Characterization of a vacuolar pyrophosphatase in Trypanosoma brucei and its localization to acidocalcisomes

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Acidification of the malaria parasite's digestive vacuole by a H+-ATPase and a H+-pyrophosphatase

As it grows within the human erythrocyte, the malaria parasite, Plasmodium falciparum, ingests the erythrocyte cytosol, depositing it via an endocytotic feeding mechanism in the "digestive vacuole," a specialized acidic organelle. The digestive vacuole is the site of hemoglobin degradation, the storage site for hemozoin (an inert biocrystal of toxic heme), the site of action of many antimalarial drugs, and the site of proteins known to be involved in antimalarial drug resistance. The acidic pH of this organelle is thought to play a critical role in its various functions; however, the mechanisms by which the pH within the vacuole is maintained are not well understood. In this study, we have used a combination of techniques to demonstrate the presence on the P. falciparum digestive vacuole membrane of two discrete H(+) pumping mechanisms, both capable of acidifying the vacuole interior. One is a V-type H(+)-ATPase, sensitive to concanamycin A and bafilomycin A(1). The other is a H(+)-pyrophosphatase, which was inhibited by NaF and showed a partial dependence on K(+). The operation of the H(+)-pyrophosphatase was dependent on the presence of a Mg(2+)-pyrophosphate complex, and kinetic experiments gave results consistent with free pyrophosphate acting as an inhibitor of the protein. The presence of the combination of a H(+)-ATPase and a H(+)-pyrophosphatase on the P. falciparum digestive vacuole is similar to the situation in the acidic tonoplasts (vacuoles) of plant cells.

Isolation and characterization of TgVP1, a type I vacuolar H+-translocating pyrophosphatase from Toxoplasma gondii. The dynamics of its subcellular localization and the cellular effects of a diphosphonate inhibitor

Here we report the isolation and characterization of a type I vacuolar-type H(+)-pyrophosphatase (V-PPase), TgVP1, from an apicomplexan, Toxoplasma gondii, a parasitic protist that is particularly amenable to molecular and genetic manipulation. The 816-amino acid TgVP1 polypeptide is 50% sequence-identical (65% similar) to the prototypical type I V-PPase from Arabidopsis thaliana, AVP1, and contains all the sequence motifs characteristic of this pump category. Unlike AVP1 and other known type I enzymes, however, TgVP1 contains a 74-residue N-terminal extension encompassing a 42-residue N-terminal signal peptide sequence, sufficient for targeting proteins to the secretory pathway of T. gondii. Providing that the coding sequence for the entire N-terminal extension is omitted from the plasmid, transformation of Saccharomyces cerevisiae with plasmid-borne TgVP1 yields a stable and functional translation product that is competent in aminomethylenediphosphonate (AMDP)-inhibitable K(+)-activated pyrophosphate (PP(i)) hydrolysis and PP(i)-energized H(+) translocation. Immunofluorescence microscopy of both free and intracellular T. gondii tachyzoites using purified universal V-PPase polyclonal antibodies reveals a punctate apical distribution for the enzyme. Equivalent studies of the tachyzoites during host cell invasion, by contrast, disclose a transverse radial distribution in which the V-PPase is associated with a collar-like structure that migrates along the length of the parasite in synchrony with and in close apposition to the penetration furrow. Although treatment of T. gondii with AMDP concentrations as high as 100 microm had no discernible effect on the efficiency of host cell invasion and integration, concentrations commensurate with the I(50) for the inhibition of TgVP1 activity in vitro (0.9 microm) do inhibit cell division and elicit nuclear enlargement concomitant with the inflation and eventual disintegration of acidocalcisome-like vesicular structures. A dynamic association of TgVP1 with the host cell invasion apparatus is invoked, one in which the effects of inhibitory V-PPase substrate analogs are exerted after rather than during host cell invasion.

An acidocalcisomal exopolyphosphatase from Leishmania major with high affinity for short chain polyphosphate

We report the cloning, overexpression, purification, and characterization of the Leishmania major exopolyphosphatase (LmPPX). The product of this gene (LmPPX), the first related to polyphosphate (polyP) metabolism isolated from an eukaryotic organism different from yeast, has 388 amino acids and a molecular mass of 48 kDa. LmPPX differs from other exopolyphosphatases previously investigated. Heterologous expression of LmPPX in Escherichia coli produced a functional enzyme that was similar to the yeast exopolyphosphatase with respect to its Mg(2+) requirement, optimum pH, and sensitivity to cations, amino acids, and heparin but that, in contrast to the yeast enzyme and other exopolyphosphatases investigated before, acts on polyP of short chain lengths with higher rates and affinity. LmPPX is a processive enzyme, and it does not hydrolyze pyrophosphate, ATP, or p-nitrophenylphosphate. Confocal immunofluorescence microscopy using affinity-purified antibodies against the recombinant enzyme indicated an acidocalcisomal and cytosolic localization. High levels of short chain (21.4 +/- 3.0 mm) and long chain polyP (55.9 +/- 5.6 mm) were detected in L. major promastigotes. The unique characteristics of LmPPX and L. major polyP metabolism may facilitate the development of novel antileishmanial agents.

