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

Area of Science:

• DNA damage response
• Cellular signaling
• Protein structure and function

Background:

• Mdc1 is a key scaffold protein at DNA double-strand break sites.
• Its C-terminal BRCT domain binds to γH2AX, anchoring it to chromatin.
• The function of Mdc1's N-terminal FHA domain in DNA damage response was unclear.

Purpose of the Study:

• To elucidate the role of the Mdc1 N-terminal FHA domain in DNA damage.
• To identify binding partners and regulatory mechanisms of the Mdc1 FHA domain.
• To understand the structural basis of Mdc1 FHA domain function.

Main Methods:

• Biochemical assays to identify Mdc1 FHA domain binding targets.
• Site-directed mutagenesis to probe Mdc1 FHA domain function.
• X-ray crystallography to determine the structure of the Mdc1 FHA domain.
• Cell-based assays in human cells to assess Mdc1 function in DNA damage response.

Main Results:

• The Mdc1 FHA domain binds to a novel ATM-dependent phosphorylation site on Mdc1 itself.
• This binding stabilizes Mdc1 FHA domain self-association into a dimer.
• X-ray structures reveal a head-to-tail dimerization mechanism for the Mdc1 FHA domain.
• This dimerization impacts Mdc1-mediated interactions in human cells during DNA damage.

Conclusions:

• The Mdc1 FHA domain has an autoinhibitory role that is regulated by ATM-dependent phosphorylation.
• Mdc1 dimerization via its FHA domain is a key regulatory mechanism in DNA damage response.
• Understanding Mdc1 FHA domain function provides insights into DNA repair pathway regulation.