Apicoplast
Encyclopedia
An apicoplast is a derived
non-photosynthetic plastid
found in most Apicomplexa
, including malaria
parasites such as Plasmodium falciparum
, but not in others such as Cryptosporidium
. It originated from an algae through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system.
, nonphotosynthetic plastid
found in most protozoa
n parasites belonging to the phylum
Apicomplexa. Among the most infamous Apicomplexa parasites is Plasmodium falciparum, a causative agent of severe malaria
. Because apicoplasts are vital to parasite survival, they provide an enticing target for antimalarial drugs. Specifically, apicoplasts' plant-like properties provide a target for herbicidal
drugs. And, with the emergence of malarial strains resistant to current treatments it is paramount that novel therapies, like herbicides, are explored and understood. Furthermore, herbicides may be able to specifically target the parasite's plant-like apicoplast and without any noticeable effect on the mammalian host's cells.
algae. An ancient cyanobacterium was first engulfed by a eukaryotic cell but was not digested. The bacterium escaped being digested because it formed a symbiotic relationship with the host eukaryotic cell; both the eukaryote and the bacterium mutually benefited from their novel shared existence. The result of the primary endosymbiosis was a photosynthetic eukaryotic alga. A descendent of this eukaryotic alga was then itself engulfed by a heterotrophic eukaryote with which it formed its own symbiotic relationship and was preserved as a plastid. The apicoplast plastid evolved in its new role to preserve only those functions and genes necessary to beneficially contribute to the host-organelle relationship. The ancestral genome of more than 150 kb was reduced through deletions and rearrangements to its present 35 kb size. During the reorganization of the plastid the apicoplast lost its nucleus and, most notably, its ability to photosynthesize. These losses of function are hypothesized to have occurred at an early evolutionary stage in order to have allowed sufficient time for the complete degradation of acknowledged photosynthetic relicts and the disappearance of a nucleomorph.
contains a single ovoid shaped apicoplast that is found at the anterior of the invading parasitic cell. The apicoplast is situated in close proximity to the cell's nucleus
and always closely associated with a mitochondrion. The small plastid, only 0.15-1.5 μm in diameter, is surrounded by four membranes. The two inner membranes are called the outer (OEM) and inner envelope membrane (IEM) and are derived from the plastid envelope. Within the apicoplast's membrane is a 35 kb long circular DNA strand that codes for approximately 30 proteins, tRNAs and some RNA
s. Particles suspected to be bacterial ribosome
s are present. The plastid, at least in the Plasmodium
species, also contains "tubular whorls" of membrane that bear a striking resemblance to the thylakoids of their chloroplast
relatives.
to the parasite's survival. Intriguingly, the destruction of the apicoplast does not immediately kill the parasite but instead prevents it from invading new host cells. This observation suggests that the apicoplast may be involved in lipid
metabolism
. If unable to synthesize sufficient fatty acid
s the parasite is unable to form the parasitophorous vacuole
(PV) that is imperative to a successful invasion of host cells. This conclusion is supported by the discovery of Type II Fatty Acid Synthase (FAS) machinery in the apicoplast.
The apicoplast has also been implicated with heme
synthesis and amino acid
synthesis. It is also suggested to have a role in cell development. These functions, however, are merely postulations and are not yet conclusively supported by experimentation.
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
non-photosynthetic plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
found in most Apicomplexa
Apicomplexa
The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or...
, including malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
parasites such as Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...
, but not in others such as Cryptosporidium
Cryptosporidium
Cryptosporidium is a protozoan that can cause gastro-intestinal illness with diarrhea in humans.Cryptosporidium is the organism most commonly isolated in HIV positive patients presenting with diarrhea...
. It originated from an algae through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system.
Significance
Apicoplasts are a relictRelict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
, nonphotosynthetic plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
found in most protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
n parasites belonging to the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Apicomplexa. Among the most infamous Apicomplexa parasites is Plasmodium falciparum, a causative agent of severe malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
. Because apicoplasts are vital to parasite survival, they provide an enticing target for antimalarial drugs. Specifically, apicoplasts' plant-like properties provide a target for herbicidal
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
drugs. And, with the emergence of malarial strains resistant to current treatments it is paramount that novel therapies, like herbicides, are explored and understood. Furthermore, herbicides may be able to specifically target the parasite's plant-like apicoplast and without any noticeable effect on the mammalian host's cells.
Evolutionary origin
Evidence suggests that the apicoplast is a product of secondary endosymbiosis, and that the apicoplast may be homologous to the secondary plastid of the closely related dinoflagellateDinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
algae. An ancient cyanobacterium was first engulfed by a eukaryotic cell but was not digested. The bacterium escaped being digested because it formed a symbiotic relationship with the host eukaryotic cell; both the eukaryote and the bacterium mutually benefited from their novel shared existence. The result of the primary endosymbiosis was a photosynthetic eukaryotic alga. A descendent of this eukaryotic alga was then itself engulfed by a heterotrophic eukaryote with which it formed its own symbiotic relationship and was preserved as a plastid. The apicoplast plastid evolved in its new role to preserve only those functions and genes necessary to beneficially contribute to the host-organelle relationship. The ancestral genome of more than 150 kb was reduced through deletions and rearrangements to its present 35 kb size. During the reorganization of the plastid the apicoplast lost its nucleus and, most notably, its ability to photosynthesize. These losses of function are hypothesized to have occurred at an early evolutionary stage in order to have allowed sufficient time for the complete degradation of acknowledged photosynthetic relicts and the disappearance of a nucleomorph.
Architecture and distribution
Most ApicomplexaApicomplexa
The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or...
contains a single ovoid shaped apicoplast that is found at the anterior of the invading parasitic cell. The apicoplast is situated in close proximity to the cell's nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and always closely associated with a mitochondrion. The small plastid, only 0.15-1.5 μm in diameter, is surrounded by four membranes. The two inner membranes are called the outer (OEM) and inner envelope membrane (IEM) and are derived from the plastid envelope. Within the apicoplast's membrane is a 35 kb long circular DNA strand that codes for approximately 30 proteins, tRNAs and some RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
s. Particles suspected to be bacterial ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s are present. The plastid, at least in the Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...
species, also contains "tubular whorls" of membrane that bear a striking resemblance to the thylakoids of their chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
relatives.
Function
The functions of apicoplasts have not been conclusively defined. However, it has been established that the apicoplast is a vital organelleOrganelle
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....
to the parasite's survival. Intriguingly, the destruction of the apicoplast does not immediately kill the parasite but instead prevents it from invading new host cells. This observation suggests that the apicoplast may be involved in lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. If unable to synthesize sufficient fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s the parasite is unable to form the parasitophorous vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
(PV) that is imperative to a successful invasion of host cells. This conclusion is supported by the discovery of Type II Fatty Acid Synthase (FAS) machinery in the apicoplast.
The apicoplast has also been implicated with heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
synthesis and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
synthesis. It is also suggested to have a role in cell development. These functions, however, are merely postulations and are not yet conclusively supported by experimentation.