The Roles of Conserved Dbf4 Motifs in DNA Replication and Checkpoint Responses in Saccharomyces cerevisiae

Abstract

The Dbf4 protein is involved in the initiation of DNA replication, in complex with Cdc7 kinase, and also plays a role in the intra-S-phase checkpoint response via an interaction with Rad53 in Saccharomyces cerevisiae. The Dbf4 protein has three highly conserved motifs, called the N, M and C motifs. In view of the fact that a comprehensive analysis of the roles of the three motifs in the initiation of DNA replication and checkpoint response was not previously available, this study was, therefore, conducted. The objectives of the study were: (1) to assess the function of the three conserved motifs, with respect to their essentiality for cell viability, (2) to determine their roles in mediating interactions with other proteins (i.e. Cdc7, Orc2, Mcm2) involved in the initiation of DNA replication and with Rad53 in the intra-S-phase checkpoint response, and (3) to obtain the three-dimensional structure of the Dbf4 N-motif by X-ray crystallography. The Dbf4 N-motif was found to be nonessential for cell viability, mediates the interaction between Dbf4 and Rad53, and as well as the interaction with Orc2. A mutant lacking the N-motif (dbf4N), was found to have a growth defect and was hypersensitive to the genotoxic agents: hydroxyurea (HU) and methyl methane sulfonate (MMS), suggesting that a disruption in the intra-S-phase checkpoint occurred because of an abrogated Dbf4-Rad53 interaction. Double point mutation of two threonine residues of the N-motif (threonines 171 and 175) to alanines also caused an abrogated Dbf4-Rad53 FHA1 domain interaction. The Dbf4 M-motif was found to be essential for cell viability and mediates the interaction between Dbf4 and Mcm2. A single proline to leucine point mutation at amino acid residue 277 conferred resistance to HU and MMS and caused disrupted Dbf4-Mcm2 and Dbf4-Orc2 interaction, while Dbf4-Rad53 interaction was maintained. Thus, the alteration of the M-motif may facilitate the role of Dbf4 as a checkpoint target. The Dbf4-C motif was also found to be essential for cell viability. Deletion and point mutations to the C-motif affected the interactions between Dbf4 and Rad53, Orc2, Mcm2 and also with Mcm4. Attempts were also made to obtain the three-dimensional structure of Dbf4, using X-ray crystallography methods. The work presented here represents a thorough functional analysis of the three conserved domains of Dbf4 in Saccharomyces cerevisiae. These results can be used as a baseline for further research involving higher eukaryotic organisims, including humans. This is particularly of relevance in light of recent evidence demonstrating an overexpression of the human Dbf4 orthologue overexpression as a cancer phenotype in human cancer cells.