Presence of a member of the mitochondrial carrier family in hydrogenosomes: conservation of membrane-targeting pathways between hydrogenosomes and mitochondria

Mitochondrion‐Derived Organelles in Protists and Fungi

The mitochondrion is generally considered to be a defining feature of eukaryotic cells, yet most anaerobic eukaryotes lack this organelle. Many of these were previously thought to derive from eukaryotes that diverged prior to acquisition of the organelle through endosymbiosis. It is now known that all extant eukaryotes are descended from an ancestor that had a mitochondrion and that in anaerobic eukaryotes the organelle has been modified into either hydrogenosomes, which continue to generate energy for the host cell, or mitosomes, which do not. These organelles have each arisen independently several times. Recent evidence suggests a shared derived characteristic that may be responsible for the retention of the organelles in the absence of the better-known mitochondrial functions--iron-sulfur cluster assembly. This review explores the events leading to this new understanding of mitochondrion-derived organelles in amitochondriate eukaryotes, the current state of our knowledge, and future areas for investigation.

Non-mitochondrial complex I proteins in a hydrogenosomal oxidoreductase complex

Trichomonas vaginalis is a unicellular microaerophilic eukaryote that lacks mitochondria yet contains an alternative organelle, the hydrogenosome, involved in pyruvate metabolism. Pathways between the two organelles differ substantially: in hydrogenosomes, pyruvate oxidation is catalysed by pyruvate:ferredoxin oxidoreductase (PFOR), with electrons donated to an [Fe]-hydrogenase which produces hydrogen. ATP is generated exclusively by substrate-level phosphorylation in hydrogenosomes, as opposed to oxidative phosphorylation in mitochondria. PFOR and hydrogenase are found in eubacteria and amitochondriate eukaryotes, but not in typical mitochondria. Analyses of mitochondrial genomes indicate that mitochondria have a single endosymbiotic origin from an alpha-proteobacterial-type progenitor. The absence of a genome in trichomonad hydrogenosomes precludes such comparisons, leaving the endosymbiotic history of this organelle unclear. Although phylogenetic reconstructions of a few proteins indicate that trichomonad hydrogenosomes share a common origin with mitochondria, others do not. Here we describe a novel NADH dehydrogenase module of respiratory complex I that is coupled to the central hydrogenosomal fermentative pathway to form a hydrogenosomal oxidoreductase complex that seems to function independently of quinones. Phylogenetic analyses of hydrogenosomal complex I-like proteins Ndh51 and Ndh24 reveal that neither has a common origin with mitochondrial homologues. These studies argue against a vertical origin of trichomonad hydrogenosomes from the proto-mitochondrial endosymbiont.

A divergent ADP/ATP carrier in the hydrogenosomes of Trichomonas gallinae argues for an independent origin of these organelles

The evolution of mitochondrial ADP and ATP exchanging proteins (AACs) highlights a key event in the evolution of the eukaryotic cell, as ATP exporting carriers were indispensable in establishing the role of mitochondria as ATP-generating cellular organelles. Hydrogenosomes, i.e. ATP- and hydrogen-generating organelles of certain anaerobic unicellular eukaryotes, are believed to have evolved from the same ancestral endosymbiont that gave rise to present day mitochondria. Notably, the hydrogenosomes of the parasitic anaerobic flagellate Trichomonas seemed to be deficient in mitochondrial-type AACs. Instead, HMP 31, a different member of the mitochondrial carrier family (MCF) with a hitherto unknown function, is abundant in the hydrogenosomal membranes of Trichomonas vaginalis. Here we show that the homologous HMP 31 of closely related Trichomonas gallinae specifically transports ADP and ATP with high efficiency, as do genuine mitochondrial AACs. However, phylogenetic analysis and its resistance against bongkrekic acid (BKA, an efficient inhibitor of mitochondrial-type AACs) identify HMP 31 as a member of the mitochondrial carrier family that is distinct from all mitochondrial and hydrogenosomal AACs studied so far. Thus, our data support the hypothesis that the various hydrogenosomes evolved repeatedly and independently.

