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Structural basis of Mcm2-7 replicative helicase loading by ORC-Cdc6 and Cdt1

Zuanning Yuan1, Alberto Riera2,3, Lin Bai1

  • 1Cryo-EM Structural Biology Laboratory, Van Andel Research Institute, Grand Rapids, Michigan, USA.

Nature Structural & Molecular Biology
|February 14, 2017
PubMed

Insights

The origin recognition complex (ORC) and Cdc6 load a Mcm2-7 hexamer onto DNA, forming a key intermediate. This study reveals the 3.9-Å structure of this Saccharomyces cerevisiae complex, detailing its DNA helicase loading mechanism.

Area of Science:

  • Molecular Biology
  • Structural Biology
  • Biochemistry

Background:

  • DNA replication initiation is crucial for cell division.
  • The origin recognition complex (ORC) and Cdc6 are key initiator proteins.
  • Mcm2-7 is the core replicative helicase, but its loading mechanism is complex.

Purpose of the Study:

  • To determine the structure of the ORC-Cdc6-Cdt1-Mcm2-7 (OCCM) helicase-loading intermediate.
  • To elucidate the mechanism of Mcm2-7 loading onto DNA by ORC-Cdc6.

Main Methods:

  • Cryo-electron microscopy (Cryo-EM) to obtain a 3.9-Å resolution structure.
  • Structural analysis of protein-protein and protein-DNA interactions within the OCCM complex.

Main Results:

  • Detailed atomic model of the Saccharomyces cerevisiae OCCM intermediate.
  • Identified key interactions between ORC, Cdc6, Cdt1, and Mcm2-7.
  • Revealed how DNA is threaded through the ORC-Cdc6 and Mcm2-7 rings.
  • Characterized the specific DNA-binding interfaces on Orc4, Orc2, and Cdc6.

Conclusions:

  • The structure provides unprecedented insight into the initial helicase loading step.
  • This work clarifies the roles of Cdt1 and the ORC-Cdc6 complex in recruiting and loading Mcm2-7.
  • The findings suggest a model for the loading of the first Mcm2-7 hexamer at the origin of replication.

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