Characterization of isolated acidocalcisomes from Toxoplasma gondii tachyzoites reveals a novel pool of hydrolyzable polyphosphate

Toxoplasma gondii tachyzoites were fractionated by modification of an iodixanol density gradient method previously used for acidocalcisome isolation from Trypanosoma cruzi epimastigotes. Fractions were characterized using electron microscopy, x-ray microanalysis, and enzymatic markers, and it was demonstrated that the heaviest (pellet) fraction contains electron-dense vacuoles rich in phosphorus, calcium, and magnesium, as found before for acidocalcisomes. Staining with 4',6-diamidino-2-phenylindole (DAPI) indicated that poly- phosphate (polyP) was preferentially localized in this fraction together with pyrophosphate (PP(i)). Using an enzyme-based method, millimolar levels (in terms of P(i) residues) of polyP chains of less than 50 residues long and micromolar levels in polyP chains of about 700-800 residues long were found to be preferentially localized in this fraction. The fraction also contained the pyrophosphatase and polyphosphatase activities characteristic of acidocalcisomes. Western blot analysis using antibodies against proteins from micronemes, dense granules, rhoptries, and plasma membrane showed that the acidocalcisomal fraction was not contaminated by these other organelles. T. gondii polyP levels rapidly decreased upon exposure of the parasites to a calcium ionophore (ionomycin), to an inhibitor of the V-H(+)-ATPase (bafilomycin A(1)), or to the alkalinizing agent NH(4)Cl. These changes were in parallel to an increase in intracellular Ca(2+) concentration, suggesting a close association between polyP hydrolysis and Ca(2+) release from the acidocalcisome. These results provide a useful method for the isolation and characterization of acidocalcisomes, showing that they are distinct from other previously recognized organelles present in T. gondii, and provide evidence for the role of polyP metabolism in response to cellular stress.

Physiological and morphological evidences for the presence acidocalcisomes in Trypanosoma evansi: single cell fluorescence and 31P NMR studies

A new Ca(2+) intracellular store, the acidocalcisome, has been reported in trypanosomatids. It has been characterized physiologically as a Ca(2+) store sensitive to nigericin. The Ca(2+)/H(+)-ATPase is the system responsible for Ca(2+) accumulation, which depends on a pH gradient formed by ATP- and PPi-dependent proton pumps. In this work we present physiological and morphological evidences for the presence of acidocalcisomes in Trypanosoma evansi. The parasites were purified and loaded with the fluorescent dye Fura 2-AM in order to detect the intracellular changes of Ca(2+) levels in individual cells. The simultaneous incubation of T. evansi cells with ionomycin and nigericin led to large release of Ca(2+) (ca. 200 nM) from intracellular stores, which was not observed with either agent alone. On the other hand, no enhancement of the nigericin-induced Ca(2+) release was observed in the presence of oligomycin. Additionally, the pretreatment with bafilomycin decreases the nigericin-induced Ca(2+) release. These results confirm the presence of an intracellular non-mitochondrial acidic Ca(2+) storage compartment. These results suggest that H(+)-ATPase is involved in the process of Ca(2+) accumulation into the acidocalcisomes. Furthermore, the cells loaded with acridine orange exhibited abundant fluorescent vacuoles, which were sensitive to nigericin or bafilomycin A(1). Electronic transmission microscopy observations demonstrated the presence of electron dense particles in the parasites. High levels of inorganic pyrophosphate and triphosphate were detected in perchloric acid extracts of T. evansi by high resolution 31P NMR. Taken together, these results present the first evidence for the presence of acidocalcisomes in T. evansi.

A vacuolar-type H+-pyrophosphatase governs maintenance of functional acidocalcisomes and growth of the insect and mammalian forms of Trypanosoma brucei

Vacuolar proton pyrophosphatases (V-H(+)-PPases) are electrogenic proton pumps found in many organisms of considerable industrial, environmental, and clinical importance. V-H(+)-PPases of several parasites were shown to be associated with acidic vacuoles named acidocalcisomes, which contain polyphosphate and calcium. In this work we functionally characterized a Trypanosoma brucei V-H(+)-PPase gene by using double-stranded RNA interference methodology to produce inducible V-H(+)-PPase-deficient strains of procyclic and bloodstream forms (PFiVP1 and BFiVP1). Acidocalcisomes of these mutated parasites lost acidity and contained 90% less polyphosphate. PFiVP1 did not release calcium after the addition of nigericin, and its total acidity was reduced by 70%. This mutant also failed to stabilize its intracellular pH on exposure to external basic pH >7.4 and recovered from intracellular acidification at a slower rate and to a more acidic final intracellular pH. In the absence of T. brucei V-H(+)-PPase expression, PFiVP1 and BFiVP1 grew at a slower rate with doubling times of 27 h instead of 15 h, and 10 h instead of 7.5 h, respectively. Moreover, BFiVP1 could not grow over 5 x 10(5) cells/ml corresponding to a cell density reduction of five times for bloodstream form stationary phase growth.

