Visualization of the MCM DNA helicase at replication factories before the onset of DNA synthesis

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• DNA synthesis occurs at replication foci (RF) in mammalian cells.
• Previous studies questioned the presence of minichromosome maintenance (MCM) proteins at RF.
• This paradox suggested MCM might not be physically present at DNA synthesis sites.

Purpose of the Study:

• To investigate the localization of MCM proteins during DNA replication.
• To resolve the apparent discrepancy between MCM presence and RF visualization.
• To elucidate the role of MCM in the context of replication foci.

Main Methods:

• Pulse-labeling of replication foci (RF) during S phase.
• Analysis of MCM protein localization at labeled DNA in the subsequent cell cycle.
• High-throughput confocal microscopy for detailed visualization.

Main Results:

• MCM proteins were found to concentrate in G1 at chromosomal sites destined to become RF in S phase.
• An active MCM eviction mechanism was identified, reducing MCM levels at RF upon DNA synthesis initiation.
• A small, detectable fraction of MCM remained at replication forks during S phase, consistent with its helicase function.

Conclusions:

• MCM proteins are recruited to future replication sites in G1.
• An eviction process regulates MCM levels at active replication foci.
• This study provides direct evidence for MCM's physical presence and dynamic regulation at sites of DNA synthesis.