Structure of the Beat–Side complex reveals their interaction is required for neuromuscular system wiring

Nervous system assembly is guided by the actions of cell surface receptors. In Drosophila, members of the beaten path (Beat) and sidestep (Side) protein families have been described as axon guidance receptor-cue pairs, in addition to roles in specifying synaptic connectivity in the optic lobe. To understand the molecular basis and specificity of Beat-Side interactions, we report here the structure of a Beat-Side complex, Beat-Vc bound to Side-VI. The structure shows a binding topology similar to other immunoglobulin superfamily receptors, especially PD-1-PD-L1, nectins, SynCAMs, Dprs, and DIPs, despite lack of established evolutionary relationships. Using a structure-based rational approach, we engineer and validate point mutations to break the binding between Beats and Sides. Using these mutant variants, we demonstrate in developing Drosophila larvae that the interaction between Beat-Ia and Side is required for establishing proper connectivity of motor neurons with muscles.