Effects of Cofilin Phosphorylation on Actin Filament Binding Cooperativity, Mechanics, and Severing

Abstract

Actin cytoskeleton remodeling is crucial for many vital cellular processes including division, growth, and motility. The actin regulatory protein cofilin contributes to actin remodeling dynamics by creating new filament ends and replenishing the actin monomer pool. Cooperative cofilin binding introduces a topological and mechanical asymmetry at boundaries of bare and cofilin decorated filament segments, which is proposed to facilitate filament severing. Cofilin binding to actin filaments is regulated by phosphorylation. However, the extent to which phosphorylation modulates human cofilin binding cooperativity and severing has not been systematically evaluated. We utilize fluorescence spectroscopy and an Ising model with cooperative nearest-neighbor interactions to quantitate wild-type and phosphomimetic mutant (S3D) human cofilin binding to actin filaments. Modulation of filament mechanics and severing upon cofilin binding is assessed by fluorescence microscopy. The S3D binds filaments with weaker intrinsic binding affinity, yet cooperative interactions are comparable to wild-type cofilin. Preliminary data suggest the phosphorylation mimic mutation also impacts the ability of cofilin to modulate filament mechanics and severing activity.