Characterization of single actin-myosin interactions.

The feedback-enhanced laser trap assay (Finer et al., 1994) allows the measurement of force and displacement produced by single myosin molecules interacting with an actin filament suspended in solution by two laser traps. The average displacement of 11 nm at low load and the average force of 4 pN near isometric conditions are consistent with the conventional swinging cross-bridge model of muscle contraction (Huxley, 1969). The durations of single actin-myosin interactions at low load, 3-7 ms, suggest a relatively small duty ratio. Event durations can be increased either by reducing the ATP concentration until ATP binding is rate-limiting or by lowering the temperature. For sufficiently long interactions near isometric conditions, low frequency force fluctuations were observed within the time frame of a single event. Single myosin events can be measured at ionic strengths that disrupt weak binding actomyosin interactions, supporting the postulate of distinct weak and strong binding states. Myosin-generated force and displacement were measured simultaneously against several different loads to generate a force-displacement curve. The linear appearance of this curve suggests that the myosin powerstroke is driven by the release of a strained linear elastic element with a stiffness of approximately 0.4 pN nm-1.