Three-dimensional structure of motor molecules

Images, calculated from electron micrographs, show the three-dimensional st ructures of microtubules and tubulin sheets decorated stoichiometrically wi th motor protein molecules. Dimeric motor domains (heads) of kinesin and nc d, the kinesin-related protein that moves in the reverse direction, each ap peared to bind to tubulin in the same way, by one of their two heads. The s econd heads show an interesting difference in position that seems to be rel ated to the directions of movement of the two motors. X-ray crystallographi c results showing the structures of kinesin and ncd to be very similar at a tomic resolution, and homologous also to myosin, suggest that the two motor families may use mechanisms that have much in common. Nevertheless, myosin s and kinesins differ kinetically. Also, whereas conformational changes in the myosin catalytic domain are amplified by a long lever arm that connects it to the stalk domain, kinesin and ncd do not appear to possess a structu re with a similar function but may rely on biased diffusion in order to mov e along microtubules.