Question #11 of the GRE Biology Practice Test says that proteins targeted for chloroplasts require two signal peptides:
Targeting of a newly synthesized protein is most likely to require two different signal peptides for which of the following destinations?
(A) Plasma membrane
(D) Chloroplast (Correct Answer)
(E) Endoplasmic reticulum
I can't find a reference for this.
I understand that a ribosome starts the synthesis of a protein in the cytosol and a signal peptide takes that to the endoplasmic reticulum (ER) which finishes protein synthesis in the ER lumen and then the protein is secreted in a vesicle to its destination (if it doesn't become an ER membrane protein). What's the purpose of the second signal peptide to enter a chloroplast?