{'fraction_non_alphanumeric': 0.025295109612141653, 'fraction_numerical': 0.01461495222034851, 'mean_word_length': 5.617100371747212, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Cdt1 Variants Offer Novel Insights Into Cdt1‐MCM Interactions and an Unexpected Mechanism for Cyclin A to Block DNA Re‐Replication

The correct execution of DNA replication is critical for normal cell proliferation. The earliest step in DNA replication is known as origin licensing. Origins are licensed by the loading of MCM complexes, an event that is restricted to G1 phase. The Cdt1 protein is essential for MCM loading, yet the molecular mechanisms of its activity are still remarkably mysterious. Here, we examined a collection of rare Cdt1 variants associated with a specific form of human primordial dwarfism (Meier‐Gorlin syndrome), plus one hypomorphic conserved Drosophila allele, to shed light on Cdt1 function. Our results indicate that two dwarfism variants (Cdt1‐R462Q, Cdt1‐E468K) are expressed normally, but load less MCM than WT Cdt1. We found the lower origin licensing activity of these variants correlates with impaired MCM binding. Likewise, the hypomorphic Drosophila allele (Cdt1‐R210C) consistently induces less MCM loading, and this deficiency also correlates with impaired MCM binding. Interestingly, this variant impacts a previously uncharacterized MCM binding domain in Cdt1. To visualize these molecular interactions, we generated human Cdt1‐MCM structural homology models based on recent yeast cryo‐EM structures. Our models position the hypomorphic mutations at direct contact points in the interface between human Cdt1 and human MCM complex. These results suggest human Cdt1 uses two domains to make multiple contacts with the MCM hexamer rather than the previously‐described single Cdt1‐MCM interaction site. Furthermore, this newly‐discovered MCM binding domain can explain how the Cdt1 inhibitor protein, geminin, blocks Cdt1‐MCM interaction.