Two structurally mobile regions control the conformation and function of metamorphic meiotic HORMAD proteins.

Metamorphic HORMA domain proteins (HORMADs) nucleate protein complex formation by refolding their mobile safety belt region to bind short closure motifs on interactors. Meiotic HORMADs (mHORMADs) bind proteinaceous axial elements to orchestrate complex chromosomal events that underpin fertility, including pairing and recombination between homologous chromosomes. However, the mechanisms supporting the diverse roles of mHORMADs remain unclear. Here, we show that mHORMADs have a second structurally mobile region, the β5-αC loop, which controls mHORMAD conformation and function. Molecular dynamics and in vivo approaches show that functional specialisation of C. elegans paralogs HTP-1 and HTP-2 depends on the interplay between their β5-αC loop and safety belt. The β5-αC loop can interact with the same HORMA core surface as the safety belt and mutations that hinder this interaction prevent HTP-1 binding to closure motifs in vitro and axis loading of HTP-1 and its paralog HTP-3 in vivo. Structural predictions of mHORMADs from yeast, plants, and mammals suggest that the β5-αC loop HORMA core interaction is a conserved feature of mHORMADs. Our study reveals that mHORMADs have expanded the bimodal folding landscape first identified in Mad2, paving the way to elucidate how non-canonical HORMAD conformations control meiotic chromosome function to ensure fertility.