Structural and evolutionary insights into DAF-12 interactions with transcriptional coactivators in parasitic nematodes.

Parasitic nematodes infect billions of humans and livestock worldwide, causing major health and economic burdens, while the spread of anthelmintic resistance threatens current control strategies. A critical step in parasite infection is the resumption of development of infective third-stage larvae (iL3) upon host entry, a process controlled by the nuclear receptor DAF-12. Activation of DAF-12 by dafachronic acids promotes developmental progression and reproductive maturation, making this receptor an attractive therapeutic target. However, the molecular mechanisms governing DAF-12 activation, particularly transcriptional coactivator recruitment, remain poorly understood. Here, we combined biophysical, cellular, structural, and bioinformatic approaches to investigate coactivator recognition by DAF-12 from the parasitic nematodes Brugia malayi and Haemonchus contortus. Crystal structures of ligand-bound DAF-12 ligand-binding domains in complex with coactivator-derived peptides reveal conserved features of ligand-dependent coactivator recruitment shared with mammalian nuclear receptors. In addition, we uncover previously unrecognized interaction features, including motif-specific contacts that extend beyond the canonical LXXLL binding mode of coactivators and distinct patterns of DAF-12 conservation across nematode clades. Structure-guided analyses redefine the interaction motif of the only described parasite-specific coactivator DIP-1 and suggest novel candidate motifs for DAF-12-interacting proteins. Together, these findings establish the structural basis of coactivator binding to nematode DAF-12 and provide mechanistic insight into the transcriptional regulation underlying parasite development. These results expand current understanding of nuclear receptor signaling in parasitic nematodes and provide a framework for the future design of strategies aimed at disrupting DAF-12 activation as a potential antiparasitic approach.