Weaker neuroligin 2-neurexin β1 interaction tethers membranes and recruits gephyrin at membrane junctions through clustering.

Single-pass transmembrane proteins neuroligin (NL) and neurexin (NRX) constitute a pair of synaptic adhesion molecules that are essential for the formation of functional synapses. Binding affinities vary by ~1000-fold between combinations of NL and NRX subtypes, which contribute to chemical and spatial specificities. Among major NL-NRX subtypes, NL2 and NRXβ1 have the lowest affinity. Here, we report structures of NL2 in complex with NRXβ1 in several conformations, along with NL2 alone. We identify mechanisms underlying the modulation of NL-NRX affinities and how the weaker NL2-NRXβ1 interaction alone is capable of tethering lipid membranes. We further show that NL2 and NRXβ1 cluster at intercellular junctions and recruit the master postsynaptic scaffolding protein gephyrin, which further clusters neurotransmitter receptors. These findings suggest a dual role of the NL2-NRXβ1 interaction-both as mechanical tether and as signaling receptor-to ensure correct spatial and chemical coordination between two cells to generate functional synapses.