Molecular characterization of two genes encoding members of the glucose transporter superfamily in the parasitic protozoan Leishmania donovani

The role of membrane transporters in Leishmania virulence

Leishmania are parasitic protozoa which infect humans and cause severe morbidity and mortality. Leishmania parasitise as extracellular promastigotes in the insect vector and as intracellular amastigotes in the mammalian host. Cycling between hosts involves implementation of stringent and co-ordinated responses to shifting environmental conditions. One of the key dynamic aspects of Leishmania biology is substrate acquisition and metabolism. Genomic analyses have revealed that Leishmania encode many putative membrane transporters, many of which are differentially expressed during the parasite life cycle. Only a small fraction of these transporters, however, have been functionally characterised. Currently, most information is available about nutrient transporters, mainly involved in carbohydrate, amino acid, nucleobase and nucleoside, cofactor, and ion acquisition. Several have apparent roles in Leishmania virulence and will be discussed in this perspective.

Biochemical and phylogenetic analyses of phosphatidylinositol production in Angomonas deanei, an endosymbiont-harboring trypanosomatid

BACKGROUND

The endosymbiosis in trypanosomatids is characterized by co-evolution between one bacterium and its host protozoan in a mutualistic relationship, thus constituting an excellent model to study organelle origin in the eukaryotic cell. In this association, an intense metabolic exchange is observed between both partners: the host provides energetic molecules and a stable environment to a reduced wall symbiont, while the bacterium is able to interfere in host metabolism by enhancing phospholipid production and completing essential biosynthesis pathways, such as amino acids and hemin production. The bacterium envelope presents a reduced cell wall which is mainly composed of cardiolipin and phosphatidylcholine, being the latter only common in intracellular prokaryotes. Phosphatidylinositol (PI) is also present in the symbiont and host cell membranes. This phospholipid is usually related to cellular signaling and to anchor surface molecules, which represents important events for cellular interactions.

METHODS

In order to investigate the production of PI and its derivatives in symbiont bearing trypanosomatids, aposymbiotic and wild type strains of Angomonas deanei, as well as isolated symbionts, were incubated with [(3)H]myo-inositol and the incorporation of this tracer was analyzed into inositol-containing molecules, mainly phosphoinositides and lipoproteins. Gene searches and their phylogenies were also performed in order to investigate the PI synthesis in symbiontbearing trypanosomatids.

RESULTS

Our results showed that the bacterium did not incorporate the tracer and that both strains produced similar quantities of PI and its derivatives, indicating that the symbiont does not influence the production of these metabolites. Gene searches related to PI synthesis revealed that the trypanosomatid genome contains an inositol transporter, PI synthase and the myo-inositol synthase. Thus, the host is able to produce PI either from exogenous myo-inositol (inositol transporter) or from myo-inositol synthesized de novo. Phylogenetic analysis using other organisms as references indicated that, in trypanosomatids, the genes involved in PI synthesis have a monophyletic origin. In accordance with experimental data, sequences for myo-inositol transport or for myo-inositol and PI biosynthesis were not found in the symbiont.

CONCLUSIONS

Altogether, our results indicate that the bacterium depends on the host to obtain PI.

Lysosomal degradation of Leishmania hexose and inositol transporters is regulated in a stage-, nutrient- and ubiquitin-dependent manner

Leishmania parasites experience variable nutrient levels as they cycle between the extracellular promastigote stage in the sandfly vector and the obligate intracellular amastigote stage in the mammalian host. Here we show that the surface expression of three Leishmania mexicana hexose and myo-inositol transporters is regulated in both a stage-specific and nutrient-dependent manner. GFP-chimeras of functionally active hexose transporters, LmGT2 and LmGT3, and the myo-inositol transporter, MIT, were primarily expressed in the cell body plasma membrane in rapidly dividing promastigote stages. However MIT-GFP was mostly rerouted to the multivesicular tubule (MVT)-lysosome when promastigotes reached stationary phase growth and all three nutrient transporters were targeted to the amastigote lysosome following transformation to in vitro differentiated or in vivo imaged amastigote stages. This stage-specific decrease in surface expression of GFP-tagged transporters correlated with decreased hexose or myo-inositol uptake in stationary phase promastigotes and amastigotes. The MVT-lysosme targeting of the MIT-GFP protein was reversed when promastigotes were deprived of myo-inositol, indicating that nutrient signals can override stage-specific changes in transporter distribution. The surface expression of the hexose and myo-inositol transporters was not regulated by interactions with the subpellicular cytoskeleton, as both classes of transporters associated with detergent-resistant membranes. LmGT3-GFP and MIT-GFP proteins C-terminally modified with mono-ubiquitin were constitutively transported to the MVT-lysosome, suggesting that ubiquitination may play a key role in regulating the subcellular distribution of these transporters and parasite adaptation to different nutrient conditions.

