Compartmentation of enzymes in a microbody, the glycosome, is essential in Trypanosoma brucei

Protists: Eukaryotic single-celled organisms and the functioning of their organelles

Organelles are membrane bound structures that compartmentalize biochemical and molecular functions. With improved molecular, biochemical and microscopy tools the diversity and function of protistan organelles has increased in recent years, providing a complex panoply of structure/function relationships. This is particularly noticeable with the description of hydrogenosomes, and the diverse array of structures that followed, having hybrid hydrogenosome/mitochondria attributes. These diverse organelles have lost the major, at one time, definitive components of the mitochondrion (tricarboxylic cycle enzymes and cytochromes), however they all contain the machinery for the assembly of Fe-S clusters, which is the single unifying feature they share. The plasticity of organelles, like the mitochondrion, is therefore evident from its ability to lose its identity as an aerobic energy generating powerhouse while retaining key ancestral functions common to both aerobes and anaerobes. It is interesting to note that the apicoplast, a non-photosynthetic plastid that is present in all apicomplexan protozoa, apart from Cryptosporidium and possibly the gregarines, is also the site of Fe-S cluster assembly proteins. It turns out that in Cryptosporidium proteins involved in Fe-S cluster biosynthesis are localized in the mitochondrial remnant organelle termed the mitosome. Hence, different organisms have solved the same problem of packaging a life-requiring set of reactions in different ways, using different ancestral organelles, discarding what is not needed and keeping what is essential. Don't judge an organelle by its cover, more by the things it does, and always be prepared for surprises.

A plasma membrane-associated glycolytic metabolon is functionally coupled to KATP channels in pancreatic α and β cells from humans and mice

The ATP-sensitive K+ (KATP) channel is a key regulator of hormone secretion from pancreatic islet endocrine cells. Using direct measurements of KATP channel activity in pancreatic β cells and the lesser-studied α cells, from both humans and mice, we provide evidence that a glycolytic metabolon locally controls KATP channels on the plasma membrane. The two ATP-consuming enzymes of upper glycolysis, glucokinase and phosphofructokinase, generate ADP that activates KATP. Substrate channeling of fructose 1,6-bisphosphate through the enzymes of lower glycolysis fuels pyruvate kinase, which directly consumes the ADP made by phosphofructokinase to raise ATP/ADP and close the channel. We further show the presence of a plasma membrane-associated NAD+/NADH cycle whereby lactate dehydrogenase is functionally coupled to glyceraldehyde-3-phosphate dehydrogenase. These studies provide direct electrophysiological evidence of a KATP-controlling glycolytic signaling complex and demonstrate its relevance to islet glucose sensing and excitability.

A Small Molecule Inhibitor of Pex3-Pex19 Interaction Disrupts Glycosome Biogenesis and Causes Lethality in Trypanosoma brucei

Trypanosomatid parasites, including Trypanosoma and Leishmania, are infectious zoonotic agents for a number of severe diseases such as African sleeping sickness and American trypanosomiasis (Chagas disease) that affect millions of people, mostly in the emergent world. The glycosome is a specialized member of the peroxisome family of organelles found in trypanosomatids. These organelles compartmentalize essential enzymes of the glycolytic pathway, making them a prime target for drugs that can kill these organisms by interfering with either their biochemical functions or their formation. Glycosome biogenesis, like peroxisome biogenesis, is controlled by a group of proteins called peroxins (Pex). Pex3 is an early acting peroxin that docks Pex19, the receptor for peroxisomal membrane proteins, to initiate biogenesis of peroxisomes from the endoplasmic reticulum. Identification of Pex3 as the essential master regulator of glycosome biogenesis has implications in developing small molecule inhibitors that can impede Pex3–Pex19 interaction. Low amino acid sequence conservation between trypanosomatid Pex3 and human Pex3 (HsPex3) would aid in the identification of small molecule inhibitors that selectively interfere with the trypanosomatid Pex3–Pex19 interaction. We tested a library of pharmacologically active compounds in a modified yeast two-hybrid assay and identified a compound that preferentially inhibited the interaction of Trypanosoma brucei Pex3 and Pex19 versus HsPex3 and Pex19. Addition of this compound to either the insect or bloodstream form of T. brucei disrupted glycosome biogenesis, leading to mislocalization of glycosomal enzymes to the cytosol and lethality for the parasite. Our results show that preferential disruption of trypanosomal Pex3 function by small molecule inhibitors could help in the accelerated development of drugs for the treatment of trypanosomiases.

Casein kinase TbCK1.2 regulates division of kinetoplast DNA, and movement of basal bodies in the African trypanosome

The single mitochondrial nucleoid (kinetoplast) of Trypanosoma brucei is found proximal to a basal body (mature (mBB)/probasal body (pBB) pair). Kinetoplast inheritance requires synthesis of, and scission of kinetoplast DNA (kDNA) generating two kinetoplasts that segregate with basal bodies into daughter cells. Molecular details of kinetoplast scission and the extent to which basal body separation influences the process are unavailable. To address this topic, we followed basal body movements in bloodstream trypanosomes following depletion of protein kinase TbCK1.2 which promotes kinetoplast division. In control cells we found that pBBs are positioned 0.4 um from mBBs in G1, and they mature after separating from mBBs by at least 0.8 um: mBB separation reaches ~2.2 um. These data indicate that current models of basal body biogenesis in which pBBs mature in close proximity to mBBs may need to be revisited. Knockdown of TbCK1.2 produced trypanosomes containing one kinetoplast and two nuclei (1K2N), increased the percentage of cells with uncleaved kDNA 400%, decreased mBB spacing by 15%, and inhibited cytokinesis 300%. We conclude that (a) separation of mBBs beyond a threshold of 1.8 um correlates with division of kDNA, and (b) TbCK1.2 regulates kDNA scission. We propose a Kinetoplast Division Factor hypothesis that integrates these data into a pathway for biogenesis of two daughter mitochondrial nucleoids.

Glycosome heterogeneity in kinetoplastids

Kinetoplastid parasites have essential organelles called glycosomes that are analogous to peroxisomes present in other eukaryotes. While many of the processes that regulate glycosomes are conserved, there are several unique aspects of their biology that are divergent from other systems and may be leveraged as therapeutic targets for the treatment of kinetoplastid diseases. Glycosomes are heterogeneous organelles that likely exist as sub-populations with different protein composition and function in a given cell, between individual cells, and between species. However, the limitations posed by the small size of these organelles makes the study of this heterogeneity difficult. Recent advances in the analysis of small vesicles by flow-cytometry provide an opportunity to overcome these limitations. In this review, we describe studies that document the diverse nature of glycosomes and propose an approach to using flow cytometry and organelle sorting to study the diverse composition and function of these organelles. Because the cellular machinery that regulates glycosome protein import and biogenesis is likely to contribute, at least in part, to glycosome heterogeneity we highlight some ways in which the glycosome protein import machinery differs from that of peroxisomes in other eukaryotes.

