Tyrosine phosphate hydrolysis of host proteins by Trypanosoma cruzi is linked to cell invasion

Invasion of cultured L6E9 myoblasts by Trypanosoma cruzi induced tyrosine dephosphorylation of proteins of 234, 205, and 50 kDa. Invasion was greatly reduced in the presence of the protein tyrosine phosphate inhibitors sodium o-vanadate and zinc chloride, and in the presence of excess phosphotyrosine or p-nitrophenyl phosphate, but not in the presence of excess phosphoserine or phosphothreonine. These results suggest an important role for protein tyrosine dephosphorylation in the invasion of host cells by T. cruzi.

Ca2+ content and expression of an acidocalcisomal calcium pump are elevated in intracellular forms of Trypanosoma cruzi

The survival of a eukaryotic protozoan as an obligate parasite in the interior of a eukaryotic host cell implies its adaptation to an environment with a very different ionic composition from that of its extracellular habitat. This is particularly important in the case of Ca2+, the intracellular concentration of which is 3 orders of magnitude lower than the extracellular value. Ca2+ entry across the plasma membrane is a widely recognized mechanism for Ca2+ signaling, needed for a number of intracellular processes, and obviously, it would be restricted in the case of intracellular parasites. Here we show that Trypanosoma cruzi amastigotes possess a higher Ca2+ content than the extracellular stages of the parasite. This correlates with the higher expression of a calcium pump, the gene for which was cloned and sequenced. The deduced protein product (Tca1) of this gene has a calculated molecular mass of 121,141 Da and exhibits 34 to 38% identity with vacuolar Ca2+-ATPases of Saccharomyces cerevisiae and Dictyostelium discoideum, respectively. The tca1 gene suppresses the Ca2+ hypersensitivity of a mutant of S. cerevisiae that has a defect in vacuolar Ca2+ accumulation. Indirect immunofluorescence and immunoelectron microscopy analysis indicate that Tca1 colocalizes with the vacuolar H+-ATPase to the plasma membrane and to intracellular vacuoles of T. cruzi. These vacuoles were shown to have the same size and distribution as the calcium-containing vacuoles identified by the potassium pyroantimoniate-osmium technique and as the electron-dense vacuoles observed in whole unfixed parasites by transmission electron microscopy and identified in a previous work (D. A. Scott, R. Docampo, J. A. Dvorak, S. Shi, and R. D. Leapman, J. Biol. Chem. 272:28020-28029, 1997) as being acidic and possessing a high calcium content (i.e., acidocalcisomes). Together, these results suggest that acidocalcisomes are distinct from other previously recognized organelles present in these parasites and underscore the ability of intracellular parasites to adapt to the hostile environment of their hosts.

[Generation of anti-rhEPO ScFv by using phage display technology]

In this paper, the recombinant phage antibody techniques was used to construct, clone, screen, and express anti-rhEPO single chain antibody ScFv. The variable region genes of antibody were amplified by PCR from a hybridoma cell line D3 which secreted monoclonal antibody to rhEPO. The ScFv gene fragments were successfully cloned into phagemid vector pCANTAB5E. The recombinant phages were panned by rhEPO which was coated on a microtiter plate. After three rounds of panning, 8 clones were determined specifically binding to rhEPO antigen. The positive recombinant phagemids were extracted and transformed into non-suppressed E. coli HB2151. The soluble single chain antibody was expressed, and the specificity of the expressed ScFv was determined by ELISA. Western blot and Dot-blot. The result of SDS-PAGE indicated that the apparent molecular weight of the target peptide which is mainly in culture supernatant is 32 kD. The DNA sequence data showed that the ScFv gene included 783 bp, encoding 261 amino acids.

