A protein is synthesized in the endoplasmic reticulum bound ribosomes

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?

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UPSC CDS-1 – 2018

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The protein is synthesized in endoplasmic reticulum-bound ribosomes, meaning it enters the ER lumen or membrane. To reach the inner thylakoid space of a chloroplast, it must move from the ER system to the chloroplast. Assuming a pathway where the protein is released from the ER lumen into the cytoplasm (e.g., via retrotranslocation, crossing the ER membrane) and then imported into the chloroplast, the path would involve crossing the following membrane layers:
1. Crossing the ER membrane to exit the lumen into the cytoplasm (1 layer).
2. Crossing the outer chloroplast membrane (1 layer).
3. Crossing the inner chloroplast membrane (1 layer).
4. Crossing the thylakoid membrane to enter the inner thylakoid space (1 layer).
Total membrane layers crossed = 1 + 1 + 1 + 1 = 4.

Reaching the inner thylakoid space from the cytoplasm requires crossing the chloroplast outer membrane, inner membrane, and thylakoid membrane (3 layers). Starting from the ER lumen adds the step of exiting the ER, which requires crossing the ER membrane.

The standard pathway for most chloroplast proteins is synthesis on free ribosomes in the cytoplasm followed by post-translational import. The mention of synthesis on ER-bound ribosomes implies a non-standard or specific targeting route, or potentially release from the ER system into the cytoplasm before import. Chloroplasts have a double membrane envelope (outer and inner membranes) and an internal thylakoid membrane system.

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