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© The Rockefeller University Press, 0021-9525/1998//875 $5.00
The Journal of Cell Biology, Volume 141, Number 4, , 1998 875-885

Articles

Nuclear Localization of Cyclin B1 Controls Mitotic Entry After DNA Damage



Pei Jin*, Stephen Hardy{ddagger}, and David O. Morgan*

* Department of Physiology, University of California, San Francisco, California 94143-0444; and {ddagger} Cell Genesys Inc., Foster City, California 94404

Mitosis in human cells is initiated by the protein kinase Cdc2-cyclin B1, which is activated at the end of G2 by dephosphorylation of two inhibitory residues, Thr14 and Tyr15. The G2 arrest that occurs after DNA damage is due in part to stabilization of phosphorylation at these sites. We explored the possibility that entry into mitosis is also regulated by the subcellular location of Cdc2-cyclin B1, which is suddenly imported into the nucleus at the end of G2. We measured the timing of mitosis in HeLa cells expressing a constitutively nuclear cyclin B1 mutant. Parallel studies were performed with cells expressing Cdc2AF, a Cdc2 mutant that cannot be phosphorylated at inhibitory sites. Whereas nuclear cyclin B1 and Cdc2AF each had little effect under normal growth conditions, together they induced a striking premature mitotic phenotype. Nuclear targeting of cyclin B1 was particularly effective in cells arrested in G2 by DNA damage, where it greatly reduced the damage-induced G2 arrest. Expression of nuclear cyclin B1 and Cdc2AF also resulted in significant defects in the exit from mitosis. Thus, nuclear targeting of cyclin B1 and dephosphorylation of Cdc2 both contribute to the control of mitotic entry and exit in human cells.

Abbreviations used in this paper: CDK, cyclin-dependent kinase; CRS, cytoplasmic retention signal; HA, hemagglutinin; NLS, nuclear localization signal; MOI, multiplicity of infection; PCC, premature chromatin condensation; tTA, tetracyclin transactivator.

Address all correspondence to David Morgan, Department of Physiology, Box 0444, University of California, San Francisco, CA 94143-0444. Tel.: 415-476-6695; Fax: 415-476-4929; E-mail: dmorgan{at}socrates.ucsf.edu


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