Mechanism of replication origin melting nucleated by CMG helicase assembly

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Eukaryotic DNA replication requires precise, timely origin activation to prevent re-replication.
• MCM helicase loading is an early step, followed by firing factor recruitment to form CMG holo-helicases.
• The mechanism of DNA melting during CMG assembly at replication origins remains unclear.

Purpose of the Study:

• To elucidate the structural mechanism of ATP-dependent CMG assembly and DNA melting at eukaryotic replication origins.
• To investigate the role of CMG formation in disrupting the MCM double hexamer and initiating DNA unwinding.
• To determine the function of specific MCM residues in DNA untwisting and replication promotion.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to visualize ATP-dependent CMG assembly on a reconstituted chromatinized yeast origin.
• In vitro reconstitution with purified yeast proteins allowed for detailed structural analysis.
• Biochemical assays assessed the function of specific Mcm2 pore-loop residues.

Main Results:

• CMG formation disrupts the MCM double hexamer interface, exposing duplex DNA between two tethered CMG helicases, forming a splayed dimer.
• Inside each MCM ring, ATP binding triggers conformational changes that untwist the DNA double helix and break base pairing.
• Specific Mcm2 pore-loop residues are essential for DNA untwisting and promoting replication, though dispensable for initial CMG formation.

Conclusions:

• ATP binding to CMG nucleates origin DNA melting, a critical step for replication initiation.
• The splayed dimer structure of CMGs contributes to replisome stability during origin activation.
• This study provides a structural basis for understanding how replication origins are activated and DNA unwinding is initiated.