Postsynaptic density scaffolding protein. Binds and cross-links cytoplasmic regions of GRM1, GRM5, ITPR1, DNM3, RYR1, RYR2, SHANK1 and SHANK3. By physically linking GRM1 and GRM5 with ER-associated ITPR1 receptors, it aids the coupling of surface receptors to intracellular calcium release (By similarity). Negatively regulates T cell activation by inhibiting the calcineurin-NFAT pathway. Acts by competing with calcineurin/PPP3CA for NFAT protein binding, hence preventing NFAT activation by PPP3CA (By similarity). Tetramer (By similarity). Encodes coiled-coil structures that mediate homo- and heteromultimerization. Interacts with NFATC2; interaction is calcium independent; interaction competes with PPP3CA for NFATC2 binding; interaction is reduced by AKT activation. Interacts with NFATC1 and NFATC4. Interacts with SHANK1; forms a high-order complex at least composed of SHANK1 and HOMER3; the complex formation is regulated by CAMK2A-mediated phosphorylation (By similarity). Postsynaptic density of neuronal cells. The WH1 domain interacts with the PPXXF motif in GRM1, GRM5, RYR1, RYR2, ITPR1, SHANK 1 and SHANK3. Belongs to the Homer family.