ELECTROSTATIC CHANGES AT THE ACTOMYOSIN SUBFRAGMENT-1 INTERFACE DURING FORCE-GENERATING REACTIONS

The ionic strength dependence of the binding of rabbit skeletal muscle myosin subfragment 1, S 1, to F-actin in the presence of saturating concentrations of MgATP or MgADP was analyzed in order to determine the association constants at zero ionic strength [K(0)] and the products of the net effective electric charges (\Z(M)Z(A)\) at the binding interfaces. K(0) and \Z(M)Z(A)\ Were 1 x 10(6) M-1 and 17 esu2 for S1-MgADP,P, and 5 X 10(7) M-1 and 7 esu2 for SI-MgADP, respectively, for binding to F-actin at 25-degrees-C. At ionic strengths near physiological, the increase in affinity is close to 10(4)-fold for this transition that may correspond to force generation in muscle fibers. The large, from 17 to 7 esu2, decrease in the electrostatic contribution to binding appears to be correlated with a much larger increase in nonelectrostatic interactions, unlike the simpler transition of actin-bound SI-MgADP to S1, which appears to be due entirely to electrostatic changes [Highsmith, S. (1990) Biochemistry 29, 10690-106941. These results for acto-S1-MgADP,P to acto-S1-MgADP suggest that a substantial transformation of the actin binding site on S1 occurs even if there is a translocation to a new interface.