Detection of the mitochondrially encoded cytochrome c oxidase subunit I in the trypanosomatid protozoan Leishmania tarentolae. Evidence for translation of unedited mRNA in the kinetoplast

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.

Differences in mitochondrial NADH dehydrogenase activities in trypanosomatids

Complex I (NADH dehydrogenase) is the first enzyme in the respiratory chain. It catalyses the electron transfer from NADH to ubiquinone that is associated with proton pumping out of the matrix. In this study, we characterized NADH dehydrogenase activity in seven monoxenous trypanosomatid species: Blechomonas ayalai, Herpetomonas tarakana, Kentomonas sorsogonicus, Leptomonas seymouri, Novymonas esmeraldas, Sergeia podlipaevi and Wallacemonas raviniae. We also investigated the subunit composition of the complex I in dixenous Phytomonas serpens, in which its presence and activity have been previously documented. In addition to P. serpens, the complex I is functionally active in N. esmeraldas and S. podlipaevi. We also identified 24-32 subunits of the complex I in individual species by using mass spectrometry. Among them, for the first time, we recognized several proteins of the mitochondrial DNA origin.

Evolutionary Insight into the Trypanosomatidae Using Alignment-Free Phylogenomics of the Kinetoplast

Advancements in next-generation sequencing techniques have led to a substantial increase in the genomic information available for analyses in evolutionary biology. As such, this data requires the exponential growth in bioinformatic methods and expertise required to understand such vast quantities of genomic data. Alignment-free phylogenomics offer an alternative approach for large-scale analyses that may have the potential to address these challenges. The evolutionary relationships between various species within the trypanosomatid family, specifically members belonging to the genera Leishmania and Trypanosoma have been extensively studies over the last 30 years. However, there is a need for a more exhaustive analysis of the Trypanosomatidae, summarising the evolutionary patterns amongst the entire family of these important protists. The mitochondrial DNA of the trypanosomatids, better known as the kinetoplast, represents a valuable taxonomic marker given its unique presence across all kinetoplastid protozoans. The aim of this study was to validate the reliability and robustness of alignment-free approaches for phylogenomic analyses and its applicability to reconstruct the evolutionary relationships between the trypanosomatid family. In the present study, alignment-free analyses demonstrated the strength of these methods, particularly when dealing with large datasets compared to the traditional phylogenetic approaches. We present a maxicircle genome phylogeny of 46 species spanning the trypanosomatid family, demonstrating the superiority of the maxicircle for the analysis and taxonomic resolution of the Trypanosomatidae.

Separating the Wheat from the Chaff: RNA Editing and Selection of Translatable mRNA in Trypanosome Mitochondria

In the mitochondria of trypanosomes and related kinetoplastid protists, most mRNAs undergo a long and sophisticated maturation pathway before they can be productively translated by mitochondrial ribosomes. Some of the aspects of this pathway (identity of the promotors, transcription initiation, and termination signals) remain obscure, and some (post-transcriptional modification by U-insertion/deletion, RNA editing, 3'-end maturation) have been illuminated by research during the last decades. The RNA editing creates an open reading frame for a productive translation, but the fully edited mRNA often represents a minor fraction in the pool of pre-edited and partially edited precursors. Therefore, it has been expected that the final stages of the mRNA processing generate molecular hallmarks, which allow for the efficient and selective recognition of translation-competent templates. The general contours and several important details of this process have become known only recently and represent the subject of this review.

High throughput sequencing revolution reveals conserved fundamentals of U-indel editing

Among Euglenozoans, mitochondrial RNA editing occurs in the diplonemids and in the kinetoplastids that include parasitic trypanosomes. Yet U-indel editing, in which open reading frames (ORFs) on mRNAs are generated by insertion and deletion of uridylates in locations dictated by guide RNAs, appears confined to kinetoplastids. The nature of guide RNA and edited mRNA populations has been cursorily explored in a surprisingly extensive number of species over the years, although complete sets of fully edited mRNAs for most kinetoplast genomes are largely missing. Now, however, high throughput sequencing technologies have had an enormous impact on what we know and will learn about the mechanisms, benefits, and final edited products of U-indel editing. Tools including PARERS, TREAT, and T-Aligner function to organize and make sense of U-indel mRNA transcriptomes, which are comprised of mRNAs harboring uridylate indels both consistent and inconsistent with translatable products. From high throughput sequencing data come arguments that partially edited mRNAs containing "junction regions" of noncanonical editing are editing intermediates, and conversely, arguments that they are dead-end products. These data have also revealed that the percent of a given transcript population that is fully or partially edited varies dramatically between transcripts and organisms. Outstanding questions that are being addressed include the prevalence of sequences that apparently encode alternative ORFs, diversity of editing events in ORF termini and 5' and 3' untranslated regions, and the differences that exist in this byzantine process between species. High throughput sequencing technologies will also undoubtedly be harnessed to probe U-indel editing's evolutionary origins. This article is categorized under: RNA Processing > RNA Editing and Modification RNA Evolution and Genomics > Computational Analyses of RNA.

