Transporters for drug delivery and as drug targets in parasitic protozoa

Parasitic protozoa cause devastating diseases across large regions of the globe, but a lack of economic incentives has resulted in the limited development of drugs against these "neglected diseases." Transporters expressed in the plasma membranes of these parasites offer potential for the development of new drugs. These permeases could be employed in two distinct strategies for drug development: (i) targeting selective delivery of drugs to the parasite and (ii) developing drugs that inhibit essential parasite permeases.

Molecular genetics of nucleoside transporters in Leishmania and African trypanosomes

Nucleoside transporters play central roles in the biochemistry of parasitic protozoa such as Leishmania and African trypanosomes, because these parasites cannot synthesize purines de novo and are absolutely reliant upon purine salvage from their hosts. Furthermore, nucleoside transporters are important to the pharmacology of these significant human pathogens, because they mediate the uptake of purine analogs, as well as some non-purine drugs, that are selectively cytotoxic to the parasites. Recent advances in molecular biology and genomics have allowed the cloning and functional expression of several nucleoside transporter genes from Leishmania donovani and Trypanosoma brucei, providing molecular reagents for a detailed functional examination of these permeases and their role in the delivery of nutrients and drugs to the parasites. Furthermore, the molecular basis of drug-resistant mutants that are deficient in nucleoside transport functions can now be fathomed.

The flagellum and flagellar pocket of trypanosomatids

The flagellum and flagellar pocket are distinctive organelles present among all of the trypanosomatid protozoa. Currently, recognized functions for these organelles include generation of motility for the flagellum and dedicated secretory and endocytic activities for the flagellar pocket. The flagellar and flagellar pocket membranes have long been recognized as morphologically separate domains that are component parts of the plasma membrane that surrounds the entire cell. The structural and functional specialization of these two membranes has now been underscored by the identification of multiple proteins that are targeted selectively to each of these domains, and non-membrane proteins have also been identified that are targeted to the internal lumina of these organelles. Investigations on the functions of these organelle-specific proteins should continue to shed light on the unique biological activities of the flagellum and flagellar pocket. In addition, work has begun on identifying signals or modifications of these proteins that direct their targeting to the correct subcellular location. Future endeavors should further refine our knowledge of targeting signals and begin to dissect the molecular machinery involved in transporting and retaining each polypeptide at its designated cellular address.

Point mutations in a nucleoside transporter gene from Leishmania donovani confer drug resistance and alter substrate selectivity

Leishmania parasites lack a purine biosynthetic pathway and depend on surface nucleoside and nucleobase transporters to provide them with host purines. Leishmania donovani possess two closely related genes that encode high affinity adenosine-pyrimidine nucleoside transporters LdNT1.1 and LdNT1.2 and that transport the toxic adenosine analog tubercidin in addition to the natural substrates. In this study, we have characterized a drug-resistant clonal mutant of L. donovani (TUBA5) that is deficient in LdNT1 transport and consequently resistant to tubercidin. In TUBA5 cells, the LdNT1.2 genes had the same sequence as wild-type cells. However, because LdNT1.2 mRNA is not detectable in either wild-type or TUBA5 promastigotes, LdNT1.2 does not contribute to nucleoside transport in this stage of the life cycle. In contrast, the TUBA5 cells were compound heterozygotes at the LdNT1.1 locus containing two mutant alleles that encompassed distinct point mutations, each of which impaired transport function. One of the mutant LdNT1.1 alleles encoded a G183D substitution in predicted TM 5, and the other allele contained a C337Y change in predicted TM 7. Whereas G183D and C337Y mutants had only slightly elevated adenosine K(m) values, the severe impairment in transport resulted from drastically ( approximately 20-fold) reduced V(max) values. Because these transporters were correctly targeted to the plasma membrane, the reduction in V(max) apparently resulted from a defect in translocation. Strikingly, G183 was essential for pyrimidine nucleoside but not adenosine transport. A mutant transporter with a G183A substitution had an altered substrate specificity, exhibiting robust adenosine transport but undetectable uridine uptake. These results suggest that TM 5 is likely to form part of the nucleoside translocation pathway in LdNT1.1

Nucleoside transporters of parasitic protozoa

Protozoan parasites are incapable of synthesizing purine nucleotides de novo and so must salvage preformed purines from their hosts. This process of purine acquisition is initiated by the translocation of preformed host purines across parasite or host membranes. Here, we report upon the identification and isolation of DNAs encoding parasite nucleoside transporters and the functional characterization of these proteins in various expression systems. These potential approaches provide a powerful approach for a thorough molecular and biochemical dissection of nucleoside transport in protozoan parasites.

