Dual role of CaMKII-dependent SAP97 phosphorylation in mediating trafficking and insertion of NMDA receptor subunit NR2A.

Synapse Associated Protein 97 (SAP97), a member of membrane-associated guanylate kinase (MAGUK) protein family, has been involved in the correct targeting and clustering of ionotropic glutamate receptors (iGluRs) at postsynaptic sites. Calcium/calmodulin kinase II (CaMKII) phosphorylates SAP97 on two major sites in vivo; one located in the N-terminal domain (Ser39) and the other in the first postsynaptic density disc large ZO1 (PDZ) domain (Ser232). CaMKII-mediated phosphorylation of SAP97-Ser39 is necessary and sufficient to drive SAP97 to the postsynaptic compartment in cultured hippocampal neurons. CaMKII-dependent phosphorylation of Ser232 disrupts SAP97 interaction with NR2A subunit, thereby regulating synaptic targeting of this NMDA receptor subunit. Here we show by means of phospho-specific antibodies that SAP97-Ser39 phosphorylation represents the driving force to release SAP97/NR2A complex from the endoplasmic reticulum. Ser39 phosphorylation does not interfere with SAP97 capability to bind NR2A. On the contrary, SAP97-Ser232 phosphorylation occurs within the postsynaptic compartment and is responsible for both the disruption of NR2A/SAP97 complex and, consequently, for NR2A insertion in the postsynaptic membrane. Thus, CaMKII-dependent phosphorylation of SAP97 in different time frames and locations within the neurons controls both NR2A trafficking and insertion.