Kinetic Studies of Actin Polymerization and Effects of Myosin A and H-Meromyosin on the Polymerization

Abstract

1. Actin polymerization was studied by the measurement of inorganic orthophosphate liberation from G-actin after the addition of MgCl₂, CaCl₂ and KCl. The time-course of phosphate liberation followed first order kine-tics, with respect to the G-actin concentration, but the dependence of the initial velocity on G-actin concentration was of the third order.
2. Changes in light scattering and viscosity accompanied with actin polymerization were studied in various environments and at dif-ferent pH values. In contrast to the reaction of phosphate liberation, these changes did not follow any simple kinetics.
3. The accelerating effects of myosin A and H-meromyosin on phosphate liberation from G-actin was studied in the presence of salts. The time course of phosphate liberation in the presence of these proteins also followed first order kinetics. The accelerating effect of H-meromyosin reached a maximum when the molar ratio of H-meromyosin to G-actin was 1:10.
We wish to thank Dr. Y. Tonomura for many valuable suggestions and criticisms and Mr. H. Naka-mura for skilled technical assistance.