The sequence of a small subunit of cytochrome c oxidase from Crithidia fasciculata which is homologous to mammalian subunit IV

Oxidative Phosphorylation Is Required for Powering Motility and Development of the Sleeping Sickness Parasite Trypanosoma brucei in the Tsetse Fly Vector

The single-celled parasite Trypanosoma brucei is transmitted by hematophagous tsetse flies. Life cycle progression from mammalian bloodstream form to tsetse midgut form and, subsequently, infective salivary gland form depends on complex developmental steps and migration within different fly tissues. As the parasite colonizes the glucose-poor insect midgut, ATP production is thought to depend on activation of mitochondrial amino acid catabolism via oxidative phosphorylation (OXPHOS). This process involves respiratory chain complexes and F₁F_o-ATP synthase and requires protein subunits of these complexes that are encoded in the parasite's mitochondrial DNA (kDNA). Here, we show that progressive loss of kDNA-encoded functions correlates with a decreasing ability to initiate and complete development in the tsetse. First, parasites with a mutated F₁F_o-ATP synthase with reduced capacity for OXPHOS can initiate differentiation from bloodstream to insect form, but they are unable to proliferate in vitro. Unexpectedly, these cells can still colonize the tsetse midgut. However, these parasites exhibit a motility defect and are severely impaired in colonizing or migrating to subsequent tsetse tissues. Second, parasites with a fully disrupted F₁F_o-ATP synthase complex that is completely unable to produce ATP by OXPHOS can still differentiate to the first insect stage in vitro but die within a few days and cannot establish a midgut infection in vivo. Third, parasites lacking kDNA entirely can initiate differentiation but die soon after. Together, these scenarios suggest that efficient ATP production via OXPHOS is not essential for initial colonization of the tsetse vector but is required to power trypanosome migration within the fly. IMPORTANCE African trypanosomes cause disease in humans and their livestock and are transmitted by tsetse flies. The insect ingests these parasites with its blood meal, but to be transmitted to another mammal, the trypanosome must undergo complex development within the tsetse fly and migrate from the insect's gut to its salivary glands. Crucially, the parasite must switch from a sugar-based diet while in the mammal to a diet based primarily on amino acids when it develops in the insect. Here, we show that efficient energy production by an organelle called the mitochondrion is critical for the trypanosome's ability to swim and to migrate through the tsetse fly. Surprisingly, trypanosomes with impaired mitochondrial energy production are only mildly compromised in their ability to colonize the tsetse fly midgut. Our study adds a new perspective to the emerging view that infection of tsetse flies by trypanosomes is more complex than previously thought.

Mitochondrial respiration and genomic analysis provide insight into the influence of the symbiotic bacterium on host trypanosomatid oxygen consumption

Certain trypanosomatids co-evolve with an endosymbiotic bacterium in a mutualistic relationship that is characterized by intense metabolic exchanges. Symbionts were able to respire for up to 4 h after isolation from Angomonas deanei. FCCP (carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone) similarly increased respiration in wild-type and aposymbiotic protozoa, though a higher maximal O2 consumption capacity was observed in the symbiont-containing cells. Rotenone, a complex I inhibitor, did not affect A. deanei respiration, whereas TTFA (thenoyltrifluoroacetone), a complex II activity inhibitor, completely blocked respiration in both strains. Antimycin A and cyanide, inhibitors of complexes III and IV, respectively, abolished O2 consumption, but the aposymbiotic protozoa were more sensitive to both compounds. Oligomycin did not affect cell respiration, whereas carboxyatractyloside (CAT), an inhibitor of the ADP-ATP translocator, slightly reduced O2 consumption. In the A. deanei genome, sequences encoding most proteins of the respiratory chain are present. The symbiont genome lost part of the electron transport system (ETS), but complex I, a cytochrome d oxidase, and FoF1-ATP synthase remain. In conclusion, this work suggests that the symbiont influences the mitochondrial respiration of the host protozoan.

Disparate phenotypic effects from the knockdown of various Trypanosoma brucei cytochrome c oxidase subunits

