Cyclin B1

  • Cyclin B1 is the main B-type cyclin, which regulates cell cycle progression during the transition from interphase to mitosis as well as the early stages of mitosis.
  • Cyclin B1 synthesis starts at the end of the S-phase and peaks during the G2/M phase transition. 
  • Tissue expression: Cyclin B1 is expressed in most dividing cells.  
  • Subcellular localization during cell cycle progression:
    • In the G2 phase, cyclin B1 is localized in the cytoplasm.
    • Accumulation of cyclin B1 on the centrosome starts in the mid-G2 phase. In late G2, the local concentration of cyclin B1 is highest at the centrosomes.
    • In prophase (first phase of M-phase), just before the nuclear envelope breakdown, it rapidly enters the nucleus. During prophase, it mostly localizes in the nucleus, however, a good amount of cyclin B1 is also observed in the cytoplasm.
    • During mitosis, after the breakdown of the nuclear envelope, it is present everywhere. Degradation of cyclin B1 starts at the beginning of the anaphase. Cyclin B1 disappears in anaphase.  
  • Disruption of the cyclin B1 gene in mice results in embryonic lethality in utero.
  • The promoter of Cyclin B1 contains CCAAT boxes. Trimeric transcriptional activator NF-Y binds to this site and regulates their transcription.  
  • Expression of cyclin B1 starts at the end of the S-phase and peak at the late G2 phase. However, activation of the CDK1-cyclin B1 complex does not occur in the G2 phase.  
  • Activation of CDK1-cyclin B1 complex occurs during the prophase of mitosis in the cytoplasm, before nuclear envelope breakdown. Initial activation of CDK1-cyclin B1 complex takes place at the centrosome.  
  • Active CDK1-cyclin B1 complex play a crucial role in many mitotic processes like cell rounding, centrosome separation, mitotic spindle formation, nuclear envelope breakdown, and chromosome condensation.  
  • CDK1-cyclin B1 complex phosphorylates many substrates which are present both in the cytoplasm as well as in the nucleus.
    • Cytoskeleton microtubule
    • Actin
    • Intermediate filament networks
    • Caspases
    • Golgi apparatus
    • Nuclear lamins
    • Nuclear pore complexes
    • Anaphase-promoting complex/cyclosome

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