A Further Study of Electron Microscopic Particle Length of F-Actin Polymerized in Vitro

The electron microscopic particle lengths of F-actin under several experimental conditions were investigated by electron microscopy. 1. Depolymerization process of preformed F-actin was followed. When KC1 was dialyzed out from an F-actin solution in 0.1 M KCl, the amount of F-actin decreased rather rapidly but the length distribution as well as the average length of remaining F-actin particles did not change significantly for as long as 24 hr of dialysis. These results suggest that the depolymerization occurs in the manner of dissociation of G-actin from the ends of F-actin. 2. The particle length of preformed F-actin was independent on temperature below the transition temperature leading to irreversible denaturation (55 to 60°C). This finding supports the view that the ATP splitting activity of F-actin at high temperature, observed previously by Asai and Tawada, is mainly due to the intrapolymer structural changes in F-actin particles. 3. The particle length of F-actin once polymerized at a certain high portein concentration, where the particle length of formed F-actin was shorter, was gradually increased as time elapsed after the protein concentration was lowered. β-Actinin inhibited this increase of the particle length probably due to the binding to the ends of F-actin. 4. It was observed under the electron microscope that heavy meromyosin bound to F-actin particles fragmented them into short particles and that the extent of fragmentation was higher when heavy meromyosin was mixed with F-actin at a molar ratio of 1/3 than at the ratio of 1/1. On the other hand, Subfragment-I, the functional part of heavy meromyosin, when bound to F-actin, did not cause any changes in the electron microscopic paticle length of F-actin. These results were explained by the assumption that the structure of F-actin was loosened when both two subunits of one heavy meromyosin molecule bound to the monomers of actin in F-actin particle(s), and such fragile particles would be easily fragmented into shorter particles when treated with uranyl acetate.