Regulation of HORMA domain proteins by the conserved remodeler Pch2/TRIP13

The AAA+ ATPase Pch2/TRIP13 is a key regulator of two related signaling protein families in eukaryotes, the meiotic HORMADs and the spindle checkpoint protein MAD2. These two families share a common domain, the HORMA domain, which can adopt two differently‐folded states: a signaling‐inactive “open” state and an active, peptide‐bound “closed” state. In the closed state, the C‐terminus of the HORMA domain, termed the “safety belt” region, wraps around a bound peptide to generate a topological linkage between the two proteins. To determine how Pch2/TRIP13 regulates HORMA domain protein function, we determined the crystal structures of Pch2/TRIP13 orthologs from C. elegans and H. sapiens . C. elegans PCH‐2 crystallizes in an asymmetric hexamer conformation, and H. sapiens TRIP13 crystallizes in a helical‐spiral conformation, with subunit‐subunit interfaces equivalent to those seen in C. elegans PCH‐2. Together, these structures reveal how ATP binding, hydrolysis, and release in the Pch2/TRIP13 hexamer power conformational changes that remodel substrate proteins. Further, we find that mammalian TRIP13, aided by the adapter protein p31(comet), converts MAD2 from its signaling‐active closed conformer to its inactive open conformer. We propose that TRIP13 partially unfolds the MAD2 C‐terminal safety belt region, allowing release of bound peptide and relaxation to the protein's open state. In the context of the spindle assembly checkpoint, TRIP13 and p31(comet) collaborate to recognize and disassemble mitotic checkpoint complexes, inactivating the checkpoint. A parallel HORMA protein complex disassembly activity likely underlies TRIP13's critical regulatory functions in meiotic chromosome structure and recombination, where it acts on the meiotic HORMAD proteins. Support or Funding Information The authors acknowledge support from the NIH (R01GM104141), the March of Dimes Foundation, and the Ludwig Institute for Cancer Research.