Lipid metabolism in Trypanosoma brucei

Trypanosoma brucei membranes consist of all major eukaryotic glycerophospholipid and sphingolipid classes. These are de novo synthesized from precursors obtained either from the host or from catabolised endocytosed lipids. In recent years, substantial progress has been made in the molecular and biochemical characterisation of several of these lipid biosynthetic pathways, using gene knockout or RNA interference strategies or by enzymatic characterization of individual reactions. Together with the completed genome, these studies have highlighted several possible differences between mammalian and trypanosome lipid biosynthesis that could be exploited for the development of drugs against the diseases caused by these parasites.

Trypanosoma cruzi epimastigotes: regulation of myo-inositol transport by effectors of protein kinases A and C

Inositol is the precursor for most Trypanosoma cruzi surface molecules, including phosphoinositides, glycosylinositolphospholipids and glycosylphosphatidylinositol anchors. As the parasite is an inositol auxotroph, the inositol transport system might be a potential target for new trypanocide drugs, as some of its properties are different from its mammalian counterpart. Here, we investigated the modulation exerted by effectors of PKA and PKC on this transport system to comply with the parasite physiology. Pre-incubation of the cells with either dibutyryl-cyclic AMP (25 microM) or forskolin (30 microM) decreased the myo-inositol uptake by half, this effect being reversed by KT5720 (PKA inhibitor). Conversely, pre-incubation of the cells with PMA (2.8 microg/ml) or serum (5%) had a approximately 50% stimulation in myo-inositol uptake, being this effect reversed by staurosporine (0.5 microM) or sphingosine (10 microM). These results allow us to conclude that the myo-inositol transport system in T. cruzi epimastigotes is inhibited by PKA and stimulated by PKC effectors.

Analysis of genetic elements regulating the methionine adenosyltransferase gene in Leishmania infantum

Methionine adenosyltransferase (MAT) is an important enzyme for metabolic processes, inasmuch as its product, S-adenosylmethionine (AdoMet), plays a key role in trans-methylation, trans-sulphuration and polyamine synthesis. Our prior studies have shown that the Leishmania infantum genome contains two identical copies of the gene encoding MAT (MAT2 gene), arranged in head-to-tail configuration and alternating with another gene, called LORIEN that contains a zinc-finger motif. Both genes are constitutively expressed throughout the promastigote stage of the parasite cell cycle, and their flanking regions were detected by RT-PCR. Luciferase (luc) reporter assays indicated the presence of regulatory elements within the MAT2 3'UTR and intergenic region, and fragments responsible for such regulation were identified by deletional analysis. By site-directed mutagenesis of the wild-type -42 AG recognized in the trans-splicing of the MAT2 gene, the AG slightly downstream (position -36) was observed to be able to generate the same levels of luc expression, thus suggesting that potentially this gene has alternative spliced leader acceptor sites. The stability of MAT2 and LORIEN transcripts was very similar in both logarithmic and stationary phases. However, cycloheximide (CHX) inhibition of protein synthesis increased MAT2 and LORIEN mRNA levels in the logarithmic phase only, an indication that these genes are regulated in promastigotes at the post-transcriptional level by protein factors that targets both transcripts for degradation. However, during the stationary phase, another CHX-independent factor also led to MAT2 and LORIEN mRNAs degradation, indicating the existence of different mechanisms operating on the post-transcriptional regulation of these two genes.

Comparative genomics of trypanosomatid parasitic protozoa

A comparison of gene content and genome architecture of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major, three related pathogens with different life cycles and disease pathology, revealed a conserved core proteome of about 6200 genes in large syntenic polycistronic gene clusters. Many species-specific genes, especially large surface antigen families, occur at nonsyntenic chromosome-internal and subtelomeric regions. Retroelements, structural RNAs, and gene family expansion are often associated with syntenic discontinuities that-along with gene divergence, acquisition and loss, and rearrangement within the syntenic regions-have shaped the genomes of each parasite. Contrary to recent reports, our analyses reveal no evidence that these species are descended from an ancestor that contained a photosynthetic endosymbiont.

Glucose and proline transport in kinetoplastids

The parasitic protozoa belonging to the kinetoplastids can use both sugars and amino acids as carbon and energy sources. In this review, Benno ter Kuile discusses nutrient acquisition and utilization and how the metabolic strategies reflect the environment encountered in host and vector. Recent genetic and physiological evidence suggests that facilitated diffusion may be the primary uptake mechanism for glucose, and possibly for proline as well, even though there is biochemical and genetic evidence suggesting that active transport occurs, if not across the plasma membrane, then across the membranes of organelles. Trypanosoma brucei seems to have a metabolic strategy that strives for maximum energy efficiency, making no storage materials and thereby limiting the control over its internal conditions. On the other hand, Leishmania donovani does create a storage buffer, entrapping glucose in the cell. In this manner, it maintains constant internal conditions at the expense of energy, enabling it to survive more adverse conditions in the macrophage and in its vector.