Identification of the human mitochondrial S-adenosylmethionine transporter: bacterial expression, reconstitution, functional characterization and tissue distribution

The mitochondrial carriers are a family of transport proteins that, with a few exceptions, are found in the inner membranes of mitochondria. They shuttle metabolites and cofactors through this membrane, and connect cytoplasmic functions with others in the matrix. SAM (S-adenosylmethionine) has to be transported into the mitochondria where it is converted into S-adenosylhomocysteine in methylation reactions of DNA, RNA and proteins. The transport of SAM has been investigated in rat liver mitochondria, but no protein has ever been associated with this activity. By using information derived from the phylogenetically distant yeast mitochondrial carrier for SAM and from related human expressed sequence tags, a human cDNA sequence was completed. This sequence was overexpressed in bacteria, and its product was purified, reconstituted into phospholipid vesicles and identified from its transport properties as the human mitochondrial SAM carrier (SAMC). Unlike the yeast orthologue, SAMC catalysed virtually only countertransport, exhibited a higher transport affinity for SAM and was strongly inhibited by tannic acid and Bromocresol Purple. SAMC was found to be expressed in all human tissues examined and was localized to the mitochondria. The physiological role of SAMC is probably to exchange cytosolic SAM for mitochondrial S-adenosylhomocysteine. This is the first report describing the identification and characterization of the human SAMC and its gene.

Role of YHM1, encoding a mitochondrial carrier protein, in iron distribution of yeast

Mitochondrial carrier proteins are a large protein family, consisting of 35 members in Saccharomyces cerevisiae. Members of this protein family have been shown to transport varied substrates from cytoplasm to mitochondria or mitochondria to cytoplasm, although many family members do not have assigned substrates. We speculated whether one or more of these transporters will play a role in iron metabolism. Haploid yeast strains each deleted for a single mitochondrial carrier protein were analysed for alterations in iron homoeostasis. The strain deleted for YHM1 was characterized by increased and misregulated surface ferric reductase and high-affinity ferrous transport activities. Siderophore uptake from different sources was also increased, and these effects were dependent on the AFT1 iron sensor regulator. Mutants of YHM1 converted into rho degrees, consistent with secondary mitochondrial DNA damage from mitochondrial iron accumulation. In fact, in the Delta yhm1 mutant, iron was found to accumulate in mitochondria. The accumulated iron showed decreased availability for haem synthesis, measured in isolated mitochondria using endogenously available metals and added porphyrins. The phenotypes of Delta yhm1 mutants indicate a role for this mitochondrial transporter in cellular iron homoeostasis.

Identification of the mitochondrial ATP-Mg/Pi transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution

The mitochondrial carriers are a family of transport proteins that, with a few exceptions, are found in the inner membranes of mitochondria. They shuttle metabolites, nucleotides, and cofactors through this membrane and thereby connect and/or regulate cytoplasm and matrix functions. ATP-Mg is transported in exchange for phosphate, but no protein has ever been associated with this activity. We have isolated three human cDNAs that encode proteins of 458, 468, and 489 amino acids with 66-75% similarity and with the characteristic features of the mitochondrial carrier family in their C-terminal domains and three EF-hand Ca(2+)-binding motifs in their N-terminal domains. These proteins have been overexpressed in Escherichia coli and reconstituted into phospholipid vesicles. Their transport properties and their targeting to mitochondria demonstrate that they are isoforms of the ATP-Mg/Pi carrier described in the past in whole mitochondria. The tissue specificity of the three isoforms shows that at least one isoform was present in all of the tissues investigated. Because phosphate recycles via the phosphate carrier in mitochondria, the three isoforms of the ATP-Mg/Pi carrier are most likely responsible for the net uptake or efflux of adenine nucleotides into or from the mitochondria and hence for the variation in the matrix adenine nucleotide content, which has been found to change in many physiopathological situations.