The Trypanosoma brucei cytochrome c oxidase (respiratory complex IV) is a very divergent complex containing a surprisingly high number of trypanosomatid-specific subunits with unknown function. To gain insight into the functional organization of this large protein complex, the expression of three novel subunits (TbCOX VII, TbCOX X and TbCOX 6080) were down-regulated by RNA interference. We demonstrate that all three subunits are important for the proper function of complex IV and the growth of the procyclic stage of T. brucei. These phenotypes were manifested by the structural instability of the complex when these indispensible subunits were repressed. Furthermore, the impairment of cytochrome c oxidase resulted in other severe mitochondrial phenotypes, such as a decreased mitochondrial membrane potential, reduced ATP production via oxidative phoshorylation and redirection of oxygen consumption to the trypanosome-specific alternative oxidase, TAO. Interestingly, the inspected subunits revealed some disparate phenotypes, particularly regarding the activity of cytochrome c reductase (respiratory complex III). While the activity of complex III was down-regulated in RNAi induced cells for TbCOX X and TbCOX 6080, the TbCOX VII silenced cell line actually exhibited higher levels of complex III activity and elevated levels of ROS formation. This result suggests that the examined subunits may have different functional roles within complex IV of T. brucei, perhaps involving the ability to communicate between sequential enzymes in the respiratory chain. In summary, by characterizing the function of three hypothetical components of complex IV, we are able to assign these proteins as genuine and indispensable subunits of the procyclic T. brucei cytochrome c oxidase, an essential component of the respiratory chain in these evolutionary ancestral and medically important parasites.

Identification and characterization of genes involved in leishmania pathogenesis: the potential for drug target selection

Identifying and characterizing Leishmania donovani genes and the proteins they encode for their role in pathogenesis can reveal the value of this approach for finding new drug targets. Effective drug targets are likely to be proteins differentially expressed or required in the amastigote life cycle stage found in the patient. Several examples and their potential for chemotherapeutic disruption are presented. A pathway nearly ubiquitous in living cells targeted by anticancer drugs, the ubiquitin system, is examined. New findings in ubiquitin and ubiquitin-like modifiers in Leishmania show how disruption of those pathways could point to additional drug targets. The programmed cell death pathway, now recognized among protozoan parasites, is reviewed for some of its components and evidence that suggests they could be targeted for antiparasitic drug therapy. Finally, the endoplasmic reticulum quality control system is involved in secretion of many virulence factors. How disruptions in this pathway reduce virulence as evidence for potential drug targets is presented.

Structural and functional association of Trypanosoma brucei MIX protein with cytochrome c oxidase complex

A mitochondrial inner membrane protein, designated MIX, seems to be essential for cell viability. The deletion of both alleles was not possible, and the deletion of a single allele led to a loss of virulence and aberrant mitochondrial segregation and cell division in Leishmania major. However, the mechanism by which MIX exerts its effect has not been determined. We show here that MIX is also expressed in the mitochondrion of Trypanosoma brucei, and using RNA interference, we found that its loss leads to a phenotype that is similar to that described for Leishmania. The loss of MIX also had a major effect on cytochrome c oxidase activity, on the mitochondrial membrane potential, and on the production of mitochondrial ATP by oxidative phosphorylation. Using a tandem affinity purification tag, we found that MIX is associated with a multiprotein complex that contains subunits of the mitochondrial cytochrome c oxidase complex (respiratory complex IV), the composition of which was characterized in detail. The specific function of MIX is unknown, but it appears to be important for the function of complex IV and for mitochondrial segregation and cell division in T. brucei.

Downregulation of the nuclear-encoded subunits of the complexes III and IV disrupts their respective complexes but not complex I in procyclic Trypanosoma brucei

The function, stability and mutual interactions of selected nuclear-encoded subunits of respiratory complexes III and IV were studied in the Trypanosoma brucei procyclics using RNA interference (RNAi). The growth rates and oxygen consumption of clonal cell lines of knock-downs for apocytochrome c1 (apoc1) and the Rieske Fe-S protein (Rieske) of complex III, and cytochrome c oxidase subunit 6 (cox6) of complex IV were markedly decreased after RNAi induction. Western analysis of mitochondrial lysates using specific antibodies confirmed complete elimination of the targeted proteins 4-6 days after induction. The Rieske protein was reduced in the apoc1 knock-down and vice versa, indicating a mutual interdependence of these components of complex III. However, another subunit of complex IV remained at the wild-type level in the cox6 knock-down. As revealed by two-dimensional blue native/SDS-PAGE electrophoresis, silencing of a single subunit resulted in the disruption of the respective complex, while the other complex remained unaffected. Membrane potential was reproducibly decreased in the knock-downs and the activities of complex III and/or IV, but not complex I, were drastically reduced, as measured by activity assays and histochemical staining. Using specific inhibitors, we have shown that in procyclics with depleted subunits of the respiratory complexes the flow of electrons was partially re-directed to the alternative oxidase. The apparent absence in T. brucei procyclics of a supercomplex composed of complexes I and III may represent an ancestral state of the respiratory chain.

