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Cell cycle progression relies on cyclin-dependent kinases phosphorylating substrates like Whi5. This study reveals how Whi5 phosphorylation regulates the G1/S transition and is crucial for timely progression through S/G2/M phases.

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Area of Science:

  • Molecular Biology
  • Cell Biology
  • Biochemistry

Background:

  • Cell cycle progression is regulated by cyclin-dependent kinases (CDKs) phosphorylating key substrates.
  • In budding yeast, Whi5 is a transcriptional inhibitor controlling the G1/S transition by inhibiting the Swi4/Swi6 (SBF) complex.
  • Whi5 phosphorylation status dictates its activity and localization, influencing cell cycle progression.

Purpose of the Study:

  • To elucidate the molecular mechanisms governing Whi5 multi-site phosphorylation.
  • To understand how Whi5 phosphorylation regulates the cell cycle, particularly the G1/S transition.
  • To investigate the role of Whi5 phosphorylation in later cell cycle phases.

Main Methods:

  • Identification of 19 phosphorylated Whi5 sites.
  • Site-directed mutagenesis to determine the role of specific phosphorylation sites in G1 hypo-phosphorylation.
  • Analysis of Whi5 hyper-phosphorylation kinetics and its dependence on priming sites.

Main Results:

  • Mutation of 7 G1 hypo-phosphorylation sites in Whi5 led to increased cell size and delayed G1/S transition.
  • Whi5 hyper-phosphorylation in late G1 is dependent on priming sites that recruit Cks1.
  • Phosphorylation is essential for Whi5 nuclear export, normal cell size, SBF target gene expression, and progression through G1/S and S/G2/M phases.

Conclusions:

  • Whi5 phosphorylation is a critical regulator of the G1/S transition in budding yeast.
  • The study demonstrates that Whi5 phosphorylation is also required for timely progression through S/G2/M phases, extending its known regulatory role.
  • Understanding Whi5 phosphorylation provides insights into cell cycle control mechanisms.