A Plasma Membrane-Type Ca2+-ATPase of 120 Kilodaltons on the Endoplasmic Reticulum from Carrot (Daucus carota) Cells (Properties of the Phosphorylated Intermediate)

The plant Ca2+ -ATPase repertoire: biochemical features and physiological functions

Ca(2+)-ATPases are P-type ATPases that use the energy of ATP hydrolysis to pump Ca(2+) from the cytoplasm into intracellular compartments or into the apoplast. Plant cells possess two types of Ca(2+) -pumping ATPase, named ECAs (for ER-type Ca(2+)-ATPase) and ACAs (for auto-inhibited Ca(2+)-ATPase). Each type comprises different isoforms, localised on different membranes. Here, we summarise available knowledge of the biochemical characteristics and the physiological role of plant Ca(2+)-ATPases, greatly improved after gene identification, which allows both biochemical analysis of single isoforms through heterologous expression in yeast and expression profiling and phenotypic analysis of single isoform knock-out mutants.

Can Ca2+ fluxes to the root xylem be sustained by Ca2+-ATPases in exodermal and endodermal plasma membranes?

The pathway of Ca2+ movement from the soil solution into the root stele has been a subject of controversy. If transport through the endodermis is assumed to be through the cytoplasm, the limiting factor is believed to be the active pumping of Ca2+ from the cytoplasm into the stele apoplast through the plasma membrane lying on the stele side of the Casparian band. By analogy, for similar transport through the exodermis, the limiting step would be the active pumping into the apoplast on the central cortical side of the layer. Such effluxes are mediated by Ca2+-ATPases. To assess whether or not known Ca2+ fluxes to the stele in onion (Allium cepa) roots could be supported by Ca2+-ATPases, the percentages of total membrane protein particles required to effect the transport were calculated using measured values of membrane surface areas, an animal literature value for Ca2+-ATPase V(max), plant literature values for Ca2+-ATPase K(m), and protein densities of relevant membranes. Effects of a putative symplastic movement of Ca2+ from the exo- or endodermis into the next cell layer, which would increase the surface areas available for pumping, were also considered. Depending on the assumptions applied, densities of Ca2+ pumps, calculated as a percentage of total membrane protein particles, varied tremendously between three and 1,600 for the endodermis, and between 0.94 and 1,900 for the exodermis. On the basis of the data, the possibility of Ca2+ transport through the cytoplasm and membranes of the exodermis and endodermis cannot be discounted. Thus, it is premature to assign an entirely apoplastic pathway for Ca2+ movement from the soil solution to the tracheary elements of the xylem. To verify any conclusion with certainty, more detailed data are required for the characteristics of exo- and endodermal Ca2+-ATPases.

A thapsigargin-sensitive Ca(2+) pump is present in the pea Golgi apparatus membrane

The Golgi apparatus behaves as a bona fide Ca(2+) store in animal cells and yeast (Saccharomyces cerevisiae); however, it is not known whether this organelle plays a similar role in plant cells. In this work, we investigated the presence of an active Ca(2+) accumulation mechanism in the plant cell Golgi apparatus. Toward this end, we measured Ca(2+) uptake in subcellular fractions isolated from the elongating zone of etiolated pea (Pisum sativum) epicotyls. Separation of organelles using sucrose gradients showed a strong correlation between the distribution of an ATP-dependent Ca(2+) uptake activity and the Golgi apparatus marker enzyme, xyloglucan-fucosyltransferase. The kinetic parameters obtained for this activity were: the rate of maximum Ca(2+) uptake of 2.5 nmol mg min(-1) and an apparent K(m) for Ca(2+) of 209 nM. The ATP-dependent Ca(2+) uptake was strongly inhibited by vanadate (inhibitor concentration causing 50% inhibition [I(50)] = 126 microM) and cyclopiazonic acid (I(50) = 0.36 nmol mg protein(-1)) and was not stimulated by calmodulin (1 microM). Addition of Cd(2+) and Cu(2+) at nanomolar concentration inhibited the Ca(2+) uptake, whereas Mn(2+), Fe(2+), and Co(2+) had no significant effect. Interestingly, the active calcium uptake was inhibited by thapsigargin (apparent I(50) = 88 nM), a well-known inhibitor of the endoplasmic reticulum and Golgi sarco-endoplasmic reticulum Ca(2+) ATPase from mammalian cells. A thapsigargin-sensitive Ca(2+) uptake activity was also detected in a cauliflower (Brassica oleracea) Golgi-enriched fraction, suggesting that other plants may also possess thapsigargin-sensitive Golgi Ca(2+) pumps. To our knowledge, this is the first report of a plant Ca(2+) pump activity that shows sensitivity to low concentrations of thapsigargin.

