9. Post Translational Processing

Question 1

What are 2 ways that proteins may be modified after translation?

Answer 1

1. Proteolytic cleavage - zymogens

2. Chemical modification - phosphorylation

Question 2

Which proteins are synthesised on free ribosomes?

Answer 2

Proteins destined for the cytosol or import into organelles (not lysosome)

Question 3

Which proteins are synthesised in the RER?

Answer 3

Proteins destined for membrane or secretory pathway via co-translational insertion, or lyosome.

Question 4

What is required for protein sorting?

Answer 4

1. A signal
2. A receptors that recognises the signal and direct it to correct membrane
3. Translocation machinery
4. Energy

Question 5

What is the protein targeting sequence for peroxisome proteins?

Answer 5

Serine-Lysine-Leucine (SKL), peroxisome targeting sequence (PTS)

Question 6

What receptor recognises the PTS?

Answer 6

PTS receptor (Pex 5) recognises the signal and binds to cargo in the cytoplasm.

Question 7

How does the PTS receptor-cargo complex enter the peroxisome?

Answer 7

13 pex proteins make up a transport channel across the peroxisome membrane, it binds to the pex5-cargo complex.

Question 8

Which stage of peroxisome target requires ATP?

Answer 8

Recycling of the PTS receptor back to the cytosol.

Question 9

Name a perixisome biogenesis disorder.

Answer 9

Zellweger syndrome

Question 10

Where is the signal sequence of secretory proteins?

Answer 10

N-terminal

Question 11

What is the signal sequence for secretory proteins?

Answer 11

5-30 amino acids with a central region rich in hydrophobic residues.
It is able to form an alpha helix which gives it the ability to cross the membrane.

Question 12

When is the signal sequence removed?

Answer 12

On entry to the ER

Question 13

What recognises the signal sequence for secretory proteins?

Answer 13

Signal recognition particle recognises the signal peptide AND the ribosome

Question 14

Following SRP binding to the signal sequence, how does the protein enter the ER?

Answer 14

When SRP recognises the sequence, it binds and stops translation on the free ribosome, the complex then binds to the SRP receptor on the ER membrane and protein synthesis re-starts with the peptide synthesised into the ER lumen. The SRP detached and the sequence is chopped off.

Question 15

What sequence is in membrane proteins?

Answer 15

Stop-transfer sequence

Question 16

How does the stop-transfer sequence allow protein integration into the membrane?

Answer 16

It holds the transfer of peptide across the ER membrane and acts as an anchor to hold the protein in position. The alpha helical membrane spanning region becomes inserted into the membrane and synthesis continues. Can have >1 transmembrane region.

Question 17

What are the functions of the ER?

Answer 17

• Insertion of proteins into membranes
• protein modification - proteolytic cleavage, glycosylation, formation of disulphide bonds
• proper folding of proteins
• assembly of multisubunit proteins
• hydroxylation of residues - collagen

Question 18

What is N-linked glycosylation and where does it take place?

Answer 18

sugar molecules are added to proteins on an asparagine side chain through a nitrogen group.
In ER

Question 19

Why is glycosylation of proteins important?

Answer 19

• correct protein folding

- protein stability

Question 20

What proteins are most likely to have disulphide bonds?

Answer 20

Extracellular proteins, as its a more hostile environment and disulphide bonds are strong, covalent.

Question 21

Which enzyme is involved in the formation of disulphide bonds in the ER?

Answer 21

Protein disulphide isomerase (PDI)

• ensures bonds are formed in the correct places
• removes incorrect bonds

Question 22

What happens if proteins are misfolded?

Answer 22

ER chaperone proteins allow refolding to try and correct the protein.
They retain unfolded proteins in the ER.

Question 23

Name some ER chaperone proteins.

Answer 23

Bip- binding immunoglobulin protein
Calnexin
Calreticulin

Question 24

What is the benefit of chaperone proteins in ER acting as ‘sensors’ to monitor the extent of protein mis-folding?

Answer 24

They can increase transcription of charter ones or reduce translation of proteins if constant misfolding.

Question 25

If a misfolded protein cannot be corrected in the ER, explain the 2 things that could happen.

Answer 25

1. returned to cytosol for degradation

2. Accumulates to toxic levels in the ER and causes disease

Question 26

How are mutations linked to protein folding?

Answer 26

A single mutation may result in protein misfolding and toxic accumulation.

Question 27

What is O-linked glycosylation and where does it occur?

Answer 27

Attachment of a sugar to an OH group of serine, threonine.

Occurs in Golgi App.