From simple to supercomplex: mitochondrial genomes of euglenozoan protists

Mitochondria are double membrane organelles of endosymbiotic origin, best known for constituting the centre of energetics of a eukaryotic cell. They contain their own mitochondrial genome, which as a consequence of gradual reduction during evolution typically contains less than two dozens of genes. In this review, we highlight the extremely diverse architecture of mitochondrial genomes and mechanisms of gene expression between the three sister groups constituting the phylum Euglenozoa - Euglenida, Diplonemea and Kinetoplastea. The earliest diverging euglenids possess a simplified mitochondrial genome and a conventional gene expression, whereas both are highly complex in the two other groups. The expression of their mitochondrial-encoded proteins requires extensive post-transcriptional modifications guided by complex protein machineries and multiple small RNA molecules. Moreover, the least studied diplonemids, which have been recently discovered as a highly abundant component of the world ocean plankton, possess one of the most complicated mitochondrial genome organisations known to date.

From simple to supercomplex: mitochondrial genomes of euglenozoan protists

Mitochondria are double membrane organelles of endosymbiotic origin, best known for constituting the centre of energetics of a eukaryotic cell. They contain their own mitochondrial genome, which as a consequence of gradual reduction during evolution typically contains less than two dozens of genes. In this review, we highlight the extremely diverse architecture of mitochondrial genomes and mechanisms of gene expression between the three sister groups constituting the phylum Euglenozoa - Euglenida, Diplonemea and Kinetoplastea. The earliest diverging euglenids possess a simplified mitochondrial genome and a conventional gene expression, whereas both are highly complex in the two other groups. The expression of their mitochondrial-encoded proteins requires extensive post-transcriptional modifications guided by complex protein machineries and multiple small RNA molecules. Moreover, the least studied diplonemids, which have been recently discovered as a highly abundant component of the world ocean plankton, possess one of the most complicated mitochondrial genome organisations known to date.

Identification of the mitochondrially encoded subunit 6 of F1FO ATPase in Trypanosoma brucei

Kinetoplast maxicircle DNA of trypanosomatids encodes eighteen proteins. RNA editing is required to confer translatability to mRNA for twelve of these. Sequence conservation of the predicted hydrophobic polypeptides indicates that they represent functional components of the respiratory chain. Yet, so far only two of those, cytochrome c oxidase subunit I and apocytochrome b of cytochrome c reductase, have been identified with biochemical methods. Here we report on identification of A6 subunit of F1FO ATPase encoded by a pan-edited mRNA in Trypanosoma brucei. The polypeptide was present among the (35)S-labeled mitochondrial translation products characterized by anomalous migration in denaturing 2D gels. It was identified as an ATPase subunit by co-migration with this complex in Blue Native 2D gels. A partial N-terminal sequence of the corresponding polypeptide present in the gel-purified ATPase complex from Leishmania tarentolae was consistent with the predicted A6 sequence.

Kinetoplast DNA-encoded ribosomal protein S12: a possible functional link between mitochondrial RNA editing and translation in Trypanosoma brucei

Mitochondrial ribosomes of Trypanosoma brucei are composed of 9S and 12S rRNAs, which are encoded by the kinetoplast genome, and more than 150 proteins encoded in the nucleus and imported from the cytoplasm. However, a single ribosomal protein RPS12 is encoded by the kinetoplast DNA (kDNA) in all trypanosomatid species examined. As typical for these organisms, the gene itself is cryptic and its transcript undergoes an extensive U-insertion/deletion editing. An evolutionary trend to reduce or eliminate RNA editing could be traced with other cryptogenes, but the invariably pan-edited RPS12 cryptogene is apparently spared. Here we inquired whether editing of RPS12 mRNA is essential for mitochondrial translation. By RNAi-mediated knockdowns of RNA editing complexes and inducible knock-in of a key editing enzyme in procyclic parasites, we could reversibly downregulate production of edited RPS12 mRNA and, by inference, synthesis of this protein. While inhibition of editing decreased edited mRNA levels, the translation of edited (Cyb) and unedited (COI) mRNAs was blocked. Furthermore, the population of SSU-related 45S complexes declined upon inactivation of editing and so did the amount of mRNA-bound ribosomes. In bloodstream parasites, which lack active electron transport chain but still require translation of ATP synthase subunit 6 mRNA (A6), both edited RPS12 and A6 mRNAs were detected in translation complexes. Collectively, our results indicate that a single ribosomal protein gene retained by the kinetoplast mitochondrion serves as a possible functional link between editing and translation processes and provide the rationale for the evolutionary conservation of RPS12 pan-editing.

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.

