Effect of calcium ion on the thermal denaturation of subfragment-1 and rod regions of squid myosin upon the heating of myofibrils

Thermal inactivation of Ca 2+ ATPase of squid myofibrils was significantly suppressed in the presence of Ca 2+ . Monomeric myosin content decreased much faster than Ca 2+ ATPase inactivation in Ca medium, which was well explained by fast rod denaturation. In contrast, rod denaturation was slower than S-1 in EDTA medium. The decrease in monomeric myosin content was explained by faster S-1 denaturation. Comparing the S-1 and rod denaturation rates at a fixed temperature, it was concluded that S-1 denaturation was suppressed by Ca 2+ whereas the rod denaturation was not. An unfolding experiment with isolated myosin rod confirmed that there was no stabilizing effect of Ca 2+ on myosin rod. It was concluded that significant stabilization of the S-1 portion by Ca 2+ generated the apparently different myosin denaturation patterns in the two media.