The removal of the bound ADP of F-actin

F-actin essentially free of bound nucleotide has been prepared by replacing bound ADP with AMP under sonic vibration and subsequently removing bound AMP by charcoal treatment. A comparison of nucleotide-deficient and normal F-actin revealed no differences between the two types in reversible depoly-merization, viscosity, combination with myosin at high ionic strength, stimulation of Mg 2+ -ATPase activity of myosin at low ionic strength or superprecipitation with myosin. Nucleotide-deficient F-actin bound ATP stoichiometrically. ATP, on combining with this F-actin, was hydrolyzed into inorganic phosphate and F-actin-bound ADP. ATP was bound to nucleotide-deficient F-actin under various conditions, both in the presence and absence of myosin, and whether or not the actomyosin was superprecipitated, e.g. in the presence of Mg 2+ , EDTA or Ca 2+ . Nucleotide-deficient F-actin did not appreciably bind deoxyATP or CTP either in the presence or absence of myosin, but stimulated the Mg 2+ -deoxy-ATPase and Mg 2+ -CTPase activity of myosin several-fold as normal F-actin does. Mg 2+ -deoxyATP and Mg 2+ -CTP caused superprecipitation of myosin with nucleotide-deficient F-actin to the same extent that it did with normal F-actin. It is suggested that the bound ADP of F-actin is not involved in the physiologically important properties of F-actin, namely, the activation of the ATPase activity of myosin and superprecipitation with myosin in the presence of ATP.