Why are many mRNAs translated to the vicinity of mitochondria: a role in protein complex assembly?

The longstanding question of the presence of mitochondria-bound polysomes has been recently revisited using new approaches. Genome-wide analyses provided evidence that many genes are actually translated on mitochondria-bound polysomes and GFP-labeling techniques have shown that, in vivo, the 3'UTR sequence of these genes contains signals which can target hybrid RNA molecules to the proximity of mitochondria. Evolutionary conservation of some of these signals will be presented. Interestingly, class I mRNA which are translated on free polysomes and class II mRNA which are translated on mitochondria-bound polysomes have, mostly, eukaryotic and prokaryotic origins respectively. Using ATP2, a typical prokaryotic-derived gene, as a model for class II mRNA, we showed that its 3'UTR sequence is essential both for a correct addressing of mRNA to mitochondria proximity and to a proper production of functional ATP synthases. These different observations suggest that prokaryotic-derived genes are, like the contemporary mitochondrial genes, translated near mitochondrial membranes. In both cases this locus specific translation process might be connected to a correct complex assembly program and the cases of ATP synthase and cytochrome c oxidase complexes will be considered in this respect.

Molecular characterisation of mitochondrial and cytosolic trypanothione-dependent tryparedoxin peroxidases in Trypanosoma brucei.

In trypanosomatids, removal of hydrogen peroxide and other aryl and alkyl peroxides is achieved by the NADPH-dependent trypanothione peroxidase system, whose components are trypanothione reductase (TRYR), trypanothione, tryparedoxin (TRYX) and tryparedoxin peroxidase (TRYP). Here, we report the cloning of a multi-copy tryparedoxin peroxidase gene (TRYP1) from Trypanosoma brucei brucei encoding a protein with two catalytic VCP motifs similar to the cytosolic TRYP from Crithidia fasciculata. In addition, we characterise a novel single copy gene encoding a second tryparedoxin peroxidase (TRYP2). TRYP2 shows 51% similarity to TRYP1, possesses a putative mitochondrial import sequence at its N-terminus and has a variant IPC motif replacing the second VCP motif implicated in catalysis in other 2-Cys peroxiredoxins. TRYP1 and TRYP2 were expressed in Escherichia coli, and the purified recombinant proteins shown to utilise hydrogen peroxide in the presence of NADPH, trypanothione, TRYR and TRYX from T. brucei, similar to the C. fasciculata cytoplasmic system. Western blots showed that TRYX, TRYP1 and TRYP2 are expressed in both bloodstream and procyclic forms of the life cycle. To determine the precise localisation of TRYX, TRYP1 and TRYP2 in the parasite, polyclonal antibodies to purified recombinant TRYX and TRYP1 and monoclonal antibody to TRYP2 were generated in mice. In-situ immunofluorescence and immunoelectron microscopy revealed a colocalisation of TRYX and TRYP1 in the cytosol, whereas TRYP2 was principally localised in the mitochondrion.

Cloning, expression and functional characterisation of a peroxiredoxin from the potato cyst nematode Globodera rostochiensis.

We report the cloning, expression and functional characterisation of a peroxidase belonging to the peroxiredoxin family from the potato cyst nematode Globodera rostochiensis, the first molecule of this type from any nematode parasitic on plants. The G. rostochiensis peroxiredoxin catalyses the breakdown of hydrogen peroxide, but not cumene or t-butyl hydroperoxide, in a trypanosomatid reducing system comprising trypanothione reductase, trypanothione and tryparedoxin. In common with its homologues from Onchocerca volvulus and Brugia malayi, the G. rostochiensis enzyme is present on the surface of invasive and post-infective juveniles despite the apparent lack of a cleavable N-terminal signal peptide. The possibility that the G. rostochiensis peroxiredoxin plays a role in protection of the parasite from plant defence responses is discussed.

The structure of reduced tryparedoxin peroxidase reveals a decamer and insight into reactivity of 2Cys-peroxiredoxins.