Life in vacuoles--nutrient acquisition by Leishmania amastigotes

Leishmania have a digenetic life cycle, involving a motile, extracellular stage (promastigote) which parasitises the alimentary tract of a sandfly vector. Bloodfeeding activity by an infected sandfly can result in transmission of infective (metacyclic) promastigotes to mammalian hosts, including humans. Leishmania promastigotes are rapidly phagocytosed but may survive and transform into non-motile amastigote forms which can persist as intracellular parasites. Leishmania amastigotes multiply in an acidic intracellular compartment, the parasitophorous vacuole. pH plays a central role in the developmental switch between promastigote and amastigote stages, and amastigotes are metabolically most active when their environment is acidic, although the cytoplasm of the amastigote is regulated at near-neutral pH by an active process of proton extrusion. A steep proton gradient is thus maintained across the amastigote surface and all membrane processes must be adapted to function under these conditions. Amastigote uptake systems for glucose, amino acids, nucleosides and polyamines are optimally active at acidic pH. Promastigote uptake systems are kinetically distinct and function optimally at more neutral environmental pH, indicating that membrane transport activity is developmentally regulated. The nutrient environment encountered by amastigotes is not well understood but the parasitophorous vacuole can fuse with endosomes, phagosomes and autophagosomes, suggesting that a diverse range of macromolecules will be present. The parasitophorous vacuole is a hydrolytic compartment in which such material will be rapidly degraded to low molecular weight components which are typical substrates for membrane transporters. Amastigote surface transporters must compete for these substrates with equivalent host transporters in the membrane of the parasitophorous vacuole. The elaboration of accumulative transporters with high affinity will be beneficial to amastigotes in this environment. The influence of environmental pH on membrane transporter function is discussed, with emphasis on the potential role of a transmembrane proton gradient in active, high affinity transport.

A model for coupling of H(+) and substrate fluxes based on "time-sharing" of a common binding site

Both prokaryotic and eukaryotic cells contain an array of membrane transport systems maintaining the cellular homeostasis. Some of them (primary pumps) derive energy from redox reactions, ATP hydrolysis, or light absorption, whereas others (ion-coupled transporters) utilize ion electrochemical gradients for active transport. Remarkable progress has been made in understanding the molecular mechanism of coupling in some of these systems. In many cases carboxylic residues are essential for either binding or coupling. Here we suggest a model for the molecular mechanism of coupling in EmrE, an Escherichia coli 12-kDa multidrug transporter. EmrE confers resistance to a variety of toxic cations by removing them from the cell interior in exchange for two protons. EmrE has only one membrane-embedded charged residue, Glu-14, which is conserved in more than 50 homologous proteins. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate-binding site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding, uptake, efflux, and exchange reactions. The studies suggest that Glu-14 is an essential part of a binding site, which is common to substrates and protons. The occupancy of this site by H(+) and substrate is mutually exclusive and provides the basis of the simplest coupling for two fluxes.

Genetics and biochemistry of Leishmania membrane transporters

The ability to clone and functionally express genes encoding membrane transporters in Leishmania and related parasitic protozoa has illuminated the processes whereby these parasites acquire nutrients from their hosts. It is now possible to probe the physiological functions of these permeases and investigate their role in drug delivery and resistance.

Complementation of a glucose transporter mutant of Schizosaccharomyces pombe by a novel Trypanosoma brucei gene

The African trypanosome Trypanosoma brucei has a digenetic life cycle that involves the insect vector and the mammalian host. This is underscored by biochemical switches in its nutritional requirements. In the insect vector, the parasite relies on amino acid catabolism, but in the mammalian host, it derives its energy exclusively from blood glucose. Glucose transport is facilitated, and constitutes the rate-limiting step in ATP synthesis. Here, we report the cloning of a novel glucose transporter-related gene by heterologous screening of a lambdaEMBL4 genomic library of T. brucei EATRO 164 using a rat liver glucose transporter cDNA clone. Genomic analysis shows that the gene is present as a single copy within the parasite genome. The gene encodes a protein with an estimated molecular mass of 55.9 kDa, which shares only segmental homology with members of the glucose transporter superfamily. Several potential post-translational modification sites including phosphorylation, N-glycosylation, and cotranslational myristoylation sites also punctuate the sequence. It is distinguished from classical transporter proteins by the absence of putative hydrophobic membrane-spanning domains. However, this protein was capable of complementing Schizosaccharomyces pombe glucose transporter mutants. The rescued phenotype conferred the ability of the cells to grow on a broad range of sugars, both monosaccharides and disaccharides. The kinetics of glucose uptake reflected those in T. brucei. In addition to complementation in yeast, we also showed that the gene enhanced glucose uptake in cultured mammalian cells.

