Question

1) What determines whether a protein will be translated in the cytosol or on the ER surface?

2) What is the difference between post-translational ER import and co-translational translocation?

3) Can you explain the steps that ensure a protein is translated in conjunction with the ER?
   ( Including names. What is the SRP/SRP receptor/translocon, etc, and how do they help?)

Answer 1

1)proteins during translation are targeted to ER if they have a signal sequence of about 20 to 30 amino acids, if they don't have these specific sequences then they remain in cytosol.

2) there are two ways for a protein to make its contact with the membrane

the nascent protein may remain associated with the translocation apparatus while being synthesized on the ribosomes. this is called as co-translational translocation.

another way is that the protein gets released from ribosomes after translation and then the complete protein is diffused to the appropriate membrane and associates with the translocation apparatus. This is called post-translational ER import.