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Nat Commun. 2016 Feb 18;7:10708. doi: 10.1038/ncomms10708.

Cryo-EM structures of the eukaryotic replicative helicase bound to a translocation substrate.

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Macromolecular Machines, Clare Hall Laboratory, The Francis Crick Institute, Blanche Lane, South Mimms EN6 3LD, UK.
National Institute for Biological Standards and Control, Microscopy and Imaging, Blanche Lane, South Mimms EN6 3QG, UK.
Section of Virology and Single Molecule Imaging Group, Department of Medicine, MRC Clinical Centre, Imperial College London, London W12 0NN, UK.
Division of Structural Biology, Wellcome Trust Centre for Human Genetics, University of Oxford, Roosevelt Drive, Oxford OX3 7BN, UK.


The Cdc45-MCM-GINS (CMG) helicase unwinds DNA during the elongation step of eukaryotic genome duplication and this process depends on the MCM ATPase function. Whether CMG translocation occurs on single- or double-stranded DNA and how ATP hydrolysis drives DNA unwinding remain open questions. Here we use cryo-electron microscopy to describe two subnanometre resolution structures of the CMG helicase trapped on a DNA fork. In the predominant state, the ring-shaped C-terminal ATPase of MCM is compact and contacts single-stranded DNA, via a set of pre-sensor 1 hairpins that spiral around the translocation substrate. In the second state, the ATPase module is relaxed and apparently substrate free, while DNA intimately contacts the downstream amino-terminal tier of the MCM motor ring. These results, supported by single-molecule FRET measurements, lead us to suggest a replication fork unwinding mechanism whereby the N-terminal and AAA+ tiers of the MCM work in concert to translocate on single-stranded DNA.

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