Characterization of the myo-inositol transport system in Trypanosoma cruzi

myo-inositol is a growth factor for mammalian cells as well as for the pathogenic protozoa Trypanosoma cruzi. Most of the cell surface molecules in this organism rely on myo-inositol as the biosynthetic precursor for phosphoinositides and glycosylated phosphatidylinositols. The aim of this work was to investigate the process of myo-inositol translocation across the parasite cell membrane. myo-Inositol uptake was concentration-dependent in the concentration range 0.1-10 microM with maximal transport obtained at 8 microM. Using sodium-free buffers, where Na+ was replaced by choline or K+, myo-inositol uptake was inhibited by 50%. Furosemide, an inhibitor of the ouabain-insensitive Na+-ATPase, inhibited the Na+-dependent and Na+-independent myo-inositol uptake by 68 and 33%, respectively. In contrast, ouabain, an (Na++/K+) ATPase inhibitor, did not affect transport. Part of the myo-inositol uptake is mediated by active transport as it was inhibited when energy metabolism inhibitors such as carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone (34%), 2,4-dinitrophenol (50%), KCN (71%) and NaN3 (69%) were added to the medium, or the temperature of the medium was lowered to 4 degrees C. The addition of glucose (5-50 mM) or mannose (10 mM) did not change the myo-inositol uptake, whereas the addition of 10 mM nonlabeled myo-inositol totally inhibited this transport, indicating that the transporter is specific for myo-inositol. Phloretin (0.3 mM) and phoridzin (5 mM), but not cytochalasin B, were efficient inhibitors of myo-inositol uptake. A portion of the accumulated myo-inositol is converted to inositol phosphates and phosphoinositides. These data show that myo-inositol transport in T. cruzi epimastigotes is mediated by at least two specific transporters - one Na+-dependent and the other Na+-independent.

The Leishmania GDP-mannose transporter is an autonomous, multi-specific, hexameric complex of LPG2 subunits

LPG2 (a gene involved in lipophosphoglycan assembly) encodes the Golgi GDP-Man transporter of the protozoan parasite Leishmania and is a defining member of a new family of eukaryotic nucleotide-sugar transporters (NSTs). Although NST activities are widespread, mammalian cells lack a GDP-Man NST, thereby providing an ideal heterologous system for probing the LPG2 structure and activity. LPG2 expression constructs introduced into either mammalian cells or a Leishmania lpg2(-) mutant conferred GDP-Man, GDP-Ara, and GDP-Fuc (in Leishmania only) uptake in isolated microsomes. LPG2 is the first NST to be associated with multiple substrate specificities. Uptake activity showed latency, exhibited an antiport mechanism of transport with GMP, and was susceptible to the anion transport inhibitor DIDS. The apparent K(m) for GDP-Man uptake was similar in transfected mammalian cells (12.2 microM) or Leishmania (6.9 microM). Given the evolutionary distance between protozoans and vertebrates, these data suggest that LPG2 functions autonomously to provide transporter activity. Using epitope-tagged LPG2 proteins, we showed the existence of hexameric LPG2 complexes by immunoprecipitation experiments, glycerol gradient centrifugation, pore-limited native gel electrophoresis, and cross-linking experiments. This provides strong biochemical evidence for a multimeric complex of NSTs, a finding with important implications to the structure and specificity of NSTs in both Leishmania and other organisms. Inhibition of essential GDP-Man uptake in fungal and protozoan systems offers an attractive target for potential chemotherapy.

Four conserved cytoplasmic sequence motifs are important for transport function of the Leishmania inositol/H(+) symporter

The protozoan Leishmania donovani has a myo-inositol/proton symporter (MIT) that is a member of a large sugar transporter superfamily. Active transport by MIT is driven by the proton electrochemical gradient across the parasite membrane, and MIT is a prototype for understanding the function of an active transporter in lower eukaryotes. MIT contains two duplicated 6- or 7-amino acid motifs within cytoplasmic loops, which are highly conserved among 50 members of the sugar transporter superfamily and are designated A(1), A(2) ((V)(D/E)(R/K)PhiGR(R/K)), and B(1) (PESPRPhiL), B(2) (VPETKG). In particular, the three acidic residues within these motifs, Glu(187)(B(1)), Asp(300)(A(2)), and Glu(429)(B(2)) in MIT, are highly conserved with 96, 78, and 96% amino acid identity within the analyzed members of this transporter superfamily ranging from bacteria, archaea, and fungi to plants and the animal kingdom. We have used site-directed mutagenesis in combination with functional expression of transporter mutants in Xenopus oocytes and overexpression in Leishmania transfectants to investigate the significance of these three acidic residues in the B(1), A(2), and B(2) motifs. Alteration to the uncharged amides greatly reduced MIT transport function to 23% (E187Q), 1.4% (D300N), and 3% (E429Q) of wild-type activity, respectively, by affecting V(max) but not substrate affinity. Conservative mutations that retained the charge revealed a less pronounced effect on inositol transport with 39% (E187D), 16% (D300E) and 20% (E429D) remaining transport activity. Immunofluorescence microscopy of oocyte cryosections confirmed that MIT mutants were expressed on the oocyte surface in similar quantity to MIT wild type. The proton uncouplers carbonylcyanide-4-(trifluoromethoxy) phenylhydrazone and dinitrophenol inhibited inositol transport by 50-70% in the wild type as well as in E187Q, D300N, and E429Q, despite their reduced transport activities, suggesting that transport in these mutants is still proton-coupled. Furthermore, temperature-dependent uptake studies showed an increased Arrhenius activation energy for the B(1)-E187Q and the B(2)-E429Q mutants, which supports the idea of an impaired transporter cycle in these mutants. We conclude that the conserved acidic residues Glu(187), Asp(300), and Glu(429) are critical for transport function of MIT.

