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MBC in Press, published online ahead of print March 7, 2007
Mol. Biol. Cell 10.1091/mbc.E06-12-1092

A more recent version of this article appeared on May 1, 2007
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Submitted on December 11, 2006
Revised on February 20, 2007
Accepted on February 26, 2007

Specialized Roles of the Two Mitotic Cyclins in Somatic Cells: Cyclin as an Activator of M Phase-promoting Factor

Tsz Kan Fung, Hoi Tang Ma, and Randy Y.C. Poon

Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China

Monitoring Editor: Mark Solomon

The role of cyclin B-CDC2 as M phase-promoting factor (MPF) is well established, but the precise functions of cyclin A remain a crucial outstanding issue. Here we show that down-regulation of cyclin A induces a G2 phase arrest through a checkpoint-independent inactivation of cyclin B-CDC2 by inhibitory phosphorylation. The phenotype is rescued by expressing cyclin A resistant to the RNA interference. In contrast, down-regulation of cyclin B disrupts mitosis without inactivating cyclin A-CDK, indicating that cyclin A-CDK acts upstream of cyclin B-CDC2. Even when ectopically expressed, cyclin A cannot replace cyclin B in driving mitosis, indicating the specific role of cyclin B as a component of MPF. Deregulation of WEE1, but not the PLK1-CDC25 axis, can override the arrest caused by cyclin A knockdown, suggesting that cyclin A-CDK may tip the balance of the cyclin B-CDC2 bistable system by initiating the inactivation of WEE1. These observations show that cyclin A cannot form MPF independent of cyclin B and underscore a critical role of cyclin A as a trigger for MPF activation.

Address correspondence to: Randy Y.C. Poon (bcrandy{at}ust.hk)






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