cours 4 - Cytoskeleton Filament

Question 1

What are the 3 types of filament protein?

Answer 1

1. Actin filaments (AFs, or microfilaments)
2. Microtubules (MTs)
3. Intermediate filaments (IFs)

Question 2

What forms the filaments?

Answer 2

• Small stable subunits existing in the cytosol as a pool
  
  • actine
  • α & β tubuline together as a stable structure
• The filaments are dynamic, they will form and disassemble multiple times.

Question 3

How are microtubule formes?

Answer 3

• Microtubule are made of 13 protofilament
  
  • the multiple bond bw fillament and the shape itself makes it more thermally stable and it il still possible to remove 1 subunit at a time.
• GTP can only be hydrolise at tubuline β, making it the (-) end where the tubuline is removed.
• The protofilament can grow on both sides but mostly at the (+) end.
• The filament is always +-+- because more thermodynamically favorable
• The hydrolysed GDP eventually distabilizes the microtubule

Question 4

How are actin filaments formed?

Answer 4

• initially goes through nucleation where actin together gives oligomers.
  
  • Once this is acheived, the process goes faster
  • Does not always use that steo because the fillament might not be destroyed entierly and the olimers might be recycled.
• The fillament will be constructed on both the positive and negative side until it reaches equilibrium.
• The actine forming the filament is made of trios of interacting actin, making the structure stable and giving it a spiral shape.

Question 5

What are ATP/GTP caps?

Answer 5

•accumuation of actin or tubulin subunit with T form (that have ATP or GTP form)

Question 6

What is a •ctical concentration (Cc)

Answer 6

where subunit addition equals subunit loss.

Question 7

Describe Treadmilling

Answer 7

1. Filaments added to ATP-actin(seed).
2. [ATP-actin] high, addition occurs at both ends.
3. [ATP-actin] drops, addition greater at plus end.
4. Steady state.
   * Look at arrow size in picture*

• same happens with microtubules.
• This is how a cell can crawl along a surface

Question 8

What is the critical concentration of T and D where threadmilling occures?

Answer 8

• It’s a range. It’s the range where the elongation rate at T end and the destruction rate at D end is the same.
• since the slope of elongation rate (y) vs concentration (x) of the Actine bound ATP at T(+) end is bigger than the slope of elongation rate (y) vs concentration (x) of the Actine bound ADP at the D(-) end, we end up with a range.
• This occurs because the T (+) end is more active since it is the end where synthesis occurs.

Question 9

What is a T and a D state?

Answer 9

T is when it is attached tubuline is attached to GTP or actine is attached to ATP

D is when it is attached tubuline is attached to GDP or actine is attached to ADP. it is the (-) end because that’s where hysrolysis occurs.

Question 10

Define Dynamic Instability in Microtubules.

Answer 10

It’s a continuous &rapid transitions of catastrophic event and rescue event during rapid growth, that depends on [Tubulin] within critical values and GTP cap.

1. Hydrolisis = constant rate
2. But rate of polymerisation (addition) is not constant because the subunit concentrations falls as polymerisation occurs.
3. The decrease in the rate of growth allows hydrolysis to catch up.
4. GDP destabilises the structure by inducing a conformational change.
5. We arrive to catastrophy because theres no more cap (it’s all hydrolysed in GDP)
6. So the tubuline link to GDP is eliminated
7. That create a high concentration of tubuline again
8. Which increase the rate of polymerisation (thats rescue)

Question 11

What leads to the change of conformation in microtubules?

Answer 11

Hydrolisis of GTP

Question 12

The microtubule rate of growth is dependant on what?

Answer 12

the concentration of tubuline.

Tubulin moving away (speed) depend on concentrtion

Question 13

What is the purpose of Treadmilling and Dynamic Instability?

Answer 13

Constant ATP consumption. Is it worth it?

• Spatial and temporal flexibility with high turnover. Allows to relocate depending on the need of the cell.
• Fastest way to grow filaments without nucleation. (not starting from scratch)
• Exploration for attachment sites and remodelling.

Question 14

What are microtubule providing structure to?

Answer 14

Microtubule are not good to be deformed because they are meant to provide structure in axons and neurons.

Question 15

describe the structure of an intermediate filament

Answer 15

coil-coil design, meant to withstand deforming forces because they are part of stuff like haire, nails. Provide a structure that withstand breakage

there is 4 types of intermediate filament:

• (A,B) Dimer formation.
• (C) Tetramer formation.
• (D) Tetramer-tetramer association.

(see pic for description)

Question 16

What is that structure?

Answer 16

Intermediate filament (fluo microscopy)

Question 17

What are the 2 types of IF?

Answer 17

1.Epithelial (e.g. keratins)

• Most diverse family
• Type I and II keratin chains
• Strength in hair, nails

2. Axonal (e.g. neurofilament, neurons)
   * Found in central and peripheral axons of vertebrate neurons.
   * Type L, M or H. (density)
   * Growth, increase axon diameter allowing better signal.
   * allow axon to produce membrannes processes and extend to tissus