A mutant of the motor protein kinesin that moves in both directions on microtubules

Molecular motors move directionally to either the plus or the minus end of microtubules or actin filaments. Kinesin moves towards microtubule plus end s, whereas the kinesin-related Ncd motor moves to the minus ends. The 'neck '-the region between the stalk and motor domain-is required for Ncd to move to microtubule minus ends(1,2), but the mechanism underlying directional m otor movement is not understood. Here we show that a single amino-acid chan ge in the Ncd neck causes the motor to reverse directions and move with wil d-type velocities towards the plus or minus end; thus, the neck is function al but directionality is defective. Mutation of a motor-core residue that t ouches the neck residue in crystal structures(2,3) also results in movement in both directions, indicating that directed movement to the minus end req uires interactions of the neck and motor core. Low-density laser-trap assay s show that a conformational change or working stroke of the Ncd motor is d irectional and biased towards the minus end, whereas that of the neck mutan t occurs in either direction. We conclude that the directional bias of the working stroke is dependent on neck/motor core interactions. Absence of the se interactions removes directional constraints and permits movement in eit her direction.