Kumagai et al. describe how cyclin-dependent kinases (Cdks) initiate DNA replication by promoting the interaction of two proteins conserved from yeast to mammals.
In budding yeast, Cdks phosphorylate two replication origin proteins called Sld2 and Sld3, allowing them to bind a third protein called Dpb11. Together, these proteins recruit Cdc45, which then promotes the activation of the MCM helicase complex to separate the two strands of DNA and initiate replication. Whether the same system operates in higher organisms is unclear because the vertebrate homologues of Sld2 and Sld3 have remained obscure.
Kumagai et al. previously identified a protein called Treslin as a candidate homologue of Sld3. Treslin is required for DNA replication in Xenopus egg extracts and human cells, and it binds to TopBP1—the vertebrate homologue of Dpb11—in the presence of Cdks. The researchers now show that Cdks phosphorylate Treslin on a specific serine residue in a region of the protein that associates with TopBP1 and that is relatively similar in sequence to yeast Sld3. When this serine was mutated to alanine, Treslin could no longer bind to TopBP1, and cells expressing this mutant instead of wild-type Treslin were unable to replicate their DNA. In addition, Treslin bound Xenopus Cdc45, suggesting that it really is the vertebrate orthologue of Sld3.
The authors now want to examine how Treslin and TopBP1 integrate Cdc45 into the replication initiation complex. They also want to investigate why Cdks apparently phosphorylate additional sites in the Treslin sequence.