Rapid Helix-Coil Transitions in the S-2 Region of Myosin

Temperature-jump studies on the long S-2 fragment (100,000 daltons) isolated from myosin show that this structure can undergo α -helix-random coil transitions in a time range approximating the cycle time of a crossbridge. Two relaxation times are observed after temperature jumps of 5 degrees C over the range 35-55 degrees C, one in the submillisecond (τf) and the other in the millisecond (τs) time ranges. Both processes exhibit maxima near the midpoint of the helix-coil transition (tm= 45 ± 2 degrees C) as determined by optical rotation melt experiments. Similar results were observed for the low temperature transition (tm= 45 degrees C) of the myosin rod. Viscosity studies reveal that the S-2 particle has significant flexibility at physiological temperature. Results are considered in terms of the Huxley-Simmons and helix-coil transition models for force generation in muscle.