Protein targeting into plastids: a key to understanding the symbiogenetic acquisitions of plastids.

Recent progress in molecular phylogenetics has proven that photosynthetic eukaryotes acquired plastids via primary and secondary endosymbiosis and has given us information about the origin of each plastid. How a photosynthetic endosymbiont became a plastid in each group is, however, poorly understood, especially for the organisms with secondary plastids. Investigating how a nuclear-encoded plastid protein is targeted into a plastid in each photosynthetic group is one of the most important keys to understanding the evolutionary process of symbiogenetic plastid acquisition and its diversity. For organisms which originated through primary endosymbiosis, protein targeting into plastids has been well studied at the molecular level. For organisms which originated through secondary endosymbiosis, molecular-level studies have just started on the plastid-targeted protein-precursor sequences and the targeting pathways of the precursors. However, little information is available about how the proteins get across the inner two or three envelope membranes in organisms with secondary plastids. A good in vitro protein-import system for isolated plastids and a cell transformation system must be established for each group of photosynthetic eukaryotes in order to understand the mechanisms, the evolutionary processes and the diversity of symbiogenetic plastid acquisitions in photosynthetic eukaryotes.