On the question of co-operative interaction of myosin heads with F-actin in the presence of ATP

Abstract

The actin-activated enzymic activity of partially modified heavy meromyosin molecules has been compared to the unmodified protein in order to examine the possibility of co-operative interactions between the two myosin heads. The modification of the myosin heads with the bifunctional thiol reagent p-phenylenedimaleimide proceeded at the same rate with subfragment-1 preparations containing either the alkali 1 chain or alkali 2 chain. The modified, inactivated heads did not bind actin. Enzymic assays of the reacted heavy meromyosin solutions, combined with the analysis of their sedimentation in the presence of F-actin, verified that the two heads reacted with p-phenylenedimaleimide independently. Well-defined heavy meromyosin samples containing fractions of zero, one and two inactivated heads were then examined for their interaction with F-actin in the presence of ATP. Maximum ATP turnover rates, Vmax, were found to be linearly dependent upon the total fraction of unmodified heads and independent of the relative fraction of the one-head modified molecules. The apparent dissociation constants of actin from heavy meromyosin, Kapp, increased somewhat with the increasing fraction of one-head modified molecules. These results are consistent with an independent binding of myosin heads to actin in the presence of ATP.