Helicase activation and establishment of replication forks at chromosomal origins of replication. Cold Spring Harb Perspect Biol doi: 10.1101/ cshperspect.a010371

Many replication proteins assemble on the pre-RC-formed replication origins and constitute the pre-initiation complex (pre-IC). This complex formation facilitates the conversion of Mcm2–7 in the pre-RC to an active DNA helicase, the Cdc45–Mcm–GINS (CMG) complex. Two protein kinases, cyclin-dependent kinase (CDK) and Dbf4-dependent ki-nase (DDK), work to complete the formation of the pre-IC. Each kinase is responsible for a distinct step of the process in yeast; Cdc45 associates with origins in a DDK-dependent manner, whereas the association of GINS with origins depends on CDK. These associations with origins also require specific initiation proteins: Sld3 for Cdc45; and Dpb11, Sld2, and Sld3 for GINS. Functional homologs of these proteins exist in metazoa, although pre-IC formation cannot be separated by requirement of DDK and CDK because of experimental limitations. Once the replicative helicase is activated, the origin DNA is unwound, and bidirectional replication forks are established. The main events at the initiation step of DNAreplication are the unwinding of double-stranded DNA and subsequent recruitment of DNA polymerases, to start DNA synthesis. Eu-karyotic cells require an active DNA helicase to unwind the origin DNA. The core components of the replicative helicase, Mcm2–7, are loaded as a head-to-head double hexamer connected via their amino-terminal rings (Evrin et al. 2009; Remus et al. 2009; Gambus et al. 2011) onto Orc-associated origins, to form the pre-RC in late M and G1 phases (see Bell and Kaguni 2013). However, Mcm2–7 alone does not show DNA helicase activity at replication origins. After the formation of the pre-RC, other repli-cation factors assemble on origins, and the pre-initiation complex (pre-IC) is formed. The pre-IC is defined as a complex formed just before the initiation of DNA replication (Zou and Still-man 1998); in yeast, it contains at least seve