Trypanosoma brucei mitochondrial respiratome: composition and organization in procyclic form

The mitochondrial respiratory chain is comprised of four different protein complexes (I-IV), which are responsible for electron transport and generation of proton gradient in the mitochondrial intermembrane space. This proton gradient is then used by F₀F₁-ATP synthase (complex V) to produce ATP by oxidative phosphorylation. In this study, the respiratory complexes I, II, and III were affinity purified from Trypanosoma brucei procyclic form cells and their composition was determined by mass spectrometry. The results along with those that we previously reported for complexes IV and V showed that the respiratome of Trypanosoma is divergent because many of its proteins are unique to this group of organisms. The studies also identified two mitochondrial subunit proteins of respiratory complex IV that are encoded by edited RNAs. Proteomics data from analyses of complexes purified using numerous tagged component proteins in each of the five complexes were used to generate the first predicted protein-protein interaction network of the Trypanosoma brucei respiratory chain. These results provide the first comprehensive insight into the unique composition of the respiratory complexes in Trypanosoma brucei, an early diverged eukaryotic pathogen.

Mitochondrial translation in trypanosomatids: a novel target for chemotherapy?

Trypanosomatids cause widespread disease in humans and animals. Treatment of many of these diseases is hampered by the lack of efficient and safe drugs. New strategies for drug development are therefore urgently needed. It has long been known that the single mitochondrion of trypanosomatids exhibits many unique features. Recently, the mitochondrial translation machinery of trypanosomatids has been the focus of several studies, which revealed interesting variations to the mammalian system. It is the aim of this article to review these unique features and to discuss them in the larger biological context. It is our opinion that some of these features represent promising novel targets for chemotherapeutic intervention that should be studied in more detail.

Mitochondrial translation is essential in bloodstream forms of Trypanosoma brucei

The parasitic protozoa Trypanosoma brucei has a complex life cycle. Oxidative phosphorylation is highly active in the procyclic form but absent from bloodstream cells. The mitochondrial genome encodes several gene products that are required for oxidative phosphorylation, but it completely lacks tRNA genes. For mitochondrial translation to occur, the import of cytosolic tRNAs is therefore essential for procyclic T. brucei. Whether the same is true for the bloodstream form has not been studied so far. Here we show that the steady-state levels of mitochondrial tRNAs are essentially the same in both life stages. Editing of the imported tRNA(Trp) also occurs in both forms as well as in mitochondria of Trypanosoma evansi, which lacks a genome and a translation system. These results show that mitochondrial tRNA import is a constitutive process that must be mediated by proteins that are expressed in both forms of the life cycle and that are not encoded in the mitochondrial genome. Moreover, bloodstream cells lacking either mitochondria-specific translation elongation factor Tu or mitochondrial tryptophanyl-tRNA synthetase are not viable indicating that mitochondrial translation is also essential in this stage. Both of these proteins show trypanosomatid-specific features and may therefore be excellent novel drug targets.

DEAD-box RNA helicase is dispensable for mitochondrial translation in Trypanosoma brucei

DEAD-box RNA helicase, a putative subunit of the mitochondrial ribosome of Trypanosoma brucei, has been down-regulated in the procyclic and bloodstream stage by RNA interference. Although ablation of the transcript leads to a week growth phenotype in the procyclic cells, the protein does not seem to be essential for mitochondrial translation under standard cultivation conditions, as shown by an assay that allows visualization of the de novo synthesized proteins. Furthermore, we show that synthesis of cytochrome c oxidase subunit I and cytochrome b does not occur in the mitochondrion of the bloodstream stage.

The genome of Naegleria gruberi illuminates early eukaryotic versatility

Genome sequences of diverse free-living protists are essential for understanding eukaryotic evolution and molecular and cell biology. The free-living amoeboflagellate Naegleria gruberi belongs to a varied and ubiquitous protist clade (Heterolobosea) that diverged from other eukaryotic lineages over a billion years ago. Analysis of the 15,727 protein-coding genes encoded by Naegleria's 41 Mb nuclear genome indicates a capacity for both aerobic respiration and anaerobic metabolism with concomitant hydrogen production, with fundamental implications for the evolution of organelle metabolism. The Naegleria genome facilitates substantially broader phylogenomic comparisons of free-living eukaryotes than previously possible, allowing us to identify thousands of genes likely present in the pan-eukaryotic ancestor, with 40% likely eukaryotic inventions. Moreover, we construct a comprehensive catalog of amoeboid-motility genes. The Naegleria genome, analyzed in the context of other protists, reveals a remarkably complex ancestral eukaryote with a rich repertoire of cytoskeletal, sexual, signaling, and metabolic modules.

Protein composition of Trypanosoma brucei mitochondrial membranes

Mitochondria consist of four compartments, outer membrane, intermembrane space, inner membrane, and matrix; each harboring specific functions and structures. In this study, we used LC-MS/MS to characterize the protein composition of Trypanosoma brucei mitochondrial (mt) membranes, which were enriched by different biochemical fractionation techniques. The analyses identified 202 proteins that contain one or more transmembrane domain(s) and/or positive GRAVY scores. Of these, various criteria were used to assign 72 proteins to mt membranes with high confidence, and 106 with moderate-to-low confidence. The sub-cellular localization of a selected subset of 13 membrane assigned proteins was confirmed by tagging and immunofluorescence analysis. While most proteins assigned to mt membrane have putative roles in metabolic, energy generating, and transport processes, approximately 50% have no known function. These studies result in a comprehensive profile of the composition and sub-organellar location of proteins in the T. brucei mitochondrion thus, providing useful information on mt functions.