Ancient Invasions: From Endosymbionts to Organelles

The acquisitions of mitochondria and plastids were important events in the evolution of the eukaryotic cell, supplying it with compartmentalized bioenergetic and biosynthetic factories. Ancient invasions by eubacteria through symbiosis more than a billion years ago initiated these processes. Advances in geochemistry, molecular phylogeny, and cell biology have offered insight into complex molecular events that drove the evolution of endosymbionts into contemporary organelles. In losing their autonomy, endosymbionts lost the bulk of their genomes, necessitating the evolution of elaborate mechanisms for organelle biogenesis and metabolite exchange. In the process, symbionts acquired many host-derived properties, lost much of their eubacterial identity, and were transformed into extraordinarily diverse organelles that reveal complex histories that we are only beginning to decipher.

Identification of a novel human subfamily of mitochondrial carriers with calcium-binding domains

Aralar1 and citrin were identified as calcium binding aspartate/glutamate carriers (AGC) in mitochondria. The presence of calcium binding motifs facing the extramitochondrial space allows the regulation of the transport activity of these carriers by cytosolic calcium and provides a new mechanism to transduce calcium signals in mitochondria without the requirement of calcium entry in the organelle. We now report the complete characterization of a second subfamily of human calcium binding mitochondrial carriers named SCaMC (short calcium-binding mitochondrial carriers). We have identified three SCaMC genes in the human genome. All code for highly conserved proteins (about 70-80% identity), of about 500 amino acids with a characteristic mitochondrial carrier domain at the C terminus, and an N-terminal extension harboring four EF-hand binding motifs with high similarity to calmodulin. All SCaMC proteins were found to be located exclusively in mitochondria, and their N-terminal extensions were dispensable for the correct mitochondrial targeting of the polypeptides. SCaMC-1 is the human orthologue of the rabbit Efinal protein, which was reported to be located in peroxisomes, and SCaMC-2 is the human orthologue of the rat MCSC protein, described as up-regulated by dexamethasone in AR42J cells. One of the SCaMC genes, SCaMC-2, has four variants generated by alternative splicing, resulting in proteins with a common C terminus but with variations in their N-terminal halves, including the loss of one to three EF-hand motifs. These results make SCaMC one of most complex subfamilies of mitochondrial carriers and suggest that the large number of isoforms and splice variants may confer different calcium sensitivity to the transport activity of these carriers.

Trichomonas vaginalis Hmp35, a putative pore-forming hydrogenosomal membrane protein, can form a complex in yeast mitochondria

An abundant integral membrane protein, Hmp35, has been isolated from hydrogenosomes of Trichomonas vaginalis. This protein has no known homologue and exists as a stable 300-kDa complex, termed HMP35, in membranes of the hydrogenosome. By using blue native gel electrophoresis, we found the HMP35 complex to be stable in 2 m NaCl and up to 5 m urea. The endogenous Hmp35 protein was largely protease-resistant. The protein has a predominantly beta-sheet structure and predicted transmembrane domains that may form a pore. Interestingly, the protein has a high number of cysteine residues, some of which are arranged in motifs that resemble the RING finger, suggesting that they could be coordinating zinc or another divalent cation. Our data show that Hmp35 forms one intramolecular but no intermolecular disulfide bonds. We have isolated the HMP35 complex by expressing a His-tagged Hmp35 protein in vivo followed by purification with nickel-agarose beads. The purified 300-kDa complex consists of mostly Hmp35 with lesser amounts of 12-, 25-27-, and 32-kDa proteins. The stoichiometry of proteins in the complex indicates that Hmp35 exists as an oligomer. Hmp35 can be targeted heterologously into yeast mitochondria, despite the lack of homology with any yeast protein, demonstrating the compatibility of mitochondrial and hydrogenosomal protein translocation machineries.

