A new motif necessary and sufficient for stable localization of the δ2 glutamate receptors at postsynaptic spines

The number of each subclass of ionotropic glutamate receptors (iGluRs) at the spines is differentially regulated either constitutively or in a neuronal activity-dependent manner. The δ2 glutamate receptor (GluRδ2) is abundantly expressed at the spines of Purkinje cell dendrites and controls synaptic plasticity in the cerebellum. To obtain clues to the trafficking mechanism of the iGluRs, we expressed wild-type or mutant GluRδ2 in cultured hippocampal and Purkinje neurons and analyzed their intracellular localization using immunocytochemical techniques. Quantitative analysis revealed that deletion of the 20 amino acids at the center of the C terminus (region E) significantly reduced the amount of GluRδ2 protein at the spines in both types of neurons. This effect was partially antagonized by the inhibition of endocytosis by high dose sucrose treatment or coexpression of dominant negative dynamin. In addition, mutant GluRδ2 lacking the E region (GluRδ2^ΔE), but not wild-type GluRδ2, was found to colocalize with the endosomal markers Rab4 and Rab7. Moreover, the antibody-feeding assay revealed that GluRδ2^ΔE was internalized more rapidly than GluRδ2^wt. These results indicate that the E region (more specifically, a 12-amino-acid-long segment of the E2 region) is necessary for rendering GluRδ2 resistant to endocytosis from the cell surface at the spines. Furthermore, insertion of the E2 region alone into the C terminus of the GluR1 subtype of iGluRs was sufficient to increase the amount of GluR1 proteins in the spines. Therefore, we propose that the E2 region of GluRδ2 is necessary, and also sufficient, to inhibit endocytosis of the receptor from postsynaptic membranes.