Functional insight into the glycosomal peroxiredoxin of Leishmania

Glycosomes of trypanosomatids are peroxisome-like organelles comprising unique metabolic features, among which the lack of the hallmark peroxisomal enzyme catalase. The absence of this highly efficient peroxidase from glycosomes is presumably compensated by other antioxidants, peroxidases of the peroxiredoxin (PRX) family being the most promising candidates for this function. Here, we follow on this premise and investigate the product of a Leishmania infantum gene coding for a putative glycosomal PRX (LigPRX). First, we demonstrate that LigPRX localizes to glycosomes, resorting to indirect immunofluorescence analysis. Second, we prove that purified recombinant LigPRX is an active peroxidase in vitro. Third, we generate viable LigPRX-depleted L. infantum promastigotes by classical homologous recombination. Surprisingly, phenotypic analysis of these knockout parasites revealed that promastigote survival, replication, and protection from oxidative and nitrosative insults can proceed normally in the absence of LigPRX. Noticeably, we also witness that LigPRX-depleted parasites can infect and thrive in mice to the same extent as wild type parasites. Overall, by disclosing the dispensable character of the glycosomal peroxiredoxin in L. infantum, this work excludes this enzyme from being a key component of the glycosomal hydroperoxide metabolism and contemplates alternative players for this function.

Proteomic analysis of glycosomes from Trypanosoma cruzi epimastigotes

In Trypanosoma cruzi, the causal agent of Chagas disease, the first seven steps of glycolysis are compartmentalized in glycosomes, which are authentic but specialized peroxisomes. Besides glycolysis, activity of enzymes of other metabolic processes have been reported to be present in glycosomes, such as β-oxidation of fatty acids, purine salvage, pentose-phosphate pathway, gluconeogenesis and biosynthesis of ether-lipids, isoprenoids, sterols and pyrimidines. In this study, we have purified glycosomes from T. cruzi epimastigotes, collected the soluble and membrane fractions of these organelles, and separated peripheral and integral membrane proteins by Na₂CO₃ treatment and osmotic shock. Proteomic analysis was performed on each of these fractions, allowing us to confirm the presence of enzymes involved in various metabolic pathways as well as identify new components of this parasite's glycosomes.

Cellular landmarks of Trypanosoma brucei and Leishmania mexicana

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Trypanosoma and Leishmania are single cell eukaryotic parasites.

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The cell organisation of these human pathogens is complex and highly structured.

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This describes an inventory of reliable reference markers for 32 cell structures.

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These light microscopy landmarks are a valuable resource for researchers.

The kinetoplastids Trypanosoma brucei and Leishmania mexicana are eukaryotes with a highly structured cellular organisation that is reproduced with great fidelity in each generation. The pattern of signal from a fluorescently tagged protein can define the specific structure/organelle that this protein localises to, and can be extremely informative in phenotype analysis in experimental perturbations, life cycle tracking, post-genomic assays and functional analysis of organelles. Using the vast coverage of protein subcellular localisations provided by the TrypTag project, an ongoing project to determine the localisation of every protein encoded in the T. brucei genome, we have generated an inventory of reliable reference organelle markers for both parasites that combines epifluorescence images with a detailed description of the key features of each localisation. We believe this will be a useful comparative resource that will enable researchers to quickly and accurately pinpoint the localisation of their proteins of interest and will provide cellular markers for many types of cell biology studies. We see this as another important step in the post-genomic era analyses of these parasites, in which ever expanding datasets generate numerous candidates to analyse. Adoption of these reference proteins by the community is likely to enhance research studies and enable better comparison of data.

Involvement of SNARE protein Ykt6 in glycosome biogenesis in Trypanosoma brucei

The kinetoplastid parasites Trypanosoma and Leishmania are etiologic agents of diseases like African sleeping sickness, Chagas and leishmaniasis that inflict many tropical and subtropical parts of the world. These parasites are distinctive in that they compartmentalize most of the usually cytosolic enzymes of the glycolytic pathway within a peroxisome-like organelle called the glycosome. Functional glycosomes are essential in both the procyclic and bloodstream forms of trypanosomatid parasites, and mislocalization of glycosomal enzymes to the cytosol is fatal for the parasite. The life cycle of these parasites is intimately linked to their efficient protein and vesicular trafficking machinery that helps them in immune evasion, host-pathogen interaction and organelle biogenesis and integrity. Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins play important roles in vesicular trafficking and mediate a wide range of protein-protein interactions in eukaryotes. We show here that the SNARE protein Ykt6 is necessary for glycosome biogenesis and function in Trypanosoma brucei. RNAi-mediated depletion of Ykt6 in both the procyclic and bloodstream forms of T. brucei leads to mislocalization of glycosomal matrix proteins to the cytosol, pronounced reduction in glycosome number, and cell death. GFP-tagged Ykt6 appears as punctate structures in the T. brucei cell and colocalizes in part to glycosomes. Our results constitute the first demonstration of a role for SNARE proteins in the biogenesis of peroxisomal organelles.

Spatial Organization of Metabolic Enzyme Complexes in Cells

The organization of metabolic multienzyme complexes has been hypothesized to benefit metabolic processes and provide a coordinated way for the cell to regulate metabolism. Historically, their existence has been supported by various in vitro techniques. However, it is only recently that the existence of metabolic complexes inside living cells has come to light to corroborate this long-standing hypothesis. Indeed, subcellular compartmentalization of metabolic enzymes appears to be widespread and highly regulated. On the other hand, it is still challenging to demonstrate the functional significance of these enzyme complexes in the context of the cellular milieu. In this review, we discuss the current understanding of metabolic enzyme complexes by primarily focusing on central carbon metabolism and closely associated metabolic pathways in a variety of organisms, as well as their regulation and functional contributions to cells.