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.

Maize cap1 encodes a novel SERCA-type calcium-ATPase with a calmodulin-binding domain

A cDNA (CAP1) isolated from maize roots shares sequence identity with genes encoding P-type Ca(2+)-ATPases and restores the growth phenotype of yeast mutants defective in Ca(2+)-pumps. CAP1 was transcribed and translated in the yeast mutant. Furthermore, the membrane-integrated product formed a Ca(2+)-dependent phosphorylated intermediate and supported Ca(2+) transport. Although CAP1 shares greater sequence identity with mammalian "endoplasmic reticulum-type" Ca(2+)-pumps, it differs from these genes by having features of calmodulin (CaM)-regulated Ca(2+)-pumps. CAP1 from yeast microsomes bound CaM, and the CAP1-dependent Ca(2+) transport in yeast was stimulated by CaM. Peptides from the C terminus of CAP1 bound CaM. Anti-CAP1 antibodies specifically recognized a maize microsomal polypeptide that also bound CaM. A similar polypeptide also formed a Ca(2+)-dependent phosphoenzyme. Our results suggest that cap1 encodes a novel form of CaM-regulated Ca(2+)-ATPase in maize. CAP1 appears to be encoded by one or two genes in maize. CAP1 RNA is induced only during early anoxia, indicating that the Ca(2+)-pump may play an important role in O(2)-deprived maize cells.

Molecular aspects of higher plant P-type Ca(2+)-ATPases

Recent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca(2+)-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca(2+)-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca(2+)-ATPase) and type IIB (ACA for autoinhibited Ca(2+)-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca(2+)-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca(2+)-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new data on a putative involvement in secretory and salt stress functions are discussed.

Two additional type IIA Ca(2+)-ATPases are expressed in Arabidopsis thaliana: evidence that type IIA sub-groups exist

High affinity Ca(2+)-ATPases play a central role in calcium homeostasis by catalysing the active efflux of calcium from the cytoplasm. This study reports the identification of two additional type IIA (SERCA-type) Ca(2+)-ATPases from Arabidopsis (AtECA2 and AtECA3), and describes the detailed sequence analysis of these genes in comparison with AtECA1 and other plant and animal Ca(2+)-ATPases. Southern analysis suggests that each of these genes is present as a single copy and also that there may be a small family of moderately related genes that encode type IIA Ca(2+)-ATPases in Arabidopsis. Evidence is also provided from RT-PCR that these genes are expressed in Arabidopsis. Hydropathy analysis predicts that the topology of the Arabidopsis type IIA proteins is similar to the animal SERCA proteins. Sequence and phylogenetic analyses suggest that the type IIA Ca(2+)-ATPases can be further divided into sub-groups.

