Membrane Trafficking

Question 1

What are the three parts of the Golgi Apparatus?

Answer 1

Cis, medial and trans

Question 2

What are the two types of secretory pathways and what is the difference between them?

Answer 2

Constitutive – it is unregulated and acts as a shuttle vesicle to the membrane
Regulatory – it is regulated. Vesicles in this pathway must receive a signal (e.g. a hormone or neurotransmitter) before the material in the vesicles can be exocytosed. Found in excitatory cells.

Question 3

Describe the passage of lysosomal enzymes in the secretory pathway.

Answer 3

Lysosomal hydrolase precursors move from the ER to the cis Golgi
The mannose on the lysosomal hydrolase is phosphorylated by phosphotransferase
The phosphorylated sugar acts as a tag
It moves through the Golgi apparatus and binds to mannose-6-phosphate receptors in the trans Golgi
Once bound, the vesicles move to the late endosome
The late endosome has a proton pump which pumps protons into the late endosome thus making it acidic
The acidity allows the dissociation of the M6P receptor
Phosphohydrolases remove the phosphate from the lysosomal hydrolase –meaning it can’t return to the Golgi
Lysosomal hydrolases accumulate in the late endosome and it matures into a lysosome

Question 4

What are the three fates of endocytosed material?

Answer 4

Degradation, transcytosis and recycling

Question 5

Give an examples of something that uses the degradation pathway.

Answer 5

LDL binds to LDL receptors and form clathrin-coated vesicles These vesicles move to the early endosome (clathrin coat is removed along the way)
From there, the LDL receptors are recycled back to the cell surface and LDL is transported to the lysosome where it is degraded to form free cholesterol

Question 6

Give an example of a disease of endocytosis

Answer 6

Familial Hypercholesterolaemia – caused by mutation in the LDL receptor