Lecture 10 - Membrane Fusion Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

Why does membrane fusion require SNARE proteins?

A
  1. The fusion of two membrane requires the displacement of water from the hydrophilic surface.
  2. Phosphate heads are negative = repel each other. SNAREs can overcome these repulsive forces.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Define the term ‘membrane fusion’.

A

Membrane fusion is the process whereby two separate lipid bilayers merge to become one.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Membranes must be in close proximity for fusion to occur. Number?

A

1.5nm.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Brief description of membrane fusion.

A

Vesicle membrane fuses and becomes part of the acceptor membrane. The vesicle contents mix with the lumenal contents of the accepting membrane.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Brief description of budding.

A

Vesicle ‘buds’ from a donor membrane. Budding vesicle will contain part of the lumenal contents of the donor membrane.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

3 key steps of membrane fusion?

A
  1. Tethering - linking the membranes, 25nm apart.
  2. Docking/formation of a trans-SNARE complex.
  3. Membrane fusion.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Step 1: tethering. How are membranes loosely attached to each other?

A

Multi-subunit tethering complexes (MTCs). They consist of 6-10 proteins.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

MTCs differ for each type of membrane. Why?

A

Provides specificity.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Main function of SNARE proteins?

A

To bring membranes in close proximity, allowing repulsion forces to be overcome, thus permitting membranes to fuse.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Structure of a SNARE protein?

A

Most are TM type II - N terminus is in the cytoplasm when on a vesicle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

For membrane fusion to occur we need a SNARE complex. Describe this structure.

A

4 alpha helical domains that bind to each other. Each SNARE protein has a SNARE domain that forms a helical structure when associated with others.

Therefore, most SNARE complexes consist of 4 SNARE proteins = one SNARE domain per protein.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Some SNARE proteins have 2 SNARE domains, and so only 2 are required to form the complex. Give an example.

A

SNAP-25.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How do these SNARE domains come together?

A

Should be noted that generally one helix comes from one membrane; the other three from the other membrane.

The N terminus of each SNARE domain coil around each other, pulling vesicles together (1.5nm). This is the ZIPPERING OF SNAREs and overcomes repulsive forces.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Trans-SNARE complex?

A

v-SNARE (vesicle) and t-SNARE (target membrane).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Key features of a SNARE complex?

A
  1. Within the bundle hydrophobic AAs point into the central core.
  2. 3 helices provide glutamine (Q); 1 arginine (R).
  3. There will always be 3 Q SNAREs and 1 R SNARE.
  4. Q SNAREs are further classified = Qa = syntaxin, Qb = SNAP-N, Qc = SNAP-P.
  5. Complex is v stable due to H bonds and salt bridges.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Q-SNAREs and R-SNAREs provide specificity to fusion events. Why?

A

A Qa, Qb, Qc and R are all required for fusion to occur.

17
Q

An experiment was performed that confirmed that SNAREs are the minimal machinery required for membrane fusion. Explain this experiment.

A

Flipped V SNAREs and T SNAREs so that they were Type I. Using fluorescent proteins found they could get the cells to fuse with each other since the domains are on the outside. Nothing else required.

18
Q

SNARE proteins can be recycled by breaking the SNARE complex. What is required to perform this?

A

Alpha-SNAP and NSF (note: alpha-SNAP is not a SNARE protein).

19
Q

What does NSF stand for?

A

NEM Sensitive Factor. NEM = N-ethyl Maleimide.

20
Q

What does alpha SNAP stand for?

A

Soluble NSF Attachment Protein.

21
Q

How does NSF and alpha SNAP function?

A

NSF is a hexamer that uses ATP to break the complex (ATPase). Alpha-SNAP is required for NSF binding, and activates the ATPase activity of the NSF.

22
Q

Sec1/Munc18 (SM) proteins regulate SNARE complex assembly. How?

A

The binding of an SM protein controls whether the SNARE complex is in an open or closed conformation. When bound = closed = unable to form complex.

23
Q

Rab proteins prime membranes for fusion. How?

A

Rab proteins reversibly bind GTP and GDP using GEF and GAP. GTP-Rab will activate effector proteins.

24
Q

SUMMARY: all membrane fusion events have?

A
  1. A 4 helix SNARE complex.
  2. SM protein.
  3. Rab protein + effector proteins.