Protein sorting and trafficking III

Question 1

How is cargo for vesicles selected?

3 types

Answer 1

Active recruitment
Selective exclusion
Passive inclusion

Question 2

Describe budding from the ER in terms of cargo

Answer 2

Some proteins are actively recruited
e.g. lumenal cargo proteins interact with cargo receptors

Some proteins are excluded from entering budding vesicles e.g. those that mis-fold in the ER

Some proteins are passively included

Question 3

Describe how KDEL is returned to the ER if it’s trafficked to the Golgi

Answer 3

KDEL recognised by KDEL receptors

• > actively recruited for COPI coated vesicles
• > retrograde transport back to ER

Question 4

What helps to direct transport vesicles to their target membrane?

Answer 4

Rab proteins
SNARE proteins

• give vesicles + target membranes molecular identities
• only if these are compatible does fusion take place

Question 5

Describe SNARE driven fusion of membranes

Answer 5

v-SNARE + t-SNARE bind as transport vesicle docks

• > water squeezed out
• > membranes coalesce
• > membranes fuse

Question 6

What is the role of chaperones?

Answer 6

Prevent mis-folded/unassembled proteins leaving the ER

Question 7

How are proteins modified in the secretory pathway?

Answer 7

Glycosylation of proteins in ER and Golgi

Question 8

What is the name of the enzyme that transfers lipid-linked oligosaccharides to the protein?

Answer 8

Oligosaccharyl trasnferase

Question 9

How are processing enzymes differentially distributed in the Golgi stack?

Answer 9

Early acting in cis cisternae

Late acting in trans-cisternae

Question 10

Describe the endocytosis of LDL

Answer 10

1. LDL receptor trafficked to cell surface of ER
2. Receptor recycles between PM and endosomes picking up LDL and releasing it in endosomes
3. LDL trafficked to lysosomes
4. Receptor returns to PM