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.

Cloning of a novel inosine-guanosine transporter gene from Leishmania donovani by functional rescue of a transport-deficient mutant

Purine transport is an indispensable nutritional function for protozoan parasites, since they are incapable of purine biosynthesis and must, therefore, acquire purines from the host milieu. Exploiting a mutant cell line (FBD5) of Leishmania donovani deficient in inosine and guanosine transport activity, the gene encoding this transporter (LdNT2) has been cloned by functional rescue of the mutant phenotype. LdNT2 encodes a polypeptide of 499 amino acids that shows substantial homology to other members of the equilibrative nucleoside transporter family. Molecular analysis revealed that LdNT2 is present as a single gene copy within the leishmanial genome and encodes a single transcript of 3 kilobase pairs. Transfection of FBD5 parasites with LdNT2 re-established their ability to take up inosine and guanosine with a concurrent restoration of sensitivity to the inosine analog formycin B. Kinetic analyses reveal that LdNT2 is highly specific for inosine (K(m) = 0.3 micrometer) and guanosine (K(m) = 1.7 micrometer) and does not recognize other naturally occurring nucleosides. Expression of LdNT2 cRNA in Xenopus oocytes significantly augmented their ability to take up inosine and guanosine, establishing that LdNT2 by itself suffices to mediate nucleoside transport. These results authenticate genetically and biochemically that LdNT2 is a novel nucleoside transporter with an unusual and strict specificity for inosine and guanosine.

Isolation and functional characterization of the PfNT1 nucleoside transporter gene from Plasmodium falciparum

Plasmodium falciparum, the causative agent of the most lethal form of human malaria, is incapable of de novo purine synthesis, and thus, purine acquisition from the host is an indispensable nutritional requirement. This purine salvage process is initiated by the transport of preformed purines into the parasite. We have identified a gene encoding a nucleoside transporter from P. falciparum, PfNT1, and analyzed its function and expression during intraerythrocytic parasite development. PfNT1 predicts a polypeptide of 422 amino acids with 11 transmembrane domains that is homologous to other members of the equilibrative nucleoside transporter family. Southern analysis and BLAST searching of The Institute for Genomic Research (TIGR) malaria data base indicate that PfNT1 is a single copy gene located on chromosome 14. Northern analysis of RNA from intraerythrocytic stages of the parasite demonstrates that PfNT1 is expressed throughout the asexual life cycle but is significantly elevated during the early trophozoite stage. Functional expression of PfNT1 in Xenopus laevis oocytes significantly increases their ability to take up naturally occurring D-adenosine (K(m) = 13.2 microM) and D-inosine (K(m) = 253 microM). Significantly, PfNT1, unlike the mammalian nucleoside transporters, also has the capacity to transport the stereoisomer L-adenosine (K(m) > 500 microM). Inhibition studies with a battery of purine and pyrimidine nucleosides and bases as well as their analogs indicate that PfNT1 exhibits a broad substrate specificity for purine and pyrimidine nucleosides. These data provide compelling evidence that PfNT1 encodes a functional purine/pyrimidine nucleoside transporter whose expression is strongly developmentally regulated in the asexual stages of the P. falciparum life cycle. Moreover, the unusual ability to transport L-adenosine and the vital contribution of purine transport to parasite survival makes PfNT1 an attractive target for therapeutic evaluation.

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.

