cours 5 - Cytoskeleton Regulation

Question 1

What order of structure do accessory protein form? Give an exemple.

Answer 1

Form higher-­order structures (e.g. mitotic spindle).

Question 2

What controles the filaments? (AFs, MTs, IFs)

Answer 2

•Filaments (AFs, MTs, IFs) are dynamic and under control of the cell.

Question 3

Which tubuline has the GTP and which one has the GDP binding site?

Answer 3

alpha: GTP
betta: GDP

Question 4

how is the GTP cap constructed in MT?

Answer 4

alpha and beta tubuline both link to GTP, creatting a cone like structure at the + end

Question 5

at what end does nucleation occurs?

Answer 5

minus end

Question 6

describe the process of nucleation for microtubules

Answer 6

The γ-tubulin combines with several other associated proteins to form a conical structure known as the γ-tubulin ring complex (γ-TuRC). This complex, with its 13-fold symmetry, acts as a template for α/β tubulin dimers during the nucleation process—speeding up the assembly of the ring of 13 protofilaments that make up the growing microtubule.

Question 7

describe centriol and the centrosome and mitotic spindle

Answer 7

centriol:

• assembly of 9 tubulin triplet
• center is gama tubulin
• (-­) ends of MTs are anchored.
• (+) ends of MTs emanate in astral configuration.

Centrosome:

• made of 2 centriols at a 90 degree angle and PCM (that containt gama tubuline)
• near nucleus.
• in plant, known as MTOC (similar but no 90 degree angle)
• devide in interphase (D) of cell division and migrate to seperate poles

mitotic spindles:

• microtubules that extends from centrosomes at the poles to reach the chromosomes

Question 8

where are actin filaments found and why?

Answer 8

• Cell periphery (cortex), where density of AF proteins is highest.
• Location related to AF function (cell shape, movement).

forms filaments of muscle cells

Question 9

How are actine filament made (nucleation)?

Answer 9

• G actine: has a pointed and a barbed end 3 interact together and assemble at each end but faster at the barbed end.
• Every subunits interact with 3 other subunit. To do that they need a specific conformation
• the initiation triplate is facilitated by formine
• Nucleation and growth of actine filament is facilitated by ABPs (actin binding proteins) including ARPs (actin related proteins).

Question 10

How do actine filament branch?

Answer 10

• branching happens at the – end
• branches are nucleated by the ARP2/3 complexe (containing 7 protein), which binds near tthe barbed ends of the microfilament and forms a new branch
• 70 degree angle if tthe most favorable for a branch in relation tto the filament

Question 11

What is the purpose of ARP2/3

Answer 11

Promote remodeling of the actin cytoskeleton tto allow movement and change in shape

Question 12

What controls the pool of actine and tubuline subunits and how do they do that?

Answer 12

• Both are maintained in the cytosol at high concentrations.( can exceed C_c)
• Accessory proteins may sequester unused subunits (sequestering proteins) by binding to them.
• Sequestered proteins are not hydrolyzed so that once bounded, it cannot be added to a chain.
• Provide control or regulation of filament elongation.

Actine:

• Thymosin makes polymerization less favourable.
• Profilin competes with thymosin and promotes assembly.

Tubuline:

• Stathmin binds to 2 tubuline subunits preventing polymerization.
  
  • Decreases effective [tubulin].
  • Promotes dynamic instability (catastrophe) by allowing GTP hydrolisis to catch up

Question 13

define MT-­Associated Proteins (MAPs)

Answer 13

They maintain stability of the structure

Interacts with subunits of tubuline within the main structure of the MT and then folds into some sorts of perpendicular handle sticking out of the MT.

They are important for the spacing of the MT.

They have multiple binding domain that dettermines how tightly packed the MT are gonna be (Tau in the axon is less packed than MAP2 domain in the cell body, allowing longer axons)

Question 14

What is one of the leading cause of alzheimer

Answer 14

• tau unable to bind to MTs
• Poorly soluble (hyperphosphorylated) tau may induce neurodegeneration.

Question 15

What are the 2 AF Binding Proteins and what do they do?

Answer 15

Cofilin destabilizes AFs

• binds to side of proteins at the - end.
• destabilise and promote dissassembly
• binding induces mechanical stress at the - end
• promotes treadmilling, turnover and cell locomotion.

Tropomyosin stabilizes AFs

• binds to side of proteins and add strenght specially to MAP protein
• muscle contraction.

Question 16

Give exemple of caps for actine filament

Answer 16

• e.g. CapZ (+), elongation occurs only at (-­) end.
• e.g. tropomodulin (-­) in muscle contraction.

Question 17

What protein can modify then end of MT

Answer 17

MAPS: stabilizes

Catastrophine (kinesins): destabilizes

• important in destroction of mitotic spindle
• initiate anaphase

Question 18

What are cross link proteins function

Answer 18

• Structural protein that space out parts of the cytosol.
• Formation of higher-­order structures..
• Spacing of 2 actin binding domains of cross-­ linking protein determines type of structure (the spacing of the actin filament overall structure)
  *

Question 19

What are the bundling cross-linking protein of AF

Answer 19

2 major groups:

bundling

• fimbrin (monomer):
  
  • has 2 actin binding domain very close together (14nm apart)
  • creates parallele bundle
  • tight packing
  • prevents myosine 2 from entering bundle
• alpha actinine (dimer of opposite direction):
  
  • has contractile bundle
  • loose packing
  • allows myosine 2 to enter the bundle
  • important for muscle spindle (bc in mucle spindle, myosine has to get inside and bind with actine)

gel-­forming.

• filamin (dimer same orientation)
  
  • allows lamellipodia in locomotion
  • Important in formation of neural tube on the dorsal aspect of the embryo
    1. formation of neural plate
    2. cell change their shape
    3. MT are alligning and the hight of cell change thicken the neural plate
    4. neural tube folding occurs bc of actin filament that change the dorsal region, contract the dorsal regoin of the cell
    5. eventually will formes spinal cord when embryo develops
• Spectrin (tetramer):
  
  • Actine forms cell cortex (mostly RBC) to give them flexibility. They modify their shape. Thats because of spectrine and actine that provide flexibility to cell structure

Question 20

describe the process of Induction by Extracellular Signals of limopotellia in a neutrophile

Answer 20

1. Extracellular signal
2. Activation of WASP
   
   • WASP activate ARP2/3
3. Nucleation and branching by Arp2/3
4. Promotion of assembly by profilin
   
   • Profiline competes with thimosine that sequester actine
5. Elongation reduced by capping proteins
   
   • caps/destabilises the one no longer needed
6. Destabilized by cofilin and return of subunits to pool

Question 21

What is the core functional unit in MTOC?

Answer 21

The γ-TuRC is typically found as the core functional unit in a microtubule organizing center (MTOC), such as the centrosome in animal cells or the spindle pole bodies in fungi and algae. it is in the cytosol for plants.

Question 22

What serves as the cap during microtubule growth? What is it’s purpose?

Answer 22

The γ-TuRC also acts as a cap of the (−) end while the microtubule continues growth from its (+) end. This cap provides both stability and protection to the microtubule (-) end from enzymes that could lead to its depolymerization, while also inhibiting (-) end growth.

Question 23

What does y-TuRC do in interphase?

Answer 23

The γ-TuRCs in the centrosome nucleate an array of microtubules in interphase, which extend their (+)-ends radially outwards into the cytoplasm towards the periphery of the cell.