Kinetoplastid RNA editing ligases 1 and 2 exhibit different electrostatic properties

Kinetoplastid RNA editing ligases 1 and 2 (KREL1 and KREL2) share a significant degree of sequence homology. However, biochemical experiments have reported that KREL1 and KREL2 differ in their functional roles during the RNA editing process. In this study, we hypothesize that dissimilar roles for KREL1 and KREL2 proteins arise from their different physicochemical characteristics. To test our hypothesis at sequence level, we plotted theoretical titration curves for KREL1, KREL2 and their binding partner proteins. The plots showed a lower isoelectric point for KREL1 compared to that for KREL2 as well as more relative alkalinity and acidity for binding partner proteins of KREL1 and KREL2 at net charge zero, respectively. At structure level, based on the available high resolution structure of KREL1 N-terminal domain and strong sequence similarity between KRELs and other ligases, we built the homology model of KREL2 N-terminal domain. Using Poisson-Boltzmann continuum approach, we calculated the electrostatic potential isosurfaces of KREL1 structure and KREL2 model. KREL1 and KREL2 coordinates differed in their electrostatic isopotential patterns. A wider negative patch on the surface of KREL1 suggests differential affinity for another protein compared to KREL2. In contrast, a larger positive patch on the KREL2 surface predicts its differential affinity and/or specificity for its RNA substrate. Subsequently, we employed in silico mutational scanning and identified the surface-exposed residues contributing to the long-range electrostatic energy of KRELs. We predict that two structurally conserved loops of KRELs, not previously reported in the literature, also recognize their RNA substrates. Our results provide important information about the physicochemical properties of RNA editing ligases that could contribute to the ligation step of RNA editing.

RNA editing and mitochondrial activity in promastigotes and amastigotes of Leishmania donovani

Kinetoplast maxicircle DNA sequence organisation was investigated in Leishmania donovani, strain 1S LdBob. Gene arrangement in the coding (conserved) region of the maxicircle is collinear with that of most trypanosomatids, with individual genes showing 80-90% nucleotide identity to Leishmania tarentolae, strain UC. The notable exception was an integration of a full-size minicircle sequence in the ND1 gene coding region found in L. donovani. Editing patterns of the mitochondrial mRNAs investigated also followed L. tarentolae UC patterns, including productive editing of the components of respiratory complexes III-V, and ribosomal protein S12 (RPS12), as well as the lack of productive editing in five out of six pan-edited cryptogenes (ND3, ND8, ND9, G3, G4) found in these species. Several guide RNAs for the editing events were localised in minicircles and maxicircles in the locations that are conserved between the species. Mitochondrial activity, including rates of oxygen consumption, the presence and the levels of respiratory complexes and their individual subunits and the steady-state levels of several mitochondrial-encoded mRNAs were essentially the same in axenically grown amastigotes and in promastigotes of L. donovani. However, some modulation of mitochondrial activity between these developmental stages was suggested by the finding of an amastigote-specific component in complex IV, a down-regulation of mitochondrial RNA-binding proteins (MRP) and MRP-associated protein (MRP-AP) in amastigotes, and by variations in the levels of RPS12, ND3, ND9, G3 and G4 pre-edited transcripts.

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.

A mosaic of RNA binding and protein interaction motifs in a bifunctional mitochondrial tRNA import factor from Leishmania tropica

Proteins that participate in the import of cytosolic tRNAs into mitochondria have been identified in several eukaryotic species, but the details of their interactions with tRNA and other proteins are unknown. In the kinetoplastid protozoon Leishmania tropica, multiple proteins are organized into a functional import complex. RIC8A, a tRNA-binding subunit of this complex, has a C-terminal domain that functions as subunit 6b of ubiquinol cytochrome c reductase (complex III). We show that the N-terminal domain, unique to kinetoplastid protozoa, is structurally similar to the appended S15/NS1 RNA-binding domain of aminoacyl tRNA synthetases, with a helix–turn–helix motif. Structure-guided mutagenesis coupled with in vitro assays showed that helix α1 contacts tRNA whereas helix α2 targets the protein for assembly into the import complex. Inducible expression of a helix 1-deleted variant in L. tropica resulted in formation of an inactive import complex, while the helix 2-deleted variant was unable to assemble in vivo. Moreover, a protein-interaction assay showed that the C-terminal domain makes allosteric contacts with import receptor RIC1 complexed with tRNA. These results help explain the origin of the bifunctionality of RIC8A, and the allosteric changes accompanying docking and release of tRNA during import.

