An isoform of microtubule-associated protein 4 inhibits kinesin-driven microtubule gliding

Recently, we revealed that microtubule-associated protein (MAP) 4 isoforms, which differ in the number of repeat sequences, alter the microtubule surface properties, and we proposed a hypothesis stating that the change in the surface properties may regulate the movements of microtubule motors [Tokuraku et al. (2003) J Biol Chem 278: 29609-29618]. In this study, we examined whether MAP4 isoforms affect the kinesin motor activity. When the MAP4 isoforms were present in an in vitro gliding assay, the five-repeat isoform but not the three- and four-repeat isoforms inhibited the movement of the microtubules in a concentration-dependent manner. The observation of individual microtubules revealed that in the presence of the five-repeat isoform, the microtubules completely stopped their movements or recurrently paused and resumed their movements, with no deceleration in the moving phase. The result can be explained by assuming that kinesin stops its movement when it encounters a microtubular region whose properties are altered by the MAPs. A sedimentation assay demonstrated that the MAP4 isoforms did not compete with kinesin for binding to microtubules, indicating that kinesin can bind to the MAP-bound microtubules, although it cannot move on them.