Vacuolar-type H+-ATPase regulates cytoplasmic pH in Toxoplasma gondii tachyzoites

Cytoplasmic pH (pHi) regulation was studied in Toxoplasma gondii tachyzoites by using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. Their mean baseline pHi (7.07+/-0.06; n=5) was not significantly affected in the absence of extracellular Na+, K+ or HCO3(-) but was significantly decreased in a dose-dependent manner by low concentrations of N, N'-dicyclohexylcarbodi-imide (DCCD), N-ethylmaleimide (NEM) or bafilomycin A1. Bafilomycin A1 also inhibited the recovery of tachyzoite pHi after an acid load with sodium propionate. Similar concentrations of DCCD, NEM and bafilomycin A1 produced depolarization of the plasma membrane potential as measured with bis-(1,3-diethylthiobarbituric)trimethineoxonol (bisoxonol), and DCCD prevented the hyperpolarization that accompanies acid extrusion after the addition of propionate, in agreement with the electrogenic nature of this pump. Confocal laser scanning microscopy indicated that, in addition to being located in cytoplasmic vacuoles, the vacuolar (V)-H+-ATPase of T. gondii tachyzoites is also located in the plasma membrane. Surface localization of the V-H+-ATPase was confirmed by experiments using biotinylation of cell surface proteins and immunoprecipitation with antibodies against V-H+-ATPases. Taken together, the results are consistent with the presence of a functional V-H+-ATPase in the plasma membrane of these intracellular parasites and with an important role of this enzyme in the regulation of pHi homoeostasis in these cells.

Specific substitutions at amino acid 256 of the sarcoplasmic/endoplasmic reticulum Ca2+ transport ATPase mediate resistance to thapsigargin in thapsigargin-resistant hamster cells

High levels of resistance to thapsigargin (TG), a specific inhibitor of intracellular Ca2+ transport ATPases (SERCAs), can be developed in culture by stepwise exposure of mammalian cells to increasing concentrations of TG. We have identified, in two independently selected TG-resistant hamster cell lines of different lineages, mutant forms of SERCA. In the TG-resistant Chinese hamster lung fibroblast cell line DC-3F/TG, a T --> C change at nucleotide 766 introduces a Phe256 --> Leu alteration within the first cytosolic loop of the SERCA. In contrast, in the TG-resistant Syrian hamster smooth muscle cell line DDT/TG 4 microM, a T --> C change at nucleotide 767 introduces a Phe256 --> Ser mutation at that position. When these specific mutations are introduced into a wild-type full-length avian SERCA1 cDNA, transfection experiments reveal that Ca2+ transport function and ATP hydrolytic activity are not altered by such mutations. However, a 4-5-fold resistance to TG inhibition of Ca2+ transport function occurs upon the introduction of either the Phe256 --> Leu or the Phe256 --> Ser mutation into wild-type SERCA1. These specific mutations also render the hydrolytic activity of the ATPase resistant to inhibition by TG. Our results not only implicate amino acid 256 in TG-SERCA interactions, but also demonstrate that specific mutations within SERCA can mediate resistance to TG.

Determination of famotidine in human plasma by high performance liquid chromatography with column switching

A rapid, sensitive and robust reverse-phase high performance liquid chromatographic (HPLC) method with column switching and an internal standard for the quantitative determination of famotidine in human plasma is described. Famotidine and the internal standard were isolated from plasma samples by cation exchange solid phase extraction with SCX cartridges. The chromatographic separation was accomplished by an Inertsil C4 column with a mobile phase of acetonitrile/phosphate aqueous solution, connected by a switching valve to a BDS Hypersil C8 column with a mobile phase of acetonitrile/sodium dodecyl sulfate and phosphate aqueous solution. UV detection was set at 267 nm. The standard curve was linear in the concentration range of 1-100 ng ml-1. The intraday coefficients of variation at all concentration levels were less than 10%. The interday consistency was assessed by running QC samples during each daily run. The limit of quantification for famotidine in human plasma was 1 ng ml-1. The method has been utilized to support clinical pharmacokinetic studies in healthy volunteers who received famotidine 10 mg orally.

