Structural Basis of Latrophilin-FLRT-UNC5 Interaction in Cell Adhesion

Fibronectin leucine-rich repeat transmembrane proteins (FLRTs) are cell-adhesion molecules with emerging functions in cortical development and synapse formation. Their extracellular regions interact with latrophilins (LPHNs) to mediate synapse development, and with Uncoordinated-5 (UNC5)/netrin receptors to control the migration of neurons in the developing cortex. Here, we present the crystal structures of FLRT3 in isolation and in complex with LPHN3. The LPHN3/FLRT3 structure reveals that LPHN3 binds to FLRT3 at a site distinct from UNC5. Structure-based mutations specifically disrupt LPHN3/FLRT3 binding, but do not disturb their interactions with other proteins or their cell-membrane localization. Thus, they can be used as molecular tools to dissect the functions of FLRTs and LPHNs in vivo. Our results suggest that UNC5 and LPHN3 can simultaneously bind to FLRT3, forming a trimeric complex, and that FLRT3 may form transsynaptic complexes with both LPHN3 and UNC5. These findings provide molecular insights for understanding the role of cell-adhesion proteins in synapse function. [Display omitted] •The crystal structure of the LPHN3/FLRT3 complex reveals the binding interface•Mutations at the binding interface disrupt the interaction of FLRT3 and LPHN3•LPHN, FLRT, and UNC5 form a trimeric complex•LPHN3/FLRT3 and FLRT3/UNC5 mediate formation of intercellular contacts FLRTs, LPHNs, and UNC5s are families of interacting neuronal cell-surface receptors that mediate brain development. The LPHN3/FLRT3 structure, reported by Lu et al., reveals that LPHN3 binds to FLRT3 at a site distinct from UNC5. FLRT3 simultaneously binds to LPHN3 and UNC5, and forms a trimeric complex.