Detergent modification of myosin function and structure in solution

Rabbit skeletal muscle myosin and myosin subfragment 1 (S1) MgATPase activities were increased 2-3-fold by the addition of a variety of molecules that contained single straight saturated 12-16-carbon chains. The nonionic detergent dodecyl nonaoxyethylene ether (C12E9) increased the activity of S1 to 50% of maximum at a free C12E9 concentration of 27 +/- 9 microM. The activation was reversible and was not due to chemical modification of S1 amino acid side chains. The Vmax for actin-activated S1 MgATPase activity was increased 3-fold by C12E9. The apparent association constant for S1 binding to pure F-actin was reduced 3-fold by C12E9. The [C12E9] dependencies of the increase in S1 and acto-S1 MgATPase activities and of the decrease in acto-S1 binding were equal, within experimental uncertainty, suggesting that a single detergent-induced S1 conformational change is sufficient to explain the results. The stoichiometry of C12E9 bound to S1 in the S1-C12E9 complex was estimated, by the S1 concentration dependence of the C12E9 activation midpoint and by the light-scattering increase when S1 and detergent were mixed, to be 7 and 57 C12E9 molecules per S1, respectively. The results are discussed in relation to possible structural aspects of the mechanism of action for S1 and acto-S1 MgATPase activities.