[Research on the regulation of glucoamylase gene(glaA) expression in A. niger. II. Analysis of the function of 5'-regulatory region of A. niger T21 and 3.795 glaA gene]

Two plasmid vectors pXH2 and pGH1 were constructed through the fusion of E. coli hph gene, the report gene and the 5' upstream regions of A. niger T21 and 3.795 respectively, as well as the terminator of A. nidulans trpC gene. The plasmid vectors were than used to transform A. niger T21 to functionally identify those different basic groups between the two 5' upstream regions responsible for high-level expression of the glaA gene. Southern analysis of two transformants XH2C and GH1C revealed that pXH2 and pGH1 were integrated respectively into the chromosome at same site with two copies in tandem array. The level resistant to HmB(3000 micrograms/ml) of XH2C was twice as high as that (1500 micrograms/ml) of GH1C, indicating that the changes of basic groups through mutation result in twice increase of functional level of region responsible for transcription and regulation of A. niger T21 glaA gene compared with that of 3.795.

Mice lacking factor VII develop normally but suffer fatal perinatal bleeding

Blood coagulation in vivo is initiated by factor VII (FVII) binding to its cellular receptor tissue factor (TF). FVII is the only known ligand for TF, so it was expected that FVII-deficient embryos would have a similar phenotype to TF-deficient embryos, which have defective vitello-embryonic circulation and die around 9.5 days of gestation. Surprisingly, we find that FVII-deficient (FVII-/-) embryos developed normally. FVII-/- mice succumbed perinatally because of fatal haemorrhaging from normal blood vessels. At embryonic day 9.5, maternal-fetal transfer of FVII was undetectable and survival of embryos did not depend on TF-FVII-initiated fibrin formation. Thus, the TF-/- embryonic lethal and the FVII-/- survival-phenotypes suggest a role for TF during embryogenesis beyond fibrin formation.

The different forms of the prolactin receptor in the rat corpus luteum: developmental expression and hormonal regulation in pregnancy

The corpora lutea of pregnancy in the rat are highly dependent on the action of PRL and PRL-like hormones to hypertrophy and to produce progesterone needed for the maintenance of gestation. Two forms of the PRL receptor (PRL-R), designated as long (PRL-RL) and short (PRL-RS), have been described in rat tissues. To determine whether both forms are present in the corpus luteum during pregnancy and to examine the developmental and hormonal regulation of their expression, total RNA isolated from corpora lutea at different stages of pregnancy and from highly luteinized granulosa cells subjected to different hormonal treatments were analyzed by semiquantitative RT-PCR. Immunoblotting of luteal proteins from early and late pregnancy was also performed to determine if the pattern of PRL-R proteins follows that of PRL-R messenger RNA (mRNA) expression. In addition, the correlation between the well characterized PRL-regulated gene, 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), and PRL-R gene expression was investigated during the time of luteolysis. Both PRL-RL and PRL-RS mRNA and protein were expressed in corpora lutea of pregnancy, with the long form being the most dominant at all stages. Whereas no changes in mRNA level of either PRL-RL or PRL-RS were found until day 20 of gestation, a profound decline in PRL-R mRNA and protein for both receptor types occurred at the end of pregnancy. This drop in PRL-R expression was accompanied by a sharp and abrupt expression of 20alpha-HSD mRNA. Studies performed in vivo and in luteinized cells in culture indicate that PRL can up-regulate the expression of the PRL-RL mRNA, an effect prevented by the tyrosine kinase inhibitor, genistein. PRL-RL mRNA was also selectively increased by cAMP. In summary, the results of this investigation have established that: 1) the corpus luteum of pregnancy expresses both the short and long forms of the PRL-R with the long form being more abundant; 2) the mRNA for both forms of the PRL-R remains at constant levels throughout pregnancy but drops before parturition; 3) the decline in PRL-R mRNA at the end of pregnancy is accompanied by a dramatic rise in 20alpha-HSD; 4) PRL is able to increase the expression of PRL-R mRNA; and that 5) both A kinase and tyrosine kinase mediated pathways appear to participate in the up-regulatory mechanism involved in PRL-R mRNA expression.

