Effect of the filamentous structure of myosin on the actomyosin ATPase activity

The ATPase activities of acto-heavy meromyosin and of acto-myosin minifilaments have been compared under the same conditions at low ATP (0.1 mM) and at several KC1 concentrations. The activities, which are strongly salt-dependent in both systems, have been found to be similar at high ionic strength (about 0.16 M) but different at lower ionic strength (0.06-0.07 M). Under this last condition, the catalytic constants kcat and Km are lower for acto-myosin minifilaments than for acto-heavy meromyosin ATPase. In addition, at low ionic strength, any decrease in the concentration of any of the ionic species (ATP, citrate, etc.) induces an increase in the interaction strength between myosin and actin filaments, as revealed by the Km changes. The presence of the troponintropomyosin complex and of Ca2+ also enhances the strength of this interaction. On the other hand, the occurrence of particular interactions between F-actin and myosin minifilaments is further substantiated by the phenomenon of superprecipitation which occurs when the ATP concentration decreases. The favourable effect of the organized structure of the myosin minifilaments on the ATPase activity of actomyosin is discussed.