Structural basis and functional roles for Toll-like receptor binding to Latrophilin adhesion-GPCR in embryo development

Latrophilins/ADGRLs are conserved adhesion-type G protein-coupled receptors associated with early embryonic morphogenesis defects, lethality, and sterility across multiple model organisms. However, their mechanistic roles in embryogenesis and the identity of their binding ligands remain unknown. Here, we identified a cell-surface receptor, TOL-1, the sole Toll-like receptor in C. elegans , as a novel ligand for the C. elegans Latrophilin, LAT-1. The extracellular lectin domain of LAT-1 directly binds to the second leucine-rich repeat domain of TOL-1. The highresolution crystal structure and the cryo-EM density map of the LAT-1–TOL-1 ectodomain complex reveal a previously-unobserved mode of one-to-one interaction enabled by a large interface. CRISPR/Cas9-mediated mutation of key interface residues selectively disrupted the endogenous LAT-1–TOL-1 interaction in C. elegans, leading to partial sterility, lethality, and malformed embryos. Thus, TOL-1 binding to LAT-1 represents a receptor-ligand axis essential for animal morphogenesis.