Substrate depletion upregulates uptake of myo-inositol, glucose and adenosine in Leishmania

Leishmania flagellates undergo a digenetic life cycle in the gut of the sandfly insect vector and in macrophage phagolysosomes of the mammalian host. This involves vast changes of the environment to which the parasite has to adapt, including temperature, pH and concentration of nutrients between different types of meals of the insect vector or within the enclosed intracellular environment of the phagolysosome. The regulation of transporters for important organic substrates in Leishmania donovani, Leishmania mexicana and Leishmania enriettii has been investigated. A pronounced upregulation of inositol (25-fold), adenosine (11-fold) or glucose (5-fold) uptake activities was found when cells were depleted of the respective substrates during culture. Inositol-depleted cells showed a half-maximal uptake rate at nanomolar inositol concentration. Depletion of inositol only affected inositol uptake but did not affect uptake of glucose analog or proline in control experiments, indicating the specificity of the mechanism(s) underlying transport regulation. Adenosine-depleted cells showed an approximately 10-fold increase in both adenosine and uridine uptake, both mediated by the L. donovani nucleoside transporter 1 (LdNT1), but no change in guanosine uptake, which is mediated by the L. donovani nucleoside transporter 2 (LdNT2). These results suggest that extracellular adenosine concentration specifically regulates LdNT1 transport activity and does not affect LdNT2. The data imply that upregulation of transport activities by substrate depletion is a general phenomenon in protozoan flagellates, which is in remarkable contrast to bacteria where upregulation typically follows an increase of extracellular organic substrate. Hence, the parasites can maximize the uptake of important nutrients from the host even under limiting conditions, whereas bacteria often have dormant stages (spores) to overcome unfavorable environmental conditions or are heterotrophic for organic substrates.

Cloning and functional expression of a gene encoding a P1 type nucleoside transporter from Trypanosoma brucei

Nucleoside transporters are likely to play a central role in the biochemistry of the parasite Trypanosoma brucei, since these protozoa are unable to synthesize purines de novo and must salvage them from their hosts. Furthermore, nucleoside transporters have been implicated in the uptake of antiparasitic and experimental drugs in these and other parasites. We have cloned the gene for a T. brucei nucleoside transporter, TbNT2, and shown that this permease is related in sequence to mammalian equilibrative nucleoside transporters. Expression of the TbNT2 gene in Xenopus oocytes reveals that the permease transports adenosine, inosine, and guanosine and hence has the substrate specificity of the P1 type nucleoside transporters that have been previously characterized by uptake assays in intact parasites. TbNT2 mRNA is expressed in bloodstream form (mammalian host stage) parasites but not in procyclic form (insect stage) parasites, indicating that the gene is developmentally regulated during the parasite life cycle. Genomic Southern blots suggest that there are multiple genes related in sequence to TbNT2, implying the existence of a family of nucleoside transporter genes in these parasites.

Characterization of a targeting motif for a flagellar membrane protein in Leishmania enriettii

The surface membranes of eukaryotic flagella and cilia are contiguous with the plasma membrane. Despite the absence of obvious physical structures that could form a barrier between the two membrane domains, the lipid and protein compositions of flagella and cilia are distinct from the rest of the cell surface membrane. We have exploited a flagellar glucose transporter from the parasitic protozoan Leishmania enriettii as a model system to characterize the first targeting motif for a flagellar membrane protein in any eukaryotic organism. In this study, we demonstrate that the flagellar membrane-targeting motif is recognized by several species of Leishmania. Previously, we demonstrated that the 130 amino acid NH(2)-terminal cytoplasmic domain of isoform 1 glucose transporter was sufficient to target a nonflagellar integral membrane protein into the flagellar membrane. We have now determined that an essential flagellar targeting signal is located between amino acids 20 and 35 of the NH(2)-terminal domain. We have further analyzed the role of specific amino acids in this region by alanine replacement mutagenesis and determined that single amino acid substitutions did not abrogate targeting to the flagellar membrane. However, individual mutations located within a cluster of five contiguous amino acids, RTGTT, conferred differences in the degree of targeting to the flagellar membrane and the flagellar pocket, implying a role for these residues in the mechanism of flagellar trafficking.

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.

