Advances in proteomic, biochemical, and in silico analyses have led to discoveries of novel lineage-specific proteins in MROs.

MRO-targeted proteins have shifted to retain mostly the internal rather than the N-terminal targeting sequence, as a result of the loss in organellar membrane potential.

MROs appear to share the use of dynamin-related proteins (DRPs) as a component of its fission machinery.

Parasitism is influenced in some cases directly or indirectly by crucial processes compartmentalized in MROs.

Despite divergence into different taxa, MROs share similarities with canonical mitochondria as well as with other MROs.

Reinvention of mitochondria occurred as novel functions and unique characteristics among MRO-possessing organisms emerged from independent events of genetic loss and lateral gene transfer, while retaining only a limited number of key components.

Mitochondria originated from the endosymbiotic event commencing from the engulfment of an ancestral α-proteobacterium by the first eukaryotic ancestor. Establishment of niches has led to various adaptations among eukaryotes. In anaerobic parasitic protists, the mitochondria have undergone modifications by combining features shared from the aerobic mitochondria with lineage-specific components and mechanisms; a diversified class of organelles emerged and are generally called mitochondrion-related organelles (MROs). In this review we summarize and discuss the recent advances in the knowledge of MROs from parasitic protists, particularly the themes such as metabolic functions, contribution to parasitism, dynamics, protein targeting, and novel lineage- specific proteins, with emphasis on the diversity among these organelles.

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The Trichomonas vaginalis hydrogenosome proteome is highly reduced relative to mitochondria, yet complex compared with mitosomes.

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

Glossary

this type of mitochondrion contains an electron transport chain with (usually) rhodoquinone adapted to anaerobic conditions, and it exists in both multicellular (e.g., Fasciola, Ascaris, and mammals) and unicellular (e.g., yeasts, Euglena, and Chlamydomonas) eukaryotes. They contain an organellar genome similar to an aerobic mitochondrion.

the stramenopile parasite inhabiting mammalian large intestines. Infection by this species may be asymptomatic or symptomatic, usually causing diarrhea. Vacuolar, granular, amoeboid, and cyst forms are commonly observed as major forms in the undefined life cycle.

β-barrel proteins found in the outer membrane of bacteria and endosymbiosis-derived organelles such as chloroplasts in primary land plants, mitochondria, and related organelles.

the apicomplexan parasite that resides in mammalian small intestines. A typical symptom of infection in healthy persons is frequent watery diarrhea. In the case of immunocompromised hosts, such as persons with AIDS, it could lead to severe chronic diarrhea.

dynamin-related proteins – GTPase family proteins that belong to the dynamin superfamily. DRPs are involved in the fusion and fission of lipid membranes by oligomerization and structural change coupled with GTP hydrolysis.

the amoebozoan parasite that inhabits the human colon. This protist causes amoebiasis, including colitis, dysentery, and extraintestinal abscesses. The life cycle of this species consists of an infectious cyst, which, when ingested, converts to and proliferates as a trophozoite in the host intestine.

a well-conserved core component of the cell-division machinery in bacteria. This protein self-assembles to form the FtsZ ring for the fission of the cell at the site where cell fission occurs. This system is also used for the fission of mitochondria in some eukaryotes.

the diplomonad parasite residing in mammalian small intestines. Giardia species cause giardiasis, of which the main symptom is diarrhea.

2

this type of MRO with electron transport chain uses protons as the terminal electron acceptor, and produces hydrogen by [Fe]-hydrogenase. The size and contents of their genome are often reduced.

this type of MRO lacks an electron transport chain but can produce ATP by substrate-level phosphorylation with [Fe]-hydrogenase, resulting in hydrogen production. The organellar genome is completely absent.

the essential cofactors for various Fe–S cluster-containing proteins in prokaryotes and eukaryotes, for example, aconitase of TCA cycle and components of electron transport chain complexes. Four enzymatic systems for the assembly are known: the ISC (iron–sulfur cluster assembly), SUF (sulfur utilization factors), NIF (nitrogen fixation), and CIA (cytosolic iron–sulfur cluster assembly) systems. The ISC is almost ubiquitous in bacteria and the mitochondria of eukaryotes. The NIF system is typically found in bacteria, including nitrogen-fixing bacteria. The SUF system is commonly found in bacteria and plastids of plants and algae. The CIA system operates in the cytosol of eukaryotes.

proteins unique to a certain taxa, with no existing homologs to other organisms, making it difficult to predict the function based on sequence similarity.

one of the quality-control systems for maintaining the functional properties of the mitochondrial population by the fusion and fission of mitochondria.

one of the mitochondrial quality-control systems that uses autophagy for specific elimination of mitochondria and MROs.

this type of MRO plays no role in ATP production but carries out metabolism associated with sulfur metabolisms (e.g., Fe–S cluster assembly in Giardia and sulfate activation in Entamoeba). They do not contain genomic material.

a presequence composed of 15 to 55 amino acids that forms positively charged amphiphatic α-helices located at the N-terminal region of mitochondrial/MRO matrix proteins, and drives the electrophoretic migration of the protein across the inner membrane.

the parabasalian parasite that causes trichomoniasis. The main symptom is vaginitis and urethritis in females and males, respectively.