Characterisation of protein isoprenylation in procyclic form Trypanosoma brucei

Flagellar targeting of an arginine kinase requires a conserved lipidated protein intraflagellar transport (LIFT) pathway in Trypanosoma brucei

Both intraflagellar transport (IFT) and lipidated protein intraflagellar transport (LIFT) pathways are essential for cilia/flagella biogenesis, motility, and sensory functions. In the LIFT pathway, lipidated cargoes are transported into the cilia through the coordinated actions of cargo carrier proteins such as Unc119 or PDE6δ, as well as small GTPases Arl13b and Arl3 in the cilium. Our previous studies have revealed a single Arl13b ortholog in the evolutionarily divergent Trypanosoma brucei, the causative agent of African sleeping sickness. TbArl13 catalyzes two TbArl3 homologs, TbArl3A and TbArl3C, suggesting the presence of a conserved LIFT pathway in these protozoan parasites. Only a single homolog to the cargo carrier protein Unc119 has been identified in T. brucei genome, but its function in lipidated protein transport has not been characterized. In this study, we exploited the proximity-based biotinylation approach to identify binding partners of TbUnc119. We showed that TbUnc119 binds to a flagellar arginine kinase TbAK3 in a myristoylation-dependent manner and is responsible for its targeting to and enrichment in the flagellum. Interestingly, only TbArl3A, but not TbArl3C interacted with TbUnc119 in a GTP-dependent manner, suggesting functional specialization of Arl3-GTPases in T. brucei These results establish the function of TbUnc119 as a myristoylated cargo carrier and support the presence of a conserved LIFT pathway in T. brucei.

Solanesyl diphosphate synthase, an enzyme of the ubiquinone synthetic pathway, is required throughout the life cycle of Trypanosoma brucei

Ubiquinone 9 (UQ9), the expected product of the long-chain solanesyl diphosphate synthase of Trypanosoma brucei (TbSPPS), has a central role in reoxidation of reducing equivalents in the mitochondrion of T. brucei. The ablation of TbSPPS gene expression by RNA interference increased the generation of reactive oxygen species and reduced cell growth and oxygen consumption. The addition of glycerol to the culture medium exacerbated the phenotype by blocking its endogenous generation and excretion. The participation of TbSPPS in UQ synthesis was further confirmed by growth rescue using UQ with 10 isoprenyl subunits (UQ10). Furthermore, the survival of infected mice was prolonged upon the downregulation of TbSPPS and/or the addition of glycerol to drinking water. TbSPPS is inhibited by 1-[(n-oct-1-ylamino)ethyl] 1,1-bisphosphonic acid, and treatment with this compound was lethal for the cells. The findings that both UQ9 and ATP pools were severely depleted by the drug and that exogenous UQ10 was able to fully rescue growth of the inhibited parasites strongly suggest that TbSPPS and UQ synthesis are the main targets of the drug. These two strategies highlight the importance of TbSPPS for T. brucei, justifying further efforts to validate it as a new drug target.

Prenyltransferase Inhibitors: Treating Human Ailments from Cancer to Parasitic Infections

The posttranslational modification of protein prenylation is a covalent lipid modification on the C-terminus of substrate proteins that serves to enhance membrane affinity. Oncogenic proteins such as Ras have this modification and significant effort has been placed into developing inhibitors of the prenyltransferase enzymes for clinical therapy. In addition to cancer therapy, prenyltransferase inhibitors have begun to find important therapeutic uses in other diseases, including progeria, hepatitis C and D, parasitic infections, and other maladies. This review will trace the evolution of prenyltransferase inhibitors from their initial use as cancer therapeutics to their expanded applications for other diseases.

Thiol-based posttranslational modifications in parasites

SIGNIFICANCE

Cysteine residues of proteins participate in the catalysis of biochemical reactions, are crucial for redox reactions, and influence protein structure by the formation of disulfide bonds. Covalent posttranslational modifications (PTMs) of cysteine residues are important mediators of redox regulation and signaling by coupling protein activity to the cellular redox state, and moreover influence stability, function, and localization of proteins. A diverse group of protozoan and metazoan parasites are a major cause of diseases in humans, such as malaria, African trypanosomiasis, leishmaniasis, toxoplasmosis, filariasis, and schistosomiasis.

RECENT ADVANCES

Human parasites undergo dramatic morphological and metabolic changes while they pass complex life cycles and adapt to changing environments in host and vector. These processes are in part regulated by PTMs of parasitic proteins. In human parasites, posttranslational cysteine modifications are involved in crucial cellular events such as signal transduction (S-glutathionylation and S-nitrosylation), redox regulation of proteins (S-glutathionylation and S-nitrosylation), protein trafficking and subcellular localization (palmitoylation and prenylation), as well as invasion into and egress from host cells (palmitoylation). This review focuses on the occurrence and mechanisms of these cysteine modifications in parasites.

