Earliest event in the heat denaturation of myosin

In a previous paper (Kimura, I., Arai, K., & Watanabe, S. (1979) J. Biochem. 86, 1629-1638), we reported that when myosin was subjected to heat treatment, inactivation of actomyosin ATPase occurred in two steps; an early fast inactivation, followed by a slow inactivation. A further study was conducted on the fast inactivation in the early stage of heat treatment of myosin, and the following results were obtained: 1. As the weight ratio of actin to heat-treated myosin increased, the rate of early inactivation of actomyosin ATPase increased. 2. The superprecipitation activity of actomyosin was also decreased in two steps by heat treatment of myosin: an early fast inactivation and a subsequent slow inactivation were distinguishable. 3. The turbidity of myosin suspensions in 0.05 M KCl increased during heat treatment of myosin. The increase also proceeded in two distinct steps, and the rate constants for the two steps of increase were comparable to those for inactivation of actomyosin ATPase. 4. In an electron microscopic study, a new change in the morphology of "thick filaments" of myosin was detectable in the early period of heat treatment, when the fast inactivation of actomyosin ATPase was found to occur; myosin filaments were shorter and thinner than the "regular thick filaments" of untreated myosin, and their size was much more heterogeneous. Heterogeneity of myosin aggregates was also detectable in the sedimentation patterns. 5. The actin-binding ability of myosin or HMM remained unaffected during heat treatment of myosin. The binding ability was estimated in three different ways; by measuring the ATP-induced changed in the turbidity of actomyosin solution, the actin-inhibition of EDTA-ATPase of myosin, and the actin-activation of Mg-ATPase of HMM. 6. In a sedimentation study, formation of myosin dimers was not detectable in the early period of heat treatment of myosin, and was detectable only in the later period. Likewise, increase in the light scattering intensity of myosin solutions occurred only in the later period of heat treatment of myosin. 7. Heat treatment caused a decrease in the ATP-induced enhancement of the HMM fluorescence. The decrease was fitted by a single exponential (first-order reaction), and not by two exponentials. 8. Heat-induced change was not detectable either in the light scattering intensity of LMM solutions in 0.5 M KCl or in the turbidity of LMM suspensions in 0.05 M KCl. Based on these results, it is strongly suggested that the earliest event in heat denaturation of myosin is loss of the ability of myosin to form "regular thick filaments."