Three-Dimensional Structural Analysis of Individual Myosin Heads Under Various Functional States

Abstract

Half a century has passed since the dedicated studies on the contraction mechanisms of muscle began, with considerable knowledge on its molecular architecture. Two major hypotheses were raised very early, one, ”sliding filament theory”,^1,2 and the other, “crossbridge theory”. ³ The former was readily accepted, because the phenomenon was apparently visible under optical microscope. The latter, however, has been hindered from thorough experimental proof even now, though nothing other than crossbridges connect thick and thin filaments enabling force development. The original idea postulated the rowing movement of actin-bound myosin head coupled with ATP hydrolysis, but it was later replaced by swinging of the “lever-arm” moiety,⁴ according to the discovery of intramolecular bending by X-ray crystallography.^5-8 One of the major reasons for such persistent difficulty to prove this simple hypothesis might be the lack of means to directly observe the actual structural change of working crossbridges with time and spatial resolution enough to visualize the fine details of the molecular nano-machine. Though the crystal structure of each component; actin^{9, 10} and myosin subfragment-1 (S1) with or without various nucleotides,^5-8 was determined ten years ago, none of their complexed form was solved nor might be the subject matter for easy crystallization.