LAUSR

Axon pathfinding is critical for nervous system development, and it is orchestrated by molecular cues that activate receptors on the axonal growth cone. Robo family receptors bind Slit guidance cues to mediate axon repulsion. In mammals, the divergent family member Robo3 does not bind Slits, but instead signals axon repulsion from its own ligand, NELL2. Conversely, canonical Robos do not mediate NELL2 signaling. Here, we present the structures of NELL-Robo3 complexes, identifying a mode of ligand engagement for Robos that is orthogonal to Slit binding. We elucidate the structural basis for differential binding between NELL and Robo family members and show that NELL2 repulsive activity is a function of its Robo3 affinity and is enhanced by ligand trimerization. Our results reveal a mechanism of oligomerization-induced Robo activation for axon guidance and shed light on Robo family member ligand binding specificity, conformational variability, divergent modes of signaling, and evolution. Robo3 is a divergent, multifunctional member of the Robo receptor family that mediates axon guidance by its ligand NELL2 instead of the canonical Slit ligands. Here, the authors present the crystal structures of human Robo3 in complex with NELL1 and NELL2, and they show through biophysical and functional assays how NELL-Robo3 affinity and ligand-induced receptor multimerization control axon guidance activity.