Format

Send to

Choose Destination
Proc Natl Acad Sci U S A. 2017 Nov 7;114(45):E9529-E9538. doi: 10.1073/pnas.1712537114. Epub 2017 Oct 25.

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

Author information

1
DNA Replication Group, Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London W12 0NN, United Kingdom.
2
Medical Research Council London Institute of Medical Sciences, London W12 0NN, United Kingdom.
3
Cryo-EM Structural Biology Laboratory, Van Andel Research Institute, Grand Rapids, MI 49503.
4
Cold Spring Harbor Laboratory, Cold Spring Harbor, NY11724 stillman@cshl.edu chris.speck@imperial.ac.uk Huilin.Li@VAI.org.
5
DNA Replication Group, Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London W12 0NN, United Kingdom; stillman@cshl.edu chris.speck@imperial.ac.uk Huilin.Li@VAI.org.
6
Cryo-EM Structural Biology Laboratory, Van Andel Research Institute, Grand Rapids, MI 49503; stillman@cshl.edu chris.speck@imperial.ac.uk Huilin.Li@VAI.org.

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.

KEYWORDS:

DNA replication; DNA unwinding; cryo-electron microscopy; helicase; mini chromosome maintenance

PMID:
29078375
PMCID:
PMC5692578
DOI:
10.1073/pnas.1712537114
[Indexed for MEDLINE]
Free PMC Article

Supplemental Content

Full text links

Icon for HighWire Icon for PubMed Central
Loading ...
Support Center