Isolation and characterization of a reservosome fraction from Trypanosoma cruzi

Reservosomes are acidic compartments present at the posterior region of epimastigote forms of Trypanosoma cruzi that store proteins and lipids. During metacyclogenesis, they consume their contents and disappear. Reservosomes are rich in cruzipain, the main proteolytic enzyme of this parasite. By centrifugation in a sucrose gradient, we have obtained a highly purified subcellular fraction containing reservosomes from 5-day-old Y strain epimastigotes. Transmission electron microscopy showed that the fraction contained well-preserved organelles. The protein profile of the organelle analyzed by SDS-PAGE depicted a wide range of protein bands, predominating those corresponding to a triplet of 60-51 kDa and a doublet of 25-23 kDa. Protease activity in substrate-containing gels, in the presence or absence of protease inhibitors, showed that cysteine proteinase is enriched and very active in the purified fraction. Enzymatic assays demonstrated the absence of pyrophosphatase, an acidocalcisome marker, and succinate cytochrome c reductase, a mitochondrial marker, although these enzymes were active in other regions of the purification sucrose gradient. Thin layer chromatographic neutral lipid analysis of purified reservosomes demonstrated that the organelle stores large amounts of ergosterol and esterified cholesterol. Phospholipid analysis indicated phosphatidylcholine and phosphatidylethanolamine as the major constituents of reservosome membranes.

Magic-angle spinning (31)P NMR spectroscopy of condensed phosphates in parasitic protozoa: visualizing the invisible

We report the results of a solid-state (31)P nuclear magnetic resonance (NMR) spectroscopic investigation of the acidocalcisome organelles from Trypanosoma brucei (bloodstream form), Trypanosoma cruzi and Leishmania major (insect forms). The spectra are characterized by a broad envelope of spinning sidebands having isotropic chemical shifts at approximately 0, -7 and -21 ppm. These resonances are assigned to orthophosphate, terminal (alpha) phosphates of polyphosphates and bridging (beta) phosphates of polyphosphates, respectively. The average polyphosphate chain length is approximately 3.3 phosphates. Similar results were obtained with whole L. major promastigotes. (31)P NMR spectra of living L. major promastigotes recorded under conventional solution NMR conditions had spectral intensities reduced with respect to solution-state NMR spectra of acid extracts, consistent with the invisibility of the solid-state phosphates. These results show that all three parasites contain large stores of condensed phosphates which can be visualized by using magic-angle spinning NMR techniques.

Evidence for a wide occurrence of proton-translocating pyrophosphatase genes in parasitic and free-living protozoa

Proton-translocating inorganic pyrophosphatases (H(+)-PPase, EC 3.6.1.1) are integral membrane proteins that have been extensively studied in higher plants, the photosynthetic bacterium Rhodospirillum rubrum and, more recently, in some human pathogenic protozoa. By using a PCR-based approach, fragments of genes coding for H(+)-PPases in a number of protists, both free-living and parasites of animals and plants, that belong to diverse taxonomic groups (trypanosomatids, ciliates, apicomplexans, euglenoids, amoeboid mycetozoa, heterokonts) have been isolated. The experimental procedure involved the use of degenerate oligonucleotides designed from protein domains conserved in H(+)-PPases from plants and bacteria. The PCR-amplified DNA fragments exhibited the characteristic genomic structure and codon usage of the corresponding protozoan group. Paralogous genes were found in some species suggesting the occurrence of protein isoforms. These results indicate that H(+)-PPases are more widely distributed among protozoa than previously thought.

