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Cryo-EM structure of Mcm2-7 double-hexamer on DNA suggests a lagging strand DNA extrusion model

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Title: Cryo-EM structure of Mcm2-7 double-hexamer on DNA suggests a lagging strand DNA extrusion model
Authors: Noguchi, Y
Yuan, Z
Bai, L
Schneider, S
Zhao, G
Stillman, B
Speck, C
Li, H
Item Type: Journal Article
Abstract: During replication initiation, the core component of the helicase—the Mcm2-7 hexamer—is loaded on origin DNA as a double hexamer (DH). The two ring-shaped hexamers are staggered, leading to a kinked axial channel. How the origin DNA interacts with the axial channel is not understood, but the interaction could provide key insights into Mcm2-7 function and regulation. Here, we report the cryo-EM structure of the Mcm2-7 DH on dsDNA and show that the DNA is zigzagged inside the central channel. Several of the Mcm subunit DNA-binding loops, such as the oligosaccharide–oligonucleotide loops, helix 2 insertion loops, and presensor 1 (PS1) loops, are well defined, and many of them interact extensively with the DNA. The PS1 loops of Mcm 3, 4, 6, and 7, but not 2 and 5, engage the lagging strand with an approximate step size of one base per subunit. Staggered coupling of the two opposing hexamers positions the DNA right in front of the two Mcm2–Mcm5 gates, with each strand being pressed against one gate. The architecture suggests that lagging-strand extrusion initiates in the middle of the DH that is composed of the zinc finger domains of both hexamers. To convert the Mcm2-7 DH structure into the Mcm2-7 hexamer structure found in the active helicase, the N-tier ring of the Mcm2-7 hexamer in the DH-dsDNA needs to tilt and shift laterally. We suggest that these N-tier ring movements cause the DNA strand separation and lagging-strand extrusion.
Issue Date: 25-Oct-2017
Date of Acceptance: 6-Oct-2017
URI: http://hdl.handle.net/10044/1/53509
DOI: https://dx.doi.org/10.1073/pnas.1712537114
ISSN: 0027-8424
Publisher: National Academy of Sciences
Start Page: E9529
End Page: E9538
Journal / Book Title: Proceedings of the National Academy of Sciences
Volume: 114
Issue: 45
Copyright Statement: Copyright © 2017 the Author(s). Published by PNAS.
Sponsor/Funder: Wellcome Trust
Biotechnology and Biological Sciences Research Council (BBSRC)
Funder's Grant Number: 107903/Z/15/Z
BB/N000323/1
Keywords: DNA replication
DNA unwinding
cryo-electron microscopy
helicase
mini chromosome maintenance
MD Multidisciplinary
Publication Status: Published
Appears in Collections:Clinical Sciences
Molecular Sciences
Faculty of Medicine



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