Kinetics and stoichiometry of a proton/myo-inositol cotransporter

Voltage clamp recording was used to measure steady-state and presteady-state currents mediated by a myo-inositol transporter cloned from Leishmania donovani and expressed in Xenopus oocytes. Application of myo-inositol resulted in inward currents, which did not require external sodium and which were increased by increasing the extracellular proton concentration and by membrane hyperpolarization. Alkalinization of the extracellular space occurred concomitantly with myo-inositol influx. Correlation of membrane currents with radiolabeled myo-inositol flux revealed that one positive charge is translocated with each molecule of myo-inositol, consistent with cotransport of one proton. The transport concentration dependence on both species suggested ordered binding of a proton followed by a molecule of myo-inositol. In the absence of myo-inositol, a voltage-dependent capacitance was observed that correlated with the transporter expression level. This charge movement obeyed a Boltzmann function, which was used to estimate a turnover of 0.70 +/- 0.06 s-1 at -60 mV. The pH and voltage dependence of the charge movements were simulated with a model involving alternating access of internal and external protons to sites within an occluded pore.

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.

Functional expression and subcellular localization of a high-Km hexose transporter from Leishmania donovani

We have used expression in Xenopus oocytes to characterize a new hexose transporter from the parasitic protozoan Leishmania donovani. This transporter utilizes the hexoses glucose, fructose, and mannose as substrates. A substrate saturation curve for 2-deoxy-D-glucose reveals a very high Km, estimated to be approximately 150 mM. Immunolocalization of the protein with an antibody directed against the COOH terminus indicates that the transporter is present primarily in the parasite plasma membrane but is not detectable in the flagellar membrane. Since this protein is expressed in the insect stage promastigotes but not in the intracellular amastigotes, it may be specialized to function following an insect sugar meal when the concentrations of sugars surrounding the parasite are high.

A family of putative receptor-adenylate cyclases from Leishmania donovani

Leishmania parasites are exposed to pronounced changes in their environment during their life cycle as they migrate from the sandfly midgut to the insect proboscis and then into the phagolysosomes of the vertebrate macrophages. The developmental transformations that produce each life cycle stage of the parasite may be signaled in part by binding of environmental ligands to receptors which mediate transduction of extracellular signals. We have identified a family of five clustered genes in Leishmania donovani which may encode signal transduction receptors. The coding regions of two of these genes, designated rac-A and rac-B, have been sequenced and shown to code for proteins with an NH2-terminal hydrophilic domain, an intervening putative transmembrane segment, and a COOH-terminal domain that has high sequence identity to the catalytic domain from adenylate cyclases in other eukaryotes. We have expressed the receptor-adenylate cyclase protein (RAC)-A protein in Xenopus oocytes and demonstrated that it functions as an adenylate cyclase. Although RAC-B exhibits no catalytic activity when expressed in oocytes, co-expression of RAC-A and RAC-B negatively regulates the adenylate cyclase activity of RAC-A, suggesting that these two proteins interact in the membrane. Furthermore, a truncated version of RAC-A functions as a dominant negative mutant that inhibits the catalytic activity of the wild type receptor. The rac-A and rac-B genes encode developmentally regulated mRNAs which are expressed in the insect stage but not in the mammalian host stage of the parasite life cycle.

Differential targeting of two glucose transporters from Leishmania enriettii is mediated by an NH2-terminal domain

Leishmania are parasitic protozoa with two major stages in their life cycle: flagellated promastigotes that live in the gut of the insect vector and nonflagellated amastigotes that live inside the lysosomes of the vertebrate host macrophages. The Pro-1 glucose transporter of L. enriettii exists as two isoforms, iso-1 and iso-2, which are both expressed primarily in the promastigote stage of the life cycle. These two isoforms constitute modular structures: they differ exclusively and extensively in their NH2-terminal hydrophilic domains, but the remainder of each isoform sequence is identical to that of the other. We have localized these glucose transporters within promastigotes by two approaches. In the first method, we have raised a polyclonal antibody against the COOH-terminal hydrophilic domain shared by both iso-1 and iso-2, and we have used this antibody to detect the transporters by confocal immunofluorescence microscopy and immunoelectron microscopy. The staining observed with this antibody occurs primarily on the plasma membrane and the membrane of the flagellar pocket, but there is also light staining on the flagellum. We have also localized each isoform separately by introducing an epitope tag into each protein sequence. These experiments demonstrate that iso-1, the minor isoform, resides primarily on the flagellar membrane, while iso-2, the major isoform, is located on the plasma membrane and the flagellar pocket. Hence, each isoform is differentially sorted, and the structural information for targeting each transporter isoform to its correct membrane address resides within the NH2-terminal hydrophilic domain.

