Novel mitochondrion‐related organelles in the anaerobic amoeba Mastigamoeba balamuthi

Abstract

SummaryUnicellular 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 freeliving 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 doublemembraned 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 e-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.

Entamoeba mitosomes play an important role in encystation by association with cholesteryl sulfate synthesis

“…An important question then arises: How ubiquitous is the involvement of CS and MROs in differentiation, i.e., is this association conserved in the supergroup Amoebozoa or is it restricted to the genus Entamoeba? To address this question, we extended the study to M. balamuthi, a free-living amoebozoan protist that is a close relative of E. histolytica (19) and, like Entamoeba, is known to possess MROs (20,21). In addition, the M. balamuthi life cycle, including the cyst stage, is similar to that of Entamoeba (22).…”

NIF-type iron-sulfur cluster assembly system is duplicated and distributed in the mitochondria and cytosol of Mastigamoeba balamuthi

“…balamuthi is a more basal member of the Archamoebae group (16). The mitochondrion-related organelles of M. balamuthi have been suggested to represent an intermediate stage between "classic" mitochondria and the reduced mitosomes of E. histolytica (16). Although aerobic respiration is absent in the modified mitochondria of M. balamuthi, they may have retained some components of the tricarboxylic acid cycle (16).…”

“…balamuthi is a more basal member of the Archamoebae group (16). The mitochondrion-related organelles of M. balamuthi have been suggested to represent an intermediate stage between "classic" mitochondria and the reduced mitosomes of E. histolytica (16).…”

“…Unique among eukaryotes studied so far, these anaerobic organisms possess genes encoding the two components of bacterial NIF machinery: NifS and NifU (16,17). Phylogenetic analyses have suggested that the amoebic NIF machinery was inherited through a common ancestor of Mastigamoeba and Entamoeba from an e-proteobacterium via lateral gene transfer (LGT).…”

“…Phylogenetic analyses have suggested that the amoebic NIF machinery was inherited through a common ancestor of Mastigamoeba and Entamoeba from an e-proteobacterium via lateral gene transfer (LGT). This transfer is hypothesized to have occurred after the split of the anaerobic Archamoebae and aerobic Mycetozoa (e.g., Dictyostelium), which possess conventional ISC machinery (16).…”