A calcium-selective channel from root-Tip endomembranes of garden cress

A Ca2+ channel from root-tip endomembranes of garden cress (Lepidium sativum L.) (LCC1) was characterized using the planar lipid-bilayer technique. Investigation of single-channel recordings revealed that LCC1 is voltage gated and strongly rectifying. In symmetrical 50 mM CaCl2 solutions, the single-channel conductance was 24 picosiemens. LCC1 showed a moderate selectivity for Ca2+ over K+ (9.4:1) and was permeable for a range of divalent cations (Ca2+, Ba2+, and Sr2+). In contrast to Bryonia dioica Ca2+ channel 1, a Ca2+-selective channel from the endoplasmic reticulum of touch-sensitive tendrils, LCC1 showed no bursting channel activity and had a low open probability and mean open time (2.83 ms at 50 mV). Inhibitor studies demonstrated that LCC1 is blocked by micromolar concentrations of erythrosin B (inhibitor concentration for 50% inhibition [IC50] = 1. 8 μM) and the trivalent cations La3+ (IC50 = 5 μM) and Gd3+ (IC50 = 10 μM), whereas verapamil showed no blocking effect. LCC1 may play an important role in the regulation of the cytoplasmic free Ca2+ concentration in root-tip and/or root-cap cells. The question of whether this ion channel is part of the gravitropic signal transduction pathway deserves further investigation.

A high-affinity Ca2+ pump, ECA1, from the endoplasmic reticulum is inhibited by cyclopiazonic acid but not by thapsigargin

To identify and characterize individual Ca2+ pumps, we have expressed an Arabidopsis ECA1 gene encoding an endoplasmic reticulum-type Ca2+-ATPase homolog in the yeast (Saccharomyces cerevisiae) mutant K616. The mutant (pmc1pmr1cnb1) lacks a Golgi and a vacuolar membrane Ca2+ pump and grows very poorly on Ca2+-depleted medium. Membranes isolated from the mutant showed high H+/Ca2+-antiport but no Ca2+-pump activity. Expression of ECA1 in endomembranes increased mutant growth by 10- to 20-fold in Ca2+-depleted medium. 45Ca2+ pumping into vesicles from ECA1 transformants was detected after the H+/Ca2+-antiport activity was eliminated with bafilomycin A1 and gramicidin D. The pump had a high affinity for Ca2+ (Km = 30 nM) and displayed two affinities for ATP (Km of 20 and 235 microM). Cyclopiazonic acid, a specific blocker of animal sarcoplasmic/endoplasmic reticulum Ca2+-ATPase, inhibited Ca2+ transport (50% inhibition dose = 3 nmol/mg protein), but thapsigargin (3 microM) did not. Transport was insensitive to calmodulin. These results suggest that this endoplasmic reticulum-type Ca2+-ATPase could support cell growth in plants as in yeast by maintaining submicromolar levels of cytosolic Ca2+ and replenishing Ca2+ in endomembrane compartments. This study demonstrates that the yeast K616 mutant provides a powerful expression system to study the structure/function relationships of Ca2+ pumps from eukaryotes.

ECA1 complements yeast mutants defective in Ca2+ pumps and encodes an endoplasmic reticulum-type Ca2+-ATPase in Arabidopsis thaliana

To understand the structure, role, and regulation of individual Ca2+ pumps in plants, we have used yeast as a heterologous expression system to test the function of a gene from Arabidopsis thaliana (ECA1). ECA1 encoded a 116-kDa polypeptide that has all the conserved domains common to P-type Ca2+ pumps (EC 3.6.1.38). The amino acid sequence shared more identity with sarcoplasmic/endoplasmic reticulum (53%) than with plasma membrane (32%) Ca2+ pumps. Yeast mutants defective in a Golgi Ca2+ pump (pmr1) or both Golgi and vacuolar Ca2+ pumps (pmr1 pmc1 cnb1) were sensitive to growth on medium containing 10 mM EGTA or 3 mM Mn2+. Expression of ECA1 restored growth of either mutant on EGTA. Membranes were isolated from the pmr1 pmc1 cnb1 mutant transformed with ECA1 to determine if the ECA1 polypeptide (ECA1p) could be phosphorylated as intermediates of the reaction cycle of Ca2+-pumping ATPases. In the presence of [gamma-32P]ATP, ECA1p formed a Ca2+-dependent [32P]phosphoprotein of 106 kDa that was sensitive to hydroxylamine. Cyclopiazonic acid, a blocker of animal sarcoplasmic/endoplasmic reticulum Ca2+ pumps, inhibited the formation of the phosphoprotein, whereas thapsigargin did not. Immunoblotting with an antibody against the carboxyl tail showed that ECA1p was associated mainly with the endoplasmic reticulum membranes isolated from Arabidopsis plants. The results support the model that ECA1 encodes an endoplasmic reticulum-type Ca2+ pump in Arabidopsis. The ability of ECA1p to restore growth of mutant pmr1 on medium containing Mn2+, and the formation of a Mn2+-dependent phosphoprotein suggested that ECA1p may also regulate Mn2+ homeostasis by pumping Mn2+ into endomembrane compartments of plants.

