Constitutive phosphorylation of MDC1 physically links the MRE11-RAD50-NBS1 complex to damaged chromatin

Area of Science:

• DNA repair
• Cellular response to DNA damage
• Molecular biology

Background:

• The MRE11-RAD50-Nijmegen breakage syndrome 1 (MRN) complex is crucial for DNA double-strand break (DSB) repair.
• MRN complex recruitment to DSBs is mediated by MDC1, which interacts with phosphorylated H2AX.
• Understanding MDC1's role in MRN complex localization is key to DNA repair mechanisms.

Purpose of the Study:

• To identify the specific region in MDC1 responsible for MRN complex focal accumulation at DNA damage sites.
• To elucidate the molecular mechanism linking MDC1 to the MRN complex during DNA repair.

Main Methods:

• Identification of a critical region in MDC1 using genetic and biochemical approaches.
• In vitro phosphorylation assays using casein kinase 2 (CK2).
• Analysis of MDC1-NBS1 interaction using phosphorylation-dependent assays.
• Depletion of CK2 using small interfering RNA (siRNA) in vivo.

Main Results:

• A specific region in MDC1, containing phosphorylated acidic motifs, was identified as essential for MRN complex focus formation.
• These motifs are phosphorylated by CK2 and directly interact with the NBS1 N-terminal forkhead-associated domain in a phosphorylation-dependent manner.
• Disruption of these motifs or CK2 depletion abrogated MRN complex accumulation at DSBs.

Conclusions:

• MDC1 physically tethers the MRN complex to damaged chromatin through a phosphorylation-dependent interaction mediated by CK2.
• This mechanism highlights the importance of post-translational modifications in orchestrating DNA damage response pathways.
• The findings provide new insights into the regulation of DNA double-strand break repair initiation.