Lecture 11 - Translation, Transduction and Insertion

Question 1

If a protein has an ER signal sequence then what is its fate?

Answer 1

Targeted to the ER where it either becomes a membrane protein or a secretory lumenal protein.

Question 2

If a protein does not have a signal sequence then what is its fate?

Answer 2

Ends up as a cytosolic protein, e.g. glycolysis enzymes.

Question 3

The ER signal sequence is found where? How long is it?

Answer 3

At the N terminus. 6-12 AAs long. Hydrophobic.

Question 4

Signal sequences can be two forms: explain this.

Answer 4

Linear: primary AA sequence, or formed when the protein is in its 3D structure.

Question 5

Process of co-translational ER targeting?

Answer 5

1. Proteins to be targeted to the ER initially have an ER signal sequence at the N terminus.
2. Signal Recognition Particle (SRP) binds to the ER signal sequence.
3. The SRP binds to the SRP receptor to dock the ribosome.
4. When the SRP binds GTP, the growing polypeptide is translocated through a hydrophilic pore created by one (or more) membrane proteins called the translocon (Sec61).
5. The SRP is released by the hydrolysis of GTP.
6. When the polypeptide is complete, a signal peptidase cleaves the signal to release the protein into the ER lumen while retaining the signal peptide in the membrane.
7. Afterwards the ribosome is released and the pore closes.

Question 6

Why does the SRP cause translational arrest?

Answer 6

Gives the ribosome time to attach to the ER membrane.

Question 7

The translocon is also called the…

Answer 7

Sec61 complex.

Question 8

The translocon consists of?

Answer 8

4 subunits = 2 alpha, 2 beta.

Question 9

Post-translation insertion into the ER can also occur. Uses sec61 but also an additional complex of?

Answer 9

Sec62, sec63, sec71 and sec72.

Question 10

What does the complex involved in post-translational ER insertion do?

Answer 10

Deposits BiP molecules onto the translocating chain as it emerges into the ER lumen. ATP-driven cycles of binding and release pull the protein into the lumen.

Question 11

What is the destiny of proteins that enter the lumen of the ER?

Answer 11

They are either going to be secreted or targeted to the lumen of an organelle.

Question 12

Membrane proteins contain start/stop transfer sequences. There are two types. Name them.

Answer 12

1. Polypeptide with an internal stop-transfer sequence and a terminal ER sequence.
2. Polypeptide with only a single start-transfer sequence.

Question 13

Polypeptide with an internal stop-transfer sequence and a terminal ER sequence.

Answer 13

• Stop-transfer sequence halts the process of translocation into the ER lumen.
• Stop-transfer sequence = 8-12 hydrophobic AAs.
• Signal peptidase cleaves the ER signal sequence, creating a TM protein with the N terminus in the lumen (type 1).

Question 14

Polypeptide with a start-transfer sequence only.

Answer 14

• Binds to the translocon.

- Translocation continues. Forms a type II membrane protein.