Structure of the eukaryotic MCM complex at 3.8 Å

DNA replication in eukaryotes is strictly regulated by several mechanisms. A central step in this replication is the assembly of the heterohexameric minichromosome maintenance (MCM2–7) helicase complex at replication origins during G1 phase as an inactive double hexamer. Here, using cryo-electron microscopy, we report a near-atomic structure of the MCM2–7 double hexamer purified from yeast G1 chromatin. Our structure shows that two single hexamers, arranged in a tilted and twisted fashion through interdigitated amino-terminal domain interactions, form a kinked central channel. Four constricted rings consisting of conserved interior β-hairpins from the two single hexamers create a narrow passageway that tightly fits duplex DNA. This narrow passageway, reinforced by the offset of the two single hexamers at the double hexamer interface, is flanked by two pairs of gate-forming subunits, MCM2 and MCM5. These unusual features of the twisted and tilted single hexamers suggest a concerted mechanism for the melting of origin DNA that requires structural deformation of the intervening DNA. Cryo-electron microscopy is used to visualize the double hexamer of the eukaryotic minichromosome maintenance complex (MCM), which is assembled during the G1 phase of DNA replication; two interdigitated hexamers have a central channel that tightly fits a DNA duplex, and the orientation of the tilted single hexamers sheds light on many functional aspects, particularly in the initial origin DNA melting. Replication-ready MCM complex In eukaryotes, DNA replication begins with the binding of a hexameric ring of minichromosome maintenance (MCM) proteins at regions known as replication origins during the G1 phase of the cell cycle. The resulting complex is dormant until the cell enters S phase, when replication occurs. This entails conversion of an MCM double hexamer into an active species, but the structure of this complex was unknown. Ning Gao and colleagues have used cryo-electron microscopy to visualize the double hexamer complex. They observe two interdigitated hexamers that have a central channel that tightly fits a DNA duplex. The orientation of the single rings suggests models in which relative movements between the two hexamers would deform the origin DNA so that other replication proteins can bind to the melted DNA double helix.