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Mol. Cell. Biol., Aug 1995, 4347-4352, Vol 15, No. 8
D Resnitzky, L Hengst and SI Reed
We have created fibroblast cell lines that express cyclin A under the
control of a tetracycline-repressible promoter. When stimulated to reenter
the cell cycle after serum withdrawal, these cells were advanced
prematurely into S phase by induction of cyclin A. In an asynchronous
population, induction of cyclin A caused a decrease in the percentage of
cells in G1. These results demonstrate that expression of cyclin A is rate
limiting for the G1-to-S transition and suggest that cyclin A can function
as a G1 cyclin. Although the level of exogenous cyclin A was constant
throughout the cell cycle, its associated kinase activity increased as
cells approached S phase. Low kinase activity in early G1 was found to
correlate with the presence of p27Kip1 in cyclin A-associated complexes,
while high kinase activity in late G1 was correlated with its absence.
These results suggest that a function of p27Kip1 in G1 is to prevent
premature activation of cyclin A-associated kinase. Cyclin A expression in
early G1 led to phosphorylation of the product of the retinoblastoma
susceptibility gene (pRb). Thus, cyclin A expression can be rate limiting
for pRb phosphorylation, implicating pRb as a physiological substrate of
the cyclin A-dependent kinase. Taken together, these results demonstrate
that deregulated expression of cyclin A can perturb the normal regulation
of the G1-to-S transition.
Copyright © 1995, American Society for Microbiology
Cyclin A-associated kinase activity is rate limiting for entrance into S phase and is negatively regulated in G1 by p27Kip1
Scripps Research Institute, La Jolla, California 92037, USA.
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