How ion binding affects actin filament stability and flexural rigidity

Actin filaments are semi-flexible biopolymers essential for the mechanical support and cell motility. Ions strongly affect actin polymerization and the flexibility of actin filaments; however, the molecular basis for how ions are coupled to the mechanics of actin filaments remains elusive. Here, we demonstrate a linkage between cation binding and both actin filament polymerization and flexural rigidity. Our results show that the thermodynamic stability and flexural rigidity of actin filament increase with cation concentration in a manner that implicates specific cation binding as opposed to general electrostatic screening. Using structural bioinformatics, we identify two distinct cation-binding sites within the F-actin structure that help explain how specific cation binding is linked to actin polymerization and flexural rigidity. Site-specific substitution of a charged amino acid residue at one of the sites modulates the cation concentration-dependence of filament bending stiffness, consistent with a bound cation at this site increasing the flexural rigidity of actin filaments. Mutation of a charged amino acid at the other site causes ``polymerization incompetent'' G-actin.