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Question #11 of the GRE Biology Practice Test says that proteins targeted for chloroplasts require two signal peptides:

  1. Targeting of a newly synthesized protein is most likely to require two different signal peptides for which of the following destinations?

    (A) Plasma membrane

    (B) Lysosome

    (C) Cytosol

    (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?

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  • $\begingroup$ Welcome to SE Biology. Please update the tags on your question to include the homework tag. Goodluck getting an answer. $\endgroup$ – Michael_A Aug 29 '18 at 7:56
  • $\begingroup$ @Michael_A Thanks for the welcome! I'm glad to tag properly, but I'm not currently in school, so this isn't homework. Is homework a broader tag for any test-based question? $\endgroup$ – freeradical Aug 29 '18 at 14:15
  • $\begingroup$ Yes, exactly. Renaming it has been debated but it seems like we're stuck with it. Your question is a model of how to ask this kind of question and I think the tag is a minor issue but including the tag should reduce the odds of people downvoting the question. $\endgroup$ – Michael_A Aug 30 '18 at 2:21
  • $\begingroup$ @Michael_A Makes sense, thanks. I've added the homework tag. $\endgroup$ – freeradical Sep 4 '18 at 1:25
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Protein import to chloroplasts (and also mitochondria) can still have multiple destinations, because these structures themselves have sub-compartments:

  • both have an inter-membrane space as well as an 'inner space' (the stroma in chloroplasts, matrix in mitochondria).
  • proteins can also be targeted towards either of the two membranes
  • additionally chloroplasts have the Thylakoid stacks in the stroma which are separated by another membrane layer.

Due to this complex structure you need multiple signal peptides to properly target proteins to their destination.

I don't remember (or can find) all the details now but basically you (can) have:

  • a 'main' signal peptide that targets proteins directly towards the stroma (which will be removed there by the stromal processing peptidase (SPP)).
  • an additional signal peptide for import to the the thylakoid stacks (which will be 'made avaibalble' by cleavage of the first peptide)
  • there should be other signal peptides or at least variants that allow import to the intermembrande space and the inner membrane (via the TIC23 complex). [I couldn't find any sources for this right now]

Sources I could find:

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