Acidocalcisomes are functionally linked to the contractile vacuole of Dictyostelium discoideum

The mass-dense granules of Dictyostelium discoideum were shown to contain large amounts of phosphorus, magnesium, and calcium, as determined by x-ray microanalysis, either in situ or when purified using iodixanol gradient centrifugation. The high phosphorus content was due to the presence of pyrophosphate and polyphosphate, which were also present in the contractile vacuoles. Both organelles also possessed a vacuolar H(+)-ATPase, an H(+)-pyrophosphatase, and a Ca(2+)-ATPase, as determined by biochemical methods or by immunofluorescence microscopy. The H(+)-pyrophosphatase activity of isolated mass-dense granules was stimulated by potassium ions and inhibited by the pyrophosphate analogs aminomethylenediphosphonate and imidodiphosphate and by KF and N-ethylmaleimide in a dose-dependent manner. The mass-dense granules and the contractile vacuole appeared to contact each other when the cells were submitted to hyposmotic stress. Acetazolamide inhibited the carbonic anhydrase activity of the contractile vacuoles and prolonged their contraction cycle in a dose-dependent manner. Similar effects were observed with the anion exchanger inhibitor 4,4' -diisothiocyanatodihydrostilbene-2, 2' -disulfonic acid and the vacuolar H(+)-ATPase inhibitor bafilomycin A(1). Together, these results suggest that the mass-dense granules of D. discoideum are homologous to the acidocalcisomes described in protozoan parasites and are linked to the function of the contractile vacuole.

Vacuolar type H+ pumping pyrophosphatases of parasitic protozoa

Trans-membrane proton pumping is responsible for a myriad of physiological processes including the generation of proton motive force that drives bioenergetics. Among the various proton pumping enzymes, vacuolar pyrophosphatases (V-PPases) form a distinct class of proton pumps, which are characterised by their ability to translocate protons across a membrane by using the potential energy released by hydrolysis of the phosphoanhydride bond of inorganic pyrophosphate. Until recently, V-PPases were known to be the purview of only plant vacuoles and plasma membranes of phototrophic bacteria. Recent discoveries of V-PPases in kinetoplastid and apicomplexan parasites, however, have expanded our view of the evolutionary reach of these enzymes. The lack of V-PPases in the vertebrate hosts of these parasites makes them potentially excellent targets for developing broad-spectrum antiparasitic agents. This review surveys the current understanding of V-PPases in parasitic protozoa with an emphasis on malaria parasites. Topological predictions suggest remarkable similarity of the parasite enzymes to their plant homologues with 15-16 membrane spanning domains and conserved sequences shown to constitute critical catalytic residues. Remarkably, malaria parasites have been shown to possess two V-PPase genes, one is an apparent orthologue of the canonical plant enzyme, whereas the other is a more distantly related paralogue with homology to a recently identified new class of K+-insensitive plant V-PPases. V-PPases appear to localise both to the plasma membrane and cytoplasmic organelles believed to be acidocalcisomes or polyphosphate bodies. Gene transfer experiments suggest that one of the malarial V-PPases is predominantly localised to the surface of intraerythrocytic parasites. We suggest a model in which V-PPase localised to the malaria parasite plasma membrane may serve as an electrogenic pump utilising pyrophosphate as an energy source, thus sparing the more precious ATP. Searching for V-PPase inhibitors could prove fruitful as a novel means of antiparasitic chemotherapy.

A novel C-terminal kinesin is essential for maintaining functional acidocalcisomes in Trypanosoma brucei

Kinesins are cytoskeletal motor proteins that play roles in a variety of fundamental cellular processes including cell division and the anterograde transport of vesicles and organelles. We purified, cloned, and functionally characterized in Trypanosoma brucei a new member of the C-terminal kinesin family, TbKIFC1. Kinetic constants of the recombinant motor domain of TbKIFC1 were estimated at 0.56 microm for the microtubule dissociation constant (K(d)) with a k(cat) of 0.2 s(-1). Immunolocalization analysis showed an association of TbKIFC1 with punctate structures. Because they were rapidly transported to the negative pole of the microtubule after NH(4)Cl treatment, these structures were considered to be associated with acidic vesicles. To determine the role of the kinesin in vivo, we produced an inducible kinesin-deficient strain by double-stranded RNA interference methodology. Mutant cells were loaded with the fluorescent reagent fura2/acetoxymethylester to measure intracellular free calcium ([Ca(2+)](i)). The resting [Ca(2+)](i) was unchanged in mutant cells; however, alkalinization of acidic vesicles induced by NH(4)Cl or nigericin was not followed by release of Ca(2+). These data and the relative importance of the ionomycin-releasable and the ionomycin-plus-NH(4)Cl-releasable Ca(2+) pools suggest a lower Ca(2+) content in acidocalcisomes and dysfunctional Ca(2+) release.

