MD: Protein import into chloroplasts: an ever-evolving story

the stroma, transit peptides are cleaved to yield proteins that can be folded, assembled into a functional complex, or tar-geted to one of the six subchloroplast compartments (Fig. 1A). The translocon at the outer envelope membrane of chloroplasts (Toc complex) is responsible for recognizing transit peptides and initiating preprotein import into plastids. It acts coordinately with the Tic (translocon at the inner en-velope membrane of chloroplasts) complex to complete translocation of preproteins into the stroma (Fig. 1B). Typically, transit peptides that mark preproteins as being destined for chloroplasts are enriched in both hydroxylated and hydrophobic amino acids and are devoid of acidic resi-dues (Von Heijne et al. 1989). However, these targeting se-quences are surprisingly variable in amino acid sequence and overall amino acid composition and range in lengt