Strain-Dependence of the Actin-Myosin Working Step

For more than 80 years, it has been known that the force applied to shortening muscle slows the chemistry of contraction. More recently it has been shown that mechanical strain slows the rate of ADP release. Here we show that mechanical strain slows the rate-limiting step in the actin-myosin ATPase cycle associated with the actin-myosin working step. In an in vitro motility assay, we measured the actin-activated ATPase activity, the rate of actin filament breaking, and actin sliding velocities at different myosin densities. At low myosin densities, changes in actin sliding velocities correlate with changes in actin-myosin ATPase activity with little or no filament breakage. At higher myosin densities, both actin-activated ATPase activity and actin sliding velocities saturate as the breaking rate increases, indicating that actin sliding velocities still correlate with ATPase activity even at high myosin densities and the saturation of both results from strain-dependent kinetics of the rate-limiting step for actin-myosin ATPase. These results imply that mechanical load slows the actin-myosin working step and that this strain-dependence has significant implications for the effects of cooperative interactions among myosin heads on muscle contraction