[Studies on the regulation of glucoamylase gene (glaA) expression in A. niger I. The overall analysis and comparison of the expression of glucoamylase in the overproducing strain A. niger T21 and its original strain A. niger 3.795]

Expression of the A. niger T21 and 3.795 glaA gene was investigated with respect to the growth of mycelia, producing of glucoamylase, copy number of glaA gene as well as glucoamylase mRNA level and its stability. Both A. niger T21 and 3.795 produced substantial glucoamylase at the stable phase when their biomass were the same. After incubating for 72 hours, while the biomass remained the same, the glucoamylase produced by T21 was 10-17 times higher than that by 3.795. Northern analysis showed that in the total RNA from T21 4.3-4.4 fold as much glucoamylase mRNA as from 3.795 was determined. This result along with results with regard to copy number of glaA gene and stability of glucoamylase mRNA supported the suggestion that the increased mRNA level of glucoamylase was contributed by the increasing at transcription level. The inconsistancy between the difference of enzyme production (10-17 fold) and that of the mRNA level (4.3-4.4 fold) revealed that some difference at translation exist probably. Furthermore, expression of both T21 and 3.795 glaA gene was regulated by carbon sources, this regulation occurred at transcription level in the same manner and also probably occurred at translation level.

[A comprehensive detection and psocessing system of visual electrophysiological information based on interconnected computers]

This paper introduce a novel comprehensive system of visual electrophysiological information processing. The weak-light stimulator and surface-electrode are used to overcome the subject's unfitted sense led by Ganzifield stimulator and the cornea-touch electrode. Based on two computers which are interconnected, this system can work on pan-character picture stimulating mode. While working on line, it can dynamically display the process of test in real-time. It also offers a clear interface and integrated modules of information processing and analysis.

PRAP, a prolactin receptor associated protein: its gene expression and regulation in the corpus luteum

We have recently identified, characterized, and cloned a luteal microsomal 32-kDa phosphoprotein that we named PRAP (for PRL-receptor associated protein), and we have demonstrated that PRAP binds to the intracellular domain of the short but not the long form of the PRL receptor. In this study, we used PRAP cDNA to examine the tissue specificity, the developmental expression, and the hormonal regulation of PRAP gene expression. Northern blot analysis revealed that in the corpus luteum, PRAP cDNA hybridized to multiple transcripts (5.5 kb, 4.3 kb, and 1.8 kb), with the smallest transcript (1.8 kb) corresponding to the size of the cDNA clone. However, none of these transcripts were detected in any other tissues examined. PRAP appears to be tightly regulated by steroids and PRL. When pregnant rats were treated with aminoglutethimide, a steroid synthesis inhibitor, all three PRAP transcripts became barely detectable. Similar results were obtained when all luteotropic support was removed by hypophysectomy and hysterectomy. Estradiol up-regulated PRAP expression and, more specifically, the two lower transcripts. PRL had no stimulatory effect on PRAP messenger RNA (mRNA) expression but caused a substantial increase in the level of PRAP protein when administered to hypophysectomized pregnant rat, suggesting that PRL may stabilize this protein. Similar dissociation between levels of mRNA and protein were observed during luteal development. Although both PRAP mRNA and protein were barely detectable in early pregnancy, their expression increased abruptly from midpregnancy; however, whereas levels of PRAP mRNA declined from day 18, those of the protein remained elevated until parturition. In summary, results of this study have defined the tissue specificity and developmental expression of PRAP mRNA during pregnancy. The data have also revealed that the gene expression of this protein is up-regulated by estradiol, suggesting a pivotal role for PRAP in the synergistic action of estradiol and PRL on the function of the rat corpus luteum.

Design, synthesis, and biological evaluation of new growth inhibitors of Trypanosoma cruzi (epimastigotes)

As a continuation of our project aimed at the search for new chemotherapeutic agents against Chagas' disease, several drugs structurally related to the insect growth regulator Fenoxycarb and the naturally occurring juvenile hormone of insects were designed, synthesized, and evaluated as antiproliferative agents against the parasite responsible of this disease. Isoprenoid derivatives (compounds 33, 34, 36, and 37) were potent growth inhibitors of Trypanosoma cruzi epimastigotes. In addition, taking into account the high activity observed for compound 30 and the inhibitory action of related compounds, the allyl ether moiety bonded at the polar extreme of these inhibitors proved to be a promising group for the design of new drugs.

