases fall into two major types: AtECA1 represents one of four or more members of the type IIA (ER-type) Ca(2+)-ATPases in Arabidopsis, and AtACA2 is one of seven or more members of the type IIB (PM-type) Ca(2+)-ATPases that are regulated by a novel amino terminal domain. Type IIB pumps are widely distributed on membranes, including the PM (plasma membrane), vacuole, and ER (endoplasmic reticulum). The regulatory domain serves multiple functions, including autoinhibition, calmodulin binding, and sites for modification by phosphorylation. This domain, however, is considerably diverse among several type IIB ATPases, suggesting that the pumps are differentially regulated. Understanding of Ca2+ transporters at the molecular level is providing insights into their roles in signaling networks and in regulating fundamental processes of cell biology.

A lipochito-oligosaccharide, Nod factor, induces transient calcium influx in soybean suspension-cultured cells

Lipochito-oligosaccharides (Nod factors) produced by Rhizobium or Bradyrhizobium are the key signal molecules for eliciting nodulation in their corresponding host legumes. To elucidate the signal transduction events mediated by Nod factors, we investigated the effects of Nod factors on the cytosolic [Ca2+] of protoplasts prepared from roots and suspension-cultured cells of soybean (Glycine max and G. soja) using a fluorescent Ca2+ indicator, Fura-PE3. NodBj-V (C18:1, MeFuc), which is a major component of Nod factors produced by Bradyrhizobium japonicum, induces transient elevation of cytosolic [Ca2+] in the cells of soybean within a few minutes. This effect is specific to soybean cells and was not observed in the tobacco BY-2 cells. Furthermore, NodBj-V without MeFuc did not induce any cytosolic [Ca2+] elevation in soybean cells. Exclusion of Ca2+ from the medium, as well as pre-treatment of the cells with an external Ca2+ chelator or with a plasma membrane voltage-dependent Ca2+ channel inhibitor, suppressed the Nod factor-dependent cytosolic [Ca2+] elevation. These results indicate that transient Ca2+ influx from extracellular fluid is one of the earliest responses of soybean cells to NodBj-V (C18:1, MeFuc) in a host-specific manner.

Properties and molecular cloning of Ca2+/H+ antiporter in the vacuolar membrane of mung bean

Kinetic and molecular properties of the Ca2+/H+ antiporter in the vacuolar membrane of mung bean hypocotyls were examined and compared with Ca2+-ATPase. Ca2+ transport activities of both transporters were assayed separately by the filtration method using vacuolar membrane vesicles and 45Ca2+. Ca2+ uptake in the presence of ATP and bafilomycin A1, namely Ca2+-ATPase, showed a relatively low Vmax (6 nmol.min-1.mg-1 protein) and a low Km for Ca2+. The Ca2+/H+ antiporter activity driven by H+-pyrophosphatase showed a high Vmax (25 nmol.min-1.mg-1) and a relatively high Km for Ca2+. The cDNA for mung bean Ca2+/H+ antiporter (VCAX1) codes for a 444 amino-acid polypeptide. Two peptide-specific antibodies of the antiporter clearly reacted with a 42-kDa protein from vacuolar membranes and a cell lysate from a Escherichia coli transformant in which VCAX1 was expressed. These observations directly demonstrate that a low-affinity, high-capacity Ca2+/H+ antiporter and a high-affinity Ca2+-ATPase coexist in the vacuolar membrane. It is likely that the Ca2+/H+ antiporter removes excess Ca2+ in the cytosol to lower the Ca2+ concentration to micromolar levels after stimuli have increased the cytosolic Ca2+ level, the Ca2+-ATPase then acts to lower the cytosolic Ca2+ level further.