Tryparedoxin peroxidase (TryP) is a recently discovered 2Cys-peroxiredoxin involved in defence against oxidative stress in parasitic trypanosomatids. The crystal structure of recombinant Crithidia fasciculata TryP, in the reduced state, has been determined using multi-wavelength anomalous dispersion methods applied to a selenomethionyl derivative. The model comprises a decamer with 52 symmetry, ten chloride ions with 23 water molecules and has been refined, using data to 3.2 A resolution (1 A=0.1 nm), to an R-factor and R(free) of 27.3 and 28.6 %, respectively. Secondary structure topology places TryP along with tryparedoxin and glutathione peroxidase in a distinct subgroup of the thioredoxin super-family. The molecular details at the active site support ideas about the enzyme mechanism and comparisons with an oxidised 2Cys-peroxiredoxin reveal structural alterations induced by the change in oxidation state. These include a difference in quaternary structure from dimer (oxidised form) to decamer (reduced form). The 2Cys-peroxiredoxin assembly may prevent indiscriminate oligomerisation, localise ten peroxidase active sites and contribute to both the specificity of reduction by the redox partner tryparedoxin and attraction of peroxides into the active site.

The structure-function relationship of functionally distinct but structurally similar hexose transporters from Trypanosoma congolense.

We have previously characterized, in Trypanosoma brucei, a multigene family encoding two developmentally regulated glucose transporters that are 80% identical at the amino-acid level. We report here the characterization of the homologous glucose transporters (TcoHT1 and TcoHT2) in Trypanosoma congolense, an African trypanosome responsible for disease in domestic animals. Both TcoHT isoforms, which are 92.4% identical, are encoded by a single cluster of genes containing two copies of TcoHT1 and three copies of TcoHT2 arranged alternately. Northern blot analysis revealed that TcoHT2 is expressed in all of the adaptive forms, while mRNA encoding TcoHT1 is only present in the metacyclic and bloodstream forms of T. congolense. When transfected with the TcoHT2 gene, Chinese Hamster Ovary cells express a hexose transporter with properties similar to those of the T. congolense procyclic forms (Km D-glucose = 41 microM versus 64 microM). In contrast to TcoHT2, TcoHT1 expressed in the Chinese hamster ovary cells appeared to be a relatively low affinity glucose transporter (Ki D-glucose = 0.8 mM). To determine the region(s) involved in the different apparent affinities for glucose, a chimera analysis was undertaken on the TcoHT isoforms. This study shows that amino-acid residues important for D-glucose recognition are located in the central region (between transmembrane domains 3 and 7) and in the C-terminal intracellular domain of TcoHT2. Site directed mutagenesis identified Ser193 located within transmembrane helix 4 as a key residue in relaxing the apparent affinity of TcoHT1 for glucose.

The high resolution crystal structure of recombinant Crithidia fasciculata tryparedoxin-I.

Tryparedoxin-I is a recently discovered thiol-disulfide oxidoreductase involved in the regulation of oxidative stress in parasitic trypanosomatids. The crystal structure of recombinant Crithidia fasciculata tryparedoxin-I in the oxidized state has been determined using multi-wavelength anomalous dispersion methods applied to a selenomethionyl derivative. The model comprises residues 3 to 145 with 236 water molecules and has been refined using all data between a 19- and 1.4-A resolution to an R-factor and R-free of 19.1 and 22.3%, respectively. Despite sharing only about 20% sequence identity, tryparedoxin-I presents a five-stranded twisted beta-sheet and two elements of helical structure in the same type of fold as displayed by thioredoxin, the archetypal thiol-disulfide oxidoreductase. However, the relationship of secondary structure with the linear amino acid sequences is different for each protein, producing a distinctive topology. The beta-sheet core is extended in the trypanosomatid protein with an N-terminal beta-hairpin. There are also differences in the content and orientation of helical elements of secondary structure positioned at the surface of the proteins, which leads to different shapes and charge distributions between human thioredoxin and tryparedoxin-I. A right-handed redox-active disulfide is formed between Cys-40 and Cys-43 at the N-terminal region of a distorted alpha-helix (alpha1). Cys-40 is solvent-accessible, and Cys-43 is positioned in a hydrophilic cavity. Three C-H...O hydrogen bonds donated from two proline residues serve to stabilize the disulfide-carrying helix and support the correct alignment of active site residues. The accurate model for tryparedoxin-I allows for comparisons with the family of thiol-disulfide oxidoreductases and provides a template for the discovery or design of selective inhibitors of hydroperoxide metabolism in trypanosomes. Such inhibitors are sought as potential therapies against a range of human pathogens.

Crystallization of recombinant Crithidia fasciculata tryparedoxin.

