Question 1

How does cofilin promote actin filament depolymerization?

Accepted Answer

Cofilin binds to ADP-actin subunits at the F-actin minus-end in a one-to-one ratio. This binding causes the F-actin to twist, reducing helical turn length and generating mechanical stress. The stress increases ATP hydrolysis rates, making the filament brittle and allowing rapid release of ADP-actin monomers from the minus-end.

Question 2

What role does calcium play in gelsolin-mediated actin filament breakdown?

Accepted Answer

Calcium ions activate gelsolin, triggering a conformational change that enables binding to F-actin sides. Activated gelsolin-calcium wedges between actin subunits, disrupting monomer interactions and splitting the filament into two fragments. One fragment has a gelsolin-capped plus-end, while the other has a minus-end with rapidly dissociating ADP-actins.

Question 3

Why does mechanical stress from cofilin binding accelerate actin monomer release?

Accepted Answer

When cofilin twists the F-actin helix, it generates mechanical stress that increases the rate of ATP hydrolysis within the filament. This heightened hydrolysis makes the filament structurally brittle, weakening the bonds holding ADP-actin monomers together and facilitating their rapid dissociation from the minus-end.

Question 4

How does glia maturation factor regulate depolymerization at branched actin networks?

Accepted Answer

GMF binds to the Arp2/3 complex at branch junctions within actin networks, preventing further nucleation of actin filaments from that site. This regulation is critical for lamellipodia formation required for cell movement and migration, allowing cells to dynamically remodel their cytoskeletal architecture during intracellular movement of viruses and bacteria.

Question 5

What is the difference between minus-end and plus-end depolymerization rates?

Accepted Answer

Actin monomers dissociate faster from the minus-end or pointed end, where ADP-actin subunits predominate. The plus-end has slower dissociation rates. This polarity difference is fundamental to actin dynamics and enables directional filament turnover, supporting the adaptability of cytoskeletal filaments during cellular processes.

Question 6

How do ADF and cofilin proteins enhance actin filament disassembly?

Accepted Answer

ADF and cofilin family proteins bind ADP-actins in a one-to-one ratio, twisting the filament and generating mechanical stress that promotes monomer dissociation. These proteins are further enhanced by association with AIP-1 (actin-interacting protein 1), which accelerates the dissociation rate at the minus-end and amplifies depolymerization efficiency.

Question 7

What structural changes occur when gelsolin inserts between actin subunits?

Accepted Answer

When activated gelsolin binds to F-actin sides, it wedges itself between actin monomers, disrupting the interactions that hold the filament together. This insertion breaks the continuous filament into two separate fragments, each with distinct structural properties and depolymerization characteristics at their newly exposed ends.

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