Figure 7.

Blocked Cdc6 ATP hydrolysis leads to formation of an ORC–Cdc6–Cdt1–MCM2–7 complex, and ORC4R promotes double hexamer formation. A model of the MCM2–7-loading process (A–D). (A) ATP–ORC is bound to DNA. (B) ATP–Cdc6 associates with ORC. (C) We show that in the absence of ATP hydrolysis, only one MCM2–7 hexamer associates with ORC–Cdc6. This complex could contain multiple Cdt1 molecules. (D) ATP hydrolysis promotes the recruitment of a second Cdt1–MCM2–7 complex potentially with the help of a second ORC–Cdc6 complex. In consequence, MCM2–7 becomes loaded, forms a stable double hexamer encircling DNA and ORC, Cdc6 and Cdt1 become released from MCM2–7. Orc1 ATPase may function to reactivate ORC for a new round of pre-RC assembly.