Cdc7-Dbf4 is a conserved, two-subunit kinase required for initiating eukaryotic DNA replication. Recent studies have shown that Cdc7-Dbf4 also regulates the mitotic exit network (MEN) and monopolar homolog orientation in meiosis I (Matos, J., Lipp, J. J., Bogdanova, A., Guillot, S., Okaz, E., Junqueira, M., Shevchenko, A., and Zachariae, W. (2008) Cell 135, 662-678 and Miller, C. T., Gabrielse, C., Chen, Y. C., and Weinreich, M. (2009) PLoS Genet. 5, e1000498). Both activities likely involve a Cdc7-Dbf4 interaction with Cdc5, the single Polo-like kinase in budding yeast. We previously showed that Dbf4 binds the Cdc5 polo-box domain (PBD) via an ϳ40-residue N-terminal sequence, which lacks a PBD consensus binding site (S(pS/pT)(P/X)), and that Dbf4 inhibits Cdc5 function during mitosis. Here we identify a non-consensus PBD binding site within Dbf4 and demonstrate that the PBD-Dbf4 interaction occurs via a distinct PBD surface from that used to bind phosphoproteins. Genetic and biochemical analysis of multiple dbf4 mutants indicate that Dbf4 inhibits Cdc5 function through direct binding. Surprisingly, mutation of invariant Cdc5 residues required for binding phosphorylated substrates has little effect on yeast viability or growth rate. Instead, cdc5 mutants defective for binding phosphoproteins exhibit enhanced resistance to microtubule disruption and an increased rate of spindle elongation. This study, therefore, details the molecular nature of a new type of PBD binding and reveals that Cdc5 targeting to phosphorylated substrates likely regulates spindle dynamics.Cell cycle progression requires the highly accurate replication and segregation of chromosomes. Although these two events occur at different times, several cell cycle kinases regulate both DNA synthesis and chromosome segregation (3). In budding yeast the Cdc7-Dbf4 kinase (also called Dbf4-dependent kinase or DDK) 2 plays such a dual role in the cell cycle. The Dbf4 regulatory subunit binds to and activates Cdc7 kinase to initiate DNA replication (4, 5). DDK also promotes other aspects of chromosome biology including cohesin loading during early S-phase in Xenopus laevis (6), centromeric cohesion in Schizosaccharomyces pombe (6), and meiotic recombination (7, 8) and the Ndt80 (early meiotic) transcriptional program in Saccharomyces cerevisiae (9). Budding yeast DDK also promotes monopolar orientation of homologs in meiosis I and inhibits chromosome segregation in the mitotic cycle (1, 2, 10, 11). Both activities are likely mediated through an interaction with Cdc5, the single Polo-like kinase in S. cerevisiae. Polo-like kinases (Plks) regulate mitotic events and are also involved in the response to DNA damage and checkpoint adaptation (12-14). Genetic and physical interactions between Dbf4 and Cdc5 were described many years ago (15, 16) raising the possibility that DDK acted beyond S phase. The DDK-Cdc5 interaction raises interesting questions regarding how these distinct kinases interact and coordinate accurate cell cycle progression.The Polo gene was na...

Cdc7p-Dbf4p is a conserved protein kinase required for the initiation of DNA replication. The Dbf4p regulatory subunit binds Cdc7p and is essential for Cdc7p kinase activation, however, the N-terminal third of Dbf4p is dispensable for its essential replication activities. Here, we define a short N-terminal Dbf4p region that targets Cdc7p-Dbf4p kinase to Cdc5p, the single Polo kinase in budding yeast that regulates mitotic progression and cytokinesis. Dbf4p mediates an interaction with the Polo substrate-binding domain to inhibit its essential role during mitosis. Although Dbf4p does not inhibit Polo kinase activity, it nonetheless inhibits Polo-mediated activation of the mitotic exit network (MEN), presumably by altering Polo substrate targeting. In addition, although dbf4 mutants defective for interaction with Polo transit S-phase normally, they aberrantly segregate chromosomes following nuclear misorientation. Therefore, Cdc7p-Dbf4p prevents inappropriate exit from mitosis by inhibiting Polo kinase and functions in the spindle position checkpoint.

Dbf4-dependent kinase (DDK) and cyclin-dependent kinase (CDK) are essential to initiate DNA replication at individual origins. During replication stress, the S-phase checkpoint inhibits the DDK-and CDK-dependent activation of late replication origins. Rad53 kinase is a central effector of the replication checkpoint and both binds to and phosphorylates Dbf4 to prevent late-origin firing. The molecular basis for the Rad53-Dbf4 physical interaction is not clear but occurs through the Dbf4 N terminus. Here we found that both Rad53 FHA1 and FHA2 domains, which specifically recognize phospho-threonine (pT), interacted with Dbf4 through an N-terminal sequence and an adjacent BRCT domain. Purified Rad53 FHA1 domain (but not FHA2) bound to a pT Dbf4 peptide in vitro, suggesting a possible phospho-threonine-dependent interaction between FHA1 and Dbf4. The Dbf4-Rad53 interaction is governed by multiple contacts that are separable from the Cdc5-and Msa1-binding sites in the Dbf4 N terminus. Importantly, abrogation of the Rad53-Dbf4 physical interaction blocked Dbf4 phosphorylation and allowed late-origin firing during replication checkpoint activation. This indicated that Rad53 must stably bind to Dbf4 to regulate its activity.T HE fidelity of chromosome replication depends on checkpoint mechanisms to stabilize stalled forks, regulate origin activation, and repair DNA damage (Hartwell and Weinert 1989;Bartek et al. 2004;Segurado and Tercero 2009). In response to replication stress, the replication checkpoint maintains replisome stability and prevents late origins from firing, which allows time for DNA repair and the completion of DNA replication prior to chromosome segregation. Incomplete DNA replication or uncoordinated origin firing following DNA damage can result in genomic instability, cancer predisposition, and premature aging (Branzei and Foiani 2010).In the budding yeast Saccharomyces cerevisiae, activation of the checkpoint sensor kinase Mec1 (vertebrate ATR, Ataxia Telangiectasia and Rad3-related) is triggered at stalled forks or sites of DNA damage (Majka et al. 2006;Labib and De Piccoli 2011). Subsequent signal amplification through the Mrc1 or Rad9 adaptors leads to activation of the checkpoint kinase Rad53 (the ortholog of the human tumor suppressor Chk2) (Branzei and Foiani 2009). Rad53 is an integral transducer of various cellular responses to replication stress or DNA damage. Rad53 induces a series of transcriptional responses through MBF-regulated genes (Bastos de Oliveira et al. 2012;Travesa et al. 2012) and also activates the Dun1 kinase, which promotes the expression of ribonucleotide reductase (RNR) subunits and additional DNA repair genes (Huang et al. 1998). In parallel, Rad53 down-regulates the RNR inhibitor Sml1 to increase deoxyribonucleotide levels and facilitate DNA synthesis (Zhao et al. 2001). In response to replication fork stalling, Rad53 prevents the activation of late replication origins by phosphorylating two proteins required for the initiation of DNA replication: Dbf4 and Sld3 (...