A dominant negative mutation in the GIM1 gene of Leishmania donovani is responsible for defects in glycosomal protein localization

Fishing for anti-leishmania drugs: principles and problems

To date, there are no vaccines against any of the major parasitic diseases including leishmaniasis, and chemotherapy is the main weapon in our arsenal. Current drugs are toxic and expensive, and are losing their effectiveness due to parasite resistance. The availability of the genome sequence of two species of Leishmania, Leishmania major and Leishmania infantum, as well as that of Trypanosoma brucei and Trypanosoma cruzi should provide a cornucopia of potential new drug targets. Their exploitation will require a multi-disciplinary approach that includes protein structure and function and high throughput screening of random and directed chemical libraries, followed by in vivo testing in animals and humans. We outline the opportunities that are made possible by recent technologies, and potential problems that need to be overcome.

Characterization of glycosomal RING finger proteins of trypanosomatids

The glycosomes of trypanosomatids are essential organelles that are evolutionarily related to peroxisomes of other eukaryotes. The peroxisomal RING proteins-PEX2, PEX10 and PEX12-comprise a network of integral membrane proteins that function in the matrix protein import cycle. Here, we describe PEX10 and PEX12 in Trypanosoma brucei, Leishmania major, and Trypanosoma cruzi. We expressed GFP fusions of each T. brucei coding region in procyclic form T. brucei, where they localized to glycosomes and behaved as integral membrane proteins. Despite the weak transmembrane predictions for TbPEX12, protease protection assays demonstrated that both the N and C termini are cytosolic, similar to mammalian PEX12. GFP fusions of T. cruzi PEX10 and L. major PEX12 also localized to glycosomes in T. brucei indicating that glycosomal membrane protein targeting is conserved across trypanosomatids.

Identification and characterization of three peroxins--PEX6, PEX10 and PEX12--involved in glycosome biogenesis in Trypanosoma brucei

Protozoan Kinetoplastida such as the pathogenic trypanosomes compartmentalize several important metabolic systems, including the glycolytic pathway, in peroxisome-like organelles designated glycosomes. Genes for three proteins involved in glycosome biogenesis of Trypanosoma brucei were identified. A preliminary analysis of these proteins, the peroxins PEX6, PEX10 and PEX12, was performed. Cellular depletion of these peroxins by RNA interference affected growth of both mammalian bloodstream-form and insect-form (procyclic) trypanosomes. The bloodstream forms, which rely entirely on glycolysis for their ATP supply, were more rapidly killed. Both by immunofluorescence studies of intact procyclic T. brucei cells and subcellular fractionation experiments involving differential permeabilization of plasma and organellar membranes it was shown that RNAi-dependent knockdown of the expression of each of these peroxins resulted in the partial mis-localization of different types of glycosomal matrix enzymes to the cytoplasm: proteins with consensus motifs such as the C-terminal type 1 peroxisomal targeting signal PTS1 or the N-terminal signal PTS2 and a protein for which the sorting information is present in a polypeptide-internal fragment not containing an identifiable consensus sequence.

Identification of trypanosomatid PEX19: functional characterization reveals impact on cell growth and glycosome size and number

Glycosomes are peroxisome-like organelles present in trypanosomatid pathogens. These organelles compartmentalize glycolysis, among other reactions, and are essential in both bloodstream and procyclic form Trypanosoma brucei. Peroxins (PEXs) are proteins necessary for biogenesis of peroxisomes and glycosomes. In each assembled trypanosomatid genome, we identified a predicted protein with approximately 20% sequence identity to human PEX19, a protein required for insertion of peroxisomal membrane proteins (PMPs) into the membrane. Functional analysis demonstrated that these proteins are indeed PEX19 orthologues. Like other PEX19s, T. brucei and Leishmania major PEX19 GFP fusion proteins are predominantly cytosolic. We further showed that LmPEX19 interacts with the glycosomal membrane protein PEX2 in the yeast two-hybrid system. Partial knockdown of TbPEX19 slowed parasite growth, particularly when glucose was present. Immunofluorescence and electron microscopic studies revealed biogenesis defect as evidenced by a sharp reduction in the number of glycosomes. Surprisingly, a four-fold increase in the size of the remaining glycosomes was observed. We propose that this phenotype of fewer but larger glycosomes results from the reduction in import of glycosomal membrane proteins.