CRITICAL ISSUES

Studies on cysteine modifications in human parasites are so far largely based on in vitro experiments.

FUTURE DIRECTIONS

The in vivo regulation of cysteine modifications and their role in parasite development will be of great interest in order to understand redox signaling in parasites.

Isoprenoid biosynthesis in the erythrocytic stages of Plasmodium falciparum

The development of new drugs is one strategy for malaria control. Biochemical pathways localised in the apicoplast of the parasite, such as the synthesis of isoprenic precursors, are excellent targets because they are different or absent in the human host. Isoprenoids are a large and highly diverse group of natural products with many functions and their synthesis is essential for the parasite's survival. During the last few years, the genes, enzymes, intermediates and mechanisms of this biosynthetic route have been elucidated. In this review, we comment on some aspects of the methylerythritol phosphate pathway and discuss the presence of diverse isoprenic products such as dolichol, ubiquinone, carotenoids, menaquinone and isoprenylated proteins, which are biosynthesised during the intraerythrocytic stages of Plasmodium falciparum.

In vitro and in vivo antiplasmodial activities of risedronate and its interference with protein prenylation in Plasmodium falciparum

The increasing resistance of malarial parasites to almost all available drugs calls for the identification of new compounds and the detection of novel targets. Here, we establish the antimalarial activities of risedronate, one of the most potent bisphosphonates clinically used to treat bone resorption diseases, against blood stages of Plasmodium falciparum (50% inhibitory concentration [IC50] of 20.3±1.0 μM). We also suggest a mechanism of action for risedronate against the intraerythrocytic stage of P. falciparum and show that protein prenylation seems to be modulated directly by this drug. Risedronate inhibits the transfer of the farnesyl pyrophosphate group to parasite proteins, an effect not observed for the transfer of geranylgeranyl pyrophosphate. Our in vivo experiments further demonstrate that risedronate leads to an 88.9% inhibition of the rodent parasite Plasmodium berghei in mice on the seventh day of treatment; however, risedronate treatment did not result in a general increase of survival rates.

Lipid metabolism in Trypanosoma brucei

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

Thematic review series: lipid posttranslational modifications. Structural biology of protein farnesyltransferase and geranylgeranyltransferase type I

More than 100 proteins necessary for eukaryotic cell growth, differentiation, and morphology require posttranslational modification by the covalent attachment of an isoprenoid lipid (prenylation). Prenylated proteins include members of the Ras, Rab, and Rho families, lamins, CENPE and CENPF, and the gamma subunit of many small heterotrimeric G proteins. This modification is catalyzed by the protein prenyltransferases: protein farnesyltransferase (FTase), protein geranylgeranyltransferase type I (GGTase-I), and GGTase-II (or RabGGTase). In this review, we examine the structural biology of FTase and GGTase-I (the CaaX prenyltransferases) to establish a framework for understanding the molecular basis of substrate specificity and mechanism. These enzymes have been identified in a number of species, including mammals, fungi, plants, and protists. Prenyltransferase structures include complexes that represent the major steps along the reaction path, as well as a number of complexes with clinically relevant inhibitors. Such complexes may assist in the design of inhibitors that could lead to treatments for cancer, viral infection, and a number of deadly parasitic diseases.

Thematic review series: lipid posttranslational modifications. Fighting parasitic disease by blocking protein farnesylation

Protein farnesylation is a form of posttranslational modification that occurs in most, if not all, eukaryotic cells. Inhibitors of protein farnesyltransferase (PFTIs) have been developed as anticancer chemotherapeutic agents. Using the knowledge gained from the development of PFTIs for the treatment of cancer, researchers are currently investigating the use of PFTIs for the treatment of eukaryotic pathogens. This "piggy-back" approach not only accelerates the development of a chemotherapeutic agent for protozoan pathogens but is also a means of mitigating the costs associated with de novo drug design. PFTIs have already been shown to be efficacious in the treatment of eukaryotic pathogens in animal models, including both Trypanosoma brucei, the causative agent of African sleeping sickness, and Plasmodium falciparum, one of the causative agents of malaria. Here, current evidence and progress are summarized that support the targeting of protein farnesyltransferase for the treatment of parasitic diseases.

