The chloroplast protein import apparatus, its components, and their roles

According to the endosymbiont theory, an early eukaryotic cell engulfed the ancestor of present-day chloroplasts - a relative of extant cyanobacteria. This was the start of a new era for the chloroplast progenitor, because it was placed under the control of the host cell. Many of the genes found originally in the cyanobacterial genome are today present in the cell nucleus. Thus, over time, the chloroplast has learned to live with less and less "home-made" proteins. Nevertheless, the chloroplast retains many of the functions found in cyanobacteria (e.g., photosynthesis, fatty acid and amino acid production). To maintain these functions, many proteins have to be transported "back" to the chloroplast. An import machinery drives this transport process, and this consists of translocons located in the outer and the inner envelope membranes, called TOC and TIC (Translocon of the Outer/Inner envelope membrane of Chloroplasts). This chapter focuses on these translocons, and summarizes how they mediate import of nucleus-encoded proteins into the chloroplast from the surrounding cytoplasm. © 2009 Springer Berlin Heidelberg.