3D Model of Vesicles Transported by Myosin Va Motor Teams through Actin Intersections Predicts Experimental Directional Outcomes

Abstract

Molecular motors transport intracellular cargo through crowded cytoskeletal environments. Here, we used experimental and computational methods to examine how a myosin Va motor team navigates a lipid vesicle in vitro through an actin intersection in 3D.In the computational model, 1) a 350 nm diameter vesicle and actin filaments are rigid; 2) motors are stiff springs connected to the vesicle via a compliant pivot; 3) the vesicle is ideally fluid, so motors diffuse rapidly across its surface; 4) motors attach to actin depending on the mechanical energy required to bind; 5) motor stepping rate along or detachment from actin depends on the vectorial force component along the actin filament; 6) motors take 36nm steps with an occasional short step; 7) the vesicle is in mechanical equilibrium.Monte-Carlo simulations show: 1) motor teams move vesicles along actin in a left-handed spiral because of a motor's occasional short step; 2) at most, three motors on the vesicle surface can be attached to an actin filament, due to geometrical constraints and the mechanics of the motors, as confirmed by laser trap data; and 3) when the vesicle encounters another actin filament oriented 90 degrees to and spaced 50-250 nm above the actin filament it's traveling on, it has a 37% probability of switching filaments, a 59% probability of continuing straight by remaining on the original filament and a 4% probability of detaching, similar to that observed experimentally. These directional outcomes are the result of a tug of war between two separate motor teams on the vesicle surface that independently engage with the intersecting actin filaments. This combination of model and experiments form the basis for understanding the complexity of intracellular vesicle transport through cortical actin networks.