Kinesin Motility on Microtubule Bundles

Abstract

Microtubules carry out numerous functions in the cell, one of which is to provide tracks along which microtubule motor proteins walk. In the cell, these filaments often form complex architectures, such as microtubule bundles. Distinct types of bundles can form depending on the orientation of microtubules within the bundle. Bundles that consist of parallel microtubules are found in axons, while bundles comprised of randomly oriented microtubules are present in dendrites. Additionally, antiparallel microtubule arrays are known to exist at the spindle midzone. How do motors navigate these complex microtubule architectures? Here, we study the motility of single kinesin-1 motors on different types of microtubule bundles in vitro. We prepared three types of microtubule bundles: bundles with tightly packed, randomly oriented microtubules formed by depletion forces; spaced, antiparallel bundles formed by the microtubule crosslinking protein, MAP65; and endogenous parallel microtubule bundles derived from neuronal-like processes of CAD cells. We observe that kinesin processivity and velocity are reduced on tightly packed microtubule bundles. This reduction in motility is likely due to staggered, overlapping microtubules within the bundle that act as obstacles for motors. We do not observe reduced motility on microtubule bundles with crosslinking MAPs. We suggest that spacing between microtubules within a bundle created by MAPs organizes the bundle architecture to prevent staggered, overlapping microtubules from acting as obstacles for kinesin motors. Interestingly, we observe single kinesin motors to switch to adjacent, oppositely oriented microtubules within bundles that contain antiparallel microtubules. We show that motors that switch to adjacent tracks during a run exhibit an enhanced processivity. This suggests that the ability of single motors to switch to adjacent tracks could be a mechanism used by kinesin to circumvent obstacles. This study provides new insights into how kinesin motors navigate complex tracks present in the cell.