Iron-dependent hydrogenases of Entamoeba histolytica and Giardia lamblia: activity of the recombinant entamoebic enzyme and evidence for lateral gene transfer

Entamoeba histolytica and Spironucleus barkhanus have genes that encode short iron-dependent hydrogenases (Fe-hydrogenases), even though these protists lack hydrogenosomes. To understand better the biochemistry of the protist Fe-hydrogenases, we prepared a recombinant E. histolytica short Fe-hydrogenase and measured its activity in vitro. A Giardia lamblia gene encoding a short Fe-hydrogenase was identified from shotgun genomic sequences, and RT-PCR showed that cultured entamoebas and giardias transcribe short Fe-hydrogenase mRNAs. A second E. histolytica gene, which encoded a long Fe-hydrogenase, was identified from shotgun genomic sequences. Phylogenetic analyses suggested that the short Fe-hydrogenase genes of entamoeba and diplomonads share a common ancestor, while the long Fe-hydrogenase gene of entamoeba appears to have been laterally transferred from a bacterium. These results are discussed in the context of competing ideas for the origins of genes encoding fermentation enzymes of these protists.

Converting bacteria to organelles: evolution of mitochondrial protein sorting

During the evolution of mitochondria from free-living alpha-proteobacteria, many bacterial genes were transferred into the nuclear genome of eukaryotic cells. This required the development of both targeting signals on the respective polypeptides and protein translocation machineries (translocases) in the mitochondrial membranes. Most components of these translocases have no obvious homologies to bacterial proteins or proteins found in other organelles. Membrane integration of many inner membrane proteins, however, apparently occurs via a conserved sorting pathway whose components and characteristics resemble protein translocation in bacteria. Consistent with this, the topogenic signals of these mitochondrial inner membrane proteins mimic those of bacterial proteins. The requirement for post-translational transport to their final destination has placed considerable constraints on the evolution of mitochondrial protein sequences.

Mitochondria and hydrogenosomes are two forms of the same fundamental organelle

Published data suggest that hydrogenosomes, organelles found in diverse anaerobic eukaryotes that make energy and hydrogen, were once mitochondria. As hydrogenosomes generally lack a genome, the conversion is probably one way. The sources of the key hydrogenosomal enzymes, pyruvate : ferredoxin oxidoreductase (PFO) and hydrogenase, are not resolved by current phylogenetic analyses, but it is likely that both were present at an early stage of eukaryotic evolution. Once thought to be restricted to a few unusual anaerobic eukaryotes, the proteins are intimately integrated into the fabric of diverse eukaryotic cells, where they are targeted to different cell compartments, and not just hydrogenosomes. There is no evidence supporting the view that PFO and hydrogenase originated from the mitochondrial endosymbiont, as posited by the hydrogen hypothesis for eukaryogenesis. Other organelles derived from mitochondria have now been described in anaerobic and parasitic microbial eukaryotes, including species that were once thought to have diverged before the mitochondrial symbiosis. It thus seems possible that all eukaryotes may eventually be shown to contain an organelle of mitochondrial ancestry, to which different types of biochemistry can be targeted. It remains to be seen if, despite their obvious differences, this family of organelles shares a common function of importance for the eukaryotic cell, other than energy production, that might provide the underlying selection pressure for organelle retention.

Multiple origins of hydrogenosomes: functional and phylogenetic evidence from the ADP/ATP carrier of the anaerobic chytrid Neocallimastix sp

A mitochondrial-type ADP/ATP carrier (AAC) has been identified in the hydrogenosomes of the anaerobic chytridiomycete fungus Neocallimastix sp. L2. Biochemical and immunocytochemical studies revealed that this ADP/ATP carrier is an integral component of hydrogenosomal membranes. Expression of the corresponding cDNA in Escherichia coli confers the ability on the bacterial host to incorporate ADP at significantly higher rates than ATP--similar to isolated mitochondria of yeast and animals. Phylogenetic analysis of this AAC gene (hdgaac) confirmed with high statistical support that the hydrogenosomal ADP/ATP carrier of Neocallimastix sp. L2 belongs to the family of veritable mitochondrial-type AACs. Hydrogenosome-bearing anaerobic ciliates possess clearly distinct mitochondrial-type AACs, whereas the potential hydrogenosomal carrier Hmp31 of the anaerobic flagellate Trichomonas vaginalis and its homologue from Trichomonas gallinae do not belong to this family of proteins. Also, phylogenetic analysis of genes encoding mitochondrial-type chaperonin 60 proteins (HSP 60) supports the conclusion that the hydrogenosomes of anaerobic chytrids and anaerobic ciliates had independent origins, although both of them arose from mitochondria.

