A COMPLEX OF NUMA AND CYTOPLASMIC DYNEIN IS ESSENTIAL FOR MITOTIC SPINDLE ASSEMBLY

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.