Cloning and characterization of the calreticulin gene from Ricinus communis L

A full-length cDNA encoding a calreticulin-like protein was isolated by immune-screening a germinating castor bean endosperm cDNA library with antisera raised to the total lumenal fraction of purified plant endoplasmic reticulum. The calcium-binding properties of the recombinant protein were characterized and shown to be essentially identical to those reported for the mammalian calreticulin. Calcium overlays and immune blot analysis confirmed the endoplasmic lumenal identity of this reticuloplasmin. Probing protein blots of endoplasmic reticulum subfractions with radio-iodinated calreticulin showed specific associations with various polypeptides including one identified as the abundant reticuloplasmin protein disulfide isomerase. Characterization of the corresponding genomic clones revealed that calreticulin is encoded by a single gene of 3 kb in castor. The full genomic sequence reveals the presence of 12 introns, 12 translated exons, and one exon containing the last three amino acids of the translated sequence and the 3'-untranslated region of the gene. Northern blot analysis of RNA isolated from various organ tissues showed a basal constitutive level of expression throughout the plant, but more abundant mRNA being detected in tissues active in secretion. This was confirmed by analysis of transgenic tobacco plants containing 1.8 kb of 5'-untranslated genomic sequence fused to the beta-glucuronidase reporter gene (GUS) showed a more localized pattern of expression. Activity being localized to the vasculature (phloem, root hairs and root tip) in vegetative tissue, and being strongly expressed in the floral organs including the developing and germinating seed.

Subcellular and tissue distribution, partial purification, and sequencing of calmodulin-stimulated Ca2+-transporting ATPases from barley (Hordeum vulgare L.) and tobacco (Nicotiana tabacum)

The subcellular distribution of plasma-membrane-type Ca2+-transporting ATPases was studied in barley leaves (Hordeum vulgare L.). A highly enriched plasma membrane (PM) fraction was analysed for Ca2+ pumps and compared with several inner-membrane preparations, including the tonoplast, the envelope of the chloroplast, and an endoplasmic reticulum (ER)-enriched fraction. The enzymes were identified and characterised with regard to the phosphointermediate formation, their nucleotide specificity and their binding to calmodulin. A Ca2+-transporting ATPase (molecular mass approximately 130 kDa), which showed high specificity for ATP and high affinity for calmodulin, was localised in the PM. A 116-kDa Ca2+-transporting ATPase, probably located in the ER, showed lower nucleotide specificity and weaker affinity for calmodulin. A comparison of the distribution of the pumps in leaves and roots indicated that the ratio of expression of the two enzymes changed in a tissue-specific manner: the PM pump was dominant in leaves, while the inner-membrane pump was expressed at a higher level in the roots. For the purification of calmodulin-binding proteins (Ca2+ pumps), microsomes isolated from tobacco cell cultures were used. Two active Ca2+ pumps were identified, and the one at 116 kDa was partially sequenced.

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.