Regulation and adaptation of glucose metabolism of the parasitic protist Leishmania donovani at the enzyme and mRNA levels

Adaptation of the glucose metabolism of Leishmania donovani promastigotes (insect stage) was investigated by simultaneously measuring metabolic rates, enzyme activities, message levels, and cellular parameters under various conditions. Chemostats were used to adapt cells to different growth rates with growth rate-limiting or excess glucose concentrations. L. donovani catabolized glucose to CO(2), succinate, acetate, and pyruvate in ratios that depended on growth rate and glucose availability. Rates of glucose consumption were a linear function of growth rate and were twice as high in excess glucose-grown cells as in glucose-limited organisms. The major end product was CO(2), but organic end products were also formed in ratios that varied strongly with growth conditions. The specific activities of the 14 metabolic enzymes measured varied by factors of 3 to 17. Two groups of enzymes adapted specific activities in parallel, but there was no correlation between the groups. The activities of only one group correlated with specific rates of glucose metabolism. Total RNA content per cellular protein varied by a factor of 6 and showed a linear relationship with the rate of glucose consumption. There was no correlation between steady-state message levels and activities of the corresponding enzymes, suggesting regulation at the posttranscriptional level. A comparison of the adaptation of energy metabolism in L. donovani and other species suggests that the energy metabolism of L. donovani is inefficient but is well suited to the environmental challenges that it encounters during residence in the sandfly, its insect vector.

Differential regulation of multiple glucose transporter genes in Leishmania mexicana

We have studied the structure and expression of glucose transporter genes in the parasitic protozoan Leishmania mexicana. Three distinct glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, are encoded by single copy genes that are clustered together at a single locus. Quantitation of Northern blots reveals that LmGT2 mRNA is present at approximately 15-fold higher level in promastigotes, the insect stage of the parasite life cycle, compared with amastigotes, the intracellular stage of the life cycle that lives within the mammalian host. In contrast, LmGT1 and LmGT3 mRNAs are expressed at similar levels in both life cycle stages. Transcription of the LmGT genes in promastigotes and axenically cultured amastigotes occurs at similar levels, as measured by nuclear run-on transcription. Consequently, the approximately 15-fold up-regulation of LmGT2 mRNA levels in promastigotes compared with amastigotes must be controlled at the post-transcriptional level. Measurement of LmGT2 RNA decay in promastigotes and axenic amastigotes treated with actinomycin D suggests that differential mRNA stability may play a role in regulating glucose transporter mRNA levels in the two life cycle stages.

Cloning of Leishmania nucleoside transporter genes by rescue of a transport-deficient mutant

All parasitic protozoa studied to date are incapable of purine biosynthesis and must therefore salvage purine nucleobases or nucleosides from their hosts. This salvage process is initiated by purine transporters on the parasite cell surface. We have used a mutant line (TUBA5) of Leishmania donovani that is deficient in adenosine/pyrimidine nucleoside transport activity (LdNT1) to clone genes encoding these nucleoside transporters by functional rescue. Two such genes, LdNT1.1 and LdNT1.2, have been sequenced and shown to encode deduced polypeptides with significant sequence identity to the human facilitative nucleoside transporter hENT1. Hydrophobicity analysis of the LdNT1.1 and LdNT1.2 proteins predicted 11 transmembrane domains. Transfection of the adenosine/pyrimidine nucleoside transport-deficient TUBA5 parasites with vectors containing the LdNT1.1 and LdNT1.2 genes confers sensitivity to the cytotoxic adenosine analog tubercidin and concurrently restores the ability of this mutant line to take up [3H]adenosine and [3H]uridine. Moreover, expression of the LdNT1.2 ORF in Xenopus oocytes significantly increases their ability to take up [3H]adenosine, confirming that this single protein is sufficient to mediate nucleoside transport. These results establish genetically and biochemically that both LdNT1 genes encode functional adenosine/pyrimidine nucleoside transporters.

Cytoskeletal association is important for differential targeting of glucose transporter isoforms in Leishmania

The major glucose transporter of the parasitic protozoan Leishmania enriettii exists in two isoforms, one of which (iso-1) localizes to the flagellar membrane, while the other (iso-2) localizes to the plasma membrane of the cell body, the pellicular membrane. These two isoforms differ only in their cytosolic NH2-terminal domains. Using immunoblots and immunofluorescence microscopy of detergent-extracted cytoskeletons, we have demonstrated that iso-2 associates with the microtubular cytoskeleton that underlies the cell body membrane, whereas the flagellar membrane isoform iso-1 does not associate with the cytoskeleton. Deletion mutants that remove the first 25 or more amino acids from iso-1 are retargeted from the flagellum to the pellicular membrane, suggesting that these deletions remove a signal required for flagellar targeting. Unlike the full-length iso-1 protein, these deletion mutants associate with the cytoskeleton. Our results suggest that cytoskeletal binding serves as an anchor to localize the iso-2 transporter within the pellicular membrane, and that the flagellar targeting signal of iso-1 diverts this transporter into the flagellar membrane and away from the pellicular microtubules.

