A MODEL FOR THE MICROTUBULE-NCD MOTOR PROTEIN COMPLEX OBTAINED BY CRYOELECTRON MICROSCOPY AND IMAGE-ANALYSIS

Kinesin motors convert chemical energy from ATP hydrolysis into unidir ectional movement. To understand how kinesin motors bind to and move a long microtubules, we fit the atomic structure of the motor domain of Ncd (a kinesin motor involved in meiosis and mitosis) into three-dimen sional density maps of Ncd-microtubule complexes calculated by cryo-el ectron microscopy and image analysis. The model reveals that Ncd share s an extensive interaction surface with the microtubule, and that a po rtion of the binding site involves loops that contain conserved residu es. In the Ncd dimer, the microtubule-bound motor domain makes intimat e contact with its partner head, which is dissociated from the microtu bule. This head-head interaction may be important in positioning the d issociated head to take a step to the next binding site on the microtu bule protofilament.