Translocation of Proteins into Four Membrane-Bound Complex Plastids of Red Algal Origin

Plastids initially evolved via endosymbiotic uptake of cyanobacteria into a non-photosynthetic eukaryote, however, in the majority of algal groups, plastids have been indirectly acquired, via endosymbiotic uptake of photosynthetic eukaryotes into other eukaryotic cells. Several types of these so-called complex plastids exist, differing mainly in the phylogenies of the former endosymbionts, and in the plastid structure, especially in the numbers and topologies of envelope membranes surrounding the plastid. Cryptophyta, Haptophyta, and Ochrophyta (the photosynthetic members of Stramenopiles) possess plastids of a similar topology, with four membranes surrounding the stroma, and the outermost membrane being part of the eukaryotic endomembrane system. These additional membranes compared to plant plastids required the establishment of further transport systems of nucleus-encoded proteins into the organelles. In this chapter, we are reviewing the targeting signals and targeting mechanisms of proteins into the complex plastids of these algal groups, and discuss the physiological and evolutionary implications.