Membrane Fusion

Question 1

Eukaryotic cells are composed from

Answer 1

many membrane bound compartments

(such as nucleus, endosomes, lysosomes, mitochondria) that are designed to perform very specialized cellular functions.

Question 2

cells need to have a mechanism that allows the transport of proteins in and out of these structures without _______. This is achieved via

Answer 2

compromising their identity.

intracellular vesicle transport

Question 3

All cellular membranes are composed from a lipid

bilayer with charged residues facing the _____ and interacting with _____

Answer 3

outside

water molecules.

Question 4

the membrane fusion thermodynamically is

Answer 4

highly inefficient event, since it needs to overcome the repulsive ionic forces and dissipate the hydration between to lipid bilayers.

Question 5

cell need to have the machinery that increases the _____. In addition, cell needs to ensure the _______.

Answer 5

efficiency of membrane fusion

specificity of membrane fusion to ensure that the vesicular cargo is delivered to the correct destination

Question 6

transport vesicle fusion steps.

Answer 6

1. delivered to fusion site

2. fuse with the target membrane.

Question 7

There are three main classes of SNAREs:

Answer 7

1. syntaxins,
2. VAMPs
3. SNAPs

Question 8

VAMP is located in _______ while syntaxin and SNAP are located in the _______.

Answer 8

transport vesicle,

target membrane

Question 9

what drives membrane fusion?

Answer 9

The formation of highly stable SNARE complex

Question 10

Once the fusion occurred, cell needs to ________.

Answer 10

recycle the SNARE proteins for another fusion events

Question 11

Because the SNARE complexes are highly stable, ___ and ___ are required to disassemble

Answer 11

1. NSF (N-ethyl-maleimide sensitive fusion protein)

2. αSNAP (soluble NSF attachment protein)

Question 12

why are proteins required to disassemble SNARE?

Answer 12

because it is so stable

Question 13

αSNAP stands for

Answer 13

soluble NSF attachment protein

Question 14

NSF stands for

Answer 14

N-ethyl-maleimide sensitive fusion protein

Question 15

The NSF uses _______ to generate energy needed to disassemble the SNARE complex.

Answer 15

ATP hydrolysis

Question 16

After disassembly of SNARE complex, the SNARE proteins need to be _________.

Answer 16

refolded to the active conformation

Question 17

Refolding of SNARE into active conformation is achieved by the ________

Answer 17

Sec1 protein

Question 18

The class of enveloped viruses (includes _______) also needs to go through _____ in order to infect cells.

Answer 18

HIV, ebola, influenza viruses

membrane fusion

Question 19

Viruses have developed _____

Answer 19

fusion machinery that is remarkably similar to SNARE fusion.

Question 20

Normally, fusion domain is _______ within viral protein.

Answer 20

folded and hidden

Question 21

Upon receiving specific signal (in case of influenza virus it is _____), the fusion domain is ________.
That is immediately followed by _____, leading to the ________.

Answer 21

change in pH

exposed and inserted into target cell membrane

refolding of fusion protein

fusion of viral and cellular membranes

Question 22

Sub-cellular protein targeting is important in ________.

Answer 22

delivering proteins required for specialized functions to different compartments within the cell.

Question 23

Cells must have a mechanism to transport proteins between these compartments without________

Answer 23

compromising the structure of the organelles or their membranes.

Question 24

Why do Cells and viruses require specialized machinery for membrane fusion?

Answer 24

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.

Question 25

SNARE proteins are

Answer 25

cellular proteins that aid in membrane fusion

Question 26

3 SNARE classes vary in structure but have one important commonality:

Answer 26

the presence of at least 1 alpha helical domain, which forms coiled coils

Question 27

When the alpha helical domains of SNARE proteins come into contact, they ______

Answer 27

zipper togther and form a highly stable complex of coiled coils

Question 28

the zippering of SNARE protein bridges the two membranes and allows

Answer 28

membrane fusion

Question 29

Because the helical complex between the SNARE proteins in highly stable,

Answer 29

the repulsive forces between the membranes are overcome and water molecules between the membranes are forced out

Question 30

Disassembly of SNARE protein complex is accomplished with a

Answer 30

"barrel-like" structured protein called NSF

Question 31

NSF contains

Answer 31

a catalytic protein called a-SNAP

Question 32

Once bound the SNARE complex, NSF function

Answer 32

twists to unwind the SNARE complex, hydrolyzing 6 molecules of the ATP process

Question 33

After the SNARE complex is disassembled, what happens to syntax in?

Answer 33

syntaxin folds onto itself and becomes inactive.

Question 34

how is syntax in reactivated?

Answer 34

n-Sec1 protein binds and refolds syntaxin to it's active conformation

Question 35

How is the membrane fusion specificity achieved?

Answer 35

variations in the structure of the SNARE proteins.

Question 36

Because there are many variations in the structure of VAMPs, SNAPs and syntax in, this allows for the vesicles to fuse with _____, meaning that _____

Answer 36

vesicles to fuse with the appropriate membranes. | Only certain variants can match and associate with each other.

Question 37

Enveloped viruses have membrane that is derived from the _____

Answer 37

host cell

Question 38

enveloped viruses have _____ proteins

Answer 38

homotrimers of ciral fusion proteins

Question 39

domains that each viral fusion protein has: (3)

Answer 39

1. transmembrane 2. helical 3. hydrophobic fusion

Question 40

the viral vision proteins of an inactive virus are folded to hide the _____ domain

Answer 40

hydrophobic fusion

Question 41

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?

Answer 41

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

Question 42

______ signals for fusion domain in the influenza virus

Answer 42

low pH.

Question 43

With HIV, signaling for ______ occur in the same step

Answer 43

fusion domain exposure and membrane recognition

Question 44

____ is the viral fusion protein for HIV

Answer 44

Gp41

Question 45

In the HIV virus inactive stage, ____ binds to and blocks gp41 from _______

Answer 45

gp120 inserting into membrane

Question 46

When CD4 antigen on the membrane of a T cell binds to gp120. Then, what happens to gp120?

Answer 46

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.

Question 47

delivery to fusion site is helped by

Answer 47

molecular motor proteins that move along actin or microtubule tracks.

Question 48

Fusion with target membrane requires:

Answer 48

SNAREs (soluble NSF attachment protein receptor).

Question 49

Sec1 protein binds and changes _____ into ____ conformation.

Answer 49

syntaxin active

Question 50

At low pH, the virus is able to

Answer 50

fuse with the endosome membranes in the cell

Question 51

The gp41 fusion domain then inserts into the target membrane. After insertion, the gp41 then does what?

Answer 51

refolds to bring the viral and target membrane into contact, gp120 dissociates and membrane fusion occurs