Biogenesis of peroxisomes and glycosomes: trypanosomatid glycosome assembly is a promising new drug target

In trypanosomatids (Trypanosoma and Leishmania), protozoa responsible for serious diseases of mankind in tropical and subtropical countries, core carbohydrate metabolism including glycolysis is compartmentalized in peculiar peroxisomes called glycosomes. Proper biogenesis of these organelles and the correct sequestering of glycolytic enzymes are essential to these parasites. Biogenesis of glycosomes in trypanosomatids and that of peroxisomes in other eukaryotes, including the human host, occur via homologous processes involving proteins called peroxins, which exert their function through multiple, transient interactions with each other. Decreased expression of peroxins leads to death of trypanosomes. Peroxins show only a low level of sequence conservation. Therefore, it seems feasible to design compounds that will prevent interactions of proteins involved in biogenesis of trypanosomatid glycosomes without interfering with peroxisome formation in the human host cells. Such compounds would be suitable as lead drugs against trypanosomatid-borne diseases.

Glucose is toxic to glycosome-deficient trypanosomes

Trypanosomatids, the etiologic agents of sleeping sickness, leishmaniasis, and Chagas' disease, compartmentalize glycolysis within glycosomes, metabolic organelles related to peroxisomes. Here, we identify a trypanosome homologue of PEX14, one of the components of the peroxisomal protein import docking complex. We have used double-stranded RNA interference to target the PEX14 transcript for degradation. Glycosomal matrix protein import was compromised, and both glycolytic bloodstream stage parasites and mitochondrially respiring procyclic stage parasites were killed. Thus, unlike peroxisomes, glycosomes are essential organelles. Surprisingly, procyclic forms, which can grow in the absence of glucose, were killed by PEX14 RNA interference only when simple sugars were present. Thus, interference with glycosome protein import makes glucose toxic to trypanosomes.

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

All kinetoplastids contain membrane-bound microbodies known as glycosomes,in which several metabolic pathways including part of glycolysis are compartmentalized. Peroxin 2 is essential for the import of many proteins into the microbodies of yeasts and mammals. The PEX2 gene of Trypanosoma brucei was identified and its expression was silenced by means of tetracycline-inducible RNA interference. Bloodstream-form trypanosomes, which rely exclusively on glycolysis for ATP generation, died rapidly upon PEX2 depletion. Insect-form (procyclic) trypanosomes do not rely solely on glycolysis for ATP synthesis. PEX2 depletion in procyclic forms resulted in relocation of most tested matrix proteins to the cytosol, and these mutants also died. Compartmentation of microbody enzymes is therefore essential for survival of bloodstream and procyclic T. brucei. In contrast, yeasts and cultured mammalian cells grow normally in the absence of peroxisomal membranes unless placed on selective media.

An essential dimeric membrane protein of trypanosome glycosomes

Kinetoplastid parasites compartmentalize the first seven enzymes of glycolysis in a peroxisome-like microbody, the glycosome. Genes encoding the most abundant protein of the glycosomal membrane, GIM5, have been cloned and the protein characterized. Two genes, GIM5A and GIM5B, encode 26 kDa proteins. Although many microbody membrane proteins are conserved in evolution, the only homologues of GIM5 in the available databases are from the closely related kinetoplastids Trypanosoma cruzi and Leishmania. The N- and C-termini are conserved between the two genes, and between species, and are oriented towards the cytosol. They are separated by a short loop that is located between two transmembrane domains and shows almost no sequence conservation. This suggests that the N- and C-terminal domains are more important for function. GIM5 forms dimers in vivo. Overexpression of GIM5B inhibits growth, whereas depletion of GIM5 to below 10% of wild-type levels is very rapidly lethal. This novel organellar membrane protein is therefore essential for bloodstream trypanosome survival.

