Involved in the development and maintenance of excitatory synapse in the vertebrate nervous system. Regulates surface expression of AMPA receptors and instructs the development of functional glutamate release sites. Acts as a ligand for the presynaptic receptors NRXN1-A and NRXN1-B (By similarity). Interacts with DLG4 and NRXN1. Localized to excitatory synapses. Expressed in neuronal tissues. Synaptogenic effects are mediated by the extracellular LRR region. Belongs to the LRRTM family. Extended N-terminus.