Part of the postsynaptic scaffold in neuronal cells. Interacts with the guanylate kinase-like domain of DLG1, DLG2, DLG3, DLG4 and AIP1. Interacts with the PDZ domain of SHANK1, SHANK2 and SHANK3. Found in a complex with DLG4 and SHANK1, SHANK2 or SHANK3. Found in a complex with DLG4 and BEGAIN. Interacts with DYL2 and LRFN1. Interacts with MPP2 (via the SH3-Guanylate kinase-like sub-module) (By similarity). Highest levels in the neocortex, part of the hippocampus, the granule cell layer of the cerebellum, the glomerular layer of the olfactory bulb, the inner plexiform layer of the retina, the ventral and dorsal horn of the spinal chord, the neuromuscular junction and the submandibular ganglion. Belongs to the SAPAP family. Extended N-terminus.