Premature initiation of mitosis in yeast lacking RCC1 or an interacting GTPase

Area of Science:

• Cell Biology
• Molecular Genetics
• Biochemistry

Background:

• The cell cycle ensures orderly progression through DNA replication and mitosis.
• Regulation of the cell cycle involves complex molecular checkpoints and signaling pathways.
• The fission yeast model system is crucial for dissecting cell cycle control mechanisms.

Purpose of the Study:

• To characterize a fission yeast mutant with uncoupled mitosis and DNA replication.
• To identify genes involved in the regulation of cell cycle progression.
• To investigate the function of novel GTPases in cell cycle coordination.

Main Methods:

• Isolation and characterization of the pim1 fission yeast mutant.
• Analysis of cell cycle progression, DNA replication, and mitotic events.
• Gene cloning, overexpression studies, and genetic interaction analysis.
• Sequence homology searches and phylogenetic analysis of novel GTPases.

Main Results:

• The pim1 mutant initiates mitosis prematurely, before S phase completion, independent of cdc25.
• pim1 encodes a homolog of the human RCC1 nuclear protein.
• Overexpression of spi1, a novel essential gene, rescues the pim1 mutant phenotype.
• spi1 and human TC4 define a new subclass of ras-like GTPases.
• Disruption of spi1 leads to nuclear abnormalities and high-frequency mitotic haploidization.

Conclusions:

• pim1 plays a role in coordinating DNA replication with mitotic entry.
• spi1 is essential for cell cycle progression and interacts with pim1.
• The spi1/TC4 subclass represents a distinct functional group within the GTPase superfamily.
• These findings provide new insights into the molecular mechanisms governing cell cycle control.