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Bioessays. 2018 Mar;40(3). doi: 10.1002/bies.201700208. Epub 2018 Feb 6.

The Eukaryotic CMG Helicase at the Replication Fork: Emerging Architecture Reveals an Unexpected Mechanism.

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Cryo-EM Structural Biology Laboratory, Van Andel Research Institute, Grand Rapids, MI 49503, USA.
Department of DNA Replication, Rockefeller University and HHMI, New York, NY 10065, USA.


The eukaryotic helicase is an 11-subunit machine containing an Mcm2-7 motor ring that encircles DNA, Cdc45 and the GINS tetramer, referred to as CMG (Cdc45, Mcm2-7, GINS). CMG is "built" on DNA at origins in two steps. First, two Mcm2-7 rings are assembled around duplex DNA at origins in G1 phase, forming the Mcm2-7 "double hexamer." In a second step, in S phase Cdc45 and GINS are assembled onto each Mcm2-7 ring, hence producing two CMGs that ultimately form two replication forks that travel in opposite directions. Here, we review recent findings about CMG structure and function. The CMG unwinds the parental duplex and is also the organizing center of the replisome: it binds DNA polymerases and other factors. EM studies reveal a 20-subunit core replisome with the leading Pol ϵ and lagging Pol α-primase on opposite faces of CMG, forming a fundamentally asymmetric architecture. Structural studies of CMG at a replication fork reveal unexpected details of how CMG engages the DNA fork. The structures of CMG and the Mcm2-7 double hexamer on DNA suggest a completely unanticipated process for formation of bidirectional replication forks at origins.


CMG helicase; DNA polymerase; DNA replication; origin initiation; replisome

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