Recombinant tryparedoxin, a thioredoxin homologue from Crithidia fasciculata, has been purified from an Escherichia coli expression system and used in crystallization trials. Orthorhombic needles in space group P212121, with unit cell dimensions of a = 38.63, b = 51. 47, and c = 73.41 A, have been obtained. The crystals present a monomer of approximate molecular mass 16 kDa in the asymmetric unit and diffract to 1.8-A resolution using synchrotron radiation. Structure determination will be carried out to further the understanding of the role tryparedoxin plays in regulating oxidative stress in parasitic trypanosomatids.

6-Phosphogluconate dehydrogenase from Lactococcus lactis: a role for arginine residues in binding substrate and coenzyme.

A gene encoding 6-phosphogluconate dehydrogenase (6-PGDH, EC 1.1.1. 44) was identified from the homofermentative lactic acid bacterium Lactococcus lactis, by complementation of Escherichia coli mutants. The cloned gene was then expressed to high levels in E. coli and the protein purified for kinetic analysis. The enzyme had a Km for 6-phosphogluconate of 15.4+/-1.4 microM and for NADP of 1.9+/-0.2 microM at pH 7.5. Sequence comparison of the L. lactis 6-PGDH with the corresponding enzyme derived from the pathogenic protozoan Trypanosoma brucei and sheep liver revealed the substrate-binding residues to be identical in all three species, although the three coenzyme-binding pockets differed slightly. A totally conserved arginine residue (Arg-447), believed to bind the 6-phosphate of substrate, was mutated to lysine, aspartate, alanine or tryptophan. In each case enzyme activity was lost, confirming an essential role for this residue on activity. A second arginine (Arg-34), believed to be critical in binding the 2'-phosphate of cofactor NADP+, was mutated to a tyrosine residue, as found in one atypical isoform of the enzyme in Bacillus subtilis. This alteration led to decrease in affinity for NADP+ of nearly three orders of magnitude. A second 6-PGDH gene has been identified from the genome of B. subtilis. This second isoform contains an arginine (Arg-34) in this position, suggesting that B. subtilis has two 6-PGDHs with different coenzyme specificities.

Cloning, expression and reconstitution of the trypanothione-dependent peroxidase system of Crithidia fasciculata.

As a consequence of aerobic metabolism, trypanosomatids are exposed to reactive oxygen intermediates such as superoxide, hydrogen peroxide and the hydroxyl radical. Metabolism of hydrogen peroxide in Crithidia fasciculata is accomplished by three distinct proteins, tryparedoxin, tryparedoxin peroxidase and trypanothione reductase, working in concert with the substrates NADPH and trypanothione. Here, we report the cloning and characterisation of the tryparedoxin (TryX) and tryparedoxin peroxidase (TryP) genes from C. fasciculata. Both genes are multicopy and organized in distinct tandem arrays in the genome. TryX encodes a 16 kDa protein, which belongs to the thioredoxin superfamily, sharing the WCPPC motif, whereas TryP encodes a 21 kDa protein belonging to a new class of peroxidases called 2-Cys peroxidoxins. Both TryX and TryP were expressed in Escherichia coli and the purified recombinant proteins shown to utilise hydrogen peroxide in the presence of NADPH, trypanothione and trypanothione reductase, similar to the native proteins. TryX is rapidly reduced by trypanothione, but weakly by glutathionylspermidine, glutathione or ovothiol A. TryP shows a broad substrate specificity and can reduced hydrogen peroxide, t-butyl hydroperoxide and cumene hydroperoxide with equal efficiency.

Identification and characterisation of a functional peroxidoxin from Leishmania major.

Leishmania spp. encounter damaging oxygen metabolites from endogenous metabolic processes as well as from exogenous sources, such as inside the gut of the sandfly vector and within host macrophages. The recently described peroxidoxin protein family form part of a novel pathway for metabolising hydrogen peroxide that, in trypanosomatids, links peroxide reduction to NADPH oxidation via trypanothione. Here we report the cloning and characterisation of the Leishmania major peroxidoxin gene, tryparedoxin peroxidase (TryP). TryP is a multi-copy gene arranged in a complex tandem array located on the size polymorphic homologues of chromosome 15. Northern analysis showed that TryP expresses a single 1.6 kb mRNA throughout promastigote development. TryP encodes a 22-kDa protein with two conserved cysteine-containing domains that defines it as a 2-Cys peroxidoxin. Purified recombinant TryP protein catabolised hydrogen peroxide in the presence of the tryparedoxin homologue from Crithidia fasciculata (Cf-TryX), trypanothione, trypanothione reductase and NADPH. The demonstration that L. major utilises a three-protein peroxidase system confirms that this is a mechanism of protection against oxidative damage in this parasite.

