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 control of DNA replication are poorly understood. The bone marrow megakaryocytes are unique in that they leave the diploid (2C) state to differentiate, synthesizing 4 to 64 times the normal DNA content within a single nucleus, a process known as endomitosis. Human erythroleukemia (HEL) cells model this process, becoming polyploid during phorbol diester-induced megakaryocyte differentiation. The mitotic arrest occurring in 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 cdk1/cyclin B1-associated H1-histone kinase activity. Constitutive over-expression of cdk1 in endomitotic cells failed to re-initiate normal mitotic events even though cdk1 was present in a 10-fold excess. This was due to an inability of cyclin-B1 to physically associate with cdk1. Nonetheless, endomitotic cyclin B1 possesses immunoprecipitable H1-histone kinase activity, and specifically translocates to the nucleus. We conclude that mitosis is abrogated during endomitosis due to the absence of cdk1 and the failure to form M-phase promoting factor, resulting in a disassociation of mitosis from the completion of S-phase. Further studies on cyclin and its interacting proteins should be informative in understanding endomitosis and cell cycle control.
Datta
N S
NS
Department of Pediatrics, University of Michigan, Ann Arbor 48109, USA.
Williams
J L
JL
Caldwell
J
J
Curry
A M
AM
Ashcraft
E K
EK
Long
M W
MW
D016203:CDC2 Protein Kinase / Q000378:metabolism*; D002453:Cell Cycle; D002455:Cell Division; D002467:Cell Nucleus / Q000378:metabolism; D019926:Cyclin B; D056744:Cyclin B1; D016213:Cyclins / Q000378:metabolism*; D006801:Humans; D008938:Mitosis; D011123:Polyploidy; D006656:Protamine Kinase / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
C534699:CCNB1 protein, human; D019926:Cyclin B; D056744:Cyclin B1; D016213:Cyclins; D006656:Protamine Kinase; D016203:CDC2 Protein Kinase; D013755:Tetradecanoylphorbol Acetate