A NEW CHROMOSOMAL PROTEIN ESSENTIAL FOR MITOTIC SPINDLE ASSEMBLY

ASSEMBLY of the mitotic spindle, the machinery responsible for chromos omal segregation, is regulated by Cdc2 kinase1-5, and requires mitotic chromatin5-8. However, the molecular identity of the kinase substrate and chromatin factor is unknown. Here we have cloned a human compleme ntary DNA encoding an evolutionarily conserved chromosomal protein of relative molecular mass 47,000 (M(r) 47K) which has three consensus mo tifs for Cdc2 kinase-mediated phosphorylation9. The protein is phospho rylated only during mitosis and is associated with polypeptides having M(r)s of 31K, 67K and 200K. Mitotic arrest is induced by antisense me ssenger RNA or by affinity-purified autoantibody. In the arrested cell s, the chromosomes remain unsegregated and the mitotic spindle is abse nt. We propose that the chromosomal protein is activated by phosphoryl ation at the interphase/mitosis transition by Cdc2 kinase, and that th e protein, alone or as a complex, is a previously unidentified Cdc2 ki nase substrate and chromatin factor necessary for spindle assembly.