The axonal transport motor kinesin‐2 navigates microtubule obstacles via protofilament switching

Abstract

Axonal transport involves kinesin motors trafficking cargo along microtubules that are rich in microtubule-associated proteins (MAPs). Much attention has focused on the behavior of kinesin-1 in the presence of MAPs, which has overshadowed understanding the contribution of other kinesins such as kinesin-2 in axonal transport. We have previously shown that, unlike kinesin-1, kinesin-2 in vitro motility is insensitive to the neuronal MAP Tau. However, the mechanism by which kinesin-2 efficiently navigates Tau on the microtubule surface is unknown. We hypothesized that mammalian kinesin-2 side-steps to adjacent protofilaments to maneuver around MAPs. To test this, we used single-molecule imaging to track the characteristic run length and protofilament switching behavior of kinesin-1 and kinesin-2 motors in the absence and presence of two different microtubule obstacles. Under all conditions tested, kinesin-2 switched protofilaments more frequently than kinesin-1. Using computational modeling that recapitulates run length and switching frequencies in the presence of varying roadblock densities, we conclude that kinesin-2 switches protofilaments to navigate around microtubule obstacles. Elucidating the kinesin-2 mechanism of navigation on the crowded microtubule surface provides a refined view of its contribution in facilitating axonal transport.

Tau directs intracellular trafficking by regulating the forces exerted by kinesin and dynein teams

“…For comparison, we modeled the effect of tau on cargoes where only kinesin‐1 or kinesin‐2 is active. When both kinesin‐1 and kinesin‐2 are engaged, the effect of tau on the unbinding rate of kinesin‐1, kinesin‐2, and dynein closely follows the results from single molecule experiments (Figure S4A). In simulations of cargoes driven by kinesin‐1 and dynein, the unbinding rate of kinesin‐1 was predicted to be less sensitive than observed for single molecules to obtain similar trajectories.…”

“…The effect of tau on the motility of kinesin and dynein has been characterized for individual motors and multiple motors of a single type . Here, we investigate the potential role for tau in directing the motility of bidirectional cargoes driven by teams of both kinesin and dynein motors.…”

“…Off‐axis displacements increase in the presence of tau for both plus‐ and minus‐end directed runs, indicating that both kinesin and dynein teams switch protofilaments to avoid obstacles on the microtubule (Figure B). Individual dynein and kinesin‐2 motors can step to adjacent protofilaments . For cargoes driven by multiple motors, sideways steps could also result from the binding and unbinding of motors to multiple protofilaments.…”

“…The effect of tau was modeled by increasing the dissociation rate of kinesin‐1, kinesin‐2, and dynein (; Table S1‐S3). Circles represent the unbinding rates determined from single‐molecule experiments . Trajectories were simulated for cargoes transported by 1 kinesin‐1, 2 kinesin‐2, and 10 dynein motors.…”

“…D, The model indicates that tau regulates the directionality of cargoes by inhibiting the processivity of kinesin‐1 more strongly than kinesin‐2 and dynein. E, Data from single‐molecule experiments was used to estimate the sensitivity of motor unbinding rates to load (closed circles) and tau (open circles) (Figure S4A). Kinesin‐1 is more sensitive than kinesin‐2 and dynein to tau.…”