Functional expression of two glucose transporter isoforms from the parasitic protozoan Leishmania enriettii

The parasitic protozoan Leishmania enriettii contains a family of tandemly repeated genes, designated Pro-1, that encode proteins with significant sequence similarity to mammalian facilitative glucose transporters. Pro-1 mRNAs are expressed almost exclusively in the promastigote or insect stage of the parasite life cycle. The Pro-1 tandem repeat encodes two isoforms of the putative transporter, iso-1 and iso-2, which have identical predicted amino acid sequences except for their NH2-terminal hydrophilic domains. We have now expressed both iso-1 and iso-2 by microinjecting their RNAs into Xenopus oocytes and assaying these oocytes for transport of various radiolabeled ligands. Both iso-1 and iso-2 transport [3H]2-deoxy-D-glucose, confirming that each protein is a bona fide glucose transporter. Each isoform also transports fructose and, to a much lesser degree, mannose. Compounds which inhibit 2-deoxy-D-glucose transport in L. enriettii promastigotes also inhibit transport in the microinjected oocytes expressing each isoform, indicating that the substrate specificities and pharmacological properties of each isoform are similar to those measured for 2-deoxy-D-glucose transport in intact parasites. The Km for transport of 2-deoxyglucose in oocytes expressing iso-1 is similar to that for oocytes expressing iso-2. These results reveal that both transporter isoforms have closely related functional properties and that the difference in their structures may serve some other purpose such as differential subcellular localization.

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.

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

The polymerase chain reaction was used to clone two genes from Leishmania donovani, each of which encodes a member of a superfamily of membrane transporters which include the mammalian facilitative glucose transporters. One of these transporters, designated D2, is similar in sequence and overall structural features to a previously cloned Leishmania transporter Pro-1. Both D2 and Pro-1 are developmentally regulated genes which are expressed primarily in the insect stage of the parasite life cycle. In contrast, the second novel transporter, D1, is structurally quite different from either D2 or Pro-1, and its expression is not regulated during the parasite life cycle. All three genes are located on different chromosomes in L. donovani.

18S rRNA sequences of Leishmania enriettii promastigote and amastigote

Dideoxy sequencing with reverse transcriptase and universal primers was used to obtain partial sequences of the 18S rRNAs from the promastigote and amastigote life-cycle stages of L. enriettii. Approximately 1400 nucleotides of sequence from the two stages were compared. Unlike Plasmodium berghei, in which 18S rRNAs from the mosquito stage and the mammalian stage of the life cycle are only 96.5% similar, the amastigote and promastigote rRNAs of L. enriettii are identical. In addition, a comparison of 1425 bases of the L. enriettii promastigote sequence with the published sequence of L. donovani revealed only four differences; the two sequences are 99.8% similar. A likely explanation for this high similarity, considering the 97% similarity between L. donovani and the related genus Crithidia fasciculata, is that the two species are closely related and of comparatively recent origin. The low diversity between the 18S rRNA sequences of Leishmania species is similar to that reported for 13 Tetrahymena species, where similarities ranged from 98.1 to 99.9%, but different from the pattern reported in the genus Naegleria, where divergence was greater.

Structural isoforms of a membrane transport protein from Leishmania enriettii

A membrane transport protein of the glucose transporter superfamily from Leishmania enriettii is encoded by a family of tandemly repeated genes. The first gene in this tandem repeat codes for a structural isoform that contains a unique amino-terminal hydrophilic domain, probably located in the cytoplasm; the remainder of the protein is identical to the polypeptide encoded by the internal genes in the tandem repeat. The unique isoform is represented by a distinct stable RNA.

Developmentally regulated transporter in Leishmania is encoded by a family of clustered genes

We have previously cloned a gene for a developmentally regulated transport protein from the trypanosomatid protozoan Leishmania enriettii. We demonstrate here that this transporter is encoded by a single family of tandemly clustered genes containing approximately 8 copies of the 3.6 kilobase repeat unit. Transcriptional mapping defines a contiguous 3.3 kilobase region of the repeat unit that encodes the mRNA. The 5' end of the mature mRNA contains the spliced leader or mini-exon previously identified in kinetoplastid protozoa, while the 3' ends of the mRNA are heterogeneous in sequence and in location of the polyadenylation site. We have identified genomic restriction fragments that flank the tandem repeat on the 5' and 3' sides and which may be linked to sequences required for expression of the gene family. Other species of Leishmania also contain sequences that hybridize to the cloned L. enriettii gene at high stringency.