Charting organellar importomes by quantitative mass spectrometry

Protein import into organelles is essential for all eukaryotes and facilitated by multi-protein translocation machineries. Analysing whether a protein is transported into an organelle is largely restricted to single constituents. This renders knowledge about imported proteins incomplete, limiting our understanding of organellar biogenesis and function. Here we introduce a method that enables charting an organelle's importome. The approach relies on inducible RNAi-mediated knockdown of an essential subunit of a translocase to impair import and quantitative mass spectrometry. To highlight its potential, we established the mitochondrial importome of Trypanosoma brucei, comprising 1,120 proteins including 331 new candidates. Furthermore, the method allows for the identification of proteins with dual or multiple locations and the substrates of distinct protein import pathways. We demonstrate the specificity and versatility of this ImportOmics method by targeting import factors in mitochondria and glycosomes, which demonstrates its potential for globally studying protein import and inventories of organelles.

Knowing the specific protein content of individual organelles is necessary for an integrated understanding of cellular physiology. Here the authors describe a mass spectrometry-based approach to identify the substrates of distinct protein import pathways and define organellar proteomes.

Cell cycle regulation and novel structural features of thymidine kinase, an essential enzyme in Trypanosoma brucei

Thymidine kinase (TK) is a key enzyme in the pyrimidine salvage pathway which catalyzes the transfer of the γ-phosphate of ATP to 2'-deoxythymidine (dThd) forming thymidine monophosphate (dTMP). Unlike other type II TKs, the Trypanosoma brucei enzyme (TbTK) is a tandem protein with two TK homolog domains of which only the C-terminal one is active. In this study, we establish that TbTK is essential for parasite viability and cell cycle progression, independently of extracellular pyrimidine concentrations. We show that expression of TbTK is cell cycle regulated and that depletion of TbTK leads to strongly diminished dTTP pools and DNA damage indicating intracellular dThd to be an essential intermediate metabolite for the synthesis of thymine-derived nucleotides. In addition, we report the X-ray structure of the catalytically active domain of TbTK in complex with dThd and dTMP at resolutions up to 2.2 Å. In spite of the high conservation of the active site residues, the structures reveal a widened active site cavity near the nucleobase moiety compared to the human enzyme. Our findings strongly support TbTK as a crucial enzyme in dTTP homeostasis and identify structural differences within the active site that could be exploited in the process of rational drug design.

Localization of a Trypanosome Peroxin to the Endoplasmic Reticulum

Trypanosoma brucei is the causative agent of diseases that affect 30,000–50,000 people annually. Trypanosoma brucei harbors unique organelles named glycosomes that are essential to parasite survival, which requires growth under fluctuating environmental conditions. The mechanisms that govern the biogenesis of these organelles are poorly understood. Glycosomes are evolutionarily related to peroxisomes, which can proliferate de novo from the endoplasmic reticulum or through the growth and division of existing organelles depending on the organism and environmental conditions. The effect of environment on glycosome biogenesis is unknown. Here, we demonstrate that the glycosome membrane protein, TbPex13.1, is localized to glycosomes when cells are cultured under high glucose conditions and to the endoplasmic reticulum in low glucose conditions. This localization in low glucose was dependent on the presence of a C‐terminal tripeptide sequence. Our findings suggest that glycosome biogenesis is influenced by extracellular glucose levels and adds to the growing body of evidence that de novo glycosome biogenesis occurs in trypanosomes. Because the movement of peroxisomal membrane proteins is a hallmark of ER‐dependent peroxisome biogenesis, TbPex13.1 may be a useful marker for the study such processes in trypanosomes.

Glycosome turnover in Leishmania major is mediated by autophagy

Autophagy is a central process behind the cellular remodeling that occurs during differentiation of Leishmania, yet the cargo of the protozoan parasite's autophagosome is unknown. We have identified glycosomes, peroxisome-like organelles that uniquely compartmentalize glycolytic and other metabolic enzymes in Leishmania and other kinetoplastid parasitic protozoa, as autophagosome cargo. It has been proposed that the number of glycosomes and their content change during the Leishmania life cycle as a key adaptation to the different environments encountered. Quantification of RFP-SQL-labeled glycosomes showed that promastigotes of L. major possess ~20 glycosomes per cell, whereas amastigotes contain ~10. Glycosome numbers were significantly greater in promastigotes and amastigotes of autophagy-defective L. major Δatg5 mutants, implicating autophagy in glycosome homeostasis and providing a partial explanation for the previously observed growth and virulence defects of these mutants. Use of GFP-ATG8 to label autophagosomes showed glycosomes to be cargo in ~15% of them; glycosome-containing autophagosomes were trafficked to the lysosome for degradation. The number of autophagosomes increased 10-fold during differentiation, yet the percentage of glycosome-containing autophagosomes remained constant. This indicates that increased turnover of glycosomes was due to an overall increase in autophagy, rather than an upregulation of autophagosomes containing this cargo. Mitophagy of the single mitochondrion was not observed in L. major during normal growth or differentiation; however, mitochondrial remnants resulting from stress-induced fragmentation colocalized with autophagosomes and lysosomes, indicating that autophagy is used to recycle these damaged organelles. These data show that autophagy in Leishmania has a central role not only in maintaining cellular homeostasis and recycling damaged organelles but crucially in the adaptation to environmental change through the turnover of glycosomes.

Comprehensive proteomics analysis of glycosomes from Leishmania donovani

Leishmania donovani is a kinetoplastid protozoan that causes a severe and fatal disease kala-azar, or visceral leishmaniasis. L. donovani infects human host after the phlebotomine sandfly takes a blood meal and resides within the phagolysosome of infected macrophages. Previous studies on host-parasite interactions have not focused on Leishmania organelles and the role that they play in the survival of this parasite within macrophages. Leishmania possess glycosomes that are unique and specialized subcellular microbody organelles. Glycosomes are known to harbor most peroxisomal enzymes and, in addition, they also possess nine glycolytic enzymes. In the present study, we have carried out proteomic profiling using high resolution mass spectrometry of a sucrose density gradient-enriched glycosomal fraction isolated from L. donovani promastigotes. This study resulted in the identification of 4022 unique peptides, leading to the identification of 1355 unique proteins from a preparation enriched in L. donovani glycosomes. Based on protein annotation, 566 (41.8%) were identified as hypothetical proteins with no known function. A majority of the identified proteins are involved in metabolic processes such as carbohydrate, lipid, and nucleic acid metabolism. Our present proteomic analysis is the most comprehensive study to date to map the proteome of L. donovani glycosomes.

