High impact information on CycB

• Indeed, in mutant embryos deficient in cyclin A, cells that accumulate only cyclin B do not enter mitosis [1].
• Thus, in vivo, cyclin B is not sufficient for mitosis [1].
• Though lacking a formal demonstration that cyclin B is essential as it is in other organisms, we propose that each of these proteins fulfills a distinct and essential role in the cell cycle [1].
• We show that a higher rate of protein sequence evolution of the neo-X-linked copy of Cyclin B relative to the neo-Y copy is driven by positive selection, which is consistent with the adaptive hypothesis for the evolution of the Y chromosome [2].
• We have purified the inactive p34cdc2/cyclin B complex from G2-arrested starfish oocytes [3].

Biological context of CycB

• Cdk1-CycB plays a key role in regulating many aspects of cell-cycle events, such as cytoskeletal dynamics and chromosome behavior during mitosis [4].
• Compared with the six cycB embryos, reducing Thr in embryos with more CycB further delays the initiation of anaphase, whereas reducing either Pim or Sse has the opposite effect [4].
• Therefore, our genetic screen has identified all three components of the complex that regulates sister chromatid separation, and our observations indicate that interactions between Cdk1-CycB and the Pim-Thr-Sse complex are dosage sensitive [4].
• Interestingly, most of the suppressors that rescue the astral microtubule phenotype also reduce Cdk1-CycB activities and are microfilament-related genes [5].
• The initiation of nuclear envelope breakdown, spindle formation, and the initial congression of the centromeric regions of the chromosomes onto the metaphase plate all take place within the surface layer occupied by cyclin B on the apical side of the blastoderm nuclei [6].

Anatomical context of CycB

• Three B-cyclins in Drosophila, Cyclins (Cyc) A, B, and B3, associate with CDK1 and play partially redundant roles in embryogenic mitosis . Here, we show that the division of Drosophila GSCs and their precursors, the primordial germ cells (PGCs), specifically requires CycB [7].
• A genetic screen for suppressors and enhancers of the Drosophila cdk1-cyclin B identifies maternal factors that regulate microtubule and microfilament stability [5].
• In larvae, cyclin A is expressed predominantly in brain and imaginal disks, whereas cyclin B transcripts are abundant in testes [8].
• In Drosophila cells cyclin B is normally degraded in two phases: (a) destruction of the spindle-associated cyclin B initiates at centrosomes and spreads to the spindle equator; and (b) any remaining cytoplasmic cyclin B is degraded slightly later in mitosis [9].
• Cyclin B is degraded by the ubiquitin pathway, a system involved in most selective protein degradation in eukaryotic cells [10].

Associations of CycB with chemical compounds

• Oocytes arrested in the G2 phase of the cell cycle contain a p34cdc2/cyclin B complex which is kept in an inactive form by phosphorylation of its p34cdc2 subunit on tyrosine, threonine and perhaps serine residues [3].
• Immunoblot analysis of embryos whose cell divisions are synchronized by inducible String (cdc25 homolog) demonstrated that cyclin B was degraded during low O2 conditions in interphase-arrested embryos but not in those arrested in metaphase [11].

Enzymatic interactions of CycB

• The reduced levels of the 55 kDa subunit correlate with the loss of protein phosphatase 2A-like, okadaic acid-sensitive phosphatase activity of brain extracts against caldesmon and histone H1 phosphorylated by p34cdc2/cyclin B kinase, but not against phosphorylase a [12].

Regulatory relationships of CycB

• Myb controls G(2)/M progression by inducing cyclin B expression in the Drosophila eye imaginal disc [13].
• Taken together, our observations support the hypothesis that cyclin B regulates cytoskeletal changes while cyclin A regulates the nuclear cycles [14].
• Antizyme is a target of sex-lethal in the Drosophila germline and appears to act downstream of hedgehog to regulate sex-lethal and cyclin B [15].
• Embryos containing pronounced anterior-posterior gradients of Gnu activity demonstrate that Gnu regulates mitotic activity by promoting cyclin B stability [16].
• We show that Pum inhibits pole-cell division by repressing translation of cyclin B messenger RNA [17].

Other interactions of CycB

• The E2-C vihar is required for the correct spatiotemporal proteolysis of cyclin B and itself undergoes cyclical degradation [18].
• Results: Rux interacted with CycA and CycB in coprecipitation experiments [19].
• A destruction box-mutated form of cyclin B (cyclin B triple-point mutant [CBTPM]-GFP) that cannot be targeted for destruction by Fzy/Cdc20, is no longer degraded on spindles in syncytial embryos [9].
• We show that both genes encode abundant maternal mRNAs, but whereas the cyclin A mRNA is relatively uniformly distributed before cell formation, the cyclin B mRNA becomes localized to the developing pole cells [8].
• Consistent with a role for Drosophila Myb during S phase, we detected Dm-Myb protein in S-phase nuclei of wild-type mitotic cells as well as endocycling cells, which lack both an M phase and cyclin B expression [20].

Analytical, diagnostic and therapeutic context of CycB

• We have studied by way of confocal laser scanning microscopy the subcellular localization of cyclin B in Drosophila-cultured cells and report here evidence that a part of the cyclin B cell pool is closely associated with the centrosome [21].

References