Rad53 Phosphorylation Site Clusters Are Important for Rad53 Regulation and Signaling

Saccharomyces cerevisiae Rad53 has crucial functions in many aspects of the cellular response to DNA damage and replication blocks. To coordinate these diverse roles, Rad53 has two forkhead-associated (FHA) phosphothreonine-binding domains in addition to a kinase domain. Here, we show that the conserved N-terminal FHA1 domain is essential for the function of Rad53 to prevent the firing of late replication origins in response to replication blocks. However, the FHA1 domain is not required for Rad53 activation during S phase, and as a consequence of defective downstream signaling, Rad53 containing an inactive FHA1 domain is hyperphosphorylated in response to replication blocks. The FHA1 mutation dramatically hypersensitizes strains with defects in the cell cycle-wide checkpoint pathways (rad9⌬ and rad17⌬) to DNA damage, but it is largely epistatic with defects in the replication checkpoint (mrc1⌬). Altogether, our data indicate that the FHA1 domain links activated Rad53 to downstream effectors in the replication checkpoint. The results reveal an important mechanistic difference to the homologous Schizosaccharomyces pombe FHA domain that is required for Mrc1-dependent activation of the corresponding Cds1 kinase. Surprisingly, despite the severely impaired replication checkpoint and also G 2 /M checkpoint functions, the FHA1 mutation by itself leads to only moderate viability defects in response to DNA damage, highlighting the importance of functionally redundant pathways.

Section: Fha1 Functions In the Mrc1 Pathwaymentioning

confidence: 81%

Rad53 Kinase Activation-independent Replication Checkpoint Function of the N-terminal Forkhead-associated (FHA1) Domain

Pike

Tenis

Heierhorst

2004

“…Extensive studies have identified the role of Mec1 in the phosphorylation of Rad53 and a key role of Mrc1 in the recruitment of Rad53 after Mrc1 is phosphorylated by Mec1 for Rad53 activation (21,33). Both Mec1 and Mrc1 are known to be recruited to the DNA replication forks via distinct mechanisms; thus they are poised to exert their activities on Rad53, which is known to help maintain DNA replication forks in response to genotoxic stresses (4,6).…”

Section: Discussionmentioning

confidence: 99%

“…Second, a direct and transphosphorylation of the N-terminal TQ sites of the Chk2 family kinases by the Mec1/ATR family kinases is also important for their activation in vivo. Analogous to the requirement of N-terminal TQ site phosphorylation of Chk2 by ATR in human (32), the activation of Rad53/Cds1 in vivo requires phosphorylation of TQ sites in their N termini by Mec1/Rad3 (33,34).…”

mentioning

confidence: 99%

Reconstitution of Rad53 Activation by Mec1 through Adaptor Protein Mrc1

Chen¹,

Zhou

2009

Upon DNA replication stress, stalled DNA replication forks serve as a platform to recruit many signaling proteins, leading to the activation of the DNA replication checkpoint. Activation of Rad53, a key effector kinase in the budding yeast Saccharomyces cerevisiae, is essential for stabilizing DNA replication forks during replication stress. Using an activity-based assay for Rad53, we found that Mrc1, a replication fork-associated protein, cooperates with Mec1 to activate Rad53 directly. Reconstitution of Rad53 activation using purified Mec1 and Mrc1 showed that the addition of Mrc1 stimulated a more than 70-fold increase in the ability of Mec1 to activate Rad53. Instead of increasing the catalytic activity of Mec1, Mrc1 was found to facilitate the phosphorylation of Rad53 by Mec1 via promotion of a stronger enzyme-substrate interaction between them. Further, the conserved C-terminal domain of Mrc1 was found to be required for Rad53 activation. These results thus provide insights into the role of the adaptor protein Mrc1 in activating Rad53 in the DNA replication checkpoint.Faithful replication of the genome is important for the survival of all organisms. During DNA replication, replication stress can arise from a variety of situations, including intrinsic errors made by DNA polymerases, difficulties in replicating repeated DNA sequences, and failures to repair damaged DNA caused by either endogenous oxidative agents or exogenous mutagens such as UV light and DNA-damaging chemicals (1-3). In eukaryotes, there is an evolutionarily conserved DNA replication checkpoint that becomes activated in response to DNA replication stress. It helps to stabilize DNA replication forks, block late replication origin firing, and delay mitosis and ultimately helps recovery from stalled replication forks after DNA repair (4 -7). Defects in the DNA replication checkpoint could result in elevated genomic instabilities, cancer development, or cell death (8, 9).Aside from replicating the genome, the DNA replication forks also provide a platform to assemble many signaling proteins that function in the DNA replication checkpoint. In the budding yeast Saccharomyces cerevisiae, Mec1, an ortholog of human ATR, 2 is a phosphoinositide 3-kinase-like kinase (PIKK) involved in sensing stalled DNA replication forks. Mec1 forms a protein complex with Ddc2 (ortholog of human ATRIP). The Mec1-Ddc2 complex is recruited to stalled replication forks through replication protein A (RPA)-coated single-stranded DNA (10, 11). The Mec3-Rad17-Ddc1 complex, a proliferating cell nuclear antigen (PCNA)-like checkpoint clamp and ortholog of the human 9-1-1 complex, was shown to be loaded onto the single-and double-stranded DNA junction of the stalled replication forks by the clamp loader Rad24-RFC complex (12). Once loaded, the Mec3-Rad17-Ddc1 complex stimulates Mec1 kinase activity (13). Dbp11 and its homolog TopBP1 in vertebrates are known components of the replication machinery (14). In addition to regulating the initiation of DNA replication, they were foun...

“…An equivalent position in fission yeast Cds1, Thr-11, is phosphorylated by Rad3 in vitro and is required for Cds1 activation in vivo (16). Similarly, efficient activation of budding yeast Rad53 requires one or more N-terminal TQ motifs that are potential substrate phosphorylation sites of Mec1 (30).…”

mentioning

confidence: 99%

Cds1 Phosphorylation by Rad3-Rad26 Kinase Is Mediated by Forkhead-associated Domain Interaction with Mrc1

Tanaka

Russell

2004