A protein is synthesized in the endoplasmic reticulum bound ribosomes and it targets to the inner thylakoid space of chloroplast. How many double-layered membrane layers it has to pass to reach its destination?

A protein synthesized on endoplasmic reticulum (ER)-bound ribosomes must first cross the ER membrane to enter the secretory pathway [6]. To reach the inner thylakoid space (thylakoid lumen) of a chloroplast, it must traverse several additional lipid bilayers. Chloroplasts are surrounded by a double-membrane envelope consisting of an outer membrane and an inner membrane [4]. Proteins typically enter via the TOC (Translocon at the outer envelope membrane) and TIC (Translocon at the inner envelope membrane) complexes [5]. Finally, to reach the thylakoid lumen, the protein must cross the thylakoid membrane itself [7]. Thus, the protein crosses four distinct double-layered membrane layers: 1) the ER membrane, 2) the chloroplast outer envelope, 3) the chloroplast inner envelope, and 4) the thylakoid membrane. This sequential translocation ensures proper targeting to the innermost compartment of the chloroplast.

Sources

1. [6] )00938-8.pdf
2. [4] https://www.sciencedirect.com/science/article/pii/S0092867403001107
3. [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC8122044/
4. [1] https://en.wikipedia.org/wiki/Protein_targeting
5. [7] https://pmc.ncbi.nlm.nih.gov/articles/PMC8790069/