Nuclear Accumulation of p21 Cip1 at the Onset of Mitosis: a Role at the G 2 /M-Phase Transition

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ABSTRACT Cell cycle arrest in G 1 in response to ionizing radiation or senescence is believed to be provoked by inactivation of G 1 cyclin-cyclin-dependent kinases (Cdks) by the Cdk inhibitor p21 Cip1/Waf1/Sdi1 . We provide evidence that in addition to exerting negative control of the G 1 /S phase transition, p21 may play a role at the onset of mitosis. In nontransformed fibroblasts, p21 transiently reaccumulates in the nucleus near the G 2 /M-phase boundary, concomitant with cyclin B1 nuclear translocation, and associates with a fraction of cyclin A-Cdk and cyclin B1-Cdk complexes. Premitotic nuclear accumulation of cyclin B1 is not detectable in cells with low p21 levels, such as fibroblasts expressing the viral human papillomavirus type 16 E6 oncoprotein, which functionally inactivates p53, or in tumor-derived cells. Moreover, synchronized E6-expressing fibroblasts show accelerated entry into mitosis compared to wild-type cells and exhibit higher cyclin A- and cyclin B1-associated kinase activities. Finally, primary embryonic fibroblasts derived from p21 −/− mice have significantly reduced numbers of premitotic cells with nuclear cyclin B1. These data suggest that p21 promotes a transient pause late in G 2 that may contribute to the implementation of late cell cycle checkpoint controls.

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