Direct visualization of MCM helicase activation and replisome coupling in situ

Direct visualization of the MCM helicase activation and replisome coupling within living cells has been achieved through the use of MINFLUX nanoscopy. This technique allows for the precise localization of individual MCM complexes at the nanometer scale in situ.

The MCM2-7 helicases are crucial for eukaryotic DNA replication, as they are loaded as double hexamers (DHs) during the G1 phase to license replication origins. Upon activation in the S phase, each DH is believed to separate into two single hexamers (SHs) that form the active CMG helicases and move bidirectionally along the DNA.

However, the scientific community has been divided on this matter, with some studies suggesting that CMG helicases are independent motors, while others propose that sister replisomes remain physically connected within replication factories.

In this study, researchers localized the MCM complexes in human cells using MINFLUX nanoscopy and directly observed the separation of DHs into SHs upon origin firing. The findings reveal that the resulting sister replisomes do not drift apart but instead remain coupled at a characteristic distance of approximately 40 nanometers throughout the S phase.

This coupling is attributed to two distinct factors: local protein-mediated tethering by the AND1 scaffold, and higher-order spatial confinement dependent on cohesin. While cohesin is not necessary for MCM loading in G1, it is essential for maintaining coupling in S phase.

By connecting the nanometer-scale architecture of the replisome to the overall genome topology of replication fountains, the researchers provide direct spatial evidence that sister forks are indeed coupled during DNA synthesis. This discovery not only sheds light on the molecular forces that organize replisomes within their native nuclear context but also resolves a long-standing debate in the field of DNA replication.