Protein processing and targeting

Question 1

Give an overview of the protein secretory pathway

Answer 1

1. free ribosome initiates protein synthesis from mRNA molecule
2. hydrophobic N terminal sequence produced
3. Signal sequence recognised and bound by SRP (signal recognition particle)
4. protein synthesis stops
5. GTP-bound SRP directs the ribosome to the SRP receptors on the cytosolic face of the ER
6. SRP dissociates
7. protein synthesis continues and the newly formed peptide is fed into the ER via the peptide translocation complex (pore in the membrane)
8. signal sequence removed by signal peptidase once the entire protein has been synthesised.
9. the ribosome dissociates and is recycled.

Question 2

where about is the SRP receptor?

Answer 2

On the cytosolic face.

Question 3

What modifications occur in the endoplasmic reticulum?

Answer 3

signal cleavage (signal peptidase)
disulphide bond formation (protein disulphide isomerase) 
N-linked glycosylation (oligosaccharide-protein transferase)

Question 4

What modifications occur in the Golgi?

Answer 4

o-linked glycosylation (glycosyl transferase)
trimming and modification of N-linked oligosaccharides
Further proteolytic processing (some proteins only)

Question 5

what is proteolytic removal?

Answer 5

Removal of the N-terminal signal sequence occurring in the ER.

Question 6

Is the N-terminus hydrophobic or hydrophilic?

Answer 6

Hydrophobic

Question 7

what is removal of the ‘pro segment’

is there an example where this type of modification occurs?

Answer 7

this is further processing that occurs in some proteins in the Golgi
Preproalbumin –> proalbumin –> albumin

Question 8

Outline the formation of insulin.

Answer 8

Preproinsulin contains a signal sequence, A, B and C peptides.
1. signal sequence cleaved to proinsulin
2. endopeptidases cleave C peptide
insulin contains A and B peptides

Question 9

What bonds form between the A and B peptides in insluin

Answer 9

disulphide bridges

Question 10

What is the basic unit of collagen

Answer 10

tropocollagen

Question 11

what is the primary sequence of tropocollagen?

Answer 11

Glycine-X-Y
X and Y are often proline and hydroxyproline
Glycine every third position

Question 12

Which Amino acid residue allows for the extended alpha chain and prevents the peptide assuming another shape?

Answer 12

Proline

Question 13

How are hydroxyproline residues formed and what is the result of these residues? (hint enzyme)

Answer 13

They form from proline residues by the enzyme prolyl hydroxylase. Increases the amount of inter-chain H-bonds.

Question 14

What does prolyl hydroxylase require for activity? What would a consequence of not having prolyl hydroxylase be?

Answer 14

Vitamin C and Fe2+.
Scurvy- lack of vitamin C therefore weak tropocollagen triple helices.
Symptoms: tiredness, joint pain, muscle pain

Question 15

What are tropocollagen units originally synthesised as?

Answer 15

Preprocollagen

Question 16

why is preprocollagen produced with N and C terminal peptides?

Answer 16

To prevent formation of collagen fibres inside the cell