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Mol. Cell. Biol., 12 1997, 7362-7374, Vol 17, No. 12
JA Diehl and CJ Sherr
Cyclins contain two characteristic cyclin folds, each consisting of five
alpha-helical bundles, which are connected to one another by a short linker
peptide. The first repeat makes direct contact with cyclin- dependent
kinase (CDK) subunits in assembled holoenzyme complexes, whereas the second
does not contribute directly to the CDK interface. Although threonine 156
in mouse cyclin D1 is predicted to lie at the carboxyl terminus of the
linker peptide that separates the two cyclin folds and is buried within the
cyclin subunit, mutation of this residue to alanine has profound effects on
the behavior of the derived cyclin D1-CDK4 complexes. CDK4 in complexes
with mutant cyclin D1 (T156A or T156E but not T156S) is not phosphorylated
by recombinant CDK- activating kinase (CAK) in vitro, fails to undergo
activating T-loop phosphorylation in vivo, and remains catalytically
inactive and unable to phosphorylate the retinoblastoma protein. Moreover,
when it is ectopically overexpressed in mammalian cells, cyclin D1 (T156A)
assembles with CDK4 in the cytoplasm but is not imported into the cell
nucleus. CAK phosphorylation is not required for nuclear transport of
cyclin D1-CDK4 complexes, because complexes containing wild-type cyclin D1
and a CDK4 (T172A) mutant lacking the CAK phosphorylation site are
efficiently imported. In contrast, enforced overexpression of the CDK
inhibitor p21Cip1 together with mutant cyclin D1 (T156A)-CDK4 complexes
enhanced their nuclear localization. These results suggest that cyclin D1
(T156A or T156E) forms abortive complexes with CDK4 that prevent
recognition by CAK and by other cellular factors that are required for
their nuclear localization. These properties enable ectopically
overexpressed cyclin D1 (T156A), or a more stable T156A/T286A double mutant
that is resistant to ubiquitination, to compete with endogenous cyclin D1
in mammalian cells, thereby mobilizing CDK4 into cytoplasmic, catalytically
inactive complexes and dominantly inhibiting the ability of transfected NIH
3T3 fibroblasts to enter S phase.
Copyright © 1997, American Society for Microbiology
A dominant-negative cyclin D1 mutant prevents nuclear import of cyclin- dependent kinase 4 (CDK4) and its phosphorylation by CDK-activating kinase
Howard Hughes Medical Institute, Department of Tumor Cell Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA.
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