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O liv ier Gav et and Jonathon P ines
P res ented b y Lily Fernand ez

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In the cell cycle, there are 2 classes of regulatory
molecules: Cyclins and Cyclin-dependent kinases (Cdk)
Cyclins form the regulatory subunit and Cdks form the
catalytic subunit of a heterodimer.
Once they join, they are activated and can phosphorylate
proteins.
Different cyclin-Cdk combinations determine the
downstream proteins targeted.
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Cyclin B1-Cdk1 complex activation causes breakdown of
nuclear envelope (NEBD) and initiation of prophase.
Once activated, the complex phosphorylates a plethora of
substrates, promoting the reorganization of cell architecture
during mitosis and cytokinesis:
o Cytoskeleton components, nuclear lamins, nuclear pore complexes
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Its deactivation causes the cell to exit mitosis.
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4. Nuclear envelope breakdown and reassembly in mitosis. At the end of G2,
the activation of cyclin-dependent kinases, including CDK1, triggers entry
into mitotic prophase. The nuclear membrane breaks down. (Chi et. al,
2009)
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How does cyclin B1-Cdk1 get imported into the nucleus just
before envelope breakdown?
How is this related to the activation of the kinase itself?
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In interphase, cyclin B1 moves between nucleus and
cytoplasm but prefers to be in the cytoplasm. Just before
nuclear envelope breakdown, cyclin B1-Cdk1 moves into
the nucleus.
Cyclin B1 has a nuclear export sequence (NES) at its Nterminus, that binds to the exportin Crm1.
The nuclear accumulation of cyclin B1 is probably caused
by the kinase Plk1 phosphorylating cyclinB1 at Ser147 in
the nuclear export sequence, thus inhibiting its export to
the cytoplasm
Is Plk1 the main regulator here? Conflicting evidence.
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Authors (and others) have shown that Plk1 phosphorylates
cyclin B1 on a different residue, Ser133.
Overexpressing Plk doesn’t cause cyclin B1 to move into the
nucleus.
Authors suggest that the rapid accumulation of cyclin B1
import in prophase is due to a phosphorylation dependent
nuclear import signal at the N-term triggered by a
combination of cyclin B1-Cdk1 and Plk1 kinase activities.
In this work they try to define the temporal relationship
between the activation of cyclin B1-Cdk1 and its nuclear
import, and the role of Plk1 and inhibition of nuclear export.
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(PEPILVDT-S-pS126-P-pS128-P-MET).
Cerulean containing the monomeric mutation A207K, linked
to YPet, with the minimal domain of the Polo-like kinase 1
polo box that is sufficient for binding to phospho-Ser/Thr
residues plus 16 amino acids from the autophosphorylation
site of human cyclin B1 with the Ala at the -1 position
(underlined) altered to Ser.
Their biosensor is specifically phosphorylated by cyclin B1Cdk1 and not by any other complexes (cyclinA/E-Cdk1).
Upon phosphorylation, the biosensor exhibits an increase in
FRET between its cyan and yellow fluorescent proteins.
Basically, measures cyclin B1-Cdk1 activity in living cells.
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Cyclin B1-Cdk1 autophosphorylates two sites on its own Nterm in the cytoplasm. When phosphorylation sites are
mutated…
o To glutamic acid: enhances nuclear import of cyclin B1-GFP in vitro
and in vivo (interphase cells).
o To alanine:delays the timing of cyclin B1 import during prophase.
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Biosensor showed that as soon as cyclin B1-Cdk1 is
activated, it triggers its own nuclear import, accumulating in
the nucleus. The import depends on continual cyclin B1Cdk1 activity but is independent of the Plk1 kinase.
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MAIN: Gavet, O, Pines, J (April 2010). “Activation of cyclin
B1-Cdk1 synchronizes events in the nucleus and the
cytoplasm at mitosis”. Journal of Cell Biology 189(2): 247259
Nigg EA (June 1995). "Cyclin-dependent protein kinases: key
regulators of the eukaryotic cell cycle". Bioessays 17 (6):
471–80.
Chi Y, Chen Z, Jeang K (October 2009). “The nuclear
envelopathies and human diseases”. Journal of Bionedical
Science 16:96
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