A putative novel nuclear-encoded subunit of the cytochrome c oxidase complex in trypanosomatids

A relatively large nuclear-encoded polypeptide, designated trCOIV, is found in the cytochrome c oxidase (CO) complex of trypanosomatids. In order to determine if this polypeptide represents a bona fide subunit of the complex, we have characterized the cDNA and the gene for this polypeptide in Leishmania tarentolae. Its nuclear gene has no sequence similarity to mammalian COIV. The trCOIV preprotein has a long mitochondrial targeting sequence of 31 residues. The mature polypeptide cofractionates with kinetoplast-mitochondria and its preferential mitochondrial localization was confirmed by immunofluorescence and immunoelectron microscopy. Based on the hydropathy plot analysis, the protein lacks pronounced transmembrane domains and likely occupies a peripheral position within the CO complex. The corresponding genes are also present in the sequenced portions of the Trypanosoma cruzi, Trypanosoma brucei and Leishmania major genomes, and the same polypeptide is found in cytochrome oxidase isolated from procyclic T. brucei and promastigote Leishmania mexicana amazonensis. However, the trCOIV gene, the mRNA and the polypeptide could not be detected in a respiration-deficient trypanosomatid Phytomonas serpens.

Further evidence for the presence of mitochondrially encoded subunits in cytochrome c oxidase of the trypanosomatid Crithidia fasciculata

Mitochondrial mRNAs in trypanosomatids are edited by uridylate insertion and deletion. The respiratory chain complexes cytochrome c reductase, cytochrome c oxidase and F0F1-ATPase of the insect trypanosomatid Crithidia fasciculata have been isolated and analysed by peptide microsequencing, but so far, proteins encoded by edited (and unedited) mitochondrial mRNAs have not been found. In this paper, we provide evidence that the mitochondrial mRNAs encoding the three large subunits of cytochrome c oxidase are indeed translated. First, purified holo cytochrome c oxidase turned out to be cysteine-rich, in agreement with the high cysteine codon-content of the sequence of mitochondrial cox subunit mRNAs. Second, in mass spectrometry measurements of cytochrome c oxidase, a protein was detected with the predicted molecular weight of cytochrome c oxidase subunit 2. Finally, an antibody generated against a fusion protein produced in Escherichia coli from constructs containing a segment of cytochrome c oxidase subunit 2 cDNA, specifically recognised protein bands present in cytochrome c oxidase following SDS PAGE. However, these proteins were present in the high molecular weight region of the gel, suggesting that cytochrome c oxidase subunit 2 aggregates in the presence of SDS.

Characterization of the respiratory chain from cultured Crithidia fasciculata

Mitochondrial mRNAs encoding subunits of respiratory-chain complexes in kinetoplastids are post-transcriptionally edited by uridine insertion and deletion. In order to identify the proteins encoded by these mRNAs, we have analyzed respiratory-chain complexes from cultured cells of Crithidia fasciculata with the aid of 2D polyacrylamide gel electrophoresis (PAGE). The subunit composition of F0F1-ATPase (complex V), identified on the basis of its activity as an oligomycin-sensitive ATPase, is similar to that of bovine mitochondrial F0F1-ATPase. Amino acid sequence analysis, combined with binding studies using dicyclohexyldiimide and azido ATP allowed the identification of two F0 subunits (b and c) and all of the F1 subunits. The F0 b subunit has a low degree of similarity to subunit b from other organisms. The F1 alpha subunit is extremely small making the beta subunit the largest F1 subunit. Other respiratory-chain complexes were also analyzed. Interestingly, an NADH: ubiquinone oxidoreductase (complex I) appeared to be absent, as judged by electron paramagnetic resonance (EPR), enzyme activity and 2D PAGE analysis. Cytochrome c oxidase (complex IV) displayed a subunit pattern identical to that reported for the purified enzyme, whereas cytochrome c reductase (complex III) appeared to contain two extra subunits. A putative complex II was also identified. The amino acid sequences of the subunits of these complexes also show a very low degree of similarity (if any) to the corresponding sequences in other organisms. Remarkably, peptide sequences derived from mitochondrially encoded subunits were not found in spite of the fact that sequences were obtained of virtually all subunits of complex III, IV and V.

Purification and characterization of cytochrome c oxidase from the insect trypanosomatid Crithidia fasciculata

Cytochrome c oxidase was purified from the mitochondrial lysate of the insect trypanosomatid Crithidia fasciculata with the aid of a methyl hydrophobic interaction column in a rapid one-step procedure. The purified complex displayed all characteristics expected from a eukaryotic cytochrome c oxidase: the presence of CuA in electron paramagnetic resonance analysis, a characteristic 605 nm peak in reduced-minus-oxidized optical spectroscopy, and the capacity to efficiently oxidize homologous, but not heterologous, cytochrome c. Two-dimensional PAGE showed that C. fasciculata cytochrome c oxidase consists of at least 10 different subunits. N-terminal sequences were obtained from the six smallest subunits of the complex, one of them showing significant similarity to Neurospora crassa cytochrome c oxidase subunit V. The N-terminus of each of the four largest subunits was found to be blocked.