Trypanosoma brucei mitochondrial ribosomes: affinity purification and component identification by mass spectrometry

Although eukaryotic mitochondrial (mt) ribosomes evolved from a putative prokaryotic ancestor their compositions vary considerably among organisms. We determined the protein composition of tandem affinity-purified Trypanosoma brucei mt ribosomes by mass spectrometry and identified 133 proteins of which 77 were associated with the large subunit and 56 were associated with the small subunit. Comparisons with bacterial and mammalian mt ribosomal proteins identified T. brucei mt homologs of L2-4, L7/12, L9, L11, L13-17, L20-24, L27-30, L33, L38, L43, L46, L47, L49, L52, S5, S6, S8, S9, S11, S15-18, S29, and S34, although the degree of conservation varied widely. Sequence characteristics of some of the component proteins indicated apparent functions in rRNA modification and processing, protein assembly, and mitochondrial metabolism implying possible additional roles for these proteins. Nevertheless most of the identified proteins have no homology outside Kinetoplastida implying very low conservation and/or a divergent function in kinetoplastid mitochondria.

Type 3 peptide deformylases are required for oxidative phosphorylation in Trypanosoma brucei

Peptide deformylase (PDF) catalyses the removal of the formyl group from the first methionine of nascent proteins. Type 1 PDFs are found in bacteria and have orthologues in most eukaryotes. Type 2 PDFs are restricted to bacteria. Type 3 enzymes are found in Archaea and trypanosomatids and have not been studied experimentally yet. Thus, TbPDF1 and TbPDF2, the two PDF orthologues of the parasitic protozoa Trypanosoma brucei, are of type 3. An experimental analysis of these enzymes shows that both are mitochondrially localized, but that only TbPDF1 is essential for normal growth. Recombinant TbPDF1 exhibits PDF activity with a substrate specificity identical to that of bacterial enzymes. Consistent with these results, TbPDF1 is required for oxidative but not for mitochondrial substrate-level phosphorylation. Ablation of TbPDF2, in contrast, does neither affect growth on standard medium nor oxidative phosphorylation. However, a reduced level of TbPDF2 slows down growth in a medium that selects for highly efficient oxidative phosphorylation. Furthermore, combined ablation of TbPDF1 and TbPDF2 results in an earlier growth arrest than is observed by downregulation of TbPDF1 alone. These results suggest that TbPDF2 is functionally linked to TbPDF1, and that it can influence the efficiency of oxidative phosphorylation.

Two-dimensional electrophoresis of membrane proteins

One third of all genes of various organisms encode membrane proteins, emphasizing their crucial cellular role. However, due to their high hydrophobicity, membrane proteins demonstrate low solubility and a high tendency for aggregation. Indeed, conventional two-dimensional gel electrophoresis (2-DE), a powerful electrophoretic method for the separation of complex protein samples that applies isoelectric focusing (IEF) in the first dimension and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) in the second dimension, has a strong bias against membrane proteins. This review describes two-dimensional electrophoretic techniques that can be used to separate membrane proteins. Alternative methods for performing conventional 2-DE are highlighted; these involve replacing the IEF with electrophoresis using cationic detergents, namely 16-benzyldimethyl-n-hexadecylammonium chloride (16-BAC) and cetyl trimethyl ammonium bromide (CTAB), or the anionic detergent SDS. Finally, the separation of native membrane protein complexes through the application of blue and clear native gel electrophoresis (BN/CN-PAGE) is reviewed, as well as the free-flow electrophoresis (FFE) of membranes.

Necessary and sufficient factors for the import of transfer RNA into the kinetoplast mitochondrion

The mechanism of active transport of transfer RNA (tRNA) across membranes is largely unknown. Factors mediating the import of tRNA into the kinetoplast mitochondrion of the protozoon Leishmania tropica are organized into a multiprotein RNA import complex (RIC) at the inner membrane. Here, we present the complete characterization of the identities and functions of the subunits of this complex. The complex contains three mitochondrion- and eight nuclear-encoded subunits; six of the latter are necessary and sufficient for import. Antisense-mediated knockdown of essential subunits resulted in the depletion of mitochondrial tRNAs and inhibition of organellar translation. Functional complexes were reconstituted with recombinant subunits expressed in Escherichia coli. Several essential RIC subunits are identical to specific subunits of respiratory complexes. These findings provide new information on the evolution of tRNA import and the foundation for detailed structural and mechanistic studies.

The mitochondrial ATP synthase of chlorophycean algae contains eight subunits of unknown origin involved in the formation of an atypical stator-stalk and in the dimerization of the complex

Mitochondrial F(1)F( O )-ATP synthase of Chlamydomonas reinhardtii and Polytomella sp. is a dimer of 1,600,000 Da. In Chlamydomonas the enzyme lacks the classical subunits that constitute the peripheral stator-stalk as well as those involved in the dimerization of the fungal and mammal complex. Instead, it contains eight novel polypeptides named ASA1 to 8. We show that homologs of these subunits are also present in the chlorophycean algae Polytomella sp. and Volvox carterii. Blue Native Gel Electrophoresis analysis of mitochondria from different green algal species also indicates that stable dimeric mitochondrial ATP synthases may be characteristic of all Chlorophyceae. One additional subunit, ASA9, was identified in the purified mitochondrial ATP synthase of Polytomella sp. The dissociation profile of the Polytomella enzyme at high-temperatures and cross-linking experiments finally suggest that some of the ASA polypeptides constitute a stator-stalk with a unique architecture, while others may be involved in the formation of a highly-stable dimeric complex. The algal enzyme seems to have modified the structural features of its surrounding scaffold, while conserving almost intact the structure of its catalytic subunits.

