Unidirectional MCM translocation away from ORC drives origin licensing

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• The MCM motor complex is crucial for eukaryotic DNA replication, functioning as a double hexamer loaded onto DNA.
• Loading of the MCM complex is mediated by the Origin Recognition Complex (ORC), Cdc6, and Cdt1, forming a helicase-recruitment complex.
• The precise mechanism by which MCM loading is completed via ATP hydrolysis remains largely unknown.

Purpose of the Study:

• To investigate the mechanism of MCM helicase loading onto DNA.
• To characterize the structural intermediates and molecular interactions during MCM loading.
• To elucidate the role of MCM ATPase activity in the loading process.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine high-resolution structures.
• ATPase-dead Arginine Finger variants of MCM subunits were engineered to trap loading intermediates.
• In vitro DNA binding assays and electron microscopy were used to assess the impact of mutations.

Main Results:

• Two distinct MCM complex structures bound to DNA, representing stages of maturation, were resolved.
• Specific Arginine Finger variants in Mcm2 and Mcm5 stabilize different DNA-binding interactions.
• Duplex DNA translocation by MCM involves specific contacts by ATPase hairpins, and mutations impair DNA binding and complex formation.

Conclusions:

• ATPase-powered duplex DNA translocation by MCM away from ORC is a key step in MCM loading.
• The study reveals critical interactions and conformational changes during helicase loading.
• Understanding MCM loading is vital for comprehending DNA replication regulation and potential therapeutic targets.