Dissecting Leishmania infantum Energy Metabolism - A Systems Perspective

Leishmania infantum, causative agent of visceral leishmaniasis in humans, illustrates a complex lifecycle pertaining to two extreme environments, namely, the gut of the sandfly vector and human macrophages. Leishmania is capable of dynamically adapting and tactically switching between these critically hostile situations. The possible metabolic routes ventured by the parasite to achieve this exceptional adaptation to its varying environments are still poorly understood. In this study, we present an extensively reconstructed energy metabolism network of Leishmania infantum as an attempt to identify certain strategic metabolic routes preferred by the parasite to optimize its survival in such dynamic environments. The reconstructed network consists of 142 genes encoding for enzymes performing 237 reactions distributed across five distinct model compartments. We annotated the subcellular locations of different enzymes and their reactions on the basis of strong literature evidence and sequence-based detection of cellular localization signal within a protein sequence. To explore the diverse features of parasite metabolism the metabolic network was implemented and analyzed as a constraint-based model. Using a systems-based approach, we also put forth an extensive set of lethal reaction knockouts; some of which were validated using published data on Leishmania species. Performing a robustness analysis, the model was rigorously validated and tested for the secretion of overflow metabolites specific to Leishmania under varying extracellular oxygen uptake rate. Further, the fate of important non-essential amino acids in L. infantum metabolism was investigated. Stage-specific scenarios of L. infantum energy metabolism were incorporated in the model and key metabolic differences were outlined. Analysis of the model revealed the essentiality of glucose uptake, succinate fermentation, glutamate biosynthesis and an active TCA cycle as driving forces for parasite energy metabolism and its optimal growth. Finally, through our in silico knockout analysis, we could identify possible therapeutic targets that provide experimentally testable hypotheses.

Identification and functional characterization of Trypanosoma brucei peroxin 16

Protozoan parasites of the family Trypanosomatidae infect humans as well as livestock causing devastating diseases like sleeping sickness, Chagas disease, and Leishmaniasis. These parasites compartmentalize glycolytic enzymes within unique organelles, the glycosomes. Glycosomes represent a subclass of peroxisomes and they are essential for the parasite survival. Hence, disruption of glycosome biogenesis is an attractive drug target for these Neglected Tropical Diseases (NTDs). Peroxin 16 (PEX16) plays an essential role in peroxisomal membrane protein targeting and de novo biogenesis of peroxisomes from endoplasmic reticulum (ER). We identified trypanosomal PEX16 based on specific sequence characteristics and demonstrate that it is an integral glycosomal membrane protein of procyclic and bloodstream form trypanosomes. RNAi mediated partial knockdown of Trypanosoma brucei PEX16 in bloodstream form trypanosomes led to severe ATP depletion, motility defects and cell death. Microscopic and biochemical analysis revealed drastic reduction in glycosome number and mislocalization of the glycosomal matrix enzymes to the cytosol. Asymmetry of the localization of the remaining glycosomes was observed with a severe depletion in the posterior part. The results demonstrate that trypanosomal PEX16 is essential for glycosome biogenesis and thereby, provides a potential drug target for sleeping sickness and related diseases.

High throughput sequencing analysis of Trypanosoma brucei DRBD3/PTB1-bound mRNAs

Trypanosomes are early-branched eukaryotes that show an unusual dependence on post-transcriptional mechanisms to regulate gene expression. RNA-binding proteins are crucial in controlling mRNA fate in these organisms, but their RNA substrates remain largely unknown. Here we have analyzed on a global scale the mRNAs associated with the Trypanosoma brucei RNA-binding protein DRBD3/PTB1, by capturing ribonucleoprotein complexes using UV cross-linking and subsequent immunoprecipitation. DRBD3/PTB1 associates with many transcripts encoding ribosomal proteins and translation factors. Consequently, silencing of DRBD3/PTB1 expression altered the protein synthesis rate. DRBD3/PTB1 also binds to mRNAs encoding the enzymes required to obtain energy through the oxidation of proline to succinate. We hypothesize that DRBD3/PTB1 is a key player in RNA regulon-based gene control influencing protein synthesis in trypanosomes.

The krebs cycle enzyme α-ketoglutarate decarboxylase is an essential glycosomal protein in bloodstream African trypanosomes

α-Ketoglutarate decarboxylase (α-KDE1) is a Krebs cycle enzyme found in the mitochondrion of the procyclic form (PF) of Trypanosoma brucei. The bloodstream form (BF) of T. brucei lacks a functional Krebs cycle and relies exclusively on glycolysis for ATP production. Despite the lack of a functional Krebs cycle, α-KDE1 was expressed in BF T. brucei and RNA interference knockdown of α-KDE1 mRNA resulted in rapid growth arrest and killing. Cell death was preceded by progressive swelling of the flagellar pocket as a consequence of recruitment of both flagellar and plasma membranes into the pocket. BF T. brucei expressing an epitope-tagged copy of α-KDE1 showed localization to glycosomes and not the mitochondrion. We used a cell line transfected with a reporter construct containing the N-terminal sequence of α-KDE1 fused to green fluorescent protein to examine the requirements for glycosome targeting. We found that the N-terminal 18 amino acids of α-KDE1 contain overlapping mitochondrion- and peroxisome-targeting sequences and are sufficient to direct localization to the glycosome in BF T. brucei. These results suggest that α-KDE1 has a novel moonlighting function outside the mitochondrion in BF T. brucei.

Molecular basis for the reverse reaction of African human trypanosomes glycerol kinase

The glycerol kinase (GK) of African human trypanosomes is compartmentalized in their glycosomes. Unlike the host GK, which under physiological conditions catalyzes only the forward reaction (ATP-dependent glycerol phosphorylation), trypanosome GK can additionally catalyze the reverse reaction. In fact, owing to this unique reverse catalysis, GK is potentially essential for the parasites survival in the human host, hence a promising drug target. The mechanism of its reverse catalysis was unknown; therefore, it was not clear if this ability was purely due to its localization in the organelles or whether structure-based catalytic differences also contribute. To investigate this lack of information, the X-ray crystal structure of this protein was determined up to 1.90 Å resolution, in its unligated form and in complex with three natural ligands. These data, in conjunction with results from structure-guided mutagenesis suggests that the trypanosome GK is possibly a transiently autophosphorylating threonine kinase, with the catalytic site formed by non-conserved residues. Our results provide a series of structural peculiarities of this enzyme, and gives unexpected insight into the reverse catalysis mechanism. Together, they provide an encouraging molecular framework for the development of trypanosome GK-specific inhibitors, which may lead to the design of new and safer trypanocidal drug(s).

