Actin-filament stochastic dynamics mediated by ADF/cofilin

Actin filaments are essential for many cellular functions, yet the mechanisms governing their dynamic behavior remain unclear. While it is known that accessory proteins influence actin filament turnover, the precise coordination of these proteins is not fully understood. Prior research has shown that actin-depolymerizing factor (ADF)/cofilin and profilin regulate filament dynamics. However, the interplay between these proteins and how they modulate filament turnover is still under investigation. This gap motivated the need to study how these proteins act together to control actin dynamics. Observing filament behavior in real time is a challenge due to the complexity of the cytoskeletal network. No prior work had resolved the exact mechanism of ADF/cofilin-induced severing. This uncertainty drove the development of a system that allows direct observation of single filaments. The study aims to address this by combining evanescent-wave microscopy with a biomimetic setup. This approach enables researchers to track filament behavior in a controlled environment.

The study aims to understand how actin-depolymerizing factor (ADF)/cofilin and profilin regulate actin filament dynamics. The specific problem is to determine how these proteins interact to modulate filament turnover. The motivation comes from the need to move beyond merely identifying the proteins involved and instead explore their synergistic effects. The researchers propose to use a combination of evanescent-wave microscopy and a biomimetic system to observe single actin filaments in real time. This method allows for tracking the effects of accessory proteins on filament dynamics. The goal is to identify the mechanisms that control actin filament turnover. By observing individual filaments, the study can reveal how proteins like ADF/cofilin and profilin influence filament behavior. The findings may provide insights into how cells regulate actin dynamics for various functions.

The researchers used evanescent-wave microscopy to visualize single actin filaments in real time. This technique allows for the observation of filament dynamics in a controlled environment. A biomimetic system was employed to simulate physiological conditions. The setup included a physiologically relevant mixture of accessory proteins. Actin filaments were labeled to track their behavior. The study focused on actin filaments with processive formin attached at their barbed ends. The presence of ADF/cofilin and profilin was monitored to observe their effects. The method enabled the researchers to track stochastic growth and shrinkage phases of individual filaments.

The study found that actin filaments oscillate between growth and shrinkage phases when ADF/cofilin and profilin are present. ADF/cofilin-induced severing is the key mechanism modulating filament shortening. The presence of a processive formin and profilin-actin promotes continuous actin polymerization. The combination of these factors leads to a 155-fold increase in filament turnover in vitro. Lateral contact between filaments reduces dynamics and favors cable formation. The kinetic simulation confirmed the observed behavior. Actin filaments with formin at their barbed ends show stochastic behavior. The results suggest that ADF/cofilin-mediated severing is central to filament dynamics.

The study concludes that ADF/cofilin-mediated severing is the dominant mechanism modulating actin filament dynamics. The observed stochastic behavior of individual filaments is attributed to this severing activity. The combination of continuous polymerization and severing increases filament turnover significantly. Lateral interactions between filaments reduce dynamics and promote cable formation. The kinetic simulation supports the observed behavior. The proposed mechanism explains how actin dynamics are controlled in cells. The findings suggest that filament bundling stabilizes structures in cells. These conclusions are based on the observed effects of ADF/cofilin and profilin on actin filaments.