An investigation of bisphosphonate inhibition of a vacuolar proton-pumping pyrophosphatase

We report the results of a three-dimensional quantitative structure-activity relationship (3D-QSAR)/comparative molecular field analysis (CoMFA) of the activity of 18 bisphosphonates and imidodiphosphate in the inhibition of a mung bean (Vigna radiata L.) vacuolar proton pumping pyrophosphatase (V/H(+)-PPase; EC 3.6.1.1). We find an experimental versus QSAR predicted pK(app)(i) R(2) value of 0.89, a cross-validated R(2) = 0.77, and a bootstrapped R(2) = 0.89 for 18 bisphosphonates plus imidodiphosphate over the 1.3 microM to 425 microM range of K(app)(i) values. We also demonstrate that this approach has predictive utility (a 0.26 pK(app)(i) rms error for three test sets of 3 activity predictions each), corresponding to about a factor of two error in K(app)(i) prediction. The 3D-QSAR/CoMFA approach provides a quantitative method for predicting the activity of V/H(+)-PPase inhibitors and is likely to be of use in the design of novel pharmacological agents since all of the major human disease-causing parasitic protozoa contain large levels of pyrophosphate, together with V-type proton-pumping pyrophosphatases located in plant-like vacuoles (acidocalcisomes), which are absent in their mammalian hosts.

The sterol composition of Trypanosoma cruzi changes after growth in different culture media and results in different sensitivity to digitonin-permeabilization

Respiration, oxidative phosphorylation. and the corresponding changes in membrane potential (deltapsi) of Trypanosoma cruzi epimastigotes grown either in liver infusion-tryptose (LIT) or brain heart infusion (BHI) culture medium were assayed in situ using digitonin to render their plasma membrane permeable to succinate, ADP, safranine O, and other small molecules. When the cells were permeabilized with 64 microM digitonin, a concentration previously used with epimastigotes, the ability of the cells grown in LIT medium to sustain oxidative phosphorylation was demonstrated by the detection of an oligomycin-sensitive decrease in mitochondrial membrane potential induced by ADP. In contrast, the cells grown in BHI medium were not able to sustain a stable membrane potential and did not respond to ADP addition. Analyses of oxygen consumption by these permeabilized cells indicated that the rate of basal respiration, which was similar in both cell types, was significantly decreased by 64 microM digitonin. Addition of ADP to the permeabilized cells grown in LIT medium promoted an oligomycin-sensitive transition from resting to phosphorylating respiration in contrast to the cells grown in BHI medium, whose respiration decreased steadily and did not respond either to ADP or CCCP. Titration of the cells grown in BHI medium with different digitonin concentrations indicated that their mitochondria have higher sensitivity to digitonin than those grown in LIT medium. Analysis of the sterol composition of epimastigotes grown in the two different media showed a higher percentage of cholesterol in total and mitochondrial extracts of epimastigotes grown in BHI medium as compared to those grown in LIT medium, suggesting the involvement of this sterol in their increased sensitivity to digitonin-permeabilization.

Rapid Changes in Polyphosphate Content within Acidocalcisomes in Response to Cell Growth, Differentiation, and Environmental Stress inTrypanosoma cruzi

Inorganic polyphosphate (polyP) has been identified and measured in different stages of Trypanosoma cruzi. Millimolar levels (in terms of P(i) residues) in chains of less than 50 residues long, and micromolar levels in chains of about 700--800 residues long, were found in different stages of T. cruzi. Analysis of purified T. cruzi acidocalcisomes indicated that polyPs were preferentially located in these organelles. This was confirmed by visualization of polyPs in the acidocalcisomes using 4',6-diamidino-2-phenylindole. A rapid increase (within 2--4 h) in the levels of short and long chain polyPs was detected during trypomastigote to amastigote differentiation and during the lag phase of growth of epimastigotes (within 12--24 h). Levels rapidly decreased after the epimastigotes resumed growth. Short and long chain polyP levels rapidly decreased upon exposure of epimastigotes to hypo-osmotic or alkaline stresses, whereas levels increased after hyperosmotic stress. Ca(2+) release from acidocalcisomes by a combination of ionophores (ionomycin and nigericin) was associated with the hydrolysis of short and long chain polyPs. In agreement with these results, acidocalcisomes were shown to contain polyphosphate kinase and exopolyphosphatase activities. Together, these results suggest a critical role for these organelles in the adaptation of the parasite to environmental changes.

Structural analysis of adenylate cyclases from Trypanosoma brucei in their monomeric state

Cyclic AMP is a major trigger of the differentiation process of Trypanosoma brucei, a bloodstream parasite causing sleeping sickness. Its generation in trypanosomes is accomplished by a unique battery of membrane-bound adenylate cyclases (ACs). We have determined the high-resolution X-ray structures of the catalytic domains of two trypanosomal ACs (tACs), GRESAG4.1 and GRESAG4.3. The tAC domains are structurally highly related to the AC domains of higher eukaryotes, but also comprise a highly conserved structural element near the active site, the Delta-subdomain. A cavity below the Delta-subdomain might correspond to an allosteric regulator site as indicated by the stereospecific binding of a single (2S,3S)-1,4- dimercapto-2,3-butanediol molecule. In three different crystal forms, the tAC domains are exclusively observed in a monomeric, catalytically inactive state. Biochemical analysis and the mutagenesis profile of GRESAG4.1 confirmed a common catalytic mechanism of tACs that involves transient dimerization of the AC domain. A low dimerization tendency might play a regulatory role in T. brucei if the activation of tACs is similarly driven by ligand-induced dimerization as in membrane-bound guanylate cyclases.

