The nucleolar phosphoprotein B23 redistributes in part to the spindle poles during mitosis

B23 is a major phosphoprotein in the interphasic nucleolus where it is invo lved in the assembly of pre-ribosomes. Using several cultured animal cells, we report that, in addition to the known redistribution of the protein dur ing mitosis, B23 also becomes associated with mitotic spindle poles startin g from early prometaphase onwards. Colocalization of B23 with the protein N uMA (Nuclear Mitotic Apparatus protein) was studied in mitotic cells and ta xol-arrested cells, During the onset of mitosis, we observed that a fractio n of B23 associates with, and dissociates from, the poles later than NuMA, At metaphase, both proteins are colocalized at the poles. The polar redistr ibution of both B23 and NuMA is mediated by microtubules. In taxol-treated cells, B23 is associated with the microtubule minus ends in the center of m itotic asters together with NuMA, Association of B23 with microtubule minus ends of mitotic asters was further confirmed with an in vitro assay, where B23 was found by western blotting to co-sediment with taxol-induced microt ubule asters formed in a mitotic cell extract. Immunolabeling demonstrated that B23 and NuMA were both present at the center of the asters, Furthermor e, an additional hyperphosphorylated form of B23 appeared when microtubule asters formed and associated with the asters, Immunodepletion of B23 from t he mitotic extract revealed that taxol-induced microtubule asters were stil l observed in B23-immunodepleted mitotic extract, indicating that the prese nce of B23 at the poles is unlikely to be essential for spindle formation o r stabilisation.