Alternative editing of cytochrome c oxidase III mRNA in trypanosome mitochondria generates protein diversity

Trypanosomes use RNA editing to produce most functional mitochondrial messenger RNA. Precise insertion and deletion of hundreds of uridines is necessary to make full-length cytochrome c oxidase III (COXIII) mRNA. We show that COXIII mRNA can be alternatively edited by a mechanism using an alternative guide RNA to make a stable mRNA. This alternatively edited mRNA is translated to produce a unique protein that fractionates with mitochondrial membranes and colocalizes with mitochondrial proteins in situ. Alternative RNA editing represents a previously unknown mechanism generating protein diversity and, as such, represents an important function for RNA editing.

An RNA-binding respiratory component mediates import of type II tRNAs into Leishmania mitochondria

Transport of tRNAs across the inner mitochondrial membrane of the kinetoplastid protozoon Leishmania requires interactions with specific binding proteins (receptors) in a multi-subunit complex. The allosteric model of import regulation proposes cooperative and antagonistic interactions between two or more receptors with binding specificities for distinct tRNA families (types I and II, respectively). To identify the type II receptor, the gene encoding RIC8A, a subunit of the complex, was cloned. The C-terminal region of RIC8A is homologous to subunit 6b of ubiquinol cytochrome c reductase (respiratory complex III), while the N-terminal region has intrinsic affinity for type II, but not for type I, tRNAs. RIC8A is shared by the import complex and complex III, indicating its bi-functionality, but is assembled differently in the two complexes. Knockdown of RIC8A in Leishmania lowered the mitochondrial content of type II tRNAs but raised that of type I tRNAs, with downstream effects on mitochondrial translation and respiration, and cell death. In RIC8A knockdown cells, a subcomplex was formed that interacted with type I tRNA, but the negative regulation by type II tRNA was lost. Mitochondrial extracts from these cells were defective for type II, but not type I, import; import and regulation were restored by purified RIC8A. These results provide evidence for the relevance of allosteric regulation in vivo and indicate that acquisition of new tRNA-binding domains by ancient respiratory components have played a key role in the evolution of mitochondrial tRNA import.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

A bifunctional tRNA import receptor from Leishmania mitochondria

In kinetoplastid protozoa, import of cytosolic tRNAs into mitochondria occurs through tRNAs interacting with membrane-bound proteins, the identities of which are unknown. The inner membrane RNA import complex of Leishmania tropica contains multiple proteins and is active for import in vitro. RIC1, the largest subunit of this complex, is structurally homologous to the conserved alpha subunit of F1 ATP synthase. The RIC1 gene complemented an atpA mutation in Escherichia coli. Antisense-mediated knockdown of RIC1/F1alpha in Leishmania resulted in depletion of several mitochondrial tRNAs belonging to distinct subsets (types I and II) that interact cooperatively or antagonistically within the import complex. The knockdown-induced defect in import of type I tRNAs was rectified in a reconstituted system by purified RIC1/F1alpha alone, but recovery of type II tRNA import additionally required a type I tRNA. RIC1/F1alpha formed stable complexes with type I, but not type II, tRNAs through the cooperation of its nucleotide binding and C-terminal domains. Thus, RIC1/F1alpha is a type I tRNA import receptor. As expected of a bifunctional protein, RIC1/F1alpha is shared by both the import complex and by respiratory complex V. Alternative use of ancient respiratory proteins may have been an important step in the evolution of tRNA import.

The effect of down-regulation of mitochondrial RNA-binding proteins MRP1 and MRP2 on respiratory complexes in procyclic Trypanosoma brucei

MRP1 and MRP2 are multifunctional mitochondrial RNA-binding proteins with a regulatory role in RNA editing and putative role(s) in RNA processing in Trypanosoma brucei. Silencing of MRP1 and/or MRP2 by RNA interference affected the assembly and functionality of respiratory complexes. The absence of several subunits of complexes I, III and IV resulted in their disintegration and subsequent decrease of specific activities and also caused a significant decrease of membrane potential. The overall respiration in the interfered cells decreased by only about 20%, since the trypanosome alternative oxidase effectively replaced the missing cytochromes and became the principal terminal oxidase.

Unexplained complexity of the mitochondrial genome and transcriptome in kinetoplastid flagellates

Kinetoplastids are flagellated protozoans, whose members include the pathogens Trypanosoma brucei, T. cruzi and Leishmania species, that are considered among the earliest diverging eukaryotes with a mitochondrion. This organelle has become famous because of its many unusual properties, which are unique to the order Kinetoplastida, including an extensive kinetoplast DNA network and U-insertion/deletion type RNA editing of its mitochondrial transcripts. In the last decade, considerable progress has been made in elucidating the complex machinery of RNA editing. Moreover, our understanding of the structure and replication of kinetoplast DNA has also dramatically improved. Much less however, is known, about the developmental regulation of RNA editing, its integration with other RNA maturation processes, stability of mitochondrial mRNAs, or evolution of the editing process itself. Yet the profusion of genomic data recently made available by sequencing consortia, in combination with methods of reverse genetics, hold promise in understanding the complexity of this exciting organelle, knowledge of which may enable us to fight these often medically important protozoans.

