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Area of Science:

  • Molecular Biology
  • Cell Biology
  • Biochemistry

Background:

  • DNA replication initiation is a fundamental process for cell division.
  • The origin recognition complex (ORC) and Mcm2-7 helicase are crucial for licensing replication origins.
  • A key step involves ORC loading two Mcm2-7 hexamers onto DNA.

Purpose of the Study:

  • To elucidate the molecular mechanism of how a single ORC loads two Mcm2-7 helicases.
  • To investigate the role of Orc6 in the helicase loading process.
  • To understand how CDK phosphorylation affects ORC function during origin licensing.

Main Methods:

  • Single-molecule Förster Resonance Energy Transfer (smFRET) assays were employed.
  • Mutational analysis of Orc6 was performed.
  • Biochemical experiments were conducted to study protein interactions.

Main Results:

  • The N-terminal half of Orc6 acts as a tether, connecting ORC to the Mcm2-7 helicase during a critical binding-site transition.
  • This Orc6-mediated tethering prevents ORC from dissociating from DNA and the helicase.
  • CDK phosphorylation of ORC disrupts this tethering interaction, inhibiting Mcm2-7 double-hexamer formation.
  • Specific Orc6 linker mutations support initial complex formation but impede stable recruitment of the second Mcm2-7 hexamer.

Conclusions:

  • Orc6 plays a pivotal role in facilitating the loading of two Mcm2-7 helicases by a single ORC.
  • The Orc6 tethering mechanism explains the efficiency and fidelity of origin licensing.
  • Orc6 is a key regulator involved in multiple stages of origin activation, linking DNA binding, helicase loading, and cell cycle control.