Branched microtubule nucleation and dynein transport organize RanGTP asters in Xenopus laevis egg extract.

Chromosome segregation relies on the correct assembly of a bipolar spindle. Spindle pole self-organization requires dynein-dependent microtubule transport along other microtubules. However, during M-phase RanGTP triggers microtubule nucleation and branching generating polarized arrays with non-astral organization in which microtubule minus ends are linked to the sides of other microtubules. This raises the question of how branched-microtubule nucleation and dynein-mediated transport cooperate to organize the spindle poles. Here, we used RanGTP-dependent microtubule aster formation in Xenopus laevis egg extract to study the interplay between these two seemingly conflicting organizing principles. Using temporally controlled perturbations of microtubule nucleation and dynein activity, we found that branched microtubules are not static but instead dynamically redistribute over time as poles self-organize. Our experimental data together with computer simulations suggest a model where dynein together with dynactin and NuMA directly pulls and move branched microtubule minus ends towards other microtubule minus ends.