Functions and origins of the chloroplast protein-import machinery

The vast majority of chloroplast proteins are nuclear-encoded and are imported into the organelle after synthesis in the cytoplasm. Targeting to chloroplasts is mediated by a variety of intrinsic targeting signals that direct the preprotein to its proper organelle subcompartment. Translocation at the envelope membrane is directed by the interactions of an N-terminal transit sequences on the preprotein and a general import machinery composed of the outer-membrane Toc machinery and the inner-membrane Tic machinery. The Toc and Tic components interact to bypass the intermembrane space and provide direct transport of preproteins from the cytoplasm to the stroma. There are at least four targeting pathways to the thylakoid membrane, the cpSec pathway, the delta pH pathway, the cpSRP pathway and the spontaneous pathway. These pathways require distinct intrinsic targeting signals, and apparently evolved to accommodate the translocation of classes of proteins with particular characteristics. Proteins similar to some components of the envelope and thylakoid translocation pathways are found in bacterial systems. However, a number of components do not have bacterial counterparts and are unique to the chloroplast pathways. It therefore appears that the chloroplast translocation systems have evolved from membrane-transport systems that were present in the original endosymbiont by incorporating proteins necessary to adapt to the constraints of endosymbiosis.