Flexible DNA Path in the MCM Double Hexamer Loaded on DNA

Area of Science:

• Molecular Biology
• Cell Biology
• Biophysics

Background:

• Pre-replicative complex (pre-RC) formation, or DNA replication licensing, is crucial for cell cycle progression.
• The precise spatial arrangement of DNA and minichromosome maintenance (MCM) 2-7 hexamers within the pre-RC remains largely uncharacterized.

Purpose of the Study:

• To elucidate the spatial organization of DNA and proteins within the pre-RC.
• To visualize the structure of the pre-RC using high-resolution microscopy.

Main Methods:

• Reconstitution of the pre-RC using purified DNA and proteins.
• Visualization of the reconstituted pre-RC using atomic force microscopy (AFM).

Main Results:

• AFM imaging demonstrated that MCM double hexamers form elliptical particles when bound to DNA.
• Analysis suggests DNA does not fully pass through both MCM hexamer holes, potentially exiting via a gap between Mcm2 and Mcm5.
• DNA loops fastened by MCM double hexamers were observed in both reconstituted and cell-purified pre-RCs.

Conclusions:

• The study provides novel insights into the structural organization of the pre-RC.
• Findings challenge the model of complete DNA threading through MCM hexamers.
• A higher-order architecture involving DNA looping and MCM double hexamers is proposed for the pre-RC.