[Transposition of the fibular skin flap of the great toe to repair the fore plantar defect]

The sole, as the body weight-bearing region, is characterized with the skin of thick stratum corneum. Wound in the region is difficult to cure. Based on the blood supply of the foot, a fibular skin flap of the great toe was designed and used to treat the defect of the planta pedis. Clinical applications in 6 cases were successful. The operation results were satisfactory.

Prolactin-mediated inhibition of 20alpha-hydroxysteroid dehydrogenase gene expression and the tyrosine kinase system

The rat luteal 20alpha-hydroxysteroid dehydrogenase plays a key role at catabolizing progesterone and at decreasing the level of this steroid secreted by the ovaries. Throughout pregnancy and before parturition neither the mRNA nor the protein for this enzyme could be detected. In this investigation we set to examine whether PRL and PRL-like hormone from placental origin silence the expression of this gene and whether PRL action involves tyrosine kinase activity and/or de novo protein synthesis. The results revealed that PRL and PRL-like hormone from rat placental origin (rPL-1 and rPL-2), but not rat growth hormone, caused a rapid and profound inhibition of 20alpha-HSD mRNA expression in highly luteinized granulosa cells. Immunoprecipition and western blot analysis indicate that PRL-R associates with JAK2 and Stat5, and this association is increased within 30 seconds with PRL treatment. Although both JAK2 and Stat5 were phosphorylated on tyrosine upon PRL treatment, the PRL mediated inhibition of 20alpha-HSD was not reversed by either tyrosine kinase inhibitors, AG18 and genistein, but was largely reversed by the protein synthesis inhibitor cycloheximide. In summary, results of this investigation indicate that although PRL can activate the JAK2/Stat5 system in the corpus luteum, the down regulation of 20alpha-HSD mRNA by PRL does not appear to involve tyrosine kinase activity but depends on de novo synthesis of protein(s).

[The immunogenicity of tooth allografts in mice]

The mice used in this study were of the in-bred strains C57BL/6 and Balb/c which differ at the major histocompatibility complex (MHC). The immunogenicity of tooth allografts was studied in vitro by means of detecting the response of Antibody-Complement Mediated Cytotoxity (ACMC) and Cell Mediated Cytotoxity (CMC) of the host. We found that there was significant difference for both ACMC and CMC activity between the allogenic and isogenic tooth transplantation (P < 0.01). The results indicate that tooth allografts are immunogens and do evoke the immune response of recipients.

Intracellular Ca2+ pool content and signaling and expression of a calcium pump are linked to virulence in Leishmania mexicana amazonesis amastigotes

Virulent and avirulent clones of Leishmania mexicana amazonensis promastigotes or amastigotes were loaded with the fluorescent reagent fura 2/AM to measure intracellular free calcium ([Ca2+]i). When the cells were treated with the calcium ionophore ionomycin in the nominal absence of extracellular Ca2+, there was an increase of [Ca2+]i that was further elevated by addition of either NH4Cl, nigericin, or the vacuolar H+-ATPase inhibitor bafilomycin A1. Similar results were obtained when the order of additions was reversed. Taking into account the relative importance of the ionomycin-releasable and the ionomycin plus NH4Cl-releasable Ca2+ pools, it is apparent that a significant amount of the Ca2+ stored in L. mexicana amazonensis promastigotes and amastigotes is present in an acidic compartment rich in Ca2+ (acidocalcisome). Results indicated that more releasable Ca2+ is stored intracellularly in virulent amastigotes than in virulent promastigotes or avirulent cells of both stages. This higher amount of releasable Ca2+ was correlated with the presence of Ca2+ signals in the virulent amastigotes during invasion of macrophages. Ca2+ signals and invasion were reduced by preloading the parasites with intracellular Ca2+ chelators (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/AM) and quin 2/AM) but not by a non-Ca2+-chelating analog (N-(2-methoxyphenyl)imidoacetic acid/AM). The gene encoding an organelle-type Ca2+-ATPase was cloned and sequenced and found overexpressed in virulent amastigotes as compared with all other forms. Together, these results demonstrate a significant link between expression of a Ca2+-ATPase, intracellular Ca2+ pool content and signaling, and virulence.

