Cytoskeleton 1

Question 1

Cytoskeleton

Answer 1

• scaffold and structural framework
• responsible for cell movements
• composed of actin, intermediate filaments, and microtubules.

Question 2

Actin assembly

Answer 2

1. Individual molecules called G actin
2. G actin has binding sites for other G actin
3. Nucleation- G active monomers form dimers and trimers
4. Elongation - monomers added to form F actin
5. actin filaments have pointed and barbed ends
6. Monomers are added to the barbed ends which are also (+)
7. Monomers removed from the pointed (-) end

Question 3

Actin Polymerization

Answer 3

1. Actin monomers bind ATP, which is cleaved to ADP
2. Polymerization of G-actin into F-actin requires ATP and cations such as Mg, K, Na
3. In F-actin ATP is cleaved to ADP and Pi
4. ADP-bound F-actin depolymerizes to release G-actin-ADP
5. ADP has to be replaced by ATP for G-actin to polymerize.
6. Steady-state - at critical concentration of monomers rate of assembly=rate of disassembly.
7. Treadmilling- the addition of monomers to the (+) end and removing of monomers at the (-) leaves the length unchanged.

Question 4

Nucleation

Answer 4

• initial rate limiting step
  1. Formin binds to actin monomers and facilitates nuclear ion of long, unbranded actin filaments.
  2. It adds new monomers to barbed end and stays attached to (+) end of growing actin filament until released.
  3. Arp2/3 binds to actin filaments near barbed ends and initiates formation of branches and stays bound to end of branch.

Question 5

Remodeling existing filaments

Answer 5

A. ADF/ cofilin (actin depolymerizing factor)
1. Binds ADP-actin at (-) end
2. Enhances rate of dissociation of monomers
3. Remains bound to monomers, preventing them from binding to filaments
4. Also severs actin filaments.
B. Profilin
1. Increases exchange of ADP for ATP on G actin monomer.
2. Recharges G-actin monomer by replacing ADP with ATP.
3. Stimulates filament formation at (+) end

Question 6

Capping proteins

Answer 6

A. CapZ
1.Binds to (+) end inhibiting polymerization
B. Tropomodulin
1. Brinds to (-) end of filament, preventing dissociation of actin monomers

Question 7

Cross-linking existing filaments into bundles and networks

Answer 7

A. Fimbrin
1. Holds two actin filaments close together in parallel
2. Supports projections of plasma membrane.
B. Alpha-actinin
1) Holds two actin filaments together so bundle can contract.
C. Filamin
1. Cross links actin filaments in network
2. Important for supporting surface of cell.

Question 8

Cell movement

Answer 8

1. Coordinated cycle of movements.
   a. Extension of leading edge, b. Attachment of leading edge to substratum, c. Retraction of the rear of cell.
2. Protrusions of leading edge.
   a. Filopodia - actin filaments
   b. Lamellopodia - actin bundles
   c. Filopodia go out first, followed by lamellopodia

Question 9

Cell Movement part 3

Answer 9

• extension requires branching and polymerization of barbed end of actin filaments.
  a. Actin binding proteins and activator recruited to plasma membrane.
  1. Arp 2/3
  2. WASP/Scar complex - activates Arp 2/3
  3. Barbed-end-tracking-proteins - connect growing actin filaments to plasma membrane.
  4. Enough force generated to move cell membrane
  5. Pointed ends of actin filaments disassembled by ADF/cofilin
  6. Carried to barbed end of filament by twinfilin
  7. Reactivated by profilin

Question 10

Wiskott-Aldrich Syndrome

Answer 10

• genetically inherited immunodeficiency disease that occurs almost exclusively in males
• function of white bloods cells is disrupted, making affected people susceptible to infections
• Affected bleed easily
• caused by mutation in gene coding for WASP protein
• affects actin cytoskeleton in immune and blood cells

Question 11

Non-muscle myosins

Answer 11

1. Myosins molecular motors- convert ATP to mechanical energy
2. Can bind to actin to allow movement
3. Myosin I
   a. Can move along actin filaments carrying cargoes

Question 12

Which of the following actin binding proteins initiates formation of actin branches?

Answer 12

Arp 2/3

Question 13

Which of the following stabilize actin filaments?

Answer 13

Fimbrin

Question 14

In a few families with congenital amyotrophic lateral sclerosis or Lou Gherig’s disease, the defective gene is profilin. Which of the following best describes its role?

Answer 14

Stimulates filament formation

Question 15

Wicket-Aldrich syndrome is a condition that affects cells of the immune system. Immune cells from Wiskott-Aldrich patients have poor motility and difficulty adhering to other cells. THe defect in the WASP protein. What is the function of WASP?

Answer 15

Activates Arp2/3