Aspartate 19 and glutamate 121 are critical for transport function of the myo-inositol/H+ symporter from Leishmania donovani

The protozoan flagellate Leishmania donovani has an active myo-inositol/proton symporter (MIT), which is driven by a proton gradient across the parasite membrane. We have used site-directed mutagenesis in combination with functional expression of transporter mutants in Xenopus oocytes and overexpression in Leishmania transfectants to investigate the significance of acidic transmembrane residues for proton relay and inositol transport. MIT has only three charged amino acids within predicted transmembrane domains. Two of these residues, Asp19 (TM1) and Glu121 (TM4), appeared to be critical for transport function of MIT, with a reduction of inositol transport to about 2% of wild-type activity when mutated to the uncharged amides D19N or E121Q and 20% (D19E) or 4% (E121D) of wild-type activity for the conservative mutations that retained the charge. Immunofluorescence microscopy of oocyte cryosections showed that MIT mutants were expressed on the oocyte surface at a similar level as MIT wild type, confirming that these mutations affect transport function and do not prevent trafficking of the transporter to the plasma membrane. The proton uncouplers carbonylcyanide-4-(trifluoromethoxy)phenylhydrazone and dinitrophenol inhibited inositol transport by 50-70% in the wild-type as well as in E121Q, despite its reduced transport activity. The mutant D19N, however, was stimulated about 4-fold by either protonophore and 2-fold by cyanide or increase of pH 7.5 to 8.5 but inhibited at pH 6.5. The conservative mutant D19E, in contrast, showed an inhibition profile similar to MIT wild type. We conclude that Asp19 and Glu121 are critical for myo-inositol transport, while the negatively charged carboxylate at Asp19 may be important for proton coupling of MIT.

Kinetoplastid glucose transporters

Protozoa of the order kinetoplastida have colonized many habitats, and several species are important parasites of humans. Adaptation to different environments requires an associated adaptation at a cell's interface with its environment, i.e. the plasma membrane. Sugar transport by the kinetoplastida as a phylogenetically related group of organisms offers an exceptional model in which to study the ways by which the carrier proteins involved in this process may evolve to meet differing environmental challenges. Seven genes encoding proteins involved in glucose transport have been cloned from several kinetoplastid species. The transporters all belong to the glucose transporter superfamily exemplified by the mammalian erythrocyte transporter GLUT1. Some species, such as the African trypanosome Trypanosoma brucei, which undergo a life cycle where the parasites are exposed to very different glucose concentrations in the mammalian bloodstream and tsetse-fly midgut, have evolved two different transporters to deal with this fluctuation. Other species, such as the South American trypanosome Trypanosoma cruzi, multiply predominantly in conditions of relative glucose deprivation (intracellularly in the mammalian host, or within the reduviid bug midgut) and have a single, relatively high-affinity type, transporter. All of the kinetoplastid transporters can also transport d-fructose, and are relatively insensitive to the classical inhibitors of GLUT1 transport cytochalasin B and phloretin.

Hexose uptake in Trypanosoma cruzi: structure-activity relationship between substrate and transporter

The gene encoding a hexose transporter, TcrHt1, from Trypanosoma cruzi has been functionally expressed in mammalian Chinese hamster ovary cells. Kinetic parameters of the heterologously expressed protein are very similar to those of the transporter identified in T. cruzi epimastigotes, confirming that TcrHT1 is the major transporter functioning in these parasites. A detailed analysis of substrate recognition using analogues of D-glucose substituted at each carbon position has been performed. The glucose transporter of T. cruzi does not recognize C-3 or C-6 analogues of D-glucose, whereas these analogues were recognized by the glucose transporter of bloodstream-form T. brucei. As for other kinetoplastid transporters, but in stark contrast to the mammalian GLUT family, TcrHT1 can also transport D-fructose, with relatively high affinity (Km = 0.682 +/- 0.003 mM). Amino acid side-chain-modifying reagents were also used to identify residues of the transporter present at the substrate-binding site. While specific modifiers of cysteine, histidine and arginine all inhibited catalytic activity, protection using substrate was only observed using the arginine-specific reagent, phenylglyoxal. Reagents which modify lysine residues had no effect on transport.

Glucose uptake in Trypanosoma vivax and molecular characterization of its transporter gene

A gene, TvHT1, encoding a glucose transporter protein, has been cloned from the haemoflagellate protozoon, Trypanosoma vivax, which has an active Kreb's cycle in the mammalian stage. The deduced polypeptide is similar in amino acid sequence to other kinetoplastid hexose transporters from Trypanosoma brucei (THT1 and THT2), Trypanosoma cruzi (TcrHT1) and Leishmania (Pro-1). The similarity is higher with THT2 (expressed in T. brucei insect forms) than with the other isoforms. The kinetic properties of glucose uptake in Chinese Hamster Ovary (CHO) cells expressing TvHT1 and in trypanosomes show s a saturable transport mechanism typical of a facilitated carrier system, with a similar affinity for D-glucose as that of the T. brucei bloodstream form carrier, THT1 (Km = 0.548 +/- 0.01 mM, Vmax = 4.26 +/- 0.12 nmol.min-1.mg protein-1 in CHO cells and Km = 0.585 +/- 0.068 mM, Vmax = 88.5 +/- 6.2 nmol.min-1.mg protein-1 in T. vivax). The specificity of the TvHT1 protein for various D-glucose analogues, as judged by inhibition of 2-deoxy-D-arabinose-hexose transport, shows properties that are intermediate between those of THT1 on the one hand and TcrHT1 and THT2 on the other. As with the hexose transporters in the other members of Kinetoplastida, the TvHT1-encoded system differs from erythrocyte-type glucose transport by its moderate sensitivity to cytochalasin B and its capacity to transport fructose.