Assembly of complete, functionally active herpes simplex virus DNA replication compartments and recruitment of associated viral and cellular proteins in transient cotransfection assays

Early during the herpes simplex virus (HSV) lytic cycle or in the presence of DNA synthesis inhibitors, core viral replication machinery proteins accumulate in intranuclear speckled punctate prereplicative foci, some of which colocalize with numerous sites of host cellular DNA synthesis initiation known as replisomes. At later times, in the absence of inhibitors, several globular or large irregularly shaped replication compartments are formed; these compartments also contain progeny viral DNA and incorporate the IE175(ICP4) transcription factor together with several cellular proteins involved in DNA replication and repair. In this study, we demonstrate that several forms of both prereplication foci and active viral replication compartments that display an appearance similar to that of the compartments in HSV-infected cells can be successfully assembled in transient assays in DNA-transfected cells receiving genes encoding all seven essential HSV replication fork proteins together with oriS target plasmid DNA. Furthermore, bromodeoxyuridine (BrdU)-pulse-labeled DNA synthesis initiation sites colocalized with the HSV single-stranded DNA-binding protein (SSB) in these replication compartments, implying that active viral DNA replication may be occurring. The assembly of complete HSV replication compartments and incorporation of BrdU were both abolished by treatment with phosphonoacetic acid (PAA) and by omission of any one of the seven viral replication proteins, UL5, UL8, UL9, UL42, UL52, SSB, and Pol, that are essential for viral DNA replication. Consistent with the fact that both HSV IE175 and IE63(ICP27) localize within replication compartments in HSV-infected cells, the assembled HSV replication compartments were also able to recruit both of these essential regulatory proteins. Blocking viral DNA synthesis with PAA, but not omission of oriS, prevented the association of IE175 with prereplication structures. The assembled HSV replication compartments also redistributed cotransfected cellular p53 into the viral replication compartments. However, the other two HSV immediate-early nuclear proteins IE110(ICP0) and IE68(ICP22) did not enter the replication compartments in either infected or transfected cells.

Expression, purification and characterization of the rat luteal 20 alpha-hydroxysteroid dehydrogenase

The enzyme, rat ovarian 20 alpha-hydroxysteroid dehydrogenase (20 alpha HSD), plays a central role in luteolysis and parturition. It catalyzes the reduction of progesterone, leading to the formation of progestationally inactive steroid, 20 alpha-hydroxypregn-4-ene-3-one (20 alpha-hydroxyprogesterone). Recently, we reported the cloning, sequencing, and deduced amino acid sequence of the rat luteal 20 alpha HSD. To further investigate whether phosphorylation and/or glycosylation affect the activity of 20 alpha HSD and to study its kinetic and biochemical properties, we established both bacterial and insect expression systems for obtaining large quantities of enzyme. The recombinant (rec) 20 alpha HSD expressed as glutathione-S-transferase-20 alpha HSD fusion protein was purified from bacterial lysates by affinity binding to glutathione-Sepharose beads followed by thrombin digestion, whereas the rec enzyme expressed in baculovirus-insect cell system was purified to apparent homogeneity by ion exchange chromatography, followed by dye affinity chromatographies. Both rec preparations of 20 alpha HSD demonstrated a single polypeptide chain of 37 kDa with similar K(m) values for 20 alpha-hydroxyprogesterone and NADP, although the corresponding maximum velocity values were slightly lower for the rec 20 alpha HSD expressed in the insect cells. The rec 20 alpha-HSD showed preference for progesterone/20 alpha-hydroxyprogesterone. 17 alpha-Hydroxyprogesterone was only 30% as effective. The enzyme also used various substrates specific for aldo-keto reductases, although with much less efficiency. The rec enzyme preparations showed an absolute requirement for NADP(H). In vitro phosphorylation of rec bacterial enzyme with either protein kinase A or protein kinase C had no demonstrable effect on its activity. Finally, no differences in enzyme activity were noted between glycosylated (expressed in insect cells) and nonglycosylated (expressed in bacteria) forms of the enzyme. In conclusion, these studies demonstrate that rat luteal 20 alpha HSD can be prepared in large amounts from either bacterial or insect expression systems in a catalytically active form. Indirect evidence also suggests that the catalytic activity of 20 alpha HSD may be independent of phosphorylation and glycosylation states of the enzyme protein, i.e. posttranslational modification of 20 alpha HSD may not be required for the maximal expression of enzyme activity.