Conserved organization of genes in trypanosomatids.

Trypanosomatids are unicellular protozoan parasites which constitute some of the most primitive eukaryotes. Leishmania spp, Trypanosoma cruzi and members of the Trypanosoma brucei group, which cause human diseases, are the most studied representatives of this large family. Here we report a comparative analysis of a large genomic region containing glucose transporter genes in three Salivarian trypanosomes (T. brucei, T. congolense and T. vivax), T. cruzi and Leishmania donovani. In T. brucei, the 8 kb (upstream) and 14 kb (downstream) regions flanking the glucose transporter genes cluster contain two and six new genes, respectively, six of them encoding proteins homologous to known eukaryotic proteins (phosphatidylinositol 3 kinase, ribosomal protein S12, DNAJ and three small G-proteins--Rab1, YPT6 and ARL3). This gene organization is identical in T. brucei, T. congolense and T. vivax suggesting that Salivarian trypanosomes have a high level of conservation in gene organization. In T. cruzi and Leishmania, the overall organization of this cluster is conserved, with insertion of additional genes when compared with T. brucei. Phylogenetic reconstitution based on glucose transporters is in accord with the monophyly of the genus Trypanosoma and the early separation of T. vivax within Salivarian trypanosomes. On the basis of gene organization, biochemical characteristics of isoforms and phylogeny, we discuss the genesis of the glucose transporter multigene family in Salivarian trypanosomes.

Cloning and characterization of the two enzymes responsible for trypanothione biosynthesis in Crithidia fasciculata.

Protozoa of the order Kinetoplastida differ from other organisms in their ability to conjugate glutathione (gamma-Glu-Cys-Gly) and spermidine to form trypanothione (N1,N8-bis(glutathionyl)spermidine), which is involved in maintaining intracellular thiol redox and in defense against oxidants. In this study, the genes from Crithidia fasciculata, Cf-GSS and Cf-TRS, which encode, respectively, glutathionylspermidine synthetase (EC 6.3.1.8) and trypanothione synthetase (EC 6.3.1.9) have been cloned and expressed. The deduced amino acid sequence of both Cf-GSS and Cf-TRS share 50% sequence similarity with the Escherichia coli glutathionylspermidine synthetase/amidase. Both genes are present as single copies in the C. fasciculata genome. When expressed in E. coli and Saccharomyces cerevisiae, neither protein was present in an active soluble form. However, thiol analysis of S. cerevisiae demonstrated that cells transformed with the Cf-GSS gene contained substantial amounts of glutathionylspermidine, whereas cells expressing both the Cf-GSS and Cf-TRS genes contained glutathionylspermidine and trypanothione, confirming that these genes encode the functional glutathionylspermidine and trypanothione synthetases from C. fasciculata. The translation products of Cf-GSS and Cf-TRS show significant homology to the amidase domain present in E. coli glutathionylspermidine synthetase, which can catalyze both synthesis and degradation of glutathionylspermidine. Glutathionylspermidine synthetase isolated from C. fasciculata was found to possess a similar amidase activity.

Crystallization and preliminary X-ray diffraction studies of 6-phosphogluconate dehydrogenase from Lactococcus lactis.

6-Phosphogluconate dehydrogenase is one of the seven enzymes involved in the pentose phosphate pathway. Crystals of a mammalian and a protozoan enzyme have been obtained previously and structures determined. It is reported here that a bacterial 6-phosphogluconate dehydrogenase, from Lactococcus lactis, has been purified and used in crystallization trials. Large prisms suitable for a detailed structural analysis have been obtained and characterized as orthorhombic, space group F222, with a = 70.4, b = 105.7, c = 474.6 A. Diffraction has been observed to 2.2 A resolution using synchrotron radiation. Structural analysis, in combination with ongoing biochemical characterization, will assist the elucidation of the structure-activity relationships of this enzyme.

Kinetoplastid glucose transporters.

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

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

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

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

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

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

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

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

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