Promotes neurite outgrowth in hippocampal neurons (By similarity). Involved in the regulation of the differentiation and maintenance of excitatory synapses. Induces the clustering of excitatory postsynaptic proteins, including DLG4, DLGAP1, GRIA1 and GRIN1. Forms heteromeric complexes with LRFN2, LRFN4 and LRFN5; binding to LRFN2 and LRFN5 may be weaker than that to LRFN4. Also interacts with LRFN3 (By similarity). Forms homomeric complexes, but not across cell junctions (By similarity). Interacts with DLG1, DLG2 and DLG4, but not with MAGI2, not CASK. Interacts with DLG3 (By similarity). Interacts with 2 AMPA receptor subunits GRIA1 and GRIA2 and NMDA receptor subunit GRIN1. Detected in excitatory, but not inhibitory, synaptic plasma membrane. Mainly expressed in brain (at protein level) and testis. In brain, found in cerebral cortex (including pyramidal neurons), hippocampus (including CA3 and CA1 neurons), dentate gyrus, cerebellum (including Purkinje neurons) (at protein level) (at protein level). Also expressed in the olfactory bulb. Expression increases steadily during postnatal rat brain development. The PDZ-binding motif is required for neurite outgrowth promotion (By similarity). This motif is also involved in DLG1-, DLG3- and DLG4-binding. Glycosylated. Belongs to the LRFN family.