Conformational changes induced in plasmodium actin polymer by Ca2+ in the presence of muscle native tropomysin.

The flexible polymer of plasmodium actin (Mg-polymer) combines with muscle native tropomyosin up to a weight ratio of actin: tropomyosin:troponin = 6:1:1. Mg-polymer which combined with muscle native tropomyosin undergoes a conformational change when the Ca2+ concentration of the solution is decreased by adding 0.5 mM ethylene glycol bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA): the viscosity of the solution increases by 50% and its ATPase activity decreases to about 20% of the original value, which suggests that the structure of Mg-polymer becomes more rigid. This change is reversible with respect to the Ca2+ concentration in the solution. The threshold concentration of Ca2+ for the conformational change is about 10(-6) M. The possible role of this transformation in vivo is discussed.