Design and Synthesis of Peptidomimetic Protein Farnesyltransferase Inhibitors as Anti-Trypanosoma brucei Agents

On the basis of the structure of the CVIM tetrapeptide substrate of mammalian protein farnesyltransferase, a series of imidazole-containing peptidomimetics was designed and synthesized, and their inhibition activity against Trypanosoma brucei protein farnesyltransferase (TbPFT) was evaluated. Peptidomimetics where the 5-position of the imidazole ring was linked to the hydrophobic scaffold showed over 70% inhibition activity at 50 nM in the enzyme assay, whereas the corresponding C-4 regioisomers were less potent. The ester prodrug 23 was found to be a potent inhibitor against cultured Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense cells with ED(50) values of 0.025 and 0.0026 microM, respectively. Furthermore, introducing a second imidazole group into 23 led to 31, which showed the highest inhibition activity against the parasite with an ED(50) of 0.0015 microM. The potency of the TbPFT inhibitors and the cytotoxicity of the corresponding esters to T. brucei cells were shown to be highly correlated. These studies validate TbPFT as a target for the development of novel therapeutics against African sleeping sickness.

Protein farnesyl and N-myristoyl transferases: piggy-back medicinal chemistry targets for the development of antitrypanosomatid and antimalarial therapeutics

To accelerate progress in the development of therapeutics for protozoan parasitic diseases, we are studying enzymes active in co- and post-translational protein modification that are already the focus of drug development in other eukaryotic systems. Inhibitors of the protein farnesyltransferases (PFT) are well-established antitumour agents of low cytotoxicity and known pharmokinetic properties, while inhibitors of N-myristoyl transferase show both selectivity and specificity in the treatment of fungal infections. Here, we summarise the current evidence that supports the targeting of these ubiquitous eukaryotic enzymes for drug development against trypanosomatid infections and malaria.

Trypanosoma brucei prenylated-protein carboxyl methyltransferase prefers farnesylated substrates

Carboxyl methylation of the C-terminal prenylated cysteine, which occurs in most farnesylated and geranylgeranylated proteins, is a reversible step and is implicated in the regulation of membrane binding and cellular functions of prenylated proteins such as GTPases. The gene coding for prenylated-protein carboxyl methyltransferase (PPMT) of the protozoan parasite Trypanosoma brucei has been cloned and expressed in the baculovirus/Sf9 cell system. The protein of 245 amino acids has 24-28% sequence identity to the orthologues from other species including human and Saccharomyces cerevisiae. Methyltransferase activity was detected in the membrane fraction from Sf9 cells infected with the recombinant baculovirus using N -acetyl- S -farnesylcysteine (AFC) and S -adenosyl[ methyl -(3)H]methionine ([(3)H]AdoMet) as substrates. Recombinant T. brucei PPMT prefers AFC to N -acetyl- S -geranylgeranylcysteine (AGGC) by 10-50-fold based on the V (max)/ K (m) values. Native PPMT activity detected in the membrane fraction from T. brucei procyclics displays similar substrate specificity ( approximately 40-fold preference for AFC over AGGC). In contrast, mouse liver PPMT utilizes both AFC and AGGC as substrates with similar catalytic efficiencies. Several cellular proteins of the T. brucei bloodstream form were shown to be carboxyl methylated in a cell-free system. Incorporation of [(3)H]methyl group from [(3)H]AdoMet into most of the proteins was significantly inhibited by AFC but not AGGC at 20 microM, suggesting that T. brucei PPMT acts on farnesylated proteins in the cell. Cells of the T. brucei bloodstream form show higher sensitivity to AFC and AGGC (EC(50)=70-80 microM) compared with mouse 3T3 cells (EC(50)>150 microM).

Protein farnesyltransferase and protein prenylation in Plasmodium falciparum

Comparison of the malaria parasite and mammalian protein prenyltransferases and their cellular substrates is important for establishing this enzyme as a target for developing antimalarial agents. Nineteen heptapeptides differing only in their carboxyl-terminal amino acid were tested as alternative substrates of partially purified Plasmodium falciparum protein farnesyltransferase. Only NRSCAIM and NRSCAIQ serve as substrates, with NRSCAIM being the best. Peptidomimetics, FTI-276 and GGTI-287, inhibit the transferase with IC(50) values of 1 and 32 nm, respectively. Incubation of P. falciparum-infected erythrocytes with [(3)H]farnesol labels 50- and 22-28-kDa proteins, whereas [(3)H]geranylgeraniol labels only 22-28-kDa proteins. The 50-kDa protein is shown to be farnesylated, whereas the 22-28-kDa proteins are geranylgeranylated, irrespective of the labeling prenol. Protein labeling is inhibited more than 50% by either 5 microm FTI-277 or GGTI-298. The same concentration of inhibitors also inhibits parasite growth from the ring stage by 50%, decreases expression of prenylated proteins as measured with prenyl-specific antibody, and inhibits parasite differentiation beyond the trophozoite stage. Furthermore, differentiation specific prenylation of P. falciparum proteins is demonstrated. Protein labeling is detected predominantly during the trophozoite to schizont and schizont to ring transitions. These results demonstrate unique properties of protein prenylation in P. falciparum: a limited specificity of the farnesyltransferase for peptide substrates compared with mammalian enzymes, the ability to use farnesol to label both farnesyl and geranylgeranyl moieties on proteins, differentiation specific protein prenylation, and the ability of peptidomimetic prenyltransferase inhibitors to block parasite differentiation.