Maturation of the unusual single-cysteine (XXXCH) mitochondrial c-type cytochromes found in trypanosomatids must occur through a novel biogenesis pathway

The c-type cytochromes are characterized by the covalent attachment of haem to the polypeptide via thioether bonds formed from haem vinyl groups and, normally, the thiols of two cysteines in a CXXCH motif. Intriguingly, the mitochondrial cytochromes c and c1 from two euglenids and the Trypanosomatidae contain only a single cysteine within the haem-binding motif (XXXCH). There are three known distinct pathways by which c-type cytochromes are matured post-translationally in different organisms. The absence of genes encoding any of these c-type cytochrome biogenesis machineries is established here by analysis of six trypanosomatid genomes, and correlates with the presence of single-cysteine cytochromes c and c1. In contrast, we have identified a comprehensive catalogue of proteins required for a typical mitochondrial oxidative phosphorylation apparatus. Neither spontaneous nor catalysed maturation of the single-cysteine Trypanosoma brucei cytochrome c occurred in Escherichia coli. However, a CXXCH variant was matured by the E. coli cytochrome c maturation machinery, confirming the proposed requirement of the latter for two cysteines in the haem-binding motif and indicating that T. brucei cytochrome c can accommodate a second cysteine in a CXXCH motif. The single-cysteine haem attachment conserved in cytochromes c and c1 of the trypanosomatids is suggested to be related to their cytochrome c maturation machinery, and the environment in the mitochondrial intermembrane space. Our genomic and biochemical studies provide very persuasive evidence that the trypanosomatid mitochondrial cytochromes c are matured by a novel biogenesis system.

Mitochondrial initiation factor 2 of Trypanosoma brucei binds imported formylated elongator-type tRNA(Met)

The mitochondrion of Trypanosoma brucei lacks tRNA genes. Its translation system therefore depends on the import of nucleus-encoded tRNAs. Thus, except for the cytosol-specific initiator tRNA(Met), all trypanosomal tRNAs function in both the cytosol and the mitochondrion. The only tRNA(Met) present in T. brucei mitochondria is therefore the one which, in the cytosol, is involved in translation elongation. Mitochondrial translation initiation depends on an initiator tRNA(Met) carrying a formylated methionine. This tRNA is then recognized by initiation factor 2, which brings it to the ribosome. To guarantee mitochondrial translation initiation, T. brucei has an unusual methionyl-tRNA formyltransferase that formylates elongator tRNA(Met). In the present study, we have identified initiation factor 2 of T. brucei and shown that its carboxyl-terminal domain specifically binds formylated trypanosomal elongator tRNA(Met). Furthermore, the protein also recognizes the structurally very different Escherichia coli initiator tRNA(Met), suggesting that the main determinant recognized is the formylated methionine. In vivo studies using stable RNA interference cell lines showed that knock-down of initiation factor 2, depending on which construct was used, causes slow growth or even growth arrest. Moreover, concomitantly with ablation of the protein, a loss of oxidative phosphorylation was observed. Finally, although ablation of the methionyl-tRNA formyltransferase on its own did not impair growth, a complete growth arrest was observed when it was combined with the initiation factor 2 RNA interference cell line showing the slow growth phenotype. Thus, these experiments illustrate the importance of mitochondrial translation initiation for growth of procyclic T. brucei.

The Effect of RNA Interference Down-regulation of RNA Editing 3′-Terminal Uridylyl Transferase (TUTase) 1 on Mitochondrial de Novo Protein Synthesis and Stability of Respiratory Complexes in Trypanosoma brucei

Inhibition of RNA editing by down-regulation of expression of the mitochondrial RNA editing TUTase 1 by RNA interference had profound effects on kinetoplast biogenesis in Trypanosoma brucei procyclic cells. De novo synthesis of the apocytochrome b and cytochrome oxidase subunit I proteins was no longer detectable after 3 days of RNAi. The effect on protein synthesis correlated with a decline in the levels of the assembled mitochondrial respiratory complexes III and IV, and also cyanide-sensitive oxygen uptake. The steady-state levels of nuclear-encoded subunits of complexes III and IV were also significantly decreased. Because the levels of the corresponding mRNAs were not affected, the observed effect was likely due to an increased turnover of these imported mitochondrial proteins. This induced protein degradation was selective for components of complexes III and IV, because little effect was observed on components of the F(1).F(0)-ATPase complex and on several other mitochondrial proteins.

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.

