Step I Ques RE: proteins synthed at RER

[audramill]

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Which of the following best explains why proteins synthesized in the rough endoplasmic reticulum and secreted from eukaryotic cells have a shorter length than would be predicted by their respective mRNAs?

A. Cleavage of 5¢ segments of nascent polypeptides being synthesized on polyribosomes in the endoplasmic reticulum

B. Covalent binding of nascent polypeptides to specific membrane proteins during their synthesis

C. Nonspecific protease activity during passage of these proteins through the endoplasmic reticulum membrane

D. Shortening of the carboxy terminus because hydrophobic amino acids remain within the membrane

 

[tfom08]

Haha, I remember this because I had to look it up when I took the test. By the way, you should put a spoiler on this thread if you got this question from where I think it's from. I went with A although I was unsure because I don't know if peptides have a 5' end. But I was guessing by 5', that means amino terminus of the peptide, which is what is cleaved off as the hydrophobic signaling part of the RER synthesized peptide. I think D is a distractor because as far as I know, the carboxy terminus does not get cleaved in the RER membrane, it is the amino terminus. Perhaps a cell bio expert can help.

 

[rox]

I'd choose A.

 

[Its_MurDAH]

I'd choose A.

yea, that just sounds right...

I did that a lot on the real thing: pick an answer the "seemed" right. Bad way to approach such an important test. whatever.