Mitosome
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A mitosome is an organelle found in some unicellular eukaryotic organisms. The mitosome has only recently been found and named,<ref name="Tovar99">Tovar, J., Fischer, A. & Clark, C. G. (1999). The mitosome, a novel organelle related to mitochondria in the amitochondrial parasite Entamoeba histolytica. Molecular Microbiology 32, 1013–1021.</ref> and its function has not yet been well characterized. It is sometimes termed a crypton.
The mitosome has been detected only in anaerobic or microaerophilic organisms which do not have mitochondria. These organisms do not have the capability of gaining energy from oxidation, which is normally performed by mitochondria. The mitosome was first described in Entamoeba histolytica, an intestinal parasite of humans.<ref name="Tovar99" /><ref name="Bakatselou03">Bakatselou, C., Beste, D., Kadri, A. O., Somanath, S. & Clark, C. G. (2003). Analysis of genes of mitochondrial origin in the genus Entamoeba. J. Eukaryotic Microbiology 50, 210–214.</ref> Mitosomes have also been identified in several species of Microsporidia<ref name="Vivares02">Vivares, C. P. (2002). Functional and evolutionary analysis of a eukaryotic parasitic genome. Current Opinion in Microbiology 5(5):499-505.</ref><ref name="Vavra05">Vavra, J. (2005) "Polar vesicles" of microsporidia are mitochondrial remnants ("mitosomes"). Folia Parasitology 52(1-2):193-5.</ref> and in Giardia intestinalis<ref name="Regoes05">Regoes A, et al. (2005). Protein import, replication, and inheritance of a vestigial mitochondrion. J. Biological Chemistry 2005 Aug 26 280(34):30557-63.</ref>.
Mitosomes are almost certainly derived from mitochondria. Like mitochondria, they have a double-wall membrane,<ref name="Vavra05" /> and proteins are delivered to them by a targeting sequence of amino acids.<ref name="Tovar99" /> The targeting sequence is very similar to that used for mitochondria, and in fact the mitochondria form of sequence will function for mitosomes.<ref name="Tovar99" /> A number of proteins associated with mitosomes have been shown to be closely related to those of mitochondria.<ref name="Bakatselou03" />
Unlike mitochondria, mitosomes do not have genes within them. The genes for mitosomal components are contained in the nuclear genome.<ref name="Tovar99" /> An early report suggested the presence of DNA in this organelle,<ref name="Ghosh00">Ghosh, S., Field, J., Rogers, R., Hickman, M. & Samuelson, J. (2000). The Entamoeba histolytica mitochondrion-derived organelle (crypton) contains double-stranded DNA and appears to be bound by a double membrane. Infectious Immunology 68, 4319–4322.</ref> but more recent research has shown this not to be the case.<ref name="Leon04">Leon-Avila, G. & Tovar, J. (2004). Mitosomes of Entamoeba histolytica are abundant mitochondrion-related remnant organelles that lack a detectable organellar genome. Microbiology 150, 1245–1250.</ref>
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