Two central steps for initiating eukaryotic DNA replication involve loading of the Mcm2–7 helicase onto double-stranded DNA and its activation by GINS–Cdc45. To better understand these events, we determined the structures of Mcm2–7 and the CMG complex by using single-particle electron microscopy. Mcm2–7 adopts two conformations—a lock-washer-shaped spiral state and a planar, gapped-ring form—in which Mcm2 and Mcm5 flank a breach in the helicase perimeter. GINS and Cdc45 bridge this gap, forming a topologically closed assembly with a large interior channel; nucleotide binding further seals off the discontinuity between Mcm2 and Mcm5, partitioning the channel into two smaller pores. Together, our data help explain how GINS and Cdc45 activate Mcm2–7, indicate that Mcm2–7 loading may be assisted by a natural predisposition of the hexamer to form open rings, and suggest a mechanism by which the CMG complex assists DNA strand separation.
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The authors would like to thank A. Lyubimov and F. Bleichert for comments and help with the manuscript; and G. Lander, P. Grob, R. Hannah, R. Hall, M. Cianfrocco and C. Ciferri for technical help. This work was supported by a European Molecular Biology Organization long-term postdoctoral fellowship (to A.C.), a PhD fellowship from the Boehringer Ingelheim Fonds (to T.P.), the Human Frontier Science Program (RPG0039, to E.N.), the National Institute of General Medical Sciences (GM071747, to J.M.B.) and the National Cancer Institute (CA R37-30490, to M.R.B.). E.N. is a Howard Hughes Medical Institute investigator.
The authors declare no competing financial interests.
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Costa, A., Ilves, I., Tamberg, N. et al. The structural basis for MCM2–7 helicase activation by GINS and Cdc45. Nat Struct Mol Biol 18, 471–477 (2011). https://doi.org/10.1038/nsmb.2004
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