Enables heparan sulfate proteoglycan binding activity. Involved in regulation of synapse assembly. Acts upstream of or within AMPA glutamate receptor clustering; positive regulation of synapse assembly; and regulation of presynaptic membrane organization. Located in glutamatergic synapse and postsynaptic membrane. Part of AMPA glutamate receptor complex. Is expressed in several structures, including central nervous system; embryo mesenchyme; and future midbrain. Orthologous to human LRRTM4 (leucine rich repeat transmembrane neuronal 4). [provided by Alliance of Genome Resources, Apr 2022]