Endoplasmic Reticulum

ID #1273

My question is regarding translocons and targeting signals. It seems clear to me that the reason why the translocon closes is because it encounters a stop transfer sequence. The stop transfer sequence tells the translocon to close and the stop transfer sequence itself becomes part of the lipid bilayer membrane. However, it doesn't seem completely clear as to the mechanism or reason why the translocon closes in one of our lecture figures where there is no final stop transfer sequence. I got the impression that it closed because the ribosome dissociated after completing protein synthesis and the ER doesn't want to let extraneous material in to the lumen; is that a detailed enough understanding of why it closes? Or is there another sequence that indicates the translocon to close, thereby locking the internal signal sequence into the membrane of the ER?

Your understanding and explanation of the situation is quite good. The translocon will close on its own when translation is finished because the ribosome dissociates its subunits and leaves. But you're also correct in pointing out that the internal signal sequences are signals for the protein to take that region and expel it from the translocon, but leave it open and continue to thread more protein in.

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