Secretion: Taking the Rough ER Path

Question 1

How do polypeptides enter the ER, co or post-translationally?

Answer 1

Although we used to believe all proteins entered cotranslationally, we now know that the majority enter cotranslationally, but some enter post-translationally.

Question 2

What happens after homogenizing the ER?

Answer 2

after breaking open the cel, ER sheets break apart and form microsomes - pieces of ER in ball form that have been resealed

Question 3

After centrifugation of microsomes, which kind are denser?

Answer 3

Rough microsomes are denser

Question 4

How to create detached polysomes?

Answer 4

Collect rough microsomes via centrifugation, then treat them with nonionic detergents that solubilize the membrane’s lipids without denaturing proteins.

Question 5

What to do to rough ER microsomes to create polysomes?

Answer 5

Add a non ionic detergent that solubilizes the membrane’s lipids without denaturing proteins

Question 6

Describe the signal model of co-translational import into the ER.

Answer 6

1. The ribosome attaches to mRNA.
2. The first amino acids translated create a code that signals for the SRP (signal recognition particle) protein complex to come bind. When the SRP binds to the signal sequence, translation is arrested.
3. The nascent polypeptide is inserted into the translocon, providing passage into the lumen of the ER.
4. The SRP binds to the SRP receptor in the cytosolic side of the ER membrane, bringing along the nascent polypeptide.
   5.