7 - Membrane Trafficking

Question 1

What are the 3 types of intracellular transport?

Answer 1

• gated tranport
• trans-membrane transport
• vesicular transport

Question 2

What is the role of the gold apparatus?

Answer 2

to package and send the proteins from the ER

Question 3

What is the cis face of the Golgi apparatus and what is the trans face?

Answer 3

cis face - closest to the ER

trans face - facing the membrane

Question 4

Give an overview of exocytosis (the order of organelles)

Answer 4

ER
Cis Golgi network
Medial Golgi network
Trans Golgi network
Cell surface via different types of secretory vesicles

Question 5

Give an overview of endocytosis

Answer 5

Materials recognised at the plasma membrane are brought in via endocytic pathway:

• first pathway to early endosome (material from this is recycles back to the plasma membrane and keep the cycle continuously)
• late endosome
• if the material is destined for destruction, it will be taken to the lysosome

Question 6

Describe the secretory/endocytic pathway

Answer 6

• membrane bound polyribosomes (on RER) make the proteins
• they are incorporated into vesicles, which bid off and are transported to the Cis Golgi apparatus
• proteins pass through the Golgi and undergo post translational modification
• reaches the Trans Golgi apparatus, where the are processed into specific transport vesicles, which go to different destinations

Question 7

How are the proteins checked in the ER (quality control)?

Answer 7

unassembled/misfolded proteins are retained in the ER and exported back into the cytosol, where they are degraded

Question 8

early vs late endosomes

Answer 8

‘Early’ endosomes are positioned close to sites of active endocytosis, where they can act as a recycling compartment for vesicles budding from the plasma membrane. In this way, early endosomes provide a sorting environment for molecules contained within endocytic vesicles.

Sorting signals on transmembrane and soluble proteins provide instructional cues regarding whether a protein will return to the plasma membrane or be packaged into membrane-bound organelles destined for the ‘late’ endosome.

Membrane-bound organelles destined for the late endosome bud from the early endosome and are transported towards the cell interior.

The late endosome is a key sorting point for membrane vesicles and their contents. Proteins destined for the lysosome arrive at the late endosome from the trans-Golgi and early endosome compartments; vesicles bud from the late endosome to travel to the trans-Golgi or lysosome.

Question 9

How does cynic fibrosis affect membrane trafficking?

What is the mechanism behind this?

Answer 9

blocks the exit of proteins from the ER

Mutation means that CFTr does not fold properly and is degraded in the ER.
CFTR is a chloride channel in epithelial cell plasma membranes. Mutations of the CFTR gene affects the functioning of the chloride channels, which causes CF.

Question 10

What happens to the proteins in the Golgi?

Answer 10

the carbohydrate structures are modified:
- removal/addition of sugars
in the Trans Golgi network, the proteins are sorted into vesicles

Question 11

How do proteins ensure that they are transported to the correct compartment?

Answer 11

• targeting/sorting signals (lysosomes)
• retention signals
• retrieval signals