Fig. 2.

Core components of the chloroplast general protein import system evolved from bacterial protein export systems. (a) Protein import is mediated by TOC and TIC complexes in the outer and inner envelope membranes of plastids, respectively. Import occurs simultaneously through TOC and TIC supercomplexes to avoid mistargeting of preproteins to the intermembrane space. Supercomplexes appear to be assembled by the binding of Tic236 to the TOC import channel, Toc75, and the core TIC component, Tic110. Tic110 is associated with the TIC import channel and the ATP-dependent import motor that drives preprotein import. (b) Tic236 and its binding partner in the TOC complex, Toc75, are related to TamB and BamA/TamA of the BAM/TAM protein export complexes from Gram-negative bacteria, respectively. Therefore, it appears that key elements of the chloroplast protein import system were derived from the bacterial protein export system during the evolutionary assimilation of the photosynthetic bacterial ancestor by a host cell during endosymbiosis. The adaptation of the chloroplast import apparatus from BAM/TAM components required the acquisition of additional TOC and TIC components to reverse the directionality of protein translocation and confer specificity for the import of nuclear-encoded plastid preproteins.