Apicoplasts Are Organelles and Have Genomes
The apicoplast (aka plastid) is an organelle found exclusively in the apicomplexan phylum. The apicoplast has a 35kb genome and is surrounded by 4 membranes. Although its genome can be isolated and sequenced, the apicoplast itself cannot be isolated. The apicoplast is also known as a plastid, as drugs against chloroplasts and prokaryotes also kill apicomplexan parasites.
The apicoplast likely became encased in four membranes via a double endosymbiotic event. The chloroplast arose by engulfment of a cyanobacteria by a plant/algae ancestor. An algae was then engulfed by the ancestor of all apicomplexans. Thus an apicoplast organelle arose with four membranes.
Apicoplasts Encode Localized Proteins
Phylogenetics and GFP tagging reveal that nuclear-encoded proteins of apicoplast origin localize to the apicoplast. Thus, localization of apicoplast proteins is regardless of where they are encoded. Apicoplast proteins have three domains: hydrophobic signal sequence; transit peptide; and mature protein domain. Localization requires the signal sequence and transit peptide.
Identifying protein function via proteomics requires isolated organelles. Unfortunately, the apicoplast cannot be isolated. Thus a common sense approach has been used: all the proteins required for fatty acid synthesis were found in the apicoplast; thus, the apicoplast likely synthesizes fatty acids.
What Is It Related To?
The apicoplast genome was sequenced and found to be plastid-like. It encodes rRNA, tRNAs, ribosomal proteins and 5-6 genes related to chloroplast genes. Furthermore, drugs targeting prokaryotic and chloroplast enzymes also kill apicomplexans. Thus, it seems that the apicoplast arose from a chloroplast. Also, nuclear-encoded plastid-like proteins -- ie, acyl carrier protein (ACP) or p59 -- localize to the apicoplast. The same goes for proteins of plastid origin. But how?
How Does It Localize?
Deletion and restoration identified an evolutionarily conserved apicoplast targeting signal at the N-terminus. Known as the ACP, this bipartite signal contains: a signal sequence to engage secretion; and a plastid-targeting domain to target the plastid. Fusing GFP to ACP targeted it the plastid. Analyzing numerous ACPs led to a consensus amino acid sequence. All translated proteins were screened for this motif at their N-terminus. This identifies putative apicoplast proteins.
So How Did It Arise?
Nuclear genes, morphology, biochemistry and pharmacology places apicomplexans with ciliates and dinoflagellates. However, presence of plastids suggest placement with plants and algae. This paradox is resolved by two stage lateral genetic transfer: an ancestral plant/algae cell engulfed a cyanobacteria; this evolved into a free-living algae; this algae was engulfed by an ancestral apicomplexan. Plastids and mitochondria arose via engulfment of eubacteria by eukaryotes.