Membrane Fusion Flashcards
Eukaryotic cells are composed from
many membrane bound compartments
(such as nucleus, endosomes, lysosomes, mitochondria) that are designed to perform very specialized cellular functions.
cells need to have a mechanism that allows the transport of proteins in and out of these structures without _______. This is achieved via
compromising their identity.
intracellular vesicle transport
All cellular membranes are composed from a lipid
bilayer with charged residues facing the _____ and interacting with _____
outside
water molecules.
the membrane fusion thermodynamically is
highly inefficient event, since it needs to overcome the repulsive ionic forces and dissipate the hydration between to lipid bilayers.
cell need to have the machinery that increases the _____. In addition, cell needs to ensure the _______.
efficiency of membrane fusion
specificity of membrane fusion to ensure that the vesicular cargo is delivered to the correct destination
transport vesicle fusion steps.
- delivered to fusion site
2. fuse with the target membrane.
There are three main classes of SNAREs:
- syntaxins,
- VAMPs
- SNAPs
VAMP is located in _______ while syntaxin and SNAP are located in the _______.
transport vesicle,
target membrane
what drives membrane fusion?
The formation of highly stable SNARE complex
Once the fusion occurred, cell needs to ________.
recycle the SNARE proteins for another fusion events
Because the SNARE complexes are highly stable, ___ and ___ are required to disassemble
- NSF (N-ethyl-maleimide sensitive fusion protein)
2. αSNAP (soluble NSF attachment protein)
why are proteins required to disassemble SNARE?
because it is so stable
αSNAP stands for
soluble NSF attachment protein
NSF stands for
N-ethyl-maleimide sensitive fusion protein
The NSF uses _______ to generate energy needed to disassemble the SNARE complex.
ATP hydrolysis
After disassembly of SNARE complex, the SNARE proteins need to be _________.
refolded to the active conformation
Refolding of SNARE into active conformation is achieved by the ________
Sec1 protein
The class of enveloped viruses (includes _______) also needs to go through _____ in order to infect cells.
HIV, ebola, influenza viruses
membrane fusion
Viruses have developed _____
fusion machinery that is remarkably similar to SNARE fusion.
Normally, fusion domain is _______ within viral protein.
folded and hidden
Upon receiving specific signal (in case of influenza virus it is _____), the fusion domain is ________.
That is immediately followed by _____, leading to the ________.
change in pH
exposed and inserted into target cell membrane
refolding of fusion protein
fusion of viral and cellular membranes
Sub-cellular protein targeting is important in ________.
delivering proteins required for specialized functions to different compartments within the cell.
Cells must have a mechanism to transport proteins between these compartments without________
compromising the structure of the organelles or their membranes.
Why do Cells and viruses require specialized machinery for membrane fusion?
Because membrane fusion is a thermodynamically unfavorable process due to repulsive ionic forces between the membranes and the presence of a layer of water coating the surfaces of membranes.
SNARE proteins are
cellular proteins that aid in membrane fusion
3 SNARE classes vary in structure but have one important commonality:
the presence of at least 1 alpha helical domain, which forms coiled coils
When the alpha helical domains of SNARE proteins come into contact, they ______
zipper togther and form a highly stable complex of coiled coils
the zippering of SNARE protein bridges the two membranes and allows
membrane fusion
Because the helical complex between the SNARE proteins in highly stable,
the repulsive forces between the membranes are overcome and water molecules between the membranes are forced out
Disassembly of SNARE protein complex is accomplished with a
“barrel-like” structured protein called NSF
NSF contains
a catalytic protein called a-SNAP
Once bound the SNARE complex, NSF function
twists to unwind the SNARE complex, hydrolyzing 6 molecules of the ATP process
After the SNARE complex is disassembled, what happens to syntax in?
syntaxin folds onto itself and becomes inactive.
how is syntax in reactivated?
n-Sec1 protein binds and refolds syntaxin to it’s active conformation
How is the membrane fusion specificity achieved?
variations in the structure of the SNARE proteins.
Because there are many variations in the structure of VAMPs, SNAPs and syntax in, this allows for the vesicles to fuse with _____, meaning that _____
vesicles to fuse with the appropriate membranes.
Only certain variants can match and associate with each other.
Enveloped viruses have membrane that is derived from the _____
host cell
enveloped viruses have _____ proteins
homotrimers of ciral fusion proteins
domains that each viral fusion protein has: (3)
- transmembrane
- helical
- hydrophobic fusion
the viral vision proteins of an inactive virus are folded to hide the _____ domain
hydrophobic fusion
When the virus receives a certain signal, the protein stretches out and the fusion domain is exposed and inserted into the cell membrane.
Then the fusion membrane does what?
Then the fusion membrane ten folds back into a coiled coil, alpha-helical complex to bring the membrane in close proximity and allow for fusion
______ signals for fusion domain in the influenza virus
low pH.
With HIV, signaling for ______ occur in the same step
fusion domain exposure and membrane recognition
____ is the viral fusion protein for HIV
Gp41
In the HIV virus inactive stage, ____ binds to and blocks gp41 from _______
gp120
inserting into membrane
When CD4 antigen on the membrane of a T cell binds to gp120. Then, what happens to gp120?
gp120 undergoes a conformational change that allows for both exposure of sites that bind to a receptor on the T cell membrane surface and exposure of the gp41 fusion domain.
delivery to fusion site is helped by
molecular motor proteins that move along actin or microtubule tracks.
Fusion with target membrane requires:
SNAREs (soluble NSF attachment protein receptor).
Sec1 protein binds and changes _____ into ____ conformation.
syntaxin
active
At low pH, the virus is able to
fuse with the endosome membranes in the cell
The gp41 fusion domain then inserts into the target membrane. After insertion, the gp41 then does what?
refolds to bring the viral and target membrane into contact, gp120 dissociates and membrane fusion occurs
