Effect of temperature on the working stroke of muscle myosin

Muscle contraction is due to myosin motors that transiently attach with their globular head to an actin filament and generate force. After a sudden reduction of the load below the maximum isometric force (T0), the attached myosin heads execute an axial movement (the working stroke) that drives the sliding of the actin filament toward the center of the sarcomere by an amount that is larger at lower load and is 11 nm near zero load. Here, we show that an increase in temperature from 2 to 17 degrees C, which increases the average isometric force per attached myosin head by 60%, does not affect the amount of filament sliding promoted by a reduction in force from T0 to 0.7T0, whereas it reduces the sliding under low load by 2.5 nm. These results exclude the possibility that the myosin working stroke is due to the release of the mechanical energy stored in the initial endothermic force-generating process and show that, at higher temperatures, the working stroke energy is greater because of higher force, although the stroke length is smaller at low load. We conclude the following: (i) the working stroke is made by a series of state transitions in the attached myosin head; (ii) the temperature increases the probability for the first transition, competent for isometric force generation; and (iii) the temperature-dependent rise in work at high load can be accounted for by the larger free energy drop that explains the rise in isometric force.