ATPase activity of plasmodium actin polymer formed in the presence of Mg 2+

Abstract

1. In the presence of Mg 2+, plasmodium actin forms a polymer different from F-actin. This polymer, termed Mg-polymer, has an ATPase activity. Its specific activity is about one-hundredth of the activity of myosin from muscle or plasmodium and of the same order as that of muscle F-actin under sonic vibration. 2. The formation of Mg-polymer and its ATPase activity are not due to modification of the protein in the preparation procedure. The ATPase activity is not due to contamination in the preparation but is closely related to the amount of Mg-polymer formed. 3. The ATPase activity of Mg-polymer is markedly inhibited by high concentration of ATP (about 1 mM) in the presence of KCl; but it is not inhibited in the absence of KCl. 4 Mg-polymer releases its bound [ 14C]ADP in a cold ATP solution. The half time of the release is about 10 min at 22°, which is of the same order as the rate of ATP splitting. This is understandable if both the ATP splitting and the ADP exchange are associated with the same cyclic process of intrapolymer reaction occurring everywhere along the Mg-polymer. 5. Copolymers of plasmodium and muscle G-actins are formed in the presence of Mg 2+. The ATPase activity of copolymers decreases rapidly with increasing content of muscle actin; it is suggested that interaction of neighboring plasmodium actin molecules in the polymer is required for the ATPase activity.