Apicoplast

The apicoplast is a relict, non-photosynthetic plastid found in Apicomplexa, including malaria parasites such as Plasmodium falciparum. It is proposed that it evolved via secondary endosymbiosis and is surrounded by four membranes with the outermost part of the endomembrane system.

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Significance

Apicoplasts are a relic, nonphotosynthetic plastid found in all protozoan parasites belonging to the phylum Apicocomplexa ^[1]. Among the most infamous Apicocomplexa parasites is Plasmodium falciparum, a causative agent of severe malaria. Because apicoplasts are vital to parasites' survival they provide an enticing target for antimalarial drugs ^[2]. Specifically, apicoplasts' plant-like properties provide a target for herbicidal drugs ^[1]. And, with the emergence of malarial strains resistant to current treatments it is paramount that novel therapies, like herbicides, are explored and understood ^[2]. 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 ^[3]. 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 ^[4]. The result of the primary endosymbiosis was a photosynthetic eukaryotic alga. This eukaryotic alga, suspected to have been a dinoflagellate ^[1], was then itself engulfed by a heterotrophic eukaryote with which it formed its own symbiotic relationship and was preserved as a plastid^[5]. 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 ^[1]. During the reorganization of the plastid the apicoplast lost its nucleus and, most notably, its ability to photosynthesize^[5].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 relics ^[1] and the disappearance of a nucleomorph^[5].

Architecture and Distribution

Each Apicocomplexa contains a single ovoid shaped apicoplast that is found at the anterior of the invading parasitic cell ^[1]. 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 ^[1], is surrounded by four membranes^[5]. The two inner membranes are called the outer (OEM) and inner envelope membrane (IEM) and are derived from the plastid envelope ^[1]. Within the apicoplast's membrane is a 35 kb long circular DNA strand that codes for approximately 30 proteins, tRNAs and some RNAs^[5]. Particles suspected to be bacterial ribosomes are present ^[2]. The plastid, at least in the Plasmodium species, also contains "tubular whorls" of membrane that bear a striking resemblance to the thylakoids ^[1] of their chloroplast relatives^[5].

Function

The functions of apicoplasts have not been conclusively defined. However, it has been established that the apicoplast is a vital organelle to the parasite's survival^[1]. 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 acids 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 Synthesase (FAS) machinery in the apicoplast. ^[2]

The apicoplast has also been implicated with heme synthesis ^[2] 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 ^[1].

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