Member of the NMDAR signaling complex that may play a role in control of AMPAR potentiation and synaptic plasticity in excitatory synapses (PubMed:15312654). Promotes clustering of HT2RC at the cell surface (PubMed:11150294). Interacts with F11R/JAM, CLDN1, NG2, CXADR, CRB1, MPP4 and PALS1, HTR2A, HTR2B, PLEKHA1/TAPP1 and PLEKHA2/TAPP2. Interacts with CXADR (By similarity). Interacts with HTR2C, CLDN5, DLG4, GRIN1, SYNGAP1, CAMK2A and CAMK2B. Interacts with FAT4 (via cytoplasmic domain) (By similarity). Interacts with DLL1 (By similarity). Associated with membranes. Enriched at the tight junctions of epithelial cells. Association to the tight junctions depends on CXADR. In the retina, localizes to the sub-apical region adjacent to the adherens junction complex at the outer limiting membrane (By similarity). Colocalizes with HTR2C on the apical membrane of epithelial choroid plexus cells (PubMed:11150294). Localized mainly in the Schmidt-Lanterman incisures of myelinating Schwann cells. Highly enriched in postsynaptic densities (PSD). Localized to punctae on dendrites of hippocampal neurons and colocalizes with the synaptic marker DLG4. Abundant in all cerebral cortical layers, especially the piriform cortex, the pyramidal cells of the CA1-CA3 subfields of the hippocampus, as well as the granular layer of the dentate gyrus. Detected in the internal granular layer and the mitral cell layer of the olfactory bulb; in the medial habenular nucleus; and in amygdaloid, thalamic, hypothalamic, and pontine nuclei. In the cerebellum, found at high levels in the granular layer. Detected in the lateral ventricle. Expression overlaps with 5-HT2C receptor expression in all regions of the brain including the choroid plexus, where 5-HT2C receptors are highly enriched. The PDZ domain 1 binds NG2. The PDZ domain 2 mainly binds CAMK2A and CAMK2B. The PDZ domain 9 binds F11R. The PDZ domain 10 binds the C-terminus of CLDN1 and KIT. The PDZ domains 10 and 13 bind PLEKHA1 and PLEKHA2. The PDZ domain 13 binds CXADR and SYNGAP1 (By similarity). The PDZ domains 7 and 10 bind the Ad9 E4-ORF1 oncoprotein. The PDZ domain 10 binds the C-terminal PDZ-binding motif of HTR2C. Sequestered into cytoplasmic, detergent-resistant bodies upon Ad9 E4-ORF1 oncoprotein expression. Targeted to degradation upon HPV-18 E6 oncoprotein expression.