Depletion of the RNA-binding protein RBP33 results in increased expression of silenced RNA polymerase II transcripts in Trypanosoma brucei

We have characterized the RNA-binding protein RBP33 in Trypanosoma brucei, and found that it localizes to the nucleus and is essential for viability. The subset of RNAs bound to RBP33 was determined by immunoprecipitation of ribonucleoprotein complexes followed by deep sequencing. Most RBP33-bound transcripts are predicted to be non-coding. Among these, over one-third are located close to the end of transcriptional units (TUs) or have an antisense orientation within a TU. Depletion of RBP33 resulted in an increase in the level of RNAs derived from regions that are normally silenced, such as strand-switch regions, retroposon and repeat sequences. Our work provides the first example of an RNA-binding protein involved in the regulation of gene silencing in trypanosomes.

High-throughput chemical screening for antivirulence developmental phenotypes in Trypanosoma brucei

In the bloodstream of mammalian hosts, the sleeping sickness parasite, Trypanosoma brucei, exists as a proliferative slender form or a nonproliferative, transmissible, stumpy form. The transition between these developmental forms is controlled by a density-dependent mechanism that is important for the parasite's infection dynamics, immune evasion via ordered antigenic variation, and disease transmissibility. However, stumpy formation has been lost in most laboratory-adapted trypanosome lines, generating monomorphic parasites that proliferate uncontrolled as slender forms in vitro and in vivo. Nonetheless, these forms are readily amenable to cell culture and high-throughput screening for trypanocidal lead compounds. Here, we have developed and exploited a high-throughput screen for developmental phenotypes using a transgenic monomorphic cell line expressing a reporter under the regulation of gene control signals from the stumpy-specific molecule PAD1. Using a whole-cell fluorescence-based assay to screen over 6,000 small molecules from a kinase-focused compound library, small molecules able to activate stumpy-specific gene expression and proliferation arrest were assayed in a rapid assay format. Independent follow-up validation identified one hit able to induce modest, yet specific, changes in mRNA expression indicative of a partial differentiation to stumpy forms in monomorphs. Further, in pleomorphs this compound induced a stumpy-like phenotype, entailing growth arrest, morphological changes, PAD1 expression, and enhanced differentiation to procyclic forms. This not only provides a potential tool compound for the further understanding of stumpy formation but also demonstrates the use of high-throughput screening in the identification of compounds able to induce specific phenotypes, such as differentiation, in African trypanosomes.

Arginase in Leishmania

The presence of different sets of several enzymes that participate in the Krebs-Henseleit cycle has been used to identify several genera of trypanosomatids. One of these enzymes is arginase (L-arginine amidinohydrolase, E.C. 3.5.3.1), a metalloenzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. Arginase activity has been detected in Leishmania, Crithidia and Leptomonas but not in Trypanosoma, Herpetomonas or Phytomonas. The ureotelic behavior of some trypanosomatids is not due to urea excretion but to the production of ornithine to supply the polyamine pathway, which is essential for replication. Leishmania is found inside macrophages in the mammalian host and to live in these cells, the parasite must escape from several microbicidal mechanisms, such as nitric oxide (NO) production mediated by inducible nitric oxide synthase (iNOS). Since arginase and iNOS use the L-arginine as substrate, the amount of this amino acid available for both pathways is critical for parasite replication. In both promastigotes and amastigotes, arginase is located in the glycosome indicating that arginine trafficking in the cell is used to provide the optimal concentration of substrate for arginase. Arginine uptake by the parasite is also important in supplying the arginase substrate. Leishmania responds to arginine starvation by increasing the amino acid uptake. In addition to the external supply, the internal L-arginine pool also governs the uptake of this amino acid, and the size of this internal pool is modulated by arginase activity. Thus, arginine uptake and arginase activity are important in establishing and maintaining Leishmania infection.

Explicit consideration of topological and parameter uncertainty gives new insights into a well-established model of glycolysis

Previous models of glycolysis in the sleeping sickness parasite Trypanosoma brucei assumed that the core part of glycolysis in this unicellular parasite is tightly compartimentalized within an organelle, the glycosome, which had previously been shown to contain most of the glycolytic enzymes. The glycosomes were assumed to be largely impermeable, and exchange of metabolites between the cytosol and the glycosome was assumed to be regulated by specific transporters in the glycosomal membrane. This tight compartmentalization was considered to be essential for parasite viability. Recently, size-specific metabolite pores were discovered in the membrane of glycosomes. These channels are proposed to allow smaller metabolites to diffuse across the membrane but not larger ones. In light of this new finding, we re-analyzed the model taking into account uncertainty about the topology of the metabolic system in T. brucei, as well as uncertainty about the values of all parameters of individual enzymatic reactions. Our analysis shows that these newly-discovered nonspecific pores are not necessarily incompatible with our current knowledge of the glycosomal metabolic system, provided that the known cytosolic activities of the glycosomal enzymes play an important role in the regulation of glycolytic fluxes and the concentration of metabolic intermediates of the pathway.

Examination of the mode of action of the almiramide family of natural products against the kinetoplastid parasite Trypanosoma brucei

Almiramide C is a marine natural product with low micromolar activity against Leishmania donovani, the causative agent of leishmaniasis. We have now shown that almiramide C is also active against the related parasite Trypanosoma brucei, the causative agent of human African trypanosomiasis. A series of activity-based probes have been synthesized to explore both the molecular target of this compound series in T. brucei lysates and site localization through epifluorescence microscopy. These target identification studies indicate that the almiramides likely perturb glycosomal function through disruption of membrane assembly machinery. Glycosomes, which are organelles specific to kinetoplastid parasites, house the first seven steps of glycolysis and have been shown to be essential for parasite survival in the bloodstream stage. There are currently no reported small-molecule disruptors of glycosome function, making the almiramides unique molecular probes for this understudied parasite-specific organelle. Additionally, examination of toxicity in an in vivo zebrafish model has shown that these compounds have little effect on organism development, even at high concentrations, and has uncovered a potential side effect through localization of fluorescent derivatives to zebrafish neuromast cells. Combined, these results further our understanding of the potential value of this lead series as development candidates against T. brucei.

