Part of the postsynaptic scaffold in neuronal cells. Interacts with 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) (PubMed:27756895). Expressed in brain and testis. Highest level of isoform 1 in the brain of newborn rats. Increasing levels of isoforms 2, 3, 4, and 5 in the brain of newborn rats from birth to 6 weeks of postnatal development. Increasing but low level of isoform 6 is expressed in the brain from 2 to 6 weeks of postnatal development. Belongs to the SAPAP family.