Post-translational Protein Processing

Question 1

What are the 2 main types of additional processing proteins can undergo after translation?

Answer 1

1. Proteolytic cleavage (= breaking peptide bonds to remove part of the polypeptide chain)
2. Chemical modification (= addition of functional groups to aa residues)

Question 2

What are the functions of the ER in post-translational protein processing?

Answer 2

1. Protein insertion into membranes
2. Proper protein folding
3. Assembly of multi-subunit proteins
4. Protein modification

Question 3

Which types of protein modification occur in the ER?

Answer 3

1. Proteolytic cleavage
2. Hydroxylation of prolyl and lysyl residues
3. Disulphide bond formation - protein disulphide isomerase
4. N-linked glycosylation

Question 4

What type of modification is likely to occur in extracellular proteins secreted into hostile environment?

Answer 4

Disulphide bond formation (PDI)

Question 5

What is N-linked glycosylation and why is it important?

Answer 5

• Addition of sugars on asparagine side chain.
• Important for:
  
  1. correct protein folding
  2. protein stability
  3. facilitates interactions with other molecules (cell-cell recognition via glycosylated receptors)

Question 6

What do deficiencies in N-linked glycoslyation cause?

Answer 6

Severe inherited CONGENITAL DISORDERS OF GLYCOSYLATION.

Question 7

How does protein disulphide isomerase mediate disulphide bone formation?

Answer 7

Going from oxidised PDI to reduced PDI.

Question 8

What problems might occur in protein folding?

Answer 8

Protein may be:

• trapped in mis-folded conformation
• contain mutation resulting in mis-folding
• incorrectly associated with other sub-units

Question 9

How might incorrect protein folding be corrected?

Answer 9

ER chaperone proteins (e.g. Calnexin, calreticulin and immunoglobulin protein) attempt to correct problem.

Question 10

What happens if protein misfolding cannot be corrected?

Answer 10

1. Retain unfolded protein in ER - acts as sensor to ‘monitor’ extent of protein mis-folding: mediate increased transcription of chaperones or reduced translation.
2. Protein returned to cytosol for degradation (e.g. CFTR in CF).
3. Protein accumulates to toxic level in ER causing disease. May arise due to single mutation, e.g. Connexin in CHARCOT-MARIE-TOOTH SYNDROME.

Question 11

What type of protein modification occur in the Golgi?

Answer 11

O-linked glycosylation = addition of sugar to hydroxyl (-OH) of serine/threonine.

Question 12

In which type of protein is O-linked glycosylation important?

Answer 12

Proteoglycans (ECM & mucus secretions component)

Question 13

Describe the post-translational modification of insulin.

Answer 13

Endoplasmic reticulum

1. Proteolytic cleavage of signal peptide: preproinsulin to proinsulin.
2. Disulphide bond formation between A chain and C chain.

Golgi

3. Proteolytic cleavage of B chain.
4. Trimming of carboxyl termini.