Lecture 14 - Protein sorting: the Golgi apparatus

Question 1

Secretory pathway vs endocytic pathway

Answer 1

the secretory pathway moves outward from the ER towards the golgi and cell surface
the endocyctic pathway moves inward from the plasma membrane

Question 2

Where do most transport vesicles form? (think about what you wear in the winter)

Answer 2

they form from specialized, coated regions of membranes
- they bud off as coated vesicles, which have a distinctive cage of proteins

Question 3

What are the 4 types of coated vesicles discussed?

Answer 3

1. Clathrin-coated
2. COPI-coated
3. COPII-coated
4. Retromer-coated

each type is used for different transport steps

Question 4

What are vesicle coats?

Answer 4

geometrical structures that assemble into vesicle cages

Question 5

Rab proteins

Answer 5

guide transport vesicles to their target membrane

Question 6

SNARE proteins

Answer 6

mediate membrane fusion by putting membranes in close proximity

Question 7

Proteins leaving the ER leave in what type of vesicle? And from what site?

Answer 7

leave in COPII-coated transport vesicles that form at the ER exit site (ERES)

Question 8

What ensures vesicles are loaded?

Answer 8

cargo receptors inside the vesicles
- activated receptors recruit adaptor proteins that recruit the coat proteins

Question 9

What do COPII transport vesicles accommodate?

Answer 9

they accommodate large cargoes by assembling tubes instead of vesicles

Question 10

Protein destination codes

Answer 10

glycosylation and phosphorylation are destination codes
- sugar coats serve as destination tags
- some sugar coats are transient
- glycosylation steps are compartmentalized in different cisternae
- codes are additive: sometimes things must be added in earlier cisternae to add something later

Question 11

What are the two mechanisms of transport through the golgi?

Answer 11

1. vesicle transport mechanism: vesicles transport molecules between cisternae
2. cisternal maturation mechanism: cisternae maturate from cis to trans together with cargo molecules

Question 12

Original evidence for the vesicular and maturation models

Answer 12

fixed samples
vesicular model: vesicles bud and fuse, COPI vesicles surround the golgi
maturation model: proto collagen is bigger than vesicles, PC is not detected inside vesicles, it localizes to enlarged golgi cisternae

Question 13

Recent evidence for the vesicular and maturation models

Answer 13

fluorescent live imaging
- make fusion proteins (fuse a protein to a fluorescent protein)
- track GFP (or other fluorescent protein) through the Golgi

Question 14

Are cisternae stacked in all species?

Answer 14

No

Question 15

What type of vesicle brings ER proteins back from the golgi?

Answer 15

COPI vesicles

Question 16

KDEL sequence

Answer 16

A sequence on ER proteins
- KDEL receptors initiate vesicle formation

KDEL = lysine - aspartic acid - glutamic acid - leucine
- a protein with these 4 amino acids in sequence will not be secreted from ER

Question 17

Non-selective constitutive secretory pathway

Answer 17

transports most other proteins directly to cell surface
- no specific signal = secrete the protein (default pathway)
- specific signals are needed to direct secretory proteins into secretory vesicles and lysosomal proteins into different specialized transport vesicles