FIGURE 15.

Model for activation of the replication fork helicase. A, Sld3-Cdc45 binds to Mcm2–7 in G₁, and Sld3 blocks the interaction between GINS and Mcm2–7 (36). B, Mcm2–7 ring opens, and ssDNA is extruded from the interior of the Mcm2–7 ring (7, 8, 42). The ssDNA exposed binds to Sld3, releasing Sld3 from Mcm2–7 (38). This event allows Sld2 to bind to the Mcm2–7 complex, and Sld2 blocks GINS from binding to Mcm2–7. C, CDK phosphorylates Sld2, and CDK-phosphorylated Sld2 binds to origin ssDNA, releasing Sld2 from Mcm2–7. This event allows GINS to bind to Mcm2–7, and the replication fork helicase (CMG complex) is assembled.