(37g) Transport of Molecular Motors on Networks of Cytoskeletal Filaments

Molecular motor proteins use the energy from ATP hydrolysis to perform work and facilitate a variety of cellular processes. Motors of the myosin family move along actin filaments, and their directed motion facilitates processes including vesicle and organelle transport, cytoplasmic streaming, and cell division. The organization of actin filaments can vary both within a cell and between different cells. Thus, it is important to understand how the organization of the cytoskeletal network influences the cellular-scale transport of motors.

In this work, we use stochastic, particle-based computer simulations to investigate the motion of molecular motors on networks of filaments. We vary properties including the density and length of filaments, the organization of filaments, and the density of motors. We are particularly interested in the effects of filament bundling on transit times for a motor to cross a spatial domain. Actin bundling is common in cells, and has been implicated in cytoplasmic streaming, in which myosin motors induce bulk flow of the cytosol in plant and fungal cells. In our model, motors can bind to and unbind from filaments, undergo diffusive motion in the cytoplasm, and undergo directed motion when bound to a filament. We generate spatially resolved stochastic simulation trajectories using the Gillespie algorithm. To characterize the transport of motors, we measure the first passage times for motors to traverse spatial domains of varying sizes. At intermediate filament densities, the mean first passage time decreases with increasing number of filaments, and for a fixed number of filaments, shorter first passage times are promoted by longer filament lengths and bundling of filaments. We find that spatiotemporal correlations and rebinding of motors to filaments in the bundle facilitate the faster transport, even with a reduced bulk density of filaments. We further explore the effects of excluded volume at higher densities, which can produce density heterogeneities and introduce traffic jams on filaments. We introduce analytical calculations to help interpret our simulation results at low motor densities.