Metabolism 8 - Membrane Trafficking

Question 1

How are proteins directed to the correct compartment?

Answer 1

Signal sequences present in the protein direct them to the correct compartment.

Question 2

List the three types of intracellular transport.

Answer 2

• Gated transport
• Transport across membranes
• Vesicular transport

Question 3

How are proteins imported inot the nucleus?

Answer 3

• This occurs by gated transport. The nuclear pores contain componants that form mesh like structures to prevent passage of large molecules, allowing small water-soluble molecules to pass freely.
• Nuclear proteins can be imported by import receptors that recognise nuclear localisation signals.

Question 4

What is an example of transport across membranes?

Answer 4

The translocation of newly synthesised proteins into the ER.

Question 5

What happens if proteins are destined for other organelles?

Answer 5

The proteins are first directed into the ER, and then move through the golgi.

Question 6

What happens to unassembled or misfolded proteins?

Answer 6

They are retained in the ER and exported back to the cytosol where they are degraded.

Question 7

What occurs in cystic fibrosis?

Answer 7

• Cystic fibrosis is an example of a disease which occurs due to misfolding of a protein.
• Mutstions in the CFTR gene result in non-functioning chloride channels being produced. These CFTRs are degraded.

Question 8

What do discodin domain receptor 2 mutations cause?

Answer 8

SMED-SL. This is a disease caused by misfolding of a protien causing short limbs and abnormal calcifications.

Question 9

What are the stages in the inward endocytic pathway?

Answer 9

Extracellular molecules are injested, forming the early endosome, they are delivered to late endosomes and then to lysosomes.

Question 10

What occurs during transport through the golgi apparatus?

Answer 10

The golgi has a cis face (entry face), mid revion, and a trans face (exit face). The cis face points towards the ER. Sugar chains added to the protein are modified here, with different enzymes present in different regions of the golgi.

Question 11

What are the two types of exocytosis?

Answer 11

• Constitutive happens constantly in all cells.

- Regulated only happens following an extracellular signal, and occurs in specific cell types.

Question 12

Describe the process of vesicle budding.

Answer 12

• Clatherin coated vesicles contain clatherin. They assemble into a basket like shape, and then pinch off due to the action of dynamin.
• Adaptin attaches the vesicle receptors to clatherin.
• Uncoating occurs, allowing the vesicles to interact with target cell membranes.y

Question 13

Describe the process of membrane fusion.

Answer 13

• Membranes must be 1.5nm apart to fuse.
• Tethering/docking proteins bind to receptors, bringing the vesicle and target membrane together.
• This produces a signal in the cell, causing the vesicle to release its contents inside.

Question 14

Describe the process of receptor mediated endocytosis using the LDL receptor.

Answer 14

• The LDL molecule binds to receptors in the cell membrane, forming a clatherin coated pit.
• A clatherin coated vesicle is formed, and uncoating occurs.
• The vesicle fuses with the endosome
• LDL is delivered to the lysosome (free cholesterol is released)
• LDL receptors are returned to the plasma membrane via transport vesicles.