Calmodulin-stimulated Ca(2+)-ATPases in the vacuolar and plasma membranes in cauliflower

The subcellular locations of Ca(2+)-ATPases in the membranes of cauliflower (Brassica oleracea L.) inflorescences were investigated. After continuous sucrose gradient centrifugation a 111-kD calmodulin (CaM)-stimulated and caM-binding Ca(2+)-ATPase (BCA1; P. Askerlund [1996] Plant Physiol 110: 913-922; S. Malmström, P. Askerlund, M.G. Plamgren [1997] FEBS Lett 400: 324-328) comigrated with vacuolar membrane markers, whereas a 116-kD caM-binding Ca(2+)-ATPase co-migrated with a marker for the plasma membrane. The 116 kD Ca(2+)-ATPase was enriched in plasma membranes obtained by aqueous two-phase partitioning, which is in agreement with a plasma membrane location of this Ca(2+)-ATPase. Countercurrent distribution of a low-density intracellular membrane fraction in an aqueous two-phase system resulted in the separation of the endoplasmic reticulum and vacuolar membranes. The 111-kD Ca(2+)-ATPase co-migrated with a vacuolar membrane marker after countercurrent distribution but not with markers for the endoplasmic reticulum. A vacuolar membrane location of the 111-kD Ca(2+)-AtPase was further supported by experiments with isolated vacuoles from cauliflower: (a) Immunoblotting with an antibody against the 111-kD Ca(2+)-ATPase showed that it was associated with the vacuoles, and (b) ATP-dependent Ca2+ uptake by the intact vacuoles was found to be CaM stimulated and partly protonophore insensitive.

Distinction between Endoplasmic Reticulum-Type and Plasma Membrane-Type Ca2+ Pumps (Partial Purification of a 120-Kilodalton Ca2+-ATPase from Endomembranes)

Two biochemical types of Ca2+-pumping ATPases were distinguished in membranes that were isolated from carrot (Daucus carota) suspension-cultured cells. One type hydrolyzed GTP nearly as well as ATP, was stimulated by calmodulin, and was resistant to cyclopiazonic acid. This plasma membrane (PM)-type pump was associated with PMs and endomembranes, including vacuolar membranes and the endoplasmic reticulum (ER). Another pump ("ER-type") that was associated mainly with the ER hydrolyzed ATP preferentially, was insensitive to calmodulin, and was inhibited partially by cyclopiazonic acid, a blocker of the animal sarcoplasmic/ER Ca2+ pump. Oxalate stimulation of Ca2+ accumulation by ER-type, but not PM-type, pump(s) indicated a separation of the two types on distinct compartments. An endomembrane 120-kD Ca2+ pump was partially purified by calmodulin-affinity chromatography. The purified polypeptide bound calmodulin reacted with antibodies to a calmodulin-stimulated Ca2+ pump from cauliflower and displayed [32P]phosphoenzyme properties that are characteristic of PM-type Ca2+ pumps. The purified ATPase corresponded to a phosphoenzyme and a 120-kD calmodulin-binding protein on endomembranes. Another PM-type pump was suggested by a 127-kD PM-associated protein that bound calmodulin. Thus, both ER- and PM-type Ca2+ pumps coexist in most plant tissues, and each type can be distinguished from another by a set of traits, even in partially purified membranes.

A Single Gene May Encode Differentially Localized Ca2+-ATPases in Tomato

Previously, a partial-length cDNA and a complete genomic clone encoding a putative sarcoplasmic reticulum-type Ca2+-ATPase (LCA, Lycopersicon Ca2+-ATPase) were isolated from tomato. To determine the subcellular localization of this Ca2+-ATPase, specific polyclonal antibodies raised against a fusion protein encoding a portion of the LCA polypeptide were generated. Based on hybridization of the LCA cDNA and of the nucleotide sequence encoding the fusion protein to genomic DNA, it appears that LCA and the fusion protein domain are encoded by a single gene in tomato. Antibodies raised against the LCA domain fusion protein reacted specifically with two polypeptides of 116 and 120 kD that are localized in the vacuolar and plasma membranes, respectively. The distribution of vanadate-sensitive ATP-dependent Ca2+ transport activities in sucrose gradients coincided with the distribution of the immunodetected proteins. The ATP-dependent Ca2+ transport activities associated with tonoplast and plasma membrane fractions shared similar properties, because both fractions were inhibited by vanadate but insensitive to carbonyl cyanide m-chlorophenylhydrazone, nitrate, and calmodulin. Moreover, antibodies raised against the LCA domain fusion protein inhibited ATP-dependent Ca2+ uptake activity associated with both the tonoplast and plasma membrane fractions. These data suggest that a single gene (LCA) may encode two P-type Ca2+-ATPase isoforms that are differentially localized in the tonoplast and plasma membrane of tomato roots.

