Cdt1 variants reveal unanticipated aspects of interactions with Cyclin/CDK and MCM important for normal genome replication

The earliest step in DNA replication is origin licensing which is the DNA loading of MCM helicase complexes. The Cdt1 protein is essential for MCM loading during G1 phase of the cell cycle, yet the mechanism of Cdt1 function is still incompletely understood. We examined a collection of rare Cdt1 variants that cause a form of primordial dwarfism (Meier-Gorlin syndrome) plus one hypomorphic Drosophila allele to shed light on Cdt1 function. Three hypomorphic variants load MCM less efficiently than WT Cdt1, and their lower activity correlates with impaired MCM binding. A structural homology model of the human Cdt1-MCM complex position the altered Cdt1 residues at two distinct interfaces rather than the previously described single MCM interaction domain. Surprisingly, one dwarfism allele ( Cdt1-A66T ) is more active than WT Cdt1. This hypermorphic variant binds both Cyclin A and SCFSkp2 poorly relative to WT Cdt1. Detailed quantitative live cell imaging analysis demonstrated no change in stability of this variant however. Instead, we propose that Cyclin A/CDK inhibits Cdt1 licensing function independently of the creation of the SCFSkp2 phosphodegron. Together, these findings identify key Cdt1 interactions required for both efficient origin licensing and tight Cdt1 regulation to ensure normal cell proliferation and genome stability.