Cyclin A-associated kinase activity is rate limiting for entrance into S phase and is negatively regulated in G1 by p27Kip1.
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.
Resnitzky
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D
Scripps Research Institute, La Jolla, California 92037, USA.
Hengst
L
L
Reed
S I
SI
D000818:Animals; D018797:Cell Cycle Proteins; D002460:Cell Line; D050760:Cyclin-Dependent Kinase Inhibitor p27; D016213:Cyclins / Q000378:metabolism*; D016193:G1 Phase / Q000502:physiology*; D006801:Humans; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D016160:Retinoblastoma Protein / Q000378:metabolism; D016196:S Phase / Q000502:physiology*; D025521:Tumor Suppressor Proteins
C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D016213:Cyclins; D008869:Microtubule-Associated Proteins; D016160:Retinoblastoma Protein; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D011494:Protein Kinases