Lecture 7: Defects in protein targeting and folding

Question 1

What disease is associated with defective mitochondrial targeting

Answer 1

Congenital lactic acidosis

Question 2

How does targeting defects cause congenital lactic acidosis?

Answer 2

1. Point mutation in pyruvate dehydrogenase targeting sequence
2. Mutant MTS has insufficient import and not enough pyruvate dehydrogenase in mitochondria
3. Pyruvate accumulates
4. Produces lactate which causes lactic acid build up in blood
5. Causes congenital lactic acidosis

Question 3

What types of proteins will a defective ER targeting sequence effect?

Answer 3

Secretory

Question 4

Example of a disease associated with defective ER targeting

Answer 4

Early onset diabetes

Question 5

Defective ER targeting : Early onset diabetes

Answer 5

1. Point mutation in insulin signal sequence
2. Doesn’t interact properly with Sec61 so doesn’t enter ER properly
3. Less insulin is made
4. Incorrectly localised to cytosol and form toxic aggregates
5. Causes B cell death

Question 6

What are the two things that can occur when proteins are incorrectly targeted?

Answer 6

Localise incorrectly to cytosol because its the default
Broken down into amino acids by proteolysis

Question 7

What occurs when proteins are broken down into amino acids by proteolysis?

Answer 7

1. Large protease complex degrades proteins by proteolysis
2. Short lived and misfolded proteins are degraded
3. They are marked for proteolysis by being attached to ubiquitin
4. The polyubiquitin chain is recognised by the proteasome

Question 8

How do misfolded proteins cause disease?

Answer 8

They are prevented from reaching its target site by chaperones
The target lacks a functional protein which causes disease

Question 9

Cystic fibrosis

Answer 9

Recessive genetic dissorder
1/25 are carriers
caused by mutations in a chloride channel called CFTR

Question 10

Role of CFTR in CF

Answer 10

1. Chloride channel CFTR pumps Cl- ions out of epithelial cell
2. This keeps mucus on surface of cells hydrated
3. Allows cilia to beat and remove bacteria
4. Lack of CFTR channel dehydrates the mucus and cilia can’t function

Question 11

What is the most common mutation in the CFTR that causes cystic fibrosis?

Answer 11

Deletion of phenylalanine at position 508
dF508 can’t fold properly and is kept by ER quality control

Question 12

What happens to misfolded CFTR

Answer 12

Recognised by ER quality control
Tagged with ERAD (ER-associated degradation)
Moved back into cytosol and degraded by proteasome

Question 13

What is one proposed method of treating dF508 cystic fibrosis?

Answer 13

The misfolded protein still works as a channel
Drugs could enhance folding and allow it to escape from ER quality control
This can be done through pharmacological chaperones or correctors

Question 14

How can misfolded proteins cause a build up?

Answer 14

The misfolded protein isn’t degraded properly and causes a build up in the ER
Triggers unfolded protein response

Question 15

What is the process of the unfolded protein response?

Answer 15

Increase expression of chaperones and stop protein synthesis
If homeostasis isn’t restored it causes apoptosis

Question 16

What are three methods of treating diseases caused by misfolded protein accumulation?

Answer 16

Reducing synthesis of mutant protein
Stimulating degradation of mutant protein
Drugs altering UPR signalling

Question 17

What is the treatment of reducing synthesis of a mutant protein?

Answer 17

Gene editing
RNA interference

Question 18

What is the treatment of stimulating degradation of mutant protein?

Answer 18

Proteasome activators
Autophagy enhancers

Question 19

What is the treatment of drugs altering UPR signalling?

Answer 19

Prevent activation of apoptosis
Increase protein ER folding and degradation capacity