Lecture 5: The endomembrane system Flashcards
What is vesicular transport?
How proteins and lipids are moved between membrane enclosed organelles in vesicles
What is symmetric growth in the synthesis of membrane lipids?
- Enzymes on cytosolic face of the ER membrane synthesise new phospholipids which adds to cytosolic half of bilayer
- Scramblase catalyses the transfer of random phospholipids from one monolayer to another
- This means both sides of the bilayer grow symmetrically
What is asymmetric growth in the synthesis of membrane lipids?
- New membrane is delivered from the ER
- Flippase catalyses the transfer of specific phospholipids to the cytosolic membrane
- Only certain ones being transported allows asymmetric growth
Where are flippase transporters found?
Golgi apparatus
What is the secretory pathway?
Vesicular transport
Proteins and lipids move from ER through the golgi and plasma membrane
Move outwards
What is the endocytic pathway?
Vesicular transport
Proteins and other molecules move from the plasma membrane to the cell interior
Moves inwards
In what way is the membrane orientation maintained during vesicular transport?
Cytosolic surface stays facing cytosol
Other side faces outwards
How are vesicles formed?
- Protein coat is formed and helps deform the membrane and capture correct cargo
- Dynamin assembles and helps the bud pinch off
- Vesicle is released and protein coats are removed through uncoating
What are some different types of coated vesicles?
Clathrin coated
COPII coated
COPI coated
2 variants of Clathrin coated vesicles
Clathrin + adaptin 1 or adaptin 2
Clathrin + adaptin 1
Origin : Golgi
Destination : Lysosome
Clathrin + adaptin 2
Origin : Plasma membrane
Destination : Endosome
COPI
Origin : Golgi
Destination : ER
COPII
Origin : ER
Destination : Golgi
What is clathrin?
Has a role at the plasma membrane and golgi
Part of endocytic pathway
Shapes membrane into bud and vesicle
Captures cargo
What are the three stages of forming a vesicle?
Cargo selection
Vesicle budding
Vesicle uncoating
What occurs during cargo selection?
- Adaptins help clathrin attach to membrane
- Clathrin coated pit forms on cytosolic face of membrane
- Or will bind to cargo receptors and recruit them to recognise specific sorting signals on the cargo
What happens during vesicle budding?
- Clathrin assembles into a cage
- Membrane invaginates into a bud
- Dynamin makes a ring around the neck of the bud and helps it pinch off
What happens during vesicle uncoating?
- After budding coat proteins are removed
- Needs molecular chaperones and ATP
- Produces naked transport vesicle that can fuse with the target membrane
What is dynamin and how does it work?
- When it hydrolyses GTP it changes conformation
- This constricts the neck of the bud and the vesicle pinches off
What is mutant dynamin?
- A temperature sensitive type in Drosophila
- The vesicles can’t pinch off which causes loss of neurotransmitter vesicles in synapses
How does the vesicle reach the right destination?
Transported along microtubules to reach the correct place
What are the 2 proteins that help vesicles tether to a membrane
- Rab proteins
- SNARE proteins
What happens when a vesicle fuses to the target membrane using Rab proteins?
- Rab protein on vesicle binds to a tethering protein on target membrane
- There are Rab and tether pairs so this happens correctly