Regulation of cyclin-dependent kinase activity during mitotic exit and maintenance of genome stability by p21, p27, and p107

To study the regulation of cyclin-dependent kinase (CDK) activity during mitotic exit in mammalian cells, we constructed murine cell lines that constitutively express a stabilized mutant of cyclin A (cyclin A47). Even though cyclin A47 was expressed throughout mitosis and in G, cells, its associated CDK activity was inactivated after the transition from metaphase to anaphase. Cyclin A47 associated with both p21 and p27 during mitotic exit, implicating these proteins in CDK inactivation. However, cyclin A47 was fully inhibited during the M-to-G(1) transition in p21(-/-) p27(-/-) fibroblasts. Also, the CDKs associated with cyclin A47 were not inactivated by phosphorylation at tyrosines. The protein responsible for CDK inactivation during mitotic exit in p21/p27 null cells was the Rb family member, p107. p107 bound to cyclin A47 when p21 and p27 were absent, and cyclin A47-CDK activity was not inactivated during the M-to-G, transition in p21(-/-) p27(-/-) p107(-/-) null fibroblasts. Enforced expression of cyclin A in cells lacking all three CDK inhibitors induced rapid tetraploidization, indicative of mitotic failure/endoreduplication. We concluded that cyclin proteolysis and CDK inhibitors constitute redundant pathways that control cyclin A-CDK activity during mitotic exit in mammalian cells and that loss of these pathways can cause genetic instability.