Lectures 13&14 - Protein Trafficking

Question 1

What is the anterograde pathway

Answer 1

Forward - ER to Golgi, Golgi to plasma membrane

Question 2

What is the retrograde pathway

Answer 2

Retrieval - Golgi to ER eg.

Question 3

How do molecules move from one organelle to the next

Answer 3

1) Vesicle buds from donor compartment
2) Vesicle pinches off and translocates
from donor to acceptor compartment
3) Vesicle docks with acceptor compartment
4) Vesicle fuses with acceptor compartment releasing contents into lumen

Question 4

How do buds form

Answer 4

Driven by the assembly of protein coats onto membrane

Question 5

What is a Cathrin Coat

Answer 5

a protein that coats vesicles - made of Cathrin proteins and ‘adaptor’ proteins)

Question 6

What is the triskelion structure

Answer 6

Three clathrin molecules join at a common hub to form a three-legged “triskelion”, which is the basic building block of the coats.

Question 7

Animation 15.5 on Cathrin coat formation - can form cages

Question 8

What are the processes in pinching off

Answer 8

Fission and scission

Scission event is carried out by a protein called dynamic that separates the membrane associated with the vesicle and the rest of the membrane

Question 9

What pathways are associated with clathrin coats

Answer 9

Budding from the Golgi and from pasta membrane are associated with clathrin coated vesicles

Question 10

What other types of coat are there

Answer 10

COPI and COPII coats (involved in retrograde trafficking and ER respectively)

Question 11

What is clathrin coats used for

Answer 11

budding from the plasma membrane (endocytosis) and from the Golgi netowrk

Question 12

What are COPII coats involved in

Answer 12

Anterograde transport from the ER.

Question 13

What are COPI coats involved in

Answer 13

Retrograde transport from gold apparatus

Question 14

In a clathrin-coated vesicle with coat proteins clathrin and adaptin 1 - where is the origin and the destination

Answer 14

O - Golgi apparatus
D - Lysosome (via endosomes)

Question 15

In clathrin coated vesicles with clathrin and adaptin 2 - where is the origin and where is the destination

Answer 15

O - Plasma Membrane
D - Endosomes

Question 16

In COP-coated proteins, made from COP proteins - what is the origin(s) and the destination

Answer 16

O1 - ER D1 - Golgi apparatus
O2 - Golgi cisterna D2- Golgi cisterna
O3 - Golgi apparatus D3 - ER

Question 17

What controls coat formation

Answer 17

Coat recruitment GTPases

Question 18

How is cargo selected

Answer 18

Active recruitment
Selective exclusion
Passive inclusion

Question 19

What is bulk sorting

Answer 19

Molecules are passively included in vesicles (bulk sorting)

Question 20

How does budding from the ER occur

Answer 20

Some proteins are selectively recruited into buds. For example some integral membrane proteins have cytoplasmic domains that will interact with coat proteins. This will concentrate them into buds. Lumenal cargo proteins can also be selectively recruited into buds bey interacting with cargo receptors –that span the ER membrane and interact with coat proteins. The cargo and coat are indirectly linked via protein-protein interactions.
Some proteins are excluded from entering budding vesicles-for example proteins that mis-fold in the ER.
Some proteins are passively included in the budding vesicle. Some of these proteins may need to be returned to the ER.

Question 21

What is a DXE motif

Answer 21

Diacidic motifs consist of two acidic amino acid residues, separated by any other amino acid

Question 22

How are vesicles uncoated

Answer 22

Vesicle uncoating is mediated by phosphorylation of adaptor subunits. The phosphatases involved are recruited to the vesicle by chaperone proteins such as Hsc70 and auxilin.

Question 23

slide 19

Question 24

Why many some proteins be excluded from budding vesicles

Answer 24

Antibodies do not get packaged into ER transport vesicles until they are correctly assembled. The ER can be considered as a ‘quality control’ station in the secretory pathway.

Question 25

How are vesicles targeted to a membrane

Answer 25

Rabs and SNAREs help direct transport vesicles to their target membrane through:
Tethering
Docking and
Fusion
(slide 25)

Question 26

What do SNARE proteins do

Answer 26

Brings two membranes close together and ‘forces’ them to fuse

Question 27

As 2 membranes are forced together - water is squeezed out. what is produced as a result of this

Answer 27

Fusion of two bilayers to form a fusion pore proceeds through stalk and hemifusion intermediates. In the last picture the lumenal contents of one compartment are in direct contact with the contents of the lumen of the other compartment.

Question 28

How do v and t SNARES drive fusion

Answer 28

v-SNARE on vesicle interlocks and winds with t-SNARE on target membrane

Transport vesicle docks

Membranes coalesce

Membranes fuse

Question 29

What do BoTox do

Answer 29

Botulinum Neurotoxin prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromuscular junction, thus causing flaccid paralysis - also used for cosmetic procedures