A plasma membrane-type Ca(2+)-ATPase co-localizes with a vacuolar H(+)-pyrophosphatase to acidocalcisomes of Toxoplasma gondii

Ca(2+)-ATPases are likely to play critical roles in the biochemistry of Toxoplasma gondii, since these protozoa are obligate intracellular parasites and the Ca(2+) concentration in their intracellular location is three orders of magnitude lower than in the extracellular medium. Here, we report the cloning and sequencing of a gene encoding a plasma membrane-type Ca(2+)-ATPase (PMCA) of T.gondii (TgA1). The predicted protein (TgA1) exhibits 32-36% identity to vacuolar Ca(2+)-ATPases of Trypanosoma cruzi, Saccharomyces cerevisiae, Entamoeba histolytica and Dictyostelium discoideum. Sequencing of both cDNA and genomic DNA from T.gondii indicated that TgA1 contains two introns near the C-terminus. A hydropathy profile of the protein suggests 10 transmembrane domains. TgA1 suppresses the Ca(2+) hypersensitivity of a mutant of S.cerevisiae that has a defect in vacuolar Ca(2+) accumulation. Indirect immunofluorescence and immunoelectron microscopy analysis indicate that TgA1 localizes to the plasma membrane and co-localizes with the vacuolar H(+)-pyrophosphatase to intracellular vacuoles identified morphologically and by X-ray microanalysis as the acidocalcisomes. This vacuolar-type Ca(2+)-ATPase could play an important role in Ca(2+) homeostasis in T.gondii.

Cloning and functional expression of a gene encoding a vacuolar-type proton-translocating pyrophosphatase from Trypanosoma cruzi

Acidocalcisomes are acidic Ca(2+)-storage organelles found in trypanosomatids that are similar to organelles known historically as volutin granules. Acidification of these organelles is driven in part by a vacuolar H(+)-pyrophosphatase (V-H(+)-PPase), an enzyme that is also present in plant vacuoles and in some bacteria. Here, we report the cloning and sequencing of a gene encoding the acidocalcisomal V-H(+)-PPase of Trypanosoma cruzi. The protein (T. cruzi pyrophosphatase, TcPPase) predicted from the nucleotide sequence of the gene has 816 amino acids and a molecular mass of 85 kDa. Several sequence motifs found in plant V-H(+)-PPases were present in TcPPase, explaining its sensitivity to N-ethylmaleimide and N,N'-dicyclohexylcarbodi-imide. Heterologous expression of the cDNA encoding TcPPase in the yeast Saccharomyces cerevisiae produced a functional enzyme. Phylogenetic analysis of the available V-H(+)-PPase sequences indicates that TcPPase is nearer to the vascular plant cluster and the branch containing Chara, a precursor to land plants, than to any of the other pyrophosphatase sequences included in the analysis. The apparent lack of such a V-H(+)-PPase in mammalian cells may provide a target for the development of new drugs.

31P NMR spectroscopy of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major. Evidence for high levels of condensed inorganic phosphates

High resolution (31)P nuclear magnetic resonance spectra at 303.6 MHz (corresponding to a (1)H resonance frequency of 750 MHz) have been obtained of perchloric acid extracts of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major, the causative agents of African sleeping sickness, Chagas' disease, and leishmaniasis. Essentially complete assignments have been made based on chemical shifts and by direct addition of authentic reference compounds. The results indicate the presence of high levels of short chain condensed polyphosphates: di-, tri-, tetra-, and pentapolyphosphate. (31)P NMR spectra of purified T. brucei, T. cruzi, and L. major acidocalcisomes, calcium and phosphorus storage organelles, indicate that polyphosphates are abundant in these organelles and have an average chain length of 3.11-3.39 phosphates. In the context of the recent discovery of several pyrophosphate-utilizing enzymes in trypanosomatids, the presence of these inorganic polyphosphates implies a critical role for these molecules in these parasites and a potential new route to chemotherapy.

Two special organelles found in Trypanosoma cruzi

We review here two unique organelles from Trypanosoma cruzi. One of them is the acidocalcisome, cytoplasmic vacuoles containing a very high Ca2+ concentration and a Ca2+ - H+ translocating ATPase activity, present in all trypanosomatids. The other organelle is the reservosome, site of accumulation of endocytosed macromolecules, very rich in cysteine proteinase, that is present only in epimastigote forms of trypanosomes belonging to the Schyzotrypanum sub-genus.

