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nbmeanswersownersucks
I was under the impression that translation of transmembrane proteins begins with ribosomes in the cytoplasm that then translocate to the rough ER once the signal sequence is reached by the ribosome? i.e. technically translation begins in the cytoplasm but finishes in the rough ER. Am I wrong about that?
+5
nbmeanswersownersucks
It was UWORLD 6544 about insulin translation. They state that the translation is initiated in the cytoplasm then relocates to the RER (d/t the signal sequence) and is finished there. So is there a difference in translation steps for proteins that are excreted like insulin and transmembrane proteins?
+2
nsinghey
Same, I am not sure about this. My best guess is that since insulin is not a functional protein, it is not synthesized in the RER (even though it it excreted from the cell). Actual proteins are made in the RER
+2
kevster123
I just put F cause it said transmembrane domains and I know the rough ER got a lot of balls on it that translate it through and to translate through the balls you're passing through membranes.
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drdoom
@nbmeanswersownersucks @nsinghey et al. There is extensive discussion of this on an NBME 24 thread. This link will take you to the comments (just don't scroll up to spoil the answer for yourself!): https://nbmeanswers.com/exam/nbme24/939#1379
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brise
The question is saying where is it initially produced? It is produced in the RER, therefore F.
Not asking where it's production starts- asking where is it produced etc.
+3
chj7
I'm not sure if this is what the question was trying to ask but technically the "polypeptide" is initially sequenced in the cytosol; once the N-terminal signal sequence is synthesized, SRP translocates ribosomes to the rER where translation continues/completes and the "protein" is folded/formed in rER. (I like UW #757's diagram on this)
So if they truly mean where the precursor "protein" is initially formed, rER is correct.
But honestly the above is a way too complex form of thinking that I feel would NOT help on the actual exam and most likely strays away from the learning objective of this question; more likely the question writers were trying to distinguish btw proteins translocated to the rER (membrane proteins, secretory proteins, ER/Golgi/lysosomal proteins) vs. proteins that are synthesized by free ribosomes (cytosol/nucleosol proteins, peroxisomal/ some mitochondrial proteins).
+1
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submitted by โazibird(279)
This is the most poorly drawn cell diagram. I see zero ribosomes, so I figured F was the smooth endoplasmic reticulum. However, now I can see that the curved organelle is the golgi apparatus and F must represent the whole endoplasmic reticulum.
I believe plasma membrane proteins are synthesized in the rough endoplasmic reticulum.
FA2020 p46 Rough endoplasmic reticulum Site of synthesis of secretory (exported) proteins and of N-linked oligosaccharide addition to lysosomal and other proteins.
Free ribosomesโunattached to any membrane; site of synthesis of cytosolic, peroxisomal, and mitochondrial proteins.
Smooth endoplasmic reticulum Site of steroid synthesis and detoxification of drugs and poisons. Lacks surface ribosomes. Location of glucose-6-phosphatase (last step of glycogenolysis).