HIGHLY PROCESSIVE MOTILITY IS NOT A GENERAL FEATURE OF THE KINESINS

Evidence is presented that the kinesin-related ncd protein is not as p rocessive as kinesin. In low surface density motility experiments, a d imeric ncd fusion protein behaved mechanistically more similar to non- processive myosins than to the highly processive kinesin. First, there was a critical microtubule length for motility; only microtubules lon ger than this critical length moved in low density ncd surfaces, which suggested that multiple ncd proteins must cooperate to move microtubu les in the surface assay. Under similar conditions, native kinesin dem onstrated no critical microtubule length, consistent with the behavior of a highly processive motor. Second, addition of methylcellulose to decrease microtubule diffusion decreased the critical microtubule leng th for motility. Also, the rates of microtubule motility were microtub ule length dependent in methylcellulose; short microtubules, that inte racted with fewer ncd proteins, moved more slowly than long microtubul es that interacted with more ncd proteins. In contrast, short microtub ules, that interacted with one or a few kinesin proteins, moved on ave rage slightly faster than long microtubules that interacted with multi ple kinesins. We conclude that a degree of processivity as high as tha t of kinesin, where a single dimer can move over distances on the orde r of one micrometer, may not be a general mechanistic feature of the k inesin superfamily.