Relationships between bacterial drug resistance pumps and other transport proteins

We have used three reference sequences representative of bacterial drug resistance pumps and sugar transport proteins to collect the 91 most closely related sequences from a composite, nonredundant protein sequence database. Having eliminated certain very close relatives, the remainder were subjected to analysis and alignment by using two different similarity matrices: one of these was a matrix based on structural conservation of amino acid residues in proteins of known conformation and the other was based on the more familiar mutational matrix. Unrooted similarity trees for these proteins were constructed for each matrix and compared. A systematic analysis of the differences between these trees was undertaken and the sequences were analyzed for the presence or absence of certain sequence motifs. The results show that the clades created by the two methods are broadly comparable but that there are some clusters of sequences that are significantly different. Further analysis confirmed that (1) the sequences collected by this objective method are all known or putative 12-helix (in some cases reported as 14-helix) transmembrane proteins, (2) there is evidence for few cases of an origin based on gene duplication, (3) the bacterial drug resistance pumps are distributed in more than one clade and cannot be regarded as a definitive subset of these proteins, and that (4) the diversity is such that there is no evidence of a single ancestral protein. The possible extension of the methods to other cases of divergent protein sequences is discussed.

Functional expression and characterization of the Trypanosoma brucei procyclic glucose transporter, THT2

The gene encoding THT2, one of two hexose-transporter isoforms present in Trypanosoma brucei, has been expressed in both Xenopus laevis oocytes and a stably transfected line of Chinese hamster ovary (CHO) cells. The heterologously expressed gene encodes a protein with pharmacological and kinetic parameters similar to those of the hexose transporter measured in procyclic-culture-form trypanosomes. The substrate recognition of the THT2 transporter differed from that of the THT1 isoform, which is expressed only in bloodstream forms, in that: (i) it has a relatively high affinity for substrate with a Km of 59 microM for 2-deoxy-D-glucose (2-DOG) and a similar high affinity for D-glucose (compared with Km of 0.5 mM for 2-DOG in bloodstream forms); (ii) the affinity for 6-deoxy-D-glucose (6-DOG) is two orders of magnitude lower than that for D-glucose, whereas the bloodstream-form transporter recognizes D-glucose and its 6-DOG analogue with similar affinity; (iii) the bloodstream-form transporter, but not THT2, recognizes 3-fluoro-3-deoxy-D-glucose. D-Fructose-transport capacity and insensitivity to D-galactose was also found in THT2-expressing CHO cells and procyclic trypanosomes. We conclude from these cumulative results that the THT2 gene encodes the transporter responsible for hexose transport in procyclic trypanosomes. The transport of 2-DOG in procyclic organisms was inhibited by both the protonophore, carbonyl cyanide 4-trifluoromethoxy phenylhydrazone (FCCP), and KCN, suggesting a requirement for a protonmotive force. However, sensitivity to these reagents depended on the external substrate concentration, with uptake being unaffected at substrate concentrations higher than 2 mM. THT2 expressed in CHO cells behaved as a facilitated transporter, and was unaffected by FCCP or KCN over the whole substrate concentration range tested.

Functional expression of a myo-inositol/H+ symporter from Leishmania donovani

The vast majority of surface molecules in such kinetoplastid protozoa as members of the genus Leishmania contain inositol and are either glycosyl inositol phospholipids or glycoproteins that are tethered to the external surface of the plasma membrane by glycosylphosphatidylinositol anchors. We have shown that the biosynthetic precursor for these abundant glycolipids, myo-inositol, is translocated across the parasite plasma membrane by a specific transporter that is structurally related to mammalian facilitative glucose transporters. This myo-inositol transporter has been expressed and characterized in Xenopus laevis oocytes. Two-electrode voltage clamp experiments demonstrate that this protein is a sodium-independent electrogenic symporter that appears to utilize a proton gradient to concentrate myo-inositol within the cell. Immunolocalization experiments with a transporter-specific polyclonal antibody reveal the presence of this protein in the parasite plasma membrane.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

Characterization of glucose transport and cloning of a hexose transporter gene in Trypanosoma cruzi

A gene from Trypanosoma cruzi, TcrHT1, which encodes a member of the glucose transporter superfamily has been cloned. The gene is similar in sequence to the T. brucei hexose transporter THT1 and the Leishmania transporter Pro-1 and is present in the T. cruzi genome as a cluster of at least eight tandemly reiterated copies. Northern blot analysis revealed two mRNA transcripts which differ in size with respect to their 3' untranslated regions. When injected with in vitro transcribed TcrHT1 mRNA, Xenopus oocytes express a hexose transporter with properties similar to those of T. cruzi. Glucose transport in T. cruzi is mediated via a carrier with unique properties when compared with the other glucose transporters already characterized among the Kinetoplastida. It is a facilitated transporter with a high affinity for D-glucose (Km = 84.1 +/- 7.9 microM and Vmax = 46 +/- 9.4 nmol/min per mg of protein) that shares with other kinetoplastid hexose transporters the ability to recognize D-fructose, which distinguishes these carriers from the human erythrocyte glucose transporter GLUT1.

