ER

Question 1

What do proteins targeted to the ER have?

Answer 1

N-terminal signal sequences

Question 2

What are signal sequences?

Answer 2

Short hydrophobic sequences

Question 3

What are signal sequences recognised by?

Answer 3

Signal recognition paritcle - SRP

Question 4

What process does SRP pause and how?

Answer 4

Pauses translation preventing proteins with signal sequences from folding in cytosol. SRP54 subunit binds to singal sequence.

Question 5

What does the SRP deliver to the ER

Answer 5

The ribosome, SRP binds to STP-receptor on ER membrane, SRP is deplaced and ribosome docks onto SEC61

Question 6

On binding onto what translocon does translation resumes?

Answer 6

SEC61, which forms a pore across ER membrane.

Question 7

What is BiP?

Answer 7

Chaperone protein, luminal HSP70. Helps proteins fold by binding to unfolded regions

Question 8

What type of environment is the ER?

Answer 8

Oxidising

Question 9

What is correct disulphide bond formation between Cys res is promoted by?

Answer 9

Protein disulphide isomerases

Question 10

What do most proteins in the ER undergo?

Answer 10

N-linked glycosylation

Question 11

What is added to proteins in the ER (N-linked glycosylation)

Answer 11

An oligosaccharide chain is added by oligosaccharyltransferase to asparagine residues

Question 12

What are oligosaccharyltransferases associated with?

Answer 12

SEC61 translocon and glycosulates proteins

Question 13

What does the oligosaccharide chain is comprised of?

Answer 13

2 N-acetylgluosamines, 9 mannoses and
3 glucoses

Question 14

Monitoring protein folding with glycosylation

Answer 14

• 1st the N-linked oligosaccharide is trimmed leaving a single glucose
  residue
• The protein is then recognised by the chaperone calnexin and the
  associated protein ERp57, which bind to the oligosaccharide and free
  cysteines respectively and prevent unfolded proteins aggregating
  together
• When the protein is released by calnexin/ERP57 the remaining glucose
  is trimmed
• If the protein is properly folded it can exit the ER
• If the protein fails to fold a glucose is added to the oligosaccharide by a
  glucosyl transferase that recognises unfolded proteins and it reenters the
  cycle.

Question 15

How does ER remove misfolded protein?

Answer 15

ERAD

Question 16

What is ERAD?

Answer 16

ER-associated degradtion

Question 17

What catalyses the slow removal of mannose residues from N-linked oligosaccharide chains?

Answer 17

Mannosidases (EDEMs) which stop re-addition of glucose and marks protein for degradation

Question 18

What is recognised by ERAD machinery?

Answer 18

Mannose-trimmed N-glycans, which are then ubiquitinated and translocated and translocated in the cytosol.

Question 19

What is recognised by cytosolic proteasome?

Answer 19

Ubiquitinated proteins (mannose-trimmed glycans)

Question 20

CFTR

Answer 20

Plasma membrane channel that transports Cl- ions, expressed in epithelial cells

Question 21

CFTR delta-F508 mutant

Answer 21

Autosomal recessive mutation, loss of res508.

Question 22

Is all of CFTR deltaF508 degraded by ERAD

Answer 22

Yes

Question 23

Does CFTR deltaF508 retain some capacity to transport chlorine ions

Answer 23

Yes

Question 24

How can we get CFTR deltaF508 out of the ER?

Answer 24

Lumcaftor which is a chemical chaperone, limited benefit.

Question 25

What can lumcaftor be used in combination with

Answer 25

Ivacaftor, channel opener

Question 26

How does cytomegalovirus hijack the ERAD pathway

Answer 26

US2 and US11 are ER membrane proteins and can cause MHC I class molecules to enter ERAD. HCMV infected cells can escape detection by cytotoxic T cells

Question 27

Molecular mechanism of cholera toxin

Answer 27

B subunit binds to GM1 glycolipid on gut lining cells. CTA-1 once in cytosol activated adenylate cyclase increase cAMP, which activates protein kinase A. PKA phsophorylates CFTR promoting Cl- secretion