Differential endocytic functions of Trypanosoma brucei Rab5 isoforms reveal a glycosylphosphatidylinositol-specific endosomal pathway

We demonstrate the presence of a glycosylphosphatidylinositol (GPI) anchor-specific endosomal pathway in the protozoan pathogen Trypanosoma brucei. In higher eukaryotes evidence indicates that GPI-anchored proteins are transported in both the endocytic and exocytic systems by mechanisms involving sequestration into specific membrane microdomains and consequently sorting into distinct compartments. This is potentially extremely important in trypanosomatids as the GPI anchor is the predominant mechanism for membrane attachment of surface macromolecules, including the variant surface glycoprotein (VSG). A highly complex developmentally regulated endocytic network, vital for nutrient uptake and evasion of the immune response, exists in T. brucei. In common with mammalian cells an early endosomal compartment is defined by Rab5 small GTPases, which control transport processes through the endosomal system. We investigate the function of two trypanosome Rab5 homologues. TbRAB5A and TbRAB5B, which colocalize in the procyclic stage, are distinct in the bloodstream form of the parasite. TbRAB5A endosomes contain VSG and transferrin, endocytosed by the T. brucei GPI-anchored transferrin receptor, whereas TbRAB5B endosomes contain the transmembrane protein ISG(100) but neither VSG nor transferrin. These findings indicate the presence of trypanosome endosomal pathways trafficking proteins through specific routes depending on the mode of membrane attachment. Ectopic expression of mutant TbRAB5A or -5B indicates that TbRAB5A plays a role in LDL endocytosis, whereas TbRAB5B does not, but both have a role in fluid phase endocytosis. Hence TbRAB5A and TbRAB5B have distinct functions in the endosomal system of T. brucei. A developmentally regulated GPI-specific endosomal pathway in the bloodstream form suggests that specialized transport of GPI-anchored proteins is required for survival in the mammalian host.

Isolation and characterization of subnuclear compartments from Trypanosoma brucei. Identification of a major repetitive nuclear lamina component

Protozoan parasites of the order Kinetoplastida are responsible for a significant proportion of global morbidity and economic hardship. These organisms also represent extremely distal points within the Eukarya, and one such organism, Trypanosoma brucei, has emerged as a major system for the study of evolutionary cell biology. Significant technical challenges have hampered the full exploitation of this organism, but advances in genomics and proteomics provide a novel approach to acquiring rapid functional data. However, the vast evolutionary distance between trypanosomes and the higher eukaryotes presents significant problems with functional assignment based on sequence similarity, and frequently homologues cannot be identified with sufficient confidence to be informative. Direct identification of proteins in isolated organelles has the potential of providing robust functional insight and is a powerful approach for initial assignment. We have selected the nucleus of T. brucei as a first target for protozoan organellar proteomics. Our purification methodology was able to reliably provide both nuclear and subnuclear fractions. Analysis by gel electrophoresis, electron microscopy, and immunoblotting against trypanosome subcellular markers indicated that the preparations are of high yield and purity, maintain native morphology, and are well resolved from other organelles. Minor developmental differences were observed in the nuclear proteome for the bloodstream and procyclic stages, whereas significant morphological alterations were visible. We demonstrate by direct sequencing that the NUP-1 nuclear envelope antigen is a coiled coil protein, containing approximately 20 near-perfect copies of a 144-amino acid sequence. Immunoelectron microscopy localized NUP-1 to the inner face of the nuclear envelope, suggesting that it is a major filamentous component of the trypanosome nuclear lamina.

