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Mol. Cell. Biol., May 1995, 2612-2624, Vol 15, No. 5
M Ohtsubo, AM Theodoras, J Schumacher, JM Roberts and M Pagano
Cyclin E was first identified by screening human cDNA libraries for genes
that would complement G1 cyclin mutations in Saccharomyces cerevisiae and
has subsequently been found to have specific biochemical and physiological
properties that are consistent with it performing a G1 function in
mammalian cells. Most significantly, the cyclin E-Cdk2 complex is maximally
active at the G1/S transition, and overexpression of cyclin E decreases the
time it takes the cell to complete G1 and enter S phase. We have now found
that mammalian cells express two forms of cyclin E protein which differ
from each other by the presence or absence of a 15-amino-acid
amino-terminal domain. These proteins are encoded by alternatively spliced
mRNAs and are localized to the nucleus during late G1 and early S phase.
Fibroblasts engineered to constitutively overexpress either form of cyclin
E showed elevated cyclin E-dependent kinase activity and a shortened G1
phase of the cell cycle. The overexpressed cyclin E protein was detected in
the nucleus during all cell cycle phases, including G0. Although the cyclin
E protein could be overexpressed in quiescent cells, the cyclin E-Cdk2
complex was inactive. It was not activated until 6 to 8 h after readdition
of serum, 4 h earlier than the endogenous cyclin E-Cdk2. This premature
activation of cyclin E-Cdk2 was consistent with the extent of G1 shortening
caused by cyclin E overexpression. Microinjection of affinity-purified
anti-cyclin E antibodies during G1 inhibited entry into S phase, whereas
microinjection performed near the G1/S transition was ineffective. These
results demonstrate that cyclin E is necessary for entry into S phase.
Moreover, we found that cyclin E, in contrast to cyclin D1, was required
for the G1/S transition even in cells lacking retinoblastoma protein
function. Therefore, cyclins E and D1 control two different transitions
within the human cell cycle.
Copyright © 1995, American Society for Microbiology
Human cyclin E, a nuclear protein essential for the G1-to-S phase transition
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA.
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