Interaction of myosin subfragment-1 with F- and G-actin in equilibrium

The structural changes of the F-actin-myosin head (S1) complex during the cross-bridge cycle are essential in muscle contraction. Although a large body of evidence has accumulated showing that the actin: S1 stoichiometry in the decorated F-actin-S1 filament is 1:1 at saturation by S1, a recent report by Andreev and Borejdo (1991, Biochem. Biophys. Res. Comm. 177, 350-356) indicated that under some conditions, the actin: S1 stoichiometry could be 2:1 at saturation by S1. Because of the important implications of this result in the mechanism of acto-myosin motility, we have re-investigated this issue. It is shown here that evidence for the 2:1 stoichiometry was circumstantial and was only observed under conditions where 50% of the actin was F-actin, i.e. at a total actin concentration twice as large as the critical concentration. The interaction of S1 with both F- and G-actin in dynamic equilibrium is studied in detail. The present data fully support the 1:1 actin: S1 stoichiometry in the decorated filament at saturation by S1.