Peroxisomal targeting signal-1 receptor protein PEX5 from Leishmania donovani. Molecular, biochemical, and immunocytochemical characterization

The human pathogens of the Leishmania and Trypanosoma genera compartmentalize glycolytic and other key metabolic pathways in unique subcellular microbodies called glycosomes, organelles related to the peroxisomes of mammals and yeast. The molecular machinery that carries out the specific targeting of glycosomal proteins to the organelle has not been characterized, although the bulk of glycosomal proteins contain the COOH-terminal tripeptide glycosomal peroxisomal targeting signal-1 (PTS-1) similar to the mammalian and fungal peroxisomal targeting signal. To characterize the mechanisms of glycosomal targeting, the gene encoding PEX5, designated LdPEX5, has been isolated from Leishmania donovani. LdPEX5 encodes a 625-amino acid protein with a molecular mass of 69.7 kDa. Like its homologs in yeast and humans, LdPEX5 predicts a protein with seven copies of a tetratricopeptide repeat in its COOH-terminal half proposed to mediate PTS-1 binding and three copies of a WXXX(Y/F) motif in its NH(2) terminus conjectured to be essential for protein translocation into the organelle. LdPEX5 was overexpressed in Escherichia coli and purified to homogeneity for binding experiments and generation of antibodies. Recombinant LdPEX5 bound xanthine phosphoribosyltransferase (XPRT), a PTS-1 containing glycosomal protein with a K(D) of 4.2 nm, but did not bind an XPRT in which the PTS-1 had been deleted. Moreover, binding studies with the COOH-terminal half of the LdPEX5 confirmed that this portion of the PEX5 protein was capable of binding the XPRT PTS-1 with an affinity of 17.3 nm. Confocal microsocopy revealed that LdPEX5 was predominantly in the cytosolic milieu, and genetic analysis implied that LdPEX5 was an essential gene.

Isolation and characterization of Leishmania mutants defective in glycosomal protein import

Kinetoplastid parasites contain a unique microbody organelle called the glycosome. Several important metabolic pathways found in the cytoplasm of higher eukaryotes are compartmentalized within the glycosome in these pathogens. This fundamental difference between the host and parasite has led to consideration of the glycosome as a potential chemotherapeutic target. The genetic basis of glycosome biogenesis is therefore of great interest. This report describes the isolation of multiple Leishmania mutant cell lines defective in glycosomal protein import, and the detailed characterization of three such lines. The mutants examined partially mislocalize a subset of glycosomal proteins to the cytosol yet retain wild-type numbers of glycosomes. One of the mutants has a mutation in the previously identified LdPEX2 (GIM1) gene. The other two mutants are demonstrated to contain cell-specific lesions in one or more genes distinct from PEX2. The identification of multiple genetically distinct mutants with defects in glycosome import provides an important genetic tool to facilitate the identification of genes involved in glycosome biogenesis.

Cloning and analysis of the PTS-1 receptor in Trypanosoma brucei

Kinetoplastid organisms, such as the protozoan parasite Trypanosoma brucei, compartmentalise several important metabolic pathways in organelles called glycosomes. Glycosomes are related to peroxisomes of yeast and mammalian cells. A subset of glycosomal matrix proteins is routed to the organelles via the peroxisome-targeting signal type 1 (PTS-1). The PEX5 gene homologue has been cloned from T. brucei coding for a protein of the translocation machinery, the PTS-1 receptor. The gene codes for a polypeptide of 654 amino acids with a calculated molecular mass of 70 kDa. Like its homologue in other organisms T. brucei PTS-1 receptor protein (TbPEX5) is a member of the tetratricopeptide repeat (TPR) protein family and contains several copies of the pentapeptide W-X-X-X-F/Y. Northern and Western blot analysis showed that the protein is expressed at different stages of the life cycle of the parasite. The protein has been overproduced in Escherichia coli and purified using immobilized metal affinity chromatography. The purified protein specifically interacts in vitro with glycosomal phosphoglycerate kinase-C (PGK-C) of T. brucei, a PTS-1 containing protein. The equilibrium dissociation constant (Kd) of PGK-C for purified TbPEX5 is 40 nM. Using biochemical and cytochemical techniques a predominantly cytosolic localization was found for TbPEX5. This is consistent with the idea of receptor cycling between the glycosomes and the cytosol.