Eg5 is static in bipolar spindles relative to tubulin: evidence for a static spindle matrix

We used fluorescent speckle microscopy to probe the dynamics of the mitotic kinesin Eg5 in Xenopus extract spindles, and compared them to microtubule dynamics. We found significant populations of Eg5 that were static over sev eral seconds while microtubules flux towards spindle poles. Eg5 dynamics ar e frozen by adenylimidodiphosphate. Bulk turnover experiments showed that E g5 can exchange between the spindle and the extract with a half life of < 5 5 s. Eg5 distribution in spindles was not perturbed by inhibition of its mo tor activity with monastrol, but was perturbed by inhibition of dynactin wi th p50 dynamitin. We interpret these data as revealing the existence of a s tatic spindle matrix that promotes Eg5 targeting to spindles, and transient immobilization of Eg5 within spindles. We discuss alternative interpretati ons of the Eg5 dynamics we observe, ideas for the biochemical nature of a s pindle matrix, and implications for Eg5 function.