Dynamic properties of an inositol 1,4,5-trisphosphate- and thapsigargin-insensitive calcium pool in mammalian cell lines

The functional characteristics of a nonacidic, inositol 1,4,5-trisphosphate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ pool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensitive store with an agonist coupled to phospholipase C activation and/or thapsigargin. The amount of Ca2+ trapped within this pool increased significantly after a prolonged elevation of intracellular Ca2+ concentration elicited by activation of Ca2+ influx. This pool was affected neither by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distribution across the cytosol, as revealed by confocal microscopy, and its insensitivity to brefeldin A make localization within the Golgi complex unlikely. A proton gradient as the driving mechanism for Ca2+ uptake was excluded since ionomycin is inefficient in releasing Ca2+ from acidic pools and Ca2+ accumulation/release in/from this store was unaffected by monensin or NH4Cl, drugs known to collapse organelle acidic pH gradients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, distinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cytological nature and functional role of this Ca2+ storage compartment are discussed.

Ca(2+)-transporting ATPase(s) of the reticulum type in intracellular membranes of Saccharomyces cerevisiae: biochemical identification

Several lines of evidence are presented to show that the Ca(2+)-ATPase activity of total yeast membranes is due to the reticulum (R) type of Ca(2+)-ATPase: (1) Neither calmodulin nor low concentrations of calmodulin antagonists change Ca2+ uptake; (2) removal of plasma membranes (PM) following Con A treatment of spheroplasts (SP) does not significantly alter Ca2+ uptake by the remaining membranes, but increases its specific activity 3.5-fold; (3) after incubation of membranes with [gamma-32P]ATP, SDS-PAGE shows the formation of acyl phosphate intermediates with molecular masses of around 100, 180-190 and 205 kDa; formation of these acyl phosphates requires Ca2+ and is blocked by cyclopiazonic acid, La3+ ions and in the absence of Ca2+. The data on fractionation of yeast membranes are consistent with the suggestion that both the ER and the Golgi are equipped with Ca(2+)-ATPase(s).

Modulation of an Intracellular Calmodulin-Stimulated Ca2+-Pumping ATPase in Cauliflower by Trypsin (The Use of Calcium Green-5N to Measure Ca2+ Transport in Membrane Vesicles)

The effect of controlled trypsin digestion of a calmodulin-stimulated Ca2+-ATPase in low-density intracellular membranes from cauliflower (Brassica oleracea L.) inflorescences was investigated. Ca2+ uptake into vesicles was measured either continuously with the fluorescent Ca2+ indicator Calcium Green-5N or with a radio-active filter technique. Trypsin treatment of vesicles resulted in a 3-fold activation of Ca2+ uptake and loss of calmodulin sensitivity. Immunoblotting experiments with an antiserum raised against the Ca2+-ATPase showed that the trypsin activation was accompanied by a decrease in the amount of intact Ca2+-ATPase (111 kD) and by successive appearances of polypeptides of 102 and 99 to 84 kD. 125I-Calmodulin overlays showed that only the intact Ca2+-ATPase bound calmodulin. Removal of the calmodulin-binding domain (about 9 kD) was not enough to obtain full activation. Trypsin proteolysis resulted in a Ca2+ concentration necessary for half-maximal activity of 0.5 [mu]M, whereas a value of about 2 [mu]M was obtained with untreated membranes in the presence of calmodulin. Without trypsin treatment or calmodulin the activity was not saturated even at 57 [mu]M free Ca2+. The data suggest that trypsin digestion and calmodulin activate the cauliflower Ca2+-ATPase by at least partly different mechanisms.

Molecular cloning of an intracellular P-type ATPase from Dictyostelium that is up-regulated in calcium-adapted cells

Results from a number of laboratories suggest that intracellular Ca2+ is involved in the regulation of Dictyostelium discoideum growth and development. To learn more about the regulation and function of intracellular Ca2+ in this organism, we have cloned and sequenced cDNAs that encode a putative P-type Ca2+ ATPase designated patA. The deduced protein product of this gene (PAT1) has a calculated molecular mass of 120,718 daltons. It exhibits about 46% amino acid identity with Ca2+ ATPases of the plasma membrane Ca2+ ATPase family and lower identity with sarco(endo)plasmic reticulum Ca2+ ATPase family members and monovalent cation pumps. However, PAT1 lacks the highly conserved calmodulin-binding domain present in the C-terminal region of most plasma membrane Ca2+ ATPase-type enzymes. When Dictyostelium amoebae are adapted to grow in the presence of 80 mM CaCl2, both the patA message and protein product are up-regulated substantially. These cells also exhibit an increase in the rate and magnitude of intracellular P-type Ca2+ uptake activity. Immunofluorescence analysis indicates that PAT1 colocalizes with bound calmodulin to intracellular membranes, probably components of the contractile vacuole complex. The presence of PAT1 on the contractile vacuole suggests that in Dictyostelium this organelle might function in Ca2+ homeostasis as well as in water regulation.

