The molecular basis of ATM-dependent dimerization of the Mdc1 DNA damage checkpoint mediator

Mdc1 is a large modular phosphoprotein scaffold that maintains signaling and repair complexes at double-stranded DNA break sites. Mdc1 is anchored to damaged chromatin through interaction of its C-terminal BRCT-repeat domain with the tail of γH2AX following DNA damage, but the role of the N-terminal forkhead-associated (FHA) domain remains unclear. We show that a major binding target of the Mdc1 FHA domain is a previously unidentified DNA damage and ATM-dependent phosphorylation site near the N-terminus of Mdc1 itself. Binding to this motif stabilizes a weak self-association of the FHA domain to form a tight dimer. X-ray structures of free and complexed Mdc1 FHA domain reveal a ‘head-to-tail' dimerization mechanism that is closely related to that seen in pre-activated forms of the Chk2 DNA damage kinase, and which both positively and negatively influences Mdc1 FHA domain-mediated interactions in human cells prior to and following DNA damag

The chromatin-remodeling factor CHD4 coordinates signaling and repair after DNA damage

The CHD4 helicase is identified as a new component of the genome surveillance machinery in a proteomic screen for factors enriched on chromatin after ionizing radiation (see also related paper by Smeenk et al. in this issue)

The molecular basis of ATM-dependent dimerization of the Mdc1 DNA damage checkpoint mediator

Mdc1 is a large modular phosphoprotein scaffold that maintains signaling and repair complexes at double-stranded DNA break sites. Mdc1 is anchored to damaged chromatin through interaction of its C-terminal BRCT-repeat domain with the tail of γH2AX following DNA damage, but the role of the N-terminal forkhead-associated (FHA) domain remains unclear. We show that a major binding target of the Mdc1 FHA domain is a previously unidentified DNA damage and ATM-dependent phosphorylation site near the N-terminus of Mdc1 itself. Binding to this motif stabilizes a weak self-association of the FHA domain to form a tight dimer. X-ray structures of free and complexed Mdc1 FHA domain reveal a ‘head-to-tail’ dimerization mechanism that is closely related to that seen in pre-activated forms of the Chk2 DNA damage kinase, and which both positively and negatively influences Mdc1 FHA domain-mediated interactions in human cells prior to and following DNA damage

The NBS1-Treacle complex controls ribosomal RNA transcription in response to DNA damage

Chromosome breakage elicits transient silencing of ribosomal RNA synthesis, but the mechanisms involved remained elusive. Here we discover an in trans signalling mechanism that triggers pan-nuclear silencing of rRNA transcription in response to DNA damage. This is associated with transient recruitment of the Nijmegen breakage syndrome protein 1 (NBS1), a central regulator of DNA damage responses, into the nucleoli. We further identify TCOF1 (also known as Treacle), a nucleolar factor implicated in ribosome biogenesis and mutated in Treacher Collins syndrome, as an interaction partner of NBS1, and demonstrate that NBS1 translocation and accumulation in the nucleoli is Treacle dependent. Finally, we provide evidence that Treacle-mediated NBS1 recruitment into the nucleoli regulates rRNA silencing in trans in the presence of distant chromosome breaks