Control of the Initiation and Termination of Kinesin-1-Driven Transport by Myosin-Ic and Non-Muscle Tropomyosin

Intracellular transport is largely driven by processive microtubule- and actin-based molecular motors. Non-processive motors have also been localized to trafficking cargos, but their roles are not well understood [ 1 – 7 ]. Myosin-Ic (Myo1c), a non-processive actin motor, functions in a variety of exocytic events, although the underlying mechanisms are not yet clear. To investigate the interplay between myosin-I and the canonical long distance transport motor kinesin-1, we attached both motor types to lipid membrane-coated bead (MCB) cargo, using an attachment strategy that allows motors to actively reorganize within the membrane in response to the local cytoskeletal environment. We compared the motility of kinesin-1-driven cargos in the absence and presence of Myo1c at engineered actin/microtubule intersections. We found that Myo1c significantly increases the frequency of kinesin-1-driven microtubule-based runs that begin at actin/microtubule intersections. Myo1c also regulates the termination of processive runs. Beads with both motors bound have a significantly higher probability of pausing at actin/microtubule intersections, remaining tethered for an average of 20 s, with some pauses lasting longer than 200 s. The actin-binding protein non-muscle tropomyosin (Tm) provides spatially-specific regulation of interactions between myosin motors and actin filaments in vivo [ 8 , 9 , 11 , 13 , 14 ]; in the crossed-filament in vitro assay, we found that Tm2-actin abolishes Myo1c-specific effects on both run initiation and run termination. Together these observations suggest Myo1c is important for the selective initiation and termination of kinesin-driven runs along microtubules at specific actin filament populations within the cell.