Major constituent of the PSD essential for postsynaptic signaling. Inhibitory regulator of the Ras-cAMP pathway. Member of the NMDAR signaling complex in excitatory synapses, it may play a role in NMDAR-dependent control of AMPAR potentiation, AMPAR membrane trafficking and synaptic plasticity. Regulates AMPAR-mediated miniature excitatory postsynaptic currents. Exhibits dual GTPase-activating specificity for Ras and Rap. May be involved in certain forms of brain injury, leading to long-term learning and memory deficits. Isoforms containing the PDZ-binding domain associate with DLG4 and DLG3 to form the PSD protein complex colocalized with GRIN2B at synapses. Interacts with MPDZ, KLHL17 CAMK2A and CAMK2B. Mostly in excitatory glutamatergic synapses. Additional isoforms seem to exist. Highly expressed in brain; predominantly in the cortex, hippocampus and olfactory bulb. Present in the postsynaptic density of central excitatory synapses. The PDZ-binding domain interacts with all three PDZ domains of DGL4. The C2 domain is required for RapGAP activity. Phosphorylated by CaM-kinase II. Dephosphorylated upon NMDA receptor activation or SYNGAP1/MPDZ complex disruption. Phosphorylation by PLK2 promotes its activity. It is uncertain whether Met-1 or Met-16 is the initiator methionine.