10 - Cell Membrane Trafficking

Question 1

Why do eukaryotic cells need membrane trafficking?

Answer 1

• For sequential modifications proteins need to be exposed to different sets of enzymes.
• Its important for the retrieval of proteins back to their resident compartment.

Question 2

What are some major features of membrane trafficking pathways?

Answer 2

• Secretory/Exocytic (Biosynthetic)
• Endocytic (recycling/degredation)

Question 3

What is the secretory pathway?

Answer 3

Traffics from the ER to the Golgi to the endosome/lysosome/Plasma membrane.

Question 4

What is the endocytic pathway?

Answer 4

Traffics from the cell surface to the endosome to the Golgi/ER/lysosome

Question 5

How are proteins modified?

Answer 5

By the addition of oligosaccharides in the ER and processing in the Golgi.

Question 6

How are proteins modified in the ER specifically?

Answer 6

By the addition of pre-formed oligosaccharides to an asparagine amino acid in a consensus sequence.

Question 7

How are proteins modified in the Golgi?

Answer 7

The oligosaccharide group is trimmed and later further sugars can be added and the structure can be further branched.

Question 8

What kind of glycosylation can happen in proteins?

Answer 8

Both N and O linked
- N from asparagine and OH from groups om serine or theanine.

Question 9

Why do we need glycolysation?

Answer 9

• To assist in folding
• Acts as a ligand

Question 10

What makes a model suitable for studies on membrane trafficking?

Answer 10

• Simplicity- single celled organisms will be better for showing things that occur on a cellular scale.

Question 11

What was the rationale behind using yeast to identify genes involved in the secretory pathway?

Answer 11

If proteins couldn’t be secreted the cell will increase in density as the vesicles accumulate.

Question 12

What was the experimental analysis for the use of yeast to identify genes involved in cell trafficking?

Answer 12

1 - Cells were analysed for their ability to secrete enzymes (invertase and acid phosphatase) at permissive and restrictive temperatures. the secretory mutants could not secrete these but continued to synthesise
2 - The alteration ultrastructure of cells can be observed by microscopy (accumulation of vesicles)

Question 13

What was the result of the experiment?

Answer 13

At least 23 out of 25 genes were required to ensure the transport of proteins from the ER to the membrane.

Question 14

What happens to alpha factor as it moves through the secretory pathway/

Answer 14

Its glycosylated (Long sugar chains added) and cleared at different stages allowing us to follow its process.

Question 15

What is endocytosis?

Answer 15

When the plasma membrane invaginates into the cell resulting in the production of a vesicle

Question 16

What is endocytosis important for?

Answer 16

• The retrieval of molecules that formed part of the secretory vesicle for recycling.
• A means for entry of pathogens and toxins.

Question 17

What is a major function of the lysosome?

Answer 17

The degradation of extracellular material taken up by endocytosis

Question 18

How are the lysosomes resident enzymes transported to it?

Answer 18

By the secretory pathway

Question 19

How are vascular protein sorting (VPS) screens carried out?

Answer 19

• Carboxypeptidase Y goes to the lysosome via the ER and the Golgi.
• Anything that secreted CPY was analysed microscopically and biochemically —-> over 60 genes were identified.

Question 20

What are the 4 possible destinations for trafficking pathways from the late Golgi?

Answer 20

1) to the plasma membrane
2) to the eraly endosome
3) to the late endosome (MVB)
4) to the vacuole