Myosin-I modulates the dynamics and architecture of motile actin ‘comet tails’

Actin and myosin are molecular machineries that convert free energy released from ATP hydrolysis into mechanical force. Polymerizing actin filaments generate force that powers membrane deformation and drive many important cellular processes. Myosin-Is are single-headed, membrane associated members of the myosin superfamily that have force-generating working strokes directed to actin's barbed end. Recent studies show that myosin-I isoforms frequently colocalize in areas of Arp2/3 complex-mediated actin polymerization at the leading edge of cell membrane, which implies an association of the two molecular complexes. To further investigate how myosin-Is affect the actin assembly, we reconstituted an in vitro actin-based motility system, where branched actin networks were nucleated by Arp2/3 complex from a micron-sized bead surface-coated with Arp2/3 activating factors. Actin filaments first formed a symmetric shell around the bead, which transitioned into a polarized comet tail after symmetry breaking and propelled the bead forward. We site-specifically coupled a range of densities of myosin-Is to the bead surface and assessed their effects on actin polymerization, network architecture, and symmetry breaking. We found that myosin changed the growth rate and the architecture of actin networks in comet tails, and also facilitated symmetry breaking. These studies show synergy between myosin activity and actin polymerization to power morphological changes at the cell membrane.