Endoplasmic Reticulum

ID #1259

I was just wondering why we have not discussed why no ATP or any other energy source is required to bring the ribosome to the outside of the ER. Energy is needed to bring the protein to TOM/TIM in the mitochondria so why wouldn't it be needed to bring the ribosome to ER?


If you'll recall our discussion of import of mitochondrial proteins, they are completely translated out in the cytoplasm and then kept unfolded until they reach the mitochondria. The reason you use ATP in the first step in import is to remove those chaperone proteins. Now contrast this with the situation for an ER-directed protein. You pause translation, not complete it, so there is no need for a multitude of chaperones. So when you get to the ER, you don't have to supply ATP to peel off chaperones, and the docking of SRP with SRP receptor opens the translocon for you.

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