Membrane Fusion Flashcards
(32 cards)
what is the concept behind membrane fusion?
- cells are compartmentalized but need movement
- requires membrane fusion
- displacement of water from the hydrophillic surface of membrane is energetically unfavourable
- requires SNARE protein
why do membranes repel each other?
phosphates have a negative charge so phospholipids repel each other
what is membrane fusion?
the process where two separate lipid bilayers merge to become one
- need to be brought together closer than 1.5nm
what do membranes contains?
lipids and proteins
describe the vesicle budding
- contains part of the lumenal contents of the donor membrane
- contains membrane proteins and lipids from donor membrane
how does the vesicle fuse?
- contents mixes with the lumen of the accepting membrane or is released extracellularly
- proteins and lipids become part of the accepting membrane
what are the 3 steps of fusion?
- tethering - bridge between two membranes
- docking/the formation of a trans-SNARE complex
- fusion
what are the different stages of fusion?
- fusion initiation
- hemi fusion
- fusion pore opening
what are MTCs?
- multi subunit tethering complexes that provide a protein bridge
- tether one membrane to another
- bind a verity proteins on opposing membranes, this provides some specifity
what i the role of SNARE proteins?
- proteins that bring the membranes into close proximity
- allows membranes to overcome the repulsive forces
what is the structure of SNARE proteins?
- most are transmembrane
- contain a SNARE domain
- when it comes into contact another SNARE protein they form an alpha helical coil
what is a SNARE complex?
- a SNARE complex consists of 4 helical coils
- either 4 individual or a SNAP proteins and 2 others
- SNARE proteins are unstructured until they form a SNARE complex
- 4 helices are usually one helix from one membrane and 3 from another
- one of the strongest complexes
what is zippering?
zippering of the SNAREs overcomes the electrostatic repulsion of the phospholipids
what is post-tethering?
post tethering the SNAREs on opposite membranes starts to interact and then coil up forming the SNARE commplexing
- if you coil up a protein it becomes shorter, bringin the membranes into close proximity and overcoming the membrane repulsive forces
what is the centre of the SNARE complex?
- 4 amino acids, one from each helix pointing inwards
- either glutamine (Q) or arginine
- always 3Q’s and 1R
- Q SNAREs, Qa, Qb, Qc
- lots of hydrogen bonding and salt bridges
- hydrophobic central core
how is the SNARE complex specific?
- you need the right combinations of SNARE proteins for membrane fusion
- Qa,Qb,Qc and R SNARE are required for every membrane fusion event
how are SNAREs the minimal machinery for membrane fusion?
- experiment show that if you can make the SNAREs type I membrane proteins can get cells to fuse with each other
describe the process of SNARE proteins fusing (the experiment)
- SNARE domains need to be in the cytoplasm for membrane fusion
- N terminus in cytoplasm and C terminus in the lumen are type II membrane proteins
- converted them into type II membrane proteins
- flipped v SNARE (made the cytoplasm red)
- flipped t SNARE (made the nucleus blue)
- cells then fused - had multinucleated cells
why is the SNARE complex good for fusion?
once formed it is extremely stable
how are you abale to resuse the SNARE proteins/
- need to break the stable complex
- with a ALPHA-SNAP and NSF
what is NSF?
- hexamer
- ATPase, hydrolyses ATP
- Sensitive to NEM
what is alpha SNAP?
- required for NSF bidning to membranes
- binds and activates NSF ATPase activity
how do NSF and alpha SNAP work?
form a complex and with hydrolysis of ATP break open the SNARE complex
- SNARES bind to SNAP, they are SNAP receptors
what happens if you inhibit NSF?
you would still get one round of fusion
