NuMA is a nuclear protein during interphase but redistributes to the s
pindle poles early in mitosis. To investigate its role during spindle
formation, we tested spindle assembly in frog egg extracts from which
NuMA was immunodepleted. Immunodepletion revealed that NuMA forms a co
mplex with cytoplasmic dynein and dynactin. The depleted extracts fail
ed to assemble normal mitotic spindles, producing, instead, chromatin-
associated irregular arrays of microtubules lacking characteristic spi
ndle poles. A subdomain of the NuMA tail was shown to induce microtubu
le aster formation by mediating microtubule bundling. Our findings sug
gest that NuMA forms bifunctional complexes with cytoplasmic dynein an
d dynactin that can tether microtubules at the spindle poles and that
are essential for mitotic spindle pole assembly and stabilization.