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A more recent version of this article appeared on December 1, 2007
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Submitted on June 20, 2006
Revised on August 9, 2007
Accepted on September 6, 2007
Department of Cell Biology, Harvard Medical School, Boston, MA 02115
Monitoring Editor: Ted Salmon
The mitotic cyclins promote cell division by binding and activating cyclin-dependent kinases (CDKs). Each cyclin has a unique pattern of subcellular localization that plays a vital role in regulating cell division. During mitosis, cyclin B1 is known to localize to centrosomes, microtubules and chromatin. To determine the mechanisms of cyclin B1 localization in M-phase, we imaged full-length and mutant versions of human cyclin B1-EGFP in live cells using spinning disk confocal microscopy. In addition to centrosome, microtubule and chromatin localization, we found that cyclin B1 also localizes to unattached kinetochores following nuclear envelope breakdown. Kinetochore recruitment of cyclin B1 required the kinetochore proteins Hec1 and Mad2, and was stimulated by microtubule destabilization. Mutagenesis studies revealed that cyclin B1 is recruited to kinetochores through both CDK1-dependent and independent mechanisms. In contrast, localization of cyclin B1 to chromatin and centrosomes is independent of CDK1 binding. The N-terminal domain of cyclin B1 is necessary and sufficient for chromatin association, whereas centrosome recruitment relies on sequences within the cyclin box. Our data support a role for cyclin B1 function at unattached kinetochores, and demonstrate that separable and distinct sequence elements target cyclin B1 to kinetochores, chromatin and centrosomes during mitosis.