Dynamics of Actin Filaments

What are the three phases of in vitro G-actin polymerization?

1. Nucleation (lag phase) 2. Elongation phase 3. Steady state phase

What occurs during the nucleation phase of actin polymerization?

Three ATP–G-actin monomers come together to form a stable nucleus or "seed", which is an inefficient initial step.

What occurs during the steady state phase of actin polymerization?

G-actin monomers exchange at the filament ends at equal rates, resulting in no net change in total filament length.

Why does ‘tread-milling’ occur in actin filaments at steady state?

ATP-actin assembles ~10x faster at the (+) end, while ADP-actin disassembles preferentially from the (-) end.

What is the role of Profilin in actin dynamics?

It catalyzes the exchange of ADP for ATP on G-actin and directs ATP–G-actin assembly exclusively to the (+) end.

What is the role of Cofilin in actin dynamics?

It binds to ADP-actin regions of the filament and fragments them, creating more (-) ends to accelerate depolymerization.

What is the function of Thymosin-β4?

It acts as a buffer/reservoir that sequesters ATP–G-actin at high concentrations and releases it when free monomer levels drop.

What is the function of CapZ?

It Binds to & caps the (+) end of an actin filament, preventing subunit addition

What is the function of Tropomodulin?

It caps the (-) end of an actin filament, preventing disassembly and stabilizing the filament.

How does Gelsolin alter actin filaments, and what activates it?

It severs actin filaments and caps the newly formed (+) end; it is activated by a rise in Ca2+ concentration.