Structure of the eukaryotic MCM complex at 3.8 Å

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• DNA replication is a fundamental process in eukaryotes, tightly regulated by multiple mechanisms.
• The minichromosome maintenance (MCM2-7) helicase complex is crucial for initiating DNA replication at origins.
• Assembly of the MCM2-7 double hexamer during the G1 phase is a key regulatory step.

Purpose of the Study:

• To determine the near-atomic structure of the MCM2-7 double hexamer from yeast G1 chromatin.
• To elucidate the structural basis for the inactive double hexamer formation.
• To understand the mechanism of origin DNA melting and replication initiation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to visualize the MCM2-7 double hexamer.
• Purification of the MCM2-7 double hexamer from yeast G1 chromatin was performed.
• Structural analysis focused on the arrangement of subunits and DNA interaction interfaces.

Main Results:

• A near-atomic structure of the yeast MCM2-7 double hexamer in its G1 state was obtained.
• The structure revealed two single hexamers arranged in a tilted and twisted conformation.
• A kinked central channel with a narrow passageway, formed by interdigitated domains and β-hairpins, was observed, tightly binding duplex DNA.
• Gate-forming subunits (MCM2 and MCM5) flank the DNA-binding channel.

Conclusions:

• The unique twisted and tilted arrangement of single hexamers suggests a mechanism for DNA melting at replication origins.
• Structural deformation of intervening DNA is likely required for origin melting.
• The findings provide insights into the regulation of DNA replication initiation in eukaryotes.