The Interaction of Actin with Myosin and Heavy Meromyosin in Solution at Low Ionic Strength

Abstract The interaction of actin with myosin and heavy meromyosin was studied in the presence of Mg++ ion and adenosine triphosphate at low ionic strength. When the heavy meromyosin ATPase was 17-fold activated by actin, the actoheavy meromyosin complex responsible for this activation did not give rise to a measurable increase in the viscosity of the sample above that predicted for a noninteracting mixture of actin and myosin. In the case of myosin, simultaneous measurements of turbidity, viscosity, and ATPase activity in the presence of actin showed, first, that considerable activation of the myosin ATPase could occur in the absence of any measurable precipitation of the actomyosin, and second, that in contrast to the behavior of the acto-heavy meromyosin system, ATPase activation was always accompanied by a corresponding viscosity increase indicating the formation of a relatively stable actomyosin complex in the cleared state. Comparison of the ATPase activity of acto-heavy meromyosin and actomyosin, under conditions where both systems were in solution (0.1 m KCl), indicated a distinct difference in the response of the actomyosin and acto-heavy meromyosin ATPases to falling ATP concentration.