Kinesin-1 Motility Inhibited by Microtubule Defects

Abstract

Microtubules (MTs) provide the basic highway system for the cell, with molecular motors providing the cargo transport. Molecular motors traverse the MT network to deliver vital nutrients throughout the cell. It has been proposed that in vivo MTs, which are free of defects, provide the best tracks for motor transport, yet no study has investigated the effect of microtubule defects on transport. Moreover, in vivo defects may result when mechanisms that maintain MTs break down. Therefore, studying motor motility in the presence of defects is an important area of interest. Unlike in vivo MTs, microtubules polymerized in vitro can have many dislocation defects in their structure, such as seam or point defects. Here, we systematically study the effects of microtubule lattice defects on the transport abilities of kinesin motor proteins. We have studied the effect a step edge defect has on kinesin-1 motility, using single molecule fluorescence imaging. We created the step edge defects by end-to-end annealing Taxol-stabilized MTs with 12-13 protofilaments to GMPCPP MTs with 14 protofilaments. We find that fewer than 20% of kinesin-1 motors traverse a defect location while the rest dissociate from the MT lattice just before the defect. We speculate that kinesin-1 motility past the defect is inhibited because the step edge results in either a change in radius, or a shift in the protofilament lattice. Consequently, kinesin-1 is unable to overcome the increased step distance. This suggests that defects do play an important role in motility.