Monte Carlo simulation of MyoVa modes of motion. (For details of the model simulations, see Discussion and Supporting Material.) (*A*) Simulation of myoVa processively stepping on single, straight actin filament, as seen in single-molecule TIRF assays. The step detection algorithm recovers a mean step size of 70 ± 23 nm (*n* = 139) similar to the 72 nm step size used in generating the trajectories. (*B*) Simulated myoVa HMM trajectory of a processive random walk through a complex actin filament network (see filament tracing) is described by a highly convoluted trajectory. Individual steps are apparent during trajectory segments and are similar to experimental results. A tracing of the cortical actin network taken from Svitkina et al. (28) is used to define the distance between actin-actin intersections. (*C*) Ensemble MSD analysis on log-log axes of 1000 simulations as in *B* (case 2, *solid line*) reveals a relationship with an initial slope of 1.3, which falls to 1.0 at longer timescales. MSD for simulations of a purely diffusive Qdot:myoVa (case 4, *dotted line*) has a slope, *α*, of 1.0 at all timescales, as expected for a diffusive process. For processive trajectories on a single straight actin filament, the MSD simulation results in *α* = 1.8 at all timescales (case 1, *dashed line*). (*D*) Cumulative probability distribution of *α*_{(0-0.2 s)} for trajectory simulations: case 1, processive movement of straight actin filament; case 2, processive movement on actin filament network as in *B*; case 3, processive movement on actin network 50% of the time and purely diffusive for the remaining 50%; case 4, purely diffusive.

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