Mitosome
Encyclopedia
A mitosome is an organelle
found in some unicellular eukaryotic
organisms. The mitosome has only recently been found and named, and its function has not yet been well characterized. It was termed a 'crypton' by one group, but that name is no longer in use.
The mitosome has been detected only in anaerobic
or microaerophilic organisms that do not have mitochondria
. These organisms do not have the capability of gaining energy from oxidative phosphorylation, which is normally performed by mitochondria. The mitosome was first described in Entamoeba histolytica
, an intestinal parasite of humans. Mitosomes have also been identified in several species of Microsporidia
and in Giardia intestinalis
.
Mitosomes are almost certainly derived from mitochondria. Like mitochondria, they have a double membrane and most proteins are delivered to them by a targeting sequence
of amino acids. The targeting sequence is similar to that used for mitochondria and true mitochondrial presequences will deliver proteins to mitosomes. A number of proteins associated with mitosomes have been shown to be closely related to those of mitochondria or hydrogenosome
s (which are also degenerate mitochondria).
Current knowledge indicates mitosomes probably play a role in Fe-S cluster assembly, since they do not display any of the proteins involved in other major mitochondrial functions (aerobic respiration, haem biosynthesis) and they do display proteins required for Fe-S cluster biosynthesis (like frataxin
, cysteine desulfurase
, Isu1 and a mitochondrial Hsp70
).
Unlike mitochondria, mitosomes do not have gene
s within them. The genes for mitosomal components are contained in the nuclear genome. An early report suggested the presence of DNA in this organelle, but more recent research has shown this not to be the case.
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
found in some unicellular eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
organisms. The mitosome has only recently been found and named, and its function has not yet been well characterized. It was termed a 'crypton' by one group, but that name is no longer in use.
The mitosome has been detected only in anaerobic
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
or microaerophilic organisms that do not have mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
. These organisms do not have the capability of gaining energy from oxidative phosphorylation, which is normally performed by mitochondria. The mitosome was first described in Entamoeba histolytica
Entamoeba histolytica
Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide...
, an intestinal parasite of humans. Mitosomes have also been identified in several species of Microsporidia
Microsporidia
The microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...
and in Giardia intestinalis
Giardia lamblia
Giardia lamblia is a flagellated protozoan parasite that colonizes and reproduces in the small intestine, causing giardiasis. The giardia parasite attaches to the epithelium by a ventral adhesive disc, and reproduces via binary fission...
.
Mitosomes are almost certainly derived from mitochondria. Like mitochondria, they have a double membrane and most proteins are delivered to them by a targeting sequence
Signal peptide
A signal peptide is a short peptide chain that directs the transport of a protein.Signal peptides may also be called targeting signals, signal sequences, transit peptides, or localization signals....
of amino acids. The targeting sequence is similar to that used for mitochondria and true mitochondrial presequences will deliver proteins to mitosomes. A number of proteins associated with mitosomes have been shown to be closely related to those of mitochondria or hydrogenosome
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s (which are also degenerate mitochondria).
Current knowledge indicates mitosomes probably play a role in Fe-S cluster assembly, since they do not display any of the proteins involved in other major mitochondrial functions (aerobic respiration, haem biosynthesis) and they do display proteins required for Fe-S cluster biosynthesis (like frataxin
Frataxin
Frataxin is a protein that in humans is encoded by the FXN gene.Frataxin is localized to the mitochondrion. The function of frataxin is not entirely clear, but it seems to be involved in assembly of iron-sulfur clusters...
, cysteine desulfurase
Cysteine desulfurase
In enzymology, a cysteine desulfurase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-cysteine and [enzyme]-cysteine, whereas its two products are L-alanine and [enzyme]-S-sulfanylcysteine....
, Isu1 and a mitochondrial Hsp70
Hsp70
The 70 kilodalton heat shock proteins are a family of ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms...
).
Unlike mitochondria, mitosomes do not have gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s within them. The genes for mitosomal components are contained in the nuclear genome. An early report suggested the presence of DNA in this organelle, but more recent research has shown this not to be the case.