A form of cell death with some features resembling apoptosis in the amitochondrial unicellular organism Trichomonas vaginalis

One of hallmarks of apoptosis is the degradation and concomitant compaction of chromatin. It is assumed that caspases and caspase-independent pathways are rate limiting for the development of nuclear apoptosis. The caspase-independent pathway involves apoptosis-inducing factor (AIF) and leads to DNA fragmentation and peripheral chromatin condensation. Both pathways are the result of activation of death signals that the mitochondrion receives, integrates, and responds to with the release of various molecules (e.g., cytochrome c and AIF). In fact, both pathways have in common the final point of the DNA fragmentation and the mitochondrial origin of molecules that initiate the apoptotic events. Here, we examine the question of whether apoptosis or apoptotic-like processes exist in a unicellular organism that lacks mitochondria. We herein show that a form of cell death with some features resembling apoptosis is indeed present in Trichomonas vaginalis. Characterization of morphological aspects implicated in this event together with the preliminary biochemical data provided may lead to new insight about the evolutionary relationships between the different forms of programmed cell death identified so far.

Mitochondrial-type hsp70 genes of the amitochondriate protists, Giardia intestinalis, Entamoeba histolytica and two microsporidians

Genes encoding putative mitochondrial-type heat shock protein 70 (mit-hsp70) were isolated and sequenced from amitochondriate protists, Giardia intestinalis, Entamoeba histolytica, and two microsporidians, Encephalitozoon hellem and Glugea plecoglossi. The deduced mit-hsp70 sequences were analyzed by sequence alignments and phylogenetic reconstructions. The mit-hsp70 sequence of these four amitochondriate protists were divergent from other mit-hsp70 sequences of mitochondriate eukaryotes. However, all of these sequences were clearly located within a eukaryotic mitochondrial clade in the tree including various type hsp70 sequences, supporting the emerging notion that none of these amitochondriate lineages are primitively amitochodrial, but lost their mitochondria secondarily in their evolutionary past.

Conserved properties of hydrogenosomal and mitochondrial ADP/ATP carriers: a common origin for both organelles

Mitochondria are one of the hallmarks of eukaryotic cells, exporting ATP in exchange for cytosolic ADP using ADP/ATP carriers (AAC) located in the inner mitochondrial membrane. In contrast, several evolutionarily important anaerobic eukaryotes lack mitochondria but contain hydrogenosomes, peculiar organelles of controversial ancestry that also supply ATP but, like some fermentative bacteria, make molecular hydrogen in the process. We have now identified genes from two species of the hydrogenosome-containing fungus Neocallimastix that have three-fold sequence repeats and signature motifs that, along with phylogenetic analysis, identify them as AACs. When expressed in a mitochondrial AAC- deficient yeast strain, the hydrogenosomal protein was correctly targeted to the yeast mitochondria inner membrane and yielded mitochondria able to perform ADP/ATP exchange. Characteristic inhibitors of mitochondrial AACs blocked adenine nucleotide exchange by the Neocallimastix protein. Thus, our data demonstrate that fungal hydrogenosomes and yeast mitochondria use the same pathway for ADP/ATP exchange. These experiments provide some of the strongest evidence yet that yeast mitochondria and Neocallimastix hydrogenosomes are but two manifestations of the same fundamental organelle.

