Two paths diverged in the stroma: targeting to dual SEC translocase systems in chloroplasts

Chloroplasts inherited systems and strategies for protein targeting, translocation, and integration from their cyanobacterial ancestor. Unlike cyanobacteria however, chloroplasts in green algae and plants contain two distinct SEC translocase/integrase systems: the SEC1 system in the thylakoid membrane and the SEC2 system in the inner envelope membrane. This review summarizes the mode of action of SEC translocases, identification of components of the SEC2 system, evolutionary history of SCY and SECA genes, and previous work on the co- and post-translational targeting of lumenal and thylakoid membrane proteins to the SEC1 system. Recent work identifying substrates for the SEC2 system and potential features that may contribute to inner envelope targeting are also discussed.