Cofilin dissociates Arp2/3 complex and branches from actin filaments

Actin filaments form branched networks that drive cellular movement. These networks are regulated by proteins like Arp2/3 complex and cofilin. While Arp2/3 complex promotes branching, cofilin is known to sever filaments and remove branches. However, the precise mechanism by which cofilin influences Arp2/3 complex binding and branch stability remains unclear. Prior research has shown that cofilin affects filament structure and dynamics. That uncertainty drove this investigation into the interplay between cofilin and Arp2/3 complex. This gap motivated the use of fluorescence and TIRF microscopy to explore binding kinetics and debranching. No prior work had resolved how cofilin occupancy affects Arp2/3 complex dissociation. This study addresses that limitation by examining cooperative effects. The results clarify how cofilin concentrations influence branch stability. The findings offer new insights into actin network remodeling.

The goal of this research was to determine how cofilin and Arp2/3 complex interact to regulate actin filament branching. This paper focuses on the spatial transitions between branched and unbranched networks. The study aimed to clarify whether cofilin removes branches directly or through structural changes. The researchers tested the hypothesis that cofilin occupancy affects Arp2/3 complex binding. They sought to measure the kinetics of cofilin binding and Arp2/3 complex dissociation. The study also aimed to determine the threshold of cofilin occupancy for effective debranching. This work sought to distinguish between direct competition and cooperative effects. The findings would help explain how actin networks are remodeled during motility.

The researchers used fluorescence spectroscopy to track cofilin binding and Arp2/3 complex dissociation. They measured the kinetics of cofilin association with actin filaments. TIRF microscopy was employed to visualize filament severing and branch loss. The setup allowed observation of single filaments and branch dynamics. The experiments tested how cofilin occupancy affects Arp2/3 complex binding affinity. They varied cofilin concentrations to determine occupancy thresholds. The study compared direct competition with cooperative effects on branch stability. The results were analyzed to determine how structural changes influence debranching.

Cofilin binding reduces Arp2/3 complex affinity for actin filaments. The study found that cofilin occupancy decreases branch stability. At low occupancy, cofilin severs filaments but does not remove branches. At higher occupancy, cofilin rapidly dissociates Arp2/3 complex and branches. Effective debranching occurs at lower cofilin concentrations than expected. Structural changes in the filament reduce Arp2/3 complex binding affinity. Cofilin acts through direct competition and cooperative binding effects. The results suggest that cofilin promotes debranching via multiple mechanisms.

The authors propose that cofilin promotes debranching through two mechanisms. First, cofilin competes with Arp2/3 complex for binding sites on actin filaments. Second, cofilin induces structural changes that lower Arp2/3 complex affinity. Effective debranching occurs at lower cofilin concentrations than previously assumed. The study shows that direct competition is not the only pathway for debranching. Structural changes in the filament enhance cofilin's debranching effect. The findings suggest that cofilin occupancy and filament structure are linked. The results support a model where cofilin acts cooperatively at low occupancy. These conclusions clarify how actin networks are remodeled during motility.