Identification of the Plasma Membrane Ca2+-ATPase and of Its Autoinhibitory Domain

The effect of controlled proteolysis on the plasma membrane (PM)Ca2+-ATPase was studied at the molecular level in PM purified from radish (Raphanus sativus L.) seedlings. Two new methods for labeling the PM Ca2+-ATPase are described. The PM Ca2+-ATPase can be selectively labeled by treatment with micromolar fluorescein isothiocyanate (FITC), a strong inhibitor of enzyme activity. Both inhibition of activity and FITC binding to the PM Ca2+-ATPase are suppressed by millimolar MgITP. The PM Ca2+-ATPase maintains the capability to bind calmodulin also after sodium dodecyl sulfate gel electrophoresis and blotting; therefore, it can be conveniently identified by 125l-calmodulin overlay in the presence of calcium. With both methods a molecular mass of 133 kD can be calculated for the PM Ca2+-ATPase. FITC-labeled PM Ca2+-ATPase co-migrates with the phosphorylated intermediate of the enzyme[mdash]labeled by incubation with [[gamma]-32P]GTP in the presence of calcium[mdash]on acidic sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Controlled trypsin treatment of purified PM determines a reduction of the molecular mass of the PM Ca2+-ATPase from 133 to 118 kD parallel to the increase of enzyme activity. Only the 133-kD but not the 118-kD PM Ca2+-ATPase binds calmodulin. These results indicate that trypsin removes from the PM Ca2+-ATPase an autoinhibitory domain that contains the calmodulin-binding domain of the enzyme.

Characterisation of an intracellular Ca2+ pump in Dictyostelium

Calcium uptake by microsomal membranes from the cellular slime mould Dictyostelium discoideum was measured using Calcium Green-2 as a fluorescent probe of external free Ca2+ concentration. High-affinity Ca2+ uptake was found to be completely inhibited by low concentrations of vanadate, but not by thapsigargin, suggesting that the activity is mediated by a Ca(2+)-ATPase distinct from sarco(endo)plasmic reticulum type of higher animal cells. On sucrose density gradients, Ca2+ uptake distributes with vacuolar proton pump activity and part of the observed Ca2+ uptake is dependent on the pH gradient generated by the vacuolar-type H(+)-ATPase, indicating that the Ca2+ pump is located on both acidic and non-acidic vesicles, possibly derived from the H(+)-ATPase-rich contractile vacuole complex.

A Study of the Effect of Inhibitors of the Animal Sarcoplasmic/Endoplasmic Reticulum-Type Calcium Pumps on the Primary Ca2+-ATPases of Red Beet