Developmentally regulated gene from Leishmania encodes a putative membrane transport protein

We have cloned a developmentally regulated gene from the parasitic protozoan Leishmania enrietti. The mRNA from this gene accumulates to a much higher level in the promastigote stage of the parasite life cycle that lives in the gut of the insect vector than in the amastigote stage of the parasite that lives inside the macrophages of the mammalian host. The predicted protein encoded by this gene is homologous to the human erythrocyte glucose transporter and to several sugar-transport proteins from Escherichia coli. These structural similarities strongly suggest that the cloned gene encodes a membrane transport protein that is developmentally induced when the parasite enters its insect vector. Regulated membrane transporters may be required for the parasite to adapt to the environment of the insect gut.

Transcriptional mapping of Leishmania enrietti tubulin mRNAs

We have mapped the mRNAs for alpha- and beta-tubulins of Leishmania enriettii promastigotes and amastigotes and have demonstrated that each RNA contains a 35 nucleotide sequence on its 5' end which is not encoded contiguously with the rest of the message. Sequencing of the 5' end of the alpha- and beta-tubulin mRNAs revealed that this 35 nucleotide leader sequence is identical in both messages and that it is homologous to the spliced leader found on the 5' end of mRNAs in the related parasite Trypanosoma brucei. Additionally, we have sequenced regions of the alpha- and beta-tubulin genomic clones upstream from the mRNA encoding regions and have shown that the leader sequence is not encoded within these regions of DNA. Hence, Leishmania tubulin mRNAs may be synthesized by a discontinuous transcription process that links together the transcription products of two separate gene families, as suggested previously by others for the assembly of T. brucei mRNAs. Homologies between the Leishmania alpha- and beta-tubulin genes themselves and between these genes and the T. brucei alpha- and beta-tubulin genes have revealed sequences which may be important in synthesis or processing of Leishmania tubulin mRNAs.

Cloning and characterization of a Leishmania gene encoding a RNA spliced leader sequence

Recent studies on leishmania enriettii tubulin mRNAs revealed a 35 nucleotide addition to their 5' end. The gene that codes for this 35 nucleotide leader sequence has now been cloned and sequenced. In the Leishmania genome, the spliced leader gene exists as a tandem repeat of 438 bases. There are approximately 150 copies of this gene comprising 0.1% of the parasite genome. This gene codes for a 85 nucleotide transcript that contains the spliced leader at its 5' end. The 35 nucleotide sequence and the regions immediately 5' and 3' to it are highly conserved across trypanosomatids. We have detected a RNA molecule that is a putative by-product of the processing reaction in which the 35 nucleotide spliced leader has been transferred to mRNA. We suggest that this molecule is the remnant of the spliced leader transcript after removal of the 35 nucleotide spliced leader.

Structure of mRNA encoded by tubulin genes in Leishmania enriettii

Both the alpha- and beta-tubulin genes of Leishmania enriettii are encoded by mRNA of 2.0-2.2 kb in length. We have shown previously that the alpha- and beta-tubulin genes are arranged in separate, tandem repeats of 2 and 4 kb, respectively, and now report the mapping of mature mRNA onto these cloned genes. Here we show that the alpha-tubulin gene contains a very short intergenic region (100-200 bases) whereas the larger, tandemly repeated beta-tubulin gene contains a 1.8-2.0 kb region not found in mature mRNA. Comparison of S-1 mapping and primer extension results indicates that the messenger RNAs for both alpha- and beta-tubulin contain a sequence of about 35 base pairs located at the 5' end that is not encoded contiguously with the rest of the mRNA. This short 5' sequence may be added to the body of the tubulin mRNAs either through splicing of a precursor RNA molecule or by a novel post-transcriptional processing reaction. The alpha- and beta-tubulin genes are arranged identically in the two developmental stages of the parasite life cycle and are present in equal copy number.

