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Activation of cyclin b1 cdk1 synchronizes events
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Activation of cyclin b1 cdk1 synchronizes events

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  • 1.   O liv ier Gav et and Jonathon P ines P res ented b y Lily Fernand ez
  • 2.     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. 2
  • 3.   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. 3
  • 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) 4
  • 5.   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? 5
  • 6.     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. 6
  • 7.     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. 7
  • 8.      (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. 8
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  • 11.  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.  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. 25
  • 12.    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 26