Role of CDK in replication initiation

Abstract

Cyclin-dependent kinases (CDKs) are master regulators of cell-cycle progression in eukaryotes. The onset of the synthesis phase (S phase), which is the initiation of chromosomal DNA replication, is a major cell-cycle event that is also regulated by CDKs. Chromosomal DNA replication occurs as a two-step reaction in eukaryotes. In the first reaction, which is termed replication licensing or helicase loading, replicative helicases (the Mcm2-7 complex) are loaded onto replication origins in an inactive form. In the second reaction, which is termed initiation reaction or helicase activation, replicative helicases are activated and double-stranded DNA is unwound to initiate DNA synthesis. The active replicative helicase complex consists of Cdc45, the Mcm2-7 complex, and the GINS complex, and is called the CMG (Cdc45-Mcm2-7-GINS) complex. In the initiation reaction, many replication proteins assemble onto the Mcm2-7 complex to facilitate the formation of the CMG complex, and this reaction is triggered by two protein kinases, S-phase-specific CDK (S-CDK) and Dbf4-dependent kinase (DDK). Each kinase is responsible for a distinct step of the initiation reaction in yeast: first, Cdc45 is loaded onto licensed origins in a DDK-dependent manner. Subsequently, GINS is loaded onto the origins in an S-CDK-dependent manner. The components of the CMG complex and the requirements of kinases are highly conserved in model eukaryotes. Therefore, the overall rationale of the initiation reaction seems to be highly conserved in eukaryotes, although other replication proteins that are required for the reaction seem to be less conserved, and the set of factors under the control of CDK might be slightly different between species. In this chapter, we focus on the role of S-CDK in the initiation reaction.