Active movement of synthetic myosin filaments observed by dark-field light microscopy

Long bipolar filaments synthesized from rabbit skeletal myosin have been visualized directly under a dark-field light microscope. It was also possible to observe movements of single myosin filaments in the presence of F-actin, which was invisible. When the two kinds of filaments were mixed (final concentrations: 17 micrograms/ml myosin and 0.6 mg/ml actin) in a medium containing 0.13 M KCl, 3 mM MgCl2, and 1 mM ATP, myosin filaments did not aggregate for a long period of time but were distributed separately in the field of microscope. Under these conditions, myosin filaments intermittently moved along their length in a bidirectional manner with a maximum instantaneous velocity of 5 micron/s. Individual filaments alternated, at random, between motile and nonmotile periods of time, with the motile fraction of filaments at any moment being roughly 10%. The in vitro movement of myosin filaments may be caused by their active sliding relative to F-actin in the background.