Proteins and lipids of glycosomal membranes from Leishmania tarentolae and Trypanosoma brucei

In kinetoplastid protists, several metabolic pathways, including glycolysis and purine salvage, are located in glycosomes, which are microbodies that are evolutionarily related to peroxisomes. With the exception of some potential transporters for fatty acids, and one member of the mitochondrial carrier protein family, proteins that transport metabolites across the glycosomal membrane have yet to be identified. We show here that the phosphatidylcholine species composition of Trypanosoma brucei glycosomal membranes resembles that of other cellular membranes, which means that glycosomal membranes are expected to be impermeable to small hydrophilic molecules unless transport is facilitated by specialized membrane proteins. Further, we identified 464 proteins in a glycosomal membrane preparation from Leishmania tarentolae. The proteins included approximately 40 glycosomal matrix proteins, and homologues of peroxisomal membrane proteins - PEX11, GIM5A and GIM5B; PXMP4, PEX2 and PEX16 - as well as the transporters GAT1 and GAT3. There were 27 other proteins that could not be unambiguously assigned to other compartments, and that had predicted trans-membrane domains. However, no clear candidates for transport of the major substrates and intermediates of energy metabolism were found. We suggest that, instead, these metabolites are transported via pores formed by the known glycosomal membrane proteins.

Endogenous sterol biosynthesis is important for mitochondrial function and cell morphology in procyclic forms of Trypanosoma brucei

Sterol biosynthesis inhibitors are promising entities for the treatment of trypanosomal diseases. Insect forms of Trypanosoma brucei, the causative agent of sleeping sickness, synthesize ergosterol and other 24-alkylated sterols, yet also incorporate cholesterol from the medium. While sterol function has been investigated by pharmacological manipulation of sterol biosynthesis, molecular mechanisms by which endogenous sterols influence cellular processes remain largely unknown in trypanosomes. Here we analyse by RNA interference, the effects of a perturbation of three specific steps of endogenous sterol biosynthesis in order to dissect the role of specific intermediates in proliferation, mitochondrial function and cellular morphology in procyclic cells. A decrease in the levels of squalene synthase and squalene epoxidase resulted in a depletion of cellular sterol intermediates and end products, impaired cell growth and led to aberrant morphologies, DNA fragmentation and a profound modification of mitochondrial structure and function. In contrast, cells deficient in sterol methyl transferase, the enzyme involved in 24-alkylation, exhibited a normal growth phenotype in spite of a complete abolition of the synthesis and content of 24-alkyl sterols. Thus, the data provided indicates that while the depletion of squalene and post-squalene endogenous sterol metabolites results in profound cellular defects, bulk 24-alkyl sterols are not strictly required to support growth in insect forms of T. brucei in vitro.

Channel-forming activities in the glycosomal fraction from the bloodstream form of Trypanosoma brucei

BACKGROUND

Glycosomes are a specialized form of peroxisomes (microbodies) present in unicellular eukaryotes that belong to the Kinetoplastea order, such as Trypanosoma and Leishmania species, parasitic protists causing severe diseases of livestock and humans in subtropical and tropical countries. The organelles harbour most enzymes of the glycolytic pathway that is responsible for substrate-level ATP production in the cell. Glycolysis is essential for bloodstream-form Trypanosoma brucei and enzymes comprising this pathway have been validated as drug targets. Glycosomes are surrounded by a single membrane. How glycolytic metabolites are transported across the glycosomal membrane is unclear.

METHODS/PRINCIPAL FINDINGS

We hypothesized that glycosomal membrane, similarly to membranes of yeast and mammalian peroxisomes, contains channel-forming proteins involved in the selective transfer of metabolites. To verify this prediction, we isolated a glycosomal fraction from bloodstream-form T. brucei and reconstituted solubilized membrane proteins into planar lipid bilayers. The electrophysiological characteristics of the channels were studied using multiple channel recording and single channel analysis. Three main channel-forming activities were detected with current amplitudes 70-80 pA, 20-25 pA, and 8-11 pA, respectively (holding potential +10 mV and 3.0 M KCl as an electrolyte). All channels were in fully open state in a range of voltages ±150 mV and showed no sub-conductance transitions. The channel with current amplitude 20-25 pA is anion-selective (P(K+)/P(Cl-)∼0.31), while the other two types of channels are slightly selective for cations (P(K+)/P(Cl-) ratios ∼1.15 and ∼1.27 for the high- and low-conductance channels, respectively). The anion-selective channel showed an intrinsic current rectification that may suggest a functional asymmetry of the channel's pore.

CONCLUSIONS/SIGNIFICANCE

These results indicate that the membrane of glycosomes apparently contains several types of pore-forming channels connecting the glycosomal lumen and the cytosol.

Leishmania amazonensis arginase compartmentalization in the glycosome is important for parasite infectivity

In Leishmania, de novo polyamine synthesis is initiated by the cleavage of L-arginine to urea and L-ornithine by the action of arginase (ARG, E.C. 3.5.3.1). Previous studies in L. major and L. mexicana showed that ARG is essential for in vitro growth in the absence of polyamines and needed for full infectivity in animal infections. The ARG protein is normally found within the parasite glycosome, and here we examined whether this localization is required for survival and infectivity. First, the localization of L. amazonensis ARG in the glycosome was confirmed in both the promastigote and amastigote stages. As in other species, arg⁻ L. amazonensis required putrescine for growth and presented an attenuated infectivity. Restoration of a wild type ARG to the arg⁻ mutant restored ARG expression, growth and infectivity. In contrast, restoration of a cytosol-targeted ARG lacking the glycosomal SKL targeting sequence (argΔSKL) restored growth but failed to restore infectivity. Further study showed that the ARGΔSKL protein was found in the cytosol as expected, but at very low levels. Our results indicate that the proper compartmentalization of L. amazonensis arginase in the glycosome is important for enzyme activity and optimal infectivity. Our conjecture is that parasite arginase participates in a complex equilibrium that defines the fate of L-arginine and that its proper subcellular location may be essential for this physiological orchestration.

The significance of peroxisomes in secondary metabolite biosynthesis in filamentous fungi

Peroxisomes are ubiquitous organelles characterized by a protein-rich matrix surrounded by a single membrane. In filamentous fungi, peroxisomes are crucial for the primary metabolism of several unusual carbon sources used for growth (e.g. fatty acids), but increasing evidence is presented that emphasize the crucial role of these organelles in the formation of a variety of secondary metabolites. In filamentous fungi, peroxisomes also play a role in development and differentiation whereas specialized peroxisomes, the Woronin bodies, play a structural role in plugging septal pores. The biogenesis of peroxisomes in filamentous fungi involves the function of conserved PEX genes, as well as genes that are unique for these organisms. Peroxisomes are also subject to autophagic degradation, a process that involves ATG genes. The interplay between organelle biogenesis and degradation may serve a quality control function, thereby allowing a continuous rejuvenation of the organelle population in the cells.

