Journal Article

DNA unwinding mechanism of a eukaryotic replicative CMG helicase

Yuan, Zuanning, O’Donnell, Michael E., Bai, Lin, Zhang, Dan, Georgescu, Roxana, Li, Huilin

2020

Overview

High-resolution structures have not been reported for replicative helicases at a replication fork at atomic resolution, a prerequisite to understanding the unwinding mechanism. The eukaryotic replicative CMG (Cdc45, Mcm2-7, GINS) helicase contains a Mcm2-7 motor ring, with the N-tier ring in front and the C-tier motor ring behind. The N-tier ring is structurally divided into a zinc finger (ZF) sub-ring followed by the oligosaccharide/oligonucleotide-binding (OB) fold ring. Here we report the cryo-EM structure of CMG on forked DNA at 3.9 Å, revealing that parental DNA enters the ZF sub-ring and strand separation occurs at the bottom of the ZF sub-ring, where the lagging strand is blocked and diverted sideways by OB hairpin-loops of Mcm3, Mcm4, Mcm6, and Mcm7. Thus, instead of employing a specific steric exclusion process, or even a separation pin, unwinding is achieved via a “dam-and-diversion tunnel” mechanism that does not require specific protein-DNA interaction. The C-tier motor ring contains spirally configured PS1 and H2I loops of Mcms 2, 3, 5, 6 that translocate on the spirally-configured leading strand, and thereby pull the preceding DNA segment through the diversion tunnel for strand separation. The DNA duplex is known to be split apart in a steric exclusion manner during replication, but the specific mechanism has remained unclear. Here the authors present a cryo-EM structure of a eukaryotic replicative CMG helicase on forked DNA, revealing the mechanism of DNA unwinding.

• Publisher: Nature Publishing Group UK,Nature Publishing Group,Nature Portfolio
• Subject: 101/28 / 631/45/147 / 631/45/173 / 631/535/1258/1259 / Cdc45 protein / Cell Cycle Proteins - chemistry / Cell Cycle Proteins - genetics / Cell Cycle Proteins - metabolism / Deoxyribonucleic acid / DNA / DNA helicase / DNA Replication / DNA, Fungal - chemistry / DNA, Fungal - genetics / DNA, Fungal - metabolism / Humanities and Social Sciences / Minichromosome Maintenance Complex Component 3 - chemistry / Minichromosome Maintenance Complex Component 3 - genetics / Minichromosome Maintenance Complex Component 3 - metabolism / Minichromosome Maintenance Complex Component 4 - chemistry / Minichromosome Maintenance Complex Component 4 - genetics / Minichromosome Maintenance Complex Component 4 - metabolism / Minichromosome Maintenance Complex Component 6 - chemistry / Minichromosome Maintenance Complex Component 6 - genetics / Minichromosome Maintenance Complex Component 6 - metabolism / Minichromosome Maintenance Complex Component 7 - chemistry / Minichromosome Maintenance Complex Component 7 - genetics / Minichromosome Maintenance Complex Component 7 - metabolism / multidisciplinary / Oligonucleotides / Oligosaccharides / Presenilin 1 / Replication / Saccharomyces cerevisiae - chemistry / Saccharomyces cerevisiae - enzymology / Saccharomyces cerevisiae - genetics / Saccharomyces cerevisiae Proteins - chemistry / Saccharomyces cerevisiae Proteins - genetics / Saccharomyces cerevisiae Proteins - metabolism / Science / Science (multidisciplinary) / Separation / Unwinding / Zinc finger proteins