Lecture 14- Vesicular trafficking I

Question 1

Outline the secretory pathway

Answer 1

1. Newly synthesised proteins are made on ribosomes on the ER
2. Proteins are moved and packaged into vesicles
3. Vesicle moves through the Golgi complex and sorted at the TGN
4. Either targeted to lysosomes, secretory vesicles or the cell surface

Question 2

What mediates the secretory pathway movement?

Answer 2

Vesicles and tubules

Question 3

What is the role of SNAREs?

Answer 3

• SNAREs mediate the targeting of vesicles

* SNAREs ensure the correct vesicle docks with the correct target compartment

Question 4

Give 3 transport vesicle coats

Answer 4

1. COPI
2. COPII
3. Clathrin

Question 5

What is post-translational translocation?

Answer 5

Where cytosolic ribosomes complete the synthesis of the protein and release it into the ER prior to translocation

Question 6

What is co-translational translocation?

Answer 6

The proteins are being delivered into the ER lumen as they are being transcribed (before the complete synthesis of the polypeptide chain)

Question 7

What is the signal hypothesis?

Answer 7

A protein to be translocated into the ER will have a signal sequence on it which is recognised by translocating machinery

Question 8

Which translocon is involved in ER translocation?

Answer 8

Sec61

Question 9

How is the signal peptide removed?

Answer 9

Cleaved by a signal peptidase

Question 10

Outline how membrane proteins are inserted into the ER

Answer 10

1. The co-translational translocation process is initiated by an N-terminal ER start-transfer signal sequence which opens the translocator
2. As the protein is being translated, it’s recognised by Sec61
3. The protein also contains a stop-transfer sequence which enters the translator and leads to the protein being discharged
4. The signal peptidase is cleaved and protein is freed from Sec61
5. Left with a protein in the membrane which is anchored by its hydrophobic TM domain

Question 11

Compare type I and type II membrane proteins

Answer 11

Type I: N-terminal in the ER lumen, C-terminal in the cytosol

Type II: N-terminal in the cytosol, C-terminal in the ER lumen

Question 12

What is the role of chaperone proteins in the ER?

Answer 12

Help and ensure correct protein folding and structure

Question 13

Give an example of a chaperone protein and it’s role in the ER?

Answer 13

BiP

• each antibody chain is folded separately and remains associated with BiP until all chains have assembled
• the incomplete antibody molecules are retained in the ER as associated with BiP
• only when they are fully formed can they be transported via a COPII vesicle through the secretory pathway

Question 14

What are the 3 essential components for all transport vesicle formation?

Answer 14

1. Small GTPases
2. Adaptor proteins
3. Coat

Question 15

States the components of COPII

Answer 15

1. GTPase: Sar1
2. Adaptor protein: Sec23/24
3. Coat: Sec13/31

Question 16

Outline how a COPII vesicle is formed

Answer 16

1. Exit signal on cargo protein is recognised by cargo receptor to ensure it is sorted into a COPII vesicle
2. Sar1-GTP is activated by a GEF on the ER membrane
3. This activations causes the recruitment of the adaptor proteins Sec23/24
4. Sec23 interacts with Sar1-GTP and Sec24 interacts with the cargo
5. The activated adaptor proteins recognise the cargo and directly link to the coat