Singular features of trypanosomatids' phosphotransferases involved in cell energy management

Trypanosomatids are responsible for economically important veterinary affections and severe human diseases. In Africa, Trypanosoma brucei causes sleeping sickness or African trypanosomiasis, while in America, Trypanosoma cruzi is the etiological agent of Chagas disease. These parasites have complex life cycles which involve a wide variety of environments with very different compositions, physicochemical properties, and availability of metabolites. As the environment changes there is a need to maintain the nucleoside homeostasis, requiring a quick and regulated response. Most of the enzymes required for energy management are phosphotransferases. These enzymes present a nitrogenous group or a phosphate as acceptors, and the most clear examples are arginine kinase, nucleoside diphosphate kinase, and adenylate kinase. Trypanosoma and Leishmania have the largest number of phosphotransferase isoforms ever found in a single cell; some of them are absent in mammals, suggesting that these enzymes are required in many cellular compartments associated to different biological processes. The presence of such number of phosphotransferases support the hypothesis of the existence of an intracellular enzymatic phosphotransfer network that communicates the spatially separated intracellular ATP consumption and production processes. All these unique features make phosphotransferases a promising start point for rational drug design for the treatment of human trypanosomiasis.

Membrane curvature during peroxisome fission requires Pex11

Pex11p is required for peroxisome proliferation. This study demonstrates that the N-terminus of Pex11p forms an amphipathic helix that generates membrane curvature required for peroxisome fission.

Pex11 is a key player in peroxisome proliferation, but the molecular mechanisms of its function are still unknown. Here, we show that Pex11 contains a conserved sequence at the N-terminus that can adopt the structure of an amphipathic helix. Using Penicillium chrysogenum Pex11, we show that this amphipathic helix, termed Pex11-Amph, associates with liposomes in vitro. This interaction is especially evident when negatively charged liposomes are used with a phospholipid content resembling that of peroxisomal membranes. Binding of Pex11-Amph to negatively charged membrane vesicles resulted in strong tubulation. This tubulation of vesicles was also observed when the entire soluble N-terminal domain of Pex11 was used. Using mutant peptides, we demonstrate that maintaining the amphipathic properties of Pex11-Amph in conjunction with retaining its α-helical structure are crucial for its function. We show that the membrane remodelling capacity of the amphipathic helix in Pex11 is conserved from yeast to man. Finally, we demonstrate that mutations abolishing the membrane remodelling activity of the Pex11-Amph domain also hamper the function of full-length Pex11 in peroxisome fission in vivo.

Comparison of the peroxisomal matrix protein import system of different organisms. Exploration of possibilities for developing inhibitors of the import system of trypanosomatids for anti-parasite chemotherapy

In recent decades, research on peroxisome biogenesis has been particularly boosted since the role of these organelles in metabolism became unraveled. Indeed in plants, yeasts and fungi, peroxisomes play an important role in the adaptation of metabolism during developmental processes and/or altered environmental conditions. In mammals their importance is illustrated by the fact that several severe human inherited diseases have been identified as peroxisome biogenesis disorders (PBD). Particularly interesting are the glycosomes - peroxisome-like organelles in trypanosomatids where the major part of the glycolytic pathway is sequestered - because it was demonstrated that proper compartmentalization of matrix proteins inside glycosomes is essential for the parasite. Although the overall process of peroxisome biogenesis seems well conserved between species, careful study of the literature reveals nonetheless many differences at various steps. In this review, we present a comparison of the first two steps of peroxisome biogenesis - receptor loading and docking at the peroxisomal membrane - in yeasts, mammals, plants and trypanosomatids and highlight major differences in the import process between species despite the conservation of (some of) the proteins involved. Some of the unique features of the process as it occurs in trypanosomatids will be discussed with regard to the possibilities for exploiting them for the development of compounds that could specifically disturb interactions between trypanosomatid peroxins. This strategy could eventually lead to the discovery of drugs against the diseases caused by these parasites.

Leishmania amazonensis META2 protein confers protection against heat shock and oxidative stress

The META cluster of Leishmania amazonensis contains both META1 and META2 genes, which are upregulated in metacyclic promastigotes and encode proteins containing the META domain. Previous studies defined META2 as a 48.0-kDa protein, which is conserved in other Leishmania species and in Trypanosoma brucei. In this work, we demonstrate that META2 protein expression is regulated during the Leishmania life cycle but constitutive in T. brucei. META2 protein is present in the cytoplasm and flagellum of L. amazonensis promastigotes. Leishmania META2-null replacement mutants are more sensitive to oxidative stress and, upon heat shock, assume rounded morphology with shortened flagella. The increased susceptibility of null parasites to heat shock is reversed by extra-chromosomal expression of the META2 gene. Defective Leishmania promastigotes exhibit decreased ability to survive in macrophages. By contrast, META2 expression is decreased by 80% in RNAi-induced T. brucei bloodstream forms with no measurable effect on survival or resistance to heat shock.

Penicillium chrysogenum Pex14/17p--a novel component of the peroxisomal membrane that is important for penicillin production

By genome analysis, we previously identified Pex14/17p as a putative novel peroxin of Penicillium chrysogenum. Here, we show that Pex14/17p is a component of the peroxisomal membrane that is essential for efficient peroxisomal targeting signal 1 and peroxisomal targeting signal 2 matrix protein import, implying that the protein is indeed a genuine peroxin. Additionally, a PEX14/17 deletion strain is affected in conidiospore formation. Pex14/17p has properties of both Pex14p and Pex17p, in that the N-terminus of this protein is similar to the highly conserved Pex5p-binding region present in the N-termini of Pex14p proteins, whereas its C-terminus shows weak similarity to yeast Pex17p proteins. We have identified a novel motif in both Pex17p and Pex14/17p that is absent in Pex14p. We show that an N-terminally truncated, but not a C-terminally truncated, Pex14/17p is able to complement both the matrix protein import and sporulation defects of a Delta pex14/17 strain, implying that it is the Pex17p-related portion of the protein that is crucial for its function as a peroxin. Possibly, this compensates for the fact that P. chrysogenum lacks an authenthic Pex17p. We also show that, in P. chrysogenum, Pex14/17p plays a role in making the penicillin biosynthesis process more efficient.

Tsetse flies, trypanosomes, humans and animals: what is proteomics revealing about their crosstalks?

