Abstract
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.
Highlights
It has long been assumed that the interaction of actin and myosin in the presence of adenosine triphosphate and Mg* at low ionic strength always results in superprecipitation of t,he actomyosin complex as well as activation of the myosin ATPase
HMM at two different KC1 concentrations, as well as the viscosity after all the ATP had been hydrolyzed. Both ATPase activity and viscosity are expressed as multiples of the theoretical values to be expected in the absence of any interaction between the actin and HMM
Complex responsible for the 17-fold ATPase activation does not give rise to a measureable increase in the viscosity of the sample above that predicted for a noninteracting mixture of actin and ATPase Activity during Clearing-In the hope of correlating the studies on acto-HMM with similar studies on actomyosin in a dissolved state, we turned to an investigation of the interaction of actin and myosin during clearing
Summary
The interaction of actin with myosin and heavy meromyosin was studied in the presence of Mgf+ ion and adenosine triphosphate at low ionic strength. This suggests either that the actomyosin complex can be soluble at low ionic strength or that strong physical interaction does not always accompany enzymatic interaction The latter possibility is supported by studies on the acto-heavy meromyosin system by Leadbeater and Perry [7] and Perry, Cotterill, and Hayter [8] who found that actin activated the ATPase of HMMr under conditions where viscosity measurements indicated that the two proteins might be dissociated. When these data were extrapolated to the actomyosin system, the latter authors argued that the interaction of actin and myosin, which results in activation of the myosin ATPase, is not accompanied by as strong a physical interaction as that which occurs between the two proteins in the absence of ATP. In the case of actomyosin in the cleared state, simultaneous measurements of turbidity, viscosity, and ATPase activity have shown that considerable activation of the myosin ATPase can occur in the absence of any measurable precipitation of the actomyosin and that, in contrast to the acto-HMM sysd.?n; &hi,- ~rqvnmii~ linkrs~iinr Gp ~~~kwys- acevrnpa&u’ dy- a corresponding viscosity increase, which indicates formation of a relatively stable actomyosin complex in the cleared state
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