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Nat Struct Mol Biol. 2016 Mar;23(3):217-24. doi: 10.1038/nsmb.3170. Epub 2016 Feb 8.

Structure of the eukaryotic replicative CMG helicase suggests a pumpjack motion for translocation.

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Department of Biochemistry &Cell Biology, Stony Brook University, Stony Brook, New York, USA.
Biology Department, Brookhaven National Laboratory, Upton, New York, USA.
DNA Replication Laboratory, Rockefeller University, New York, New York, USA.
Howard Hughes Medical Institute, Rockefeller University, New York, New York, USA.
Department of Pathology and Laboratory Medicine, University of Texas Medical School at Houston, Houston, Texas, USA.


The CMG helicase is composed of Cdc45, Mcm2-7 and GINS. Here we report the structure of the Saccharomyces cerevisiae CMG, determined by cryo-EM at a resolution of 3.7-4.8 Å. The structure reveals that GINS and Cdc45 scaffold the N tier of the helicase while enabling motion of the AAA+ C tier. CMG exists in two alternating conformations, compact and extended, thus suggesting that the helicase moves like an inchworm. The N-terminal regions of Mcm2-7, braced by Cdc45-GINS, form a rigid platform upon which the AAA+ C domains make longitudinal motions, nodding up and down like an oil-rig pumpjack attached to a stable platform. The Mcm ring is remodeled in CMG relative to the inactive Mcm2-7 double hexamer. The Mcm5 winged-helix domain is inserted into the central channel, thus blocking entry of double-stranded DNA and supporting a steric-exclusion DNA-unwinding model.

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