Actin Polymerization and Its Relationship to Adenosine Triphosphatase Activity

F-actin was prepared from pig heart myofibrils without the use of organic solvents. G-actin, prepared from F-actin by sonication, had adenosine tri- phosphatase activity. Dephosphorylation of added ATP by G-actin had a broad pH optimum between pH 7.5 and pH 9.0, specifically required Mg 2 + , and was activated by EGTA. G-actin prepared from skeletal muscle of rabbits by similar methods also showed ATPase activity. Cardiac G-actin and G-actin from skeletal muscle had equimolar nucleotide bound to them. Polymerization of cardiac G-actin was greatest at a pH of about 6.5 and declined above pH 6.8. KC1 was necessary for polymerization, and EGTA inhibited the con- version to F-actin. By selectively acetylating G-actin, using acetic anhydride, a preparation of G-actin was obtained that did not have adenosine triphosphatase activity but did polymerize under optimal conditions. It is concluded that dephosphorylation of ATP and the polymerization process in cardiac G-actin are two different reactions occurring at different sites on the actin molecule.