A 42-kilodalton annexin-like protein is associated with plant vacuoles

A 42-kD, calcium-dependent, membrane-binding protein (VCaB42) was associated with partially purified vacuole membrane. Membrane-dissociation assays indicated that VCaB42 binding to vacuole membranes was selective for calcium over other cations and that 50% of VCaB42 remained membrane bound at 61 +/- 11 nM free calcium. A 13-amino acid sequence obtained from VCaB42 showed 85% similarity with the endonexin fold, a sequence found in the annexin family of proteins that is thought to be essential for calcium and lipid binding. The greatest similarity in amino acid sequence was observed with annexin VIII (VAC-beta). The calcium-binding properties and sequence similarities suggest that VCaB42 is a member of the annexin family of calcium-dependent, membrane-binding proteins. Functional assays for VCaB42 on vacuole membrane transport processes indicated that it did not significantly affect the initial rate of calcium uptake into vacuole membrane vesicles. Because VCaB42 is vacuole localized (likely on the cytosolic surface of the vacuole) and is 50% dissociated within the physiological range of cytosolic free calcium, we hypothesize that this protein is a sensor that monitors cytosolic calcium levels and transmits that information to the vacuole.

Induction of a Putative Ca-ATPase mRNA in NaCl-Adapted Cells

A cDNA clone was isolated that encodes the partial sequence of a putative endoplasmic reticulum Ca(2+)-ATPase of tobacco. The 1.497-kb insert had an open reading frame of 1.149 kb. The deduced peptide had the greatest homology to the endoplasmic reticulum Ca(2+)-ATPases of Drosophila and Artemia, followed by the mammalian and avian enzymes (SERCA2 and 3). The cDNA insert hybridized to a single mRNA of 4.4 kb from tobacco cultured cells or plant tissues. The level of this transcript was induced about 2-fold by NaCl shock in 428 mm NaCl-deadapted tobacco cells that were maintained in medium without salt, but not in unadapted cells. The level of this transcript was 3- to 4-fold higher in 428 mm NaCl-adapted cells growing in salt than in unadapted cells growing without salt.

The role of the plasma-membrane Ca(2+)-ATPase in Ca (2+) homeostasis in Sinapis alba root hairs

The regulation of cytosolic Ca(2+) has been investigated in growing root-hair cells of Sinapis alba L. with special emphasis on the role of the plasmamembrane Ca(2+)-ATPase. For this purpose, erythrosin B was used to inhibit the Ca(2+)-ATPase, and the Ca(2+) ionophore A23187 was applied to manipulate cytosolic free [Ca(2+)] which was then measured with Ca(2+)-selective microelectrodes. (i) At 0.01 μM, A23187 had no effect on the membrane potential but enhanced the Ca(2+) permeability of the plasma membrane. Higher concentrations of this ionophore strongly depolarized the cells, also in the presence of cyanide. (ii) Unexpectedly, A23187 first caused a decrease in cytosolic Ca(2+) by 0.2 to 0.3 pCa units and a cytosolic acidification by about 0.5 pH units, (iii) The depletion of cytosolic free Ca(2+) spontaneously reversed and became an increase, a process which strongly depended on the external Ca(2+) concentration, (iv) Upon removal of A23187, the cytosolic free [Ca(2+)] returned to its steady-state level, a process which was inhibited by erythrosin B. We suggest that the first reaction to the intruding Ca(2+) is an activation of Ca(2+) transporters (e.g. ATPases at the endoplasmic reticulum and the plasma membrane) which rapidly remove Ca(2+) from the cytosol. The two observations that after the addition of A23187, (i) Ca(2+) gradients as steep as-600 mV could be maintained and (ii) the cytosolic pH rapidly and immediately decreased without recovery indicate that the Ca(2+)-exporting plasma-membrane ATPase is physiologically connected to the electrochemical pH gradient, and probably works as an nH(+)/Ca(2+)-ATPase. Based on the finding that the Ca(2+)-ATPase inhibitor erythrosin B had no effect on cytosolic Ca(2+), but caused a strong Ca(2+) increase after the addion of A23187 we conclude that these cells, at least in the short term, have enough metabolic energy to balance the loss in transport activity caused by inhibition of the primary Ca(2+)-pump. We further conclude that this ATPase is a major Ca(2+) regulator in stress situations where the cytosolic Ca(2+) has been shifted from its steady-state level, as may be the case during processes of signal transduction.

