DONSON is required for CMG helicase assembly in the mammalian cell cycle

DONSON is one of 13 genes mutated in a form of primordial microcephalic dwarfism known as Meier‐Gorlin syndrome. The other 12 encode components of the CDC45‐MCM‐GINS helicase, around which the eukaryotic replisome forms, or are factors required for helicase assembly during DNA replication initiation. A role for DONSON in CDC45‐MCM‐GINS assembly was unanticipated, since DNA replication initiation can be reconstituted in vitro with purified proteins from budding yeast, which lacks DONSON. Using mouse embryonic stem cells as a model for the mammalian helicase, we show that DONSON binds directly but transiently to CDC45‐MCM‐GINS during S‐phase and is essential for chromosome duplication. Rapid depletion of DONSON leads to the disappearance of the CDC45‐MCM‐GINS helicase from S‐phase cells and our data indicate that DONSON is dispensable for loading of the MCM2‐7 helicase core onto chromatin during G1‐phase, but instead is essential for CDC45‐MCM‐GINS assembly during S‐phase. These data identify DONSON as a missing link in our understanding of mammalian chromosome duplication and provide a molecular explanation for why mutations in human DONSON are associated with Meier‐Gorlin syndrome.