Protein transport in organelles

Abstract

Plant cells contain two organelles of endosymbiotic origin: mitochondria and chloroplasts. During the endosymbiotic process, the once free-living predecessors of mitochondria and chloroplasts lost most, but not all, of their genomes to the host nucleus. Therefore, organelle biogenesis and function today are under nuclear control. However, a close cross-talk between nuclear and organellar gene expression exists to guarantee full cellular functionality. Gene transfer from the endosymbiont to the host made it obligatory that the products of transferred genes, namely the proteins, could be transported back to their destination. This required the development of targeting systems that could distinguish between all cellular compartments, and not only between chloroplasts and mitochondria. Simultaneously, it should be kept in mind that chloroplasts evolved in a cellular background that already contained mitochondrial organelles. Furthermore, the arising endosymbiontic organelles had to be equipped with receptor systems to recognize targeting signals, as well as transport capacities to facilitate the transfer of the large biopolymer, protein, across the organellar membranes. Owing to their origin from Gram-negative-like proteobacteria, chloroplasts and mitochondria are surrounded by two membranes, and this required the establishment of two translocons, which, to our present knowledge, was achieved not by simple duplication, but by independent events. The translocation systems present in the bacterial endosymbiont either were largely lost during evolution, such as in mitochondria, which contain only the YidC homologue Oxa1, but no Sec or Tat translocation systems, or such as in chloroplasts, which still contain a Sec, Tat and YidC derived system. These, however, are located on the thylakoid membrane and are required for its biogenesis and maintenance. This minireview series on protein translocation describes the targeting and import process into chloroplasts and the further processes involved in insertion and translocation into and across the thylakoid membranes. The processes at the chloroplast outer membrane are highly regulated by two GTPases: Toc34 and Toc159. The overall process is superficially reminiscent of the GTPase cycle involved in the signal recognition particle pathway. Completion of translocation into the chloroplasts by the Tic complex shows the potential of environmental as well as metabolic regulation. Within the chloroplast, routing to and translocation into the thylakoids involves three distinct pathways. Although organelle identity requires specific targeting and import processes into each cellular compartment, a subset of proteins is dual targeted to both mitochondria and chloroplasts; a phenomenon that is more widespread than previously anticipated. These minireviews provide an up-to-date view on recent developments in the field and also comprise an overview for the nonspecialist.