Short and long range functions of amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase. A mutational study

Mutational analysis of several amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase was performed by expressing wild type ATPase and 32 site-directed mutants in COS-1 cells followed by functional characterization of the microsomal fraction. Four different phenotype characteristics were observed in the mutants: (a) functions similar to those sustained by the wild type ATPase; (b) Ca2+ transport inhibited to a greater extent than ATPase hydrolytic activity; (c) inhibition of transport and hydrolytic activity in the presence of high levels of phosphorylated enzyme intermediate; and (d) total inhibition of ATP utilization by the enzyme while retaining the ability to form phosphoenzyme by utilization of P(i). Analysis of experimental observations and molecular models revealed short and long range functions of several amino acids within the transmembrane region. Short range functions include: (a) direct involvement of five amino acids in Ca2+ binding within a channel formed by clustered transmembrane helices M4, M5, M6, and M8; (b) roles of several amino acids in structural stabilization of the helical cluster for optimal channel function; and (c) a specific role of Lys297 in sealing the distal end of the channel, suggesting that the M4 helix rotates to allow vectorial flux of Ca2+ upon enzyme phosphorylation. Long range functions are related to the influence of several transmembrane amino acids on phosphorylation reactions with ATP or P(i), transmitted to the extramembranous region of the ATPase in the presence or in the absence of Ca2+.

Acidocalcisomes in Toxoplasma gondii tachyzoites

Toxoplasma gondii tachyzoites were loaded with the fluorescent indicator fura 2 to investigate the transport mechanisms involved in maintaining their intracellular Ca2+ homoeostasis. The mitochondrial ATPase inhibitor oligomycin and the endoplasmic-reticulum Ca(2+)-ATPase inhibitor thapsigargin increased the intracellular Ca2+ concentration ([Ca2+]i), thus indicating the requirement for ATP and the involvement of the endoplasmic reticulum in maintaining intracellular Ca2+ homoeostasis. The effect of thapsigargin was more accentuated in the presence of extracellular Ca2+, clearly showing that, as occurs with other eukaryotic cells, depletion of intracellular Ca2+ pools led to an increase in the uptake of Ca2+ from the extracellular medium. In addition to these results, we found evidence that, in contrast with what occurs in mammalian cells, T. gondii tachyzoites possess a significant amount of Ca2+ stored in an acidic compartment, termed the acidocalcisome, as indicated by: (1) the increase in [Ca2+]i induced by bafilomycin A1 (a specific inhibitor of H(+)-ATPases), nigericin (a K+/H+ exchanger) or the weak base NH4Cl, in the nominal absence of extracellular Ca2+ to preclude Ca2+ entry; and (2) the effect of ionomycin, a Ca(2+)-releasing ionophore that cannot take Ca2+ out of acidic organelles and that was more effective after alkalinization of these compartments by addition of bafilomycin A1, nigericin or NH4Cl. Considering the relative importance of the ionomycin-releasable and the ionomycin + NH4Cl-releasable Ca2+ pools, it is apparent that T. gondii tachyzoites contain a significant amount of Ca2+ stored in acidocalcisomes.