A spatial gradient coordinates cell size and mitotic entry in fission yeast.

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Cell cycle progression in eukaryotes is tightly regulated by cyclin-dependent kinases, notably Cdk1.
• Size-dependent cell cycle transitions are crucial for cell division but the underlying mechanisms remain unclear.
• Fission yeast, *Schizosaccharomyces pombe*, provides a model for studying reproducible cell size control.

Purpose of the Study:

• To investigate the role of the cell polarity protein kinase Pom1 in regulating cell size and mitotic entry.
• To elucidate the signaling network that links cell size monitoring to Cdk1 activity.
• To understand how Pom1 influences the localization and function of key cell cycle regulators.

Main Methods:

• Utilized fission yeast (*Schizosaccharomyces pombe*) as a model organism.
• Investigated the localization and function of Pom1, Wee1, Cdr1, Cdr2, and Mid1.
• Analyzed the Pom1 polar gradient and its effect on cortical nodes controlling Cdk1 activity.

Main Results:

• Pom1, localized to cell ends, forms a polar gradient that inhibits mitotic entry in a dose-dependent manner.
• A signaling network involving Wee1, Cdr1, and Cdr2 is localized at medial cortical nodes.
• Cdr2's localization is hierarchical and negatively regulated by Pom1, linking Pom1 levels to mitotic entry.
• Decreasing Pom1 levels at the cell middle during cell elongation correlates with mitotic entry.

Conclusions:

• The Pom1 polar gradient and medial cortical nodes act as a cell size-sensing mechanism.
• This system coordinates cell size with mitotic entry by regulating Cdk1 activity through the Pom1-Cdr2-Cdr1-Wee1 pathway.
• This provides a novel insight into how cell size is linked to cell cycle progression in eukaryotes.