Control of tubulin gene expression in the parasitic protozoan Leishmania enriettii

The life cycle of parasitic protozoa of the genus Leishmania consists of two morphologically distinct forms: (1) amastigotes, the form of the parasite that resides inside macrophages of the mammalian host, which are non-motile and possess only a residual flagellum; and (2) promastigotes, the extracellular forms that live in the gut of the sandfly vector, which are highly motile and possess a single prominent flagellum. During the developmental transformation of amastigotes to promastigotes, the biosynthesis of alpha- and beta-tubulin proteins is dramatically increased, presumably to provide sufficient tubulin for synthesis and maintenance of the flagellum. We show here that the level of alpha- and beta-tubulin mRNA in Leishmania enriettii promastigotes is significantly greater than that in amastigotes, as determined by both Northern blot analysis and by in vitro translation of cellular RNA. These results show that the regulated expression of the tubulin genes is controlled at the level of mRNA accumulation in L. enriettii , by contrast with Leishmania mexicana, in which the control of gene expression has been reported to be at the level of translation.

Tandem arrangement of tubulin genes in the protozoan parasite Leishmania enriettii

The arrangement of developmentally regulated alpha- and beta-tubulin genes has been studied in the parasitic protozoan Leishmania enriettii by using Southern blot hybridization analysis. The alpha-tubulin genes occur in a tandem repeat whose monomeric unit may be represented by a 2-kilobase PstI fragment. Similarly, the beta-tubulin genes probably occur in a separate tandem repeat consisting of approximately 4-kilobase units unlinked to the alpha-tubulin repeats.

Transcriptional control of gene expression during development of Dictyostelium discoideum

During development of the cellular slime mold Dictyostelium discoideum, approximately 2,000 to 3,000 regulated mRNAs are induced when amoebae enter multicellular aggregates. We used in vitro transcription in isolated nuclei to follow the synthesis of individual mRNA precursors during development; these were quantitated by hybridization to cloned cDNAs or genomic DNAs. Those RNAs that are present at all stages of development--the common RNAs--were transcribed by nuclei from cells at all stages of development. By contrast, those RNAs that are present only after cells begin to aggregate--here called aggregation stage RNAs--were transcribed only by nuclei from cells at the aggregation and postaggregation stages of development. The temporal pattern of in vitro transcription correlated well with the time course of accumulation of different aggregation stage mRNAs. Continued expression of aggregation stage genes normally depends upon cell-to-cell contact or cyclic AMP (cAMP); when cells are disaggregated, the regulated mRNAs are rapidly and specifically degraded. When cAMP is subsequently added to the disaggregated cells, most of the mRNAs reaccumulate. We show here that disaggregation reduced 2- to 10-fold the relative transcription of several aggregation stage RNAs, whereas addition of cAMP to disaggregated cells reinduced the level of regulated gene transcription to values approximating those found in normal postaggregation cells. These results indicate that a representative set of Dictyostelium aggregation stage genes are under transcriptional control; both the transcription and the stability of these mRNAs require either continued cell-to-cell interactions or cAMP.

Synthesis and stability of developmentally regulated dictyostelium mRNAs are affected by cell--cell contact and cAMP

Postaggregation Dictyostelium discoideum cells contain 3000 mRNA species that are absent from preaggregation cells; these aggregation-dependent sequences comprise 30% of the mass of mRNA in these cells. We show that the synthesis and stability of these regulated mRNA sequences are affected by both cell--cell contact and cAMP. Three independent assays are used to quantitate these mRNAs: in vitro translation followed by two-dimensional gel analysis of the protein products; hybridization of gel-separated RNAs to cloned DNAs; and hybridization of mRNA to a cDNA probe specific for the population of regulated sequences. In postaggregation cells, the half-life of both the developmentally regulated mRNAs and the constitutive mRNAs present throughout growth and differentiation is the same--about 4 hr. Following disaggregation, all of the late mRNA sequences are degraded and decay with a half-life of 25 to 45 min. The constitutive species are unaffected; 2.5 hr after disaggregation, the ratio of late to constitutive mRNAs is about 6% that of normal plated cells. Addition of cAMP to cells that have been disaggregated for 2.5 hr (or longer) restores the level of most late mRNAs within 3 hr. We conclude that cAMP stimulates the synthesis of these mRNAs and may also act to stabilize them in the cytoplasm. This effect of cAMP is dependent on the cells having been in contact with other cells; cAMP has no effect on the levels of mRNA in suspension-starved, aggregation-competent cells that have never formed cell--cell aggregates.