Limonene arrests parasite development and inhibits isoprenylation of proteins in Plasmodium falciparum

Isoprenylation is an essential protein modification in eukaryotic cells. Herein, we report that in Plasmodium falciparum, a number of proteins were labeled upon incubation of intraerythrocytic forms with either [(3)H]farnesyl pyrophosphate or [(3)H]geranylgeranyl pyrophosphate. By thin-layer chromatography, we showed that attached isoprenoids are partially modified to dolichol and other, uncharacterized, residues, confirming active isoprenoid metabolism in this parasite. Incubation of blood-stage P. falciparum treated with the isoprenylation inhibitor limonene significantly decreased the parasites' progression from the ring stage to the trophozoite stage and at 1.22 mM, 50% of the parasites died after the first cycle. Using Ras- and Rap-specific monoclonal antibodies, putative Rap and Ras proteins of P. falciparum were immunoprecipitated. Upon treatment with 0.5 mM limonene, isoprenylation of these proteins was significantly decreased, possibly explaining the observed arrest of parasite development.

TcRho1, a farnesylated Rho family homologue from Trypanosoma cruzi: cloning, trans-splicing, and prenylation studies

Rho GTPases are members of the Ras superfamily and are involved in signal transduction pathways, including maintenance of cell morphology and motility, cell cycle progression, and transcription activation. We report the molecular identification in trypanosomatids (Trypanosoma cruzi) of the first member of the Rho family. The cloned Rho protein, TcRho1, shares approximately 40% homology with other members of the Rho family. Southern blot analysis revealed that TcRHO1 is a single copy gene per haploid genome, and Northern blot assays showed a transcript of 1200 nucleotides in length. Mapping the 5'-untranslated region of TcRHO1 transcripts revealed at least five different transcripts derived from differential trans-splicing. Three of the five transcripts contain the trans-splicing site within the coding region of the TcRHO1 gene. TcRho1 also contains the C-terminal sequence CQLF (CAAX motif), which is predicted to direct post-translation prenylation of the cysteine residue. A synthetic peptide containing this C-terminal motif, when tested against Q-Sepharose chromatography fractions from T. cruzi cytosol, was shown to be efficiently farnesylated, but not geranylgeranylated, despite the fact that the CAAX motif with X = Phe specifies geranylgeranylation by mammalian protein geranylgeranyltransferase I. Furthermore, immunoblot analyses of epimastigote protein with anti-S-farnesylcysteine methyl ester and anti-TcRho1 antisera strongly suggested that TcRho1 is farnesylated in vivo. The farnesylation of proteins such as Rho GTPases could be the basis for the selective cytotoxic action of protein farnesyltransferase inhibitors on trypanosomatids versus mammalian cells.

Bisphosphonates inhibit the growth of Trypanosoma brucei, Trypanosoma cruzi, Leishmania donovani, Toxoplasma gondii, and Plasmodium falciparum: a potential route to chemotherapy

We have investigated the effects in vitro of a series of bisphosphonates on the proliferation of Trypanosoma cruzi, Trypanosoma brucei rhodesiense, Leishmania donovani, Toxoplasma gondii, and Plasmodium falciparum. The results show that nitrogen-containing bisphosphonates of the type used in bone resorption therapy have significant activity against parasites, with the aromatic species having in some cases nanomolar or low-micromolar IC(50) activity values against parasite replication (e.g. o-risedronate, IC(50) = 220 nM for T. brucei rhodesiense; risedronate, IC(50) = 490 nM for T. gondii). In T. cruzi, the nitrogen-containing bisphosphonate risedronate is shown to inhibit sterol biosynthesis at a pre-squalene level, most likely by inhibiting farnesylpyrophosphate synthase. Bisphosphonates therefore appear to have potential in treating parasitic protozoan diseases.

The adaptative mechanisms of Trypanosoma brucei for sterol homeostasis in its different life-cycle environments

Bloodstream forms of Trypanosoma brucei do not synthesize sterols de novo and therefore cannot survive in medium devoid of lipoproteins. Growth of parasites is essentially supported by receptor-mediated endocytosis of low-density lipoproteins (LDLs), which carry phospholipids and cholesteryl esters. These lipids are released from internalized LDL after apoprotein B-100 is degraded by acidic thiol-proteases in the endolysosomal apparatus and then metabolized, as in mammalian cells. The LDL receptor is recycled and its expression is regulated by the sterol stores. Documented pharmacological and immunological interferences with LDL receptor-mediated lipid supply to the bloodstream forms are summarized, and the potential for new approaches to fight against these parasites is evaluated. In contrast to bloodstream forms, cultured procyclic forms can acquire sterols from both exogenous (lipoprotein endocytosis) and endogenous (biosynthesis of ergosterol) sources. The rate-limiting steps of both endocytosis (surface LDL receptor expression) and biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase activity) are regulated by the cellular content of sterol. These two pathways thus complement each other to yield a balanced sterol supply, which demonstrates adaptative capacities to survive in totally different environments and fine regulatory mechanisms of sterol homeostasis.