Membrane-related processes and overall energy metabolism in Trypanosoma brucei and other kinetoplastid species

An electrochemical proton gradient exists across the plasma membrane and the mitochondrial membrane of the bloodstream form of Trypanosoma brucei. The membrane potential across the plasma membrane and the regulation of the internal pH depend on the temperature. Leishmania donovani regulates its internal pH and maintains a constant electrochemical proton gradient across its plasma membrane under all conditions examined. The mitochondrion of the T. brucei bloodstream form is energized, even though the reactions taking place in it do not result in net ATP synthesis and the Kreb's cycle and the respiratory chain are absent. Glucose is transported across the plasma membrane of T. brucei by a facilitated diffusion carrier, that can transport a wider range of substrates than its mammalian counterparts. Pyruvate exits the cell via a facilitated diffusion transporter as well. Conflicting evidence exists for the mechanism of glucose transport in L. donovani; biochemical evidence suggests proton/glucose symport, while facilitated diffusion is indicated by physiological data.

Biochemistry and molecular genetics of Leishmania glucose transporters

Glucose is utilized as a significant source of metabolic energy by Leishmania parasites. This sugar is accumulated by the parasite via a specific carrier-mediated transport system located in the parasite membrane. Parasites may also contain another transporter that shuttles glucose between the cytoplasm and the glycosome, a membrane-bound organelle where the early steps of glycolysis occur. The transport systems of both the insect stage promastigotes and the intracellular amastigotes have been characterized and shown to have kinetic properties that are consistent with the different physiological environments of the insect gut and the macrophage phagolysosome. Several genes have been cloned from Leishmania species which encode proteins with substantial sequence similarity to glucose transporters from mammals and lower eukaryotes. Two of these genes are expressed preferentially in the promastigote stage of the life cycle, where glucose is more readily available and more rapidly transported and metabolized than in the intracellular amastigotes. One of these two developmentally-regulated genes has been functionally expressed in Xenopus oocytes and shown to encode a glucose transporter. A third gene encodes a protein that is also a member of the glucose transporter family on the basis of sequence similarity and proposed secondary structure. However, the significant differences between this protein and the other two suggest that it is likely to transport a different substrate. Functional expression will be required to define the specific biochemical role of each gene within the parasite.

Mutational analysis of a signal sequence required for protein secretion in Leishmania major

In eukaryotes, amino-terminal extensions, signal sequences, mediate the translocation of lysosomal, membrane and secreted proteins into the lumen of the endoplasmic reticulum (ER). Structure/function studies indicate that eukaryotic signal sequences are composed of 3 distinct domains, a positively charged amino-terminal domain, a central hydrophobic domain, and a polar carboxy-terminal domain. In an attempt to better understand protein trafficking in Leishmania we have constructed strains of Leishmania major that secrete an exogenous protein, interferon-gamma (IFN-gamma), under the control of a mammalian signal sequence. In this report we present a mutational analysis of this signal sequence. Deletion of the entire signal sequence or the hydrophobic core region prevents secretion of IFN-gamma and results in cytoplasmic expression of the protein. Mutations in the amino-terminal domain indicate that a net positive charge is not required for efficient secretion of IFN-gamma. Mutations in the carboxy-terminal domain are more complex and display two phenotypes, either they prevent expression of IFN-gamma or they have no effect on protein secretion. These results indicate that the function of the signal sequence in targeting proteins to the ER in Leishmania is similar to that observed in yeasts and higher eukaryotes and suggests that the Leishmania protein secretory apparatus may also be similar.

Uptake and turnover of glucose in Leishmania donovani

Glucose uptake and metabolism by Leishmania donovani promastigotes was studied using D-[14C]glucose in combination with the silicone oil centrifugation technique on organisms preadapted to different growth rates and glucose availability in the chemostat. The uptake step was differentiated from the subsequent metabolism by separation in time rather than by using non-metabolisable analogues. The uptake of glucose was measured as a function of time and/or the external glucose concentration on cells grown at high or low growth rate with glucose either as growth rate-limiting substrate, or present in excess. Glucose uptake as a function of its external concentration could be described as consisting of two components (1) a rapid equilibration owing to facilitated diffusion, followed by (2) a much slower uptake that involves an enzymatic component. This slower accumulation of label could be explained as the conversion of glucose into metabolites and a storage carbohydrate. Uptake experiments in the presence of inhibitors indicated that the conversion of glucose was an energy dependent process. These experiments indicate that the active uptake of glucose by L. donovani, as reported by others does not occur across the plasma membrane and should be reinterpreted as the intracellular conversion of glucose into metabolites and storage carbohydrate.