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 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . This licensing control is achieved through the activities of geminin 10 , 11 , 12 and cyclin-dependent kinases 9 , 13 , 14 . Geminin binds tightly to Cdt1, an essential component of the replication licensing system 6 , 15 , 16 , 17 , 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 Cdt1 and the MCM helicase 19 , 20 . Here we describe the crystal structure of the mouse geminin–Cdt1 complex using tGeminin (residues 79–157, truncated geminin) and tCdt1 (residues 172–368, truncated Cdt1). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdt1. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdt1 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 tCdt1, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdt1 through steric hindrance.