ACTIVATION OF H-MEROMYOSIN ATPASE BY POLYMERS OF ACTIN AND CARBOXYMETHYLATED ACTIN

Carboxymethylation of actin was carried out in the state of F-actin by the reaction with bromoacetic acid. By Carboxymethylation the ability of F-actin to activate the Mg-ATPase of H-meromyosin was reduced to less than 50% of normal F-actin, without breaking the polymer structure. The relation of the ATPase activation to the amount of added F-actin showed that Carboxymethylation decreases the binding constant between H-meromyosin having ATP and F-actin, with no influence on the ATP splitting rate of the complex of two proteins. An important role of the increased negative charge of Carboxymethylated F-actin in decreasing the binding constant was suggested by the fact that the effect of Carboxymethylation became very small at low pH or at high ionic strength. G-actin obtained by depolymerization of Carboxymethylated F-actin was polymerizable at nearly the same rate with normal G-actin. The ATPase activation by copolymers of this Carboxymethylated G-actin and normal G-actin was compared with that by simple mixtures of homopolymers, Carboxymethylated F-actin and normal F-actin. At the same composition, the activation by copolymers is lower than that by mixtures of homopolymers. This result excludes the possibility that in the activation of the ATPase each actin monomer in the copolymer interacts with an H-meromyosin molecule, independently of neighboring monomers. The relation observed between the extent of activation and the composition of copolymers can be explained very well by assuming that the activation of the ATPase needs simultaneous binding of two subunits of an H-meromyosin molecule with two neighboring actin monomers in F-actin. © 1969.