The cell biology of secondary endosymbiosis--how parasites build, divide and segregate the apicoplast.

Protozoan parasites of the phylum Apicomplexa harbour a chloroplast-like organelle, the apicoplast. The biosynthetic pathways localized to this organelle are of cyanobacterial origin and therefore offer attractive targets for the development of new drugs for the treatment of malaria and toxoplasmosis. The apicoplast also provides a unique system to study the cell biology of endosymbiosis. This organelle is the product of secondary endosymbiosis, the marriage of an alga and an auxotrophic eukaryote. This origin has led to a fascinating set of novel cellular mechanisms that are clearly distinct from those employed by the plant chloroplast. Here we explore how the apicoplast interacts with its 'host' to secure building blocks for its biogenesis and how the organelle is divided and segregated during mitosis. Considerable advances in parasite genetics and genomics have transformed apicomplexans, long considered hard to study, into highly tractable model organisms. We discuss how these resources might be marshalled to develop a detailed mechanistic picture of apicoplast cell biology.