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

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.