L6- The secretary pathway

Question 1

What do all mechanisms of protein import require?

Answer 1

Energy- usually in the form of ATP and GTP

Question 2

What determines a protein’s final destination within the secretary pathway?

Answer 2

Specific sorting/trafficking signals

Question 3

What does a KDEL sequence at the C-terminus do?

Answer 3

Retains soluble proteins including molecular chaperones such as BiP + PDI at the ER

Question 4

What retains membrane proteins in the Golgi?

Answer 4

Unusually short transmembrane domains

Question 5

What results in proteins being sorted to the lysosomes?

Answer 5

Addition of Mannose-6-phosphate

Question 6

Where do these go?

1. Short TMs
2. M-6-P
3. KDEL

Answer 6

1. short transmembrane domains- Golgi
2. Mannose-6-phosphate - lysosome
3. KDEL- ER

Question 7

What are the 2 sides of the Golgi?

Answer 7

Cis face for entry, Trans face for exit

Question 8

What is the path of new proteins?

Answer 8

Leave ER in transport vesicles and enter cis Golgi network. Cargo exits trans Golgi network in transport vesicles.

Question 9

What are the 2 models (theories) for movement through sub-compartments in the Golgi?

Answer 9

1. Vesicular transport model- Golgi is static and vesicles carry proteins between the cisternae.
2. Cisternal maturation model- cisternae move with their cargo and are replaced by new cisternae. (explains proteins too big to fit into vesicles)

Question 10

What are the Golgi’s functions?

Answer 10

1. protein sorting
2. protein modification
3. O-linked glycosylation

Question 11

What happens in O-linked glycosylation?

Answer 11

The -OH side chains of Serine and Threonine residues- oligosaccharides are added on.
Tyrosine has an OH group but can;t be glycosylated.

Question 12

What do budding vesicles have?

Answer 12

Characteristic protein coat on their cytosolic face. These coat proteins help to shape the membrane into a bud and capture cargo. The coat proteins are removed after budding to allow fusion.

Question 13

What’s an example of a coat protein?

Answer 13

Clathrin, COP1, COP2

Question 14

What are the steps and the proteins involved in vesicle budding?

Answer 14

1. Clathrin coats assemble into network on cytosolic face of membrane
2. Adaptins help capture specific cargo by trapping (binding) cargo receptors into budding vesicle.
3. Dynamin- assembles around the neck of bus and pinches off vesicle.
4. Coat removed after budding

Question 15

What 2 components does recognition of transport vesicles by the appropriate target organelles rely on?

Answer 15

1. Rab proteins (small GTP binding proteins)

2. SNAREs (membrane proteins)

Question 16

What do Rab proteins do?

Answer 16

They are on the vesicle surface. Tetherin proteins on the target organelle recognise the Rab proteins. Tether pulls it towards organelle.

Question 17

What do SNARE proteins do?

Answer 17

They’re integral membrane proteins that contribute to specifity after the intial Rab mediated step. vSNAREs on vesicle. tSNAREs on target. Complementary SNAREs interact and dock the vesicle in place.