Actin

Question 1

Where can you find actin micro filaments in cells?

Answer 1

Microvilli, cell cortex, filopodia, lamelopodia, stress fibers, phagocytosis and contractile ring

Question 2

How many subdomains of G-actin?

Answer 2

4 subdomains which organise into F-actin in 4 steps

Question 3

What are the 4 stages of F-actin formation?

Answer 3

1. Nucleation
2. Elongation
3. Branching
4. Steady state

Question 4

Explain nucleation step of actin filament formation

Answer 4

3 globulins form a stable nuc

Question 5

What and Why is there a twist in F-actin?

Answer 5

Conformational change in F-actin makes it more flat, 20 degrees rotation of inner relative to outer domain.

Question 6

What about elongation step of actin filament formation?

Answer 6

Actin binds ATP and has ATPase activity. Polar structure assembles into a double helix sub globular structure with a twist. Repeating unit is 72 nm and 28 subunits. The “-“ or pointed end and “+” barbed end of filament.

Question 7

Why do you need ATP for F-actin formation?

Answer 7

ATP/ADP binding regulates actin conformation, but not needed for energy! G-actin will be incorporated into Factin only when bound to ATP.

Question 8

Explain actin treadmiling

Answer 8

At steady state condition, when total incorporation of g-actins= total dissociation, actin is treadmilling. Length stays the same, but filaments are moving.

Question 9

ABPs

Answer 9

Actin binding proteins: profilin, tropomyosin, capping protein, cofilin and thymosin

Question 10

Function of tropomyosin-
Of capping protein
Of cofilin
Thymosin-

Answer 10

Tropomyosin- lateral binding, stabilizes filament
Capping protein- prevents assembly and disassembly at + end
Cofilin- binds ADP-actin filaments, accelerates disassembly!
Thymosin- binds and sequesteres ATP- actin; competes with profilin for atp-actin binding. Profilin is speeding elongation!

Question 11

Give an example of capping proteins of actin

Answer 11

Tropomodulin

Question 12

What is the role of Arp2/3?

Answer 12

Actin branching by nucleation factor arp2/3

Question 13

How are NFs regulated?

Answer 13

Gtpase stimulate NFs at plasma membrane. So GTPases indirectly promote actin elongation, branching

Question 14

Tell me about formins!

Answer 14

Formins are activated via activation of GTPase Rho. They stimulate actin nucleation by binding profilin ATP G actin complex. They remain bound to + end to prevent capping of filament. Formin domains FH1,2 are loading g-actins on the elongating filament