Mitochondrial type iron-sulfur cluster assembly in the amitochondriate eukaryotes Trichomonas vaginalis and Giardia intestinalis, as indicated by the phylogeny of IscS

Pyridoxal-5'-phosphate-dependent cysteine desulfurase (IscS) is an essential enzyme in the assembly of FeS clusters in bacteria as well as in the mitochondria of eukaryotes. Although FeS proteins are particularly important for the energy metabolism of amitochondrial anaerobic eukaryotes, there is no information about FeS cluster formation in these organisms. We identified and sequenced two IscS homologs of Trichomonas vaginalis (TviscS-1 and TviscS-2) and one of Giardia intestinalis (GiiscS). TviscS-1, TviscS-2, and GiiscS possess the typical conserved regions implicated in cysteine desulfurase activity. N-termini of TviscS-1 and TviscS-2 possess eight amino acid extensions, which resemble the N-terminal presequences that target proteins to hydrogenosomes in trichomonads. No presequence was evident in GiiscS from Giardia, an organism that apparently lacks hydrogenosmes or mitochondria. Phylogenetic analysis showed a close relationship among all eukaryotic IscS genes including those of amitochondriates. IscS of proteobacteria formed a sister group to the eukaryotic clade, suggesting that isc-related genes were present in the proteobacterial endosymbiotic ancestor of mitochondria and hydrogenosomes. NifS genes of nitrogen-fixing bacteria, which are IscS homologs required for specific formation of FeS clusters in nitrogenase, formed a more distant group. The phylogeny indicates the presence of a common mechanism for FeS cluster formation in mitochondriates as well as in amitochondriate eukaryotes. Furthermore, the analyses support a common origin of Trichomonas hydrogenosomes and mitochondria, as well as secondary loss of mitochondrion/hydrogenosome-like organelles in Giardia.

Alpha and beta subunits of pyruvate dehydrogenase E1 from the microsporidian Nosema locustae: mitochondrion-derived carbon metabolism in microsporidia

Microsporidia are highly adapted eukaryotic intracellular parasites that infect a variety of animals. Microsporidia contain no recognisable mitochondrion, but recently have been shown to have evolved from fungi and to possess heat shock protein genes derived from mitochondria. These findings make it clear that microsporidian ancestors were mitochondrial, yet it remains unknown whether they still contain the organelle, and if so what its role in microsporidian metabolism might be. Here we have characterised genes encoding the alpha and beta subunits of pyruvate dehydrogenase complex E1 (PDH, EC 1.2.4.1) from the microsporidian Nosema locustae. All other amitochondriate eukaryotes studied to date have lost the PDH complex and replaced it with pyruvate:ferredoxin oxidoreductase (PFOR). Nevertheless, molecular phylogeny shows that these Nosema enzymes are most closely related to mitochondrial PDH from other eukaryotes, demonstrating that elements of mitochondrial metabolism have been retained in microsporidia, and that PDH has not been wholly lost. However, there is still no evidence for a mitochondrion in microsporidia, and neither PDH subunit is predicted to encode an amino terminal leader sequence that could function as a mitochondrion-targeting transit peptide, raising questions as to whether these proteins function in a relic organelle or in the cytosol. Moreover, it is also unclear whether these proteins remain part of the PDH complex, or whether they have been retained for another purpose. We propose that microsporidia may utilise a unique pyruvate decarboxylation pathway involving PDH, demonstrating once again the diversity of core metabolism in amitochondriate eukaryotes.

