Nucleation of microtubules from mitotic centrosomes is modulated by a phosphorylated epitope.

At the onset of mitosis a class of proteins appears that possess a phosphorylated epitope recognized by the monoclonal antibody MPM-2. Immunofluorescence staining shows that a subset of these proteins is associated with the centrosomes of the mitotic apparatus. The appearance of these proteins coincides with the increased microtubule nucleating capacity of the centrosomes. We have tested whether growth of microtubules from mitotic centrosomes in a lysed cell model is dependent on the availability of the phosphorylated epitope by blocking the epitope with a specific antibody or by modifying it by removal of the phosphate. Centrosomes incubated with purified tubulin nucleate microtubule asters. However, preincubating the centrosomes with MPM-2 blocks all microtubule nucleation. Pretreating mitotic centrosomes with alkaline phosphatase also inhibits nucleation. These data suggest that the phosphorylated epitope recognized by MPM-2 is important for microtubule nucleation.