Protein Targeting and Collagen synthesis

Question 1

Where do protein synthesised by free ribosomes go? Where do proteins synthesised by ribosomes on the RER go?

Answer 1

• Proteins destined for cytoskeleton or post-translational import into organelles.
• Protein destined for membrane or secretory pathways via co-translational insertion.

Question 2

What are the two types of secretion

Answer 2

1. Constitutive
2. Regulated
   - Endocrine cells - secreting hormones
   - Exocrine cells - secreting digestive juices
   - Neurocrine cells - secreting neurotransmitters

Question 3

What is required for protein sorting?

Answer 3

• A signal, intrinsic to the protein (where to take the protein)
• A receptor to recognise the signal and which directs it to the correct membrane
• Translocation machinery
• Energy to transfer the protein to its new place

Question 4

What is a signal sequence?

Answer 4

• Always at the N terminal as sequence
• 5- 30 amino acids in length.
• Central region rich in hydrophobic residues.
• Able to form a-helix

E.g. preproalbumin - the pre part defines the signal sequence which is removed during processing.

Pg 50

Question 5

What occurs in the translation of a secreted protein?

Answer 5

1. We start to make the secreted protein in free ribosomes, as soon as the signal sequence comes out of the ribosome, it is recognised the Signal Recognition Particle (SRP).
2. The SRP has affinity for the signal sequence and it will bind strongly to it and when it binds, its stops any further translation.
3. The complex containing the SRP and the newly created protein binds to the endoplasmic reticulum via a signal recognition particle receptor.
4. Associated with the SRP receptor is a translocon, which is a channel that allows the newly synthesised protein to pass through into the lumen at endoplasmic reticulum.
5. Once binding occurs, there is a hydrolysis of GTP for GDP, to produce energy that drives the opening of the channel and allows translation to start again.
6. The newly formed protein is then extruded into the lumen of the endoplasmic reticulum.
7. As soon as the signal sequence entered the endoplasmic reticulum it is chopped off by signal peptidase, as we don’t need it anymore.
8. Then the rest of the protein is formed and folds.

Question 6

What are the functions of the endoplasmic reticulum?

Answer 6

Proteins go into the endoplasmic reticulum to undergo further modification, this is post translational modification.

• Insertion of proteins into membranes
• Specific proteolytic cleavage
• Glycosylation
• Formation of S-S bonds
• Proper folding of proteins
• Assembly of multisubunit proteins
• Hydroxylation of selected Lys and Pro residues

Question 7

What is the function of the Golgi apparatus?

Answer 7

• Movement of proteins to the cis-Golgi through budding
• Further protein modifications
• Release through trans-Golgi to membrane/organelles
• Endoplasmic reticulum is continuous with the Golgi apparatus, the Golgi is important for further protein modification.

Question 8

What is needed by prolyl hydroxylase?

Answer 8

• Associated with PDI (protein disulphide isomerase) in the ER.
• Requires vitamin C and Fe++ ions for activity
• allows increased H-binding to stabilise triple helix
• Scurvy is due to weak tropocollagen triple helices

Question 9

How is the triple helix formed?

Answer 9

• Pro alpha collagen chains come together to form the triple helix.
• In the ER, there are 3 pro alpha collagen chains, which are initially aligned by the formation of disulphide bonds in the C terminal region, then the triple helix forms.

Question 10

What enzyme converts procollagen to tropocollagen?

Answer 10

Procollagen peptidase

Question 11

What enzyme forms covalent binds between lysine residues?

Answer 11

Lysyl oxidase