Fig. 6: Model illustrating the action of DDK on Mcm2-7 complex to regulate helicase activation and replisome activities.

To summarize the high-resolution snapshots of the DH-DDK structures into a congruent series of motion frames that represent the step-by-step action of DDK on its MCM substrates. The low-pass filtered cryo-EM maps are used to depict DH, DDK, and replisome (EMD-10227). a Once assembled at replication origin, the DH encircles dsDNA with the NSD of Mcm4 nested on its own NTD-A to impose an inhibitory effect on helicase activation. b The unstructured NTE of Mcm2’ engages with motif N of Dbf4 and/or other region of DDK to facilitate the docking of motif N onto the NTD-A of Mcm2’. c When motif N and M6NB of Dbf4 bind to the NTD-As of Mcm2’ and Mcm6 respectively, the floppy kinase core of DDK is positioned proximal to the NTD of Mcm4. d The kinase core is then docked onto the NTD-A of Mcm4 with the NSD of Mcm4 displaced by the M4NB of Dbf4 to be captured by Cdc7. During this docking process, the regulatory loop of Dbf4 occupies the activity center of the kinase to prevent premature substrate capturing by Cdc7. e DDK is docked onto each of the coupled MCM hexamers to function independently. f Conformational change in the kinase core and its subsequent dissociation from Mcm4 help to remove the regulatory loop of Dbf4 from Cdc7, enabling kinase center accessible to substrates. g After DDK phosphorylation, the NSD of Mcm4 becomes highly flexible, and exposes potential binding surfaces for origin firing factors to convert the DH into two active replisomes. h In the replisome, the binding surfaces of Dbf4 on Mcm4 and Mcm6 are covered by Tof1, which effectively redirects the DDK docked on the NTD-A of Mcm2 to other substrates. Note: the “replisome subcomplex” (CMG-Ctf4-Csm3-Tof1, EMD-10227) only depicts part of the replisome. i The DDK, recruited to the NTD-A of Mcm2 located at the front end of the translocating replisome, can extend its flexible kinase core to target substrates ahead of the replication fork.