Phosphoinositide-Dependent Protein Kinases Regulate Cell Cycle Progression Through the SAD Kinase Cdr2 in Fission...

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Cell cycle control is crucial for preventing diseases like cancer.
• Ksg1, a fission yeast homolog of mammalian phosphoinositide-dependent protein kinase 1 (PDK1), is implicated in cell cycle progression, but its mechanism is unclear.
• PDK1 is a known signaling hub in human tumorigenesis.

Purpose of the Study:

• To elucidate the mechanism by which Ksg1 regulates cell cycle progression in *Schizosaccharomyces pombe*.
• To identify novel proteins interacting with Ksg1 and influencing its function.
• To investigate the roles of identified interacting kinases in cell cycle control.

Main Methods:

• Genomic library screen to identify multicopy suppressors of a *ksg1-208* mutant.
• Analysis of cell cycle transition defects in single and double mutants.
• Assessment of protein localization and abundance using microscopy and Western blotting.
• Evaluation of Cdc25 protein levels in response to gene overexpression and deletion.

Main Results:

• Serine/threonine kinases Cdr2 and Ppk21 were identified as multicopy suppressors of the *ksg1-208* thermosensitive phenotype.
• Overexpression of Ppk21 or Cdr2 rescued the cell cycle defects of the *ksg1-208* mutant.
• Double mutant *ksg1-208* Δ*ppk21* exhibited more severe cell cycle defects than single mutants.
• Ksg1 mutation affected Cdr2 localization and abundance, which was exacerbated by Ppk21 deletion.
• Cdc25 protein levels were decreased in *ksg1-208*, Δ*ppk21*, and Δ*cdr2* cells, and Ppk21/Cdr2 overexpression partially restored Cdc25 levels.

Conclusions:

• Cdr2 is a novel downstream effector of Ksg1 and Ppk21, both PDK1 homologs.
• Ksg1 and Ppk21 cooperatively regulate cell cycle progression in fission yeast.
• Cdc25 is involved in the cell cycle regulatory pathway involving Ksg1, Ppk21, and Cdr2.