Molecular motors require ATP to move along microtubules or actin filaments. To understand how molecular motors function, it is crucial to know how binding 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 accepted that conformational changes in the motor proteins(1-3) occur when they bind to microtubules or actin filaments, facilitating the release of ADP. Here we report that a mutation in the motor domain of the microtubule motor proteins Kar3 and Ncd uncouples nucleotide- and microtubule-binding by the proteins, 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-binding sites are blocked. The region of the motor that includes the mutated amino arid could transmit or undergo a conformational change required to convert the motor ATPase into a microtubule-stimulated state.