Section 7A: Basic steps of vesicle transport

Question 1

What is the disease THES caused by?

Answer 1

It is caused by proteins not sorting in the right direction

Question 2

Why doesn’t sorting happen?

Answer 2

• Because of a defect in vesicle traffic

Question 3

What is vesicle traffic?

Answer 3

The movement of material from one spot of the cell to the other

Question 4

What do you have to do if you want to transport a protein that’s wedged in a lipid bilayer membrane, and that protein needs to travel to the next membrane-bound compartment?

Answer 4

You have to break off a piece of the membrane into something called a vesicle

Question 5

What is the function of a vesicle?

Answer 5

Packaging of membrane bound and other proteins from one compartment and deliver the cargo (contents) to the destination compartment

Question 6

Vesicular Transport

Answer 6

• first step involves fission: separating the vesicle from the donor membrane
• once vesicle is loaded with the right cargo, it needs to go to the right destination: this step involves fusion
• anything that’s in the lipid bilayer of the vesicle is going to fuse with and become part of the donor membrane

Question 7

What is fusion?

Answer 7

Membrane bound vesicle that has a lipid bilayer is going to fuse with the target compartment

Question 8

What is topology?

Answer 8

Continuity and ability to exchange without “crossing” the membrane/lipid bilayer
- This is possible because cargo molecule gets packaged in a vesicle, carried, and then delivered
- Exchange happens because of vesicle traffic

Question 9

Topology of the cell

Answer 9

Two locations are said to have the “same topology” if a molecule can be transported between the two locations using only vesicle fusion/fission and thus not requiring a membrane transport pore/translocator

Question 10

Vesicle traffic at the plasma membrane:

Answer 10

Endocytosis and Exocytosis

Question 11

What is Endocytosis?

Answer 11

• making vesicles from the plasma membrane budding into the cell
• how the cell takes in material

Question 12

What is Exocytosis?

Answer 12

• the delivery of vesicles so that the fuse with the plasma membrane
• if you have anything in the interior of that vesicle and you fuse the lipid bilayers of the vesicle with the plasma membrane, it gets released to the outside environment
• e.g. Acetylcholine releasing to the muscle

Question 13

Vesicle transport pathways and their functions

Answer 13

1. Biosynthetic pathway
2. Exocytosis pathway
3. Endocytosis pathway

Question 14

Biosynthetic pathway

Answer 14

Delivery of newly synthesized proteins and membrane from ER to most other organelles:
- Golgi
- Plasma membrane
- endosomes
- lysosomes

Question 15

Exocytosis pathway

Answer 15

Delivery of newly synthesized or stored proteins into the cellular extracellular space:
- Neurotransmitters
- Proteolytic enzymes to kill bacteria
- Release of signaling peptides like growth factors and insulin

Question 16

Endocytosis pathway

Answer 16

Uptake of external and plasma membrane bound proteins and ligands:
- vitamins and nutrients
- Cholesterol (LDL)
- growth factors and receptors (control of cell growth)
- changing the properties of the cell surface
- Destination: to the Endosome then Lysosome for degradation OR elsewhere in the cell

Question 17

Why are membrane bound compartments different from one another?

Answer 17

Because they have different proteins, lipids, and functions

Question 18

How do membrane compartments exchange material between one another?

Answer 18

By breaking off a piece of one organelle, packaging as a vesicle, and then delivering that to the target compartment where that vesicle now becomes part of that target compartment

Question 19

There is a constant and rapid flow between organelles, why is it that all these organelles become homogenized?

Answer 19

You prevent homogeneity by a selection process where only the molecules that are meant to go to the destination are packaged

Question 20

Organelle identity markers

Answer 20

Proteins specific to each organelle determine the identity of that organelle: what to import (fuse) and what to export (bud off)

Question 21

In order to bend a membrane…

Answer 21

you need to invest energy into it (bubble analogy)

Question 22

How do you make a vesicle?

Answer 22

• coat proteins latches onto a donor membrane provides energy to force it to bend into a vesicle
• non spontaneous
• coat proteins also help sort the right cargo (content) into that vesicle

Question 23

Four well-characterized protein coats

Answer 23

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

Question 24

Clathrin

Answer 24

happens primarily at the plasma membrane and some places of the golgi or trans golgi network

Question 25

COPII

Answer 25

only happens in the ER to make vesicles to go to the golgi

Question 26

Functions needed for protein coats for generating vesicles

Answer 26

1. Provide the required machinery and energy for membrane budding (bending) and scission (cutting) from the donor membrane
2. Select for the appropriate cargo (e.g. specific proteins) to be included in the vesicle protein