Cloning, heterologous expression, and distinct substrate specificity of protein farnesyltransferase from Trypanosoma brucei

Protein prenylation occurs in the protozoan that causes African sleeping sickness (Trypanosoma brucei), and the protein farnesyltransferase appears to be a good target for developing drugs. We have cloned the alpha- and beta-subunits of T. brucei protein farnesyltransferase (TB-PFT) using nucleic acid probes designed from partial amino acid sequences obtained from the enzyme purified from insect stage parasites. TB-PFT is expressed in both bloodstream and insect stage parasites. Enzymatically active TB-PFT was produced by heterologous expression in Escherichia coli. Compared with mammalian protein farnesyltransferases, TB-PFT contains a number of inserts of >25 residues in both subunits that reside on the surface of the enzyme in turns linking adjacent alpha-helices. Substrate specificity studies with a series of 20 peptides SSCALX (where X indicates a naturally occurring amino acid) show that the recombinant enzyme behaves identically to the native enzyme and displays distinct specificity compared with mammalian protein farnesyltransferase. TB-PFT prefers Gln and Met at the X position but not Ser, Thr, or Cys, which are good substrates for mammalian protein farnesyltransferase. A structural homology model of the active site of TB-PFT provides a basis for understanding structure-activity relations among substrates and CAAX mimetic inhibitors.

The farnesyltransferase inhibitor manumycin A is a novel trypanocide with a complex mode of action including major effects on mitochondria

Eukaryotes modify numerous proteins, including small GTPases of the ras superfamily, with isoprenes as a mechanism for membrane attachment. Inhibition of farnesylation of ras has been successfully exploited to control cell growth, with promise in the clinic for treatment of human tumours. Using an in vitro screen of mammalian farnesyltransferase inhibitors, we have identified manumycin A as potently active against growth of both bloodstream and procyclic forms of Trypanosoma brucei. Other structural classes of farnesyltransferase inhibitors were far less effective. Exposure of T. brucei for brief periods to lethal concentrations of manumycin A resulted in subsequent cell death whilst the concentration required to achieve killing was dependent on serum concentration, suggesting partitioning of manumycin A into hydrophobic cellular sites. Manumycin A did not affect trypanosomal protein and DNA synthesis or cell cycle progression but altered incorporation of prenyl groups into several polypeptides indicating a specific effect on the prenylation without effect on other mevalonate pathway products, most importantly prenyl pyrophosphate levels. Morphological analysis indicated that manumycin A caused significant mitochondrial damage suggesting an additional site of action. Structural analogues of manumycin A containing a quinone were also highly trypanocidal and altered mitochondrial morphology, suggesting interference with electron/proton transport systems. Furthermore, manumycin A also elicited mitochondrial alterations in mammalian cells indicating that the effect is not confined to lower eukaryotes. Manumycin A is well tolerated in vivo but failed to cure experimental trypanosomiasis in mice.

GTPases in protozoan parasites: tools for cell biology and chemotherapy

Small G proteins belong to a superfamily of GTPases related to the protooncogene ras, and function as master control elements for a range of cellular functions. This ability is related to their low rate of substrate turnover; GTPases catalyse the conversion of GTP to GDP, but with a rate in the order of one substrate per second, orders of magnitude slower than 'good' enzyme catalysis, but placing the reaction into the temporal frame of many cellular processes including signal transduction, cytoskeletal reorganization and vesicle trafficking. In this article, Mark Field, Bassam Ali and Helen Field describe some recent advances in G-protein studies in the parasite field, concentrating on the protozoan parasites. Because of their numerous roles in cell biology, understanding parasite G proteins has great potential for increasing our knowledge of parasite cellular physiology, as well as providing important inroads into vital processes for potential therapeutic exploitation.

A novel gene encoding a ras-like GTP-binding protein from Trypanosoma brucei: an evolutionary ancestor of the ras and rap genes of higher eukaryotes?

The ras superfamily of GTP binding proteins encompasses a wide range of family members, related by conserved amino-acid motifs, and act as molecular binary switches that play key roles in cellular processes. Gene duplication and divergence has been postulated as the mechanism by which such family members have evolved their specific functions. We have cloned and sequenced a ras-like gene, tbrlp, from the primitive eukaryote Trypanosoma brucei. The gene encodes a protein of 227 amino acids and contains the six conserved subdomains that designate it as a ras/rap subfamily member. However, the presence of key diagnostic residues characteristic of both the ras and rap families of GTP confuse the familial classification of this gene. Phylogenetic analysis of the GTP binding domain places its origins at the divergence point of the ras/rap families and suggests that tbrlp is an ancestral gene to the ras/rap genes of higher eukaryotes.