Characterization of isolated acidocalcisomes of Trypanosoma cruzi

The acidocalcisome is an acidic calcium store in trypanosomatids with a vacuolar-type proton-pumping pyrophosphatase (V-H(+)-PPase) located in its membrane. In this paper, we describe a new method using iodixanol density gradients for purification of the acidocalcisome from Trypanosoma cruzi epimastigotes. Pyrophosphatase assays indicated that the isolated organelle was at least 60-fold purified compared with the large organelle (10,000 x g) fraction. Assays for other organelles generally indicated no enrichment in the acidocalcisome fraction; glycosomes were concentrated 5-fold. Vanadate-sensitive ATP-driven Ca(2+) uptake (Ca(2+)-ATPase) activity was detectable in the isolated acidocalcisome, but ionophore experiments indicated that it was not acidic. However, when pyrophosphate was added, the organelle acidified, and the rate of Ca(2+) uptake increased. Use of the indicator Oxonol VI showed that V-H(+)-PPase activity generated a membrane potential. Use of sulfate or nitrate in place of chloride in the assay buffer did not affect V-H(+)-PPase activity, but there was less activity with gluconate. Organelle acidification was countered by the chloride/proton symport cycloprogidiosin. No vacuolar H(+)-ATPase activity was detectable in isolated acidocalcisomes. However, immunoblots showed the presence of at least a membrane-bound V-H(+)-ATPase subunit, while experiments employing permeabilized epimastigotes suggested that vacuolar H(+)-ATPase and V-H(+)-PPase activities are present in the same Ca(2+)-containing compartment.

Vacuolar proton pyrophosphatase activity and pyrophosphate (PPi) in Toxoplasma gondii as possible chemotherapeutic targets

The addition of PP(i) promoted the acidification of a subcellular compartment in cell homogenates of Toxoplasma gondii tachyzoites, implying the presence of a proton-translocating pyrophosphatase. The proton gradient was collapsed by addition of the K(+)/H(+) antiporter nigericin, and was also inhibited by addition of the PP(i) analogue aminomethylenediphosphonate (AMDP). Both proton transport and PP(i) hydrolysis were dependent upon K(+), but Na(+) caused partial inhibition of these activities. PP(i) hydrolysis was sensitive in a dose-dependent manner to AMDP, imidodiphosphate, NaF and to the thiol reagent N-ethylmaleimide. This activity was unaffected by common inhibitors of phosphohydrolases, except that NaO(3)V (sodium orthovanadate) stimulated the activity by 87%. Immunofluorescence microscopy, using antisera raised against conserved peptide sequences of a plant vacuolar pyrophosphatase, suggested that the pyrophosphatase in T. gondii tachyzoites was located in the plasma membrane and intracellular vacuoles of the parasite. High-field (31)P-NMR spectroscopy showed that PP(i )was more abundant than ATP in tachyzoites. Bisphosphonates (PP(i) analogues), drugs that are used in the treatment of bone diseases, inhibited proton transport and PP(i) hydrolysis in tachyzoite homogenates, and also inhibited intracellular proliferation of tachyzoites in tissue culture cells.

Presence of a Na(+)/H(+) exchanger in acidocalcisomes of Leishmania donovani and their alkalization by anti-leishmanial drugs

Acidocalcisomes are acidic vacuoles present in trypanosomatids that contain most of the cellular calcium. The data presented here demonstrate that Leishmania donovani acidocalcisomes possess a Na(+)/H(+) exchanger. 3,5-Dibutyl-4-hydroxytoluene, in the concentration range of 0-20 microM, inhibited the Na(+)/H(+) exchanger, and strongly stimulated the activity of the vacuolar H(+)-ATPase responsible for vacuolar acidification. As occurs with Na(+), the cationic anti-leishmanial drugs pentamidine, WR-6026, and chloroquine promoted a fast and extensive alkalization of the L. donovani acidocalcisomes.

