Structural and biochemical insights into the interaction mechanism underlying HORMAD1 and its partner proteins

The mammalian HORMA domain-containing protein 1 (HORMAD1) regulates DNA mismatch repair and homologous recombination (HR) repair in many cancers. Here, we show that the structure of human HORMAD1 adopts a self-closed conformation and displays an intra-molecular HORMA domain-closure motif interaction mode. Structural and biochemical data suggest that the interaction modes of the peptide motifs from HORMAD2 and MCM9 with HORMAD1 are highly similar to that of HORMAD1 own closure motif. The peptide motifs from diverse binding partners of HORMAD1 share a conserved Ser-Glu-Pro sequence. Additionally, structural comparison unveiled the HORMA-peptide motif interaction mode diversity among HORMA-containing proteins. Finally, cell-based assays revealed that this HORMA-closure motif interaction pattern contributes to DNA mismatch repair and is required for HORMAD1-dependent HR repair. Together, our results provide structural and biochemical insights into the common theme and functional plasticity of the HORMA domain-containing protein family, and also reveal a universal regulation mechanism for HORMAD1.

Disassembly of the Shieldin Complex by TRIP13

In the past decade, the study of the major DNA double strand break (DSB) repair pathways, homologous recombination (HR) and classical non-homologous end joining (C-NHEJ), has revealed a vast and intricate network of regulation.  The choice between HR and C-NHEJ is largely controlled at the step of DNA end-resection. A pro-C-NHEJ cascade commencing with 53BP1 and culminating in the newly discovered REV7-Shieldin complex impedes end resection and therefore HR. Importantly, loss of any component of this pathway confers PARP inhibitor resistance in BRCA1-deficient cells; hence, their study is of great clinical importance. The newest entrant on the scene of end resection regulation is the ATPase TRIP13 that disables the pro-C-NHEJ cascade by promoting a novel conformational change of the HORMA protein REV7. Here, we tie these new findings and factors with previous research on the regulation of DSB repair and HORMA proteins, and suggest testable hypotheses for how TRIP13 could specifically inactivate REV7-Shieldin to promote HR. We also discuss these biological questions in the context of clinical therapeutics.