NOVEL ALTERATIONS IN CDK1 CYCLIN B1 KINASE COMPLEX-FORMATION OCCUR DURING THE ACQUISITION OF A POLYPLOID DNA CONTENT/

The pathways that regulate the S-phase events associated with the cont rol of DNA replication are poorly understood. The bone marrow megakary ocytes are unique in that they leave the diploid (2C) state to differe ntiate, synthesizing 4 to 64 times the normal DNA content within a sin gle nucleus, a process known as endomitosis. Human erythroleukemia (HE L) cells model this process, becoming polyploid during phorbol diester -induced megakaryocyte differentiation. The mitotic arrest occurring i n these polyploid cells involves novel alterations in the cdk1/cyclin B1 complex: a marked reduction in cdk1 protein levels, and an elevated and sustained expression of cyclin B1. Endomitotic cells thus lack cd k1/cyclin B1-associated H1-histone kinase activity. Constitutive overe xpression of cdk1 in endomitotic cells failed to re-initiate normal mi totic events even though cdk1 was present in a 10-fold excess. This wa s due to an inability of cyclin-Bl to physically associate with cdk1. Nonetheless, endomitotic cyclin B1 possesses immunoprecipitable H1-his tone kinase activity, and specifically translocates to the nucleus. We conclude that mitosis is abrogated during endomitosis due to the abse nce of cdk1 and the failure to form M-phase promoting factor, resultin g in a disassociation of mitosis from the completion of S-phase. Furth er studies on cyclin and its interacting proteins should be informativ e in understanding endomitosis and cell cycle control.