Endoplasmic Reticulum

ID #1260

I was confused over reading something in the book compared to the notes I took in class (probably didn't understand what I wrote). Is BiP only for posttranslational import of proteins into ER (like in yeast) or is BiP used to pull in polypeptide in cotranslational translocation as well?


Yeah, I purposely didn't mention that other function of BiP because it gets pretty confusing and seemingly contradictory. What we learned in class is that ER-targeted proteins get translated directly into the ER lumen, and the mechanical force of the emerging amino acids from the ribosome "drives" or "pushes" the protein through the translocon. Once in the lumen, the protein is bound by BiP and assisted in folding properly.

Now of course, that's not the whole story (it never is!). This will NOT be on the exam, but it turns out some proteins that need to go through the ER are actually translated completely out in the cytoplasm and then inserted into the lumen of the ER. These proteins will be shielded (and kept unfolded) by chaperones until they get to the ER membrane, and then inserted into the translocon. But in contrast with a protein that is in the process of being translated, there's no "force" pushing the protein through the translocon, so BiP's other job is to grab these proteins and drag them into the ER lumen. That's what the book was trying to describe, and you can probably see why I don't go into that much detail for MCB 150.

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