Structure, function and biogenesis of the secondary plastid of apicomplexan parasites.
Résumé
This review gives a brief overview of our current knowledge on the origin, biology and metabolic functions of the relict, nonphotosynthetic plastid of Apicomplexan parasites, named the apicoplast. The Apicomplexa phylum comprises highly diverse obligateintracellular parasites, including the taxons Plasmodium, Toxoplasma, Eimeria and Cryptosporidia, causing as severe diseases as malaria, toxoplasmosis, coccidiosis and cryptosporidiosis, respectively. The apicoplast is vital for the parasites, but the exact reason for its maintenance is not yet understood. This organelle is surrounded by a complex set of multiple membranes and is believed to derive from a red algal plastid acquired by a secondary endosymbiosis. Massive gene transfer to the nucleus has considerably reduced the size of its genome, which is the smallest plastid genome described so far. A small number of genes essentially involved in transcription and translation is encoded by the apicoplast DNA. The vast majority of apicoplast proteins is encoded in the nucleus and imported in the plastid by the courtesy of a bipartite N-terminal presequence. Some typical plastid functions were characterized in apicomplexan plastids, including fatty acid, isoprenoid and heme biosyntheses. These metabolic processes are potentially excellent drug targets and intense research is currently undergone to target the apicoplast as a potential 'Achilles heel' of this group of devastating parasites.