Structural basis for inhibition of the replication licensing factor Cdt1 by geminin

To maintain chromosome stability in eukaryotic cells, replication origins must be licensed by loading mini-chromosome maintenance (MCM2-7) complexes once and only once per cell cycle 1-9 . This licensing control is achieved through the activities of geminin 10-12 and cyclin-dependent kinases 9,13,14 . Geminin binds tightly to Cdtl, an essential component of the replication licensing system 6,15-18 , and prevents the inappropriate reinitiation of replication on an already fired origin. The inhibitory effect of geminin is thought to prevent the interaction between Cdtl and the MCM helicase 19,20 . Here we describe the crystal structure of the mouse geminin-Cdtl complex using tGeminin (residues 79-157, truncated geminin) and tCdtl (residues 172-368, truncated Cdtl). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdtl. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdtl and, in particular, Pro 181, Ala 182, Tyr 183, Phe 186 and Leu 189. The crystal structure, in conjunction with our biochemical data, indicates that the N-terminal region of tGeminin might be required to anchor tCdtl, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdtl through steric hindrance.