Figure 4.

Model fits small ensemble force-velocity measurements. (a) Cartoon of experimental setup. (Top) Actin, manipulated by two beads trapped in lasers, is brought close to a small ensemble of myosin molecules that can then bind. The position of one laser-trapped bead is measured as a function of time x(t), and the position of the laser is dynamically adjusted to keep a constant force on the system. (Bottom left) Typical simulation of bead position versus time (compare to Fig. 1, B and C, of Debold et al. (30)). These simulations were for 11 active skeletal muscle myosin heads at 100 μM ATP. Applied force is shown below. (Right) Five position-versus-time simulations, aligned to emphasize how applied force slows actin speed. Force-velocity plots are shown for (b) skeletal and (c) smooth muscle myosin. (Solid circles) Data (30,43); (open circles) simulations. A quantity of 11 and 8 active myosin heads were simulated for skeletal and smooth myosin, respectively. (Error bars) Standard error. (Solid curves) Hill fits to the data (31).