Phylogenetic relationships of class II fumarase genes from trichomonad species

Class II fumarase sequences were obtained by polymerase chain reaction from five trichomonad species. All residues known to be highly conserved in this enzyme were present. Nuclear run-on assays showed that one of the two genes identified in Tritrichomonas foetus was expressed, whereas no fumarase transcripts were detected in the related species Trichomonas vaginalis. These findings corroborate previous biochemical data. Fumarase genes were also expressed in Monocercomonas sp. and Tetratrichomonas gallinarum but not in Pentatrichomonas hominis, Trichomonas gallinae, Trichomonas tenax, and Trichomitus batrachorum under the culture conditions used. Molecular trees inferred by likelihood methods reveal that trichomonad sequences have no affinity to described class II fumarase genes from other eukaryotes. The absence of functional mitochondria in protists such as trichomonads suggests that they diverged from other eukaryotes prior to the alpha-proteobacterial symbiosis that led to mitochondria. Furthermore, they are basal to other eukaryotes in rRNA analyses. However, support for the early-branching status of trichomonads and other amitochondriate protists based on phylogenetic analyses of multiple data sets has been equivocal. Although the presence of hydrogenosomes suggests that trichomonads once had mitochondria, their class II iron-independent fumarase sequences differ markedly from those of other mitochondriate eukaryotes. All of the class II fumarase genes described from other eukaryotes are of apparent alpha-proteobacterial origin and hence a marker of mitochondrial evolution. In contrast, the class II fumarase from trichomonads emerges among other eubacterial homologs. This is intriguing evidence for an independent acquisition of these genes in trichomonads apart from the mitochondrial endosymbiosis event that gave rise to the form present in other eukaryotes. The ancestral trichomonad class II fumarase may represent a prokaryotic form that was replaced in other eukaryotes after the divergence of trichomonads with the movement of endosymbiont genes into the nucleus. Alternatively, it may have been acquired via a separate endosymbiotic event or lateral gene transfer.

Evolutionary relationship of Rickettsiae and mitochondria

Phylogenetic data support an origin of mitochondria from the alpha-proteobacterial order Rickettsiales. This high-rank taxon comprises exceptionally obligate intracellular endosymbionts of eukaryotic cells, and includes family Rickettsiaceae and a group of microorganisms termed Rickettsia-like endosymbionts (RLEs). Most detailed phylogenetic analyses of small subunit rRNA and chaperonin 60 sequences consistently show the RLEs to have emerged before Rickettsiaceae and mitochondria sister clades. These data suggest that the origin of mitochondria and Rickettsiae has been preceded by the long-term mutualistic relationship of an intracellular bacterium with a pro-eukaryote, in which an invader has lost many dispensable genes, yet evolved carrier proteins to exchange respiration-derived ATP for host metabolites as envisaged in classic endosymbiont theory.

Pyruvate : NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum: a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists

Most eukaryotes perform the oxidative decarboxylation of pyruvate in mitochondria using pyruvate dehydrogenase (PDH). Eukaryotes that lack mitochondria also lack PDH, using instead the O(2)-sensitive enzyme pyruvate : ferredoxin oxidoreductase (PFO), which is localized either in the cytosol or in hydrogenosomes. The facultatively anaerobic mitochondria of the photosynthetic protist Euglena gracilis constitute a hitherto unique exception in that these mitochondria oxidize pyruvate with the O(2)-sensitive enzyme pyruvate : NADP oxidoreductase (PNO). Cloning and analysis of Euglena PNO revealed that the cDNA encodes a mitochondrial transit peptide followed by an N-terminal PFO domain that is fused to a C-terminal NADPH-cytochrome P450 reductase (CPR) domain. Two independent 5.8-kb full-size cDNAs for Euglena mitochondrial PNO were isolated; the gene was expressed in cultures supplied with 2% CO(2) in air and with 2% CO(2) in N(2). The apicomplexan Cryptosporidium parvum was also shown to encode and express the same PFO-CPR fusion, except that, unlike E. gracilis, no mitochondrial transit peptide for C. parvum PNO was found. Recombination-derived remnants of PNO are conserved in the genomes of Saccharomyces cerevisiae and Schizosaccharomyces pombe as proteins involved in sulfite reduction. Notably, Trypanosoma brucei was found to encode homologs of both PFO and all four PDH subunits. Gene organization and phylogeny revealed that eukaryotic nuclear genes for mitochondrial, hydrogenosomal, and cytosolic PFO trace to a single eubacterial acquisition. These findings suggest a common ancestry of PFO in amitochondriate protists with Euglena mitochondrial PNO and Cryptosporidium PNO. They are also consistent with the view that eukaryotic PFO domains are biochemical relics inherited from a facultatively anaerobic, eubacterial ancestor of mitochondria and hydrogenosomes.

Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads

Land, K. M., Clemens, D. L., and Johnson, P. J. 2001. Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads. Experimental Parasitology 97, 102-110. In trichomonads, metronidazole is activated to its cytotoxic form in a specialized energy-producing organelle called the hydrogenosome. Electron transport components in the organelle, pyruvate:ferredoxin oxidoreductase and ferredoxin, donate a single electron to the drug, converting it to a cytotoxic free radical. Previous biochemical analyses of enzyme activities of highly resistant strains of both Trichomonas vaginalis and Tritrichomonas foetus reveal undetectable activity for pyruvate:ferredoxin oxidoreductase and another hydrogenosomal enzyme, hydrogenase. We have chosen to analyze a highly drug-resistant strain of T. foetus and its parental drug-sensitive strain from which it was derived to study the molecular basis for these enzyme defects. Quantitation of pyruvate:ferredoxin oxidoreductase and ferredoxin levels in sensitive and resistant cells shows a marked reduction of these proteins in the resistant strain. RNA analysis reveals an approximately 60% reduction in pyruvate:ferredoxin oxidoreductase mRNA and 90-98% reduction in mRNA levels encoding hydrogenosomal proteins hydrogenase, ferredoxin, and malic enzyme. We have measured the levels of transcription of these genes and observed 60% reduction of pyruvate:ferredoxin oxidoreductase gene transcription and 85% reduction in malic enzyme gene transcription in the resistant strain. The reduction or absence of these organellar proteins is likely to reduce or eliminate the ability of the cell to activate the drug, giving rise to the highly resistant phenotype. Ultrastructural analysis of thin sections revealed that resistant cells are 20% smaller in size and hydrogenosomes in resistant cells are approximately one-third the size of those in the drug-sensitive parental strain. These data suggest that altered gene expression of multiple hydrogenosomal proteins results in the modification of the organelle and leads to drug resistance.

Hydrogenosomes: convergent adaptations of mitochondria to anaerobic environments

Hydrogenosomes are membrane-bound organelles that compartmentalise the final steps of energy metabolism in a number of anaerobic eukaryotes. They produce hydrogen and ATP. Here we will review the data, which are relevant for the questions: how did the hydrogenosomes originate, and what was their ancestor? Notably, there is strong evidence that hydrogenosomes evolved several times as adaptations to anaerobic environments. Most likely, hydrogenosomes and mitochondria share a common ancestor, but an unequivocal proof for this hypothesis is difficult because hydrogenosomes lack an organelle genome - with one remarkable exception (Nyctotherus ovalis). In particular, the diversity of extant hydrogenosomes hampers a straightforward analysis of their origins. Nevertheless, it is conceivable to postulate that the common ancestor of mitochondria and hydrogenosomes was a facultative anaerobic organelle that participated in the early radiation of unicellular eukaryotes. Consequently, it is reasonable to assume that both, hydrogenosomes and mitochondria are evolutionary adaptations to anaerobic or aerobic environments, respectively.

Origins of hydrogenosomes and mitochondria

Complete genome sequences for many oxygen-respiring mitochondria, as well as for some bacteria, leave no doubt that mitochondria are descendants of alpha-proteobacteria, a finding for which the endosymbiont hypothesis can easily account. Yet a wealth of data indicate that mitochondria and hydrogenosomes - the ATP-producing organelles of many anaerobic protists - share a common ancestry, a finding that traditional formulations of the endosymbiont hypothesis less readily accommodates. Available evidence suggests that a more in-depth understanding of the origins of eukaryotes and their organelles will hinge upon data from the genomes of protists that synthesize ATP without the need for oxygen.

Origins of hydrogenosomes and mitochondria: evolution and organelle biogenesis

It is becoming increasingly evident that all eukaryotes characterized to date bear some mitochondrial trait, whether it be a 'real' mitochondrion, a hydrogenosome, a mitosome or a few genes left behind from secondary losses of organelles. The implication is that the evolutionary history of the mitochondrion may reveal the history of the eukaryotic cell itself.