Protein Processing

Question 1

Describe the constitutive pathway

Answer 1

Continuous process, protein processed and released by exocytosis

Question 2

What is regulated secretion?

Answer 2

Protein released in response to a signal. Protein packaged into vesicles but not released until stimulus released.

Question 3

Which enzyme is involved in signal cleavage?

Answer 3

Signal Peptidase

Question 4

Which enzyme is involved in the formation of disulphide bonds?

Answer 4

Protein disulphide isomerase (disulphide bonds between cysteine residues on free -SH groups).

Question 5

Which enzyme is involved in N-Linked glycosylation?

Answer 5

Oligosaccharide protein transferase

Question 6

Describe O-linked glycosylation

Answer 6

• Modification of hydroxyl groups on serine and threonine

- Glycosyl transferase builds up a sugar chain from nucleotide sugar substrates

Question 7

Where does O-linked glycosylation take place

Answer 7

only in golgi

Question 8

Describe N-linked glycosylation

Answer 8

• sugars are added on an Asparagine side chain

- Occurs in ER

Question 9

Endoproteases are sequence specific, true or false?

Answer 9

True (break peptide bonds of non-terminal amino acids)

Question 10

Name two exoproteases

Answer 10

Amino peptidase, carboxypeptidase (break peptide bonds of terminal amino acids)

Question 11

Describe the formation of insulin

Answer 11

preproinsulin transcribed containing a N terminal signal sequence, A, B and C peptides

signal sequence cleaved by signal peptidase

Proinsulin left - contains A, B and C peptides

endopeptidases cleave B peptide

leave mature insulin consisting of 2 strands linked by disulphide bridges

Question 12

What is a good marker for measuring endogenous insulin

Answer 12

Peptide C

Question 13

What is a nuclear localisation sequence?

Answer 13

A run of 4-8 basic amino acids somewhere in the primary structure that targets to the nucleus

Question 14

How are ER resident proteins retained within the lumen of the ER?

Answer 14

1) the proteins have a lsy-asp-glu-leu sequence near the C-terminus which interact with KDEL receptor on the golgi
3) sent back to ER in transport proteins
4) KDEL receptor transported back

Differences in pH in the golgi and ER mediate the receptor recycling. (ATP dependent proton pump maintains a pH of 6.5 in the golgi)

Question 15

Why is glycosylation important?

Answer 15

• correct protein folding

- protein stability

Question 16

What are ER chaperone proteins?

Answer 16

• Examples include BIP, calnexin, calreticulin
• They act as sensors to measure extent of protein misfolding
• If misfolding can’t be corrected, protein may return to cytosol for degradation or it could accumulate in the ER causing toxicity