ADF/Cofilin Accelerates Actin Dynamics by Severing Filaments and Promoting Their Depolymerization at Both Ends.

The actin cytoskeleton is central to cell motility and shape changes. While ADF/cofilin proteins are known to influence actin filament dynamics, the precise mechanisms remain unclear. Prior research has shown that ADF/cofilin can sever filaments and promote depolymerization. However, the rates and specific effects on barbed and pointed ends are still debated. Existing models suggest that capping proteins block barbed ends while pointed ends depolymerize. This uncertainty has driven the need for direct measurements of ADF/cofilin activity. The classical view does not account for isoform-specific differences or the role of capping proteins in end behavior. No prior work had resolved how ADF/cofilin affects both ends simultaneously. This gap motivated the current investigation into the detailed dynamics of actin filament disassembly. Understanding these mechanisms is crucial for modeling cytoskeletal function in living systems.

This study aimed to clarify the mechanisms by which ADF/cofilin proteins influence actin filament dynamics. Specifically, the researchers focused on the three mammalian ADF/cofilin isoforms and their effects on skeletal muscle and cytoplasmic actin filaments. The goal was to directly quantify severing and depolymerization reactions at both filament ends. The team sought to determine whether ADF/cofilin promotes depolymerization at barbed ends in the absence of monomeric actin. They also wanted to assess how capping proteins interact with ADF/cofilin-decorated ends. The study aimed to reveal isoform-specific differences in depolymerization rates. The researchers proposed that these findings could challenge existing models of actin dynamics. Their work may provide new insights into how ADF/cofilin and capping proteins regulate filament turnover.

The researchers used single-molecule techniques to monitor ADF/cofilin activity on individual actin filaments. They tested the three mammalian ADF/cofilin isoforms on skeletal muscle and cytoplasmic actin. The experiments measured depolymerization rates at both barbed and pointed ends. They observed how ADF/cofilin interacts with bare filament ends in the absence of monomeric actin. The team compared depolymerization rates of ADF/cofilin-saturated filaments to those of bare filaments. They assessed the effect of capping proteins on filament ends decorated by ADF/cofilin. The researchers tracked the frequency of severing events and their outcomes. They used time-lapse imaging to capture the dynamic behavior of filament ends.

The study found that ADF/cofilin can bind to bare barbed ends and accelerate their depolymerization. ADF/cofilin-saturated filaments depolymerize faster at pointed ends and slower at barbed ends. This results in similar overall depolymerization rates at both ends. The effect varies by isoform, with ADF showing faster depolymerization than cofilin. Barbed ends of ADF/cofilin-decorated filaments resist capping and elongation. These ends undergo depolymerization even when capping protein and actin monomers are present. Severing events produce depolymerizing barbed ends in 17% of cases. Capping protein allows ADF/cofilin domains to reach barbed ends, promoting uncapping and depolymerization. These findings suggest that barbed-end depolymerization may occur in cells. The results challenge the classical view of capping protein function in actin dynamics.

The authors propose that ADF/cofilin and capping protein jointly regulate actin filament dynamics. Their findings suggest that barbed-end depolymerization may occur in cellular environments. The study highlights isoform-specific differences in depolymerization rates. ADF appears to promote faster depolymerization than cofilin. The results indicate that ADF/cofilin-decorated barbed ends resist capping and elongation. These ends undergo depolymerization even in the presence of capping protein. The researchers observed that capping allows ADF/cofilin domains to reach barbed ends. This promotes uncapping and subsequent depolymerization of the filament. The study provides new insights into how ADF/cofilin and capping proteins interact. The findings may lead to revised models of actin filament turnover in cells.