Decoupling of nucleotide- and microtubule-binding sites in a kinesin mutant

Molecular motors require ATP to move along microtubules or actin filaments. To understand how molecular motors function, it is crucial to know how bin ding of the motor to its filamentous track stimulates the hydrolysis of ATP by the motor, enabling it to move along the filament. A mechanism for the enhanced ATP hydrolysis has not been elucidated, but it is generally accept ed that conformational changes in the motor proteins(1-3) occur when they b ind to microtubules or actin filaments, facilitating the release of ADP. He re we report that a mutation in the motor domain of the microtubule motor p roteins Kar3 and Ncd uncouples nucleotide- and microtubule-binding by the p roteins, preventing activation of the motor ATPase by microtubules. Unlike the wild-type motors, the mutants bind tightly to both ADP and microtubules , indicating that interactions between the nucleotide- and microtubule-bind ing sites are blocked. The region of the motor that includes the mutated am ino arid could transmit or undergo a conformational change required to conv ert the motor ATPase into a microtubule-stimulated state.