MGD Session 6 INCOMPLETE

Question 1

What is meant by constitutive secretion?

Answer 1

It is a continuous process of protein regulation. The proteins are packaged into vesicles and are continually released by exocytosis.

Question 2

Give 2 proteins secreted by constitutive secretion.

Answer 2

Serum albumin, collagen

Question 3

What is regulated secretion?

Answer 3

Proteins are released in response to a signal. They are packaged into vesicles but not released until the stimulus is received.

Question 4

Give an example of a protein secreted by regulated secretion.

Answer 4

Insulin

Question 5

Explain the first 5 steps of the protein secretion pathway.

Answer 5

1. Free ribosome initiates protein synthesis from mRNA molecule
2. Hydrophobic N-terminal signal sequence is produced.
3. Signal recognition particle recognises newly formed protein & synthesis stops.
4. SRP (GTP-bound) directs the ribosome to the SRP receptors on the cytosolic face of the ER.
5. SRP dissociates

Question 6

Explain what happens after the SRP dissociates in the protein secretion pathway.

Answer 6

1. Protein synthesis continues and feeds the new polypeptide into the ER via a pore in the membrane (peptide translocation complex)
2. Signal sequence is removed by a signal peptidase
3. Ribosome dissociates and is recycled.

Question 7

What are the 4 things required for protein sorting?

Answer 7

A signal intrinsic to the protein
A receptor that recognises the signal and directs it to the correct membrane
A translocation machinery
Energy to transfer the protein to its new place

Question 8

Where do you find the signal sequence on a protein?

Answer 8

At the N-terminus.

Question 9

Name 3 kinds of cells where regulated secretion takes place

Answer 9

Endocrine cells
Exocrine cells
Neurocrine cells

Question 10

What is SRP?

Answer 10

Signal Recognition Particle.
It is a multidomain riboprotein that mediates 3-way association with the SRP receptor in the ER, the ribosome and the signal peptide.

Question 11

What protein modifications can take place in the ER? (name 3 and the enzymes that they require)

Answer 11

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

Question 12

What protein modifications can take places in the golgi apparatus? Name 3

Answer 12

O-linked glycosylation by glycosyl transferase
Trimming and modification of N-linked oligosaccharides
Proteolytic processing

Question 13

What is N-linked glycosylation?

Answer 13

A carbohydrate is added via N-glycosyl link to the nitrogen of the amide group of an asparagine amino acid from a Dolichol phosphate carrier molecule.

Question 14

What is Dolichol phosphate?

Answer 14

A long chain hydrocarbon molecule that inserts into the membrane of the ER with its phosphate group protruding.and an oligosaccharide attached.

Question 15

Where does O-linked glycosylation take place?

Answer 15

Golgi apparatus

Question 16

What is O-linked glycosylation?

Answer 16

Carbohydrate is added via a glycosidic link to the hydroxyl group on Serine or Threonine. Glycosyl transferase builds up a sugar chain from Nucleotide sugar substrates.