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J. Cell Biol.,
Volume 141, Number 4, May 18, 1998 875-885
* Department of Physiology, University of California, San Francisco, California 94143-0444; and 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.
Cell Genesys Inc., Foster City,
California 94404
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