Heavy meromyosin binds actin with negative cooperativity.

The association of fluorescently labeled heavy meromyosin (HMM) and F-actin was measured by time-resolved fluorescence depolarization. The effects of varying the protein concentrations, temperature, KCl concentration, and pH were determined. Measurements of HMM mobility supported a model of no interaction between the two heads in the absence of actin. Measurements of actin binding, when compared with results for myosin subfragment I, indicated that the two heads of HMM do not bind independently in the rigor complex. This could result from actin-transmitted negative cooperativity or from steric inhibition due to the structure of HMM. For HMM and actin in 0.15 7 kcl at 25 degrees C: Ka = 3.9 X 10(7) M-1, deltaHco' = 36 +/- 2 J M-1, deltaSco' = 0.26 +/- 0.02 kJ M-1 K-1; the slope of ln Ka vs. [KCl]1/2 = -3.88 and the pH of maximum association was 6.9.