Endoplasmic Reticulum

ID #2445

I have a few questions: 1) Does the signal sequence stay in the translocon because it's hydrophobic? 2) if an internal signal sequence is present, the N terminus will always be in the cytoplasm right? in class we said that a stop transfer sequence would then push the protein into the membrane. what happens if there is no stop transfer sequence? 3) Does the SRP recognize the internal sequence the same way it identifies the amino terminal sequence? 4) if a stop transfer sequence is present does it cause the ribosome to disconnect from the translocon? Then, if another internal sequence is found an SRP will bring it back to the translocon? 5) with one of our homework problems, a protein is 'read' from left to right, correct? since the N terminus is in the lumen, it had an amino terminal squence. the first zigzag is a stop transfer sequence, the second is an internal signal sequence, then it alternates to the end. is that correct? if at the end of the translation, there was no stop transfer sequence, would the N and C terminals both be in the lumen?


To address your questions: 
1) Yes, the signal stays in the translocon (and on the interior of the membrane if it is an internal signal sequence or stop transfer sequence) because it is hydrophobic. 
2) Yes, a protein displaying an internal signal sequence as its first signal will have its N terminus in the cytoplasm. If it does not display a stop transfer sequence, at the conclusion of translation, the ribosome will dissociate, the translocon will close, and the internal signal sequence is kicked out of the translocon. 
3)Yes, SRP recognizes an internal signal sequence the same way. 
4)No. Once the ribosome has been brought to the translocon by SRP, and after SRP lets go of the signal and leaves, translation will proceed. The ribosome will translate regardless of whether or not the translocon is open or closed. The internal signal sequences and stop transfer sequences are what tells the translocon to open and close. 
5) Yes, I think you have it! 

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