Characterization of chaperonin 10 (Cpn10) from the intestinal human pathogen Entamoeba histolytica

The mitosome of the anaerobic parasitic protist Entamoeba histolytica: A peculiar and minimalist mitochondrion-related organelle

The simplest class of mitochondrion-related organelles (MROs) is the mitosome, an organelle present in a few anaerobic protozoan parasites such as Entamoeba histolytica, Giardia intestinalis, and Cryptosporidium parvum. E. histolytica causes amoebiasis in humans, deemed as one of the important, yet neglected tropical infections in the world. Much of the enigma of the E. histolytica mitosome circles around the obvious lack of a majority of known mitochondrial components and functions exhibited in other organisms. The identification of enzymes responsible for sulfate activation (AS, IPP, and APSK) and a number of lineage-specific proteins such as the outer membrane beta-barrel protein (MBOMP30), and transmembrane domain-containing proteins that bind to various organellar proteins (ETMP1, ETMP30, EHI_170120, and EHI_099350) showcased the remarkable divergence of this organelle compared to the other MROs of anaerobic protozoa. Here, we summarize the findings regarding the biology of the mitosomes in E. histolytica, from their discovery up to the present understanding of its roles and interactions. We also include current advances and future perspectives on the biology, biochemistry, and evolution of the mitosomes of E. histolytica.

Molecular characterization and promoter analysis of crustacean heat shock protein 10 in Scylla paramemosain

Heat shock proteins (Hsps) are an evolutionarily conserved group of molecules present in all eukaryotic and prokaryotic organisms. Hsp10 and Hsp60 were originally described as the essential mitochondrial proteins involved in protein folding. Recent studies demonstrate that Hsp10 has additional roles including immune modulation. In our study, an homologous Hsp10 (Sp-Hsp10) was identified in the mud crab Scylla paramemosain, and its genomic DNA organization was determined. The cDNA sequence of Sp-Hsp10 contains an open reading frame of 309 bp, encoding a putative protein of 102 amino acid residues with approximately 10 kDa. The Sp-Hsp10 gene is located next to the Sp-Hsp60 gene and shares a 1916-bp intergenic region. The promoter activity of the Sp-Hsp10 flanking gene was analyzed using luciferase reporter assays in transfected endothelial progenitor cells. The upregulation of Sp-Hsp10 expression was detected after exposure of hemocytes to a heat shock of 1 h at 37 °C compared with unstressed hemocytes raised at 20 °C. To our knowledge, this is the first report characterizing the genomic organization of a new Hsp10 in a crustacean.

Mitochondrion-related organelles in eukaryotic protists

The discovery of mitochondrion-type genes in organisms thought to lack mitochondria led to the demonstration that hydrogenosomes share a common ancestry with mitochondria, as well as the discovery of mitosomes in multiple eukaryotic lineages. No examples of examined eukaryotes lacking a mitochondrion-related organelle exist, implying that the endosymbiont that gave rise to the mitochondrion was present in the first eukaryote. These organelles, known as hydrogenosomes, mitosomes, or mitochondrion-like organelles, are typically reduced, both structurally and biochemically, relative to classical mitochondria. However, despite their diversification and adaptation to different niches, all appear to play a role in Fe-S cluster assembly, as observed for mitochondria. Although evidence supports the use of common protein targeting mechanisms in the biogenesis of these diverse organelles, divergent features are also apparent. This review examines the metabolism and biogenesis of these organelles in divergent unicellular microbes, with a focus on parasitic protists.

Diversity and reductive evolution of mitochondria among microbial eukaryotes

All extant eukaryotes are now considered to possess mitochondria in one form or another. Many parasites or anaerobic protists have highly reduced versions of mitochondria, which have generally lost their genome and the capacity to generate ATP through oxidative phosphorylation. These organelles have been called hydrogenosomes, when they make hydrogen, or remnant mitochondria or mitosomes when their functions were cryptic. More recently, organelles with features blurring the distinction between mitochondria, hydrogenosomes and mitosomes have been identified. These organelles have retained a mitochondrial genome and include the mitochondrial-like organelle of Blastocystis and the hydrogenosome of the anaerobic ciliate Nyctotherus. Studying eukaryotic diversity from the perspective of their mitochondrial variants has yielded important insights into eukaryote molecular cell biology and evolution. These investigations are contributing to understanding the essential functions of mitochondria, defined in the broadest sense, and the limits to which reductive evolution can proceed while maintaining a viable organelle.