The inhibitor sensitivity of the endoplasmic reticulum (ER) and plasma membrane (PM) calcium pumps of red beet (Beta vulgaris L.) were studied by measuring the ATP-driven accumulation of 45Ca2+ into isolated membrane vesicles. Both transporters were strongly inhibited by 50 [mu]mol m-3 erythrosin B, but only by 50% in the presence of 100 mmol m-3 vanadate. A number of inhibitors considered to be specific for the sarcoplasmic reticulum (SR)/ER-type calcium pump in animal cells were used to further characterize the PM and ER Ca2+-ATPases in red beet and were compared with their effect on the transport and hydrolytic activities of the PM and tonoplast H+-ATPases. The hydroquinones 2,5-di(tert-butyl)-1,4-benzohydroquinone and 2,5-di(tert-amyl)-1,4-benzohydroquinone produced around 20 and 40% inhibition of activity, respectively, of the PM and ER calcium pumps and the PM H+-ATPase when present at concentrations of 30 mmol m-3. In contrast, the vacuolar proton pump displayed a much higher sensitivity to these two compounds. Nonylphenol appeared to have a general inhibitory effect on all four membrane transport proteins and gave almost complete inhibition when present at a concentration of 100 mmol m-3. Thapsigargin and the structurally related compound trilobolide produced 50% inhibition of both the ER and PM calcium pumps at concentrations of 12.5 and 24 mmol m-3, respectively. The PM and tonoplast proton pumps were also sensitive to these compounds. The ER and PM calcium pumps were almost completely insensitive to cyclopiazonic acid (CPA) up to a concentration of 20 mmol m-3. When present at 100 mmol m-3 CPA caused 30% inhibition of the transport properties of all four ATPases. The high concentrations of all of the inhibitors of the SR/ER Ca-ATPase required to inhibit the red beet ER calcium pump, together with the similar effects on the PM calcium pump and the PM and tonoplast proton pumps, suggests that these hydrophobic compounds have a general nonselective action in red beet, possibly through disruption of membrane lipid-protein interactions.

PM-type calcium pumps are associated with higher plant cell intracellular membranes

The supposition that all eukaryotic cell types contain a plasma membrane (PM)-type Ca pump (i.e. a Ca pump which is directly-stimulated by calmodulin and located exclusively at the PM) has been questioned by recent data from higher plant cells. These studies suggest the presence of Ca pumps directly stimulated by calmodulin associated with an intracellular membrane (probably the endoplasmic reticulum, ER) in a variety of monocotelydonous and dicotelydonous species. Thus plants have a 'PM-type' Ca pump at an intracellular membrane. The evidence for this includes studies on isolated membranes, purification and functional reconstitution and phosphorylated intermediate formation. Plant cells also contain a homologue of the sarcoplasmic reticulum/endoplasmic reticulum (SR/ER) Ca pump, probably located at the ER. The implications of these new data for our appreciation of the structure, function and location of eukaryotic Ca pumps are discussed, together with recent data from the use of inhibitors specific to mammalian ER/SR Ca pumps.

Ca2+-Calmodulin Modulates Ion Channel Activity in Storage Protein Vacuoles of Barley Aleurone Cells

Many plant ion channels have been identified, but little is known about how these transporters are regulated. We have investigated the regulation of a slow vacuolar (SV) ion channel in the tonoplast of barley aleurone storage protein vacuoles (SPV) using the patch-clamp technique. SPV were isolated from barley aleurone protoplasts incubated with CaCl2 in the presence or absence of gibberellic acid (GA) or abscisic acid (ABA). A slowly activating, voltage-dependent ion channel was identified in the SPV membrane. Mean channel conductance was 26 pS when 100 mM KCl was on both sides of the membrane, and reversal potential measurements indicated that most of the current was carried by K+. Treatment of protoplasts with GA3 increased whole-vacuole current density compared to SPV isolated from ABA- or CaCl2-treated cells. The opening of the SV channel was sensitive to cytosolic free Ca2+ concentration ([Ca2+]i) between 600 nM and 100 [mu]M, with higher [Ca2+]i resulting in a greater probability of channel opening. SV channel activity was reduced greater than 90% by the calmodulin (CaM) inhibitors W7 and trifluoperazine, suggesting that Ca2+ activates endogenous CaM tightly associated with the membrane. Exogenous CaM partially reversed the inhibitory effects of W7 on SV channel opening. CaM also sensitized the SV channel to Ca2+. In the presence of ~3.5 [mu]M CaM, specific current increased by approximately threefold at 2.5 [mu]M Ca2+ and by more than 13-fold at 10 [mu]M Ca2+. Since [Ca2+]i and the level of CaM increase in barley aleurone cells following exposure to GA, we suggest that Ca2+ and CaM act as signal transduction elements mediating hormone-induced changes in ion channel activity.