Natural and induced dyskinetoplastic trypanosomatids: how to live without mitochondrial DNA

Salivarian trypanosomes are the causative agents of several diseases of major social and economic impact. The most infamous parasites of this group are the African subspecies of the Trypanosoma brucei group, which cause sleeping sickness in humans and nagana in cattle. In terms of geographical distribution, however, Trypanosoma equiperdum and Trypanosoma evansi have been far more successful, causing disease in livestock in Africa, Asia, and South America. In these latter forms the mitochondrial DNA network, the kinetoplast, is altered or even completely lost. These natural dyskinetoplastic forms can be mimicked in bloodstream form T. brucei by inducing the loss of kinetoplast DNA (kDNA) with intercalating dyes. Dyskinetoplastic T. brucei are incapable of completing their usual developmental cycle in the insect vector, due to their inability to perform oxidative phosphorylation. Nevertheless, they are usually as virulent for their mammalian hosts as parasites with intact kDNA, thus questioning the therapeutic value of attempts to target mitochondrial gene expression with specific drugs. Recent experiments, however, have challenged this view. This review summarises the data available on dyskinetoplasty in trypanosomes and revisits the roles the mitochondrion and its genome play during the life cycle of T. brucei.

Unusual polypeptide synthesis in the kinetoplast-mitochondria from Leishmania tarentolae. Identification of individual de novo translation products

The de novo synthesis of cytochrome c oxidase subunits I, II (COI and COII), and apocytochrome b (Cyb) was investigated in kinetoplast-mitochondria of Leishmania. The organelles were isolated after breaking whole cells with nitrogen cavitation. Individual COI, COII, and Cyb polypeptides were identified by fractionation of the kinetoplast membranes, labeled with [(35)S]methionine and cysteine, using two-dimensional (9 versus 14% and 20 versus 11%) denaturing gel electrophoresis. The reaction did not require exogenous energy sources or amino acids. On the contrary, the presence of amino acids other than methionine somewhat inhibited the labeling reaction probably by competing with the uptake of labeled amino acids. The synthesis reaction was insensitive to 100 microg/ml chloramphenicol, gentamycin, paromomycin, lincomycin, hygromycin, and tetracycline, as well as cycloheximide. The process showed a linear increase in the amount of synthesized polypeptides during the first 2 h of incubation, followed by a slower accumulation of products for up to 4 h. The de novo synthesized polypeptides were stable for several additional hours. Their assembly into respiratory complexes, investigated using two-dimensional Blue Native/N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine-SDS gels, began early during the incubation and continued throughout the course of the synthesis. This work represents the first unequivocal identification of the polypeptide synthesis in kinetoplasts.

Unique aspects of mitochondrial biogenesis in trypanosomatids

Mitochondrial biogenesis consists of the sum of all processes required for the formation of the mitochondrial membranes as well as the soluble compartments they contain. Furthermore, it includes the replication of the mitochondrial genome and correct segregation of the organelles during cell division. Mitochondrial proteins come from two sources, a limited but essential set of inner membrane proteins is encoded by the mitochondrial genome, whereas the large majority (90-95%) is derived from nucleus-encoded genes and are posttranslationally imported into the organelle. Trypanosomatids belong to the earliest diverging branches of the eukaryotic evolutionary tree which have mitochondria. This is reflected in the organisation of their mitochondrial DNA that consists of a network of two classes of topologically interlocked circular DNA molecules as well as many unique features in their mitochondrial biogenesis. The proteins encoded on the mitochondrial genome are conventional for a mitochondrial genome, their expression, however, involves a complex series of processes. Many genes represent incomplete open reading frames and their primary transcripts have to remodelled by RNA editing to convert them into translatable mRNAs. RNA editing is mediated by small mitochondria-encoded transcripts, the guide RNAs, and is in that form specific for trypanosomatids and closely related organisms. Mitochondrial translation is also unconventional. No tRNA genes are encoded on the mitochondrial genome. Instead, mitochondrial protein synthesis functions exclusively with imported cytosolic, eukaryotic-type tRNAs. The composition of mitochondrial ribosomes is also unusual in that they contain the smallest known rRNAs. They are about 30% shorter than the already much reduced rRNAs in human mitochondria. Furthermore, the topological organisation of the mitochondrial genome requires an elaborate replication machinery involving topoisomerases. Finally, some trypanosomatids have life cycle stages exhibiting very different mitochondrial activities and can therefore serve as a model system for the regulation of mitochondrial biogenesis.

Leishmania tarentolae: a parallel isolation of cytochrome bc(1) and cytochrome c oxidase

A rapid and simple method which allowed for a parallel isolation of cytochrome c reductase (cytochrome bc(1) ) and cytochrome c oxidase from kinetoplast-mitochondria of Leishmania tarentolae was developed. The method involved the lysis of kinetoplasts with dodecyl maltoside in the presence of 260 mM NaCl, followed by purification of bc(1) complexes on DEAE-sepharose CL-6B. The oxidase which was found in the flow-through fractions of the first chromatographic step was diluted and then repurified on a similar DEAE-sepharose column. The investigated properties of the isolated cytochrome c oxidase and reductase, such as their absolute and difference spectrum absorption maxima, heme content, specific activity, and subunit composition, confirm the usefulness of this method for obtaining highly active preparations of the enzymes.