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
What happens when the vesicle binds to the target membrane using SNARES?
- Vesicle SNARE pairs with target SNARE
- The Rab binding draws vesicle closer
- Vesicle snare and target snare interact and vesicle is docked onto target membrane as they wrap around eachother
Process of fusion
Transport vesicle dock
Membranes coalesce
Lipid bilayers fuse
What must happen for the SNARE proteins to interact?
Bilayers are within 1.5nm
Water is displaced
What are the two methods of protein sorting in the secretory pathway?
ER retention and retrieval signal
Other sorting signals
How does the ER retention and retrieval signal work?
- KDEL sequence at C terminus of protein is recognised by KDEL receptor in golgi
- Protein then Recruited into COPI vesicles that return protein to ER
- Protein prevented from secretion
How do other sorting signals work to sort proteins?
- Addition of Mannose-6-phosphate to N-linked glycans of some glycoproteins at the golgi
- Causes sorting to lysosomes
- Short transmembrane domain retains proteins in the golgi
What is the structure of the golgi apparatus?
Near nucleus
Stack of membrane enclosed sacs called cisternae
What are the two transport models for how proteins move through the golgi?
Vesicular transport
Cisternal maturation
What is the vesicular transport model for movement through the golgi?
- Vesicles bud from one cisterna and fuse with the next
- This transports cargo through each cisterna
What is the cisternal maturation transport model for how proteins move through the golgi?
- Cargo enters Cisterna
- As each cisterna matures it migrates outwards carrying the cargo with it
- Cargo is then released once the Cisterna has fully matured and buds off on its own
What are the two functions of the golgi apparatus?
Protein modification
Protein sorting
3 examples of protein modification in the Golgi
- O-linked oligosaccharides are added to some -OH side chains of serine and threonine residues in proteins travelling across golgi
- N-linked oligosaccharides that are added to protein in the ER can be trimmed and rebuilt in golgi
- N-linked oligosaccharides on lysosomal proteins are modified with mannose-6-phosphate
How does protein sorting work in the golgi apparatus?
Cis = Resident proteins are sorted back to ER and cargo proteins carry on
Trans = Cargo proteins sorted into transport vesicles
