  Spindles are self-organized microtubule-based structures that segregate
chromosomes during cell division. The mass of the spindle is controlled by the
balance between microtubule turnover and nucleation. The mechanisms that
control the spatial regulation of microtubule nucleation remain poorly
understood. Previous work has found that microtubule nucleators bind to
microtubules in the spindle, but it is unclear if this binding regulates the
activity of those nucleators. Here we use a combination of experiments and
mathematical modeling to investigate this issue. We measure the concentration
of tubulin and microtubules in and around the spindle. We found a very sharp
decay in microtubules at the spindle interface, which is inconsistent with the
activity of microtubule nucleators being independent of their association with
microtubules and consistent with a model in which microtubule nucleators are
only active when bound to a microtubule. This strongly argues that the activity
of microtubule nucleators is greatly enhanced when bound to microtubules. Thus,
microtubule nucleators are both localized and activated by the microtubules
they generate.
