v-Src inhibits the interaction between Rad17 and Rad9 and induces replication fork collapse

The C-terminal tail of Rad17, iVERGE, binds the 9‒1‒1 complex independently of AAA+ ATPase domains to provide another clamp-loader interface

The ATR pathway plays a crucial role in maintaining genome integrity as the major DNA damage checkpoint. It also attracts attention as a therapeutic target in cancer treatment. The Rad17-RFC2-5 complex loads the Rad9-Hus1-Rad1 (9-1-1) DNA clamp complex onto damaged chromatin to activate the ATR pathway. We previously reported that phosphorylation of a polyanionic C-terminal tail of human Rad17, iVERGE, is essential for the interaction between Rad17 and the 9-1-1 complex. However, the molecular mechanism has remained unclear. Here, we show that iVERGE directly interacts with the Hus1 subunit of the 9-1-1 complex through Rad17-S667 phosphorylation independently of the AAA+ ATPase domains. An exogenous iVERGE peptide interacted with the 9-1-1 complex in vivo. The binding conformation of the iVERGE peptide was analyzed by de novo modeling with docking simulation, simulated annealing-molecular dynamics simulation, and the fragment molecular orbital method. The in silico analyses predicted the association of the iVERGE peptide with the hydrophobic and basic patches on the Hus1 protein, and the corresponding Hus1 mutants were deficient in the interaction with the iVERGE peptide in vivo. The iVERGE peptide occupied the same position as the C-terminus of Saccharomyces cerevisiae RAD24 on MEC3. The interaction energy calculation suggested that the Rad17 KYxxL motif and the iVERGE peptide are the primary and secondary interaction surfaces between the Rad17-RFC2-5 and 9-1-1 complexes. Our data reveal a novel molecular interface, iVERGE, between the Rad17-RFC2-5 and 9-1-1 complexes in vertebrates and implicate that Rad17 utilizes two distinct molecular interfaces to regulate the 9-1-1 complex.

[Novel Tyrosine Phosphorylation Signals in the Nucleus and on Mitotic Spindle Fibers and Lysosomes Revealed by Strong Inhibition of Tyrosine Dephosphorylation]

Protein-tyrosine phosphorylation is one of the posttranslational modifications and plays critical roles in regulating a wide variety of cellular processes, such as cell proliferation, differentiation, adhesion, migration, survival, and apoptosis. Protein-tyrosine phosphorylation is reversibly regulated by protein-tyrosine kinases and protein-tyrosine phosphatases. Strong inhibition of protein-tyrosine phosphatase activities is required to undoubtedly detect tyrosine phosphorylation. Our extremely careful usage of Na₃VO₄, a potent protein-tyrosine phosphatase inhibitor, has revealed not only the different intracellular trafficking pathways of Src-family tyrosine kinase members but also novel tyrosine phosphorylation signals in the nucleus and on mitotic spindle fibers and lysosomes. Furthermore, despite that the first identified oncogene product v-Src is generally believed to induce transformation through continuous stimulation of proliferation signaling by its strong tyrosine kinase activity, v-Src-driven transformation was found to be caused not by continuous proliferation signaling but by v-Src tyrosine kinase activity-dependent stochastic genome alterations. Here, I summarize our findings regarding novel tyrosine phosphorylation signaling in a spatiotemporal sense and highlight the significance of the roles of tyrosine phosphorylation in transcriptional regulation inside the nucleus and chromosome dynamics.

Casein kinase 2 promotes interaction between Rad17 and the 9-1-1 complex through constitutive phosphorylation of the C-terminal tail of human Rad17

An interaction between the Rad17-RFC2-5 and 9-1-1 complexes is essential for ATR-Chk1 signaling, which is one of the major DNA damage checkpoints. Recently, we showed that the polyanionic C-terminal tail of human Rad17 and the embedded conserved sequence iVERGE are important for the interaction with 9-1-1 complex. Here, we show that Rad17-S667 in the C-terminal tail is constitutively phosphorylated in vivo in a casein kinase 2-dependent manner, and the phosphorylation is important for 9-1-1 interaction. The serine phosphorylation of Rad17 could be seen in the absence of exogenous genotoxic stress, and was mostly abolished by S667A substitution. Rad17-S667 was also phosphorylated when the C-terminal tail was fused with EGFP, but the phosphorylation was inhibited by two casein kinase 2 inhibitors. Furthermore, interaction between Rad17 and the 9-1-1 complex was inhibited by the casein kinase 2 inhibitor CX-4945/Silmitasertib, and the effect was dependent on the Rad17-S667 residue, indicating that S667 phosphorylation is the only role of casein kinase 2 in the 9-1-1 interaction. Our data raise the possibility that the C-terminal tail of vertebrate Rad17 regulates ATR-Chk1 signaling through multi-site phosphorylation in the iVERGE.

The KYxxL motif in Rad17 protein is essential for the interaction with the 9-1-1 complex

ATR-dependent DNA damage checkpoint is the major DNA damage checkpoint against UV irradiation and DNA replication stress. The Rad17-RFC and Rad9-Rad1-Hus1 (9-1-1) complexes interact with each other to contribute to ATR signaling, however, the precise regulatory mechanism of the interaction has not been established. Here, we identified a conserved sequence motif, KYxxL, in the AAA+ domain of Rad17 protein, and demonstrated that this motif is essential for the interaction with the 9-1-1 complex. We also show that UV-induced Rad17 phosphorylation is increased in the Rad17 KYxxL mutants. These data indicate that the interaction with the 9-1-1 complex is not required for Rad17 protein to be an efficient substrate for the UV-induced phosphorylation. Our data also raise the possibility that the 9-1-1 complex plays a negative regulatory role in the Rad17 phosphorylation. We also show that the nucleotide-binding activity of Rad17 is required for its nuclear localization.

Src family kinases maintain the balance between replication stress and the replication checkpoint

Progression of DNA replication is tightly controlled by replication checkpoints to ensure the accurate and rapid duplication of genetic information. Upon replication stress, the replication checkpoint slows global DNA replication by inhibiting the late-firing origins and by slowing replication fork progression. Activation of the replication checkpoint has been studied in depth; however, little is known about the termination of the replication checkpoint. Here, we show that Src family kinases promote the recovery from replication checkpoints. shRNA knockdown of a Src family kinase, Lyn, and acute chemical inhibition of Src kinases prevented inactivation of Chk1 after removal of replication stress. Consistently, Src inhibition slowed resumption of DNA replication, after the removal of replication blocks. The effect of Src inhibition was not observed in the presence of an ATM/ATR inhibitor caffeine. These data indicate that Src kinases promote the resumption of DNA replication by suppressing ATR-dependent replication checkpoints. Surprisingly, the resumption of replication was delayed by caffeine. In addition, Src inhibition delayed recovery from replication fork collapse. We propose that Src kinases maintain the balance between replication stress and the activity of the replication checkpoint.