A role for cyclic AMP in expression of developmentally regulated genes in Dictyostelium discoideum

Starved cells of Dictyostelium discoideum begin to synthesize a new class of developmentally regulated proteins at about 13 hr of the 24-hr developmental program, concomitant with the formation of tips on the tight cell aggregates [Alton, T. H. & Lodish, H. F. (1977) Dev. Biol. 60, 180--206]. Continued synthesis of these proteins is normally dependent upon the integrity of the multicellular aggregates, because cells that have been disaggregated at 13 hr and shaken in suspension for 5 hr do not make these proteins. We show here that addition of 20 microM cyclic AMP to suspension cultures of disaggregated 13-hr cells caused synthesis of most of these late proteins to be maintained. Translation in an in vitro wheat germ system of total cellular RNA isolated from these cyclic AMP-stimulated suspension cells, or from normal aggregates, generated several proteins that were not encoded by the RNA isolated from equivalent suspension cells which had not been treated with cyclic AMP or from preaggregation cells. We conclude that cyclic AMP has a direct role in maintaining the synthesis of aggregation-dependent Dictyostelium proteins and in maintaining the level of the corresponding mRNAs.

Elimination of cooperativity in aspartate transcarbamylase by nitration of a single tyrosine residue

In a previous report [Landfear, S. M., Lipscomb, W. N. & Evans, D.R. (1978) J. Biol. Chem. 253, 3988--3996] we demonstrated that tetranitromethane can be employed to nitrate a limited number of tyrosine residues in aspartate transcarbamylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2); such modification eliminates cooperativity, feedback inhibition, and enzymatic activity, and reduces binding of the feedback inhibitor cytidine triphosphate. Cooperativity is lost more rapidly than other properties, and this loss correlates with the nitration of a single tyrosine residue. In this paper, we describe the saturation kinetics of hybrid species constructed from nitrated subunits of one type (either catalytic or regulatory) and native subunits of the other type. We conclude that the modification responsible for loss of cooperativity is on the catalytic subunit. The tryptic peptide containing this modification has been isolated and identified.

Interaction of tetraiodofluorescein with a modified form of aspartate transcarbamylase

Low concentrations of the dye tetraiodofluorescein activate native aspartate transcarbamylase (aspartate carbomoyltransferase, carbomoylphosphate:L-aspartate carbomoyltransferase, EC 2.1.3.2), while high concentrations inhibit the enzyme's activity [Jacobsberg, L. B., Kantrowitz, E. R. & Lipscomb, W. N. (1975) J. Biol. Chem. 250, 9238-9249]. This dye is now shown to produce similar effects upon a modified form of aspartate transcarbamylase produced by Escherichia coli grown in a culture medium supplemented with thiouracil. Significantly, the ATP-induced activation is reduced in the modified form of the enzyme to the same extent as is the tetraiodofluorescein-induced activation. Thus, a relationship is demonstrated between the internal mechanisms by which ATP and tetraiodofluorescein activate aspartate transcarbamylase.

Complex of aspartate carbamoyltransferase from Escherichia coli with its allosteric inhibitor, cytidine triphosphate: electron density at 5.9-angstroms resolution

Following our earlier determination of the three-dimensional structure of aspartate carbamoyltransferase (EC 2.1.3.2; carbamoylphosphate: L-aspartate carbamoyltransferase) to 5.5-A resolution [S. G. Warren, B. F. P. Edwards, D. R. Evans, D. C. Wiley & W. N. Lipscomb (1973) Proc. Nat. Acad. Sci. USA 70, 1117-1121], we report here, from a different crystal form, the three-dimensional structure at 5.9 A of this enzyme complexed with its allosteric inhibitor, cytidine triphosphate. Location of the major binding site of this inhibitor within each of the six regulatory chains is made secure by comparison of these results with those obtained upon binding of 5-iodocytidine triphosphate to the enzyme. Conformational changes in the aspartate carbamoyltransferase molecule when this inhibitor binds are described briefly at 5.9-A resolution.