Localization and targeting of an unusual pyridine nucleotide transhydrogenase in Entamoeba histolytica

Pyridine nucleotide transhydrogenase (PNT) catalyzes the direct transfer of a hydride-ion equivalent between NAD(H) and NADP(H) in bacteria and the mitochondria of eukaryotes. PNT was previously postulated to be localized to the highly divergent mitochondrion-related organelle, the mitosome, in the anaerobic/microaerophilic protozoan parasite Entamoeba histolytica based on the potential mitochondrion-targeting signal. However, our previous proteomic study of isolated phagosomes suggested that PNT is localized to organelles other than mitosomes. An immunofluorescence assay using anti-E. histolytica PNT (EhPNT) antibody raised against the NADH-binding domain showed a distribution to the membrane of numerous vesicles/vacuoles, including lysosomes and phagosomes. The domain(s) required for the trafficking of PNT to vesicles/vacuoles was examined by using amoeba transformants expressing a series of carboxyl-terminally truncated PNTs fused with green fluorescent protein or a hemagglutinin tag. All truncated PNTs failed to reach vesicles/vacuoles and were retained in the endoplasmic reticulum. These data indicate that the putative targeting signal is not sufficient for the trafficking of PNT to the vesicular/vacuolar compartments and that full-length PNT is necessary for correct transport. PNT displayed a smear of >120 kDa on SDS-PAGE gels. PNGase F and tunicamycin treatment, chemical degradation of carbohydrates, and heat treatment of PNT suggested that the apparent aberrant mobility of PNT is likely attributable to its hydrophobic nature. PNT that is compartmentalized to the acidic compartments is unprecedented in eukaryotes and may possess a unique physiological role in E. histolytica.

The essentials of protein import in the degenerate mitochondrion of Entamoeba histolytica

Several essential biochemical processes are situated in mitochondria. The metabolic transformation of mitochondria in distinct lineages of eukaryotes created proteomes ranging from thousands of proteins to what appear to be a much simpler scenario. In the case of Entamoeba histolytica, tiny mitochondria known as mitosomes have undergone extreme reduction. Only recently a single complete metabolic pathway of sulfate activation has been identified in these organelles. The E. histolytica mitosomes do not produce ATP needed for the sulfate activation pathway and for three molecular chaperones, Cpn60, Cpn10 and mtHsp70. The already characterized ADP/ATP carrier would thus be essential to provide cytosolic ATP for these processes, but how the equilibrium of inorganic phosphate could be maintained was unknown. Finally, how the mitosomal proteins are translocated to the mitosomes had remained unclear. We used a hidden Markov model (HMM) based search of the E. histolytica genome sequence to discover candidate (i) mitosomal phosphate carrier complementing the activity of the ADP/ATP carrier and (ii) membrane-located components of the protein import machinery that includes the outer membrane translocation channel Tom40 and membrane assembly protein Sam50. Using in vitro and in vivo systems we show that E. histolytica contains a minimalist set up of the core import components in order to accommodate a handful of mitosomal proteins. The anaerobic and parasitic lifestyle of E. histolytica has produced one of the simplest known mitochondrial compartments of all eukaryotes. Comparisons with mitochondria of another amoeba, Dictystelium discoideum, emphasize just how dramatic the reduction of the protein import apparatus was after the loss of archetypal mitochondrial functions in the mitosomes of E. histolytica.

All eukaryotic organisms have mitochondria, organelles cordoned by a double membrane, which are descendants of an ancestral bacterial endosymbiont. Nowadays, mitochondria are fully integrated into the context of diverse cellular processes and serve in providing energy, iron-containing prosthetic groups and some of the cellular building blocks like lipids and amino acids. In multi-cellular organisms, mitochondria play an additional vital role in cell signaling pathways and programmed cell death. In some unicellular eukaryotes which inhabit oxygen poor environments, intriguing mitochondrial adaptations have taken place resulting in the creation of specialized compartments known as mitosomes and hydrogenosomes. Several important human pathogens like Entamoeba histolytica, Giardia intestinalis, Trichomonas vaginalis and microsporidia contain these organelles and in many cases the function and biogenesis of these organelles remain unknown. In this paper, we investigated the protein import pathways into the mitosomes of E. histolytica, which represent one of the simplest mitochondria-related compartment discovered yet. In accordance with the limited organellar proteome, we show that only core components of mitochondria-related protein import machines are present in E. histolytica to serve for the import of a small set of substrate proteins.

Mitosomes in Entamoeba histolytica contain a sulfate activation pathway

Hydrogenosomes and mitosomes are mitochondrion-related organelles in anaerobic/microaerophilic eukaryotes with highly reduced and divergent functions. The full diversity of their content and function, however, has not been fully determined. To understand the central role of mitosomes in Entamoeba histolytica, a parasitic protozoon that causes amoebic dysentery and liver abscesses, we examined the proteomic profile of purified mitosomes. Using 2 discontinuous Percoll gradient centrifugation and MS analysis, we identified 95 putative mitosomal proteins. Immunofluorescence assay showed that 3 proteins involved in sulfate activation, ATP sulfurylase, APS kinase, and inorganic pyrophosphatase, as well as sodium/sulfate symporter, involved in sulfate uptake, were compartmentalized to mitosomes. We have also provided biochemical evidence that activated sulfate derivatives, adenosine-5'-phosphosulfate and 3'-phosphoadenosine-5'-phosphosulfate, were produced in mitosomes. Phylogenetic analysis showed that the aforementioned proteins and chaperones have distinct origins, suggesting the mosaic character of mitosomes in E. histolytica consisting of proteins derived from alpha-proteobacterial, delta-proteobacterial, and ancestral eukaryotic origins. These results suggest that sulfate activation is the major function of mitosomes in E. histolytica and that E. histolytica mitosomes represent a unique mitochondrion-related organelle with remarkable diversity.

