Abstract
All eukaryotic organisms contain mitochondria or organelles that evolved from the same endosymbiotic event like classical mitochondria. Organisms inhabiting low oxygen environments often contain mitochondrial derivates known as hydrogenosomes, mitosomes or neutrally as mitochondrion-like organelles. The detailed investigation has shown unexpected evolutionary plasticity in the biochemistry and protein composition of these organelles in various protists. We investigated the mitochondrion-like organelle in Trimastix pyriformis, a free-living member of one of the three lineages of anaerobic group Metamonada. Using 454 sequencing we have obtained 7 037 contigs from its transcriptome and on the basis of sequence homology and presence of N-terminal extensions we have selected contigs coding for proteins that putatively function in the organelle. Together with the results of a previous transcriptome survey, the list now consists of 23 proteins – mostly enzymes involved in amino acid metabolism, transporters and maturases of proteins and transporters of metabolites. We have no evidence of the production of ATP in the mitochondrion-like organelle of Trimastix but we have obtained experimental evidence for the presence of enzymes of the glycine cleavage system (GCS), which is part of amino acid metabolism. Using homologous antibody we have shown that H-protein of GCS localizes into vesicles in the cell of Trimastix. When overexpressed in yeast, H- and P-protein of GCS and cpn60 were transported into mitochondrion. In case of H-protein we have demonstrated that the first 16 amino acids are necessary for this transport. Glycine cleavage system is at the moment the only experimentally localized pathway in the mitochondrial derivate of Trimastix pyriformis.
Highlights
In the last decades, systematic research has considerably improved our knowledge regarding the functions of mitochondrial homologues in many eukaryotic lineages
The path of electrons in these truncated electron transport chains ends at fumarate or in the case of Blastocystis at oxygen to which the transfer is mediated by the complex of alternative oxidase [4,5,6,7]
Even if most of these extensions were not recognized as putative targeting peptides, we still consider this possibility and below present experimental evidence that the extension present in the H-protein of glycine cleavage system is required for protein targeting into mitochondrion
Summary
Systematic research has considerably improved our knowledge regarding the functions of mitochondrial homologues in many eukaryotic lineages. Many obligatory anaerobes and microaerophiles lack a respiratory chain completely [3,8,9] and the enzyme [FeFe]hydrogenase provides the sink for electrons produced by redox reactions in their organelles. This enzyme transfers these electrons to protons producing hydrogen gas, a typical feature of hydrogenosomes that represent one functional class of organelles homologous to mitochondrion. Neither the metabolism of pyruvate nor the ATP production is a function common to all mitochondrial homologues These processes are absent in the most minimalistic versions of these organelles – mitosomes of Giardia, Entamoeba, Cryptosporidium and microsporidia [8,13,14,15,16,17]. The synthesis of FeS clusters is often mentioned in this context [18]
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