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A role for the START gene-specific transcription factor complex in the inactivation of cyclin B and Cut2 destruction
1Division of Yeast Genetics, National Institute for Medical Research, London, United Kingdom.
Abstract:
Hyperactivation of Cdc2 in fission yeast causes cells to undergo a lethal premature mitosis called mitotic catastrophe. This phenotype is observed in cdc2-3w wee1-50 cells at high temperature. Eleven of 17 mutants that suppress this phenotype define a single complementation group, mcs1. The mcs1-77 mutant also suppresses lethal inactivation of the Wee1 and Mik1 tyrosine kinases and thus delays mitosis independently of Cdc2 tyrosine phosphorylation. We have cloned mcs1 by isolating suppressors of the cell cycle arrest phenotype of mcs1-77 cdc25-22 cells and found that it encodes Res2, a component of the START gene-specific transcription factor complex MBF (also known as DSC-1). The mcs1-77 mutant bears a single point mutation in the DNA-binding domain of Res2 that causes glycine 68 to be replaced by a serine residue. Importantly, two substrates of the anaphase-promoting complex (APC), the major B-type cyclin, Cdc13, and the anaphase inhibitor, Cut2, are unstable in G2-phase mcs1-77 cells. Consistent with this, we observe abnormal sister chromatid separation in mcs1-77 cdc25-22 cells at the restrictive temperature. Mutation of either Cdc10 or Res1 also deregulates MBF-dependent transcription and causes a G2 delay. We find that this cell cycle delay is abolished in the absence of the APC regulator Ste9/Srw1 and that the periodic expression of Ste9/Srw1 is controlled by the MBF complex. These data suggest that in fission yeast the MBF complex plays a key role in the inactivation of cyclin B and Cut2 destruction by controlling the periodic production of APC regulators.
Insights
The MBF complex, including Res2, regulates cell cycle progression in fission yeast by controlling the anaphase-promoting complex (APC). This regulation is crucial for timely mitosis and preventing catastrophic cell division.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- Hyperactivation of Cdc2 kinase in fission yeast leads to premature mitosis (mitotic catastrophe).
- The mcs1 mutation suppresses this lethal phenotype and delays mitosis independently of Cdc2 phosphorylation.
- The mcs1-77 mutant also suppresses inactivation of Wee1 and Mik1 tyrosine kinases.
Purpose of the Study:
- To identify the gene responsible for the mcs1 suppressor phenotype.
- To elucidate the role of the MBF complex in cell cycle regulation.
- To understand the mechanism by which MBF controls anaphase-promoting complex (APC) regulators.
Main Methods:
- Genetic screening for suppressors of cell cycle arrest.
- Cloning of the mcs1 gene.
- Site-directed mutagenesis to create mcs1-77.
- Analysis of protein stability (Cdc13, Cut2) and cell cycle progression.
- Investigating the role of APC regulators (Ste9/Srw1).
Main Results:
- The mcs1 gene encodes Res2, a component of the MBF transcription factor complex.
- The mcs1-77 mutation is a point mutation in the DNA-binding domain of Res2.
- MBF complex activity is essential for the instability of cyclin B (Cdc13) and Cut2 in G2-phase.
- MBF complex controls the periodic expression of the APC regulator Ste9/Srw1.
- Mutation of MBF components (Cdc10, Res1) causes G2 delay, abolished by loss of Ste9/Srw1.
Conclusions:
- The MBF complex plays a critical role in regulating cell cycle progression in fission yeast.
- MBF controls the timely inactivation of cyclin B and degradation of Cut2 by regulating APC activity.
- MBF achieves this control through the periodic expression of APC regulators like Ste9/Srw1.