TbRab2p, a marker for the endoplasmic reticulum of Trypanosoma brucei, localises to the ERGIC in mammalian cells

The Rab family of small GTPases is a subset of the Ras superfamily. Rabs regulate the flux through individual steps of the intracellular membrane trafficking pathway, such as ER-to-Golgi transport, probably by controlling SNARE complex assembly. In Trypanosoma brucei a number of Rab proteins have been isolated by EST analysis; here we characterise one of these, TbRab2p (originally designated Trab1p), which is a member of the Ypt1p subfamily of Rab proteins. Recombinant TbRab2p is capable of hydrolysing GTP and is post-translationally modified in vitro by addition of a geranylgeranyl prenyl group, properties of an authentic Rab GTPase. Antibodies against recombinant TbRab2p show that in trypanosomes TbRab2p is localised primarily to the endoplasmic reticulum (ER) and colocalises with BiP in wild-type trypanosomes. Over expression of TbRab2p in procyclic form T. brucei results in a cell population having a 40-fold increase in TbRab2p expression. In these cells biosynthesis of procyclin, a secretory pathway glycoprotein, is decreased, accompanied by an increase in general protein biosynthesis, suggesting that excess TbRab2p affects ER function. Heterologous expression of TbRab2p in COS cells resulted in targeting to the pre-Golgi transport intermediate (ERGIC), indicating that the targeting information is conserved between mammals and trypanosomes. Clustal and phylogenetic analyses support assignment of TbRab2p as a Rab2 homologue. In addition, over expression of TbRab2p in trypanosomes results in membrane reorganisation and formation of opaque vesicular structures visible by phase contrast microscopy, consistent with accumulation of ER-derived vesicular structures in cells highly overexpressing TbRab2p. Ultrastructural examination by electron microscopy confirmed the presence of a tubulo-vesicular membrane bound compartment in close proximity to the cis-Golgi, probably equivalent to the ERGIC. TbRab2p is therefore a new ER/ERGIC marker for T. brucei.

Protein farnesyltransferase from Trypanosoma brucei. A heterodimer of 61- and 65-kda subunits as a new target for antiparasite therapeutics

We have previously shown that protein prenylation occurs in the Trypanosomatids Trypanosoma brucei (T. brucei), Trypanosoma cruzi, and Leishmania mexicana and that protein farnesyltransferase (PFT) activity can be detected in cytosolic extracts of insect (procyclic) form T. brucei. A PFT that transfers the farnesyl group from farnesyl pyrophosphate to a cysteine that is 4 residues upstream of the C terminus of the Ras GTP-binding protein RAS1-CVIM has now been purified 60,000-fold to near homogeneity from procyclic T. brucei. By screening a mixture of hexapeptides SSCALX (X is 20 different amino acids), it was found that SSCALM binds to T. brucei PFT with sub-micromolar affinity, and affinity chromatography using this peptide was a key step in the purification of this enzyme. On SDS-polyacrylamide gel electrophoresis, the enzyme migrates as a pair of bands with apparent molecular masses of 61 and 65 kDa, and thus its subunits are approximately 30% larger than those of the mammalian homolog. The 61-kDa band was identified as the putative beta-subunit by photoaffinity labeling with a 32P-labeled analog of farnesyl pyrophosphate. Mimetics of the C-terminal tetrapeptide of prenyl acceptors have been previously shown to inhibit mammalian PFT, and these compounds also inhibit T. brucei PFT with affinities in the nanomolar to micromolar range, although the structure-activity relationship is very different for parasite versus mammalian enzyme. Unlike mammalian cells, the growth of bloodstream T. brucei is completely inhibited by low micromolar concentrations of two of the PFT inhibitors, and these compounds also block protein farnesylation in cultured parasites. These compounds also potently block the growth of the intracellular (amastigote) form of T. cruzi grown in fibroblast host cells. The results suggest that protein farnesylation is a target for the development of anti-trypanosomatid chemotherapeutics.

