Loading mechanism of DNA polymerases to replication origin

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Dpb11 of budding yeast has four BRCT (BRCA1 C-Terminus) repeats, which function for protein-protein interactions, and it is required for DNA replication and cell cycle checkpoint. We also isolated Sld(synthetic lethality with dpb11-1)1〜6. In this study, we revealed those described below.1) Dpb11 forms a complex with DNA polymerase ε and is required for loading of DNA polymerase to replication origins. 2) When DNA replication is inhibited by hydroxyl urea, associations of DNA replication enzymes with late-firing origins are blocked. In dpb11-1 cells, this block is released, like other checkpoint mutants. Thus, it seems likely that Dpb11 functions for the checkpoint through the block of late-firing origins. 3) Sld3, like Cdc45, associates with early-firing origins in G1 phase and with late-firing origins in S phase. 4) Sld3 and Cdc45 form a complex and this complex formation is required for origin associations of both proteins. 5) The origin associations of Sld3 and Cdc45 is required for the origin unwinding. 6) Sld5 and Psf1 (Partner of sld five) form a complex which associates with replication origins in S phase and is required for origin association of DNA polymerase.7) Sld2 which form a complex with Dpb11 is phosphorylated in S phase and this phosphorylation depends on S-Cdk activity. Only the phosphorylated form of Sld2 form a complex with Dpb11, which is required for further loading of DNA polymerase. Thus, S-Cdk controls the initiation of DNA replication through phosphorylation of Sld2. Taken all together, Dpb11 together with Sld proteins functions for loading of DNA polymerase to replication origins. Future analysis will give a clear view for origin lading of DNA polymerase.