PHOSPHORYLATION BY P34(CDC2) REGULATES SPINDLE ASSOCIATION OF HUMAN EG5, A KINESIN-RELATED MOTOR ESSENTIAL FOR BIPOLAR SPINDLE FORMATION IN-VIVO

We have isolated a human homolog of Xenopus Eg5, a kinesin-related mot or protein implicated in the assembly and dynamics of the mitotic spin dle. We report that microinjection of antibodies against human Eg5 (Hs Eg5) blocks centrosome migration and causes HeLa cells to arrest in mi tosis with monoastral microtubule arrays. Furthermore, an evolutionari ly conserved cdc2 phosphorylation site (Thr-927) in HsEg5 is phosphory lated specifically during mitosis in HeLa cells and by p34(cdc2)/cycli n B in vitro. Mutation of Thr-927 to nonphosphorylatable residues prev ents HsEg5 from binding to centrosomes, indicating that phosphorylatio n controls the association of th is motor with the spindle apparatus. These results indicate that HsEg5 is required for establishing a bipol ar spindle and that p34(cdc2) protein kinase directly regulates its lo calization.