Liposome Transport by Myosin Va Motors: Coupling Through Lipid Membranes Modulates Cooperative Motor Interactions and Mechanics

Abstract

Myosin Va (myoVa) is a processive, actin-based motor involved in intracellular vesicular transport. Although capable of single myoVa transport in vitro, multiple myoVa motors transport intracellular vesicles, composed of phospholipid outer membranes. To determine how membrane fluidity affects the collective transport capacity of a myoVa ensemble, we synthesized liposomes with either fluid, DOPC or rigid, DPPC phospholipids. Varying number of myoVa motors were attached, and the liposome velocity on actin tracks observed by TIRF microscopy. Fluid DOPC liposomes with physiologically relevant myoVa surface densities (8 motors/200nm liposome) move at 498±228nm/s (n=282), faster (p<0.001) than a single, unloaded myoVa (430±120nm/s, n=233) and even faster (p<0.001) than rigid DPPC liposomes (328±120nm/s, n=128) of the same size and motor density. We proposed and confirmed through supported lipid bilayer studies that myoVa motors rapidly diffuse within fluid DOPC membranes (D=0.97±0.62μm2/s, n=157) and enrich at the liposome:actin track interface, compared to relatively immobile motors in rigid DPPC membranes. We modeled this phenomenon through Monte Carlo simulations and assumed that lipid membrane properties are critical to inter-motor interactions. Based on these simulations, the slower rigid liposome velocities may result from resistive forces being transmitted between motors, thus modulating the stepping kinetics in a load-dependent manner. However, this strain is dissipated in the fluid DOPC membranes, allowing stochastically faster motors to greatly enhance liposome velocities. Higher motor densities (32 motors/200nm liposome) lead to slower velocities for both liposome species but to a greater extent for fluid liposomes, due to motor crowding and interference at the liposome:actin track interface. Therefore, changes in both motor number and vesicular membrane properties dictate the extent of inter-motor interactions within the ensemble, becoming potential in vivo modulators of intracellular cargo transport.