apicoplasten

Apicoplasten, also known as the apicoplast, is a plastid-like organelle found in most apicomplexan parasites, including Plasmodium and Toxoplasma. It originated through secondary endosymbiosis of a red alga and is surrounded by four membranes, reflecting its complex ancestry. In these organisms the plastid is non-photosynthetic and does not participate in light-driven energy capture as plant chloroplasts do.

The apicoplasten contains a small circular genome, typically about 30 to 40 kilobases, encoding a limited set of plastid genes such as ribosomal RNA and transfer RNA genes. Most plastid functions are performed by nucleus-encoded proteins that are imported into the organelle, and some components are shared with other cellular compartments.

Metabolic roles attributed to the apicoplasten include several essential biosynthetic pathways. These include type II fatty acid synthesis (FASII), the non-mevalonate (DOXP/MEP) pathway for isoprenoid biosynthesis (producing IPP and DMAPP), and portions of the heme biosynthetic pathway. The organelle also participates in iron-sulfur (Fe-S) cluster assembly via the SUF pathway. While some pathways can be supplied by the host, others are critical for parasite viability, particularly in stages outside the red blood cell cycle.

Clinical relevance arises because these pathways are attractive drug targets. Inhibitors that disrupt apicoplasten function, such as certain antibiotics that block plastidic ribosomes (e.g., doxycycline, clindamycin), can impair parasite growth and produce delayed death, highlighting the organelle’s importance for parasite survival and as a target for antiprotozoal therapies.

Evolutionarily, the apicoplasten is widespread among Apicomplexa but has been lost in some lineages, and its presence helps define the group’s plastid biology and inheritance.