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Identification of Cyclin-dependent Kinase 1 Specific Phosphorylation Sites by an In Vitro Kinase Assay
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Site-specific phosphorylation of MCM4 during the cell cycle in mammalian cells

Yuki Komamura-Kohno1, Kumiko Karasawa-Shimizu, Takako Saitoh

  • 1Mitsubishi Kagaku Institute of Life Sciences, Tokyo, Japan.

The FEBS Journal
|March 8, 2006
PubMed

Insights

This study details MCM4 phosphorylation patterns across the cell cycle, identifying distinct roles for specific phosphorylation sites regulated by cyclin-dependent kinases (CDK1 and CDK2) in DNA replication and cell function.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • MCM4 is a key subunit of the replicative helicase, crucial for DNA replication.
  • Cyclin-dependent kinases (CDKs) regulate DNA replication by phosphorylating MCM4.
  • Detailed characterization of MCM4 phosphorylation sites and their functions is needed.

Purpose of the Study:

  • To comprehensively analyze the phosphorylation of human MCM4 at seven specific sites (Ser3, Thr7, Thr19, Ser32, Ser54, Ser88, Thr110).
  • To determine the cell cycle-specific patterns of MCM4 phosphorylation.
  • To identify the specific CDKs responsible for phosphorylating MCM4 at different sites and during distinct cell cycle phases.

Main Methods:

  • Western blot analysis using anti-phosphoMCM4 sera on HeLa cells.
  • Utilized temperature-sensitive mouse CDK1 mutant and dominant-negative human CDK2 mutant.
  • Performed in vitro phosphorylation assays with CDK2/cyclin A.
  • Investigated chromatin binding and nuclear localization of phosphorylated MCM4.

Main Results:

  • MCM4 phosphorylation at Thr7, Thr19, Ser32, Ser54, Ser88, and Thr110 is enhanced in G2/M phases.
  • MCM4 phosphorylation at Ser3 is detected during interphase.
  • CDK1 is required for M-phase phosphorylation (Thr7, Thr19, Ser32, Ser88, Thr110); CDK2 is required for interphase phosphorylation (Ser3, Ser32).
  • Phosphorylated MCM4 exhibits differential chromatin affinity and is not always colocalized with replicating DNA.
  • MCM4 phosphorylated at Ser32 is enriched in the nucleolus throughout the cell cycle.

Conclusions:

  • MCM4 phosphorylation is complex and site-specific, occurring in distinct patterns throughout the cell cycle.
  • CDK1 and CDK2 play critical, distinct roles in regulating MCM4 phosphorylation.
  • Site-specific MCM4 phosphorylation suggests diverse roles beyond DNA replication, including potential functions within the nucleolus.

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