Protein prenyl transferase activities of Plasmodium falciparum

Prenylated proteins have been shown to function in important cellular regulatory processes including signal transduction. The enzymes involved in protein prenylation, farnesyl transferase and geranylgeranyl transferase, have been recent targets for development of cancer chemotherapeutics. We have initiated a systematic study of protein prenyl transferases of the malaria parasite, Plasmodium falciparum, to determine whether these enzymes can be developed as targets for antimalarial chemotherapy. We report here the identification of protein farnesyl transferase and protein geranylgeranyl transferase-I in the malaria parasite, P. falciparum. The farnesyl transferase has been partially purified from the cytosolic fraction through ammonium sulfate precipitation and Mono-Q chromatography. Farnesyl and geranylgeranyl transferase-I activities are present at all stages of P. falciparum intraerythrocytic development with maximum specific activity in the ring stage. Geranylgeranyl transferase-I specific activity is two times that of farnesyl transferase in the ring stage. Peptidomimetics and prenyl analogues of protein farnesyl transferase substrates were tested as in vitro inhibitors of partially purified P. falciparum prenyl transferase and of malaria parasite growth. The peptidomimetics were significantly more potent inhibitors than lipid substrate analogues of both the activity of Mono-Q purified enzyme and parasite growth in intraerythrocytic cultures. Exposure of the parasite to the peptidomimetic L-745,631 also showed significant inhibition of morphological development beyond the trophozoite stage. These studies suggest the potential of designing or identifying differential inhibitors of P. falciparum and mammalian prenyl transferases as an approach to novel malaria therapy.

The effects of protein farnesyltransferase inhibitors on trypanosomatids: inhibition of protein farnesylation and cell growth

Attachment of the prenyl groups farnesyl and geranylgeranyl to specific eukaryotic cell proteins by protein prenyltransferases is required for the functioning of a number of cellular processes including signal transduction. In this study it was found that previously reported inhibitors of mammalian protein farnesyltransferase (PFT) [those that mimic the substrate farnesyl pyrophosphate and those that mimic the protein acceptor of the farnesyl group (CaaX mimetic)] inhibit in vitro farnesylation catalyzed by partially purified Trypanosoma brucei (T. brucei) PFT. The most potent PFT inhibitors at concentrations of 3-10 microM inhibit the growth of insect (procyclic) and bloodstream forms of T. brucei. One of the PFT inhibitors was found to block the incorporation of radiolabeled mevalonic acid (the precursor of prenyl groups) into specific T. brucei proteins. This study also shows that protein prenylation occurs in the protozoan parasites Trypanosoma cruzi (T. cruzi) and Leishmania mexicana (L. mexicana). The growth of T. cruzi intracellular form (amastigote) is also sensitive to PFT inhibitors, whereas the insect form (epimastigote) is considerably more resistant to inhibition of protein farnesylation. On the other hand, growth of 3T3 fibroblast cells (host cells for amastigote growth) was not affected by up to 100 microM PFT inhibitors. The growth of L. mexicana insect form (promastigote) is modestly inhibited by protein farnesyltransferase inhibitors. These results suggest the potential for the development of PFT inhibitors for treating trypanosomiasis and leishmaniasis.

Tandem duplication of rab genes followed by sequence divergence and acquisition of distinct functions in Trypanosoma brucei

The Ras superfamily of small G proteins governs unidirectional cellular processes by virtue of GTP hydrolysis and concomitant conformational changes, which are in turn regulated by a number of accessory factors. Members of the Rab subfamily are important for correct targeting and fusion of intra-organellar vesicles loaded with trafficking proteins and lipids. During evolution from a prototype gene, novel functions may be acquired by duplicated daughter genes; for Rab proteins, this can be tested by location, which is specifically related to the function of each Rab. We have found an example of two rab genes in Trypanosoma brucei (trab genes) that clearly arose by tandem duplication, being highly related to each other and remaining juxtaposed in the genome, whose products have dramatically different subcellular locations, indicative of discrete functions. These two trab genes, isolated on a single genomic clone, are separated by a short intervening sequence and are in a head-to-tail orientation. The nucleotide sequences of the open reading frames and intervening sequence were determined and show that the genes are paralogues, probably arising from an ancient tandem duplication. Both genes are most homologous to ypt1 and sec4 in the Saccharomyces cerevisiae genome, while phylogenetic reconstruction indicates that although they have clearly diverged, the proteins are more closely related to each other than to other Rab protein sequences available in the data base. Immunofluorescence microscopy, using antibodies raised against the recombinant Trab proteins, clearly demonstrates that the native Trab proteins have completely distinct subcellular locations in the trypanosome. Trab1p is present in a widespread reticular location similar to BiP, suggesting an endoplasmic reticulum location, while Trab7p is observed in a discrete structure adjacent to the kinetoplast. Most interestingly, the Trab7p-positive compartment also appears to divide at the same time, or just prior to, the kinetoplast, i.e. early in mitosis, suggestive of association with structures in the flagellar pocket region. An estimate of the divergence time indicates that the trab1/trab7 duplication occurred approximately 100 million years ago, and therefore, the persistence of this pair suggests an essential role in the survival of T. brucei.