Human and animal African trypanosomoses, or sleeping sickness and Nagana, are neglected vector-borne parasitic diseases caused by protozoa belonging to the Trypanosoma genus. Advances in proteomics offer new tools to better understand host-vector-parasite crosstalks occurring during the complex parasitic developmental cycle, and to determine the outcome of both transmission and infection. In this review, we summarize proteomics studies performed on African trypanosomes and on the interactions with their vector and mammalian hosts. We discuss the contributions and pitfalls of using diverse proteomics tools, and argue about the interest of pathogenoproteomics, both to generate advances in basic research on the best knowledge and understanding of host-vector-pathogen interactions, and to lead to the concrete development of new tools to improve diagnosis and treatment management of trypanosomoses in the near future.

Immunobiology of African trypanosomes: need of alternative interventions

Trypanosomiasis is one of the major parasitic diseases for which control is still far from reality. The vaccination approaches by using dominant surface proteins have not been successful, mainly due to antigenic variation of the parasite surface coat. On the other hand, the chemotherapeutic drugs in current use for the treatment of this disease are toxic and problems of resistance are increasing (see Kennedy (2004) and Legros et al. (2002)). Therefore, alternative approaches in both treatment and vaccination against trypanosomiasis are needed at this time. To be able to design and develop such alternatives, the biology of this parasite and the host response against the pathogen need to be studied. These two aspects of this disease with few examples of alternative approaches are discussed here.

Matching the proteome to the genome: the microbody of penicillin-producing Penicillium chrysogenum cells

In the filamentous fungus Penicillium chrysogenum, microbodies are essential for penicillin biosynthesis. To better understand the role of these organelles in antibiotics production, we determined the matrix enzyme contents of P. chrysogenum microbodies. Using a novel in silico approach, we first obtained a catalogue of 200 P. chrysogenum proteins with putative microbody targeting signals (PTSs). This included two orthologs of proteins involved in cephalosporin biosynthesis, which we demonstrate to be bona fide microbody matrix constituents. Subsequently, we performed a proteomics based inventory of P. chrysogenum microbody matrix proteins using nano-LC-MS/MS analysis. We identified 89 microbody proteins, 79 with a PTS, including the two known microbody-borne penicillin biosynthesis enzymes, isopenicillin N:acyl CoA acyltransferase and phenylacetyl-CoA ligase. Comparative analysis revealed that 69 out of 79 PTS proteins identified experimentally were in the reference list. A prominent microbody protein was identified as a novel fumarate reductase-cytochrome b5 fusion protein, which contains an internal PTS2 between the two functional domains. We show that this protein indeed localizes to P. chrysogenum microbodies.

Leishmania donovani peroxin 14 undergoes a marked conformational change following association with peroxin 5

The import of PTS1 proteins into the glycosome or peroxisome requires binding of a PTS1-laden PEX5 receptor to the membrane-associated protein PEX14 to facilitate translocation of PTS1 proteins into the lumen of these organelles. Quaternary structure analysis of protozoan parasite Leishmania donovani PEX14 (LdPEX14) revealed that this protein forms a homomeric complex with a size > 670 kDa. Moreover, deletion mapping indicated that disruption of LdPEX14 oligomerization correlated with the elimination of the hydrophobic region and coiled-coil motif present in LdPEX14. Analysis of the LdPEX5-LdPEX14 interaction by isothermal titration calorimetry revealed a molar binding stoichiometry of 1:4 (LdPEX5: LdPEX14) and an in-solution dissociation constant (K(d)) of approximately 74 nm. Calorimetry, circular dichroism, intrinsic fluorescence, and analytical ultracentrifugation experiments showed that binding of LdPEX5 resulted in a dramatic conformational change in the LdPEX14 oligomeric complex that involved the reorganization of the hydrophobic segment in LdPEX14. Finally, limited tryptic proteolysis assays established that in the presence of LdPEX5, LdPEX14 became more susceptible to proteolytic degradation consistent with this protein interaction triggering a significant conformational change in the recombinant and native LdPEX14 structures. These structural changes provide essential clues to how LdPEX14 functions in the translocation of folded proteins across the glycosomal membrane.

The RNA-binding protein TbDRBD3 regulates the stability of a specific subset of mRNAs in trypanosomes

In trypanosomes, the apparent lack of regulation of RNA polymerase II-dependent transcription initiation poses a challenge to understand how these eukaryotes adjust gene expression to adapt to the contrasting environments they find during their life cycles. Evidence so far indicates that mRNA turnover and translation are the major control points in which regulation is exerted in trypanosomes. However, very little is known about which proteins are involved, and how do they regulate the abundance and translation of different mRNAs in different life stages. In this work, an RNA-binding protein, TbDRBD3, has been identified by affinity chromatography, and its function addressed using RNA interference, microarray analysis and immunoprecipitation of mRNA-protein complexes. The results obtained indicate that TbDRBD3 binds to a subset of developmentally regulated mRNAs encoding membrane proteins, and that this association promotes the stabilization of the target transcripts. These observations raise the possibility that TbDRBD3-mRNA complexes act as a post-transcriptional operon, and provide a framework to interpret how trypanosomes regulate gene expression in the absence of transcriptional control.

Depletion of dimeric all-alpha dUTPase induces DNA strand breaks and impairs cell cycle progression in Trypanosoma brucei

The enzyme deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase) is responsible for the control of intracellular levels of dUTP thus controlling the incorporation of uracil into DNA during replication. Trypanosomes and certain eubacteria contain a dimeric dUTP-dUDPase belonging to the recently described superfamily of all-alpha NTP pyrophosphatases which bears no resemblance with typical eukaryotic trimeric dUTPases and presents unique properties regarding substrate specificity and product inhibition. While the biological trimeric enzymes have been studied in detail and the human enzyme has been proposed as a promising novel target for anticancer chemotherapeutic strategies, little is known regarding the biological function of dimeric proteins. Here, we show that in Trypanosoma brucei, the dimeric dUTPase is a nuclear enzyme and that down-regulation of activity by RNAi greatly reduces cell proliferation and increases the intracellular levels of dUTP. Defects in growth could be partially reverted by the addition of exogenous thymidine. dUTPase-depleted cells presented hypersensitivity to methotrexate, a drug that increases the intracellular pools of dUTP, and enhanced uracil-DNA glycosylase activity, the first step in base excision repair. The knockdown of activity produces numerous DNA strand breaks and defects in both S and G2/M progression. Multiple parasites with a single enlarged nucleus were visualized together with an enhanced population of anucleated cells. We conclude that dimeric dUTPases are strongly involved in the control of dUTP incorporation and that adequate levels of enzyme are indispensable for efficient cell cycle progression and DNA replication.