Novel mitochondrion-related organelles in the anaerobic amoeba Mastigamoeba balamuthi

Unicellular eukaryotes that lack mitochondria typically contain related organelles such as hydrogenosomes or mitosomes. To characterize the evolutionary diversity of these organelles, we conducted an expressed sequence tag (EST) survey on the free-living amoeba Mastigamoeba balamuthi, a relative of the human parasite Entamoeba histolytica. From 19 182 ESTs, we identified 21 putative mitochondrial proteins implicated in protein import, amino acid interconversion and carbohydrate metabolism, two components of the iron-sulphur cluster (Fe-S) assembly apparatus as well as two enzymes characteristic of hydrogenosomes. By immunofluorescence microscopy and subcellular fractionation, we show that mitochondrial chaperonin 60 is targeted to small abundant organelles within Mastigamoeba. In transmission electron micrographs, we identified double-membraned compartments that likely correspond to these mitochondrion-derived organelles, The predicted organellar proteome of the Mastigamoeba organelle indicates a unique spectrum of functions that collectively have never been observed in mitochondrion-related organelles. However, like Entamoeba, the Fe-S cluster assembly proteins in Mastigamoeba were acquired by lateral gene transfer from epsilon-proteobacteria and do not possess obvious organellar targeting peptides. These data indicate that the loss of classical aerobic mitochondrial functions and acquisition of anaerobic enzymes and Fe-S cluster assembly proteins occurred in a free-living member of the eukaryote super-kingdom Amoebozoa.

A mitosome purification protocol based on percoll density gradients and its use in validating the mitosomal nature of Entamoeba histolytica mitochondrial Hsp70

Mitochondria are indispensable for aerobic respiration, but many microbial eukaryotes have lost this function through reductive evolution. Their modified mitochondria are known as hydrogenosomes or mitosomes depending on whether or not they produce molecular hydrogen. The intestinal parasite Entamoeba histolytica contains mitosomes whose role in cellular metabolism is unclear. Only three proteins have been shown thus far to reside in these organelles: the molecular chaperones Hsp10 and Hsp60 and an unusual ADP/ATP carrier. Here we describe the isolation of E. histolytica mitosomes by cellular fractionation and density gradient centrifugation and show that the mitochondrial-type chaperone Hsp70 is also housed in Entamoeba mitosomes.

Entamoeba histolytica mitosomes: Organelles in search of a function

It has been more than eight years since the discovery of mitosomes (mitochondrial remnant organelles) in the intestinal human pathogen Entamoeba histolytica. Despite detailed knowledge about the biochemistry of this parasite and the completion of the E. histolytica genome sequencing project no physiological function has yet been unequivocally assigned to these organelles. Entamoeba mitosomes seem to be the most degenerate of all endosymbiosis-derived organelles studied to date. They do not appear to participate in energy metabolism and may have dispensed completely with the proteins required for iron-sulphur cluster biosynthesis. However, the large number of mitosomes found in E. histolytica trophozoites hints at a significant biological role for these organelles in their natural environment. Identifying the protein complement of mitosomes will provide answers as to their biological significance and the reason(s) for their retention in this parasite.

Hydrogenosomal succinyl-CoA synthetase from the rumen-dwelling fungus Neocallimastix patriciarum; an energy-producing enzyme of mitochondrial origin

Hydrogenosomes are hydrogen-producing organelles that are related to mitochondria and found in a variety of evolutionarily unrelated anaerobic microbial eukaryotes. Similar to classic mitochondria, hydrogenosomes contain the enzyme catalyzing the only reaction of the citric acid cycle directly producing energy; succinyl-CoA synthetase. We have isolated and characterized the genes encoding both subunits of this enzyme from the anaerobic chytrid fungus Neocallimastix patriciarum, a model organism in hydrogenosome research. Both subunits contain all characteristic features of this enzyme, including predicted hydrogenosomal targeting signals. Phylogenetic analyses of succinyl-CoA synthetase clearly indicate its mitochondrial ancestry, both by affiliation with mitochondrially localized fungal homologues and by the sisterhood of the eukaryotic succinyl-CoA synthetase clade with alpha-proteobacteria. Our analyses of the Trichomonas vaginalis SCS sequences also confirmed the mitochondrial affiliation of these hydrogenosomal enzymes, in contrast to previous results. While both hydrogenosomal and mitochondrial succinyl-CoA synthetase homologues have been identified, no succinyl-CoA synthetase proteins were identifiable in taxa possessing another mitochondrially derived organelle, the mitosome. Our analyses further confirm the mitochondrial ancestry of the Neocallimastix hydrogenosome and sheds light upon the stepwise process by which mitochondria evolve into alternate forms of the organelle.