Section 7C: Vesicle tethering and fusion Flashcards
What does vesicle targeting to a compartment involve?
- Tethering
- Rab GTPase
What is Tethering?
- binding loosely to the correct target protein
- does not mean material can go easily, they’re just held in proximity to where they need to eventually fuse and deliver their content; but those contents aren’t delivered yet
Which proteins carry out the process of tethering?
Rab GTPases
What are the two steps for membrane fusion?
- tethering
- docking/fusion
Rab GTPase-mediated tethering to target organelles of transport vesicles (steps)
- a specific Rab can be activated either on vesicle or target membrane
- GTP-bound Rab binds to Rab “effector proteins”
- Rab effector proteins carry out the Rab regulated function
- Rab effectors are myriad: enzymes, motor proteins, tethers, etc
Every vesicle tends to have a ______ Rab protein
unique
What are the unique Rab proteins responsible for?
Each one of them are responsible for taking a vesicle to a unique target compartment
Rab 5
Early endosomes (endocytosis - degradation and recycling)
Rab 7 and Rab 9
Lysosome (endocytosis - degradation)
Rab 2
Golgi —> ER (retreival)
Rab 1
ER —> Golgi (synthesis)
What does targeting and fusing vesicles to the right organelle involve?
- Rab GTPases
- SNARE proteins
What are the two different types of SNAREs
- Vesicular SNARE (v-SNARE): on vesicle
- Target SNARE (t-SNARE): on target
- these two are good at binding to each other
What causes the lipid bilayer to fuse?
When v-SNARE and t-SNARE bind to each other very tightly they form a trans-SNARE complex
Why does at least one t-SNARE protein have to be a transmembrane or peripheral membrane protein?
- 2 proteins (v-SNARE and t-SNARE) come together, forces the membranes together because those two proteins are anchored in the lipid bilayer
- if they were not anchored, they could just get ripped off the membrane
What happens if SNARE proteins don’t work?
Neurotransmitters won’t be released
v-SNAREs
transmembrane, single chain
t-SNAREs
2 or 3 chains, at least 1 transmembrane, others may be peripheral membrane proteins
How does botox work?
It degrades certain types of SNARE proteins that are involved in neurotransmitter release; causing muscles to relax (stops muscle contraction)
Sequential steps of SNARE-mediated membrane fusion
- Trans-SNARE complex formation causes water exclusion: energetically unfavourable
- Membranes are within 1.5 nm of each other: sufficient to cause lipids to cross layers
- Stalk formation: one layer of the membrane undergoes fusion (outer leaflets fuse, the inner leaflet of the lipid bilayers do not mix yet)
- Hemifusion: 2nd, inner layer is juxtaposed, permitting lipid molecules to move across the inner layer of the membrane (inner leaflets now mix)
- Full fusion occurs, two membrane compartments become one
Are SNARE proteins going from individual v-SNARE and t-SNARE proteins into a SNARE complex favourable or unfavourable?
Favourable
How does a cell invest energy to reset for the next vesicle that has to feed?
It separates the v-SNARE from the t-SNARE after fusion has taken place; away to recycle the proteins
What is NSF?
an enzyme that hydrolyzes ATP or uses ATP hydrolysis to separate v-SNAREs from t-SNAREs (the SNARE complex)
If a cell no longer has ATP will it be able to sustain vesicle fusion?
No, because it will run out of the “free” v-SNAREs and t-SNAREs and won’t be able to regenerate them
Why are both the SNAREs in the same compartment after full fusion?
Because the vesicle became a part of the target compartment; the target compartment absorbed the vesicle as well as all its contents
