Redefining the nitroplast: Recent insights into the endosymbiontto- organelle transition

One of the most remarkable events in cellular evolution is the endosymbiosis of α-proteobacteria with a single archaean host cell, a rare evolutionary process, which eventually led to the transformation of symbionts into fully functional mitochondrial organelles in eukaryotes. Evolutionary events related to plants occurred almost 1.6 billion years ago, when eukaryotic heterotrophs acquired a β-cyanobacterium (containing 1B RUBISCO) in what is termed as primary endosymbiosis. Further, this composite cell lineage evolved into three photosynthetic lineages: green algae (plants), red algae and the glaucophytes. Thereafter, a secondary, and tertiary endosymbiosis event occurred giving rise to distinct kinds of green and red-derived photosynthetic plastids, which can be observed in a few haptophytes and dinoflagellates respectively. Eventually, these endosymbionts acquired characteristic cellular properties such as two/multiple envelope membranes and reduction of their genomes through either loss or concerted endosymbiotic gene transfer (EGT) into the nucleus, which ultimately led to the decline of more than three quarters of coding capacity and complete loss of several metabolic pathways. This loss, however, is partly compensated by import of nuclearencoded proteins as well as proteins acquired by horizontal gene transfer (HGT). For most proteins, specific transport mechanisms from nucleus/cytoplasm to organelle exist. The proteins are typically translated as a preprotein with specific signal sequences targeted to the organelle membrane. These membranes harbour receptors, in some cases soluble receptors, for recognition of these signal sequences. Proteins are then internalised using a set of translocation machineries (Gould et al. 2006).