Interaction of Actin and Myosin during Clearing

Abstract

SUPERPRECIPITATION of actomyosin has long been thought to involve both binding of the actin to the myosin and activation of the myosin adenosine triphosphatase (ATPase) by actin in the presence of magnesium¹. The clearing response, on the other hand, has been reported to result from dissociation of the actomyosin with a concomitant loss of this ATPase activation². In both situations, the physically observable binding and the activation of the myosin ATPase by actin were thought to change in parallel³, but some recent observations have been difficult to reconcile with this conclusion. In investigating the effect of tropomyosin B on actomyosin, A. M. Katz⁴ noted that actin can activate the myosin ATPase without superprecipitation occurring. In other investigations, Leadbeater and Perry⁵, and Perry and Cotterill⁶, found that actin activates the ATPase of heavy meromyosin under conditions where viscosity measurements indicated that the two proteins might be dissociated. Extrapolating these data to the actomyosin system, Perry et al. argued that the interaction of actin and myosin which results in activation of the myosin ATPase is not accompanied by physical interaction as strong as that which occurs between the two proteins in the absence of adenosine triphosphate (ATP). Maruyama and Gergely², however, noted that actomyosin undergoes a brief viscosity increase simultaneously with the rise in ATPase just before superprecipitation, which suggests that strong physical interaction does accompany the activation of the myosin ATPase by actin.