Activation of cyclin b1 cdk1 synchronizes events

Oliv ier Gavet and Jonathon Pines

Presented by Li ly Fernandez

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

Its deactivation causes the cell to exit mitosis.

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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)

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 N-

terminus, 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 B1-Cdk1 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 N-

term 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.

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 B1-

Cdk1 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): 247-

259

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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