Effect of fragmin on actin polymerization: Evidence for enhancement of nucleation and capping of the barbed end

Abstract

As reported previously, fragmin isolated form Physarum plasmodia restricts the polymerization of actin to produce short F-actin filaments in the presence of Ca²⁺ ions. Here it is shown that when actin is polymerized at low concentrations of salts, fragmin increases the critical concentration of actin for polymerization. This effect of fragmin on the critical concentration is independent of the molar ratio of fragmin to actin. The addition of actin monomers onto heavy meromyosin-decorated F-actin fragments treated with fragmin occurs unidirectionally at the pointed end of each fragment. These results suggest that fragmin binds to the barbed ends of F-actin filaments and inhibits association and dissociation of actin monomers at this end. Fragmin accelerates the initial stage of polymerization of actin. When a constant amount of G-actin is polymerized in the presence of small amounts of fragmin, the inverse of the half-polymerization time increases in proportion to the square root of the amount of fragmin added. This means that fragmin acts as a potent promoter of the nucleation step in actin polymerization. Both functions of fragmin–promotion of nucleation and capping at the barbed end of F-actin–require micromolar concentrations of Ca²⁺.