Rapid helix--coil transitions in the S-2 region of myosin.

Abstract

Temperature-jump studies on the long S-2 fragment (100,000 daltons) isolated from rabbit myosin show that this structure can undergo .alpha.-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.degree. C over the range 35-55.degree. C, 1 in the submillisecond (.tau.f) and the other in the millisecond (.tau.s) time ranges. Both processes exhibit maxima near the midpoint of the helix-coil transition (tm = 45 .+-. 2.degree. C) as determined by optical rotation melt experiments. Similar results were observed for the low temperature transition (tm = 45.degree. 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.