The eukaryotic CMG helicase pumpjack and integration into the replisome

Area of Science:

• Molecular Biology
• Structural Biology
• Genetics

Background:

• The eukaryotic replisome is a complex multiprotein machine essential for DNA replication.
• The CMG helicase (Cdc45/Mcm2-7/GINS) is the central component organizing the replisome.
• Previous models proposed a specific arrangement of DNA polymerases relative to the helicase.

Purpose of the Study:

• To elucidate the detailed structure and architecture of the eukaryotic replisome.
• To understand the mechanism of DNA translocation by the CMG helicase.
• To investigate the spatial organization of DNA polymerases within the replisome.

Main Methods:

• Cryo-electron microscopy (cryo-EM) single-particle 3D reconstruction was used to determine the CMG helicase structure.
• In vitro reconstitution of leading and lagging strand DNA synthesis was performed.
• Single-particle EM studies were conducted on staged replisome assemblies built from pure proteins.

Main Results:

• The detailed structure of the 11-subunit CMG helicase revealed pumpjack motions, suggesting a novel DNA translocation mechanism.
• The overall architecture of the eukaryotic replisome was identified through staged assembly and EM analysis.
• Surprisingly, leading and lagging strand polymerases were found to bind to opposite faces of the CMG helicase.

Conclusions:

• The CMG helicase acts as the organizing center of the replisome with an unexpected DNA translocation mechanism.
• The binding orientation of DNA polymerases challenges long-held views on replisome architecture.
• These findings provide new insights into the structural organization and functional mechanisms of the eukaryotic replisome.