Subunit composition and Ca(2+)-ATPase activity of the vacuolar ATPase from barley roots

The vacuolar ATPase was purified from a tonoplast-enriched membrane fraction from barley (Hordeum vulgare cv CM72) roots. The membranes were solubilized with Triton X-100 and the membrane proteins were separated by chromatography on Sephacryl S-400 followed by fast protein liquid chromatography on a Mono-Q column. The purified vacuolar ATPase was inhibited up to 90% by KNO3 or 80% by dicyclohexylcarbodiimide (DCCI). The ATPase was resolved into polypeptides of 115, 68, 53, 45, 42, 34, 32, 17, 13, and 12 kDa. An additional purification step of centrifugation on a glycerol gradient did not result in loss of any polypeptide bands or increased specific activity of the ATPase. Antibodies against the purified holoenzyme inhibited proton transport by the native ATPase. Two peaks of solubilized Ca(2+)-ATPase were obtained from the Sephacryl S-400 column. A peak of Ca(2+)-ATPase copurified with the vacuolar ATPase during all of the purification steps and was inhibited by NO3- and DCCI. It is proposed that this Ca(2+)-ATPase is a partial reaction of the plant vacuolar ATPase. The second Ca(2+)-ATPase was greatly retarded on the Sephacryl S-400 column and eluted after the main protein peak. It was not inhibited significantly by NO3- or DCCI. The second Ca(2+)-ATPase is a major component of ATP hydrolysis by the native membranes.

Mechanism of aluminum inhibition of net ca uptake by amaranthus protoplasts

Calcium ions serve as a second messenger in signal transduction and metabolic regulation. Effects of Al on calcium homeostasis remain to be elucidated. Short-term net (45)Ca(2+) uptake by Amaranthus tricolor protoplasts was monitored from uptake media prepared to test the influence of pH, Al, and various inhibitors. Accumulation of (45)Ca(2+) increased during the first 3 to 6 minutes and then leveled off or declined. Al and Ca(2+) channel blockers (verapamil and bepridil) decreased net (45)Ca(2+) uptake. This decrease was more pronounced when Al and bepridil were both present in uptake media, but Al did not aggravate verapamil-induced reduction of net (45)Ca(2+) uptake. Erythrosin B and calmidazolium each increased net (45)Ca(2+) uptake, probably by interfering with Ca(2+) efflux. This effect was undetectable in the presence of Al. Mycophenolic acid decreased net (45)Ca(2+) uptake; guanosine alleviated this effect. Al-induced reduction of net (45)Ca(2+) uptake was not aggravated by mycophenolic acid. Net (45)Ca(2+) uptake was generally less at pH 4.5 than at 5.5 for all treatments. It is concluded that Al ions affect net (45)Ca(2+) uptake by binding to the verapamil-specific channel site that is different from the bepridil-specific one, as well as by interfering with the action of guanosine 5'-triphosphate-binding proteins.

In vitro and in vivo phosphorylation of polypeptides in plasma membrane and tonoplast-enriched fractions from barley roots

Phosphorylation of polypeptides in membrane fractions from barley (Hordeum vulgare L. cv CM 72) roots was compared in in vitro and in vivo assays to assess the potential role of protein kinases in modification of membrane transport. Membrane fractions enriched in endoplasmic reticulum, tonoplast, and plasma membrane were isolated using sucrose gradients and the membrane polypeptides separated using sodium dodecyl sulfate polyacrylamide gel electrophoresis. When the membrane fractions were incubated with gamma-[(32)P]ATP, phosphorylation occurred almost exclusively in the plasma membrane fraction. Phosphorylation of a band at 38 kilodaltons increased as the concentration of Mg(2+) was decreased from millimolar to micromolar levels. Phosphorylation of bands at 125, 86, 58, 46, and 28 kilodaltons required millimolar Mg(2+) concentrations and was greatly enhanced by Ca(2+). When roots of intact plants were labeled with [(32)P]orthophosphate, polypeptides at approximately 135, 116, 90, 46 to 53, 32, 28, and 19 kilodaltons were labeled in the plasma membrane fraction and polypeptides at approximately 73, 66, and 48 kilodaltons were labeled in the tonoplast fraction. Treatment of the roots of intact plants with 150 millimolar NaCl resulted in increased phosphorylation of some polypeptides while treatment with 100 mm NaCl had no effect.