Acidocalcisomes and a vacuolar H+-pyrophosphatase in malaria parasites

Plasmodium berghei trophozoites were loaded with the fluorescent calcium indicator, fura-2 acetoxymethyl ester, to measure their intracellular Ca(2+) concentration ([Ca(2+)](i)). [Ca(2+)](i) was increased in the presence of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin. Trophozoites also possess a significant amount of Ca(2+) stored in an acidic compartment. This was indicated by: (1) the increase in [Ca(2+)](i) induced by bafilomycin A(1), nigericin, monensin, or the weak base, NH(4)Cl, in the nominal absence of extracellular Ca(2+), and (2) the effect of ionomycin, which cannot take Ca(2+) out of acidic organelles and was more effective after alkalinization of this compartment by addition of bafilomycin A(1), nigericin, monensin, or NH(4)Cl. Inorganic PP(i) promoted the acidification of a subcellular compartment in cell homogenates of trophozoites. The proton gradient driven by PP(i) collapsed by addition of the K(+)/H(+) exchanger, nigericin, and eliminated by the PP(i) analogue, aminomethylenediphosphonate (AMDP). Both PP(i) hydrolysis and proton transport were dependent upon K(+), and Na(+) caused partial inhibition of these activities. PP(i) hydrolysis was sensitive in a dose-dependent manner to AMDP, imidodiphosphate, sodium fluoride, dicyclohexylcarbodi-imide and to the thiol reagent, N-ethylmaleimide. Immunofluorescence microscopy using antibodies raised against conserved peptide sequences of a plant vacuolar pyrophosphatase (V-H(+)-PPase) suggested that the proton pyrophosphatase is located in intracellular vacuoles and the plasma membrane of trophozoites. AMDP caused an increase in [Ca(2+)](i) in the nominal absence of extracellular Ca(2+). Ionomycin was more effective in releasing Ca(2+) from this acidic intracellular compartment after treatment of the cells with AMDP. Taken together, these results suggest the presence in malaria parasites of acidocalcisomes with similar characteristics to those described in trypanosomatids and Toxoplasma gondii, and the colocalization of the V-H(+)-PPase and V-H(+)-ATPase in these organelles.

Isolation of Trypanosoma brucei CYC2 and CYC3 cyclin genes by rescue of a yeast G(1) cyclin mutant. Functional characterization of CYC2

Two Trypanosoma brucei cyclin genes, CYC2 and CYC3, have been isolated by rescue of the Saccharomyces cerevisiae mutant DL1, which is deficient in CLN G(1) cyclin function. CYC2 encodes a 24-kDa protein that has sequence identity to the Neurospora crassa PREG1 and the S. cerevisiae PHO80 cyclin. CYC3 has the most sequence identity to mitotic B-type cyclins from a variety of organisms. Both CYC2 and CYC3 are single-copy genes and expressed in all life cycle stages of the parasite. To determine if CYC2 is found in a complex with previously identified trypanosome cdc2-related kinases (CRKs), the CYC2 gene was fused to the TY epitope tag, integrated into the trypanosome genome, and expressed under inducible control. CYC2ty was found to associate with an active trypanosome CRK complex since CYC2ty bound to leishmanial p12(cks1), and histone H1 kinase activity was detected in CYC2ty immune-precipitated fractions. Gene knockout experiments provide evidence that CYC2 is an essential gene, and co-immune precipitations together with a two-hybrid interaction assay demonstrated that CYC2 interacts with CRK3. The CRK3 x CYC2ty complex, the first cyclin-dependent kinase complex identified in trypanosomes, was localized by immune fluorescence to the cytoplasm throughout the cell cycle.

Calcium mobilization by arachidonic acid in trypanosomatids

A recent report (Eintracht J, Maathai R, Mellors A, Ruben L. Calcium entry in Trypanosoma brucei is regulated by phospholipase A, and arachidonic acid, Biochem J 1998:336:659-66) provided evidence that calcium entry in Trypanosoma brucei bloodstream trypomastigotes is regulated via a signaling pathway involving phospholipase A2-mediated generation of arachidonic acid and stimulation of a plasma membrane-located calcium channel. Here we show that Ca2+ influx in T. brucei procyclic trypomastigotes, Leishmania donovani promastigotes and T. cruzi amastigotes was also stimulated in a dose-dependent manner (50-400 nM) by the amphiphilic peptide melittin. This effect was blocked by the phospholipase A, inhibitor 3-(4-octadecyl)-benzoylacrylic acid. The unsaturated fatty acid arachidonic acid, in the range of 10-75 microM, induced Ca2+ entry by a mechanism sensitive to LaCl3. However, both melittin and arachidonic acid induced an increase in [Ca2+]i in T. brucei procyclic trypomastigotes incubated in Ca2+-free medium implying Ca2+ mobilization from intracellular stores. This hypothesis was supported by experiments showing that arachidonic acid promoted Ca2+ release from the acidocalcisomes of these cells. The results showing changes in mitochondrial membrane potential, release of acridine orange and Ca2+ from the acidocalcisomes and Ca2+ transport across the plasma membrane suggest that in addition to the possible stimulation of a Ca2+ channel-mediated process, arachidonic acid, in the range of concentrations used here, have other nonspecific effects on the trypanosomatids membranes.

Acidocalcisome: A novel Ca2+ storage compartment in trypanosomatids and apicomplexan parasites

Acidocalcisomes are novel acidic Ca2+ storage organelles found in trypanosomatids and apicomplexan parasites, abundant in the intracellular stages of these parasites, and characterized by their high electron density, and high content of phosphorus, Ca2+, Mg2+, Na+ and Zn2+. A number of energy-utilizing pumps and exchangers have been found in these organelles, which underlines their importance in the homeostasis of different elements, as discussed here by Roberto Docampo and Silvia Moreno.