Superprecipitation induced by soluble myosin fragments

Superprecipitation (s.p.) took place when both an active myosin fragment [heavy meromyosin (HMM) or HMM subfragment-1 (S-1)] and an inactivated myosin were added to actin. The duration of the "clearing phase" decreased, while the rate and extent of s.p. increased up to a constant value when the myosin fragment concentration was raised. The extent and rate were higher while the delay time shorter for HMM, as compared to S-1 at the same concentration, No s.p. could be detected when: a) an inactivated myosin fragment or the ATPase apyrase was used; b) MgATP was replaced by Mg-pyrophosphate; c) the ability of myosin to form "rigor" complex with actin has been abolished. It is concluded that the soluble myosin fragment is probably involved in the mechanochemical process associated with s.p..