Protein sorting + trafficking II Flashcards
How are proteins targeted for the ER?
Hydrophobic signal sequences
- often at N terminus
Have a +vely charged N terminus and a cleavage sequence
Describe ER targeting + translocation
- Signal recognition particle (SRP) recognises signal sequence + binds to peptide + ribosome
- Pauses translation
- SRP-bound ribosome attaches to SRP receptor in ER membrane
- Translation continues + translocation begins
In what form are proteins transported through the ER membrane?
Unfolded
What provides the energy for the conformational change in SRP that causes it to lose its affinity for the ribosome + nascent chain?
GTP hydrolysis
What happens to proteins that’s soluble in the lumen of the ER?
Signal sequence is cleaved by a lumenally located signal peptidase protein complex
-> creates new N-terminus of the protein
What happens to ER transmembrane proteins?
Signal sequence not cleaved + remains transmembrane
What is a non-cleavable signal sequence called?
Describe their features
Signal-anchor sequence
Longer hydrophobic stretches of amino acids
No signal peptidase cleavage sequence
What is the endomembrane system?
All organelles/membranes of the secretory + endocytic pathways
How do molecules move from 1 organelle to another?
- vesicle buds from donor compartment
- translocates to acceptor compartment
- fuses with acceptor compartment releasing contents into lumen
Describe the budding process
- Clathrin coat assembly + cargo selection
- Bud formation
- Vesicle formation
- Uncoating leaves naked vesicle which gets targeted to next membrane
In the case of cathrin coated vesicles, what is the final scission event carried out by?
Dynamin
= a GTPase that constricts the neck
What pathways are associated with cathrin coats?
Budding from the Golgi and from the plasma membrane
What are the 2 other types of coat?
What types of transport are they involved in?
COPI
- retrograde from Golgi
COPII
- anterograde from ER
